49.
]
[Illustration:
3. Trench in firm ground. 4. Trench in Rock.
NOTE.—Underdrains and Wedges to be used only when Ordered by the
Engineer.
]
[Illustration:
7. Brick and Concrete Sewer showing cunette.
]
[Illustration:
5. Soft Foundation. 6. Wet ground.
]
[Illustration:
8. Brick and Concrete Sewer, Evanston, Ill., Eng. Contracting, Vol.
46, p. 227.
]
FIG. 22.
[Illustration]
1. Tunnel Sections. 2. Open Cut Sections.
───────────────────────────────────────────────────────────────────────
Type A. Type B. Type C. Type D.
Where Rock Where Rock Where Rock Where Rock 16′ 6″ Where Rock
is more is more is between drops below Sewer. 25′ is above
than 16′ than 7′ Springing Springing Fill Springing
above and less Line and 7′ Line on Line
Springing than 16′ above either
Line. above Springing Side.
Springing Line on
Line on both Sides.
both Sides.
Mill Creek Sewer, St. Louis, Eng. Record, Vol. 70, pp. 434, 435.
[Illustration:
3. Circular Concrete Section in Soft and Hard Ground, Eng. Record,
Vol. 59, p. 570.
]
[Illustration:
4. Semi-Elliptical Section, Louisville, Ky., Eng. News, Vol. 62, p.
416.
]
[Illustration:
5. Reinforced Concrete Sewer, Harlem Creek, St. Louis, Eng. News, Vol.
60, p. 131.
]
[Illustration:
6. U-Shaped Section, San Francisco, Eng. News, Vol. 73, p. 310.
]
FIG. 23.
Equivalent sections are sections of the same capacity for the same slope
and coefficient of roughness. They have not necessarily the same
dimensions, shape, nor area. The diameter of the equivalent circular
section in terms of the diameter of each special section shown is given
in Table 18. The inside height of a sewer is spoken of as its diameter.
For example let it be required to determine the rate of flow in a
54–inch egg-shaped sewer on a slope of 0.001 when _n_ = .015.
First convert to the equivalent circle. From Table 18 the diameter
of the equivalent circle is 1⁄1.295 times the diameter of the
egg-shaped sewer, which becomes in this case 43 inches. From Fig.
16 the capacity of a circular sewer of this diameter with _S_ =
0.001 and _n_ = .015 is 28 cubic feet per second, which by
definition is the flow in the egg-shaped sewer.
As an example of the reverse process let it be required to find
the velocity of flow in an egg-shaped sewer flowing full and
equivalent to a 48–inch circular sewer. Both sewers are on a slope
of 0.005 and have a roughness coefficient of _n_ = .015. It is
first necessary to find the quantity of flow in the circular
sewer, which by definition is the quantity of flow in the
equivalent egg-shaped sewer. The velocity of flow in the
egg-shaped sewer is found by dividing this quantity by the area of
the egg-shaped section. As read from the diagram the quantity of
flow is 90 cubic feet per second. From Table 18 the area of the
egg-shaped sewer is 0.51_D_^2 where _D_ is the diameter of the
egg-shaped sewer, and _D_ = 1.295_d_ where _d_ is the diameter of
the equivalent circular sewer. Therefore the area equals (0.51) ×
(1.295 × 4)^2 = 13.5 square feet and the velocity of flow is
90⁄13.5 = 6.7 feet per second. This is slightly less than the
velocity in the circular section.
Some lines for egg-shaped sewers have been shown on Fig. 17 by which
solutions can be made directly. For other shapes, and for sizes of
egg-shaped sewers not found on Fig. 17 the preceding method or the
original formula must be used for solution. Problems in partial flow in
special sections are solved similarly to partial flow in circular
sections, by converting first to the conditions of full flow or by
working in the opposite direction.
=40. Non-uniform Flow.=—In the preceding articles it is assumed that the
mean velocity and the rate of flow past all sections are constant. This
condition is known as steady, uniform flow. In this article it will be
assumed that conditions of steady non-uniform flow exist, that is, the
rate of flow past all sections is constant, but the velocity of flow
past these sections is different for different sections. Under such
conditions the surface of the stream is not parallel to the invert of
the channel. If the velocity of flow is increasing down stream the
surface curve is known as the drop-down curve. If the velocity of flow
is decreasing down stream the surface curve is known as the backwater
curve. The hydraulic jump represents a condition of non-uniform flow in
which the velocity of flow decreases down stream in such a manner that
the surface of the stream stands normal to the invert of the channel at
the point where the change in velocity occurs. Above and below this
point conditions of uniform flow may exist.
Conditions of non-uniform flow exist at the outlet of all sewers, except
under the unusual conditions where the depth of flow in the sewer under
conditions of steady, uniform flow with the given rate of discharge
would raise the surface of water in the sewer, at the point of
discharge, to the same elevation as the surface of the body of water
into which discharge is taking place. By an application of the
principles of non-uniform flow to the design of outfall sewers, smaller
sewers, steeper grades, greater depth of cover, and other advantages can
be obtained.
The backwater curve is caused by an obstruction in the sewer, by a
flattening of the slope of the invert, or by allowing the sewer to
discharge into a body of water whose surface elevation would be above
the surface of the water in the sewer, at the point of discharge, under
conditions of steady, uniform flow with the given rate of discharge.
The drop-down curve is caused by a sudden steepening of the slope of the
invert; by allowing a free discharge; or by allowing a discharge into a
body of water whose surface elevation would be below the surface of the
water in the sewer, at the point of discharge, under conditions of
steady, uniform flow with the given rate of discharge. The last
described condition is common at the outlet of many sewers, hence the
common occurrence of the drop-down curve.
The hydraulic jump is a phenomenon which is seldom considered in sewer
design. If not guarded against it may cause trouble at overflow weirs
and at other control devices, in grit chambers, and at unexpected
places. The causes of the hydraulic jump are sufficiently well
understood to permit designs that will avoid its occurrence, but if it
is allowed to occur the exact place of the occurrence of the jump and
its height are difficult, if not impossible, to determine under the
present state of knowledge concerning them. The hydraulic jump will
occur when a high velocity of flow is interrupted by an obstruction in
the channel, by a change in grade of the invert, or the approach of the
velocity to the “critical” velocity. The “critical” velocity is equal to
√(_gh_), where _h_ is the depth of flow and _g_ is the acceleration due
to gravity. The velocity in the channel above the jump must be greater
than √(_gh__{1}), where _h__{1} is the depth of flow in the channel
above the jump. The velocity in the channel below the jump must be
greater than √(_gh__{2}), where _h__{2} is the depth of flow below the
jump. The jump will not take place unless the slope of the invert of the
channel is greater than _g_⁄_C_^2,in which _C_ is the coefficient in the
Chezy formula. With this information it is possible to avoid the jump by
slowing down the velocity by the installation of drop manholes, flight
sewers, or by other expedients.
The shape of the drop-down curve can be expressed, in some cases, by
mathematical formulas of more or less simplicity, dependent on the shape
of the conduit. The formula for a circular conduit is complicated. Due
to the assumptions which must be made in the deduction of these
formulas, the results obtained by their use are of no greater value than
those obtained by approximate methods. A method for the determination of
the drop-down curve is given by C. D. Hill.[32] In this method it is
necessary that the rate of flow past all sections shall be the same;
that the depth of submergence at the outlet shall be known; and that the
depth of flow at some unknown distance up the stream shall be assumed.
The shape and material of construction of the sewer and the slope of the
invert should also be known. The problem is then to determine the
distance between cross-sections, one where the depth of flow is known,
and the other where the depth of flow has been assumed. This distance
can be expressed as follows:
_L_ = ((_d__{2} − _d__{1}) − (_H__{1} − _H__{2}))⁄(_S_ − S_{1}) = (_d_′
− _H_′)⁄_S_′,
in which _L_ = the distance between cross-sections;
_d__{1} = the depth of flow at the lower section;
_d__{2} = the depth of flow at the upper section;
_H__{1} = the velocity head at the lower section;
_H__{2} = the velocity head at the upper section;
_S_ = the hydraulic slope of the stream surface;
_S__{1} = the slope of the invert of the sewer.
In order to solve such problems with a satisfactory degree of accuracy
the difference between _d__{1} and _d__{2} should be taken sufficiently
small to divide the entire length of the sewer to be investigated into a
large number of sections. The solution of the problem requires the
determination of the wetted area, the hydraulic radius, and other
hydraulic elements at many sections. The labor involved can be
simplified by the use of diagrams, such as Fig. 19, or by specially
prepared diagrams such as those accompanying the original article by C.
D. Hill. The solution of the problem can be simplified by tabulating the
computations as follows:
DROP-DOWN CURVE COMPUTATION SHEET
Uniform discharge. Varying depth
┌───────────────────────────────────────────────────────────────────────┐
│ _D_ = _Q_ = _A_ = _V_ = │
│ _Q_⁄_A_ = _S__{1} = _L_ = (_d__{1} − _H__{1})⁄_S__{1} │
├───┬───┬───────┬───┬───┬───────┬───────┬───┬───┬───────┬───┬─────┬─────┤
│ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ 8 │ 9 │ 10 │11 │ 12 │ 13 │
├───┴───┴───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┴─────┤
│ Depth │_R_│_H_│_H__{1}│_d__{1}│_V_│_S_│_S__{1}│_L_│ Elevation │
│ │ │ │ │ − │ │ │ │ │ │
│ │ │ │ │_H__{1}│ │ │ │ │ │
├───┬───┬───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┬─────┤
│_D_│_d_│_d__{1}│ │ │ │ │ │ │ │ │Sewer│W. L.│
├───┼───┼───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┼─────┤
│ │ │ │ │ │ │ │ │ │ │ │ │ │
├───┼───┼───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┼─────┤
│ │ │ │ │ │ │ │ │ │ │ │ │ │
├───┼───┼───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┼─────┤
│ │ │ │ │ │ │ │ │ │ │ │ │ │
At the head of the computation sheet should be recorded the diameter of
the sewer in feet, the assumed volume of flow, the area of the full
cross-section of the sewer, the velocity of the assumed volume flowing
through the full bore of the sewer, and the gradient or slope of the
invert. In the 1st column enter the assumed depth in decimal parts of
the diameter for each cross-section; in the 2nd column enter the same
depth in feet; in the 3rd column enter the difference in feet between
the successive cross-sections; in the 4th column enter the hydraulic
radius corresponding to the depth at each cross-section; in the 8th
column enter the velocity, equal to the volume divided by the wetted
area, for each cross-section; in the 5th column enter the corresponding
velocity head; in the 6th column enter the difference between the
velocity heads at successive cross-sections; in the 7th column enter the
difference between the quantities in the third and in the sixth columns;
in the 9th column enter the hydraulic slope corresponding to the
velocity and hydraulic radius of each cross-section; in the 10th column
enter the difference between the hydraulic slope and the slope or
gradient of the sewer; in the 11th column enter the computed distance
between successive cross-sections; in the 12th column enter the
elevation of the bottom of the sewer at each cross-section; and in the
13th column enter the corresponding elevation of the surface of the
water.
The table should be filled in until the distance to the required section
is determined, or if the distance is known, it should be filled in until
the depth of flow with the assumed rate of discharge has been checked.
If only the depth of flow at some section is known and it is required to
know the maximum rate of flow with a free discharge, or a discharge with
a submergence at the outlet less than the depth of flow with the maximum
rate of discharge, it is necessary to make a preliminary estimate of the
maximum rate of flow in order to fill in the quantity _Q_ at the head of
the table. The procedure should be as follows:
1st. Assume a depth of flow at the outlet.
2nd. Compute the area (_A_) and the hydraulic radius (_R_) at the known
section and at the outlet.
3rd. Determine the area and the hydraulic radius half way between these
two sections as the mean of the areas and the hydraulic radii of
the two sections.
4th. Determine the rate of flow through the sewer from the condition
that the difference in head at the two sections is the head lost
due to friction caused by the average velocity of flow between
the sections (equals (_lV_^2)⁄(_C_^2_R_)) plus the gain in
velocity head (equals _V__{2}^2 − (_V__{1}^2)⁄(2_g_)), which
then combined and transposed result in the expression:
_Q_ = _AA__{1}_A__{2} √(2_Rgh_⁄(2_A__{1}^2_A__{2}^2_gl_ + (_A__{1}
− _A__{2})(_A_^2_C_^2_R_)))
in which _Q_ = rate of flow;
_A_ = the area determined in the 3rd step;
_A__{1} = the area at the upper cross-section;
_A__{2} = the area at the lower cross-section;
_C_ = the coefficient in the Chezy formula;
_g_ = the acceleration due to gravity;
_h_ = the difference in elevation of the surface of the
stream at the two cross-sections;
_l_ = the distance between the cross-sections;
_R_ = the hydraulic radius determined in the third step.
5th. Continue this process by assuming different depths at the outlet
until the maximum rate of discharge has been found by trial.
With this rate of discharge and depth of flow at the outlet, the depth
of flow at the known section can be checked. If appreciably in error a
correction should be made by the assumption of a different depth of flow
at the outlet. The approximate character of the method is scarcely
worthy of the refinement in the results which will be obtained by
checking back for the depth of flow at the known section. It will be
sufficiently accurate to assume the rate of flow obtained by trial from
the preceding expression, as the maximum rate of discharge from the
sewer.
CHAPTER V
DESIGN OF SEWERAGE SYSTEMS
=41. The Plan.=—Good practice demands that a comprehensive plan for a
sewerage system be provided for the needs of a community for the entire
extent of its probable future growth, and that sewers be constructed as
needed in accordance with this plan.
Sewerage systems may be laid out on any one of three systems: separate,
storm, or combined. A separate system of sewers is one in which only
sanitary sewage or industrial wastes or both are allowed to flow. Storm
sewers carry only surface drainage, exclusive of sanitary sewage.
Combined sewers carry both sanitary and storm sewage. The use of a
combined or a separate system of sewerage is a question of expediency.
Portions of the same system may be either separate, combined, or storm
sewers.
Some conditions favorable to the adoption of the separate system are
where:
_a._ The sanitary sewage must be concentrated at one outlet, such
as at a treatment plant, and other outlets are available for the
storm drainage.
_b._ The topography is flat necessitating deep excavation and
steeper grades for the larger combined sewers.
_c._ The sanitary sewers must be placed materially deeper than the
necessary depth for the storm-water drains.
_d._ The sewers are to be laid in rock, necessitating more
difficult excavation for the larger combined sewers.
_e._ An existing sewerage system can be used to convey the dry
weather flow, but is not large enough for the storm sewage.
_f._ The city finances are such that the greater cost of the
combined system cannot be met and sanitary drainage is imperative.
_g._ The district to be sewered is an old residential section
where property values are not increasing and the assessment must
be kept down.
Some additional points given in a report by Alvord and Burdick to the
city of Billings, Montana, are:
The separate system of sewerage should be used, where:
1st. Storm water does not require extensive underground removal,
or where it can be concentrated in a few shallow underground
channels.
2nd. Drainage areas are short and steep facilitating rapid flow of
water over street surfaces to the natural water courses.
3rd. The sanitary sewage must be pumped.
4th. Sewers are being built in advance of the city’s development
to encourage its growth.
5th. The existing sewer is laid at grades unsuitable for sanitary
sewage, it can be used as a storm sewer.
A combined system must be relatively larger than a separate storm
sewer as the latter may overflow on exceptional occasions, but the
former never.
A combined system of sewerage should be used where:
1st. It is evident that storm and sanitary sewerage must be
provided soon.
2nd. Both sanitary and storm sewage must be pumped.
3rd. The district is densely built up.
