Detention Pond to Stream in HEC-RAS? 2

[Pete710]

I'm trying to determine the 100-year BFE (base flood elevation) for an unstudied creek. The creek drains to a detention pond which is controlled by a grated riser (inlet) where water enters the top of the inlet structure and then exits the structure through a circular culvert. Once leaving this culvert, the creek picks up again.

I'm familiar with steady flow analysis using HEC-RAS, and have modeled streams with multiple obstructions (bridges, culverts, etc.) but never with a detention pond (which I'm assuming is modeled as a "reservoir"). I can develop a stage/elevation - discharge table rating curve using other software, but where do i go from there?

 

[francesca]

I would set the maximum detention basin elevation as your downstream elevation in HEC-RAS and model the detention outflow hydrograph in something like Hydraflow Hydrographs. Your upstream and downstream streams won't peak at the same time and the detention basin will attenuate the peak so the downstream peak will be lower too.

 

[RWF7437]

Unclear.

How you model this depends on several facts you've not given us. Is the pond on the stream, adjacent to the stream or on a tributary to the stream ? If the last, what is the probability that the tributary and the main stream will peak at, or near, the same time? What are the drainage areas involved ? How did you arrive at the flows you are trying to analyze?

A schematic sketch would help a great deal in visualizing your problem.

good luck

 

[Pete710]

The pond is on the "stream" (it's more like a creek, the creek travels straight to and straight from the pond). The creek travels through a culvert to enter the pond. The pond has the grated riser inlet structure as described above. Once the water level reaches the top of the grates, it drops though the top and exits the structure via a circular pipe and discharges into another small ponding area. Water exits this ponding area via a circular culvert which passes under a road, at which point the creek channel picks back up again. I hope this gives a better idea of the configuration, I could provide a sketch but I don't know if one can be posted in this forum.

The drainage area is very small (approximately 95 acres). I haven't finalized the flows yet, but as I'm dealing with a relatively small drainage area, I have a variety of methods at my disposal.

 

[RWF7437]

Thanks Pete710,

Your explanation makes things much clearer.

Francesca's advice is probably the easiest way to model it. But, unless the pond is very large it is not likely that it will attenuate the peak flow very much. But try it and see what you get.

If your BFE is to be submitted to FEMA for review you'll need to use the same methodology for determining flows that FEMA has accepted for other streams in your area. Usually this is the Regional Regression Equation approach but check published Flood Studies to be sure. Find the study for the next downstream waterway that has been studied.

For an area as small as this ( <95 acres) the rational Method would not be appropriate. Try both the Regional Reg. Eq. and the NRCS unit hydrograph methods.

good luck

 

[Pete710]

RWF7437 & Francesca, thanks for the info. The problem I'm having is this:

1) Routing the basin yields that the 100-year storm overtops the basin, thus I don't have an accurate max WSEL.

2) How do I model the entire steady state model in HEC-RAS? Do I model the pond as a reservoir or just cross-sections within the model? And if a reservoir, how do I connect to the downstream?

 

[RWF7437]

Pete710,

I'm not yet clear about what you're trying to do. Do you want to determine the BFE upstream or downstream from the basin ?

If the basin overflows can you model this as a roadway embankment with a culvert, or culverts, below the "roadway" and an overflow weir at the "roadway" elevation ? You should be able to use cross sections through the basin and can assume normal depth at your most downstream cross section on the creek.

Not incidentally, WHY are you doing this ? Is FEMA or some local reviewing agency requiring this ? If so, what are their criteria ?

good luck

 

[Pete710]

I am trying to determine the BFE for the entire system (creek-pond-creek) as it runs through an existing townhome community.

Thanks for your help with the model, I assumed that since the pond is overtopping for the 100-year that it could just be modeled as crossections with a culvert and spillway (road).

To answer your why, the creek is currently in an unstudied area (Flood Zone A) and the home-owners association wants to developed a BFE to establish whether or not flood insurance will be required for some or all of the units. I imagine if the study shows BFE's below some of the building's first floor elevations, that a FEMA submission will be made.

 

[RWF7437]

Don't forget that you need stream cross section both upstream and downstream from your property lines. The stream doesn't care about property lines.

good luck

 

[bltseattle]

My two cents: if possible, keep it simple, don't worry about modeling the pond directly in HEC-RAS, and run HEC-RAS steady state for the peak undetained flow to determine BFE up/downstream of the pond, then use the design max water surface elevation for BFE around the pond. Use the flood elevation at the pond as a boundary condition for the upstream reach.

Ignoring the pond benefit (reduced peak flow rate) would likely yield a bit wider floodplain, but if you get a 101-year storm (or clogged control structure, which is not unheard of in in-stream facilities) that overtops the pond then at least you won't have infringements into the downstream flood zone that are impacted.

 

[TerryScan]

I have no practical experience with performing a flood study. I have taken a week long course, and have contacted FEMA's Map Assistance Center to inquire about preparing a study. They were extremely helful.

I would be inclined to contact the Map Assistance Center and ask for their guidance on modeling the pond.

 

[queque]

I suggest you contact the USGS Water Resources Division Surface Water Specialist in your State. They know the errors inherrent in using the Regional Regression equations. I helped develop them for 12 states in the west. The errors are substantial for many reasons.
Your area may have a Regression Eqation with a standard error of regresssion so large, it may mask the precision you seek.