HEC-RAS: Modeling a Weir and Detention Area

[djstrick110]

Hi, all. To summarize my situation and goal, I am trying to design a solution to lower the current water surface elevations along an existing rention pond/channel. Due to various constraints on the property, the most viable option I have right now is extending the existing pond and constructing a weir across the mouth of that extension. The idea is for the extension to act as a detention area during certain storm events.

The weir will have to be placed in a practically 90 degree bend (drawing attached... sorry; it's not the prettiest I've ever done!). I've modeled the weir as a lateral structure connected to a storage area. However, I ran into a problem when experimenting with weir heights. For example, I entered a weir which would detain only the 10 and 25 year storms. However, the model reduced the flow for those events as well as the 50 and 100 year events which I would think, should have overwhelmed the weir and detention keeping the flow in the channel unchanged.

Is there a way to have HECRAS recognize the weir and reduce the flow only during those smaller events, then neglect it when flows reach an elevation above the crest of the weir?

Any tips/advice will be greatly appreciated. Thanks in advance!

 

[RWF7437]

Scant information to go on here. Try detailing your sketch with a graphic scale, dimensions, volumes, flows, inlets and outlets and elevations.

Is this really a retention pond with no outlets or is it a detention pond; or is it something else?

How can the model (HEC-RAS) change flows as you suggest ? The only flows HEC-RAS uses are those you enter.

 

[MattZ99]

Are you using an unsteady flow HEC-RAS model? That would be your best option. You could set up a plan for each storm event you would like to analyze. Then you would have to make several runs with different weir heights to determine which weir height meets your criteria. The program should take into account the effects of submergence when the weir is completely under water.