=42. Preliminary Map.=—The first step in the design of a sewerage system
is the preparation of a map of the district to be served within the
limits of its probable growth. The map should be on a scale of at least
200 feet to the inch in the built up sections or other areas where it is
anticipated that sewers may be built, and where much detail is to be
shown a scale as large as 40 feet to the inch may have to be used. The
adoption of so large a scale will usually necessitate the division of
the city or sewer district into sections. A key map should be drawn to
such a scale that the various sections represented by separate drawings
can all be shown upon it. In preparing the enlarged portions of the map
it is not necessary to include these portions of the city in which it is
improbable that sewers will be constructed, such as parks and
cemeteries.
The contour interval should depend on the character of the district and
the slope of the land. In those sections drawn to a scale of 200 feet to
the inch for slopes over 5 per cent, the contour interval need not be
closer than 10 feet. For slopes between 1 and 5 per cent the contour
interval should be 5 feet. For flatter slopes the interval should not
exceed 2 feet, and a one foot interval is sometimes desirable. In
general the horizontal distances between contours should not exceed 400
feet and they should be close enough to show important features of the
natural drainage. Elevations should also be given at street
intersections, and at abrupt changes in grade. For portions of the map
on a smaller scale the contours need be sufficiently close to show only
the drainage lines and the general slope of the land.
The following may be shown on the preliminary map: the elevation of lots
and cellars; the character of the built up districts, whether cheap
frame residences, flat-roof buildings, manufacturing plants, etc.;
property lines; width of streets between property lines and between curb
lines; the width and character of the sidewalks and pavements; street
car and railroad tracks; existing underground structures such as sewers,
water pipes, telephone conduits, etc.; the location of important
structures which may have a bearing on the design of the sewers such as
bridges, railroad tunnels, deep cuts, culverts, etc.; and the location
of possible sewer outlets and the sites for sewage disposal plants.
Fig. 24 shows a preliminary map for a section of a city, on which the
necessary information has been entered. The map is made from survey
notes. All streets are paved with brick. The alleys are unpaved. The
entire section is built up with high-class detached residences averaging
one to each lot. The lots vary from 1 to 3 feet above the elevation of
the street.
=43. Layout of the Separate System.=—Upon completion of the preliminary
map a tentative plan of the system is laid out. The lines of the sewer
pipe are drawn in pencil, usually along the center line of the street or
alley in such a manner that a sewer will be provided within 50 feet or
less of every lot. The location of the sewers should be such as to give
the most desirable combination of low cost, short house connections,
proper depth for cellar drainage, and avoidance of paved streets. Some
dispute arises among engineers as to the advisability of placing pipes
in alleys, although there is less opposition to so placing sewers than
any other utility conduit. The principal advantage in placing sewers in
alleys is to avoid disturbing the pavement of the street, but if both
street and alley are paved it is usually more economical to place the
sewer in the street as the house connections will be shorter. On
boulevards and other wide streets such as Meridian Avenue in Fig. 24,
the sewers are placed in the parking on each side of the street, rather
than to disturb the pavement and lay long house connections to the
center of the street.
All pipes should be made to slope, where possible, in the direction of
the natural slope of the ground. The preliminary layout of the system is
shown in Fig. 24. The lowest point in the portion of the system shown is
in the alley between Alabama and Tennessee Streets. The flow in all
pipes is towards this point, and only one pipe drains away from any
junction, except that more than one pipe may drain from a terminal
manhole on a summit.
=44. Location and Numbering of Manholes.=—Manholes are next located on
the pipes of this tentative layout. Good practice calls for the location
of a manhole at every change in direction, grade, elevation, or size of
pipe, except in sewers 60 inches in diameter or larger. The manholes
should not be more than 300 to 500 feet apart, and preferably as close
as 200 to 300 feet. In sewers too small for a man to enter the distance
is fixed by the length of sewer rods which can be worked successfully.
In the larger sewers the distances are sometimes made greater but
inadvisedly so, since quick means of escape should be provided for
workmen from a sudden rise of water in the sewer, or the effect of an
asphyxiating gas. In the preliminary layout the manholes are located at
pipe intersections, changes in direction, and not over 300 to 500 feet
apart on long straight runs at convenient points such as opposite street
intersections where other sewers may enter.
No standard system of manhole numbering has been adopted. A system which
avoids confusion and is subject to unlimited extension is to number the
manholes consecutively upwards from the outlet, beginning a new series
of numbers prefixed by some index number or letter for each branch or
lateral. This system has been followed with the manholes on Fig. 24.
[Illustration:
FIG. 24.—Typical Map Used in the Design of a Separate Sewer System.
]
[Illustration:
FIG. 25.—Typical Map Used in the Design of a Storm Sewer System.
]
=45. Drainage Areas.=—The quantity of dry weather sewage is determined
by the population rather than the topography. Lot lines and street
intersections or other artificial lines marking the boundaries between
districts are therefore taken as watershed lines for sanitary sewers.
The quantity of sewage to be carried and the available slope are the
determining factors in fixing the diameter of the sewer. Since there may
be no change in diameter or slope between manholes the quantity of
sewage delivered by a sewer into any manhole will determine the diameter
of the sewer between it and the next manhole above. In order to
determine the additional amount contributed between manholes a line is
drawn around the drainage area tributary to each manhole. This line
generally follows property lines and the center lines of streets or
alleys, its position being such that it includes all the area draining
into one manhole, and excludes all areas draining elsewhere. An entire
lot is usually assumed to lie within the drainage area into which the
building on the lot drains. In laying out these areas it is best to
commence at the upper end of a lateral and work down to a junction. Then
start again at the upper end of another lateral entering this junction,
and continue thus until the map has been covered.
The areas are given the same numbers as the manholes into which they
drain. The dividing lines for the drainage areas on Fig. 24 are shown as
dot and dash lines, and the areas enclosed are appropriately numbered.
If more than one sewer drains into the same manhole the area should be
subdivided so that each subdivision encloses only the area contributing
through one sewer. Such a condition is shown at manhole _C_2. The areas
are designated by subletters or symbols corresponding to the symbol used
for the sewer into which they drain. For example, the two areas
contributing to manhole _C_2 are lettered _C_2_{_K_} and _C_2_{_D_}. The
sewer from manhole _C_3 to _C_2 receives no addition, it being assumed
that all the lots adjacent to it drain into the sewer on the alley.
There is therefore no area _C_2. Likewise there is no area _A_1_{_C_}.
=46. Quantity of Sewage.=—The remaining work in the computation of the
quantity of sewage is best kept in order by a tabulation. Table 19 shows
the computations for the sewers discharging from the east into manhole
No. 142. The computation should begin at the upper end of a lateral,
continue to a junction, and then start again at the upper end of another
lateral entering this junction. Each line in the table should be filled
in completely from left to right before proceeding with the computations
on the next line. In the illustrative solution in Table 19, computations
for quantity have not been made between manholes where it was apparent
that there would be an insufficient additional quantity to necessitate a
change in the size of the pipe.
In making these computations the assumptions of quantity and other
factors given below indicate the sort of assumptions which must be made,
based on such studies as are given in Chapter III. The density of
population was taken as 20 persons per acre, the assumption being based
on the census and the character of the district. The average sanitary
sewage flow was taken as 100 gallons per capita per day. The per cent
which the maximum dry weather flow is of the average was taken as _M_ =
500⁄_P_^⅕, in which _P_ is the population in thousands. The per cent is
not to exceed 500 nor to be less than 150. The rate of infiltration of
ground water was assumed as 50,000 gallons per mile of pipe per day.
In the first line of Table 19, the entries in columns (1) to (6) are
self-explanatory. There are no entries in columns (7) to (10), as no
additional sewage is contributed between manholes 3.5 and 3.4. In column
(11), 2250 persons are recorded as the number tributary to manhole No.
3.5 in the district to the north and west. These people contribute an
average of 100 gallons per person per day, or a total of 0.346 second
foot. This quantity is entered in column (13). The figure in column (14)
is obtained from the expression _M_ = (500)⁄_P_^⅕. Column (15) is .01 of
the product of columns (13) and (14). Column (16) is the product of the
length of pipe between manholes 3.5 and 3.4, and the ground water unit
reduced to cubic feet per second. Column (17) is the sum of column (16),
and all of the ground water tributary to manhole 3.5, which is not
recorded in the table. Column (18) is the sum of columns (15) and (17).
No new principle is represented in the second and third lines.
In the fourth line the first 10 columns need no further explanation. The
(11th) column is the sum of the (10th) column, and the (11th) column in
the third line. It represents the total number of persons tributary to
manhole 3.4 on lateral No. 8. Column (13) in the fourth line is the sum
of column (13) in the third line and the (12th) column in the fourth
line, and the (15th) column in the fourth line is the product of the 2
preceding columns in the fourth line. Note that in no case is the figure
in column (15) the sum of any previous figures in column (15). With this
introduction the student should be able to check the remaining figures
in the table, and should compute the quantity of sewage entering manhole
No. 142 from the west, making reasonable assumptions for the tributary
quantities from beyond the limits of the map.
TABLE 19
COMPUTATIONS FOR QUANTITY OF SEWAGE FOR A SEPARATE SEWERAGE SYSTEM
──────────┬──────────┬──────────┬───────┬───────┬──────┬───────┬─────
On Street │ From │To Street │ From │ To │Length│Mark of│Area,
│ Street │ │Manhole│Manhole│ Feet │ Added │Acres
│ │ │ │ │ │ Areas │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┼─────
Nebraska │Map margin│Alley S. │ 3.5│ 3.4│ 338│ │
St. │ │ Grant │ │ │ │ │
│ │ St. │ │ │ │ │
Alley S. │Railroad │E. of │ 8.3│ 8.2│ 328│ 8.2│ 2.7
of Grant│ │ Missouri│ │ │ │ │
St. │ │ St. │ │ │ │ │
Alley S. │E. of │E. of │ 8.2│ 8.1│ 355│ 8.1│ 3.41
of Grant│ Missouri│ Kansas │ │ │ │ │
St. │ St. │ St. │ │ │ │ │
Alley S. │E. of │Nebraska │ 8.1│ 3.4│ 340│3.4_{8}│ 2.68
of Grant│ Kansas │ St. │ │ │ │ │
St. │ St. │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.4│ 3.3│ 380│ │
St. │ of Grant│ of │ │ │ │ │
│ St. │ Meridian│ │ │ │ │
│ │ │ │ │ │ 7.1│
Alley S. │Railroad │Nebraska │ 7.2│ 3.3│ 800│3.3_{7}│ 7.14
of │ │ St. │ │ │ │ │
Meridian│ │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.3│ 3.2│ 304│ │
St. │ of │ of Smith│ │ │ │ │
│ Meridian│ Av. │ │ │ │ │
│ │ │ │ │ │ 6.1│
Alley S. │Railroad │Nebraska │ 6.2│ 3.2│ 609│3.2_{6}│ 3.82
of Smith│ │ St. │ │ │ │ │
Ave. │ │ │ │ │ │ │
Nebraska │Alley S. │S. of │ 3.2│ 3.1│ 300│ │
St. │ of Smith│ Cordovez│ │ │ │ │
│ Ave. │ St. │ │ │ │ │
S. of │Railroad │Nebraska │ 4.1│ 3.1│ 410│3.1_{4}│ 3.10
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
S. of │Map margin│Nebraska │ 5.1│ 3.1│ 380│3.1_{5}│ 2.69
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
Nebraska │S. of │Long St. │ 3.1│ 148│ 172│ │
St. │ Cordovez│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Map margin│Nebraska │ 149│ 148│ 380│ 148│ 1.53
│ │ St. │ │ │ │ │
Long St. │Nebraska │N. │ 148│ 147│ 492│ │
│ St. │ Carolina│ │ │ │ │
│ │ St. │ │ │ │ │
Long St. │N. │Georgia │ 147│ 146│ 430│ │
│ Carolina│ St. │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Georgia │Harris St.│ 146│ 145│ 419│ 146│ 0.81
│ St. │ │ │ │ │ │
│ │ │ │ │ │ 2.1│
Long St. │Harris St.│Tennessee │ 145│ 143│ 725│143–145│ 6.6
│ │ St. │ │ │ │ │
│ │ │ │ │ │ │
Column No.│ (2) │ (3) │ (4) │ (5) │ (6) │ (7) │ (8)
(1) │ │ │ │ │ │ │
──────────┴──────────┴──────────┴───────┴───────┴──────┴───────┴─────
──────────┬──────────┬──────────┬──────────┬───────┬─────────┬─────────
On Street │ From │To Street │Population│Number │ Total │ Avg.
│ Street │ │ per Acre │ of │ Persons │Sanitary
│ │ │ │Persons│Tributary│ Flow,
│ │ │ │ │ │ C.F.S.
│ │ │ │ │ │
──────────┼──────────┼──────────┼──────────┼───────┼─────────┼─────────
Nebraska │Map margin│Alley S. │ │ │ 2250│ 0.0000
St. │ │ Grant │ │ │ │
│ │ St. │ │ │ │
Alley S. │Railroad │E. of │ 20│ 54│ 54│ .0084
of Grant│ │ Missouri│ │ │ │
St. │ │ St. │ │ │ │
Alley S. │E. of │E. of │ 20│ 68│ 122│ .0106
of Grant│ Missouri│ Kansas │ │ │ │
St. │ St. │ St. │ │ │ │
Alley S. │E. of │Nebraska │ 20│ 54│ 176│ .0084
of Grant│ Kansas │ St. │ │ │ │
St. │ St. │ │ │ │ │
Nebraska │Alley S. │Alley S. │ │ │ 2428│ .0000
St. │ of Grant│ of │ │ │ │
│ St. │ Meridian│ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ 20│ 142│ 142│ .0221
of │ │ St. │ │ │ │
Meridian│ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ │ │ 2568│ .0000
St. │ of │ of Smith│ │ │ │
│ Meridian│ Av. │ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ 20│ 76│ 76│ .0119
of Smith│ │ St. │ │ │ │
Ave. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ │ │ 2644│ .0000
St. │ of Smith│ Cordovez│ │ │ │
│ Ave. │ St. │ │ │ │
S. of │Railroad │Nebraska │ 20│ 62│ 62│ .0096
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
S. of │Map margin│Nebraska │ 20│ 54│ 54│ .0084
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
Nebraska │S. of │Long St. │ │ │ 2760│ .0000
St. │ Cordovez│ │ │ │ │
│ St. │ │ │ │ │
Long St. │Map margin│Nebraska │ 20│ 31│ 31│ .0048
│ │ St. │ │ │ │
Long St. │Nebraska │N. │ │ │ 2791│ .0000
│ St. │ Carolina│ │ │ │
│ │ St. │ │ │ │
Long St. │N. │Georgia │ │ │ 2791│1.000[33]
│ Carolina│ St. │ │ │ │
│ St. │ │ │ │ │
Long St. │Georgia │Harris St.│ 20│ 16│ 2807│ .0025
│ St. │ │ │ │ │
│ │ │ │ │ │
Long St. │Harris St.│Tennessee │ 20│ 132│ 2936│ .0205
│ │ St. │ │ │ │
│ │ │ │ │ │
Column No.│ (2) │ (3) │ (9) │ (10) │ (11) │ (12)
(1) │ │ │ │ │ │
──────────┴──────────┴──────────┴──────────┴───────┴─────────┴─────────
──────────┬──────────┬──────────┬──────────┬────────┬─────────
On Street │ From │To Street │Cumulative│Per cent│ Total
│ Street │ │ Avg. │ Max. │ Max.
│ │ │ Sanitary │Sanitary│Sanitary,
│ │ │ Flow, │ is of │ C.F.S.
