Berms are raised banks or mounds of earth that are either naturally occurring or intentionally engineered. These structures consist of a durable core material, typically compacted soil, aggregate, or sometimes organic matter like mulch or compost. Historically, they were used in military engineering as defensive earthworks. Today, berms are utilized across diverse fields to manage physical forces like sound, water, and erosion.
Defining Physical Characteristics
The effectiveness of a constructed berm depends on its specific geometry, including its height, length, and the ratio of its side slopes. The core material is often compacted earth, which is rolled to increase density and prevent internal weakness or structural failure. For large-scale applications, berms can range from five to fifty feet in height, with the intended function determining the required size.
Side slopes are important for stability and maintenance, with the ratio expressed as horizontal distance to vertical rise. While a 2:1 slope is a common maximum for stability, gentler slopes of 3.5:1 or flatter are preferred to minimize soil sloughing and erosion. The crest, or top width, is designed for purpose, such as allowing foot access (one meter wide) or vehicle access (three meters). The entire structure is often covered with vegetation, which provides a stabilizing cover against wind and water erosion.
Primary Functional Roles
A common application for engineered berms is mitigating noise pollution, particularly alongside highways and industrial sites. These earthen barriers function by physically interrupting the line of sight between the noise source and the receiver. The dense core of the berm deflects sound waves upward, contrasting with hard, vertical sound walls that can reflect noise across a roadway.
A well-designed berm several feet higher than the visual line of sight can provide noise attenuation, sometimes reducing sound levels by up to 15 dBA. Although studies suggest an earthen berm may be about 2 dBA less effective than a structural wall of the same height, its soft, inclined surface helps absorb sound energy. Berms are also widely used for visual screening, creating a private barrier around commercial operations or residential properties. Height and placement are calculated to ensure a complete visual block between the area being screened and the observer.
Berms in Environmental Management
Berms control water flow and minimize sediment movement in environmental management. In construction and land development, they are utilized to slow stormwater runoff, which reduces the water’s erosive energy. This slowing of velocity allows suspended sediments to settle out before the runoff leaves the site, preventing waterway pollution.
Permeable berms, constructed of materials like compost or wood chips, act as a three-dimensional filter, trapping pollutants such as suspended solids, metals, and grease. These structures can divert clean water away from disturbed areas or direct sediment-laden runoff toward a collection point. For ecological purposes, berms with shallow slopes (e.g., 10:1 or 20:1) create littoral shelves in wetlands, promoting microclimates that support diverse vegetation and wildlife. They also increase the hydraulic residence time in constructed wetlands, improving the system’s efficiency in naturally treating water.