Private Drive through Floodway 2

[bandpman]

I'm looking to place a private residential drive through a floodway and am told by county engr that fill/culvert is not allowed. They require a span that is 36" above grade. Actual creek is approx 30' wide. Fayette County, GA. Anyone with experience in this area? Thanks.

 

[sam74]

In Tennessee that is the standard response you would get for fill in a floodway. However here, if you can an aquatic resources alteration permit (ARAP)from Tn Dept of Environment and Conservation (TDEC) and also get approval from the Army Corps of engineers then you can do it. Usually not to hard to do for a driveway crossing as long as you are not filling very much. Check with your state's website and see if they have similar requirements as TDEC and I imagine you would atleast need a corp permit.

 

[francesca]

In my town in Middle TN, the local Street Department is authorized to give "No Rise" certificates on even the Cumberland River, which is a water of the state with a USACE flowage easement on it as we're in the back waters of Lake Barkley.

When you say the creek is 30' wide, do you mean that is the normal width of flow? You'll most likely have to design the culvert for the 100-year storm, or, if it is as you say, a private road, then perhaps for the 10-year storm but checking that the 100-year storm easily passes over the top of the roadway. You'll need to prove that your construction does not increase the 100-year flood elevation, so sizing the culvert for the 100-year storm is probably better in the long run.

Have you checked the FEMA Flood Insurance Rate Map (FIRM)? How wide is the 100-year flood zone? (Also, what is the date of the map - some of them are very old and/or the land use has changed significantly since the flood zone was determined.)

Do you have any idea what size the drainage basin is? Last year I did a HEC-RAS model of a creek with a normal flow width of approximately 30'. It had a 62 square mile drainage basin and a 100-year flood peak flow of approximately 21,000 cfs. The creek is crossed by an old bridge (circa 1950) and a recent update to the model showed that the bridge did not have sufficient capacity for the 100-year flood, causing the creek to back up behind the bridge and flow over the road.

Another subdivision has a 10' wide channel that does not hold water all year round. This drainage basin is approximately 300 acres, with a 100-year flood peak flow of approximately 650 cfs. We're crossing this one with a 8'x8 box culvert; however, we are increasing the 100-year flood elevation as the property that is affected belongs to the developer and the new elevation will be platted prior to the sale of the lots.

The moral of this tale is that you need to quantify your drainage basin before you can determine what means of crossing the creek is feasible. As long as you can prove that you will not increase the 100-year flood elevation, however, they should allow you to cross it.

 

[RWF7437]

Be careful ! The terms "floodway" and "floodplain" are NOT synonymous. Be sure you know the difference as it will greatly affect what you can and cannot do.

Get the floodplain maps for your stream. Also get a copy of the Flood Insurance Study which will tell you drainage areas and flows. If you don't feel comfortable doing this yourself, get a competant local engineer familiar with FEMA's rules and procedures.

Finally, the odds of the so called "100 year storm" being equaled or exceeded in the life of a 30 year mortgage are 1 in 4.

good luck