Stage-Storage table vs Starting elevation 1

[PAC8]

For a pond, when a "starting elevation" is input, does HydroCAD assume the volume in the pond below this elevation is dead storage (i.e. a wet pond with standing water at a normal water level (NWL) equal to the starting elevation), even if the stage-storage table lists that there is storage volume below the starting elevation?

For example, bottom of pond= 100ft, NWL=103ft. The stage-storage table lists "inc. store" and "cum. store" of zero at elevation 100ft, but greater than zero for elevations above 100ft. However, I want to ensure the model is assuming there will be water in the pond below 103ft and, therefore, is not available for storage during a rain event. Can one just input the starting elevation of 103ft or should the stage-storage table be modified to show inc. store and cum. store of zero for elevations below the NWL?

 

[psmart]

The starting elevation causes the pond to be prefilled with water to the specified elevation. This is comparable to starting the storage definition at the specified elevation, but has the advantage of allowing you to easily change the starting elevation without having to re-enter the storage data.

The two approaches will generate the same outflow hydrograph in most cases. However, the results will differ is you've using exfiltration based on pond area, since there is additional area below the starting elevation. There will also be a difference in the plug flow detention time, since the "dead storage" volume actually increases the detention time. (first-in first-out calculation).

The starting elevation can also be used to pre-fill a pond above the outlet device(s), in order to observe the draw-down. Or it can be used in conjunction with a base flow to establish an equilibrium condition with a non-zero outflow, as occurs in many "wet ponds".

In short, there are several advantages to including the entire volume (including any dead storage) in the storage definition, and then using the starting elevation to establish the pre-storm WSE.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[PAC8]

Thank you Peter. Extremely helpful!