Use and Limitations of Floodplain Maps
Floodplain maps are administrative tools which depict minimum elevations for floodproofing. Minimum floodproofing requirements can then be incorporated into building bylaws, subdivision approvals, and local government planning and regulations.
The accuracy of the location of a floodplain as shown on a map is limited by the base topography. It is generally assumed to be plus or minus one-half the increment of the ground contours.
Floodplain maps do not provide information on site-specific flood hazards, such as land erosion or sudden shifts in the channel of the watercourse.
Other sources of water, roads or other barriers can restrict water flow and effect local flood levels. As well, obstructions such as ice, debris, flooding in surrounding areas, groundwater or other phenomena can cause flood levels to exceed those indicated on the map. Land adjacent to a floodplain may be subject to flooding from tributary watercourses.
Floodplain maps do not locate legal survey boundaries. A site survey is required to reconcile the property location, ground elevations, and designated flood level information.
Map users should note the dates of topographic mapping, aerial photography, river surveys and map issue, and dates of development in the map area. Subsequent developments within the floodplain (natural or construction) may effect flood levels and render site-specific map information obsolete.
Professional assistance and detailed engineering analysis may be required to address any of the above considerations.
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