EXAMPLE 2 



Problem: A road crosses the outlet of a 40-acre watershed over a 12-inch corrugated 

 metal culvert whose top is 1 foot below the road surface. Plans call 

 for clearcutting the watershed that has an annual yield of 35 inches. 

 After cutting we expect annual water yield to increase by 15 percent. 

 Assuming a 20-year-design flood, will the presently located culvert 

 accommodate the increased flow? 



Solution: 



The expected annual flow is 40.25 inches (35 inches X 1.15). In table 2, col. 5, 

 interpolating for peak values between 40 and 41 inches, we obtain an expected peak flow 

 of 39.82 c.f.s.m. Since the area involved is 40 acres, the expected peak flow is 2.49 

 40 



c.f.s. (39.82 X . Using culvert discharge tables (e.g., Hendrickson 1957), we find 



that the presently installed culvert (use 1.0 percent slope and 0.025 roughness coeffi- 

 cient) will accommodate only 2.4 c.f.s. Because there is a present capacity of only 

 2.4 c.f.s., and the expected need is for 2.49 c.f.s., it appears that the road will 

 probably be damaged by overflowing unless a larger culvert is installed or the surface 

 of the road is raised to allow for ponding. 



EXAMPLE 3 



Problem: The annual water yield from a 5.0-sq,-mi. drainage is 40 inches. 



Plans call for clearcutting a 40-acre subwatershed. What are the 

 present and expected peak flows at the outlet of the main drainage for 

 a 10-year flood if the annual water yield of the clearcut area is ex- 

 pected to increase by 15 percent? 



Solution: 



Under present conditions (before cutting) the expected 10-year flood for the en- 

 tire drainage is 168.20 c.f.s. In table 2, col. 4, opposite 40 inches, read 33.64 

 c.f.s.m. and multiply by 5.0 sq. mi., and for the 40-acre subwatershed it is 2.10 c.f.s. 



(33.64 X^). 



After cutting, the average yield from the clearcut 40-acre subwatershed will be 



46.0 inches (40 X 1.15). In table 2, col. 4, we read a peak flow value of 36.11 c.f.s.m. 



Since the subwatershed is 40 acres, the peak flow is 2.26 c.f.s. , „ 40 ^ 



(^6.11 X ^j. 



On the clearcut area, the peak flow will increase by 0.16 c.f.s. (2.26 - 2.10). 

 Add this value to the precutting peak flow for the entire drainage: 



168.20 + 0.16 = 168.36 c.f.s. 



It is apparent that clearcutting the 40-acre subwatershed will not alter the peak flow 

 of the main watershed in any significant way. 



6 



