Des Moines River Upstream Mitigation Study April 2020 IIHR Technical Report No. 533 Prepared by: Antonio Arenas Daniel Gilles Michael Krasowski Nathan Young Larry Weber Iowa Flood Center | IIHR—Hydroscience & Engineering The University of Iowa C. Maxwell Stanley Hydraulics Laboratory Iowa City, Iowa 52242 ii | Des Moines River Upstream Mitigation Study iii Acknowledgements The Iowa Flood Center and IIHR—Hydroscience & Engineering at the University of Iowa would like to thank Des Moines Public Works Department for providing funding for and supporting this work. iv | Des Moines River Upstream Mitigation Study Table of Contents Acknowledgements ................................................................................................ iv List of Figures ....................................................................................................... vii List of Tables ........................................................................................................... x Introduction ........................................................................................................... 1 1. Literature Review ............................................................................................. 5 a. Hydrologic History of the Des Moines Lobe ............................................................................. 5 b. Impact of climate change on hydrology .................................................................................... 5 c. History of conservation in Iowa ................................................................................................. 7 d. Benefits of conservation practices ............................................................................................. 7 Contour Buffer Strips, Riparian Practices, and Grassed Waterways ....................................... 8 Water and Sediment Control Basins ......................................................................................... 8 Drainage Water Management .................................................................................................... 8 Nutrient Removal Wetlands – Water Quantity ........................................................................ 9 Nutrient Removal Wetlands – Water Quality ........................................................................... 9 Cover Crops .............................................................................................................................. 10 No-Till ....................................................................................................................................... 10 Biodiversity ............................................................................................................................... 10 e. History of Hydrologic modeling .............................................................................................. 10 f. Modern uses of hydrologic modeling to study flooding ........................................................... 11 g. Comparison of modeling approaches ...................................................................................... 12 h. Conservation siting tools ..........................................................................................................13 i. Summary ....................................................................................................................................13 2. Conditions in the Des Moines River Watershed .............................................. 14 a. Introduction ............................................................................................................................. 14 b. Geology and Soils ......................................................................................................................15 c. Topography ............................................................................................................................... 20 d. Land Use/Cover ....................................................................................................................... 21 e. Instrumentation/Data Records ............................................................................................... 23 f. Streamflow Historic Trends ..................................................................................................... 27 g. Floods of Record ...................................................................................................................... 29 h. Agricultural Conservation Planning Framework (ACPF) ....................................................... 32 i. Application of ACPF tools in representative HUC12 watersheds............................................ 33 v 3. Des Moines River Hydrologic Model Development ......................................... 39 a. Attributing the Model .............................................................................................................. 40 Surface ...................................................................................................................................... 40 Subsurface ................................................................................................................................ 41 Forcing Data and Computational Mesh .................................................................................. 41 b. Model Performance Evaluation ............................................................................................... 41 Monthly Water Cycle ................................................................................................................ 44 Annual Maximum Peak Discharge .......................................................................................... 45 Tile Drainage ............................................................................................................................ 46 4. Analysis of Watershed Scenarios .................................................................... 49 a. Increased Precipitation and Index Points ............................................................................... 49 b. Simulations with observed precipitation. 2002-2018. ........................................................... 52 Mitigating the Effects of High Runoff with Increased Infiltration ......................................... 52 Mitigating the Effects of High Runoff with Native Vegetation ............................................. 53 Mitigating the Effects of High Runoff with Cover Crops/Soil Health/No-Till .................... 56 Mitigating the Effects of High Runoff with Distributed Storage ............................................ 59 Mitigating the Effects of High Runoff with Distributed Storage and Increased Infiltration 62 Saylorville Emergency Spillway ............................................................................................... 64 c. Simulations with increased precipitation (IP) ........................................................................ 65 5. Summary and Conclusions ............................................................................. 67 a. Des Moines River Watershed Water Cycle and Conditions .................................................... 67 b. Des Moines River Watershed Hydrologic Model .................................................................... 67 c. Watershed Scenarios for the Des Moines River Watershed ................................................... 67 Appendix A – Policy and Legal Analysis ................................................................ 72 Appendix B – References ...................................................................................... 87 vi | Des Moines River Upstream Mitigation Study List of Figures Figure 1. The extent of the flooding during the 1993 and 2008 floods (Bradley, 2010). ................ 1 Figure 2. Number of flood-related federally declared disasters in Iowa counties (1988–2016). Data source: https://www.fema.gov/. ....................................................................................................... 3 Figure 3. The number of flood-related federally declared disasters in Iowa (1988–2016). Data source: https://www.fema.gov/. ....................................................................................................... 3 Figure 2.1. The Des Moines River Watershed. The Des Moines River joins the South Raccoon River near Van Meter, Iowa. ..................................................................................................................... 14 Figure 2.2. General soil and geologic characteristics within the Des Moines River Watershed. .. 15 Figure 2.3. Typical Des Moines Lobe cross-section (Prior 1991). .................................................. 16 Figure 2.4. Depth to bedrock in the Des Moines River Watershed. ...............................................17 Figure 2.5. Distribution of Hydrologic Soil Groups in the Des Moines River Watershed. Hydrologic Soil Groups reflect the degree of runoff potential a particular soil has, with Type A representing the lowest runoff potential and Type D representing the highest runoff potential. 19 Figure 2.6. Topography of the Des Moines River Watershed. ....................................................... 20 Figure 2.7. Terrain slopes derived from LiDAR data. .................................................................... 21 Figure 2.8. Land use composition in the Des Moines River Watershed, according to the 2018 Crop Data Layer provided by USDA’s National