River Hydraulics 1

[beameupscotty]

I have a client who wishes to develop riperian property to residential lots. The existing grades are as much as 12 feet below base flood elevation, however, the developed portion of the site will be outside FEMA's mapped floodway. To compound things further, a seasonal tributary flows through the site and is likely a categorized wetland. Filling the wetland will be expensive in itself (permits and mitigation). I would prefer to pipe the seasonal flow through the site rather than build a rip rap, gabion or concrete wall and divert this flow back to the main channel of the river. I believe a wall would eventually be undermined by the river.

As I have never been confronted with this situation. I'd appreciate anybody's shared thoughts or experiences.

Thank you.

 

[cvg]

A floodwall can easily be designed with sufficient footing to resist undermining. You will need to do a scour analysis as well as analyzing any long term adjustments to bed slope (aggradation, degradation, sediment transport. This is a specialized field of engineering and you should find an consultant experienced with this to discuss your alternatives.

You are correct, getting the permits may be very difficult and expensive. It sounds as if you will need a Section 404 Individual Permit. Hopefully your client is aware of this.

 

[Karstman]

The client might want to re-think developing this site if much of the area is several feet below the Base Flood Elevation (BFE). Federal regulations (44 CFR 60) require that any new building in the floodplain be elevated to at least the BFE and many communities require a foot or more of freeboard. If the properties are not elevated, then property owners will probably not be able to get flood insurance and without flood insurance they will not be able to get a federally backed mortgage, that is VHA, FHA, Freddy Mac or Fannie May. If they can get flood insurance it can be phenominally high, as much as $25 per year premium for $100 coverage.