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Town of Bridgewater flood risk project

Tim Webster, PhD, Kevin McGuigan, Kate Collins, Candace MacDonald
Applied Geomatics Research Group
Centre of Geographic Sciences
NSCC, Middleton
Tel. 902 825 5475
email: tim.webster@nscc.ca

Executive Summary

Like so many other towns in Nova Scotia, Bridgewater is located along a major river where the tide from the Atlantic Ocean can influence the water level. The combined effects of river runoff and a storm surge on top of high tide possibly put Bridgewater at risk of flooding. Projections from climate change will make the risk of flooding in the future worse with increased sea-level rise (SLR) and possible increased river runoff. Past flooding events of the town and the LaHave River were part of the background research and documented in this report. A set of river and ocean water level simulations were carried out to determine the risk of flooding to the Town of Bridgewater today and in the future under climate change. Fieldwork to support these modeling efforts included installing water level sensors at the Marine Terminal Wharf in the town and at Kraut Point near the mouth of the LaHave River, and conducting bathymetric surveys and river cross-sections to be integrated with a high-resolution elevation model derived from a lidar survey. Environment Canada measurements of flow-discharge of the LaHave River upstream of the town date back to 1913 and were analyzed to calculate the return period of specific flood events, such as March 31, 2003, and the 50 year and 100 year return periods of flood events. The flows associated with these return periods were used to define the river discharge boundary at the upstream section of the river and a tidal boundary was established near the mouth of the LaHave River for the modeling. The tidal boundary used for the modeling consisted of a normal annual high tide as well as the simulation of a 2.2 m and 3.5 m storm surge event to include SLR in the future. 

The results of the various flood risk simulations indicate that the area upstream of the Veteran’s Memorial Bridge is vulnerable to flooding from large discharge events of the LaHave River. However, the downtown waterfront does not appear to be susceptible to flooding from significant discharge events. This is probably a result of the deeper river channel in this area. However, the modeling does suggest that the downtown area is vulnerable to SLR and storm surge flooding. Based on the simulations, areas such as the Bridgewater Mall parking lot, the Marine Terminal and sections of LaHave Street and Shipyards Landing Park become inundated under a 2.2 m storm surge generated at the mouth of the LaHave River. When the storm surge level is increased to 3.5 m, the flood extent expands to cover the parking lot of the Bridgewater Mall up to LaHave Street and inundates Old Bridge Street and Davison Drive. The flooding extends from overtopping the Marine Terminal Wharf to inundating LaHave Street and Route 3 either side of the terminal. Flooding covers a larger section of LaHave Street upstream of the Veteran’s Memorial Bridge including lower Elm Street. Flooding at Shipyards Landing Park along the waterfront expands to cross King Street and the lower section of School Street. The area surrounding the southwest side of Veteran’s Memorial Bridge begins to flood under these storm surge or SLR conditions condition. The model suggests that floodwater west of Old Bridge Street appears to get trapped and does not easily drain back into the river once the storm surge levels subside while the mall parking lot is able to drain back into the river quickly. The risk of such events (2.2 and 3.5 m storm surge) occurring today are low, although a 4 m storm surge was associated with Hurricane Sandy in New Jersey in 2012, and with increased SLR ranging from present conditions of 0.32 m/century to projections of 1.56 m/century by 2100 coupled with possible increased storm intensity, the risks will increase over time of such an event occurring. Other areas of the town at risk of local flooding after extreme rainfall events include Wile Brook and Hebb Brook which were also modeled in this study.

Aerial photos dating from 2009 to 1948 were used to measure the coastline position and determine areas of erosion. The results indicate the downtown waterfront area does not appear to be vulnerable to erosion, although upstream of Veteran’s Memorial Bridge do show areas with higher erosion rates.

AttachmentSize
Town of Bridgewater final report on flood risk AGRG May 2013.pdf21.79 MB
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