Unit Hydrograph Question

[mmmbeer]

Example 20.3 Page 20-8 Cerm 8th edition

i am following this example all the way through UNTILL we go to calculate V=PA(d)

in part (a) of the problem we JUST calculated P as 1.375 in.

so WHY in the name of all thats right and good are we using 1.0 in/in for P in part ( to find the Flood Volume???

why bother to calculate P in part a if we can just use 1.0 in???

i am confused . .

can someone help me to understand this?

THANKS

 

[frazil]

The first P you calculated is for the storm hydrograph. You used that P and the storm Q to find the unit hydrograph peak discharge for this watershed. (Look at the figure 20.13)

By definition a unit hydrograph always has 1 in of precip. So they calculate the volume for this watershed for the unit hydrograph storm and then multiply it by 2.5 to get the design flood volume.

I hope that makes sense.

 

[mmmbeer]

The first P you calculated is for the storm hydrograph. You used that P and the storm Q to find the unit hydrograph peak discharge for this watershed. (Look at the figure 20.13)
By definition a unit hydrograph always has 1 in of precip. So they calculate the volume for this watershed for the unit hydrograph storm and then multiply it by 2.5 to get the design flood volume.

I hope that makes sense.

SURE DOES!!

thansk for your help . . . . .

i do ALOT of stormwater at work but never have to do this kind of stuff . . . . just design stroms and sizing pond and designing outlet structures.

I JUST HAFTA STUDY ENOUGH TO PASS THIS TIME!!!