Effects of closed roads, traffic, and road decommissioning on infiltration and sediment production: A comparative study using rainfall simulations

Abstract

Road closures and road decommissioning are increasingly being used to reduce runoff and sediment production from unpaved roads, but few studies have quantitatively assessed the effectiveness of these treatments. This study used rainfall simulations to: 1) quantify the differences in infiltration and sediment production among five treatments: undisturbed forest, closed roads, closed roads exposed to all-terrain vehicle (ATV) traffic, and two decommissioning treatments (ripping only, and ripping plus wood-strand mulch); and 2) quantify the effects of key site variables on infiltration and sediment production. Four replicate rainfall simulations were conducted for each treatment in northcentral Colorado, with 44mmh−1 of rainfall being applied to 1m2 bounded plots for 45min. The mean infiltration rate for the last 5min (“infiltration capacity”) for the forest was 28mmh−1 and highly variable, while the closed roads with and without traffic had nearly identical mean values of only 5 and 4mmh−1, respectively. Ripping only increased the mean infiltration capacity to 9mmh−1, while adding mulch more than doubled this to 20mmh−1. Mean sediment production from the forested plots was only 3gm−2 as compared to 43gm−2 from the closed roads with no traffic. Eighty passes of an ATV tripled the mean sediment production compared to the closed roads with no traffic. The mean sediment production for the ripping treatment was 72gm−2 or 67% higher than the mean value from the closed roads, while adding mulch decreased the mean sediment production to just 16gm−2. These results first show the importance of roads and even small amounts of traffic for increasing plot-scale runoff and sediment production, and second that ripping plus mulching is a more effective road decommissioning treatment than just ripping. The results provide important guidance for future road decommissioning efforts.