Fact Sheet: Slopes, Grades and Berms

Slopes provide drainage. Each landscape has an existing drainage pattern including swales, ditches, and sheet drainage, or perhaps even surface water features. Problems can occur when a site is not properly draining. Low, wet problem spots occur where ice can freeze and air spaces between soil particles become so saturated that oxygen to the plants is cut off and they die.

Grading permits reshaping of a site's slopes to create flat areas such as parking or playgrounds, or to slow surface water run-off that would otherwise cause erosion.

Berms are mounded earth forms that can be useful to screen views to the street or unsightly elements, or they can be used to provide a new shape to a space in order to create drama or a sense of enclosure.

Once you determine you need to change your site's slope, there are several considerations. First, will the slope be "cut" into the existing landform or will soil be imported to add or "fill" an area? Cut slopes are more stable than fill slopes in which the soil must be compacted.

Next, the soil type must be considered together with the size and the intended use of the site area. If you have a special use in mind, you may have to visit the library to do some research. Find out the maximum slope for the given activity and decide if you have enough space for the activity. The size and shape of the area needed can help you in determining design solutions by either using the slope or adding retaining walls.

Soil type is also important because individual soil types have their own natural angle of repose - the angle at which the soil will rest and not be subject to slope failure. For example, sand particles roll past one another, so a slope of sand must be relatively flat. Clay soil can be cut more vertically. The angle of repose for any soil will vary with the texture, water content, and vegetative cover.

Typical angles of repose are as follows:

• Non-compacted clay: 5 percent to 20 percent
• Dry sand: 33 percent
• Loam: 35 percent to 40 percent
• Boulders and cobbles: 35 percent to 40 percent
• Compacted sand with forest cover: 35 percent to 50 percent
• Compacted clay: 50 percent to 80 percent
• Bedrock: 70 percent to 90 percent

Source: Environmental Analysis for Land Use and Site Planning, William M. Marsh

Maintenance is another critical consideration. Conventional mowers cannot easily maintain turf grasses if the slope is greater than 25 percent or 4:1. Irrigation for slope plantings should encourage deep rooting - best achieved with infrequent, deep watering. However, on slopes the deep watering must be achieved through a series of several light applications of irrigation water, instead of one continuous application that would cause run off and erosion. Irrigation should provide more water at the upper slope location and less at the base of the slope where water will accumulate.

Plantings on slopes can consist of a mix of grasses (depending on the steepness) or a combination of shrubs and ground covers or even trees. Retaining walls or rock gardens can be good solutions to slope issues. A diversity of plants and root patterns will hold the slope better than a one species planting. Trees should be placed on the sides rather than tops of berms in order to collect more water from runoff. Mulches or netting should be applied over seeded slopes in exposed conditions or with a slope of 33 percent (3:1) or greater, to prevent erosion.

Where berms are used as a buffer to screen views and divert sound or for visual interest, it is important to create large, gently rolling landforms that are compatible with the surrounding landscape. A rigid berm of constant slope and height is not typically harmonious with surrounding natural landforms. All grading should be done to provide safe and visually pleasing forms and to enhance both the on-site and offsite views.

Where possible, retaining existing vegetation will assist with control of erosion by water and wind. Slopes to be planted should have a rough and irregular surface and a convex shape near the top to slow water run-off. The rough surface provides little pockets that collect soil and moisture that in turn benefits plantings. Further, a slope contoured to look natural will visually blend with the native terrain.