Floodplain Management Definitions

Apparent valley: When the watercourse is located within a valley corridor, confined by valley walls. The watercourse can contain perennial, intermittent or ephemeral flows and may range in channel configuration, from seepage and natural springs to detectable channels. A river or stream valley is apparent if

  • the valley wall slope is 3 metres or greater in height
  • the valley wall slope is 3:1 or 5 :1 (depending on soil type) or steeper


Flood fringe: The outer portion of the flood plain between the floodway and the regulatory flood plain limit. Flood depths and flow velocities are generally less severe in the flood fringe than those experienced in the floodway.

Floodway: The channel of a watercourse and that inner portion of the flood plain where flood depths and velocities are generally higher than those experienced in the flood fringe. The floodway represents that area required for safe passage of flood flows and/or where the velocities are considered such that they pose a potential threat to life and/or property damage.

100 year flood: The flood, based on analysis of precipitation, snowmelt, or a combination thereof, having a 1% chance of occurring or being exceeded in any given year.

Regulation limit:  The extent of the regulatory jurisdiction for a Conservation Authority regulation as defined by Section 2 of the applicable Section 28 CA Act regulations. The Regulation Limit is a setback of 15 metres inland from the greatest identified hazard (flood plain, steep slope, erosion hazard, etc.)

Regulatory flood plain: In eastern Ontario, the 100 Year Flood Event Standard meaning rainfall or snowmelt, or a combination of rainfall and snowmelt producing at any location in a river, creek, stream or watercourse, a peak flood level that has a probability of occurrence or exceedance of 1% during any given year.

Stable slope limit: The limit, or the setback that ensures safety if slumping or slope failure occur.  It represents the limit to which the slope would recede if it were to reach the long term stable slope inclination; at which point it would resist slumping and rotational slipping. The stability of slopes can be affected by everything from increases in loading, such as the placement of buildings, changes in drainage patterns to erosion of the toe of a slope or loss of stabilizing vegetation on the slope face.

The stable slope allowance is determined by using a horizontal allowance measured landward from the toe erosion allowance equivalent to three or five times (3:1 or 5:1) the height of the slope (depending on soil type) OR through a valid study.

Toe-erosion allowance: The setback that ensures safety if the toe of the slope adjacent to the river or stream erodes and weakens the bank, increasing the risk of slumping.

Two-zone concept: This concept identifies the floodway and the flood fringe. The floodway refers to that portion of the flood plain where development and site alteration would cause a threat to public health and safety and property damage. In other words it is that portion of the flood plain required for the safe passage of flood flow and/or that area where flood depths and/or velocities are considered to be such that they pose a potential threat to life and property damage. (See Figure 2). The flood fringe is the portion of the flood plain where development may be permitted subject to certain established standards and procedures. Because conditions vary, there is no province wide standard for determining the more hazardous areas of flood plains. But some factors to take into account include depth of water: velocity of flow, combined depth and velocity, vehicle access and structural integrity.