Northstar Hydro is part of the Design-Build Team of Reed and Reed and HDR for the replacement of the twin bridges that carry I-91 over the Williams River in Rockingham, Vermont. NHI provided hydrologic, hydraulic, and scour analysis Northbound and Southbound (NB and SB) bridges Work completed included hydrologic analysis, hydraulic modeling, and scour analysis based on site specific channel survey and FEMA data. A geomorphic assessment of the river and project reach were also prepared. Project components included consideration of FEMA elevation and floodway data, existing and proposed condition models, ice jam considerations, and scour and geomorphic assessments.
The Williams River flows into the Connecticut River downstream of the project site. The I-91 twin bridges were constructed on long fill sections to narrow the crossing length at the bridges. The bridge crossing has steep embankments on either side of the river. FEMA mapping and site observations indicate that flood levels at this bridge are impacted by backwater from the Connecticut River.
The existing bridge was constructed in 1961. At that time, the river was moved about 350’ south from its previous location. The massive embankments that support the highway constrict the floodplain and direct the river flow to the current location.