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Problems/Questions 395
Table 11.14 Examples of Flood-Flow Formulas

Individualized Variable Region Formula
None New England Q Ca 5>6 , where C 200 for a in mile 2
4/5 1>5
Rainfall intensity St. Louis, Mo. Q cia s , where s slope in %
and slope of and c 0.75 for a in acres or 480 for a
watershed in mile 2
Shape and slope Cumberland Plateau Q ca 7>6 /(l/s 1>2 ), where l length of
of water shed principal waterway in miles, s slope
of waterway in ft/mile, and c 1920d
2
for a in mile , the 10-year peak flood
and a factor d relating the basin to the
base station at Columbus, Ohio
0.7
Shape, slope, and New England Q (0.000036h 2.4 124)a 0.85 /(rl ),
surface storage where h median altitude of drainage
of watershed basin in ft above the outlet; r % of
lake, pond, and reservoir area; l av-
erage distance in miles to outlet; and
a mile 2
0.8
Frequency of flood Fuller, U.S.A. Q Ca (1 0.8 log T) (1 2a 0.3 ),
where T number of years in the
period considered, and C varies from
25 to 200 for different drainage basins
and a in mile 2
2
2
Conversion factors: 1 mi 2.59 km ; 1 mi 1.61 km; 1 ft 0.3048 m; 1 acre 0.4046 ha; 1 ft/mi 0.1894 m/km
PROBLEMS/QUESTIONS

11.1 Coastal air with a temperature of 70 F (21 C) and a dew Annual precipitation varies from more than 100 in. (2,540 mm)
point of 60 F (16 C) is forced over a mountain range that rises in parts of the northwest to only 2 or 3 in. (50.8 or 76.2 mm) in
6,000 ft (1,829 m) above sea level. The air then descends to a parts of the southwest. Determine the annual average runoff from
plain 4,000 ft (1219 m) below. If the dew point is lowered at a U.S. watersheds in the English units of million gallons per day
3
2
rate of 1.3 F in 1,000 ft (2.47 C in 1,000 m), find: per square mile (MGD/mi ) and cubic feet per second (ft /s).
(a) The height at which condensation will begin For students who are not living in the United States, please
consult with your national government for the information of
(b) The temperature at the mountain top
national average annual precipitation rate and average annual
(c) The temperature on the plain beyond the mountain
evaporation and transpiration rates.
assuming that condensed moisture precipitates be-
11.4 In the state of New York, the records of the daily average
fore the air starts downward.
flows are published annually by the U.S. Geological Survey in its
11.2 Estimate the daily average evaporation during which the
publication Surface Water Record of New York. Copies can be ob-
following averages are obtained:
tained by writing to the Water Resources Division, U.S. Geological
Water temperature 60 F (16 C) Survey (USGS), Albany, New York. This year the annual average
Maximum vapor pressure, P w 0.62 in. Hg (15.8 mm Hg) rainfall is about 45 in. (1,143 mm), of which about 23 in. (584 mm)
Air temperature 80 F (27 C) is lost to the atmosphere by evaporation and transpiration, and only
22 in. (559 mm) becomes surface runoff. Determine the average
Relative humidity 50%
surface runoff per square kilometer from New York State water-
Vapor pressure, P d 1.03 0.50 0.51 in. Hg (13 mm Hg) 3
sheds in the metric units of cubic meter per day (m /d).
Wind velocity, w 15 mi/h (24 km/h)
For U.S. students who are not living in the state of New
Barometric pressure, p a 28.0 in. Hg (711 mm Hg) York, please consult with your state government or the local
11.3 In the United States the average annual precipitation is USGS office for your state’s rainfall and evaporation/transpira-
about 30 in. (762 mm), of which about 21 in. (533 mm) is lost to tion data for an accurate determination for your home state.
the atmosphere by evaporation and transpiration. The remaining 11.5 This year the annual average rainfall is about 45 in.
9 in. (229 mm) becomes runoff into rivers and lakes. Both the (1,143 mm), of which about 23 in. (584 mm) is lost to the atmos-
precipitation and runoff vary greatly with geography and season. phere by evaporation and transpiration, and only 22 in. (559 mm)