│ │ │ C.F.S. │Average │
──────────┼──────────┼──────────┼──────────┼────────┼─────────
Nebraska │Map margin│Alley S. │ 0.346│ 425│ 1.47
St. │ │ Grant │ │ │
│ │ St. │ │ │
Alley S. │Railroad │E. of │ .0084│ 500│ 0.041
of Grant│ │ Missouri│ │ │
St. │ │ St. │ │ │
Alley S. │E. of │E. of │ .0190│ 500│ 0.095
of Grant│ Missouri│ Kansas │ │ │
St. │ St. │ St. │ │ │
Alley S. │E. of │Nebraska │ .0274│ 500│ 0.137
of Grant│ Kansas │ St. │ │ │
St. │ St. │ │ │ │
Nebraska │Alley S. │Alley S. │ .373│ 423│ 1.58
St. │ of Grant│ of │ │ │
│ St. │ Meridian│ │ │
│ │ │ │ │
Alley S. │Railroad │Nebraska │ .0221│ 500│ 0.111
of │ │ St. │ │ │
Meridian│ │ │ │ │
Nebraska │Alley S. │Alley S. │ .395│ 414│ 1.63
St. │ of │ of Smith│ │ │
│ Meridian│ Av. │ │ │
│ │ │ │ │
Alley S. │Railroad │Nebraska │ .0119│ 500│ 0.060
of Smith│ │ St. │ │ │
Ave. │ │ │ │ │
Nebraska │Alley S. │S. of │ .407│ 414│ 1.68
St. │ of Smith│ Cordovez│ │ │
│ Ave. │ St. │ │ │
S. of │Railroad │Nebraska │ .0096│ 500│ 0.048
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
S. of │Map margin│Nebraska │ .0084│ 500│ 0.042
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
Nebraska │S. of │Long St. │ .425│ 409│ 1.74
St. │ Cordovez│ │ │ │
│ St. │ │ │ │
Long St. │Map margin│Nebraska │ .0048│ 500│ 0.024
│ │ St. │ │ │
Long St. │Nebraska │N. │ .430│ 409│ 1.76
│ St. │ Carolina│ │ │
│ │ St. │ │ │
Long St. │N. │Georgia │ .430│ 409│ 1.76
│ Carolina│ St. │ │ │
│ St. │ │ │ │
Long St. │Georgia │Harris St.│ .433│ 407│ 1.76
│ St. │ │ │ │
│ │ │ │ │
Long St. │Harris St.│Tennessee │ .454│ 403│ 1.83
│ │ St. │ │ │
│ │ │ │ │
Column No.│ (2) │ (3) │ (13) │ (14) │ (15)
(1) │ │ │ │ │
──────────┴──────────┴──────────┴──────────┴────────┴─────────
──────────┬──────────┬──────────┬─────────┬──────────┬──────┬──────
On Street │ From │To Street │Increment│Cumulative│Total │ Line
│ Street │ │of Ground│ Ground │Flow, │Number
│ │ │ Water, │ Water, │C.F.S.│
│ │ │ C.F.S. │ C.F.S. │ │
│ │ │ │ │ │
──────────┼──────────┼──────────┼─────────┼──────────┼──────┼──────
Nebraska │Map margin│Alley S. │ 0.005│ 0.0187│ 1.66│ 1
St. │ │ Grant │ │ │ │
│ │ St. │ │ │ │
Alley S. │Railroad │E. of │ .0048│ .0048│ 0.046│ 2
of Grant│ │ Missouri│ │ │ │
St. │ │ St. │ │ │ │
Alley S. │E. of │E. of │ .0052│ .010│ 0.105│ 3
of Grant│ Missouri│ Kansas │ │ │ │
St. │ St. │ St. │ │ │ │
Alley S. │E. of │Nebraska │ .0050│ .015│ 0.152│ 4
of Grant│ Kansas │ St. │ │ │ │
St. │ St. │ │ │ │ │
Nebraska │Alley S. │Alley S. │ .0058│ .208│ 1.79│ 5
St. │ of Grant│ of │ │ │ │
│ St. │ Meridian│ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ .0117│ .0117│ 0.123│ 6
of │ │ St. │ │ │ │
Meridian│ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ .0045│ .224│ 1.85│ 7
St. │ of │ of Smith│ │ │ │
│ Meridian│ Av. │ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ .0089│ .0089│ 0.069│ 8
of Smith│ │ St. │ │ │ │
Ave. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ .0044│ .237│ 1.92│ 9
St. │ of Smith│ Cordovez│ │ │ │
│ Ave. │ St. │ │ │ │
S. of │Railroad │Nebraska │ .006│ .006│ 0.054│ 10
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
S. of │Map margin│Nebraska │ .0056│ .0056│ 0.048│ 11
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
Nebraska │S. of │Long St. │ .0025│ .251│ 1.99│ 12
St. │ Cordovez│ │ │ │ │
│ St. │ │ │ │ │
Long St. │Map margin│Nebraska │ .0056│ .0056│ 0.030│ 13
│ │ St. │ │ │ │
Long St. │Nebraska │N. │ .0072│ .264│ 2.02│ 14
│ St. │ Carolina│ │ │ │
│ │ St. │ │ │ │
Long St. │N. │Georgia │ .0064│ 1.27│ 3.03│ 15
│ Carolina│ St. │ │ │ │
│ St. │ │ │ │ │
Long St. │Georgia │Harris St.│ .0061│ 1.28│ 3.04│ 16
│ St. │ │ │ │ │
│ │ │ │ │ │
Long St. │Harris St.│Tennessee │ .024│ 1.30│ 3.13│ 17
│ │ St. │ │ │ │
│ │ │ │ │ │
Column No.│ (2) │ (3) │ (16) │ (17) │ (18) │
(1) │ │ │ │ │ │
──────────┴──────────┴──────────┴─────────┴──────────┴──────┴──────
TABLE 20
COMPUTATIONS FOR SLOPE AND DIAMETER OF PIPES FOR A SEPARATE SEWERAGE
SYSTEM
──────────┬──────────┬──────────┬───────┬───────┬──────┬───────────────
On Street │ From │To Street │ From │ To │Length│El. of Surface
│ Street │ │Manhole│Manhole│ Feet │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┬───────
│ │ │ │ │ │ Upper │ Lower
│ │ │ │ │ │Manhole│Manhole
──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┼───────
Nebraska │Map margin│Alley S. │ 3.5│ 3.4│ 338│ 105.8│ 102.4
St. │ │ Grant │ │ │ │ │
│ │ St. │ │ │ │ │
Alley S. │Railroad │E. of │ 8.3│ 8.2│ 328│ 113.5│ 112.0
of Grant│ │ Missouri│ │ │ │ │
St. │ │ St. │ │ │ │ │
Alley S. │E. of │E. of │ 8.2│ 8.1│ 355│ 112.0│ 107.7
of Grant│ Missouri│ Kansas │ │ │ │ │
St. │ St. │ St. │ │ │ │ │
Alley S. │E. of │Nebraska │ 8.1│ 3.4│ 340│ 107.7│ 102.4
of Grant│ Kansas │ St. │ │ │ │ │
St. │ St. │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.4│ 3.3│ 380│ 102.4│ 100.7
St. │ of Grant│ of │ │ │ │ │
│ St. │ Meridian│ │ │ │ │
Alley S. │Railroad │Kansas St.│ 7.2│ 7.1│ 400│ 111.8│ 107.0
of │ │ │ │ │ │ │
Meridian│ │ │ │ │ │ │
Alley S. │Kansas St.│Nebraska │ 7.1│ 3.3│ 400│ 107.0│ 100.7
of │ │ St. │ │ │ │ │
Meridian│ │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.3│ 3.2│ 304│ 100.7│ 99.3
St. │ of │ of Smith│ │ │ │ │
│ Meridian│ Av. │ │ │ │ │
Alley S. │Railroad │East of │ 6.2│ 6.1│ 305│ 109.3│ 105.3
of Smith│ │ Kansas │ │ │ │ │
Ave. │ │ St. │ │ │ │ │
Alley S. │East of │Nebraska │ 6.1│ 3.2│ 304│ 105.3│ 99.3
of Smith│ Kansas │ St. │ │ │ │ │
Ave. │ St. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ 3.2│ 3.1│ 300│ 99.3│ 101.1
St. │ of Smith│ Cordovez│ │ │ │ │
│ Ave. │ St. │ │ │ │ │
S. of │Railroad │Nebraska │ 4.1│ 3.1│ 410│ 100.8│ 101.1
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
S. of │Map margin│Nebraska │ 5.1│ 3.1│ 380│ 104.6│ 101.1
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
Nebraska │S. of │Long St. │ 3.1│ 148│ 172│ 101.1│ 98.7
St. │ Cordovez│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Map margin│Nebraska │ 149│ 148│ 380│ 103.8│ 98.7
│ │ St. │ │ │ │ │
Long St. │Nebraska │N. │ 148│ 147│ 492│ 98.7│ 103.8
│ St. │ Carolina│ │ │ │ │
│ │ St. │ │ │ │ │
Long St. │N. │Georgia │ 147│ 146│ 430│ 103.8│ 99.1
│ Carolina│ St. │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Georgia │Harris St.│ 146│ 145│ 419│ 99.1│ 96.9
│ St. │ │ │ │ │ │
Alley S. │End of │Harris St.│ 2.2│ 2.1│ 350│ 105.2│ 98.1
of Janis│ Janis │ │ │ │ │ │
St. │ St. │ │ │ │ │ │
Harris St.│Alley N. │Long St. │ 2.1│ 145│ 135│ 98.1│ 96.9
│ of Janis│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Harris St.│Kentucky │ 145│ 144│ 258│ 96.9│ 94.4
│ │ St. │ │ │ │ │
Long St. │Kentucky │Tennessee │ 144│ 143│ 282│ 94.4│ 93.6
│ St. │ St. │ │ │ │ │
Tarbell │Harris St.│Long St. │ 1.1│ 143│ 417│ 98.7│ 92.6
Ave. │ │ │ │ │ │ │
Long St. │Tennessee │Alley W. │ 143│ 142│ 185│ 92.6│ 92.3
│ St. │ of Tenn.│ │ │ │ │
│ │ St. │ │ │ │ │
│ │ │ │ │ │ │
Column No.│ (2) │ (3) │ (4) │ (5) │ (6) │ (7) │ (8)
(1) │ │ │ │ │ │ │
──────────┴──────────┴──────────┴───────┴───────┴──────┴───────┴───────
──────────┬──────────┬──────────┬──────┬──────┬──────┬────────┬────────
On Street │ From │To Street │Total │Slope │ Dia. │Velocity│Capacity
│ Street │ │Flow, │ │ of │ when │ when
│ │ │C.F.S.│ │Pipe, │ Full, │ Full,
│ │ │ │ │Inches│Ft. per │ Second
│ │ │ │ │ │ Second │ Feet
──────────┼──────────┼──────────┼──────┼──────┼──────┼────────┼────────
│ │ │ │ │ │ │
│ │ │ │ │ │ │
──────────┼──────────┼──────────┼──────┼──────┼──────┼────────┼────────
Nebraska │Map margin│Alley S. │ 1.66│0.0108│ 10│ 3.25│ 1.78
St. │ │ Grant │ │ │ │ │
│ │ St. │ │ │ │ │
Alley S. │Railroad │E. of │ 0.046│.00575│ 8│ 2.00│ 0.71
of Grant│ │ Missouri│ │ │ │ │
St. │ │ St. │ │ │ │ │
Alley S. │E. of │E. of │ 0.105│ .0110│ 8│ 2.78│ 0.98
of Grant│ Missouri│ Kansas │ │ │ │ │
St. │ St. │ St. │ │ │ │ │
Alley S. │E. of │Nebraska │ 0.152│ .0156│ 8│ 3.27│ 1.18
of Grant│ Kansas │ St. │ │ │ │ │
St. │ St. │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 1.79│.00385│ 12│ 2.28│ 1.79
St. │ of Grant│ of │ │ │ │ │
│ St. │ Meridian│ │ │ │ │
Alley S. │Railroad │Kansas St.│ │ .0120│ 8│ 2.90│ 1.03
of │ │ │ │ │ │ │
Meridian│ │ │ │ │ │ │
Alley S. │Kansas St.│Nebraska │ 0.123│ .0157│ 8│ 3.28│ 1.18
of │ │ St. │ │ │ │ │
Meridian│ │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 1.85│ .0042│ 12│ 2.36│ 1.85
St. │ of │ of Smith│ │ │ │ │
│ Meridian│ Av. │ │ │ │ │
Alley S. │Railroad │East of │ │ .0131│ 8│ 3.00│ 1.08
of Smith│ │ Kansas │ │ │ │ │
Ave. │ │ St. │ │ │ │ │
Alley S. │East of │Nebraska │ 0.069│ .0197│ 8│ 3.70│ 1.32
of Smith│ Kansas │ St. │ │ │ │ │
Ave. │ St. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ 1.92│.00213│ 15│ 2.00│ 2.45
St. │ of Smith│ Cordovez│ │ │ │ │
│ Ave. │ St. │ │ │ │ │
S. of │Railroad │Nebraska │ │.00574│ 8│ 2.00│ 0.71
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
S. of │Map margin│Nebraska │ 0.054│.00854│ 8│ 2.46│ 0.87
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
Nebraska │S. of │Long St. │ 1.99│.00213│ 15│ 2.00│ 2.45
St. │ Cordovez│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Map margin│Nebraska │ 0.030│ .0134│ 8│ 3.04│ 1.08
│ │ St. │ │ │ │ │
Long St. │Nebraska │N. │ 2.02│.00213│ 15│ 2.00│ 2.45
│ St. │ Carolina│ │ │ │ │
│ │ St. │ │ │ │ │
Long St. │N. │Georgia │ 3.03│ .0016│ 18│ 2.00│ 3.50
│ Carolina│ St. │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Georgia │Harris St.│ 3.04│ .0016│ 18│ 2.00│ 3.50
│ St. │ │ │ │ │ │
Alley S. │End of │Harris St.│ │ .0203│ 8│ 3.78│ 1.35
of Janis│ Janis │ │ │ │ │ │
St. │ St. │ │ │ │ │ │
Harris St.│Alley N. │Long St. │ │ .0088│ 8│ 2.53│ 0.89
│ of Janis│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Harris St.│Kentucky │ │.00353│ 18│ 2.98│ 5.20
│ │ St. │ │ │ │ │
Long St. │Kentucky │Tennessee │ │.00635│ 18│ 4.00│ 7.00
│ St. │ St. │ │ │ │ │
Tarbell │Harris St.│Long St. │ │ .0146│ 8│ 3.18│ 1.14
Ave. │ │ │ │ │ │ │
Long St. │Tennessee │Alley W. │ 3.13│ .0016│ 18│ 2.00│ 3.50
│ St. │ of Tenn.│ │ │ │ │
│ │ St. │ │ │ │ │
│ │ │ │ │ │ │
Column No.│ (2) │ (3) │ (9) │ (10) │ (11) │ (12) │ (13)
(1) │ │ │ │ │ │ │
──────────┴──────────┴──────────┴──────┴──────┴──────┴────────┴────────
──────────┬──────────┬──────────┬───────────────┬──────
On Street │ From │To Street │ El. of Invert │ Line
│ Street │ │ │Number
│ │ │ │
│ │ │ │
│ │ │ │
──────────┼──────────┼──────────┼───────┬───────┼──────
│ │ │ Upper │ Lower │
│ │ │Manhole│Manhole│
──────────┼──────────┼──────────┼───────┼───────┼──────
Nebraska │Map margin│Alley S. │ 97.80│ 94.40│ 1
St. │ │ Grant │ │ │
│ │ St. │ │ │
Alley S. │Railroad │E. of │ 105.50│ 103.62│ 2
of Grant│ │ Missouri│ │ │
St. │ │ St. │ │ │
Alley S. │E. of │E. of │ 103.61│ 99.70│ 3
of Grant│ Missouri│ Kansas │ │ │
St. │ St. │ St. │ │ │
Alley S. │E. of │Nebraska │ 99.69│ 94.40│ 4
of Grant│ Kansas │ St. │ │ │
St. │ St. │ │ │ │
Nebraska │Alley S. │Alley S. │ 94.07│ 92.61│ 5
St. │ of Grant│ of │ │ │
│ St. │ Meridian│ │ │
Alley S. │Railroad │Kansas St.│ 103.80│ 99.00│ 6
of │ │ │ │ │
Meridian│ │ │ │ │
Alley S. │Kansas St.│Nebraska │ 98.99│ 92.70│ 7
of │ │ St. │ │ │
Meridian│ │ │ │ │
Nebraska │Alley S. │Alley S. │ 92.37│ 91.09│ 8
St. │ of │ of Smith│ │ │
│ Meridian│ Av. │ │ │
Alley S. │Railroad │East of │ 101.30│ 97.30│ 9
of Smith│ │ Kansas │ │ │
Ave. │ │ St. │ │ │
Alley S. │East of │Nebraska │ 97.29│ 91.30│ 10
of Smith│ Kansas │ St. │ │ │
Ave. │ St. │ │ │ │
Nebraska │Alley S. │S. of │ 90.84│ 90.20│ 11
St. │ of Smith│ Cordovez│ │ │
│ Ave. │ St. │ │ │
S. of │Railroad │Nebraska │ 92.80│ 90.62│ 12
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
S. of │Map margin│Nebraska │ 96.60│ 93.10│ 13
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
Nebraska │S. of │Long St. │ 90.04│ 89.87│ 14
St. │ Cordovez│ │ │ │
│ St. │ │ │ │
Long St. │Map margin│Nebraska │ 95.80│ 90.70│ 15
│ │ St. │ │ │
Long St. │Nebraska │N. │ 89.86│ 88.94│ 16
│ St. │ Carolina│ │ │
│ │ St. │ │ │
Long St. │N. │Georgia │ 88.69│ 88.00│ 17
│ Carolina│ St. │ │ │
│ St. │ │ │ │
Long St. │Georgia │Harris St.│ 87.99│ 87.32│ 18
│ St. │ │ │ │
Alley S. │End of │Harris St.│ 97.20│ 90.10│ 19
of Janis│ Janis │ │ │ │
St. │ St. │ │ │ │
Harris St.│Alley N. │Long St. │ 90.09│ 88.90│ 20
│ of Janis│ │ │ │
│ St. │ │ │ │
Long St. │Harris St.│Kentucky │ 87.31│ 86.40│ 21
│ │ St. │ │ │
Long St. │Kentucky │Tennessee │ 86.39│ 84.60│ 22
│ St. │ St. │ │ │
Tarbell │Harris St.│Long St. │ 90.70│ 84.60│ 23
Ave. │ │ │ │ │
Long St. │Tennessee │Alley W. │ 83.77│ 83.47│ 24
│ St. │ of Tenn.│ │ │
│ │ St. │ │ │
│ │ │ │ │
Column No.│ (2) │ (3) │ (14) │ (15) │
(1) │ │ │ │ │
──────────┴──────────┴──────────┴───────┴───────┴──────
=47. Surface Profile.=—A profile of the surface of the ground along the
proposed lines of the sewers should be drawn after the completion of the
computations for quantity. An example of a profile is shown in Fig. 26
for the line between manholes No. 3.5 and No. 147. The vertical scale
should be at least 10 times the horizontal. A horizontal scale of 1 inch
to 200 feet can be used where not much detail is to be shown, but a
scale of one 1 to 100 feet is more common and more satisfactory and even
one inch to 10 feet has been used. The information to be given and the
method of showing it are illustrated on Fig. 26. The profile should show
the character of the material to be passed through and the location of
underground obstacles which may be encountered. The method of obtaining
this information is taken up in Chapter II. The collection of the
information should be completed as far as possible previous to design,
and borings and other investigations made as soon as the tentative
routes for the sewers have been selected.
