Lagoons and Modelling

[martin888888]

Hi,
I have an odd stormwater modeling question. We have a 50ac man made lagoon with a single 12" culvert discharging water to a ditch from the lagoon (no outlet structure). NO stormwater reches the lagoon from outside the 50ac. We are just doing a model of the existing lagoon and outlet structure to determine drawdown times and what size pipe we are able to replce the 12" with. In modeling the 25yr storm hitting the lagoon I would think you would use a CN (santa barbara methology) number of the existing soils in the lagoon as you will still have infiltration. If it was a lined lagoon would you use a CN of 98 and treat it as impervious? I think its just a simple catchment to pond to outlet model, with the catchment being the lagoon/pond area.
Thanks

 

[psmart]

Since the rainfall is essentially falling on the surface of the pond, you would use a high CN (98+) to allow for (almost) complete interception of the rainfall. If there is no watershed area beyond the lagoon, and you assume no infiltration or discharge, the peak depth in the lagoon will be the same as the rainfall depth, correct? So a 5" event will give 5" of water in the lagoon (Assuming a flat bottom) Adding an outlet device will obviously reduce the peak WSE. But it's going to take a long time to drain the entire 50 acres, unless the bottom is sloped to create more head at the outlet. Of course, all this assumes the lagoon is empty at the start of the storm.

Peter Smart
HydroCAD Software

http://www.hydrocad.net