Soil Berms

A berm is a level space, shelf, or raised barrier separating two areas. It can serve as a fortification line or a border/separation barrier. The word berm originates in the Middle Dutch and German berme and came into usage in English via French.

A berm is a mound of earth with sloping sides that is located between areas of approximately the same elevation. Berms or mounds may serve one or a combination of the following functions: Add interest to a flat landscape.

Berms are also used to control erosion and sedimentation by reducing the rate of surface runoff. The berms either reduce the velocity of the water, or direct water to areas that are not susceptible to erosion, thereby reducing the adverse effects of running water on exposed topsoil.

For general applications, a berm is a physical, stationary barrier of some kind. For example, in modern highway construction, a berm is a noise barrier constructed of earth, often landscaped, running along a highway to protect adjacent land users from noise pollution.

Soil

Berms of any significant size will require large quantities of soil. Consequently, a berm may be rather expensive to create. However, skimping on size will result in what appears to be just a leftover heap of soil. A berm may consist solely of high-quality topsoil; as a cheaper alternative, only the top foot needs to consist of high-quality topsoil with well-drained soil making up the remainder of the berm. Using gravel in the layers directly underneath the topsoil is not recommended because of the tendency of the soil to wash through the gravel. In some municipalities, ordinances or specification codes require that clay make up the majority of the bottom layer because of its cohesive quality. Still, in many areas, it is possible to use a fill material such as rubble, asphalt, or gravel for the bulk of the berm if the material is capable of retaining stability without deteriorating or eroding and will compact well. The advantage of using a coarser fill over a clay substrate is that it will be much more cost-effective in most cases. The best combination of materials is illustrated in the diagram below. It consists of a bottom layer of fill material (whichever type is more readily available and affordable) followed by an impervious layer of clay at least 1′ 0″ thick, and finally the surface layer of good quality topsoil. The slope and shape of the clay layer should be correct before the topsoil layer is added.

Slope

The angle of repose for the soil used in the berm should determine the maximum slope of the berm with consideration to aesthetics, drainage, and maintenance needs. The angle of repose refers to the steepest grade at which a particular type of soil will remain in place. The slope is another term commonly used when referring to elevation changes. It is defined as the vertical distance over the horizontal distance of an elevation, rise over run. If a berm is to be mowed, the slope probably should be less than the maximum possible steepness for that berm. Moreover, berms with steep slopes are more difficult to mow without “scalping”. Scalping occurs when the mower blade strikes the ground. For easy and safe mowing, the slope should not exceed a 4:1 ratio of rise over run. This means that for every one-foot vertical drop in elevation four feet of horizontal distance must be covered. Other sources, however, recommend a more gradual slope in the range of 5:1.

As a natural element in the landscape, berms should be visually compatible with the surrounding environment. One way to help encourage the natural appearance of the berm in the landscape is to vary the slopes used within a berm by applying gradual transitions in elevation. Trees should be planted on a shallower slope of 5:1 to 7:1. Other types of plantings may tolerate the gradient (slope) of 3:1, but water will run off steeper slopes more quickly. As a result, the water may not be absorbed in sufficient quantities to support plants with slopes steeper than 3:1. Although mulch may help to slow down the water on a 3:1 gradient, a 4:1 to 5.1 gradient is preferable. A 5:1 ratio may still encourage wood chips to wash down the berm. On slopes, shredded or elongated wood chips are recommended, as they will slide less easily than the round pieces of wood mulch. If rock mulch is used, irregular, elongated rock will not move as easily as washed or rounded rock and is therefore recommended.

Drainage

A designer should consider drainage in the entire area around the proposed berm. Construction of a berm may affect drainage patterns of surface water. By acting as a dam or by redirecting runoff to other areas, a berm may encourage undesirable ponding and flooding of surface water runoff, particularly after a rainstorm. A berm may also affect the normal flow of surface water across a landscape, which in turn, could lead to changes in the groundwater conditions of the site. The key to incorporating a functional berm into the landscape is to alter the existing drainage as little as possible. The only time you want to alter drainage patterns of surface water flow or runoff is when that alteration will positively affect the site. In areas where it is impossible to avoid damming surface water runoff, installation of a culvert through the berm is a hopeful remedy. In most cases where extensive regrading of a site is necessary, it is wise to hire a qualified professional, such as a civil engineer or landscape architect, to prepare a grading plan. They will be able to evaluate the site and make sure that underground utilities, surface and groundwater flow, and mature trees are not adversely affected by the desired grading changes.

Supplies

Soil, mulch, and drainage supplies for residential uses are available at most nurseries and garden centers. Fill can be obtained from many larger nurseries. Various fill materials can also be obtained from construction and mining refuse sources such as road repair sites, mining spoils, and rubble from building demolition sites for much lower costs.

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