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396 Chapter 11 Hydrology: Rainfall and Runoff
becomes surface runoff. Determine the average surface runoff Table 11.15 Rain Gauge Record for Problem 11.8
per square mile from New York State watersheds in the English
2
units of million gallons per day per square mile (MGD/mi ) and Time from Beginning Cumulative
3
cubic feet per second (ft /s). of Storm (min) Rainfall (in.)
11.6 Data from the Hudson River watershed area in the state 5 0.47
of New York has been recorded since 1946 at the Green Island 10 0.93
Station, near the city of Albany. According to the record, the 15 1.32
2
2
watershed is 8,090 mi (21,000 km ), the average annual 20 2.02
runoff is 21.9 in. (556 mm), and the average Hudson River 25 2.44
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3
flow is 13,060 ft /s (370 m /s). Obtain the most recent 30 3.15
recorded information from the government or the internet and 35 3.96
double check the obtained data. 40 4.76
11.7 To be able to withdraw water continuously at or near the 45 5.10
average rate of flow from a watershed, water must be stored 50 5.49
during periods of high flow. The safe yield of a watershed area 60 5.75
is the maximum rate at which the water can be withdrawn con- 80 6.23
tinuously over a long period of time. Without storage, the safe 100 6.62
yield would be equal to the minimum rate of flow to be ex- 120 6.88
pected in the future. It is obvious that over the long period water Conversion factor: 1 in. 25.4 mm
could not be withdrawn from an area at a rate that exceeded the
3
3
long-term average flow. To do this, water runoff from the area 11.10 Determine the mean runoff flow (ft /s or m /s) assum-
would have to be stored. As a practical matter, the safe yield of ing the runoff coefficient c for a semiresidential area is 0.35, the
any watershed area will fall somewhere between the minimum mean rainfall intensity i is 4.57 in./h (116 mm/h), and the
expected flow rate and the long-term average. The exact drainage area a is 24.7 acres (10 ha).
amount depends on the storage capacity of a surface water 11.11 Determine the mean runoff flow (ft /s or m /s) assum-
3
3
reservoir. Figure 11.12 shows the relationship of safe yield of ing the runoff coefficient for a semiresidential area is 0.35, the
surface water supplies versus the reservoir storage capacity for mean rainfall intensity is 0.55 in./h (14 mm/h), and the drainage
New York State. Determine the safe yield of a reservoir if the area is 131 acres (53 ha).
following information is known: (a) The reservoir storage ca- 11.12 Determine the mean runoff flow (ft /s) from a drainage
3
pacity is 2 billion gallons or 2,000 MG (7,570 ML), (b) the total area of 120 acres using the English equation for a 2-year fre-
2
2
contributory watershed area is about 10 mil (26 km ), and (c) the quency i (in./h) 206>(t 30). The area is sloping at 5% and
watershed is in the state of New York.
is covered with average grass with a c value of 0.35. The fur-
thest point from the inlet is 60 m.
11.13 A residential area with an average estimated runoff co-
1.0 slope of 5%. The overland and across streets’ water flow veloc-
efficient of c 0.65 is shown in Fig. 11.13. The streets have a
Safe yield-MGD/mi 2 of watershed 0.8 ity is 3 m/min. The average flow velocity in the gutters is 2 m/s.
0.6
0.4
0.2 Inlets
0 20 m MH 70
100 200 300 400
2
Storage capacity-MG/mi of watershed 30 m
30 m
Figure 11.12 Safe Yield v.s. Reservoir Storage Capacity for
2
Problem 11.5. Conversion factor: 1 MGD/mi 1.461 MLD/km 2 20 m MH 69
30 m
11.8 Given the record of an automatic rain gauge (Table 11.15),
30 m
find the progressive arithmetic mean rates, or intensities, of pre-
20 m MH 68
cipitation for various durations.
11.9 Determine the mean rainfall intensity (in./h) using the
Length 1000 m
U.S. customary system equation for a 2-year frequency, if the
time of concentration, t, is 15 min. Figure 11.13 Layout of Drainage System for Problem 11.13