Brad Hansen

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.

Improvements in Fluvial Stability Associated with Two-Stage Ditch Construction in Mower County, Minnesota

Water quality and stream habitat in agricultural watersheds are under greater scrutiny as hydrologic pathways are altered to increase crop production. Agricultural drainage ditches function to remove water quickly from farmed landscapes. Conventional ditch designs lack the form and function of natural stream systems and tend to be unstable and provide inadequate habitat. In October of 2009, 1.89 km of a conventional drainage ditch in Mower County, Minnesota, was converted to an alternative system with a two-stage channel to investigate the improvements in water quality, stability, and habitat. Longitudinal surveys show a 12-fold increase in the pool-riffle formation. Cross-sectional surveys show an average increase in bankfull width of approximately 10% and may be associated to an increased frequency in large storm events. The average increase in bankfull depth was estimated as 18% but is largely influenced by pool formation. Rosgen Stability Analyses show the channel to be highly stable and the banks at a low risk of erosion. The average bankfull recurrence interval was estimated to be approximately 0.30 years. Overall, the two-stage ditch design demonstrates an increase in fluvial stability, creating a more consistent sediment budget, and increasing the frequency of important instream habitat features, making this best management practice a viable option for addressing issues of erosion, sediment imbalance, and poor habitat in agricultural drainage systems. (KEY TERMS: fluvial processes; best management practices; physical stability; drainage ditches; erosion; instream habitat.) Krider, Lori, Joseph Magner, Brad Hansen, Bruce Wilson, Geoffrie Kramer, Joel Peterson, and John Nieber, 2017. Improvements in Fluvial Stability Associated with Two-Stage Ditch Construction in Mower County, Minnesota. Journal of the American Water Resources Association (JAWRA) 53(4): 886-902. https://doi.org/10.1111/ 1752-1688.12541