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.