Slope Retention and Erosion Prevention

 

 

 

 

 

 

 

 

 

 

 

 

Erosion Control and Slope Stabilization are imperative when working on a steeply-pitched site, or part of a site. Surface water moving over large or steep sloped areas can cause the surface to erode, wash out, or worse, cause large volumes of soil to separate and slide downhill.

When working on a site with existing slopes in a stable condition (fully covered with plants, or otherwise secured), the first way one can mitigate the impact of site improvement is to minimize the amount of disturbance to occur in the first place. Designing site features to work with existing topography can save significant effort and construction cost while minimizing the amount of disruption to the land. For instance, if a design includes a new proposed pond or rain garden, this feature should be located in a lower-elevation part of the site where water will flow naturally without much site engineering.

Designing the Components of a Slope

Preparing a slope begins with the subgrade below it. For large areas of slope exceeding 5:1 (5 feet of run to 1 foot of vertical elevation change), it is recommended that the subgrade be stepped and scarified, in order to give the top layers of soil more resistance to sliding downhill. An aggregate or gravel base should be provided directly above this to allow for subsurface water to drain freely – water can saturate planting soil on slopes and make it heavy, which in turn can cause soil layers to slide downhill with gravity.

The next step is to select appropriate soil. Usually this will mean soil that does not easily become saturated and heavy, which will often mean a sandier soil with good drainage properties. Depending on what slope stabilization method(s) may be employed, the soil may or may not need to be fertile (with some silt and clay particles) to support plant colonization on the slope.

When applying planting soil, place soil up to the desired depth in 6-8” lifts, or layers, rolling each one or lightly compacting it to be sure it is stable in place. Ensure proper soil stability throughout the full depth of the soil will prevent settlement from occurring and make the slope more resistant to washout or displacement. Until the slope is fully stabilized (either fully colonized with plants or secured by some other method), hay bales and silt fences should be employed to catch any rainwater-borne sediment which may have washed off the slope.

When designing a site as a part of a sloping or hilly landscape, steep conditions may sometimes be inevitable. If a slope must be steep, design to minimize its length or break it up into shorter lengths with flatter areas in between. This holds especially true if the slope falls toward a site structure or building, as water will drain off the slope directly toward it.

Engineered Solutions/Products

Aside from continuous, dense planting, there are a variety of products which are commonly used for slope stabilization projects:

-Riprap/angular aggregate, used as a surface treatment for stability and resistance to washout.

-Gabion mattresses, function similarly to riprap surface since the contents are essentially the same.

-Geogrid or similar slope reinforcement

Paving the slope with concrete or modular pavers.

 

Plant Cover

For many site designs, a slope stabilized by stone mulch or an engineered system will not be the final condition envisioned by the site designer, who may wish to add plants to improve the aesthetics and further secure the slope. Plants secure a slope’s soil in place in two ways; first, the root masses act as ties to help prevent downward soil migration with gravity and subsurface water flow. Second, the foliage covers the surface of the slope, redirecting rainwater and reducing the likelihood of soil being washed downhill.