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# HEC-RAS 5.0.7 cannot calculate infiltration for each cells?

## HEC-RAS 5.0.7 cannot calculate infiltration for each cells?

(OP)
Just started to use HEC-RAS 2D to calculate the flow depth and velocities. I am using HEC-RAS 5.0.7. I noticed it cannot calculate the infiltration for each cells. I have to calculate infiltration by myself when Computing Excess Rainfall Hyetograph using a averaged CN for the whole site. And then use the Excess Rainfall Hyetograph as a boundary condition.

Here are the equations I use:
Fa=S*(P-Ia)/(P-Ia+S)
where, P, Fa, S and Ia have the same unit of length [L]. Usually total rainfall (P) is known, and S is computed by using the CN (assumed to be given for this exercise) as shown in the equation below:
S= (1000/CN)-10)
Where S is in inches. the continuous abstraction term Fa gives the amount of rainfall that is lost as infiltration from the total rainfall.

I do realize this is not accurate to use an averaged CN. Am I not using the software properly? What is the correct way? Any guidance will be greatly appreciated! T.I.A.

### RE: HEC-RAS 5.0.7 cannot calculate infiltration for each cells?

HEC-RAS 5.0.7 does not account for infiltration or initial abstraction. The precipitation data on SA/2D boundary condition has be be the excess rainfall hyetograph. Unless you want to be conservative and assume no infiltration and all the rainfall runoff are conveyed to your 2D flow area. It is more of hydraulics computation and less hydrology. Your other alternative will be switch to HEC-HMS which does little of both.

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