Storing billions of gallons of water, Winooski River Flood Control dam reservoirs prevent damaging floods, produce hydroelectricity, and support recreation. The Vermont Department of Environmental Conservation hired Schnabel to conduct hydrology and hydraulic analyses, dam breach analyses, and flood inundation mapping to support new emergency action plans for three rolled-embankment, high-hazard flood control dams.

Because Tropical Storm Irene in 2011 was the flood of record for the majority of the stream gages, we used this storm to calibrate and validate the HEC-HMS model parameters. These are the hydrologic modeling system parameters set by the Hydrologic Engineering Center (HEC) within the U.S. Army Corps of Engineers. For all three dams, we used the Corps’ two-dimensional river analysis system tool. We assessed their potential for downstream flooding due to dam failure during the spillway design flood, which for all three dams was the full probable maximum flood.

To execute the analysis, we developed and calibrated an HEC-HMS hydrologic model of the watershed, and then used an integrated 2D HEC-RAS hydraulic model to route the breach flood wave downstream. We conducted detailed breach analyses and inundation mapping of the resultant flood wave for each dam using a 2D HEC-RAS hydraulic model to simulate breach and non-breach scenarios for probable maximum flood, sunny-day, and sunny-day gate failure scenarios.