Evaluation of Problems and Solutions relating to Stormwater Runoff from Roadside Ditches

Abstract

Ditches along roads in rural areas are a dominant conveyor of stormwater to streams along the north shore. Loss of forest cover as well as increased rural development can increase runoff amounts, but ditches provide the structure that moves this water quickly to nearby stream channels (Forman and Alexander 1998, Wemple et al. 1996). Ditches capture overland flow from nearby forests, wetlands, homes, and businesses in rural areas and channel the flows to discharge points, often into streams (e.g., Duke et al. 2006). Much of this runoff would have otherwise infiltrated into the ground, evaporated, drained into wetlands, or flowed slowly across the landscape until reaching a natural stream channel (Forman and Alexander 1998). The result is more water reaching streams much faster after rainfall events (Trombulak and Frissell 2000). The resulting high stream flows during and after storms can increase the erosion of susceptible clay banks, increase bank failure rates, damage aquatic habitats, and impair water quality (Forman and Alexander 1998, Wemple et al. 1996). Road runoff and excess sediment are then delivered to the sensitive nearshore zone of oligotrophic Lake Superior. Eleven north shore streams are currently on the state’s impaired waters list for turbidity (http://www.pca.state.mn.us/water/tmdl/tmdl-303dlist.html), and evaluations on both the Knife and Poplar rivers have indicated that increased flows and bank erosion are major contributing factors (Brady and Breneman 2007, 2008). We have also recently seen an increase in the frequency of large rainfall events, consistent with climate change predictions for this region, indicating that the need for runoff control will only increase in importance. Despite their critical role, ditches are seldom the target of active runoff management programs. Governmental officials who are responsible for ditches in the region have not had the resources and techniques available to address this issue comprehensively, even though ditch maintenance and culvert repair often use up large amounts of local government road budgets, and local and state governments (LGUs) are actively seeking solutions to ditch issues. We brought together a cross-section of experts on rural road and ditch issues to identify the role of ditches in protecting water quality, enumerate the most pressing issues and problems with existing ditches, identify ditch stormwater best management practices (BMPs), and prioritize research, technical, and educational needs on ditch runoff management for the North Shore. The outcomes of this project identify the major ditch problems in northeastern Minnesota, identify appropriate BMPs to solve these problems, the obstacles to the installation of these BMPs, and suggest solutions to overcoming these obstacles. We also identify needed research on ditch BMPs and suggest next steps for improving ditches so that they can help protect our streams and our coast.