Dirt Moves Downhill: Advances in Equipment and Techniques for Measuring Erosion

Abstract

Advancements in measuring soil erosion at the hillslope scale have allowed for evaluation of erosion control treatments and validation of erosion prediction technologies. These advancements can be applied across multiple situational settings. Relevant examples include measuring the effectiveness of mulch treatments on hillslopes in the post-wildfire environment, evaluating the impact of tillage practices in an agricultural field or determining the benefit of streamside vegetation buffers. Simulated rill experiments utilize concentrated rill flow to measure rill erodibility parameters and treatment effectiveness on steeper slopes. Rill flow is initiated at the top of the plot via the application of concentrated flow. The concentrated flow is applied through an energy dissipater at various inflow rates for 10-12 min durations. Runoff is collected at timed intervals to determine runoff volume and sediment concentrations. The runoff velocity is measured using a dyed calcium chloride solution and two conductivity probes placed a known distance apart. To measure the effectiveness of vegetation buffers or stream buffers at reducing erosion that reaches a stream channel, we applied “dirty” or sediment-laden runoff and measured the amount of reduction (deposition) of sediment through the buffer area. These rill experiments used 4 flow rates which changed every 10 min. The first flow rate was 50 L min^-1 of clean water, the second flow rate included the addition of 25 g L^-1 of sediment to the original flow. The flow was then increased to 100 and 150 L min^-1 for the last two intervals while maintaining the 25 g L^-1 sediment concentration. Locations where sufficient flow can reasonably be sampled were identified along each rill, at approximately the top of the rill, the mid-point, and near the endpoint. The exact locations of the two downslope sample points were determined and adjusted in real-time in response to flow progression. Sediment concentration and flow amounts decreased by distance were then used to calculated buffer lengths or effectiveness. Under natural rainfall and at the hillslope scale (< 0.2 ha), high tensile strength woven geotextile silt fences are an inexpensive method to collect sediment. Silt fences can be used for plots ranging up to 15 m wide and 70 m in length, with a delineated uphill contributing area and with wire mesh reinforcement for catchments up to 5 ha. The fabric is held in place with wooden or steel posts, and the bottom edge is buried or stapled in the ground. These silt fences can be cleaned out by event or periodically. Additional information such as rainfall and ground cover provide the necessary supplementary data to interpret results. At swale scales over 5 ha, hand-built low-cost sediment retention structures are used to measure sediment yield. These retention structures are constructed with standard framing lumber, wood stakes, welded mesh wire, and high tensile strength woven geotextile silt fence fabric. Lumber framed structures spanning the channel widths are secured to the channel bed and banks. The structures have side walls that extend out from the center spillway wall and angle slightly up-channel. The structures' center spillway walls are approximately 1 m in height depending on the channel's shape. Additional instrumentation can be added to allow for calculating runoff rates, though emptying the sediment retention structure to measure sediment yield must be done manually. All of these methods are easily adaptable to diverse environments such as agriculture, rangeland, disturbed lands, and forests. These data can be used to compare erosion control treatments and validate erosion prediction technology.