Unpaved road conservation planning at the catchment scale.

Abstract

In addition to soil losses on hillslopes, unpaved rural roads, especially when poorly designed and maintained, can be a significant contributor to the erosive processes seen at the catchment scale. In areas with deep soils, the solutions primarily focus on channeling excess surface runoff into settling ponds or terraces. However, few studies have addressed runoff control from roads on steep slopes in areas of shallow soil. Modeling hydrological processes at the catchment scale is a useful strategy for choosing the most effective and least costly conservation practices to control surface runoff. This study applies a mathematical model to a monitored catchment in southern Brazil to better understand the effects of conservation practices on unpaved roads and their impact on the hydrological and erosive dynamics of a small rural catchment. We calibrated the LISEM model using data from eight stormwater events and evaluated how three different road conservation scenarios-low (LI), medium (MI), and high intensity (HI)-contributed to sediment yield (SY), surface runoff volume (Qe), and peak flow (Qp) reduction. The LI and MI scenarios involved installation of hydraulic structures to control the road surface runoff (i.e. road ditch graveling, diversion weirs and grass waterways) while the HI scenario added surface runoff control practices (grass strips) to surrounding crop fields, in addition to the practices included in the MI scenario. Based on these scenarios, the results showed a Qe reduction at the catchment outlet from - 3.5% (LI) to - 22.5% (HI). The Qp and SY varied from + 6.0% (LI) to - 292.5% (HI) and from + 20.0% (LI) to - 963.9% (HI), respectively. These results show that the low- and medium-intensity practices were not effective in controlling surface runoff from roads, based on the Qe, Qb, and SY observed at the catchment's outlet. On the other hand, when MI scenarios were complemented with practices to control surface runoff in the cultivated areas, a significant reduction in surface runoff (Qe and Qp) and SY was verified.