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Coastal Flood Risk mapping

Lidar processing and Flood Risk Mapping for the Communities of the District of Lunenburg, Oxford-Port Howe, Town and District of Yarmouth, Chignecto Isthmus and Minas Basin
Tim Webster, Kevin McGuigan and Candace MacDonald
Applied Geomatics Research Group
Centre of Geographic Sciences
Nova Scotia Community College, Middleton
Tel. 902 825 5475
email: timothy [dot] webster [at] nscc [dot] ca

Executive Summary

The Canadian coastlines have been assessed for sensitivity to future sea-level change and it has been determined that the east coast of Canada is highly vulnerable to erosion and flooding. (Shaw et al., 1998). The Atlantic Climate Adaptation Solutions (ACAS) project aims to address some of these issues. This report deals with the construction of flood risk maps to support communities that are vulnerable to coastal flooding from storm surges and long term sea-level rise. Our coasts are presently at risk to the occurrence of storm surges during high tide which results in flooding and erosion. For future planning we need to identify the current areas, then look to future sea-level predictions and determine the areas at long term risk. The latest Intergovernmental Panel on Climate Change (IPCC) Assessment Report 4 (AR4) has projected global mean seal-level to rise between 0.18 and 0.59 m from 1990 to 2095 (Meehl et al. 2007). However as Forbes et al. (2009) point out, these projections do not account for the large ice sheets melting and that measurements of actual global sea-level rise (SLR) are higher than the previous predictions of the third assessment report. Rhamstorf et al. (2007) compared observed global sea-level rise to that projected and found it exceeded the IPCC AR3 projections and have suggested a rise between 0.5 and 1.4 m from 1990 to 2100. Forbes et al. (2009) used an upper limit of 1.3 m of SLR over 100 years as a precautionary approach to SLR projections in the Halifax region. The selection of an upper limit of flooding is dependent on realistic projections of SLR. However, because of the variations in SLR projections we have generated flood risk maps to a maximum level of 5 and12 m depending on the study site and tidal range. The tidal range from mean sea-level varies from 2 m on the Northumberland Straight and Atlantic to 7 m in the upper Bay of Fundy communities thus requiring the different flood level ranges. The number of flood level GIS layers ensures that the flood extent information is available whatever the water level used in the projections in the future.
In order to generate accurate flood inundation maps along the coastal communities we developed elevation models using an airborne laser scanning technique, lidar, to acquire a new generation of maps during the project. The ACAS communities have the benefit of having the detailed 1-2 m bare earth elevation model accurate to 15 cm in the vertical, they also have the surface model which incorporates the tops of the trees and buildings. These two types of maps can provide a wealth of information to various engineering or resource based projects that require GIS analysis. The bare earth map was used to increase the ocean sea-level and determine the inundation area every 10 cm of flooding. Hydraulic pathways, as represented by culverts and bridges, have been applied to the bare earth model connecting the ocean to inland low lying areas. For the areas protected by dykes we have generated a set of flood layers that represent dyke overtopping. These layers highlight the areas at risk of flooding if the ocean water level were to exceed the dyke elevation. We have also implemented a set of flood layers associated with dyke breaching, where we have simulated a break in the dyke and allowed the water to flood immediately behind the dyke. We have used benchmark storms for each of the ACAS communities to highlight past flooding events. This was done to show decision makers and citizens that these coastal areas have been vulnerable to storm surges in the past and allows people to better understand the potential implications of climate change and sea-level rise in the future.
We have used the water level time series data from available tide gauges (Halifax, Yarmouth and Saint John, NB) and no-longer operating (Pictou) to estimate the return periods of high water events and the benchmark storms. These events represent storm surges where the atmospheric pressure and wind have caused the water level to exceed that of a normal predicted tide. We have calculated the return period of such events based on current relative sea-level (RSL) rise as well imposed higher rates of sea-level rise (73 cm/century and 146 cm/century) from climate change. We have also used the time series data to calculate the water level of the 100 year storm events at a 95% probability of occurrence under these different RSL conditions. Under current RSL conditions the 100 year water level (CGVD28) for the Lunenburg area is 2.2 m, for the Port Howe area is 2.4 m and for Yarmouth is 3.5 m. These levels increase under different RSL predictions with climate change. The benchmark storms for these areas approach the 100 year events, Hurricane Juan in Sept. 2003 water level in Halifax was 2.1 m and along the Northumberland Strait in Dec. 1993 and more recently in Dec. 2010 water levels reached 2.3 and 2.2 m respectively. The Groundhog Day storm in Yarmouth in Feb. 1976 reached a level of 3.4 m.
Each ACAS municipality has taken delivery of various GIS layers lidar derived surface model and elevation model as well as the different flood layers. Presentations have been made to each of the municipalities to transfer the results and technology and provide the scientific explanation of how these maps were generated and how to use them. This report is the summary document to be used in association with those GIS data files.


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