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CANADA
- BRITISH COLUMBIA
Floodplain
Mapping Program
an
essential step for reducing flood damage
About
the Program
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
maped 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 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.
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.
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.
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.
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 accomodate 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.
Having
determined the geodetic ground elevation,
flood levels and depths can be calculated as follows.
- Scale
the distance from the desired location
to the upstream flood level isogram (x).
- Scale
the distance between the flood level isograms
located upstream and downstream of the desired location (y).
- Subtract
the downstream flood level from the upstream level (z).
- Calculate
the flood level.
Flood Level = Upstream Flood Level - {(x/y)z}
- Calculate
flood depth
Flood Depth = Flood Level - Ground Elevation*
*Determined by ground survey
For
more detailed definitions, see "Canada/British Columbia
Agreement Respecting Floodplain Mapping" (amended Aug.
2, 1994)
Designation:
a
declaration of a floodplain by the federal and provincial Environment
Ministers, within which floodproofing measures are required.
Designated
Flood: a
significant historic flood or a flood having a 200-year recurrence
interval based on analysis of historic flow records.
Flood
fringe: the portion of the floodplain not in the floodway
to which floodproofing requirements apply.
Flood
level: the calculated elevation of the designated flood,
including an allowance for uncertainty (freeboard) based on
site specific conditions.
Floodplain:
a lowland area, whether dyked, floodproofed, or unprotected,
which is at an elevation susceptible to flooding.
Floodproofing:
the alteration of land or other precautions to reduce flood
damages. Floodproofing may include adding fill to raise the
elevation of a building site, structural measures such a foundation
walls or columns to raise a building, or combinations of fill
and structural measures. Floodproofing includes the use of setbacks
to maintain a floodway and allow for erosion.
Floodway:
the channel of the watercourse and those portions of the
floodplain which are reasonably required to convey the designated
flood.
Freeboard:
a vertical distance added to the actual calculated flood
level to accommodate uncertainties (hydraulic and hydrologic
variables), potential for waves, surges, and other natural phenomena.
Interim
Designation: a declaration of a floodplain by the federal
and provincial Environment Ministers, within which floodproofing
measures are required. An interim designation is made if the
Ministers are satisfied the mapping is adequate but may not
fully comply with the procedures set out in the agreement.
Natural
Boundary: means the visible high water mark of any lake,
river, stream or other body of water where the presence and
action of the water are so common and usual and so long continued
in all ordinary years as to mark upon the soil of the bed of
the lake, river, stream or other body of water a character distinct
from that of the banks thereof, in respect to vegetation, as
well as in respect to the nature of the soil itself. In addition,
the natural boundary includes the best estimate of the edge
of dormant or old side channels and marsh areas.
Setback:
a distance which an undertaking or landfill must be set
back from a natural boundary or other reference line to maintain
a floodway and allow for erosion. The minimum required setback
is 30 metres from the natural boundary unless otherwise approved.
Undertaking:
the use of land, including the construction of, or major
addition to, any structure.
Key
Plans have been prepared for floodplain mapping areas.
Several options, listed below, are available to assist you in
locating your floodplain area of interest. Links to Design Brief's
are provided when available.
To obtain
a printed copy, please contact Cown
Publications located in Victoria, BC
For information on local bylaws regulating construction in flood-prone
areas, contact your local government.
Further general information on flood hazards is available
from Environment Canada at:
http://www.ec.gc.ca/water/en/manage/flood/e_cont.htm
Information
on topographic base mapping specifications is available from:
Base
Mapping and Geomatic Services Branch
Ministry of Sustainable Resource Management
2nd Floor, 395 Waterfront Crescent
Victoria BC V8W 5K7
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