Handbook of Civil Engineering Calculations

TABLE 20. Storm-Sewer Inlet Capacity per Foot (Meter)

of Length

Depression Capacity per foot

Flow depth in depth, in length, fVVs

gutter, ft (mm) (mm) (m^3 /s)

0.2(0.06) O (O) 0.062(5.76)

1 (25.4) 0.141(13.10)

2 (50.8) 0.245(22.76)

3 (76.2) 0.358(33.26)

4(101.6) 0.500(46.46)

0.3(0.09) O (O) 0.115(10.69)

1 (25.4) 0.205(19.05)

2 (50.8) 0.320(29.73)

3 (76.2) 0.450(41.81)

4(101.6) 0.590(54.82)

taken as 0.015; s = gutter slope, in/ft; other symbols as before. Where the computed inlet

length is 5 ft (1.5 m) or more, some engineers assume that a portion of the water will pass

the first inlet and enter the next one along the street.

STORM-SEWER DESIGN

Design a storm-sewer system for a 30-acre (1.21 * 105 -m^2 ) residential area in which the
storm-water runoff rate is computed to be 24 ft^3 /s (0.7 m^3 /s). The total area is divided into
10 plots of equal area having similar soil and runoff conditions.

Calculation Procedure:

1. Sketch a plan of the sewer system
   Sketch the area and the 10 plots as in Fig. 15. A scale of 1 in = 100 ft (1 cm = 12 m) is
   generally suitable. Indicate the terrain elevations by drawing the profile curves on the plot
   plan. Since the profiles (Fig. 15) show that the terrain slopes from north to south, the main
   sewer can probably be best run from north to south. The sewer would also slope down-
   ward from north to south, following the general slope of the terrain.
   Indicate a storm-water inlet for each of the areas served by the sewer. With the terrain
   sloping from north to south, each inlet will probably give best service if it is located on
   the southern border of the plot.
   Since the plots are equal in area, the main sewer can be run down the center of the plot
   with each inlet feeding into it. Use arrows to indicate the flow direction in the laterals and
   main sewer.


2. Compute the lateral sewer size
   Each lateral sewer handles 24 ft
   3
   /s/10 plots = 2.4 fWs (0.07 m
   3
   /s) of storm water. Size
   each lateral, using the Manning formula with n = 0.013 and full flow in the pipe. Assume
   a slope ratio of 0.05 for each inlet pipe between the inlet and the main sewer. This means