Depression Storage 1

[SwampMuskrat]

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.

 

[LHA]

If you are using TR55, there is a correction factor to apply only to the peak, in the Graphical analysis. It is in Table 4-2, page 4-2. I don't know if it is valid to apply to the entire hydrograph in Tabular TR55, or if it can be applied to the peak from a Rational or Regression analysis.

Engineering is the practice of the art of science - Steve

 

[psmart]

You may be able to consider the depressions as part of the "Potential Maximum Retention", which would allow you to include them as part of an adjusted Curve Number.

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

 

[blueoak]

It has been some time but I believe the SCS methods disregard depressions of less than 10%. I am also hesitant to adjust the curve number. I think it would be better to take it out of the hyetograph as an initial loss.

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).

 

[LHA]

psmart's seems valid, for smaller storms. blueoaks concerns are intuitively right on for floods; by my observations, by the time anything approaching the peak rainfall comes along, those depressions are under several inches of rapidly moving water.

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