Introduction
Storm water management plays a crucial role in maintaining the outlook of cities, structures and facilities in the event of precipitation. Natural, undeveloped land manages rainfall through a combination of evaporation, transpiration and infiltration (Brook 2013). Urban, developed land however contains many altered impermeable surfaces such as roads and buildings that create the need for storm water management. The goal of storm water management is to improve water quality, maintain water balance and to protect waterways from flooding and erosion (Marsalek and Watt 1984). Storm sewers collect water in roadway catch basins that divert the water through a pipe network and eventually into a river or body of water. The result of this water diversion can also cause flooding and erosion in the case of intense storms or during thaw cycles. Designing a storm sewer outfall that can handle the effects of the stormwater discharge and river flow erosion ensures the longevity of the structure and the surrounding area. The focus of this literature review will be to identify the factors that cause embankment erosion and to showcase design practices that are used for bank stabilization.
Factors
Bank erosion can be affected by fluvial erosion and/or bank failure. Fluvial erosion is the breakdown of material in water and can occur due to the following physical processes; abrasion, attrition, hydraulic action and solution (Goudie 2003). Although not the primary focus of this review, it is crucial to understand how these processes affect the embankment before discussing methods of prevention. Abrasion occurs when particles from the riverbed are transported within the flow of water and cause scour along the bank and shoreline. Attrition is the process of which rocks along the bed of the river roll and collide creating more debris that can break down the river boundaries. Hydraulic action affects the shoreline due to the repeated force of waves hitting the shoreline and the air being compressed into cracks and notches along the shoreline (Morgan and Nearing 2011). Solution involves the dissolving of rock due to the chemical composition and pH of the water. These processes are a function of many factors including the grain size distribution, density, flow rate, temperature, shear stress and angle of friction. The complexity of these variable factors should be recognized and can be investigated, but are ultimately a combination of hydrologic, geotechnical and hydraulic processes (Luppi et al. 2009). The scope of this review will deal primarily with the geotechnical aspect including methods and techniques used to prevent fluvial erosion as well as mass bank failure. Mass bank failure is dependent on the weathering and weakening of the embankment. The forces holding the riverbank in place are overcome by repeated wave action until the force of gravity causes slope shear failure (Robert 2014). The banks of the channels can contain a range of soil types. Banks with fine grained soil such as silt and clay have an increased shear strength and better resist erosion through the inter-particle bonds. Coarse grained soils such as sand do not have cohesion between particles and can lead to collapse. The figure on the right shows the collapse of an embankment made of course grained material due to mass bank failure. Not discussed in this literature review are factors including channel geometry, flow quantity, wave height, location of junctions and bank slope.
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In summary, engineers must consider the wide range of factors that can cause bank erosion but it is their primary role to implement designs to prevent erosion and bank failure.
Prevention
The effects of erosion can be mitigated through two treatment methods. Engineering systems use non-living materials for the construction of the slope stability. Bioengineering systems use live plant materials to achieve slope stability. Each method has positive and negative aspects that will factor into the selection of the most suitable option for the scenario.
Engineering Treatments
Bank stabilization through engineering treatments is often more effective than bioengineering treatments but can be damaging to wildlife or unpleasant to look at. Summarized below are some of the most commonly used methods.
Riprap
Riprap armouring is constructed from suitable rocks along the bank of a slope for erosion protection (Ministry of Transportation 2015). Riprap may contain varying stone sizes, weights and densities and can follow an array of design guides (Goudie 2003). The main consideration factor is rock size. Size selection should be based on the local experience of its usage on the same river under similar conditions, government standard guidelines and the hydraulic relationships found through analytical models and experiments (Ministry of Transportation 2015).
Advantages Disadvantages
- Useful in soils where vegetation is not easily established
- Effective in areas of high velocities such as culvert outlets
- Easy installation
- Long lasting/maintenance free • Expensive to install as heavy equipment is required for transport
- Not feasible in areas where rock is not readily available
- Does not support vegetation or fish habitats
Bioengineering Treatments
Soil bioengineering is the use of living plant materials to provide some engineering function and stabilize the soil (Polster 2002). In general, soil bioengineering is a sustainable option that protects against surface erosion, promotes ecological activity, is considerably more cost effective than the engineering solutions covered above. Below are a few bioengineered options that can be used to increase the slope stability.
Battle Fences
Live bank protection, otherwise referred to as wattle fences, are natural terraced walls built at creek level to prevent scour along slopes (Polster 2002). They thrive in moist areas, are inexpensive, function as silt fences and are biodegradable as they are made of natural fibre (Ministry of Transportation 2015). Wattle fences are a suitable option for steep slopes but are labour intensive to install and are only sustainable in low flow velocity applications. See figure 6 for an example of this type of bioengineering treatment.
Live Palisades
Live palisades incorporate the use of cottonwood (or similar) posts installed in trenches at an offset from the area of riparian erosion. The posts are planted 3-4m below ground and are expected to root along the bank to protect it from erosion (Polster 2002). Palisades are typically used in low flow velocity environments. Much like other bioengineering treatments, the embankment is also subject to erosion until plants and wildlife develop. Once established they are aesthetically pleasing and grow stronger as roots grow deeper (Di Pietro 2009).
Conclusion
In summary, erosion protection has many contributing factors that affect the stability of embankments. These factors vary for each location and must be investigated both theoretically and experimentally before considering a solution (Neill 1998). Engineering treatments are generally more resilient to erosion and faster to install but at the sacrifice of ecological sustainability and poor aesthetics. Bioengineering treatments typically take more time to become effective, do not provide the same degree of erosion protection but instead facilitate a healthy habitat (Polster, 2002). Of the many erosion control methods available, this literature review summarizes a few of the most widely used options and gives insight to their advantages and shortcomings.
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