Achieving Peak Flow and Sediment Loading Reductions through Increased Water Storage in the Le Sueur Watershed, Minnesota: A Modeling Approach

Abstract

Climate change, land clearing, and artificial drainage have increased the Minnesota River Basin’s stream flows and the rates at which channel banks and bluffs are eroded. Increasing erosion rates have contributed to higher sediment-loading rates, excess turbidity levels, and increases in sedimentation rates in Lake Pepin further downstream. These issues have motivated the discussion of flood management through either wetland restoration or the implementation of simple detention basins. This study uses the Soil and Water Assessment Tool (SWAT) to assess a wide variety of water retention site (WRS) implementation scenarios in the Le Sueur watershed in south-central Minnesota, a subwatershed of the Minnesota River Basin. Projected flows were used in conjunction with an empirical relationship developed from gauging data to assess changes in sediment-loading rates from near-channel features in the lower watershed. The WRS term is used as a general term for depressional storage areas, and sites could be made into wetlands or detention basins. Sites were delineated as topographic depressions with specific land uses, minimum areas (3000 m2), and relatively high compound topographic index (CTI) values. Contributing areas were manually measured for the WRS delineated. These contributing areas were used with existing depression depths, and different site characteristics to create 210 initial WRS scenarios. The contributing areas measured for the initial scenarios were used to create a generalized relationship between WRS area and contributing area. This relationship was used with different design depths, placement scenarios, and K values to create 225 generalized WRS scenarios. Reductions in peak flow volumes and sediment-loading rates are generally maximized by placing sites with high K values in the upper half of the watershed. High K values allow sites to lose more water through seepage, emptying their storages between precipitation events and preventing frequent overflowing. Reductions in peak flow volumes and sediment-loading rates also level off as WRS extent increases. This reduction in cost effectiveness with increasing site extent is due to the decreasing frequencies of high-magnitude events. The generalized WRS scenarios were used to create a simplified empirical model capable of generating peak flows and sediment-loading rates from near-channel features in the lower watershed. This simplified model is being incorporated into a decision-analysis model portraying a wide variety of management options in the Le Sueur watershed. This tool may better enable local stakeholders to evaluate, select, and promote management scenarios that best address the issues faced in the region.