Slash Mulch Berms in Sediment Barrier Applications

An under-utilized sediment barrier practice that does not require any synthetic materials and is composed entirely of natural material that is often a by-product of land-clearing is slash mulch berms. Slash mulch berms are composed of chipped and shredded woody material and can be left on-site after the completion of a project. Despite the environmental benefits of the practice, little research has been conducted on the effectiveness of slash mulch berms in sediment barrier applications. The sediment barrier testing apparatus at the Auburn University - Stormwater Research Facility was used to evaluate various slash mulch berm installations under commonly found conditions on construction projects to determine performance and allow for comparison to other sediment barrier practices.

Full Abstract: On construction projects, sediment barriers are typically installed downstream of disturbed areas and on the perimeter of construction projects to remove sediment from runoff; these practices typically rely on sedimentation within impoundments formed behind the practice. Common sediment barrier practices on construction projects include silt fences, wattles, and straw bales, with silt fences being the most prevalent. However, silt fences typically contain plastics that are either landfilled after use or can break down and cause polluted runoff if left on a construction site for excessive time after project completion. One sediment barrier practice that does not have the same risk of pollution and is not required to be landfilled after use is slash mulch berms; slash mulch berms are composed of shredded or chipped woody material that is a byproduct of land clearing operations, especially in areas where burning is not possible, and can be left on site after the completion of a project. Despite the environmental benefits of slash mulch berms, very little research has been conducted on their effectiveness in sediment barrier applications. The objective of this research was to evaluate slash mulch berm installations under large-scale testing conditions at the Auburn University – Stormwater Research Facility (AU-SRF) to determine if slash mulch berms can be effective in sediment barrier applications when compared to other commonly used sediment barrier practices. Various slash mulch berm installations, including the Nebraska Department of Transportation standard installation and modifications, were evaluated using a Modified ASTM D7351 testing apparatus at the AU-SRF under simulated stormwater runoff events that mimic conditions found on highway construction projects. Flow and sediment, in the form of sediment-laden sheet flow, equivalent to a 2-yr, 24-hr storm event for a selected region, runs into a 6.1 m (20 ft) wide slash mulch berm installation. Data collected during testing included impoundment potential, flow-through rates, sediment capture upstream of and within the berms, and water quality improvements for turbidity and total suspended solids. Results of testing of slash mulch berm installations indicate high sediment capture; a standard installation captured 98.1% of introduced sediment. However, the standard installation only facilitated a maximum impoundment depth of 10.2 cm (4 in.). To improve impoundment potential, modified installations that used less total material but relied on compaction were tested and were found to increase impoundment potential to 12.4 cm (5 in.); the increased compaction also increased sediment capture and water quality treatment. Slash mulch berms could capture more sediment and indicate a greater treatment potential for turbidity and total suspended solids than other tested sediment barriers under the same conditions. This research indicates that slash mulch berms can be effective as sediment barrier practices by showing that they can capture more sediment and lower turbidity more effectively without using synthetic materials that must be removed from construction projects or can be harmful when broken down.