Overland Flow

From: http://environment.alberta.ca/01655.html

Update July 17, 2013

The Flood Hazard Identification Program assists Alberta communities to prepare for a flood event with a one per cent chance of occurring annually (this is also known as a 1:100 year flood event).

Information is collected in a format known as a flood hazard study for a specific location within the province. The study focuses on a single river but may also include information about other smaller tributaries that may also influence a flood event.

To create a flood hazard study an assessment of the river system’s hydrology and topography needs to occur and be modelled with known river data. The outcome of the study is to create a visual representation (a flood hazard map) for anticipated areas that would be impacted by a flood event. These maps only need to be replaced or updated if there has been a change in the hydraulic system.

Areas denoted include floodway zones and flood fringe zones. '''Overland flow areas are considered to be part of a flood fringe zone. '(emphasis ours -Greg'')

The flood hazard maps seen on this site are up-to-date and final. These maps will be used for the Disaster Recovery Program (DRP) in assessing the location of flood ways and flood fringes. The map is interactive and users can scroll to see specific locations along the river systems. If an area is not mapped and you were impacted by flood events, please contact 310-0000 or the DRP office at 1-888-671-1111 for an assessment.

Disaster Recovery Program (DRP) Policy The current Flood Hazard Maps are being used for the DPR policy. Fact sheets: Flood Hazard Identification Program (pdf) Components of a Flood Hazard Study (pdf) Appropriate Use of a Flood Hazard Study (pdf)
 * 1) The current flood hazard maps are the final approved data set for use in determining land designations within the provincial program.
 * 2) Move pointer over map and select a location then magnify the view, alternatively you can search by lot, block, plan; legal land description; postal code and community under the ‘Find’ tab and magnify the view.
 * 3) The different zones are color coded Floodway is dark pink, Flood Fringe (per 2013 policy decision) includes both the fringe and overland flow designations on this map. A legend is available on the ‘Introduction’ tab.

Terminology and Definitions
Flood Hazard Area - The area affected by the design flood under encroachment conditions. The flood hazard area is typically divided into floodway and flood fringe zones, and may also include areas of overland flow.

Floodway - The portion of the flood hazard area where flows are deepest, fastest and most destructive. The floodway typically includes the main channel of a stream and a portion of the adjacent overbank area. The floodway is required to convey the design flood. New development is discouraged in the floodway and may not be permitted in some communities.

Flood Fringe - The portion of the flood hazard area outside of the floodway. Water in the flood fringe is generally shallower and flows more slowly than in the floodway. New development in the flood fringe may be permitted in some communities and should be floodproofed.

Overland Flow - Areas of overland flow are part of the flood hazard area outside of the floodway, and typically considered special areas of the flood fringe.

Design Flood - The current design standard in Alberta is the 100-year flood, determined when a flood hazard study is undertaken. A 100-year flood is defined as a flood whose magnitude has a one percent chance of being equalled or exceeded in any year. The design flood can also reflect a computed 100-year water level resulting from an ice jam or be based on a historical flood event.

Design Flood Levels - Flood hazard area water elevations computed to result from a design flood under encroachment conditions. Design flood levels do not change as a result of development or obstruction of flows within the flood fringe.

Encroachment Conditions - The flood hazard design case that assumes a scenario where the flood fringe is fully developed and flood flows are conveyed entirely within the floodway.