Paper reports on models for predicting delivery of sediment to streams

Forest road stream crossings, when constructed with inadequate implementation of best management practices, have potential to deliver sediment to streams. Models have been developed to estimate soil erosion and sediment delivery and can be used by managers to predict erosion-prone areas. Models that accurately predict erosion and sediment delivery also could be used to assess the impacts of forestry best management practices (BMPs) on stream sediment.

However, most models used to predict erosion and sediment delivery in forest settings are modified agriculture models. Thus, there is a need to improve understanding of how well these models predict actual erosion and sediment delivery for forest roads.

Recently, Virginia Tech, WestRock, the U.S. Department of Agriculture, the U.S. Forest Service, and NCASI collaborated to (1) evaluate performance of three erosion models by comparing model simulations to trapped sediment measurements, and (2) assess model utility for identifying stream crossing approaches that may require additional BMPs.

A paper presenting results from the study has been published in the Transactions of the American Society of Agricultural and Biological Engineers. Authors of the paper include A.J. Lang of the North Carolina Forest Service; W. Michael Aust, M. Chad Bolding, and Kevin J. McGuire of Virginia Tech; and Erik B. Schilling of NCASI.

In the paper, the authors identify strengths and limitations of the models and identify opportunities for improvement. The abstract for the paper follows.

“Soil erosion and sediment delivery models have been developed to estimate the inherent complexities of soil erosion, but most models are not specifically modified for forest operation applications. Three erosion models, the Universal Soil Loss Equation for forestry (USLE-Forest), Revised Universal Soil Loss Equation Version 2 (RUSLE2), and Water Erosion Prediction Project (WEPP), were compared to one year of trapped sediment data for 37 forest haul road stream crossings. We assessed model performance from five variations of the three erosion models: USLE-Roadway, USLE-Soil Survey, RUSLE2, WEPP-Default, and WEPP-Modified. Each road approach was categorized into one of four levels of erosion (very low, low, moderate, and high) based on trapped erosion rate data and erosion rates reported in recent peer-reviewed literature. Model performance metrics included: (1) summary statistics and nonparametric analysis, (2) linear relationships, (3) percent agreement within erosion categories and tolerable error ranges, and (4) contingency table metrics. Sediment trap data varied from negligible (<0.1) to hundreds of Mg ha-1 year⁻¹. The soil erosion models evaluated could estimate erosion within 5 Mg ha⁻¹ year-1 for most approaches having erosion rates less than 11.2 Mg ha-1 year⁻¹, while models estimates varied widely for approaches that eroded at rates above 11.2 Mg ha⁻¹ year⁻¹. Kruskal-Wallis nonparametric analyses revealed that only WEPP-Modified estimates were not significantly different from trapped sediment data (p ≥ 0.107). While WEPP-Modified ranked best for most model performance metrics, the time, effort, modeling expertise, and uncertainty associated with model results may discourage the use of WEPP as a forest management tool. WEPP is better suited for researchers and government agencies that have the capability to measure extensive parameter data. Additional sensitivity analysis is needed to expand default parameters for forest roads within the WEPP and USLE models.”  

Reference 

Lang, A.J., W.M. Aust, M.C. Bolding, K.J. McGuire, and E.B. Schilling. 2017. Comparing sediment trap data with erosion models for evaluation of forest haul road stream crossing approaches. Transactions of the American Society of Agricultural and Biological Engineers 60(2):393-408. https://dx.doi.org/10.13031/trans.11859