Floodplain studies and FEMA submittals

Floodplain work sits inside our grading and drainage practice, so the flood answer and the site design come from one team. The first question is how flood conditions change the land you can use, the layout and the approvals you need.

Three levels of work

• Floodplain determination. We check the FEMA flood map for the site and, in Texas, Base Level Engineering data where it exists, then tell you what the mapped zone means for your layout. Many sites need nothing more.
• Drainage study and model update. When the design depends on flood elevations, we run the hydrology and hydraulics, update the HEC-RAS model and carry the results into the grading and detention design.
• A FEMA map change based on fill. When the grading places fill to raise part of a site above the base flood elevation, we revise the grading plan and, once the fill is in, prepare the Letter of Map Revision based on Fill (LOMR-F), filed on FEMA’s MT-1 forms.

Who decides

In a community that takes part in the National Flood Insurance Program, building in a mapped floodplain needs a floodplain development permit from the local floodplain administrator. FEMA decides any change to its map. A mapped floodplain does not always rule a site out, and a map change is not always needed or available. When the flood study itself has to change, that is a different FEMA request, and we tell you which one your site needs.

What we need to start

The site location, any boundary and topographic survey, and the concept plan. With those, we can tell you whether the floodplain changes the layout and what the next step is.