Flood Mitigation: Berms

What is a berm?

A berm can be a man-made sediment barrier placed at the edge of a slope or a wall built adjacent to a ditch to guard against potential flooding. Berms are placed in flood-prone areas to protect against erosion, run off and high water. Typically berms are made of compost, sand, mulch or gravel materials, their density enable them to slow down and retain flood waters[1].

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

Berms contain dense sediment materials that decrease water velocity, control flow rates and absorb excess water in the event of a flood. Normally placed in communities or sites prone to flooding, berms act as a barrier further mitigating the impacts of flood water. Berms are an effective form of flood mitigation so long as they are built large enough to handle flood flows. For this reason, berms are often coupled with other mitigation measures such as dams and dry-ponds[2].

Are berms already built in Alberta?

Berms have been built in high risk areas of southern Alberta along the Bow and Elbow rivers in Calgary and along parts of the Highwood River around the Town of High River. In 2005, berms successfully contained water levels throughout southern Alberta. During the 2013 Flood event, berms managed to hold back some of the flow volume. More specifically, berms built along the Red Deer River in the 1980’s in Drumheller successfully mitigated 2013 flood flow volumes[3].

What are the costs to build and maintain?

The cost of building a lateral berm ranges depending on size, use (transverse or lateral) and materials. For flood mitigation, transverse berms are typically larger and more expensive. In the 1980’s, Drumheller built a system of berms throughout the city which proved beneficial for all subsequent flooding, including the 2013 event. In order to upgrade and maintain lateral berms, Drumheller is requesting another $12 to $15 million from the Province to mitigate future floods[4].

How long does it take to build?

Depending on the size of the berm, construction times are quicker than other infrastructure options such as dams and diversions (transverse structures). In Alberta, proposed berms in the hamlet of Exshaw and in the City of Calgary could be built within 12 months in comparison to other mitigation options that could take a few years[5].

Operations lifetime expectancy?

Typically, the concern for lifetime expectancy of a berm is the river’s breach of berm, therefore, these structures will be upgraded over time to accommodate higher river flows. For example, Drumheller’s recent request to upgrade current berm infrastructure has come 30 years after initial construction.

What are the associated risks?

Berms built along smaller streams and rivers tend to increase the water’s velocity and cause downstream erosion and riverbank damage. Berms do interrupt the natural floodplain function because water is confined thus creating significant erosion and subsequent berm failure causing damage to property[6].

What are the environmental and watershed impacts of berms?

The environmental impacts of berms include; riverbank erosion, habitat destruction and stream instability. These impacts are mainly caused by the increase in velocity that berms cause by limiting the room for water to flow[7].

Berms are built to contain flood waters and prevent riverbank breaches that could flood developments. Impacts on the surrounding watershed can be significant due to high flow rates and subsequent erosion. The function of floodplains is also disrupted because water is being confined to the stream rather than allowing flow to expand across the floodplain[8].

Would a berm help in a drought?

Berms are intended to aid in flood situations, therefore, they cannot be used in a drought.