Hillslope sediment fence catch efficiencies and particle sorting for post‐fire rain storms

Abstract

Sediment fences are often used to monitor hillslope erosion, but these can underestimate sediment yields due to overtopping of runoff and associated sediment. We modified four sediment fences to collect and measure the runoff and sediment that overtopped the fence in addition to the sediment deposited behind the fence. Specific objectives were to: (1) determine the catch efficiency of sediment fences measuring post‐fire hillslope erosion; (2) assess particle sorting of sand, silt/clay, and organic matter from each hillslope through the sediment fence and subsequent runoff collection barrels; (3) evaluate how catch efficiency and particle size sorting relate to site and rainfall‐runoff event characteristics; and (4) use runoff simulations to estimate sediment fence volumes for future post‐fire monitoring. Catch efficiency ranged from 28 to 100% for events and 38 to 94% per site for the entire sampling season, indicating a relatively large underestimation of sediment yields by sediment fences. Most of the eroded sediment had similar proportions of sand and silt/clay as the hillslope soils, but the sediment behind the fence was significantly enriched in sand while the sediment that overtopped the fence was more strongly enriched in silt/clay. The sediment fences had capacities of 3 m3 for hillslopes of 0.19–0.43 ha, but simulations of runoff for 2‐ to 100‐year storms indicate that the sediment fences would need a capacity of up to 240 m3 to store all of the runoff and associated sediment. More accurate measurements of sediment yields with sediment fences require either increasing the storage capacity of the sediment fence(s) to accommodate the expected volume of runoff and sediment, reducing the size of the contributing area, or directly measuring the runoff and sediment that overtop the fence. © 2020 John Wiley & Sons, Ltd.

Connectivity of post‐fire runoff and sediment from nested hillslopes and watersheds

“…In the dynamic post‐fire environment obtaining accurate measurements of runoff and sediment transport is extremely difficult, and the values presented here have fairly high uncertainty. Hillslope runoff measurements are affected by the accuracy of the weir equation and stage readings from cameras, which exclude any water that ponds or infiltrates inside the sediment fence (e.g., Wilson et al, 2020). Streamflow and in‐stream sediment yields are affected by the accuracy of the rating curves.…”

“…In summer 2014 two additional sediment fences and one additional rain gauge were installed within the headwaters of Skin Gulch. These two new sediment fences and two existing sediment fences in the headwaters of Hill Gulch were instrumented to continuously monitor runoff and the amount of sediment that overtopped the sediment fence (Figures 1 and 2; Wilson et al, 2020). Contributing areas for the 31 hillslope monitoring sites ranged from 0.1 to 2.8 ha ( = 0.5 ha) with average slope of 5–33°, and elevations from 1780–2670 m.…”

Effects of forest residue mulching on organic matter and nutrient exports after wildfire in North-Central Portugal

“…They also found that 86 % of P was exported in particulate form. As P is preferentially transported associated to sediment particles (Ferreira et al, 2016a;Serpa et al, 2020), the use of sediment fences, which capture mostly particulate fractions (Robichaud and Brown, 2002;Wilson et al, 2021) may be more effective at retaining P than runoff plots. On the other hand, runoff tanks have a limited storage capacity and other limitations associated to sample homogenization (Bagarello and Ferro, 1998).…”

Wildfire and earth surface processes

“…Consequently, in soils where matrix flow dominates, K fs should be measured with tension infiltrometers or simulated rainfall, whereas in locations where macropores dominate, ponded head methods are preferable (Ebel, 2019). Wilson et al (2020) focused on best practices for measuring sediment production. They show how particle sorting occurs after sediment fences are overtopped and provide practical suggestions to appropriately size sediment fences by modeling runoff.…”