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FEMA FLOODPLAIN EVALUATIONS

Often, proposed construction activities need to be evaluated to determine what impacts the project will have on the surrounding floodplain.

This is particularly important if the site is located in a FEMA designated floodway or if base flood elevations exist for the floodplain. Bridge & Stream has the expertise to evaluate the hydraulic impacts of a project using a one-dimensional or a two-dimensional hydraulic model. For large floodplains, the analysis often involves a bridge over the main channel with numerous relief bridges in the floodplain that also convey floodwater. GIS software packages are often used to manage and assemble the data for the hydrologic and hydraulic models.

The recent proliferation of LiDAR data across the U.S., combined with advances in two-dimensional (2D) modeling techniques, allows for more robust floodplain evaluations. Publicly-available LiDAR data provides topographic details of vast areas that previously would have required extensive on-site survey.  When used in a 2D model, LiDAR digital elevation models answer questions surrounding split flows, ponding and flood attenuation, and complex flow scenarios.  LiDAR data can allow for a cost-efficient desktop study ahead of or in lieu of on-site reconnaissance.  Where LiDAR data is unavailable, Bridge & Stream can collect topographic data using imagery collected from UAS or drone systems

 

The hydraulic impacts of a project may require modifications to the existing regulatory hydraulic model and the associated Flood Insurance Rate Maps (FIRM). In such cases, Bridge & Stream’s Certified Floodplain Managers (CFMs) would coordinate with federal, state, and local floodplain authorities to submit an application for a Conditional Letter of Map Revision (CLOMR).