Flood Mitigation: Dry Dams

What is a dry dam?

Dry dams are catchment areas that have been designed to hold excess water in times of flooding and allow water to move freely in normal conditions. This is a short term flood mitigation solution that stores water for approximately 24 to 48 hours before slowly releasing the water to prevent downstream flooding. There are no permanent areas for holding water, which is why these structures are called “dry dams”[1].This form of flood mitigation is useful for smaller, more frequent floods that can erode river banks[2].

How does this option help to mitigate the impacts of a flood?

Dry dams are designed to retain excess flood water for a short-period of time (24-48 hours). Once the time has lapsed the water is then released at a slow and controlled rate through gates located at the outlet. While dry dams on their own would only be capable of addressing smaller floods, this mitigation method combined with other strategies, such as larger reservoir dams, have the potential to mitigate for larger floods such as those seen in 2013[3].

Have dry dams already been built in Alberta?

Dry dams are not currently built in Alberta, however, a Government of Alberta feasibility study is currently underway to determine whether or not this form of flood mitigation could be used.

The following is a dry-dam located in Japan.

This is another dry-dam located in Ohio.

What are the costs to build and maintain?

Construction costs to build a dry dam vary depending on size and landscaping requirements, therefore, it is difficult to discern a specific cost. Also, regular maintenance and upgrading costs are needed which, range from 3 to 5% of the original construction costs and must occur every 10 to 20 years. Maintenance requires unclogging sediment and removing overgrowth[4].

For example, after major flooding in Miami in 1913 a system of five dry dams was built to retain flood waters and prevent future damage. This original system cost $30.36 million to construct. In today’s dollars, this system of dry dams would cost $912 million to replace[5].

How long does it take to build?

Construction start and completion dates depend on how many dry dams are being built and their size. For example, the five dry dams constructed on the Miami River took five years to construct[6].

Operations lifetime expectancy?

Dry dams are considered a long-term, permanent flood mitigation strategy. Maintenance is required especially after the event of a flood or storm surge to remove debris and sediment deposits.

What are the associated risks?

Risks associated with the construction and use of dry dams include; structural and hydraulic design, sediment management, water quality, clogging problems due to debris, and ecosystem and land-use issues in the surrounding area[7]. Lastly, ongoing costs for maintenance and upgrading can be significant.

What are the environmental and watershed impacts of dry dams?

Given the design of dry dams as a short-term retention area for excess water, this inundation can lead to the loss of vegetation, forests and natural areas. To prevent adverse impacts to the local environment and biodiversity, dry dams should be built to accommodate both regular and rare floods, which require large storage capacity[8]. The local watershed can also be impacted due to interruption of the sediment transport process. River flows carry sediment downstream for aquatic species and erosion prevention. Interrupting this process can cause turbidity issues downstream and soil erosion on riverbanks[9].

Would a dry dam help in a drought?

Dry dam infrastructure cannot mitigate for drought given the design of the infrastructure to hold excess water and manage flooding.

 

[1]   “How a dam works: Dry Dams.” Miami Conservancy District. https://www.miamiconservancy.org/flood/works.asp  

[2]  “Dry Detention Ponds.” U.S. Environmental Protection. Accessed November 18, 2013. http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=browse&Rbutton=detail&bmp=67

[3] Ibid.

[4]  “Water Quality: Dry Detention Basins.” Government of B.C. Ministry of Environment: Environmental Protection Division. Accessed January 15, 2014. http://www.env.gov.bc.ca/wat/wq/nps/BMP_Compendium/Municipal/Urban_Runoff/Treatment/Detention.htm

[5]  “Flood Protection: MCD system.” Miami Conservancy District. http://www.miamiconservancy.org/flood/dams.asp

[6]  Ibid. 

[7]  Sumi, Tetsuya. “Designing and operating of flood retention ‘dry’ dams in Japan and USA.” Kyoto University. Pg. 1.

[8] Brown, W and T. Schueler. “Flood Detention Basin.” The Economics of Storm water BMPs in the Mid-Atlantic Region. (1997) Accessed January 17, 2014. http://www.floodengage.com/option.php?id=10

[9] Sumi, Tetsuya. “Designing and operating of flood retention ‘dry’ dams in Japan and USA.” Kyoto University. Pg. 7.