=48. Slope and Diameter of Sewers.=—After the quantity of sewage to be
carried has been determined, and the profile of the ground surface has
been drawn, it is possible to determine the slope and diameter of the
sewer. A table such as No. 20 is made up somewhat similar to No. 19, or
which may be an extension of Table 19 since the first 6 columns in both
tables are the same. The elevation of the surface at the upper and lower
manholes is read from the profile.
The depth of the sewer below the ground surface is first determined.
Sewers should be sufficiently deep to drain cellars of ordinary depth.
In residential districts cellars are seldom more than 5 feet below the
ground surface. To this depth must be added the drop necessary for the
grade of the house sewer. Six-inch pipe laid on a minimum grade of 1.67
per cent is a common size and slope restriction for house drains or
sewers. An additional 12 inches should be allowed for the bends in the
pipe and the depth of the pipe under the cellar floor. Where the
elevation of the street and lots is about the same, and the street is
not over 80 feet in width between property lines, a minimum depth of 8
feet to the invert of sewers, 24 inches or less in diameter is
satisfactory. This is on the assumption that the axes of the house drain
and the sewer intersect. For larger pipes the depth should be increased
so that when the street sewer is flowing full, sewage will not back up
into the cellars or for any great distance into the tributary pipes.
[Illustration:
FIG. 26.—Typical Profile Used in the Design of a Separate Sewer
System.
]
The grade or slope at which a sewer shall be may be fixed by: the slope
of the ground surface; the minimum permissible self-cleansing velocity;
a combination of diameter, velocity, and quantity; or the maximum
permissible velocity of flow. Sewers are laid either parallel to the
ground surface where the slope is sufficient or where possible without
coming too near the surface they are laid on a flatter grade to avoid
unnecessary excavation. The minimum permissible slope is fixed by the
minimum permissible velocity.
The velocity of flow in a sewer should be sufficient to prevent the
sedimentation of sludge and light mineral matter. Such a velocity is in
the neighborhood of 1 foot per second. Since sewers seldom flow full
this velocity should be available under ordinary conditions of dry
weather flow. The minimum velocity when full should therefore be about 2
feet per second. Under this condition, the velocity of 1 foot per second
is not reached until the sewer is less than 18 per cent full. The
velocity in small sewers should be made somewhat faster than in large
sewers since the velocity of flow for small depths in small pipes is
less than for the same proportionate depth in large pipes. The maximum
permissible velocity of flow is fixed at about 10 feet per second in
order to avoid excessive erosion of the invert. If the sewer is
carefully laid this limit may be exceeded in sanitary sewers.
The method for determining the grade and diameter of sewers is best
explained through an illustrative problem which is worked out in Table
20 for the profile shown on Fig. 26. The figures are inserted in the
table from left to right in each line, one line being completed before
the next one is commenced. The headings in the first 6 columns are
self-explanatory. The elevations of the surface at the upper and lower
manholes are read from the profile. The total flow is read from column
(18) in Table 19. The slope of the ground surface is then computed, and
with the quantity, slope, and coefficient of roughness, the diameter of
the pipe and the velocity of flow are read from Fig. 15.
The following conditions may arise:
(1) The diameter required is less than 8 inches. Use a diameter of
8 inches as experience has shown that the use of smaller diameters
is unsatisfactory.
(2) The velocity of flow when the sewer is full is less than 2
feet per second. Increase the slope until the velocity when full
is 2 feet per second.
(3) The diameter of the pipe required is not one of the commercial
sizes shown in Fig. 15. Use the next largest commercial size.
(4) The slope of the ground surface is steeper than necessary to
maintain the required minimum velocity and the upper end of the
sewer is deeper than the required minimum depth. Place the sewer
on the minimum permissible grade, or upon such a grade that its
lower end will be at the minimum permissible depth.
(5) The slope of the ground surface is so steep as to make the
velocity of flow greater than the maximum rate permissible. Reduce
the grade by deepening the sewer at the upper manhole and using a
drop manhole at this point.
It is not permissible to use a pipe larger than that called for by the
above conditions. This is attempted sometimes in order to reduce the
grade and thereby save excavation, under the rule of a minimum velocity
of 2 feet per second when full. It is better to use the smaller pipe on
the flat grade as the quantity of sewage is insufficient to fill the
larger sewer and the minimum permissible velocity is more quickly
reached.
Having determined the slope, the diameter, and the capacity of the pipe
to be used, these values are entered in the table. The elevations of the
invert of the pipe at the upper and lower manholes are next computed and
entered in the table. This method is followed until all of the
diameters, slopes, and elevations have been determined.
The slopes are computed from center to center of manholes, but an extra
allowance of 0.01 of a foot is allowed by some designers for the
increased loss in head in passing through the manhole. When it becomes
necessary to increase the diameter of the sewer the top of the outgoing
sewer is placed at the same elevation or below the top of the lowest
incoming sewer. No extra allowance is made to compensate for loss in
head in the manhole in this case. This case is illustrated in columns
(14) and (15) in lines (16) and (17) of Table 20. All of the conditions
listed above are illustrated in Table 20, except the condition for a
velocity greater than 10 feet per second.
The first condition is met at the head of practically every lateral, and
is illustrated in the second line.
The second condition is also illustrated in the second line. The slope
of the ground surface is 0.0046, which gives a velocity of only 1.8 feet
per second in an 8–inch pipe. The slope is therefore increased to
0.00575, on which the full velocity is 2 feet per second.
The third condition is met in the first line. The diameter called for to
carry 1.66 cubic feet per second on a slope of 0.0108 is slightly less
than 10 inches. A 10–inch pipe is therefore used and its full capacity
and velocity are recorded.
The fourth condition is illustrated in the fourteenth line. The cut at
manhole No. 3.1 is 11.1 feet. The slope of the ground is 0.014, much
steeper than is necessary to maintain the minimum velocity in a 15–inch
pipe. The pipe is therefore placed on the minimum permissible slope, and
excavation is saved. The student should check the figures in Table 20
and be sure that they are understood before an attempt is made to make a
design independently.
=49. The Sewer Profile.=—The profile is next completed as shown in Fig.
26, the pipe line being drawn in as the computations are made. The cut
is recorded to the nearest ⅒th of a foot at each manhole, or change in
grade. It should not be given elsewhere as it invites controversy with
the contractor. The cut is the difference of the elevation of the invert
of the lowest pipe in the trench at the point in question, and the
surface of the ground.
The stationing should be shown to the nearest ⅒th of a foot. It should
commence at 0 + 00 at the outlet and increase up the sewer. The station
of any point on the sewer may show the distance from it to the outlet,
or a new system of stationing may be commenced at important junctions or
at each junction.
Elevations of the surface of the ground should be shown to the nearest
⅒th of a foot, and the invert elevation to the nearest 1/100th of a
foot.
Only the main line sewer is shown in profile in Fig. 26. The profiles of
the laterals computed in Table 20, have not been shown. The approximate
location of all house inlets are shown on the profile and located
exactly, and are made a matter of record during construction.
DESIGN OF A STORM WATER SEWER SYSTEM
=50. Planning the System.=—Storm sewer systems are seldom as extensive
as separate or combined sewer systems, since storm sewage can be
discharged into the nearest suitable point in a flowing stream or other
drainage channel, whereas dry weather or combined sewage must be
conducted to some point where its discharge will be inoffensive. The
need of a comprehensive general plan of a storm sewer system is quite as
great, however, as for a separate system. The haphazard construction of
sewers at the points most needed for the moment results in the
duplication of forgotten drains, expense in increasing the capacity of
inadequate sewers, and difficult construction due to underground
structures thoughtlessly located. A comprehensive plan permits the
construction of sewers where they are needed as they are required, and
enables all probable future needs to be cared for at a minimum of
expense.
The same preliminary survey, map, and underground information are
necessary for the design of a storm sewer system as for a separate sewer
system. The map shown on Fig. 25 has been used for the design of a
storm-water sewer system.
The steps in the design of a storm-water sewer system are:
1st. Note the most advantageous points to locate the inlets and lay out
the system to drain these inlets. 2nd. Determine the required capacity
of the sewers by a study of the run-off from the different drainage
areas. 3rd. Draw the profile and compute the diameter and slope of the
pipes required.
=51. Location of Street Inlets.=—The location of storm sewers is
determined mainly by the desirable location of the street inlets. The
inlets must therefore be located before the system can be planned. In
general the inlets should be located so that no water will flow across a
street or sidewalk, in order to reach the sewer. This requires that
inlets be placed on the high corners at street intersections, in
depressions between street intersections, and at sufficiently frequent
intervals that the gutters may not be overloaded. City blocks are seldom
so long as to necessitate the location of inlets between crossings
solely on account of inadequate gutter capacity. The capacity of a
gutter can be computed approximately by the application of Kutter’s
formula. Inlet capacities are discussed in Chapter VI. When the area
drained is sufficiently large to tax the capacity of the gutter or
inlet, an inlet should be installed regardless of the location of the
street intersections.
The street inlets are located on the map as shown in Fig. 25. The sewer
lines are then located so as to make the length of pipe to pass near to
all inlets a minimum. Storm sewers are seldom placed near the center of
a street because of the frequent crowded condition on this line.
=52. Drainage Areas.=—The outline of a drainage area is drawn so that
all water falling within the area outlined will enter the same inlet,
and water falling on any point beyond the outline will enter some other
inlet. This requires that the outline follow true drainage lines rather
than the artificial land divisions used in locating the drainage lines
in the design of sanitary sewers. The drainage lines are determined by
pavement slopes, location of downspouts, paved or unpaved yards, grading
of lawns and the many other features of the natural drainage which are
altered by the building up of a city. The location of the drainage lines
is fixed as the result of a study of local conditions.
The watershed or drainage lines are shown on Fig. 25 by means of dot and
dash lines. A drainage line passes down the middle of each street
because the crown of the street throws the water to either side and
directs it to different inlets. A watershed line is drawn about 50 feet
west of such streets as Kentucky St., Florida St., etc., because the
downspouts from the houses on those streets discharge or will discharge
into the street on which they face. The location of any watershed line
within 20 feet more or less is, in most cases, a matter of judgment
rather than exactness. Each area is given an identifying number or mark
which is useful only in design. It usually corresponds to the inlet
number.
=53. Computation of Flood Flow by McMath Formula.=—McMath’s Formula is
used as an example of the method pursued when an empirical formula is
adopted for the computation of run-off, and because of its frequent use
in practice. Other formulas may be more satisfactory under favorable
conditions.
Computations should be kept in order by a tabulation such as is shown in
Table 21, in which the quantity of storm flow discharged from the sewer
at the foot of Tennessee St., on Fig. 25, has been computed by means of
the McMath Formula, using the constants suggested for St. Louis
conditions, _i_ = 2.75, and _c_ = 0.75. The solutions of the formula
have been made by means of Fig. 11. The column headings in the Table are
explanatory of the figures as recorded. The computation should begin at
the upper end of a lateral, proceed to the first junction and then
return to the head of another lateral tributary to this junction. They
should be continued in the same manner until all tributary areas have
been covered. Special computations will be necessary for the
determination of the maximum quantity of storm water entering each inlet
to avoid the flooding of an inlet or gutter. These computations have not
been shown as they are so easily made by the application of McMath’s
Formula to each area concerned.
The determination of the average slope ratio is a matter of judgment,
based on the average natural slope of the surface of the ground and an
estimate of the probable future conditions.
=54. Computation of Flood Flow by Rational Method.=—The rational method
for the computation of storm-water run-off is described in Chapter III.
An example of its application to storm sewer design is given here for
the district shown in Fig. 25.[34] The computations are shown in Table
21. As in the preceding designs the table has been filled in from left
to right and line by line. Computations have started at the upper end of
laterals tributary to each junction. The column headed _I_ represents
the imperviousness factor in the expression _Q_ = _AIR_. It is based on
judgment guided by the constants given in Chapter III concerning
imperviousness. The column headed “Equivalent 100 per cent _I_ acres” is
the product of the two preceding columns. It reduces all areas to the
same terms so that they can be added for entry in the column headed
“Total 100 per cent _I_ acres.” It may be necessary to record the values
for this column on several lines where the imperviousnesses of the
tributary areas are different. This condition is illustrated in the last
line of the table, for the length of sewer nearest the outlet. In the
preceding lines the imperviousness recorded represents an average for
all the tributary areas.
