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Learn: How Are Flood Maps Created In Alberta?

Hydraulic Modelling

Hydraulic modelling is a form of modelling that replicates flows and fluid transport processes. A hydraulic model can calculate flood levels, flow patterns, model the effects of backwater, overtopping of embankments, waterway confluences, bridge constrictions, and other hydraulic structure behaviour[1]. This replication of the environment can be used to recreate previous floods, project the impact of future flooding, and illustrate the areas at a risk of flooding. 

As of 2011, the Flood Hazard Maps in Alberta have been created using the Hydrologic Engineering Center River Analysis System (HEC-RAS), a hydraulic model from the Hydrologic Engineering Center, Corps of Engineers, U.S. Army, Davis, California. HEC-RAS is a free program that may be downloaded here: http://www.hec.usace.army.mil/  

In order for the HEC-RAS model to be applicable to Alberta it must be populated with Alberta specific information and calibrated based on experienced flows and river conditions.

Populating the Model

In order for a hydraulic model to emulate Alberta it requires information about the environment it is modelling. Information on the terrain, cross section surveys, stream junctions, storage structures, diversions, weirs, gated spillways, bridges, culverts, pump stations, river ice, steady flow, and unsteady flow are loaded into the model.

Model Calibration

Historical flow conditions, including droughts and floods, can be used to calibrate the model. The known flood impacts of previously experienced floods are measured against the modelled flood scenario. If the modelled flood scenario does not produce the same impacts as the experienced flood than further calibration is required.

The following information is gathered by the Alberta Government to calibrate flood scenarios.

Highwater Mark Information 

After large flood events there are marks where the water peaked on structures such as bridges and buildings. That peak is recorded and used to calibrate hydraulic models.

Aerial Photography

Photos taken from the air can provide perspective as to the aerial flood inundation zone.  Aerial photography can also provide a historical record of flooding.

Want to see historic flood photos from your area? Check out http://esrd.alberta.ca/forms-maps-services/air-photos/default.aspx

Rating Curves

Gauging stations measure the flow rates and water levels which can then be used to create rating curves. Rating curves show the relationship between the water surface elevation and the discharge, based on measurements taken at the gauge[2]. However, during a flood gauging stations are often subject to damage which can impact their ability to gather data.

Other Historic Information

Newspaper articles, photos, or personal recollections for historic floods help to provide a snapshot of historic floods before gauging, aerial photography, or high water mark data was recorded. Additionally, they can provide information for areas that are not gauged.

Creating the Design Flood

The design flood is the flood that is used to delineate the flood hazard area. The design flood is statistically created and based off the 1-100 year return period, 1% chance of occurring in any given year, calculated at the time of the study[3]. The design flood may also be a historical open water or ice jam flood[4]

More complex math and statistics are used to determine the certainty of the return period that has been calculated.

In cases where there are not many recorded years of data, a statistical model is created.

Flood Hazard Area

Once the model has been completed the design flood can be used to create the flood hazard maps. 

The flood hazard area is the identified area of land that experiences a flood during the design flood due to overland flows.

The Alberta Government then separates the flood hazard area into two zones: the floodway and the flood fringe. The Government of Alberta defines the floodway and flood fringe as follows.

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. 

The floodway includes areas where the water is 1 m deep or greater, the local velocities are 1 m/s or faster and if the river were encroached upon, the water level rise would be 0.3 m or more. Typically the floodway includes the river channel and adjacent overbank areas.

Flood fringe 

The portion of the flood hazard area outside 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 flood-proofed. 

The flood fringe is the land along the edges of the flood hazard area that has relatively shallow water (less than 1 m deep) with lower velocities (less than 1 m/s).

Updating Flood Hazard Studies

Following the 2013 June Flood and after significant media coverage about the accuracy of the flood hazard maps, it was announced that money would be set aside to complete further studies[5]. The Flood Hazard Identification Program Guidelines also had the following to say about updating flood hazard studies:

If a significant  change in a river valley occurs or if an error is discovered in the flood hazard study, it will be reviewed. As a result of the review the flood hazard identification study may be amended.

[1] Earth Science Australia. (n.d.). Flood Model Development and Calibration. Retrieved from http://www.earthsci.org/flood/J_Flood04/flood/J_Flood_2.html 

[2]  Flood Hazard Identification Program Guidelines, 2011

[3] Ibid.

[4] Ibid.

[5]   Government of Alberta, Environment and Sustainable Resource Development. (2013). New funding earmarked for flood hazard studies [Press Release]. Retrieved from http://alberta.ca/release.cfm?xID=350598DC3F212-DA7A-D3E5-75048A3014A64ADC