Depression Storage
Depression Storage
(OP)
I am modeling a fairly large watershed in HydroCAD that has a substantial amount of "Depression Storage" within it. Rather than trying to model numerous smaller subwatersheds that drain to the numerous "Depressions" (Ponds), I am wondering if there is a way to account for this "Depression Storage". The Tc does seem to account for some degree of the flatness of the watershed. However, I have to believe that there is still substantial storage within the depressions (wetlands, ponds, natural depressed areas) that could be accounted for, thereby reducing the outflow.





RE: Depression Storage
Engineering is the practice of the art of science - Steve
RE: Depression Storage
Start with your normal CN value and calculate the Potential Maximum Retention:
S = 1000/CN - 10
Then add the depression storage (as an average depth over the subcatchment) to the S value, and calculate a final CN:
CN = 1000 / (S+10)
For example, with a normal CN value of 70 and depression storage of one inch, the adjusted CN would be 65.4
RE: Depression Storage
We have some discussions on this from time to time. I like to see if there is any outlet or eroded low spot. In this case it is likely that at least the 100yr isn't trapped. Then if this area is large as it fills how do you adjust your routing. If you have flow through then you have water on water and have to either consider a modified puls or shrinking your flow path.
It's like karst features, I know they take out my water from a shed, but do they put it back in above or below my dam or dike?
I would see if there are any gages though. Only look at events that produce runoff from an area of your shed equal or larger than your event if possible (if you are doing 1% make sure the storms are 50 yr or greater if possible).
RE: Depression Storage
Maybe you could benefit by applying psmart's to the 2-yr and lower storms. This might help avoid those obnoxious, debris-catchers - orifices of less than 3' - we've all been forced to design and/or approve, while not compromising the protection from flood-level runoff.
Engineering is the practice of the art of science - Steve