Modeling a Floodplain in the Desert

[655321]

I’m defining a floodplain in a desert/rangeland area that's very flat and broad. The 100-year storm has 600 cfs flowing through the area from a 400Ac watershed. According to the FEMA FIRM study downstream from me the flow spreads up to 800' across. Our site is about a mile and half from the upstream watershed boundary.

The problem for me is that it’s so flat that there is no defined channel, nothing that even remotely resembles a bed and bank. We have actual survey of the area I've walked it as well. Its 1% or 2% for hundreds of feet across the bottom of my cross sections.

Can I just define the main channel somewhere? It becomes important since we want to grade some building pads in…I feel I need a defined floodway.

We weren't planning on submitting our study to FEMA; but I do need to get county drainage approval for permits.

If anyone has any input I'd really appreciate it. Thanks!

 

[cvg]

it sounds like you have distributary flow or possibly an alluvial fan. I would advise talking with the county to get their help before you waste your time.

 

[655321]

Thanks CVG, I'm certain this isn't an alluvial fan, but yes I think this is distributary-flow area.

The limit of the county's study ends just inside of our clients property. They used HEC-RAS to define the flood plain up to that point. We had orginally thought we'd just expand their model through our site, and locate the new building pad out of floodway. It seems its more complicated than that...

Does anyone have any suggestion how they would approach this? Since this is an upper reach used for AG purposes, It doesn't seem like it should warrent a complicated mulit-dimensional model.

By the way, we do plan to meet with the county. I'd just like to be prepared.

Thanks,

 

[cvg]

HEC-RAS can be used, but is not ideal for this. Especially since flow may split and breakout of the low flow channels depending upon the size of the storm and on the amount of sediment blocking the channel. If HEC-RAS is used, I would recommend that you and your surveyors go out to the site to precisely locate the limits of the main wash channels using your engineering judgement.

A two-dimensional analysis may be more useful. Flo-2D is one software I would recommend if you decide to go this route. The need for 2D may depend on your plans for the site as well as the County's evaluation of the flood risk.