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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