A mountain stream at flood stage. 



EXAMPLES FOR FIELD APPLICATION 



Three examples are given to show how the tables can be used for field application. 

 EXAMPLE 1 



Frohlem: A road will be built across the outlet of a 1.5-sq.-mi. drainage having 

 an expected annual water yield of 40 inches. What is the maximum 

 rate of flow the culvert should accommodate: 



(a) if designed for a 20-year flood? 



(b) if designed for a 50-year flood? 



Solution: 



(a) For an annual yield of 40 inches, table 2, col. 5, shows a peak flow of 39.69 

 c.f.s.m. Since the drainage area is 1.5 sq. mi., we can expect a 20-year flood of 59.53 

 c.f.s. (39.69 X 1.5). 



(b) The tables do not show peak flows for return periods exceeding 20 years. 

 Extrapolation is required, but this procedure is fraught with great uncertainty. In 

 table 2, opposite an average 40-inch annual yield, we find the expected peak flows for 

 the 10- and 20-year floods to be 33.64 and 39.69 c.f.s.m., respectively. Plot the 

 paired values (year versus flow) on log- log paper, connect them with a straight line, 

 and extend the line to the 50-year flood. The peak flow is 49.3 c.f.s.m. Inasmuch as 

 the area is 1.5 sq. mi., the expected 50-year flood is 74 c.f.s. (49.3 X 1.5 = 74). 



It must be understood that such extrapolation results in only approximate values. 



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