TABLE 21
COMPUTATIONS FOR THE QUANTITY OF STORM SEWAGE AT THE FOOT OF TENNESSEE
STREET ON FIGURE 25
─────────┬──────────┬──────────┬───────────┬─────────────────────────────────
On Street│ From │To Street │Identifying│ By McMath’s Formula
│ Street │ │ Number of │
│ │ │ Acres │
│ │ │ Drained │
─────────┼──────────┼──────────┼───────────┼──────────┬───────┬───────┬──────
│ │ │ │Additional│ Total │ Slope │ Run
│ │ │ │ Acres │ Acres │ of │Off in
│ │ │ │ Drained │Drained│Surface│C.F.S.
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
─────────┼──────────┼──────────┼───────────┼──────────┼───────┼───────┼──────
State │N. │S. │ 91 and 92 │2.35 │ 2.35│ 0.005│ 5.5
│ Carolina│ Carolina│ │ │ │ │
State │S. │Georgia │88, 89 and │3.0 │ 5.35│ .005│ 10.8
│ Carolina│ │ 90 │ │ │ │
State │Georgia │Florida │85, 86 and │3.0 │ 8.35│ .007│ 16.5
│ │ │ 87 │ │ │ │
State │Florida │Kentucky │81, 83 and │3.0 │ 11.35│ .009│ 22.0
│ │ │ 84 │ │ │ │
State │Kentucky │Tennessee │79, 80 and │3.0 │ 14.35│ .010│ 28.0
│ │ │ 82 │ │ │ │
State │Texas │Louisiana │ 76 and │3.8 │ 3.8│ .005│ 8.3
│ │ │ others │ │ │ │
State │Louisiana │Alabama │73, 74 and │3.7 │ 7.5│ .007│ 15.0
│ │ │ 75 │ │ │ │
State │Alabama │Tennessee │70, 71 and │3.0 │ 10.5│ .006│ 19.0
│ │ │ 72 │ │ │ │
Tennessee│State │Talon │68, 69, 77 │4.3 │ 29.15│ .15│ 52
│ │ │ and 78 │ │ │ │
Talon │Albemarle │Tennessee │65, 66 and │2.8 │ 2.8│ .018│ 8.4
│ │ │ 67 │ │ │ │
Tennessee│Talon │Burnside │ 64 and │0.7 │ 29.85│ .15│ 55
│ │ │ 64_a_ │ │ │ │
Burnside │N. │S. │57, 58 and │2.84 │ 2.84│ .008│ 7.2
│ Carolina│ Carolina│ 59 │ │ │ │
Burnside │S. │Georgia │54, 55 and │3.88 │ 6.72│ .010│ 14.9
│ Carolina│ │ 56 │ │ │ │
Burnside │Georgia │Florida │50, 52 and │3.88 │ 10.60│ .012│ 22
│ │ │ 53 │ │ │ │
Burnside │Florida │Kentucky │47, 48 and │3.88 │ 14.48│ .013│ 30
│ │ │ 51 │ │ │ │
Burnside │Kentucky │Tennessee │44, 45 and │3.88 │ 18.36│ .013│ 36
│ │ │ 46 │ │ │ │
Tennessee│Burnside │Elm │ 42 and 43 │2.84 │ 51.05│ .015│ 82
Elm │Above │Chetwood │ Included in next line below │ │
│ Chetwood│ │ │ │
Elm │Chetwood │Albemarle │31, 32 and │2.75 │ 2.75│ .007│ 7.0
│ │ │ 33 │ │ │ │
Elm │Albemarle │Tennessee │27, 28, 29 │5.75 │ 8.50│ .016│ 20
│ │ │ and 30 │ │ │ │
Tennessee│Elm │Varennes │25, 26 and │2.62 │ 62.17│ .017│ 100
│ │ │ 41 │ │ │ │
Varennes │S. │Georgia │17, 18 and │3.17 │ 3.17│ .010│ 8.3
│ Carolina│ │ 19 │ │ │ │
Varennes │Georgia │Florida │14, 15 and │3.17 │ 6.34│ .011│ 14.5
│ │ │ 16 │ │ │ │
Varennes │Florida │Kentucky │11, 12 and │3.17 │ 9.51│ .013│ 21
│ │ │ 13 │ │ │ │
Varennes │Kentucky │Tennessee │8, 9 and 10│3.17 │ 12.68│ .013│ 26
Tennessee│Varennes │Boulevard │ 6 and 7 │2.32 │ 77.17│ .017│ 120
Tennessee│Boulevard │Outlet │1, 2, 3, 4,│4.72 │ 81.89│ .017│ 122
│ │ │ and 5 │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
─────────┴──────────┴──────────┴───────────┴──────────┴───────┴───────┴──────
─────────┬──────────┬──────────┬───────────────────────────────────────────
On Street│ From │To Street │ By Rational Method
│ Street │ │
│ │ │
│ │ │
─────────┼──────────┼──────────┼─────┬─────┬──────────┬─────┬──────────────
│ │ │Area,│ _I_ │Equivalent│Total│ Time of
│ │ │Acres│ │ 100 Per │ 100 │Concentration,
│ │ │ │ │ Cent _I_ │ Per │ Minutes
│ │ │ │ │ Acres │Cent │
│ │ │ │ │ │ _I_ │
│ │ │ │ │ │Acres│
─────────┼──────────┼──────────┼─────┼─────┼──────────┼─────┼──────────────
State │N. │S. │ 2.35│ 0.50│ 1.17│ 1.17│ 7.0
│ Carolina│ Carolina│ │ │ │ │
State │S. │Georgia │ 3.00│ .50│ 1.50│ 2.67│ 8.1
│ Carolina│ │ │ │ │ │
State │Georgia │Florida │ 3.00│ .50│ 1.50│ 4.17│ 9.0
│ │ │ │ │ │ │
State │Florida │Kentucky │ 3.00│ .50│ 1.50│ 5.67│ 9.9
│ │ │ │ │ │ │
State │Kentucky │Tennessee │ 3.00│ .50│ 1.50│ 7.17│ 10.7
│ │ │ │ │ │ │
State │Texas │Louisiana │ 3.80│ .35│ 1.33│ 1.33│ 10.0
│ │ │ │ │ │ │
State │Louisiana │Alabama │ 3.70│ .40│ 1.48│ 2.81│ 11.9
│ │ │ │ │ │ │
State │Alabama │Tennessee │ 3.00│ .45│ 1.35│ 4.16│ 12.9
│ │ │ │ │ │ │
Tennessee│State │Talon │ 4.30│ .50│ 2.15│13.48│ 14.5
│ │ │ │ │ │ │
Talon │Albemarle │Tennessee │ 2.80│ .40│ 1.12│ 1.12│ 8.0
│ │ │ │ │ │ │
Tennessee│Talon │Burnside │ 0.70│ .20│ 0.14│14.74│ 15.3
│ │ │ │ │ │ │
Burnside │N. │S. │ 2.84│ .55│ 1.56│ 1.56│ 10.0
│ Carolina│ Carolina│ │ │ │ │
Burnside │S. │Georgia │ 3.88│ .55│ 2.13│ 3.69│ 11.1
│ Carolina│ │ │ │ │ │
Burnside │Georgia │Florida │ 3.88│ .55│ 2.13│ 5.82│ 12.2
│ │ │ │ │ │ │
Burnside │Florida │Kentucky │ 3.88│ .55│ 2.13│ 7.95│ 13.1
│ │ │ │ │ │ │
Burnside │Kentucky │Tennessee │ 3.88│ .55│ 2.13│10.08│ 13.8
│ │ │ │ │ │ │
Tennessee│Burnside │Elm │ 2.84│ .45│ 2.28│26.10│ 15.7
Elm │Above │Chetwood │ │ │ │ │
│ Chetwood│ │ │ │ │ │
Elm │Chetwood │Albemarle │ 2.75│ .40│ 1.10│ 1.10│ 8.0
│ │ │ │ │ │ │
Elm │Albemarle │Tennessee │ 5.75│ .45│ 2.59│ 3.69│ 9.5
│ │ │ │ │ │ │
Tennessee│Elm │Varennes │ 2.62│ .50│ 1.31│30.00│ 16.2
│ │ │ │ │ │ │
Varennes │S. │Georgia │ 3.17│ .55│ 1.74│ 1.74│ 9.0
│ Carolina│ │ │ │ │ │
Varennes │Georgia │Florida │ 3.17│ .55│ 1.74│ 3.48│ 9.9
│ │ │ │ │ │ │
Varennes │Florida │Kentucky │ 3.17│ .55│ 1.74│ 5.22│ 10.8
│ │ │ │ │ │ │
Varennes │Kentucky │Tennessee │ 3.17│ .55│ 1.74│ 6.96│ 11.4
Tennessee│Varennes │Boulevard │ 2.32│ .55│ 1.28│32.84│ 16.5
Tennessee│Boulevard │Outlet │ 0.18│ .80│ 0.14│ Area No. 1
│ │ │ │ │ │
│ │ │ 1.38│ .50│ 0.69│ Area No. 2
│ │ │ 2.80│ .55│ 1.54│ Areas No. 3 and 4
│ │ │ 0.36│ .75│ 0.27│35.48│ 16.9
│ │ │ │ │ │ │
─────────┴──────────┴──────────┴─────┴─────┴──────────┴─────┴──────────────
─────────┬──────────┬──────────┬─────────────────────────────────┬──────
On Street│ From │To Street │ By Rational Method │ Line
│ Street │ │ │Number
│ │ │ │
│ │ │ │
─────────┼──────────┼──────────┼───┬────┬─────┬────┬───────┬─────┼──────
│ │ │_R_│_Q_ │ _S_ │_V_ │ Sewer │Time │
│ │ │ │ │ │ │Length,│ in │
│ │ │ │ │ │ │ Feet │Sewer│
│ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │
─────────┼──────────┼──────────┼───┼────┼─────┼────┼───────┼─────┼──────
State │N. │S. │4.8│ 5.6│0.011│ 4.6│ 300│ 1.1│ 1
│ Carolina│ Carolina│ │ │ │ │ │ │
State │S. │Georgia │4.6│12.2│ .010│ 5.5│ 300│ 0.9│ 2
│ Carolina│ │ │ │ │ │ │ │
State │Georgia │Florida │4.4│18.3│ .012│ 5.8│ 300│ 0.9│ 3
│ │ │ │ │ │ │ │ │
State │Florida │Kentucky │4.2│23.9│ .009│ 6.0│ 300│ 0.8│ 4
│ │ │ │ │ │ │ │ │
State │Kentucky │Tennessee │4.1│29.3│ .009│ 6.2│ 300│ 0.8│ 5
│ │ │ │ │ │ │ │ │
State │Texas │Louisiana │4.2│ 5.6│ .009│ 3.2│ 370│ 1.9│ 6
│ │ │ │ │ │ │ │ │
State │Louisiana │Alabama │3.9│11.0│ .011│ 5.2│ 300│ 1.0│ 7
│ │ │ │ │ │ │ │ │
State │Alabama │Tennessee │3.8│15.8│ .002│ 3.2│ 300│ 1.6│ 8
│ │ │ │ │ │ │ │ │
Tennessee│State │Talon │3.6│48.5│ .019│ 9.8│ 450│ 0.8│ 9
│ │ │ │ │ │ │ │ │
Talon │Albemarle │Tennessee │4.6│ 5.2│ .004│ 3.0│ 210│ 1.2│ 10
│ │ │ │ │ │ │ │ │
Tennessee│Talon │Burnside │3.5│51.5│ .006│ 5.0│ 120│ 0.4│ 11
│ │ │ │ │ │ │ │ │
Burnside │N. │S. │4.2│ 6.5│ .008│ 4.5│ 300│ 1.1│ 12
│ Carolina│ Carolina│ │ │ │ │ │ │
Burnside │S. │Georgia │4.0│14.8│ .007│ 4.7│ 300│ 1.1│ 13
│ Carolina│ │ │ │ │ │ │ │
Burnside │Georgia │Florida │3.9│22.7│ .011│ 5.8│ 300│ 0.9│ 14
│ │ │ │ │ │ │ │ │
Burnside │Florida │Kentucky │3.7│29.4│ .016│ 7.5│ 300│ 0.7│ 15
│ │ │ │ │ │ │ │ │
Burnside │Kentucky │Tennessee │3.7│37.3│ .019│ 9.2│ 300│ 0.5│ 16
│ │ │ │ │ │ │ │ │
Tennessee│Burnside │Elm │3.4│88.8│ .015│10.2│ 280.│ 0.5│ 17
Elm │Above │Chetwood │ │ │ │ │ │ │ 18
│ Chetwood│ │ │ │ │ │ │ │
Elm │Chetwood │Albemarle │4.6│ 5.1│ .020│ 5.3│ 480│ 1.5│ 19
│ │ │ │ │ │ │ │ │
Elm │Albemarle │Tennessee │4.3│15.8│ .012│ 6.1│ 410│ 1.1│ 20
│ │ │ │ │ │ │ │ │
Tennessee│Elm │Varennes │3.4│ 102│ .012│10.2│ 180│ 0.3│ 21
│ │ │ │ │ │ │ │ │
Varennes │S. │Georgia │4.4│ 7.7│ .012│ 5.2│ 270│ 0.9│ 22
│ Carolina│ │ │ │ │ │ │ │
Varennes │Georgia │Florida │4.2│14.5│ .010│ 5.7│ 300│ 0.9│ 23
│ │ │ │ │ │ │ │ │
Varennes │Florida │Kentucky │4.1│21.4│ .017│ 7.7│ 300│ 0.6│ 24
│ │ │ │ │ │ │ │ │
Varennes │Kentucky │Tennessee │4.0│27.8│ .015│ 7.8│ 300│ 0.6│ 25
Tennessee│Varennes │Boulevard │3.3│ 108│ .012│10.2│ 230│ 0.4│ 26
Tennessee│Boulevard │Outlet │ │ │ │ │ │ │ 27
│ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │ 28
│ │ │ │ │ │ │ │ 29
│ │ │3.3│ 117│Areas No. 1–5 inclusive │ 30
│ │ │ │ │ │ │ │ │
─────────┴──────────┴──────────┴───┴────┴─────┴────┴───────┴─────┴──────
The time of concentration in minutes is assumed by judgment for the
first area. For all subsequent areas it is the sum of the time of
concentration for the area or areas tributary to the inlet next above
and the time of flow in the sewer from the inlet next above to the inlet
in question. For example, in line 2 the time 8.1 minutes is the sum of
7.0 minutes time of concentration to the inlet at the corner of State
and North Carolina St., and the time of flow of 1.1 minute in the sewer
on State St. from North Carolina St. to South Carolina St. Where two
sewers are converging as at the corner of Varennes Road and Tennessee
St. the longest time is taken. For example, the time of concentration
down Varennes Road to Tennessee St. is shown in line 25 as 11.4 + 0.6 =
12.0 minutes. The time to the same point down Tennessee St. is shown in
line 21 as 16.2 + 0.3 = 16.5 minutes. This time is therefore used in
line 26.
_R_, the rate of rainfall in inches per hour is determined by Talbot’s
formula.
_Q_, is in cubic feet per second and is the product of the 8th and 10th
columns. Since the 8th column is the sum of the products of the 5th and
the 6th columns for the lines representing tributary areas, then the
11th column is the product of _A_, _I_, and _R_.
_S_, is the slope on which it is assumed that the sewer will be laid. It
is usually assumed as parallel to the ground surface unless the velocity
for this slope becomes less than 2 feet per second. In such a case the
slope is taken as one which will cause this velocity.
_V_, the velocity in feet per second, is computed from diagrams for the
solution of Kutter’s formula. The length in feet is scaled from the map
as the distance between inlets or groups of inlets, and the time is the
length in feet divided by the velocity in feet per minute.
