Jacob Beatty, P.E.Hydrology & hydraulics · Birmingham, AL Get in touch
Cahaba Basin screening model

Where the water actually goes

391 square miles under a 100-year storm. FEMA's regulatory floodplain and a 2D rain-on-mesh model of the same ground, overlaid.

Basemap

Layers

FEMA floodplain 1% annual chance Regulatory floodway
Rain-on-mesh flood depth 0.5 to 25 ft

Screening-grade. For ranking and scoping, not a determination about any property, and no substitute for a Flood Insurance Study, a Letter of Map Change or site-specific analysis. FEMA's floodplain governs; the model is an additional view, not a correction.

Building-level results, the per-city breakdowns and the calibration are not published here. Ask me for your city →

Floodplain: FEMA National Flood Hazard Layer · Depth grid: HEC-RAS 2D, J. Beatty · Basemap and imagery: Esri

Why rain-on-mesh sees more

A stream jumping its banks is fluvial flooding. That is what the FEMA map shows, using an older 1-D model of the channel. But much of a community's flood damage is pluvial: rain falling faster than the ground and storm drains can move it, pooling in streets, yards and low spots, often nowhere near a mapped stream.

Run that same storm as a 2-D rain-on-mesh model, tracing where every drop flows across the whole surface, and it floods far more ground than the FEMA map shows.

FEMA regulatory map
  • 1-D hydraulic model
  • Fluvial only: channels and rivers
  • Not built for rain-driven street and low-spot flooding
Rain-on-mesh model
  • 2-D rain-on-mesh model
  • Pluvial + fluvial: the whole surface
  • Every drop, across streets, yards and creeks
The river overflows its banks in the valley (in the FEMA floodplain) while rain pools at low points on higher ground away from the river (only rain-on-mesh modeling shows it). A terrain cross-section during one rainstorm. In the valley the river rises above its normal level, overflows its banks, and floods homes on its floodplain, inside the FEMA regulatory floodplain. The land climbs to an upland bench where the rain cannot soak in and pools at a low point, flooding a home well above the river and outside the mapped floodplain, which only 2D rain-on-mesh modeling reveals. FEMA regulatory floodplain FLUVIAL the river overflows its banks IN THE FEMA FLOODPLAIN normal river level PLUVIAL rain pools at low points ONLY RAIN-ON-MESH SHOWS IT One storm, two floods: the river overflows its banks in the valley (fluvial, in the FEMA floodplain) while rain pools at a low point on higher ground (pluvial), which only rain-on-mesh modeling shows. A terrain cross-section: a low valley on the left where the river overtops its banks and floods homes inside the FEMA floodplain, rising to higher ground on the right where rain pools in a low spot and floods a home the FEMA map never covers. FLUVIAL the river overflows its banks IN THE FEMA FLOODPLAIN PLUVIAL rain pools at low points ONLY RAIN-ON-MESH SHOWS IT FEMA floodplain normal river level
The river overflows its banks and floods homes on its floodplain (fluvial, in the FEMA floodplain); rain pools at a low point on higher ground and floods a home there (pluvial), which only rain‑on‑mesh modeling reveals illustrative section