Flood Mitigation: Diversion Channel
What is a diversion channel?
Diversion channels or floodways are manmade channels built to offer a different route for excess water to flow further, thus mitigating the effects of flooding and restoring rivers to their natural water level. Typically, diversion channels are built around communities or economic centres to prevent extensive flood damage [i].
How does this option help to mitigate the impacts of a flood?
Diversion channels mitigate the impacts of a flood by offering an alternative route for excess water. Normally, diversion channels are built around major cities and towns to mitigate the effects of flooding in communities and urban centres. For example, the Red River Diversion Channel in Manitoba (phot above) is an artificial waterway built around the City of Winnipeg. Flood water is diverted down this channel in springtime when water levels are high, rejoining the Red River further downstream [ii].
The following is the outlet structure of the Red River Floodway, a diversion channel around Winnipeg, located near Lockport, Manitoba.
What are the costs to build and maintain?
The costs to build a diversion channel are high; however, once constructed, diversion channels are permanent and used frequently to attenuate high river flows. For example, the Portage Diversion in Manitoba cost $20.5 million upon its completion in 1970 [iii].
How long does it take to build?
Depending on the size and length of the proposed diversion channel, mitigation projects such as this take a number of years. For example, the Portage Diversion in Manitoba took five years to build, between 1965 and 1970.
What is the operations lifetime expectancy?
Channel diversions are built to attenuate high river flows, providing an alternative route for flows. Once built, these structures are permanent in nature. However, diversion channels are typically only used in times of need. The Portage Diversion, for example, has operated 29 out of 43 years since construction.
What are the associated risks?
Diversion channels can become blocked with upstream ice jams that cause surges of water, further overflowing diversion channels and flooding surrounding areas. This was a major concern after the Portage Diversion in Manitoba was opened in spring 2013 to accommodate surging water [iv]. High water flows resulting from heavy precipitation can also overwhelm diversion channels. For example, in 2011, the Assiniboine headwaters in Manitoba experienced heavy precipitation resulting in a 1:330 event that strained diversion infrastructure [v]. Concerns over the risks of climate change and potential for higher river flows that exceed the capacity of diversion channels have also risen.
What are the environmental and watershed impacts of a diversion channel?
The downstream environmental impacts of diversion channels include loss of habitat, due to altered flows, the loss of flood plains and wetlands, habitat fragmentation for aquatic species and wildlife, and impacts to biodiversity in the region [vi].
Diversion channels, like dams, disrupt the natural flow of rivers and the function of flood plains. For example, the Portage Diversion used in Manitoba during flood season protects the larger City of Winnipeg; however, flooding can occur where the waters are diverted, especially in the region near Lake Manitoba [vii]. Therefore, the increase in flow from one watershed may be diverted to another watershed, potentially causing flooding in another region.
Would a diversion channel help in a drought?
Water diversion channels can redistribute water from one region to another that is experiencing severe drought. For example, a river diversion project in China provides water from the Yangtze River to the Yellow River, where extreme droughts occur [viii]. A system of canals carries the water from one location to the other. Another Chinese diversion scheme, the “South-to-North” diversion, sends water from China’s southern region to drier northern regions [ix].
[iv] Canadian Broadcasting Corporation, 2013, Portage diversion pushing flood waters north. http://www.cbc.ca/news/canada/manitoba/portage-diversion-pushing-flood-waters-north-1.1400442. Accessed 2023-05-01.
[v] Environment Canada, 2011, Canada’s Top Weather Stories for 2011. http://ec.gc.ca/meteo-weather/default.asp?lang=En&n=0397DE72-1. Accessed 2023-05-01.
[vi] World Meteorological Organization, 2006, Associated Programme on Flood Management Technical Document No. 3, Flood Management Policy Series – Environmental Aspects of Integrated Flood Management. https://www.floodmanagement.info/publications/policy/ifm_env_aspects/Environmental_Aspects_of_IFM_En.pdf. Accessed 2023-05-26.
[vii] Government of Manitoba, 2013, 2011 Flood: Technical Review of Lake Manitoba, Lake St. Martin and Assiniboine River Water Levels. https://www.gov.mb.ca/mit/floodinfo/floodproofing/reports/pdf/assiniboine_lakemb_lsm_report_nov2013.pdf. Accessed 2023-05-26.
[viii] Soonawala, N., 2017, Yangtze River Diversion Problems. https://sciencing.com/yangtze-river-diversion-problems-20123.html. Accessed 2023-05-26.
[ix] Baiyu, G., 2020, Vast river diversion plan afoot in western China. https://chinadialogue.net/en/nature/11762-vast-river-diversion-plan-afoot-in-western-china-2/. Accessed 2023-05-01.