Having computed the quantity of flow to be carried in the sewer, the
design is completed by drawing the profile and computing the diameters
and slopes by the same method as used in the design of separate sewers.
CHAPTER VI
APPURTENANCES
=55. General.=—The appurtenances to a sewerage system are those devices
which, in addition to the pipes and conduits, are essential to or are of
assistance in the operation of the system. Under this heading are
included such structures and devices as: manholes, lampholes,
flush-tanks, catch-basins, street inlets, regulators, siphons,
junctions, outlets, grease traps, foundations and underdrains.
=56. Manholes.=—A manhole is an opening constructed in a sewer, of
sufficient size to permit a man to gain access to the sewer. Manholes
are the most common appurtenances to sewerage systems and are used to
permit inspection and the removal of obstructions from the pipes. The
details of the Baltimore standard manholes are shown in Fig. 27 and a
manhole on a large sewer in Omaha is shown in Fig. 28. The features of
these designs which should be noted are the size of the opening and
working space, and the strength of the structure. Manhole openings are
seldom made less than 20 inches in diameter and openings 24 inches in
diameter are preferable. A man can pass through any opening that he can
get his hips through provided he can bend his knees and twist his
shoulders immediately on passing the hole. For this reason the manhole
should widen out rapidly immediately below the opening, as shown in Fig.
27 and 38.
The walls of the manhole may be built either of brick or of concrete.
Brick is more commonly used, as the forms necessary for concrete make
the work more expensive unless they can be used a number of times. The
walls of the manhole should be at least 8 inches thick. Greater
thicknesses are used in treacherous soils and for deep manholes, or to
exclude moisture. A rough expression for the thickness of the walls of a
brick manhole more than 12 feet deep in ordinary firm material is _t_ =
_d_⁄2 + 2, in which _t_ is the thickness in inches and _d_ is the depth
in feet. The thickness of brick walls may be changed every 5 to 10 feet
or so. Concrete walls may be built thinner than brick walls.
[Illustration:
FIG. 27.—Baltimore Standard Manhole Details.
]
The bottoms of brick manholes are frequently made of concrete as shown
in Fig. 27. The floor slopes towards the center and is constructed so
that the sewage flows in a half round or U-shaped channel of greater
capacity than the tributary sewers. The sides of the channel should be
high enough to prevent the overflow of sewage onto the sloping floor,
which should have a pitch of about one vertical to 10 or 12 horizontal.
In manholes where two or more sewers join at approximately the same
level the channels in the bottom should join with smooth easy curves.
Where the inlet and outlet pipes are not of the same diameter the tops
of the pipes should ordinarily be placed at the same elevation to
prevent back flow in the smaller pipes when the larger pipes are flowing
full.
The dimensions of the manhole should not be less than 3 feet wide by 4
feet long for a height of at least 4 feet, when built in the form of an
ellipse, or 4 feet in diameter when built circular. No standard method
for the reduction of the diameter of the manhole near the top is
observed, the rate being more or less dependent on the depth of the
manhole. The use of sloping sides above the frost line is desirable as
such a form is more resistant to heaving by frost action.
For sewers up to 48 inches in diameter the manhole is usually centered
over the intersection of the pipes and has a special foundation. For
larger sewers the manhole walls spring from the walls of the sewer as
shown in Fig. 28.
[Illustration:
FIG. 28.—Details of a Manhole and a Well Hole.
]
In the case of a decided drop in the elevation of a sewer, or of a
tributary sewer appreciably higher than an outlet in any manhole, the
sewage is allowed to drop vertically at the manhole, hence the name drop
manhole. The Baltimore standard drop manhole is shown in Fig. 27. A well
hole is an unusually deep drop manhole in which the force of the
vertical drop of sewage is broken by a series of baffle plates, or by a
sump at the bottom of the well hole. Fig. 28 shows a well hole at St.
Paul, Minn. The use of drop manholes can be avoided in large sewers by
the construction of a flight of steps or flight sewer as shown in Fig.
29, which allows the use of a steep grade and serves to break the
velocity of the sewage.
The specifications of the Sanitary District of Chicago, covering the
construction of manhole covers and frames are:
All castings shall be of tough, close grained, gray iron, free
from blow holes, shrinkage and cold shuts, and sound, smooth,
clean and free from blisters and all defects.
All castings shall be made accurately to dimensions to be
furnished and shall be planed where marked or where otherwise
necessary to secure perfectly flat and true surfaces. Allowance
shall be made in the patterns so that the specified thickness
shall not be reduced.
All castings shall be thoroughly cleaned and painted before
rusting begins and before leaving the shop with two coats of high
grade asphaltum or any other varnish that the Engineer may direct.
After the castings have been placed in a satisfactory manner, all
foreign adhering substances shall be removed and the castings
given one additional coat of asphaltum. No castings shall be
accepted the weight of which shall be less than that due to its
dimensions by more than 5 per cent.
[Illustration:
FIG. 29.—Flight Sewer at Baltimore.
Eng. Record, Vol. 59, p. 161.
]
[Illustration:
FIG. 30.—Baltimore Standard Manhole Frame and Cover.
]
The weights of frames and covers in use vary from 200 to 600 pounds, the
weight of the frame being about 5 times that of the cover. The lightest
weights are used where no traffic other than an occasional pedestrian
will pass over the manhole. Frames and covers weighing about 400 pounds
are commonly used on residential streets, whereas 600 pound frames and
covers are desirable in streets on which the traffic is heavy. The
frames should be so designed that the pavement will rest firmly against
it and wear at the same rate as the surrounding street surface.
Experience has shown that vertical sides should be used for the outside
of the frame to approach this condition, and that the frame should not
be less than 8 inches high. The cover should be roughened in some
desirable pattern as shown in Fig. 30. Smooth covers become dangerously
slippery. Where the ventilation of the sewers is not satisfactory the
manhole covers are sometimes perforated. This is undesirable from other
points of view as the rising odors and vapors are obnoxious at the
surface and the entering dirt and water are detrimental to the operation
of the sewer. The stealing and destruction of manhole covers and the
unauthorized entering of sewers has occasionally required the locking of
the covers to the frame when in place. The locks commonly used consist
of a tumbler which falls into place when the manhole is closed, and
which can be opened only by a special wrench or hook. Adjustable frames
are sometimes used where the street grade is settling, or may be raised
in order that the elevation of the top of the cover may be made to
conform to that of the street surface, without reconstructing the top of
the manhole. One type of adjustable cover is shown in Fig. 31. Manhole
covers should be so marked that the sanitary sewer can be distinguished
from the storm-water sewer, and both from the telephone conduit, etc.
[Illustration:
FIG. 31.—Adjustable Manhole Frame and Cover.
]
Iron steps are set into the walls of the manhole about 15 inches apart
vertically to allow entrance and exit to and from the manhole.
Galvanized iron is preferable to unprotected metal as the action of rust
is particularly rapid in the moist air of the sewer. One type of these
manhole steps is shown in Fig. 27. The steps should have a firm grip in
the wall as a loose step is a source of danger.
[Illustration:
FIG. 32.—Baltimore Standard Lamphole.
]
=57. Lampholes.=—A lamphole is an opening from the surface of the ground
into a sewer, large enough to permit the lowering of a lantern into the
sewer. Lampholes are used in the place of manholes to permit the
inspection or the flushing of sewers, and to avoid the expense of a
manhole. They are located from 300 to 400 feet from the nearest manhole
in such a manner that a lamp lowered in the lamp hole can be seen from
the two nearest manholes.
Lampholes should be constructed of 8– to 12–inch tile or cast-iron pipe.
The lower section should be a cast-iron T on a firm foundation, but if
constructed of tile it should be reinforced with concrete to take up the
weight of the shaft. The details of the Baltimore standard lamphole are
shown in Fig. 32. Lampholes are not commonly used on sewerage systems on
account of their lack of real usefulness and the troubles encountered by
breaking of the pipe below the shaft.
=58. Street Inlets.=—A street inlet is an opening in the gutter through
which storm water gains access to the sewer. The types used in different
cities vary widely. Details of an inlet in successful use are shown in
Fig. 33. The figure shows also a common form of connection to the sewer.
A water-seal trap is sometimes used to prevent the escape of odors from
the sewer. Care must be taken in design that such traps do not freeze in
winter nor dry out in summer, although it is not always possible to
prevent these contingencies.
[Illustration:
FIG. 33.—Details of an Untrapped Street Inlet, without Catch-Basin.
]
The important features to be observed in the design of a street inlet
are: height and length of opening, character of grating, and location.
The general location of inlets is discussed in Chapter V. The clear
height of opening commonly used is from 5 to 6 inches, with a clear
length of 24 to 30 inches or longer. Inlets of this size have given
satisfaction on paved streets with moderate slopes, where the drainage
area is not greater than 10,000 to 12,000 square feet of pavement. W. W.
Horner states:[35]
The St. Louis type of inlet “old” style was a vertical opening in
the curb 8 inches high and 4 feet in length with a horizontal bar
making the net opening about 5 inches. It has a broad sill
extending under the sidewalk. The “new” style inlet is 4½ feet
long with a clear opening of 6 inches and no bar. The sill is done
away with and the opening drops down directly from the curb. Tests
were made of the capacity of this inlet on pavements on different
slopes with sumps of depths varying from 0 to 6 inches in front of
the inlet, extending out 3 feet from the gutter, and returning to
the elevation of the gutter at a slope of 3 inches to the foot.
The results of these tests are shown in Table 22. The capacity of
the inlet is expressed as the amount of water entering just before
some water begins to lap past. If a large amount of water is
allowed to flow past much more water will enter the inlet thus
furnishing a factor of safety for large storms. It was noted that
by beginning the rise in the pavement about opposite the middle of
the inlet the capacity of the inlet was increased from 20 to 50
per cent.
TABLE 22
CAPACITIES OF ST. LOUIS STREET INLETS
From tests by W. W. Horner. Cubic feet per second
─────────┬───────────────────┬───────────────────
Slope in │ 0.42 │ 1.5
Per Ct. │ │
─────────┼────┬────┬────┬────┼────┬────┬────┬────
Depth of │ 0.0│ 2│ 4│ 6│ 0│ 2│ 4│ 6
Sump, │ │ │ │ │ │ │ │
Inches │ │ │ │ │ │ │ │
Capacity,│ │ │1.27│ │ │ │ │
old │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
Capacity,│ 0.1│ 0.5│ 1.5│ 2.5│0.08│ 0.4│ 1.1│ 2.1
new │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
─────────┴────┴────┴────┴────┴────┴────┴────┴────
─────────┬───────────────────┬───────────────────
Slope in │ 2.85 │ 4.5
Per Ct. │ │
─────────┼────┬────┬────┬────┼────┬────┬────┬────
Depth of │ 0│ 2│ 4│ 6│ 0│ 2│ 4│ 6
Sump, │ │ │ │ │ │ │ │
Inches │ │ │ │ │ │ │ │
Capacity,│0.03│0.25│0.78│1.49│ │ │ │
old │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
Capacity,│0.03│0.28│0.87│1.62│0.02│0.15│0.45│ 1.0
new │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
─────────┴────┴────┴────┴────┴────┴────┴────┴────
Gratings with horizontal bars will admit more water than gratings with
vertical bars, but they will also admit more rubbish such as sticks,
papers, leaves, etc., which serve to clog the sewers. Vertical barred
gratings and gratings in the bottom of the gutter clog more quickly than
other types. In the selection of the type of grating to be used a
decision must be made as to whether it is more desirable to clean the
sewer or catch-basin, or to flood the street as a result of clogged
inlets. Where catch-basins are used or the sewers are large, horizontal
bars are more satisfactory. The openings between bars should be small
enough to prevent the entrance of a horse’s hoof or objects of
sufficient size to clog the sewer. Four inches in the clear for vertical
openings and 6 inches for horizontal openings are reasonable sizes.
The location of the inlets at the intersection of the two curb lines at
a corner results in a lower first cost but on heavily traveled streets
this may result in a higher maintenance cost than for other locations
because of the concentration of traffic at street corners, hammering the
inlet casting out of shape or position. Vehicles making short turns will
tend to climb the curb and will intensify the wear upon the inlet. These
objections can be overcome by the use of two inlets at each corner, set
back far enough from the curb intersection to avoid interference with
the cross-walks. This also makes it possible to raise the cross-walks
without the use of gutters under them.
The size of the pipe from the inlet to the catch-basin or sewer should
be large enough to care for all of the water which may enter the inlet.
As the capacity of the inlet is seldom known with accuracy and the
capacity of the pipe is difficult of determination, it has become
customary to use a 10–inch or a 12–inch connecting pipe for each
ordinary independent inlet.
=59. Catch-basins.=—Catch-basins are used to interrupt the velocity of
sewage before entering the sewer, causing the deposition of suspended
grit and sludge and the detention of floating rubbish which might enter
and clog the sewer. A separate catch-basin may be used for each inlet,
or to save expense, the pipes from several inlets may discharge into one
catch-basin.
[Illustration:
FIG. 34.—Catch-basin.
Outlets are not always trapped.
]
The types in successful use are extremely varied, but the distinguishing
feature of all is an outlet located above the floor of the basin. A
common form of catch-basin is shown in Fig. 34. It is constructed
similar to a manhole with a diameter of about 4 or 4½ feet and a depth
of retained water from 3 to 4 feet. Catch-basins of this size will care
for the sewage from the inlets at the four corners of a street
intersection, each draining a city block. In unusual situations it may
be necessary to install a larger basin, but too large a catch-basin is
less desirable than one which is too small, as the former stinks and the
latter is useless. Traps are sometimes used to prevent the escape of
odors from the sewer into the street, but odors are often created in the
catch-basins themselves. Some engineers arrange the trap so that it can
be opened for observation down the sewer as in Fig. 34, thus combining
the advantages of a manhole with the catch-basin.
The use of catch-basins is objectionable because: they furnish a
breeding place for mosquitoes and other flying insects; the septic
action in them produces offensive odors; if on a combined sewer they
permit the escape of offensive odors in dry weather when the water seal
in the trap has evaporated; and the freezing of the water seal in the
trap prevents the entrance of water to the sewer. The sole advantage
lies in the prevention of the clogging of the sewers, but this may be
sufficient to overbalance all of the disadvantages. In general
catch-basins should be provided on paved streets which are cleaned by
flushing the material into the sewers, or where the drainage is from an
unimproved or macadamized street, sandy country, or into sewers in which
the velocity of flow is less than 2 feet per second.
[Illustration:
FIG. 35.—Diagrammatic Section through a Grease Trap.
]
=60. Grease Traps.=—The presence of grease in sewers results in the
formation of incrustations which are difficult to remove and which cause
a material loss in the capacity of the sewer. The presence of oil and
gasoline has resulted in violent and destructive explosions as is
described in Chapter XII. A type of grease trap used on the drains from
hotels, restaurants, or other large grease producing industries is shown
in Fig. 35. The trap is similar to a catch-basin except that it is too
small for a man to enter, and the outlet is necessarily trapped in order
to prevent the escape of grease. The details of a gasoline and oil
separator approved by the New York City Fire Department are shown in
Fig. 36.[36]
[Illustration:
FIG. 36.—Gasoline and Oil Separator.
]
=61. Flush-Tanks.=—These are devices to hold water used in flushing
sewers. They are required only on sanitary and combined sewers. Their
use tends to prevent the clogging of sewers laid on flat grades and
permits flatter grades in construction than could otherwise be adopted.
