HEC-RAS Workshop a Success!

Posted on: August 16th, 2019 by Tim Koch

The Ashokan Watershed Stream Management Program recently hosted a three-day workshop on how to use HEC-RAS, a powerful computer program used to model flow in stream channels. HEC-RAS is an acronym for the Hydrologic Engineering Center’s River Analysis System. First released in 1995, its capabilities have grown significantly over time. HEC-RAS is now on its fifth version. It is often used to delineate the extent of the 1% annual chance floodplain (aka, the 100-year floodplain) among other regulatory, technical, and environmental uses.

Workshop participants use digital models of the terrain to help model how rivers behave during flood events.

Workshop participants use digital models of the terrain to help model how rivers behave during flood events.

This 3-day workshop focused on using HEC-RAS to aid in the assessment and design of bridges and culverts. Milone and MacBroom, Inc. (MMI) were contracted to conduct the hands-on workshop to an audience of twenty people. Participants included staff and managers from County Departments of Public Works and Town Highway Departments within the West of Hudson Water Supply watersheds. Others in attendance included flood hazard mitigation personnel from NYC DEP, Stream Management Program staff, DEC hydrologists, and folks from Riverkeeper.

HEC-RAS requires site-specific input data to accurately model flows and floods. Thus, the workshop had a field component where people were taught where to place stream cross sections in relation to the bridge, how to conduct pebble counts to determine size distribution of sediment particles on the stream bed, and how to measure specific components of bridges and culverts required to build a HEC-RAS model. Only local data were used, and the workshop centered around modeling existing conditions and proposed alternatives for an under-sized bridge in the Ashokan Reservoir watershed.

Workshop participants investigate the Fox Hollow Road bridge over the Esopus Creek. Measurements taken on site were used to model different bridge replacement scenarios in order to increase community resilience during floods.

Workshop participants investigate the Fox Hollow Road bridge over the Esopus Creek. Measurements taken on site were used to model different bridge replacement scenarios in order to increase community resilience during floods.

It is important that bridges and culverts are sized properly to pass flows that the structure is likely to see over the course of its life. Undersized bridges and culverts not only worsen flooding, but also fragment aquatic ecosystems and can create instability in the stream channel that can propagate significant distances upstream and downstream from the structure and lead to other damage.

This workshop was aimed at empowering local engineers and highway department staff to make informed decisions when managing road-stream crossings (i.e., bridges and culverts.) Properly sized crossings help to increase community resilience to climate change, improve aquatic habitat, and help to maintain water quality in the Esopus Creek and its tributaries.

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