St. Paul’s School, a private boarding school in New Hampshire, sustained over $5 million in property damage to campus facilities as a result of a major flood in May 2006. The peak flow levels in the Turkey River, which traverses the school grounds, were the highest recorded in more than 150 years. Based on flow gauging data in other nearby streams, the US Geological Survey and US Army Corps of Engineers had subsequently determined that the May 2006 event was close to a 100-year storm.
Shortly after this event, the School retained VHB to conduct a flood remediation study and identify various alternatives to prevent or minimize future damages to school grounds from similar flood events. VHB engineers and hydrologists simulated the observed flood elevations using the recorded rainfall data. The modeling results produced flood elevations that were within 0.2 feet of the observed flood elevations.
At issue were several undersized bridges and culverts at key road crossings, as well as the discharge capacity at an historic mill dam located in the center of campus, being less than that for a 50-year storm. VHB developed three viable options including: Modifying an upstream dam to increase flood storage and flow control above the campus; installing a major bypass structure around the flow constraints within the school grounds or reconstructing the smaller dam to increase its discharge capacity and increase passage; or, adding culverts at the undersized road crossings. Subsequent to this study, St. Paul’s enlisted VHB to design a flexible spillway gate system for the historic dam.