Detention pond routing with variable tailwater - program?

[jonrpatrick]

I have a detention pond outleting into a floodplain. we have a 100 year detention requirement.
Obviously, as the flood waters rise in the floodplain, my tailwater will be increasing and decreasing the head my outlet structure is working with.
Primarily I use Hydroflow Hydrograph2004, and it allows you to set a 'fixed' tailwater... but I'm trying to figure if there's a program that would allow you to have not only your inflow hydrograph, but also another component to reflect the dynamic nature of the outfall floodplain?

I'd be happy to use Hec1, if it can do this...if someone can point me in the right direction.
Thanks,
JP

 

[gbam]

Inter Connected Pond Routing will allow for a variable tailwater. But I must ask what are the odds of the timing between the two events? Will the flood on the channel be flowing and or peaking during your design storm? There are approximations outlined in HEC22 that can provide the tailwater conditions.

 

[TerryScan]

Hydrocad allows one to enter tailwater as a table or a tidal cycle. One can download an evaluation version on their website:

http://www.hydrocad.net.

 

[env21tech]

We use the FHA culvert analysis program that allows tailwater depth to increase with flow rate.

 

[cvg]

I would assume that your tailwater is directly downstream of your outlet structure, so that the level is directly related to the flow through the outlet structure. I generally calculate the rating curve for the outlet structure independantly using FHWA HY8 or other analysis and then enter the rating curve data points into the routing program (as recommended by Terryscan) to route the flow through the detention basin. This allows a more rigourous, generally more accurate estimation of the outlet works rating curve.

 

[psmart]

You need to investigate the floodplain behavior in more detail. The previous posts assume that your tailwater is directly related to your discharge. However, the tailwater timing may not coincide with your discharge at all. The floodplain may peak before OR after your site. If so, this will require that you determine an elevation-vs-time relationship for the floodplain and use this as the tailwater for your site as TerryScan suggested. Or you may find that the floodplain level is relatively constant during the design period for your site, in which case you can use a constant tailwater.

 

[blueoak]

It is likely that analyzing timing between your site and the floodplain will not provide a lot of info unless you have pretty big low intensity storms.

Depending on your site you may just look at a low tailwater scenario and a high tailwater scenario and see if you have a problem. If you do it might be useful to look at the probability of the 100 yr event on your site along with the whole watershed since your site 100-yr event may occur with areal reductions for the floodplain.

 

[cvg]

unless your floodplain is quite flat, flooding at a distance downstream of your basin in the floodplain may have minimal if any backwater effect on tailwater immediately downstream of your outlet structure. On the other hand, flow rate through your outlet structure could have a large effect on the tailwater, depending upon the size and capacity of your outfall channel.

 

[blueoak]

Good point, last time I had a tailwater problem I also was dealing with flap gates and pumps.