Wiswall Dam Repair and Fish Ladder
Durham, NH
The Town of Durham engaged SA to perform Dam Engineering for evaluation and design of repairs and improvements to this 22-foot high, 380-foot long, significant hazard, concrete gravity and embankment Dam providing water supply and recreation to the Town of Durham and the University of New Hampshire. The Dam was originally constructed in 1911.
Stephens Associates Consulting Engineers (SA) designed upgrades and repairs to the dam consisting of replacement of the left abutment and gate structure to improve stability and replace deteriorated sections. SA designed a Denil fish ladder in consultation with New Hampshire Fish and Game and the United States Fish and Wildlife Service. SA also provided geotechnical and hydraulic engineering for reconstruction of Wiswall Road Bridge, just upstream of the dam. SA has worked closely with the Town to develop and meet their goals and the needs of other parties, including NHDES Water and Dam Bureaus, the Lamprey River Advisory Council, the US Army Corps of Engineers, NH Fish and Game, US Fish and Wildlife, the National Park Service, and local residents.
During SA’s services, floods approaching the estimated 100-year flood occurred in May 2006 and April 2007. SA seized the opportunity to measure water elevations at the Dam and evaluate the Dam performance under design flows – a rare opportunity. Based on USGS stream gage data downstream of the Dam, SA revised the predicted 100-year flood discharge by regression analysis of the data and developed a rating curve for the Dam. The Dam hydraulics are further complicated by modifications to the opening of Wiswall Road Bridge, just upstream of the Dam, the fish ladder construction, and requirements imposed on the Town by NHDES that restrict water withdrawal rate and minimum reservoir elevation during low flows, all of which are intimately connected. SA evaluated these hydraulic and hydrologic factors in designing improvements to the Dam to safely pass the 100-year design flood in accordance with NHDES Rules.
SA performed the following engineering:
- Phase I dam safety evaluation by U.S. Army Corps of Engineers (USACE) guidelines
- Visual (including underwater) inspection;
- Stability analysis of existing dam for normal and unusual loading conditions (e.g. ice and seismic) for sliding and overturning;
- Hydraulic and hydrologic study;
- Subsurface exploration by geotechnical borings;
- Design of repairs and upgrades to dam and new Denil fish ladder, including:
- Geotechnical and structural design of new gate structure, left abutment and fish ladder;
- Rock anchors for stabilization of existing spillway and new gate structure;
- Site improvements (e.g. access road, etc.);
- Preparation of design plans and specifications;
- Engineer’s cost estimate;
- Dam breach analysis and inundation mapping;
- Preparation and submittal of permit applications for NHDES Wetlands and Dam Bureaus;
- Consultation with Town during Contractor bidding/selection and as Design Engineer during construction.