Flush-tanks may be operated either by hand or automatically. Automatic
operation is more common than hand operation. The hand-operated tanks
are similar to manholes so arranged that the inlet and outlet sewers can
be plugged while the manhole or tank is being filled with water either
from a hose or a special service connection. When sufficient water has
been accumulated the outlet is opened and the sewer is flushed by the
rush of water. A sluice gate, flap valve, or a specially fitted board is
sufficient to fit over the mouth of the inlet and outlet during the
filling of the tank. Such an arrangement has the advantage of being
cheap, simple, and satisfactory, though somewhat crude. In some cases
water is run into the tank at the same rate that it is discharged
through the open outlet, maintaining a depth of 4 or 5 feet in the tank
until the water passing the manhole below runs clean. The volume of
water required by this method is large. Flushing water under a
relatively high head is sometimes obtained by the use of tank wagons
which are quickly emptied into the sewer through a canvas pipe dropped
down a manhole. In all such cases if not well constructed the manhole is
subject to caving due to the rush of water around the outlet.
Precautions should be taken to minimize this danger by limiting the
depth of water which may be accumulated. This can be done by
constructing an overflow at a height of 4 or 5 feet above the bottom of
the manhole, discharging into the sewer through an outside drain.
Automatic flush-tanks are constructed similar to a manhole, but special
care should be taken to make them water-tight. The apparatus for
providing the automatic discharge may operate either with or without
moving parts, the latter being preferable as they require less attention
and are not so liable to get out of order. An automatic flush-tank of
the Miller type is shown in Fig. 37. It is a patented device
manufactured by the Pacific Flush Tank Company. The small pipe at the
left is a service connection to the water main. Water is allowed to flow
continuously into the tank at such a rate as to fill it in the required
interval between discharges. The tanks are discharged as nearly once a
day as it is practicable to regulate them. The rate of flow into the
tank is determined by trial and varies to some extent with the water
pressure. The regulator shown in the figure is desirable as the
continuous flow through the ordinary cock soon wears it away. Some
waters will cause deposits to form in the small passages of the cocks or
regulators, thus cutting off the flow.
[Illustration:
FIG. 37.—Automatic Flush-Tank.
Pacific Flush Tank Co.
]
The tank operates as follows: when the water rising in the tank reaches
the bottom of the bell, air is trapped in the bell and prevented from
escaping through the main trap by the water at _A_. As the water
continues to rise in the tank the air in the bell is compressed, the
water level at _A_ is driven down and water trickles from the siphon at
_C_. The height of the water in the tank above the level of the water in
the bell is equal at all times to the height of _C_ above the lowering
position of _A_. When _A_ reaches the position of _B_ a small amount of
air is released through the short leg of the trap and a corresponding
volume of water enters the bell. The head of water above the bell then
becomes greater than the head of water in the short leg of the trap,
which results in the discharge of all of the air in the long leg of the
trap and the rapid discharge of the water in the tank through the
siphon. The discharge is continued until the siphonic action is broken
by the admission of air when the water level in the tank is lowered to
the bottom of the bell. The size of the siphons is fixed by the diameter
of the leg of the siphon. Table 23 shows the capacity and size of sewers
for which the different sizes of siphons are recommended by the
manufacturers.[37]
TABLE 23
SIZES OF SIPHONS TO BE USED WITH AUTOMATIC FLUSH-TANKS
──────────┬───────────┬───────────┬───────────┬───────────┬────────────
│ │ │ │ │ Height of
│Diameter of│ Total │ │ │ the
│Tank at the│ Discharge │ Average │ │ Discharge
Diameter │ Discharge │ for One │ Rate of │Diameter of│ Line above
of Siphon │ Line in │ Flush in │ Discharge │ Sewer in │the Edge of
in Inches │ Feet │ Gallons │in Sec.-ft.│ Inches │ the Bell
──────────┼───────────┼───────────┼───────────┼───────────┼────────────
4│ 3│ 60│ 0.35│ 4 to 6│1 ft. 2 in.
5│ 3│ 100│ 0.73│ 6 to 8│1 ft. 11 in.
6│ 4│ 240│ 1.06│ 8 to 10│2 ft. 6 in.
8│ 4│ 280│ 2.12│ 12 to 15│2 ft. 11 in.
──────────┴───────────┴───────────┴───────────┴───────────┴────────────
When flush-tanks are placed at the upper end of laterals provision
should be made for inspecting and cleaning the sewer by the construction
of a separate manhole, or by combining the features of a manhole and a
flush-tank in the same structure. Such a combination is shown in Fig. 38
from a design by Alexander Potter.
Except under unusual conditions flush-tanks are used only on separate
sewers. They should be placed at the upper end of laterals in which the
velocity of flow when full is less than 2 to 4 feet per second. The
capacity of the tank or the volume of the dose is dependent on the
diameter and slope of the sewer. The most effective flush is obtained by
a volume of water traveling at a high velocity and completely filling
the sewer. A large volume allowed to run slowly through the sewer will
have but little if any flushing action. Data on the quantity of flushing
water needed are given in Table 24.[38] As the result of a series of
experiments conducted by Prof. H. N. Ogden on the flushing of
sewers,[39] the conclusion was reached that the effect of a flush of
about 300 gallons in an 8–inch sewer on a grade less than 1 per cent
would not be effective beyond 800 to 1,000 feet, but that on steeper
grades much smaller quantities of water would produce equally good
results.
[Illustration:
FIG. 38.—Automatic Flush-Tank and Manhole.
Miller-Potter Design. Pacific Flush Tank Co.
]
TABLE 24
GALLONS OF WATER NEEDED FOR FLUSHING SEWERS
─────────────────┬─────────────────────────────────────────────────────
Slope │ Diameter of Sewer in Inches
─────────────────┼─────────────────┬─────────────────┬─────────────────
│ 8│ 10│ 12
─────────────────┼─────────────────┼─────────────────┼─────────────────
0.005 │ 80│ 90│ 100
.0075 │ 55│ 65│ 80
.01 │ 45│ 55│ 70
.02 │ 20│ 30│ 35
.03 │ 15│ 20│ 24
─────────────────┴─────────────────┴─────────────────┴─────────────────
Engineers do not agree upon the advisability of the use of automatic
flush-tanks, some believing that they are a needless expense that can be
avoided by hand flushing, and others feeling that a flush-tank should be
placed at the upper end of every lateral. These diverse opinions are the
result of different experiences in different cities.
=62. Siphons.=—There are two forms of siphons used in sewerage practice,
a true siphon and an inverted siphon. A true siphon is a bent tube
through which liquid will flow at a pressure less than atmospheric,
first upwards and then downwards, entering and leaving at atmospheric
pressure. An inverted siphon is a bent tube through which liquid will
flow at a pressure greater than atmospheric first downwards and then
upwards, entering and leaving at atmospheric pressure.
In sewerage practice the word siphon refers to an inverted siphon unless
otherwise qualified. Siphons, both true and inverted, are used in
sewerage systems to pass above or below obstacles. True siphons are
seldom used as they must be kept constantly filled with liquid.[40]
Accumulated gas must be removed in order to prevent the breaking of the
siphon which results in the cessation of flow. By the breaking of a true
siphon is meant the stoppage of siphonic action due to the accumulation
of air or gas at the peak of the siphon. Since the rate of flow of
sewage fluctuates widely it is extremely difficult to control the flow
so that a true siphon may be completely filled with liquid at all times.
In the design of inverted siphons care must be taken to prevent
sedimentation, and to permit inspection and cleaning. Sedimentation is
prevented by maintaining a velocity greater than a fixed minimum,
usually taken at about 2 feet per second. This minimum is attained by
providing a number of channels. The smallest channel is designed to
convey the least expected flow at the minimum velocity. Each of the
other channels is made as small as possible, within the limits of
economy and simplicity, in order that the minimum velocity shall be
exceeded quickly after flow has commenced in them. The last channel or
channels to be filled are made somewhat larger, because the sewage
conveyed in them contains less settleable matter than is contained in
the more concentrated dry weather flow. The type of siphon used in New
York to pass under the subway is shown in Fig. 39. Note should be taken
of the clean-out manhole provided on the 14–inch pipe. The other pipes
are large enough for a man to enter and clean.
[Illustration:
FIG. 39.—Sewer Siphon under New York Subway.
Eng. News Vol. 76, p. 443.
]
The computations involved in the design of a siphon are illustrated in
the following example, in which it is desired to construct a siphon to
pass under the railway cut shown in Fig. 40. The first step is to
determine the limiting diameter and slope of the smallest pipe in the
siphon. One-sixth of the capacity of the 6–foot approach sewer or 19
cubic feet per second will be assumed as the minimum flow. The diameter
of the pipe necessary to carry 19 cubic feet per second at a velocity of
2 feet per second is 42 inches. The hydraulic gradient should have a
slope of 0.0005 if the material used has a roughness coefficient
of .015. This is the minimum permissible slope of the siphon. The
selection of a steeper slope will necessitate the laying of the sewer at
a greater depth, and will permit the use of smaller pipes for the
siphon. The selection of the exact slope must then be based on judgment
with the minimum limitation above placed. The slope will be arbitrarily
selected as 0.001, the same as that of the approach sewer. The diameter
of the dry weather pipe will therefore be 36 inches, with a capacity of
18 second-feet, which is approximately the assumed dry weather flow. The
velocity of flow will be 2.5 feet per second. The length of flow along
the siphon is 150 feet.
[Illustration:
FIG. 40.—Diagram for the Design of an Inverted Siphon.
]
The next step should be the determination of the elevation at the lower
end of the 36–inch pipe. This is done by multiplying the assumed grade
by the equivalent length of straight pipe, and subtracting the product
from the elevation at the upper end. The length of straight pipe which
will give the same loss of head as the siphon is called the equivalent
pipe. It is determined as follows:
First, determine the head loss at entrance. This will vary between
nothing and one velocity head, dependent on the arrangement at the
entrance.[41] The length of straight pipe which will give this same loss
can be computed from the expression _l_ = _h_⁄_S_, using for _S_ the
assumed slope of the hydraulic gradient.
Second, determine the head loss due to the bends, This is determined
from the expression
_h_ = (_fl_)⁄(_d_) (_V_^2)⁄(2_g_)
in which _h_ = the head loss in the bend;
_l_ = the length of the bend;
_d_ = the diameter of the pipe;
_v_ = the average velocity of flow;
_g_ = the acceleration due to gravity;
_f_ = a factor dependent on the radius (_R_) of the bend and
_d_.
The relation between _f_, _R_, and _d_, for 90° bends is shown as
follows:[42]
_R_⁄_d_ 24 16 10 6 4 2.4
_f_ 0.036 0.037 0.047 0.060 0.062 0.072
After the head loss has been determined, the equivalent length of
straight pipe is determined as above.
Third. The equivalent length of pipe will be the sum of the actual
length of pipe and the equivalent lengths as found above.
In the problem in hand the head lost at the entrance will be assumed as
one-third of a velocity head, or 0.0324 foot. With the assumed slope of
0.001 this is equivalent to 32 feet of pipe. The radius of the bend is
about 20 feet and the length for a 45° central angle is about 16 feet.
The head loss for this angle will probably be a little more than
one-half that for a 90° angle. The expression will therefore be taken as
about 0.2(_V_^2)⁄(2_g_) and for two bends is equivalent to about 40 feet
of pipe. The equivalent length of pipe is therefore 150 + 32 + 40 = 222
feet. The elevation at the lower end should therefore be: the elevation
at the upper end, 92.07 − 222 × .001 = 91.85.
The diameters of the remaining pipes in the siphon are determined so
that the sewage in the approach sewer is backed up as little as is
consistent with good judgment before each pipe comes into action. This
is done satisfactorily by a method of cut and try. Let it be assumed
that the siphon will be composed of three pipes: the dry weather pipe
taking 18 second-feet, the second pipe taking 28 second-feet, and the
third pipe taking the remaining 70 second-feet. The diameters of the two
larger pipes on the assumed slope of 0.001 will therefore be 42 inches
and 60 inches respectively. Other combinations might be used which would
be equally satisfactory. There are many methods by which the sewage can
be diverted into the different channels of the siphon. For example, the
openings into the different pipes may be placed at the same elevation,
and the sewage allowed to enter them in turn through automatically or
hand-controlled gates, or in another method of control the openings may
be placed at such elevations that when the capacity of one pipe has been
exceeded the sewage will flow into the next largest pipe as shown in
Fig. 40. The outlets from the different pipes are ordinarily placed at
the same elevation, thus leaving each pipe standing full of sewage. Stop
planks should be provided at the outlet in order that the pipes may be
pumped out for cleaning. The objection to this arrangement is that the
larger pipes may operate at a velocity less than 2 feet per second, and
they will be standing full of sewage which might become septic. However,
as they will take nothing but the storm flow near the top of the sewer
no difficulty should be encountered from sedimentation in them, and all
are large enough for a man to enter for inspection or cleaning.
[Illustration:
FIG. 41.—Coffin Sewer Regulator.
]
=63. Regulators.=—Regulators are commonly used to divert the direction
of flow of sewage in order to prevent the overcharging of a sewer or to
regulate the flow to a treatment plant. Sewer regulators are of two
types, those with moving parts and those without moving parts. An
example of the moving part type is shown in Fig. 41. In this type as the
sewage rises the float closes the gate to the inlet sewer, thus
preventing the entrance of sewage under head from the larger sewer.
There are many variations in the details of float controlled regulators,
but the principle of operation is similar in all. These regulators can
be adjusted to fix the maximum rate of flow to a relief channel or
sewage treatment plant, or during times of storm to cut off the outlet
to the dry weather channel. Another form of the moving part type is
shown in Fig. 42.[43] It has been used extensively by the Milwaukee
Sewerage Commission. In its operation the dry weather flow is diverted
by the dam into the intercepter. It passes under the movable gate on its
way to the treatment plant. As the flow increases the dam is overtopped
and flood waters are discharged down the storm channel. The movable gate
is hung on a pivot placed below center. As the water rises in the
intercepter, the pressure against the upper portion of the gate becomes
greater than that against the lower portion, and the gate closes. An
opening is left at the bottom to allow an amount of sewage equal to the
dry weather flow to escape beneath the gate to prevent clogging or
sedimentation in the intercepter channel.
Objections to all moving part regulators are their need of attention and
liability to become clogged.
[Illustration:
FIG. 42.—Moving Part Regulator without Float.
]
[Illustration:
FIG. 43.—Leaping Weir at Danville, Illinois.
]
[Illustration:
FIG. 44.—Overflow Weir at San Francisco.
Eng. News, Vol. 73, p. 307.
]
[Illustration:
FIG. 45.—Overflow Weir in Action.
Shadow of steel knife edge which forms the lip of the weir can be seen
through the falling sewage.
]
The overflow weir and the leaping weir have no moving parts and are used
for the regulation of the flow in sewers. A leaping weir is formed by a
gap in the invert of a sewer through which the dry weather flow will
fall and over which a portion or all of the storm flow will leap. One
form of leaping weir is shown in Fig. 43. An overflow weir is formed by
an opening in the side of a sewer high enough to permit the discharge of
excess flow into a relief channel. A weir at San Francisco is shown in
Fig. 44. A series of tests were run on leaping weirs and overflow weirs
in the hydraulic laboratory of the University of Illinois. The type of
leaping weir tested was formed by the smooth spigot end of a standard
vitrified sewer pipe. The overflow weirs were formed by a steel knife
edge in the side of the pipe parallel to its axis as shown in Fig. 45.
Tests were made in 18–inch and 24–inch pipes on various slopes from
0.018 to 0.005, for both leaping weirs and overflow weirs. The overflow
weirs were varied in length from 16 inches to 42 inches and were placed
at various heights from 25 per cent to 50 per cent of the diameter above
the invert of the sewer. As the result of the observations the following
formulas were developed. For the leaping weir the expressions for the
coordinates of the curve of the surfaces of the falling stream, are:
For the outside surface _x_ = 0.53_V_^⅔ + _y_^{4⁄7}
For the inside surface _x_ = 0.30_V_^{4⁄7} + _y_^¾
in which _x_ and _y_ are the coordinates. The origin is in the upper
surface of the stream vertically above the end of the invert of the
pipe. The ordinate _y_ is measured vertically downwards. _V_ is the
velocity of approach in feet per second. These expressions are
applicable to any diameter of sewer up to 10 or 15 feet. They should
_not_ be used for depths of flow greater than about 14 inches; nor for
slopes of more than 25 per 1,000; nor for velocities less than 1 foot
per second nor more than 10 feet per second. The expression for the
ordinate of the inside curve is not good for less than 6 inches nor more
than 5 feet. The expression for the ordinate of the outside curve is
limited to values between the origin and 5 feet below it.
