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Problems/Questions 553
180 m and all pipes have an n 0.013. The areas and their co-
Drainage Draining Overland Flow
efficients of runoff are as follows:
Area Area, ha c to MH Time, min
Drainage Area Area, ha c
A 10 0.50 40 10
1 1.2 0.45 B 7 0.65 20 12
2 2.4 0.45 C 6 0.65 30 5
3 1.2 0.15 D 15 0.70 20 15
E 8 0.65 10 8
The storm sewers are set on a slope of 0.0020. The rainfall in-
tensity i (mm/h) as a function of rainfall duration t (min) is:
14.27 Izzard developed the following watershed hydrology
3,300
i = equation for calculation of concentration time, t c :
t + 19
1/3
1/3 2/3
14.26 Design the stormwater drainage system for the area t c 41 L i [(0.0007i C r )>s ]
shown in Fig. 14.43. Draw a neat profile along the route of MH
where t c time of concentration, min; s slope of ground, di-
30 - MH 20 - MH 10 - Stream edge. On the profile show all pipe
mensionless; i intensity of rainfall, in./h; C r retardance co-
sizes, slopes and ground and Invert elevations. 2
efficient varying from 6.0 10 for pervious turf areas, 3.2
The rainfall intensity (mm/h) as a function of rainfall dura- 2 2
10 for pervious bare, packed areas, 1.2 10 for pave-
tion (min) is: ments, to 1.7 10 2
1,000 for flat, gravel roofs.
i = Based on the above equation, develop further relationships for
t + 50
the determination of:
The minimum allowed velocity in sewers is 0.80 m/s and the
maximum is 3 m/s. The minimum cover over sewers is 1.50 m. (a) overland flow distance, L
The maximum water level in stream is 615.00 m. (b) retardance coefficient, C r
The following data concerning the drainage area are known:
(c) slope, s
Point Ground Elevation, m 14.28 Conduct a university library and an Internet search to
find other researchers’ mathematical models for determination
MH 30 625.30 of concentration time, overland flow distance, retardance coef-
MH 20 624.20 ficient, and slope within a watershed. Write a literature review
MH 10 622.20 report.
MH 40 622.38
Stream edge 622.50 14.29 A metric Izzard equation using the SI System of units
t c (min), L (m), i (mm/h), C r (dimensionless), and s (dimension-
less) has been developed by the authors:
Stream edge
Outlet to MH 10 MH 40 t c 526 L i [(0.00002756i C r )>s ]
1/3
1/3 2/3
stream
L 300 m L 250 m Determine the time of concentration in both U.S. customary
L 400 m A sity i 1 in./h (25.4 mm/h).
E and SI units assuming the overland flow distance L 1,000 ft
(304.8 m), C r 0.06, the slope s 0.02, and the rainfall inten-
14.30 Review Appendix 18, Simplified Overland Flow Time
Determination for Urban Drainage. If the distance of water flow
MH 20 is 60 m, what is the time of concentration for a flow passing
over an ordinary grass surface with a slope of 4%? Discuss the
L 200 m B accuracy of your results.
D
MH 30 14.31 The general form of intensity-duration curves is i
A>(t B). The values of A and B vary from one region to another.
C The rainfall curves must be derived from historical regional
storm records. Assuming you are an environmental engineer in
charge of a local drainage project, conduct a search on local and
government storm records (such as USGS) and recommend an
intensity-duration curve for use in the project. Give a
Figure 14.43 Layout of Storm Drainage for Problem 14.26. PowerPoint presentation to your class showing your classmates
Conversion factor: 1 m 3.2808 ft. how you reached to your results.