Integrated River and Coastal Hydrodynamic Flood Risk Mapping of the LaHave River Estuary and Town of Bridgewater, Nova Scotia, Canada

Abstract

Bridgewater, Nova Scotia, is located 20 km inland from the mouth of the LaHave River estuary on the Atlantic Coast of Canada. Bridgewater is at risk of flooding due to the combined effects of river runoff and a storm surge on top of high tide. Projected increases in sea-level and possible increased river runoff with climate change increase the risk of future flooding. A set of river and ocean water level simulations were carried out to determine the risk of flooding to Bridgewater today and in the future under climate change. The hydrodynamic simulation developed incorporates return periods of a time series of river discharge measurements for the LaHave watershed, ocean water dynamics at the mouth of the river under normal tidal conditions and with two levels of storm surge, near shore and river bathymetry, as well as high precision topographic lidar derived ground elevations and survey grade GPS. The study was supported by data from two tide gauge sensors, and qualitative evidence provided by the community such as historical flood levels and photographs. Results show that areas upstream of the town are vulnerable to large discharge events of the LaHave River. The downtown waterfront and infrastructure are not susceptible to fluvial flooding, but is vulnerable to sea-level rise and storm surge flooding.

Determinación de la inundación en tramos de ríos afectados por marea basada en la simulación continúa de nivel

“…La mayoría de los estudios de riesgo de inundación en áreas costeras no tienen en cuenta de forma correcta la probabilidad de ocurrencia conjunta de varios eventos extremos al mismo tiempo (Archetti et al, 2011). En general, los eventos extremos se llegan a asumir como independientes o se usa algún tipo de combinación arbitraria de periodos de retorno (Teakle et al, 2005;Webster et al, 2014).…”

Flood Hazard Assessment for a Hyper-Tidal Estuary as a Function of Tide-Surge-Morphology Interaction

“…Nonlinear interactions between extreme river discharge and storm surges can elevate residual water levels up to 0.35 m in idealised estuaries (Maskell et al 2014), and have been shown to influence sub-tidal friction and the timing of high and low water in the River Mahakam, Indonesia (Sassi and Hoitink 2013). Heavy rainfall and spring meltwater, which result in high discharges have been shown to inhibit tide-surge propagation up-estuary in the LaHave Estuary, Nova Scotia (Webster et al 2014); however, dredging activities in the Modaomen Estuary, China, facilitates inland propagation of surges and can alter salinisation within an estuary (Cai et al 2012). Flood hazard and inundation extents are largely controlled by surge elevation, except in the most extreme river discharge events (Maskell et al 2014).…”

“…Extreme sea levels, caused by the combination of astronomical high tides and meteorological storm surges, are a major threat to coastal communities and infrastructure (Elliott et al 2014;Quinn et al 2014;Webster et al 2014;Prime et al 2015). This is of particular significance in hyper-tidal estuaries, where tidal range exceeds 6 m (Davies 1964;Robins et al 2016).…”

Representing hydrodynamically important blocking features in coastal or riverine lidar topography

“…Hydrodynamic models are useful tools for exploring how climate change, rising sea levels, and hydrological regime alterations might affect the interaction between tides, rivers, and coastlines (Purvis et al, 2008;Bhuiyan and Dutta, 2012;Nardin and Edmonds, 2014), as well as urban coastal flooding (Gallien et al, 2013;Webster et al, 2014). Similarly, such models are vital in analysis of river hydrodynamics and floodplain inundation that might be affected by changing climate patterns (Wen et al, 2013;Vastila et al, 2010).…”

“…As an alternative to grid coarsening, grid nesting (from fine to coarse grids) can be applied to somewhat reduce computational costs (e.g., Webster et al, 2014), but such methods inherently require an expert modeler to make a judgement as to where model resolution might be coarsened. Artificial porosity approaches seem appropriate for urban areas with multiple pathways around unresolved objects , but it is not clear that they are necessarily useful in broader natural settings or where narrow objects are blocking over multiple cells.…”