The expression for the length of an overflow weir of the type shown in
Fig. 45, necessary to discharge a given quantity, is in the form,
_l_ = 2.3_Vd_ log _h__{1}⁄_h__{2}
in which _l_ = the length of the weir in feet;
_V_ = the velocity of approach in feet per second;
_d_ = the diameter of the pipe in feet;
_h__{1} = the head of water on the upper end of the weir;
_h__{2} = the head of water on the lower end of the weir.
In the design of an overflow weir by this formula the height of the weir
above the invert of the sewer and the flow over the weir should be
determined arbitrarily. The height should be subtracted from the
computed depth of water above the weir to determine the value of
_h__{1}. The difference between the flow over the weir and the flow
above the weir will represent the rate of flow in the sewer below the
weir. The value of _h__{2} can then be computed as the difference in the
depth of flow below the weir and the height of the weir above the
invert. The value of _V_ is computed from Kutter’s formula. The length
of the weir is determined by substituting these values in the formula.
=64. Junctions.=—At the junction of two or more sewers the elevation of
the inverts should be such that the normal flow lines are at the same
elevation in all sewers. The sewers should approach the junction on a
steep grade to prevent sewage backing up in one when the other is
flowing full. The velocity of flow at the junction should not be
decreased and turbulence should be avoided in order to prevent
sedimentation and loss of head. The junction should be effected on
smooth easy curves with radii at least five times the diameter of the
sewer where possible. Curves with short radii cause backing up of sewage
thus reducing the capacity of the sewers.
The terms bellmouth or trumpet arch are sometimes applied to the
junction of sewers large enough to be entered by a man. In small sewers
the Y branches and special junctions are manufactured so that the center
lines of the pipes intersect, and the junctions of mains and laterals
are made in manholes. In the construction of a bellmouth the arch is
carried over all the sewers. A manhole should be constructed at these
junctions as clogging frequently occurs there, due to swirling and back
eddies which cannot be avoided.
=65. Outlets.=—The outlets to a sewerage system discharging into a
swiftly running stream must be protected against wash and floating
debris. In a stream or other body of water subject to wide variations in
elevation the backing up of the sewage during high water should be
avoided. Where tidal flats or low ground about the outlet may be
alternately submerged and uncovered the discharge should always be into
swiftly running water. In quiescent bodies of water such as lakes and
harbors, and in rivers where the dilution is low, and in many other
cases, the sewer outlet should be submerged.
[Illustration:
FIG. 46.—Tide Gate.
]
Outlets are protected against wash and the impact of debris by the
construction of deep foundations and heavy protecting walls. Although
the construction of an outlet in a slow current or a back eddy would
avoid danger from wash and debris, the discharge of the sewage into the
most rapid current possible aids in the prevention of a local nuisance.
A row of batter piles on the upstream or exposed side of the sewer is
desirable, or it may be necessary to construct a break-water to prevent
the wash of the current from dislodging the pipe. These break-waters are
low dams of wood or broken stone, more or less loosely thrown together.
The backing up of water into the sewer can be prevented by constructing
the sewer above the outlet on a steep grade. Where this is not possible
the use of tide gates will be helpful. A tide gate, one form of which is
shown in Fig. 46, is a special form of check valve placed on the end of
the sewer.
[Illustration:
FIG. 47.—Sewer Outlet on a Trestle.
Eng. News, Vol. 49, p. 9.
]
Sewer outlets are sometimes constructed on long trestles in order to
reach deep or running water. Such a trestle is shown in Fig. 47. In
Boston the outlet sewers are submerged under the harbor and discharge
through outlets well out in the tidal currents. The sewage is discharged
under pressure and the pumps are operated at some of the stations only
at such times as the tidal currents will carry the sewage away from the
harbor. It is not always necessary in a combined sewerage system to
carry the storm flow to a distant submerged outlet. A double outlet can
be constructed as shown in Fig. 48 in which the dry weather flow is
carried to the channel in a submerged sewer and the storm flow is
discharged on the bank.[44] Cast-iron pipe should be used for submerged
outlets as the sewer is subject to disturbance by the currents, anchors,
ice, and other causes.
[Illustration:
FIG. 48.—Dry Weather and Storm Sewer Outlet at Minneapolis, Minnesota.
Eng. Record, Vol. 63, p. 383.
]
=66. Foundations.=—Sewers constructed in firm dry soil require no
special foundation to distribute the weight over the supporting medium.
In soft materials the lower half of the sewer ring may be spread as
shown in Fig. 22, and in rock the pressures on sewer pipes are evenly
distributed by a cushion of sand. In wet ground such as quicksand, mud,
swamp land, etc., a foundation must be constructed if the water cannot
be drained off.
The permissible intensities of pressure on foundations in various
classes of material allowed by the building codes in different cities
are given in Table 25. These figures are based on the assumption that
the material is restrained laterally, which is generally the condition
in sewer construction. In the softer materials it becomes necessary to
spread the foundations not only to reduce the intensity of pressure, but
also to care for the thrust of the sewer arch. The arch thrust may be
found by one of the methods of arch analysis, and the haunches spread to
care for this, or the sewer invert may be transversally reinforced to
assist in caring for this action. Some sewer sections in hard and soft
material are shown in Fig. 22 and 23.
TABLE 25
ALLOWABLE BEARING VALUE ON SOILS IN VARIOUS CITIES
From Proc. Am. Soc. Civil Engrs., Vol. 46, 1920, p. 906
─────────────────────────┬─────────────────────────────────────────────
Quicksand and alluvial │½ to 1 ton per sq. ft. for Providence, R. I.,
soil │ ½ ton per sq. ft. for 6 cities
─────────────────────────┼─────────────────────────────────────────────
Soft clay │1 ton per sq. ft. for 27 cities, ¾ ton per
│ sq. ft. for New Orleans, 2 to 3 tons for
│ Providence, R. I.
─────────────────────────┼─────────────────────────────────────────────
Moderately dry clay and │2 tons for 7 cities, 1¾ to 2¼ for Chicago, 2½
fine sand, clean and │ tons for Louisville, 2 to 4 tons for
dry │ Providence, 3 tons for Grand Rapids and Los
│ Angeles
─────────────────────────┼─────────────────────────────────────────────
Clay and sand in │2 tons for 19 cities, 1¾ to 2¼ for Chicago, 3
alternate layers │ to 5 tons for Providence
─────────────────────────┼─────────────────────────────────────────────
Firm and dry loam or │3 tons for 24 cities, 2½ tons for 2 cities, 2
clay, or hard dry clay │ to 3 tons for Atlanta, 3½ tons for
or fine sand │ Philadelphia, 4 tons for 6 cities
─────────────────────────┼─────────────────────────────────────────────
Compact coarse sand, │4 tons for 25 cities, 3½ tons for Buffalo, 3
stiff gravel or natural│ to 4 tons for Atlanta, 4 to 5 tons for
earth │ Cincinnati, 5 tons for Denver, 4 to 6 tons
│ for 3 cities, 6 tons for Rochester, N. Y.
─────────────────────────┼─────────────────────────────────────────────
Coarse gravel, stratified│6 tons for 3 cities, 5 tons for 2 cities, 8
stone and clay, or rock│ tons for 1 city
inferior to best brick │
masonry │
─────────────────────────┼─────────────────────────────────────────────
Gravel and sand well │8 tons for 5 cities, 6 tons for 2 cities, 8
cemented │ to 10 tons for 1 city
─────────────────────────┼─────────────────────────────────────────────
Good hard pan or hard │10 tons for 4 cities, 6 tons for 2 cities, 8
shale │ tons for 1 city
─────────────────────────┼─────────────────────────────────────────────
Good hard pan or hard │8 tons for 1 city, 10 to 15 tons for 1 city,
shale unexposed to air,│ 12 to 18 tons for 1 city
frost or water │
─────────────────────────┼─────────────────────────────────────────────
Very hard native bed rock│20 tons for 5 cities, 15 tons for 1 city, 10
│ tons for 1 city, 25 to 50 tons for 1 city
─────────────────────────┼─────────────────────────────────────────────
Rock under caisson │24 tons for Baltimore, 25 tons for Cleveland
─────────────────────────┴─────────────────────────────────────────────
On soft foundations such as swamps or for outfalls on the muck bottom of
rivers the sewer may be carried on a platform. For small sewers 2–inch
planks, 2 to 4 feet longer than the diameter of the pipe are laid across
the trench, and the sewer rests directly upon them. For large sewers
imposing a heavy concentrated load, a pile foundation should be
constructed. The foundation may consist of piles alone, pile bents, or a
wooden platform supported on pile bents. The load which can be carried
by a pile is determined with accuracy only by driving a test pile and
placing a load on it. Where piles do not penetrate to a firm stratum the
load they will support can be determined by any one of the various
formulas, either theoretical or empirical, which have been devised.
Probably the best known of these formulas are the so-called Engineering
News formulas one of which is:
_P_ = 2_Wh_⁄(_S_ + 1) for a pile driven by a drop hammer,
in which _P_ = the safe load on the pile in pounds. The factor of
safety is 6;
_W_ = the weight of the hammer in pounds;
_h_ = the fall of the hammer in feet;
_S_ = the penetration of the pile in inches at the last
driving blow. The blow is assumed to be driven on
sound wood without rebound of the hammer.
Reference should be made to engineering handbooks for other forms of
pile formulas. The accuracy of all of these formulas is not of a high
degree.
The piles are driven at about 2 to 4 feet centers, to a depth of from 8
to 20 feet, unless hard bottom is struck at a lesser depth. The butt
diameter of the piles used for the smallest sewers is about 6 to 8
inches. If bents are used, 2 or 3 piles are driven in a row across the
line of the sewer and are capped with a timber. For brick, block, pipe,
and some concrete sewers, a wooden platform must be constructed between
the pile bents for the support of the sewer.
=67. Underdrains.=—The construction of special foundations can sometimes
be avoided by laying drains under the sewers, thus removing the water
held in the soil. The laying of the underdrains facilitates the
construction of the sewer and reduces the amount of ground water
entering the sewer. The underdrains usually consist of 6– or 8–inch
vitrified tile laid with open joints from 1 to 2 feet below the bottom
of the sewer as shown in Fig. 1. If the sewers are large, parallel lines
of drains may be laid beneath them. An observation hole should be
constructed from the bottom of the manhole to each underdrain. This hole
usually consists of a 6– or 8–inch pipe, embedded in concrete, connected
to the drain and open at the top. It is too small to permit effective
cleaning of the underdrains, which are usually neglected after
construction, and which as a result clog and cease to function. Since
the principal period of usefulness of the drains is during construction,
their stoppage after the work is completed is not serious. The hollow
tile used in vitrified block sewers serve as underdrains after
construction, but are of little or no assistance to the draining of the
trench during construction.
CHAPTER VII
PUMPS AND PUMPING STATIONS
=68. Need.=—In the design of a sewerage system it is occasionally
necessary to concentrate the sewage of a low-lying district at some
convenient point from which it must be lifted by pumps. In the
construction of sewers in flat topography the grade required to cause
proper velocity of sewage flow necessitates deep excavation. It is
sometimes less expensive to raise the sewage by pumping than to continue
the construction of the sewers with deep excavation.
In the operation of a sewage-treatment plant a certain amount of head is
necessary. If the sewage is delivered to the plant at a depth too great
to make possible the utilization of gravity for the required head, pumps
must be installed to lift the sewage. In the construction of large
office buildings, business blocks, etc., the sub-basements are
frequently constructed below the sewer level. The sewage and other
drainage from the low portion of the building must therefore be removed
by pumping. Because pumps are often an essential part of a sewerage
system, their details should be understood by the engineer who must
write the specifications under which they are purchased and installed.
=69. Reliability.=—If the only outlet from a sewerage system is through
a pumping station, the inability of the pumps to handle all of the
sewage delivered to them may so back up the sewage as to flood streets
and basements, endangering lives and health and destroying property.
Such an occurrence should be guarded against by providing sufficient
pumping capacity and machinery of the greatest reliability.
=70. Equipment.=—The equipment of a sewage pumping station, in addition
to pumping machinery, may include a grit chamber, a screen, and a
receiving well. The grit chamber and screen are necessary to protect the
pumps from wear and clogging. Grit chambers are not necessary in sewage
devoid of gritty matter, such as the average domestic sewage, unless
reciprocating pumps are used. Sufficient gritty matter is found in
average domestic sewage to have an undesirable effect on reciprocating
pumps. Receiving wells are used in small pumping stations where the
capacity of the pumps is greater than the average rate of sewage flow.
The pumps are then operated intermittently, the pumps standing idle
during the time that the receiving well is filling.
Except for a few types of pumps of which the valve openings are
unsuitable, any machine capable of pumping water is capable of pumping
sewage which has been properly screened. The principles of sewage pumps
are then similar to principles of water pumps. The conditions under
which these principles are applied differ but slightly in the character
of the liquid, and a smaller range of discharge pressures. Pumps with
large passages, discharging under low heads are more commonly found
among sewage pumps.
[Illustration:
FIG. 49.—Calumet Sewage Pumping Station, Chicago, Illinois.
]
=71. The Building.=—The pumping station should, if possible, be of
pleasing design and should be surrounded by attractive grounds. The
Calumet Sewage Pumping Station in Chicago is shown in Fig. 49. Its
architecture is pleasing particularly in contrast with its location and
immediate surroundings. Such structures tend to remove the popular
prejudice from sewerage and to arouse interest in sewerage questions.
Service to the public is of value. It can be rendered more easily by
arousing public interest and cooperation by the erection of attractive
structures, than by feeding popular prejudice by the construction of
miserable eyesores.
=72. Capacity of Pumps.=—The capacity of the pumping equipment should be
sufficient to care for the maximum quantity of sewage delivered to it,
with the largest pumping unit shut down, and the provision of such
additional capacity as, in the opinion of the designer, will provide the
necessary factor of safety.
Pumps can usually be operated under more or less overload. Power pumps
and centrifugal pumps driven by constant speed electric motors have no
overload capacity. A power pump or a centrifugal pump may be overloaded
up to the maximum horse-power of any variable speed motor or steam
engine driving it, provided the pump has been designed to permit it.
Direct-acting steam pumps which are designed for proper piston speed and
valve action at normal loads, can carry a 50 per cent overload for short
periods, although the strain on the pump is great. They will carry a 20
to 25 per cent overload for about eight hours with less vibration and
strain. The use of pumps capable of working at an appreciable overload
is somewhat of an additional factor of safety, but the overload factor
should not be taken into consideration in determining the capacity of
the pumping equipment.
The load on a pumping station consists of the quantity of sewage to be
pumped and the height it must be lifted. Variations in the quantity are
discussed in Chapter III. The head against which the pumps must operate
fluctuates with the level in the tributary sewer or pump well, and in
the discharge conduit. For a free discharge or discharge into a short
force main the greater the rate of sewage flow the smaller the lift, as
the depth of flow in the tributary sewer increases more rapidly than
that in the discharge conduit. If the discharge is into a large body of
water or under other conditions where the discharge head is
approximately constant, the fluctuations in total head should not exceed
the diameter of the tributary sewer. Such fluctuations are of minor
importance in the operation of direct-acting steam pumps, but may be of
great importance in the operation of centrifugal pumps, as is brought
out in Art. 78.