Learn: What is Flooding?

 An extremely simple definition of flooding is “too much water in a new place” but a more technical description is when water has overflown into an area that is normally dry. In Alberta there exists a potential for flooding along all rivers and streams[1] and there is also potential for flooding from rising groundwater levels or an abundance of stormwater. When floods occur there is the potential for property damage, hardship to people, and loss of life. However, flooding is also a natural hydrological process required by some plants and animals to thrive[2]

Types of Floods

There are three types of flooding of which have the potential to impact Albertans. They are Fluvial Flooding, Pluvial Flooding, and Groundwater Flooding. 

Fluvial (Riverine) Flooding

Fluvial flooding occurs when surface runoff washes into a river causing the water to breach the river’s banks[3]. The water then overflows into the surrounding area.

There are numerous contributing factors to fluvial flooding. They include:

  • Rainfall conditions, such as intensity, amount, and distribution; and
  • Ground conditions, such as amount of soil moisture, seasonal variations in vegetation, depth of snow cover, imperviousness due to urbanization[4]

In addition to regular fluvial flooding, there are some special subtypes of fluvial flooding[5]  that are defined by how the water breaches its banks, or the time it takes for the flood to occur. They include:

Flash Flooding – Rapid flooding caused by combination of intense rainfall and fast runoff. Flash flooding generally occurs in less than 6 hours[6]  and is therefore difficult to predict.  

Ice Jam Flooding – When an ice jam forms on the river, the flow of water behind it backs up until the jam breaks, causing a rush of water downstream[7]

Alluvial Flooding – Alluvial fans are naturally occurring deposits of unconsolidated sediment that have accumulated at the mouth of a mountain canyon[8]. As sediment comes downstream it accumulates until the riverbed rises high enough that water spills over the banks and begins to create a new path for the river in a lower area. The process is then repeated in a new area. This process causes the alluvial fan river to move either gradually over time or rapidly during flash flooding [9][10] .

Dam Break Floods – Improper care of dams or the accumulation of water that is higher than the maximum holding threshold can contribute to a dam break. A dam break will cause overflowing of the river downstream due to the suddenly released volume of water[11]. This is also called catastrophic flooding.  

Pluvial Flooding

Pluvial flooding is when rainfall or snowmelt is not absorbed into the ground forcing the water to flow overland. The area will remain flooded until water has drained away through stormwater systems or waterways[12]. In instances where there is no drainage system, such as the prairie pothole region, the water ponds remain until the excess water evaporates, sublimates or transpires[13].

Factors that contribute to pluvial flooding include:

  • Rainfall conditions, such as the severity of rainfall; 
  • Ground and soil conditions, such as, soil type, amount of soil moisture, seasonal variations in vegetation, depth of snow cover, imperviousness due to urbanization;
  • Sewer infrastructure, such as overland stormwater flow conveyance capacity, the state of the sewer infrastructure; and 
  • Lot level actions by property owners[14]. 

 imperviousurbanflood

Figure 1 – Imperviousness due to urbanization’s effect on flooding[15].

The predominate subtype of pluvial flooding is stormwater flooding, which is sometimes called urban flooding. Although urban areas may be flooded when riverine flooding occurs, urban flooding is often used to refer to pluvial flooding in urban areas.

Urban flooding occurs when the runoff from a heavy downpour of rain overwhelms the drainage system[16]. The excess water cannot be absorbed by the ground or contained within drainage systems. This results in water flowing overland into nearby lakes and streams or creating temporary pools in low(er) lying areas.

Groundwater Flooding

Groundwater flooding occurs when the water table rises up through permeable sediments to the ground surface or basements[17]. Groundwater flooding can occur weeks or months after a large rain event, and may not be linked to any specific rainfall event[18].

 

[1]  Flood Hazard Identification Program Guidelines. (2011). http://alberta.ca/albertacode/images/Flood-Hazard-Identification-Program-Guidelines.pdf  

[2]  Cottonwood Riparian Ecosystems of the Southern Interior (1997). http://www.env.gov.bc.ca/wld/documents/cottonwood.pdf  

[3] Jha, A.K., Bloch, R. &  Lamond, J. (2012).Cities and Flooding: A Guide to Integrated Urban Flood Risk Management for the 21st Century. Global Facility for Disaster Reduction and Recovery. Retrieved from https://www.gfdrr.org/sites/gfdrr.org/files/urbanfloods/pdf/Cities%20and%20Flooding%20Chapter%201.pdf  

[4] "Facing geologic and hydrologic Hazards: Earth-science considerations". Hays, W. W (Ed.). (1981). U. S Government Printing Office: Washington D.C. http://books.google.ca/books?id=aqxrINwzD7AC&pg=SL2-PA48...  

[5] The Drace Project. Types of Flooding. (n.d.). Retrieved from http://drace-project.org/index.php/floods/types  

[6]  "Flash Flood Early Warning System Reference Guide". World Meteorological Organization. (2010). http://www.meted.ucar.edu/communities/hazwarnsys/ffewsrg/FF_EWS.Chap.2.pdf  

[7]  Ice Jam (ice formation).(n.d.). In Encyclopædia Britannica online. Retrieved from http://www.britannica.com/EBchecked/topic/281150/ice-jam  

[8]  Alluvial Fan. (n.d.). In Encyclopædia Britannica online. Retrieved from http://www.britannica.com/EBchecked/topic/16659/alluvial-fan 

[9]  County of San Diego. (n.d.). Alluvial Fan Flooding and Protection. Retrieved from http://www.sdcounty.ca.gov/dpw/floodcontrol/alluvial_fan.html  

[10] The Drace Project. Types of Flooding. (n.d.). Retrieved from http://drace-project.org/index.php/floods/types  

[11]  National Flood Insurance Program. (n.d.). Flood Risk Scenarios: Which Scenario Applies to You? Retrieved from https://www.floodsmart.gov/floodsmart/flash/main_experience.swf  

[12]  Jha, A.K., Bloch, R. &  Lamond, J. (2012).Cities and Flooding: A Guide to Integrated Urban Flood Risk Management for the 21st Century. Global Facility for Disaster Reduction and Recovery. Retrieved from https://www.gfdrr.org/sites/gfdrr.org/files/urbanfloods/pdf/Cities%20and%20Flooding%20Chapter%201.pdf  

[13]  Shook, Kevin. "A new tool for modelling Prairie flooding." University of Calgary. Hotel Alma, Calgary, AB. February 18th. Flood Forecasting Methods and Models Workshop.

[14]  Kovacs , P & Sandink, D. (2013). Best practices for reducing the risk of future damage to homes from riverine and urban flooding: A report on recovery and rebuilding in southern Alberta. Institute for Catastrophic Loss Reduction. Retrieved from http://www.iclr.org/images/Alberta_flood_risk_2013_PDF.pdf 

[15]  World Meteorological Organization & Global Water Partnership. (2008). Urban Flood Risk Management: A tool for Integrated flood management. Retrieved from http://www.apfm.info/publications/tools/Tool_06_Urban_Flood_Risk_Management.pdf  

[16]  National Institute of Disaster Management, India. (n.d.).  Urban Flooding and its Management. Retrieved from http://nidm.gov.in/idmc/Proceedings/Flood/B2%20-%2036.pdf 

[17]  British Geological Society. (n. d.). Groundwater Flooding FAQs. Retrieved from http://www.bgs.ac.uk/research/groundwater/flooding/groundwaterHomesFAQ.html  

[18]  Ibid