A Flood Risk Assessment of the LaHave River Watershed, Canada Using GIS Techniques and an Unstructured Grid Combined River-Coastal Hydrodynamic Model

Kevin Mcguigan,Kate Collins,Tim Webster

doi:10.3390/jmse3031093

Kevin Mcguigan, Kate Collins + Show 1 more

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https://doi.org/10.3390/jmse3031093

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Abstract

A flexible mesh hydrodynamic model was developed to simulate flooding of the LaHave River watershed in Nova Scotia, Canada, from the combined effects of fluvial discharge and ocean tide and surge conditions. The analysis incorporated high-resolution lidar elevation data, bathymetric river and coastal chart data, and river cross-section information. These data were merged to generate a seamless digital elevation model which was used, along with river discharge and tidal elevation data, to run a two-dimensional hydrodynamic model to produce flood risk predictions for the watershed. Fine resolution topography data were integrated seamlessly with coarse resolution bathymetry using a series of GIS tools. Model simulations were carried out using DHI Mike 21 Flexible Mesh under a variety of combinations of discharge events and storm surge levels. Discharge events were simulated for events that represent a typical annual maximum runoff and extreme events, while tide and storm surge events were simulated by using the predicted tidal time series and adding 2 and 3 m storm surge events to the ocean level seaward of the mouth of the river. Model output was examined and the maximum water level for the duration of each simulation was extracted and merged into one file that was used in a GIS to map the maximum flood extent and water depth. Upstream areas were most vulnerable to fluvial discharge events, the lower estuary was most vulnerable to the effect of storm surge and sea-level rise, and the Town of Bridgewater was influenced by the combined effects of discharge and storm surge. To facilitate the use of the results for planning officials, GIS flood risk layers were intersected with critical infrastructure, identifying the roads, buildings, and municipal sewage infrastructure at risk under each flood scenario. Roads were converted to points at 10 m spacing for inundated areas and appended with the flood depth calculated from the maximum water level subtracted from the lidar digital elevation model.

Highlights

The LaHave River is an estuarine river in southern Nova Scotia that has a history of both fluvial and coastal flooding
Maximum water level from each of the nine simulations were output from Mike as flexible mesh elements and interpolated to a surface which was intersected with the digital elevation model (DEM)
Analysis of the results of all nine simulations revealed that the area upstream of Bridgewater is dominated by discharge events with limited flooding from elevated sea-level (Figure 8), the area around Bridgewater is influenced by the combined effects of discharge and sea-level (Figure 9), and downstream of Bridgewater is dominated by elevated sea-levels and less by major discharge events (Figure 10)

Summary

Introduction

The LaHave River is an estuarine river in southern Nova Scotia that has a history of both fluvial and coastal flooding. Heavy rain and snowmelt can cause the river to overflow its banks, flooding the rural communities and roads within the watershed, while storm surge and sea-level rise can cause flooding near the coast; when a heavy rainfall event is combined with a storm surge, infrastructure within the watershed is especially at risk. This project builds on a previous study which focused on the town of Bridgewater, Nova Scotia, which is located within the LaHave River watershed [1].

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