Two-stage ditch assessment using the CONCEPTS model

There are approximately 21,000 miles of drainage ditches in Minnesota. Historic and current ditch design utilizes a trapezoidal cross section. Sediment deposition in trapezoidal channels results in a smaller inset channel being formed, termed a two-stage ditch, in which: 1) an inset channel conveys the bankfull flow; 2) a floodplain-like bench above the inset channel accommodates design/flood flows. Formation of two-stage channels has been observed in Minnesota's drainage ditches. Retrofitting existing drainage ditches by over-widening the bench area or applying the two-stage concept to new ditches is expected to reduce maintenance by decreasing cleanout frequency and improve water quality and ecology. Two-stage ditches constructed in retrofit projects in Ohio have shown through limited monitoring that channel widths have not increased and floodplain bench areas have not experienced notable deposition, which are indicators of channel stability. However, channel stability must be evaluated on a long-term basis. The COnservational Channel Evolution and Pollutant Transport System (CONCEPTS) model was used to evaluate long-term channel stability and sediment transport characteristics of a 1.9 km-long ditch segment recently retrofitted with a two-stage design. The study area is in southern Mower Co., MN, one mile north of the MN/IA border. A long-term hydrograph was generated using the SWMM model, using 29 years of 15-minute precipitation data from nearby Spring Valley, MN. Results from the CONCEPTS model show that the model appears to be capable of simulating channel morphology. Site specific soil characterization is expected to improve model results.