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

Abstract:

A combination of factors including climate change, land clearing, and artificial drainage have increased many agricultural regions’ stream flows and rates at which channel banks and bluffs are eroded. Increasing erosion rates within the Minnesota River Basin have contributed to higher sediment-loading rates, excess turbidity levels, and increases in sedimentation rates in Lake Pepin further downstream. Water storage sites (e.g., wetlands) have been discussed as a means to address these issues. This study uses the Soil and Water Assessment Tool (SWAT) to assess a range of water retention site (WRS) implementation scenarios in the Le Sueur watershed in south-central Minnesota, a subwatershed of the Minnesota River Basin. Sediment loading from bluffs was assessed through an empirical relationship developed from gauging data. Sites were delineated as topographic depressions with specific land uses, minimum areas (3000 m2), and high compound topographic index values. Contributing areas for the WRS were manually measured and used with different site characteristics to create 210 initial WRS scenarios. A generalized relationship between WRS area and contributing area was identified from measurements, and this relationship was used with different site characteristics (e.g., depth, hydraulic conductivity (K), and placement) to create 225 generalized WRS scenarios. Reductions in peak flow volumes and sediment-loading rates are generally maximized by placing site 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 at high WRS extents due to the decreasing frequencies of high-magnitude events. The generalized WRS scenarios were also 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 help local stakeholders to evaluate, select, and promote management scenarios that best address the issues faced in the region.