Minnesota agricultural ditch reach assessment for stability (Madras): A decision support tool

Selected ditches in Minnesota and throughout the upper Midwestern USA have become morphologically unstable via geotechnical failure, channel enlargement and/or aggradation. Most ditches adjust channel form over time; some remain stable whereas other ditches unravel and require thousands of dollars worth of maintenance. Unstable ditch channels in Minnesota have also resulted in loss of biotic habitat and excessive sediment transport to downstream water bodies resulting in an impaired waters designation under the Clean Water Act, Section 303(d). There are climatic, geologic and land use reasons why ditch channels become unstable over time. We provide an assessment tool for evaluating channel and bank processes occurring within a given ditch reach. The tool systematically considers factors driving ditch channel instability and offers potential remediation actions related to nutrient attenuation. MADRAS is a relatively rapid assessment tool that considers both channel hydraulics and geotechnical factors associated with channel instability. A ditch reach must be walked by an evaluator to gather field evidence and determine processes such as toe slope erosion, bank seepage, bank angle, vegetation, slumping and the relative in-channel sediment storage and transport. Observations of physical processes and hydrologic pathways are documented and then interpreted to diagnosis the ditch condition. Localized ground water seepage induced slumps require a different solution compared to bank slumping induced by systematic hydrologic changes within a watershed. Ditch reach assessment offers the local drainage authority a means to define and prioritize the nature of ditch channel instability and a framework for guiding the maintenance response to unstable ditches.