1. For the watershed sketched below, subareas A and B are identical in size, sha

Question

1. For the watershed sketched below, subareas A and B are identical in size, shape, slope, and channel length. 1-hr UHs are provided for natural and fully developed conditions for each subarea. Note that the UHs are not for the entire watershed, only for each subarea. a) Assuming both subareas A and B are undeveloped (i.e., under natural condition), evaluate peak outflow at point 1 if 2.5 in/hr of rain falls for 2 hr. Assume total infiltration loss of 1-in occurs at constant rate (i.e., 0.5 in/hr) throughout the rain duration. b) Determine runoff hydrograph for the watershed at point 1 if both subareas A and B are fully developed (i.e., urbanized) and if effective rainfall of 2 in/hr falls for only 1 hr UHnat cfs 0 12 32 62 108 180 208 182 126 80 53 32 18 Time (hr) UHdev (cfs 0 0.00 196 290 268 185 4 30 0 10 12 14

Explanation / Answer

1 hour unit hydrograph has been given which is same for sub areas A and B

Peak outflow at point 1 = sum of outflows of sub area A and B

Now ordinates of 2.5 in/ hr hydrograph = 2.5 * ordinates of unit hydrograph

Thus peak flow for 2.5 in / hr hydrograph = 2.5*( 208+ 208)= 1040 cfs

however for 2.5 in/ hr and 2 hr duration hydrograph we get peak flow = 2.5*( 208+ 182 + 208+182)= 1950 cfs

however as infiltration rate is 0.5 in / hr throughout rainfall duration hence we will have peak outflow at point 1 due to both the sub areas A and B as = 2*(208+182+208+182) = 1560 cfs which is the answer for question a

for question b we need to determine the runoff hydrograph which is given as difference of unit hydrograph and infiltration

thus for question b runoff = effective rainfall - infiltration = 2-0.5 = 1.5 in/ hr

thus ordinates of runoff hydrograph at point 1 due to fully developed sub areas A anand B are :

0 ; 1.5*(40+40) ; 1.5 * (196+196) ; 1.5 *( 290+290) ; 1.5*( 268+268); 1.5*( 185+185); 1.5* (90+90); 1.5*( 30+30) ; 0

corresponding to times 0; 1; 2 ; 3; 4; 5; 6; 7 and 8 hr respectively

the ordinates of hydhydrograph in cfs are as follows :

0; 120; 588; 870; 804; 555; 270; 90; 0 cfs corresponding to the time periods as already discussed

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