Flood Mitigation: Erosion Control

Erosion control is a catch-all phrase encompassing a variety of projects and methods for reducing river and stream erosion. Typically, there are three different categories of erosion control projects including; channel bank armouring, vegetative erosion control, and hard engineered structures.  Channel bank armouring includes riprap projects where rocks are placed along river banks to prevent soil erosion and gabion structures where rocks are held back using meshed wiring at different levels to mitigate flooding. Vegetative options include cement-sand bagging and planting more water-retaining vegetation along river and stream beds. Hard engineered structures refer mainly to retaining walls built from concrete that prevent soil and vegetation erosion but increase the rate of channel flow. 

Oftentimes erosion control projects are described as river bank protection methods. For our purposes, we will use the term erosion control because this is the language used by the Government of Alberta to describe past, present, and future projects. 

What is erosion control?

Erosion control is the practice of re-stabilizing river and stream banks after a flood event to secure developments in the surrounding area. Erosion control measures can use hard engineering such as retaining walls to prevent future erosion, however, this prevents natural meandering of the river. More naturalized options include gabion structures, rip rap, and vegetation to soak up excess water and prevent bank destruction. Erosion control is important for protecting man-made structures. 

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

Erosion control measures may reduce the damage of future floods, especially in urban areas where developments are located near rivers and streams. There are three options for erosion control measures including channel bank armouring (riprap, channels, gabions), vegetative erosion control (soil retention, vegetation, sand-cement bagging), and harder control structures such as walls[1]. Each option uses a different technique to prevent erosion and damage to river banks, however, some are more permanent than others. 

Do erosion control measures already exist in Alberta? 

Yes, erosion control measures do exist in Alberta. After the June 2013 flooding, the Government of Alberta initiated the Flood Recovery Erosion Control Program and pledged an initial $20 million in immediate support to communities suffering extensive damage. This amount was later increased to $116 million[2]. Municipalities that experienced erosion damage and declared a state of emergency were eligible for funding to rebuild after the flood. Areas that received immediate attention included; Cougar Creek  in Canmore, Exshaw, Lac Des Arc, six areas in Calgary, Crowsnest Pass, Highwood River, Sheep River in Turner Valley and Bragg Creek[3]. More specifically, some of the immediate construction projects used retaining walls, debris removal, dredging, river bank protection and overland flow protection measures[4].

The following picture shows erosion control construction on the Elbow River at the Calgary Stampede grounds.

Erosion Control at the Calgary Stampede Grounds

What are the costs of erosion control? 

In Alberta, the provincial government has provided $116 million in funding for erosion control projects to restore river and stream banks. To date, approximately 75 projects have been approved ranging from $67,000 to just over $6 million dollars[5]. Each project has utilized specific types of erosion control measures as well as a combination of options to increase results. 

How long does it take to build? 

In the case of Alberta, there was great urgency in rebuilding river and stream banks as quickly as possible in areas that were greatly affected by the 2013 flood. In those cases, erosion control projects were completed in just a number of weeks. In all, however, the Flood Recovery Erosion Control Project is focused on completing all projects by December 31, 2015[6]

Lifetime expectancy of erosion control?

Depending on the type of erosion control project, lifetime expectancy can vary. Structures such as retaining walls will require continual maintenance over time while natural options will require less. Due to the natural meandering of the river erosion control projects will change over time, therefore, measures that can adapt to the ever-changing river will be more beneficial. 

What are the associated risks? 

A major concern with erosion control projects is that soil removed from the site in the construction process increases the potential for poor water quality. Sedimentation and toxic pollution are two examples of water quality issues than can result from the construction process of erosion control projects. Furthermore, soil removal can actually increase the rate of erosion because important water-retaining vegetation is removed[7].  

What are the impacts on the surrounding watershed system and environment? 

While erosion control measures are implemented to prevent damage to development, these projects can impact the environment and local watershed. Specifically, when soil and vegetation is removed for engineered structures this increases potential storm water runoff leading to water quality issues and sedimentation[8]. For this reason, it is important for every erosion control project to maintain vegetation so that rain, river and flood water can be absorbed and released into the river or stream properly and without large amounts of sediment. 

Would erosion control help in a drought?

Similar to flooding, drought events can cause erosion due to dry climates where wind can remove top soil important for the growth of vegetation. Erosion control measures can help to mitigate the impacts of a drought in the following ways; maintaining residue such as top soil and clay, letting vegetation over-grow, and building shelter beds that protect specific areas[9]

 

[1] “Stream bank protection and erosion damage mitigation measures.” McKinney Texas. Accessed February 28, 2014. http://www.mckinneytexas.org/DocumentCenter/View/419 

[2] Based on an interview with Jay Litke, Flood Recovery Coordinator for the Bow Basin. February 25, 2014. 

[3] “2013 Flood Recovery Erosion Control Program.” Government of Alberta. Revised October 2013. Accessed February 28, 2014. http://environment.alberta.ca/documents/FREC_Program_Guide_October_2013.pdf 

[4] “Flood Recovery Erosion Control Program.” Alberta Environmental and Sustainable Resource Development. Updated February 10, 2014. Accessed February 24, 2014. http://environment.alberta.ca/04248.html

[5] Ibid. 

[6] Ibid. 

[7] “Sediment and Erosion Control.” Environmental Protection Agency. http://www.epa.gov/region6/6en/w/sw/sediment.pdf 

[8] Ibid. 

[9] “Drought increases erosion concerns.” Government of Canada. Agriculture and Agri-food Canada. Modified April 25, 2012. Accessed February 28, 2014. http://www.agr.gc.ca/eng/?id=1326746188555