Name: Englishman River Flood Extents

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Description: 1. The flood extents and flood level contours are associated with the designated flood event for the riverine portion of the Englishman River including a freeboard allowance of 0.6 m. Flooding may still occur to levels above the estimated flood levels, and outside the estimated flood extents. The Regional District of Nanaimo does not assume any liability for the accuracy of the estimated flood extents and flood level contours. 2. The flood extents and flood level contours represent a combination of the river flood level and the coastal stillwater flood level that were derived from independently modelled flood scenarios. The highest flood levels for a given location were selected. Floodplain limits are not delineated for tributary creeks. The flood extents and flood level contours should be considered together with complementary studies such as master drainage plans. 3. The designated flood event for the riverine portion of the Englishman River is a 200-year return period peak instantaneous flood event having a statistical annual exceedance probability (probability of a flood event of equal or greater magnitude occurring in a given year) of approximately 0.5%. The river flow has been increased by 25% over the current (2020) value to account for projected climate change impacts. A freeboard allowance of 0.6 m has been added to account for: (1) hydrologic and hydraulic modelling uncertainties, (2) local hydraulic effects such as waves and surges, and (3) some degree of river and delta processes. However, the freeboard allowance does not account for significant changes in channel shape or location. 4. The downstream coastal flood level reflects astronomical tides, a 200-year return period storm surge, and a freeboard allowance of 0.6 m, but does not include an allowance for wave effects. The coastal flood level has been increased by 1.0 m to account for the effects of relative sea level rise to Year 2100. 5. The flood extents and flood level contours do not include areas below the flood level contour of 5 m CGVD2013, as these areas lie within the coastal floodplain limits, which has not been defined as part of this work. 6. The flood extents and flood level contours do not provide information on the potential for site-specific flood-related hazards such as bank erosion, aggradation, debris accumulation and sudden shifts in river channel alignment. 7. Topography used to develop the flood extents and flood level contours is based on LiDAR data that may not accurately reflect the current ground elevation. For site-specific application, the flood level can be related to the ground elevation by topographic survey. 8. When adopted by bylaw, the Regional District of Nanaimo is responsible for land use regulation within the floodplain limits, which are the limits that contain the flood extents. Information on floodplain management can be found in the BC Flood Hazard Area Land Use Management Guidelines.

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Copyright Text: Regional District of Nanaimo

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