Modeling Soil Erosion from Insloping Forest Roads with Impoundment or Surface Cross Drain Structures

Soil erosion is one of the key concerns in forest resource management. Human activities often aggravate sediment production and transport, leading to significantly elevated sediment levels in forest streams and adversely impacting stream water quality, channel stability, and aquatic habitat. Presently, the health and viability of fish stocks in the northwest US is becoming a critical issue for the government and the general public. Improvement of our understanding of forest soil erosion is of pressing importance. The main purpose of this study is to evaluate forest road erosion processes using a modeling approach. A refined version of WEPP (Water Erosion Prediction Project), a physically-based, distributed-parameter erosion prediction model, was chosen as the foundation for the modeling effort. A segment of an insloping forest road with an impoundment or surface cross drain structure, together with the roadside ditch channel and a waterway channel below the drainage structure, was conceptualized and modeled as a small watershed. WEPP was applied to simulate gross sediment yield within the watershed and sediment delivery at its outlet. Different road system configurations with respect to the density of the drainage structures along a road and downslope road gradient were examined under climate and soil conditions for a representative forest watershed in Idaho State. Soil erosion and delivery ratios resulting from the two road drainage system designs were compared. In addition, cost analyses based on the standard information about material and labor cost were performed to identify road drainage structure designs that can lead to minimized soil erosion and sediment delivery, while proving economically justifiable. Results from this study show that WEPP is a useful tool in predicting water erosion from insloping forest roads with impoundment or cross drain structures as well as in helping establish optimum road drainage system designs.