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548 Chapter 14 Design of Sewer Systems
14.10 Using Kerby’s relationship, determine the overland 100 m
flow time for a bare and moderately rough surface 60 ft long A B
and having a slope of 0.010 that drains toward a storm outlet.
Overland flow Gutter
14.11 Calculate the amount of surface detention on a land sur- flow
face having a ground slope of 0.01, length of overland flow of 40 m
100 ft, rainfall intensity of 1 in./h, overland flow of 0.8 in./h, Overland flow Inlet D
–2
and a coefficient of roughness 6 10 .
14.12 The drainage area shown in Fig. 14.30 contributes its C D MH
total flow to the inlet and henceforth into the storm sewer. The
area is not homogenous and its runoff coefficient c is estimated Overland flow
for the different portions as follows: Storm
sewer
% of Area c
Overland flow
20 0.90
40 0.80
10 0.60
30 0.30
Figure 14.31 Layout of Storm Drainage for Problem 14.13.

Overland flow time 12 min Gutter flow mogenous, and its runoff coefficient c is estimated for the dif-
time 4 min
Area 6,000 m 2 ferent portions as follows:
Overland flow time 18 min Inlet % of area c
MH 30 0.50
40 0.80
10 0.60
20 0.40
Storm sewer
The rainfall intensity i (mm/h) as a function of rainfall duration
t (min) is:
Figure 14.30 Layout of Storm Drainage for Problem 14.12. 1,220
2
2
Conversion factor: 1 m 10.76 ft . i =
t + 12
Find:
The rainfall intensity i (mm/h) as a function of rainfall duration (a) The concentration time at inlet D.
t (min) is: (b) The overall runoff coefficient c for the area ABCD

1,680 (c) The total flow Q (L/s) entering Inlet D.
i =
t + 19 (d) What is the maximum and minimum pipe diameter you
can use for the storm sewer? Assume that the required
(a) Find the overall coefficient c for the drainage area
slopes can be economically accommodated in each case.
(b) Find the total flow Q in the storm sewer
14.14 Blocks I, II and III contribute all their runoff into the
(c) Determine the maximum and minimum storm sewer street ABC (see Fig. 14.32). Inlet C intercepts all the flow and
diameter. Assume that the required slope can be eco- discharges it into the storm sewer. The drainage areas and their
nomically accommodated in each case. corresponding runoff coefficients are as follows:
14.13 Area ABCD contributes all its flow into the gutter BD 2
(see Fig.14.31). The overland flow time from A to B is 10 Block Drainage Area (m ) Runoff Coefficient
min, from C to D is 20 min, and the flow time in the gutter BD I 10,000 0.80
is 5 min. II 12,000 0.75
Inlet D intercepts all the flow from gutter BD and dis- III 12,000 0.70
charges it into the storm sewer. Surface area ABCD is not ho-