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About the Program - An Essential Step for Reducing Flood Damage
The Floodplain Mapping Program was a joint initiative by the federal and B.C. governments to provide information to help minimize flood damage in British Columbia. The program identified and mapped areas that were highly susceptible to flooding. These areas were designated as floodplains by the federal and provincial Environment Ministers.
Designated floodplains are subject to development restrictions. Crown agencies such as the Canada Mortgage and Housing Corporation do not support development on designated floodplains unless adequate floodproofing measures are taken. Local governments may impose further restrictions.
The Floodplain Mapping Agreement
The Floodplain Mapping Program was delivered under the Canada/British Columbia Agreement Respecting Floodplain Mapping. Signed in December 1987, the agreement marked an acceleration of a provincial mapping program which commenced in 1974. Following is a summary of the objectives of the agreement.
- restricted both governments from further undertakings (including construction of, or a major addition to, structures or a change in the use of land) in areas that are vulnerable to flooding and are located in designated floodplains;
- discouraged financial assistance for development of undertakings in flood-prone areas;
- accommodated measures to encourage local authorities to restrict undertakings in flood-prone areas under their jurisdiction; and
- required adequate floodproofing measures to have been incorporated into new development in a floodplain after designation to be eligible for disaster assistance.
An Amending Agreement was signed in August 1994. Following are highlights of the Amending Agreement.
- recognized the need to ensure the continued effectiveness of the floodplain mapping program and contribute towards integrated water resource management,
- provided additional funding to March 31, 1998 to phase down the program in British Columbia by completing a number of the high priority projects,
- allowd for a review of the existing floodplain mapping projects to maintain their adequacy, accuracy and usefulness,
- terminated the Agreement on the 31st day of March 2003.
What is a Floodplain Map?
A floodplain map delineates the area that can be expected to flood, on average, once every 200 years. This is called the 200-year flood. A 200-year flood can occur at any time in any given year; the indicated flood level may be exceeded; and portions of the floodplain can flood more frequently.
Floodplain maps show the location of the normal channel of a water course, surrounding features or developments, ground elevation contours, flood levels and floodplain limits (the elevation and horizontal extent of the high water marks of a 200-year flood).
Within the floodplain, flood level isograms show the water elevation during a 200-year flood. (The maps may also include the 20-year flood level, which is used in applying Health Act requirements for septic tanks.) A flood level isogram is a line which spans the floodplain, plotting the location at which the floodwater is expected to reach the indicated elevation. The elevation of floodwater between each isogram can be interpolated.
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.
Key Elements of a Typical Floodplain Map
Click here for a typical view of a floodplain map.
Flood Levels and Depths
The location of a floodplain is determined by plotting calculated flood levels on a topographic map. Within the floodplain, flood level information is calculated at cross-sections along the watercourse and plotted as flood level isograms.
Each isogram represents regular vertical increments. Isograms indicate the 200-year flood level, which is the designated flood in British Columbia.
Flood levels between isograms can be interpolated. To determine the depth of the flood, ground elevation must be determined by ground survey. The depth of the flood at any given location is the difference between the flood level and the ground elevation at that location.
Professional assistance and site surveys may be warranted to ensure accuracy.
Flood levels as indicated on a floodplain map usually make an allowance to accommodate uncertainties (hydrologic and hydraulic variables). Referred to as freeboard, this allowance varies based on site-specific conditions. It must be remembered that flood levels indicated on the maps are the minimum recommended elevations for floodproofing requirements.