Geographically Isolated Wetlands (GIWs)-To regulate or not to regulate

Waters of the United States (WOTUS) have been a hot topic for over a decade. Questions arise, such as: What qualifies as navigable water? What is a significant nexus? And how does this affect floodplain management?

In the summer of 2023, the U.S. Supreme Court narrowed WOTUS by limiting the Clean Water Act (CWA) coverage to waters or wetlands with a continuous surface connection to navigable waters. Previously, Geographically Isolated Wetlands (GIWs) were protected if a “significant nexus” to navigable or interstate waters could be shown.

Although GIWs are not typical FEMA-regulated floodplains, they remain an interconnected environmental issue with floodplain management implications. Beyond water quality and ecological benefits, GIWs might also provide floodplain benefits such as increased watershed storage, enhanced flood protection, and decreased channel velocities. With a growing push for state-level regulation, many states are now debating this issue.

Recently, an organization in Tennessee contacted Red Mountain Engineering to assess GIWs’ impacts from a hydrology and hydraulics standpoint. Modern floodplain technology has simplified such evaluations. A 1,452-acre rural watershed was analyzed, comprising a small city and mainly undeveloped rural and agricultural lands. Approximately 91 acres (6.3% of the watershed) were identified as GIWs. A two-dimensional rain-on-grid watershed model was run for existing conditions, and a proposed scenario in which all GIWs were developed. The baseline used Atlas-14 rainfall data, SCS curve numbers for infiltration, LiDAR topography, field surveys, and aerial photography to derive Manning’s roughness values. In the proposed scenario, infiltration rates and roughness values were reduced to simulate an impervious surface.

Preliminary results confirm expectations: land use changes affect runoff, and the significance depends on the extent of change. A hydrograph of a road crossing in the headwaters of the watershed is shown above, and substantial flow increases and minor water surface elevation increases are seen. Other locations saw a significant jump in channel velocity. While stormwater regulations may mitigate increased flow in some areas, reduced watershed storage is inevitable. The key question is whether these impacts justify regulation without unduly burdening landowners and developers. Finding a balanced, science-based regulatory framework remains essential for ensuring environmental protection and sustainable land use practices under the CWA—one that echoes issues seen in Biggert-Waters reforms and NFIP Risk Rating 2.0.

The project has not been wrapped up, so full conclusions have yet to be drawn. Other proposed scenarios are still to be modeled. Stay tuned for more updates on this project from RME!