A GIS-BASED HYDRAULIC MODELING TOOL FOR STREAM CROSSING REPLACEMENT PROJECTS IN MASSACHUSETTS

Historically, the design and installation of stream crossing structures (culverts and bridges) rarely prioritized stream habitat connectivity and resiliency to flooding and erosion. Therefore, many of the over 25,000 stream crossings in Massachusetts are currently undersized, which can result in substantial impacts to aquatic organism passage, stream health and morphology, and the capacity to withstand floods. Many municipalities with these undersized stream crossings lack the funding and resources to properly design robust replacements. The U.S. Geological Survey (USGS), in cooperation with the Massachusetts Department of Environmental Protection and the University of Massachusetts at Amherst, have developed a GIS-based hydraulic modeling tool to assist with evaluating stream crossing replacement projects. This tool has been integrated into the USGS StreamStats web application (https://streamstats.usgs.gov/ss/) to provide preliminary culvert designs that freely convey the estimated 10- and 4-percent annual exceedance probability (AEP) flood flows and meet the Massachusetts Stream Crossing Standards (SCS) for box, conspan arch, and pipe culverts.

The GIS-based hydraulic modeling tool uses automated Python scripts to create hydraulic input data and simulate flood flows in the U.S. Army Corps of Engineers HEC-RAS hydraulic modeling program. The cross-sections used in HEC-RAS are created using lidar data and estimated bankfull channel geometry from regional regression equations. The 10- and 4-percent AEP flood flows are estimated using Massachusetts regional regression equations in StreamStats. The preliminary stream crossing designs are optimized by iteratively increasing the culvert dimensions in the HEC-RAS simulations until they freely convey the 10- and 4-percent AEP flood flows and meet the Massachusetts SCS. Users can access this tool in StreamStats to generate a report containing the stream crossing location, flood flows estimations, preliminary culvert dimensions, and hydraulic model input and output files. This web application provides municipalities, engineers, water managers, and others a stream crossing planning tool that improves flood and erosion resiliency, stream health, and aquatic organism passage for replacement projects.