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Salt Creek Floodplain Resiliency Study
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Final Salt Creek Floodplain Resiliency Study Recommendations Report
August 15, 2020
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Salt Creek Floodplain Resiliency Study
(5.4 M PDF)
Cover and Table of Contents
Executive Summary
Introduction
Section 1 – National Floodplain Best Management Practices (BMP) Review
1.1 Federal Emergency Management Agency (FEMA) Levee Policy/Guidance
1.2 National Oceanic and Atmospheric Administration
1.3 Pew Charitable Trusts’ Flood-prepared Communities Project
1.4 FEMA’s Resilient Nation Partnership Network
1.5 Technical Mapping Advisory Council
1.6 National Academy of Sciences
Section 2 – Community Best Management Practices Review
2.1 Current Review of Practices Across the U.S.
2.2 Beatrice, Nebraska
2.3 Boulder County, Colorado
2.4 Cedar Falls, Iowa
2.5 City of Fort Collins, Colorado
2.6 Mecklenburg County, North Carolina
2.7 Papillion, Nebraska
2.8 Platte County, Missouri
2.9 Shawnee, Kansas
Section 3 – Floodplain Practice Summary for Lincoln and the Salt Creek Watershed
3.1 Review of Current Practices
3.2 Summary
Section 4 – Summary of National and Local Flood History
4.1 Review of Past Studies
4.2 Summary
Section 5 – Local Climate Evaluations and Resiliency Standards
5.1 Historical Precipitation and Existing Conditions Hazards
5.1.1 Updated Conditions Precipitations Estimates
5.1.2 Updated Conditions Discharge Estimates
5.2 Probable Future Storm Magnitudes
5.2.1 Ratio of Future Precipitation to Historic Precipitation
5.3 Flood Hazards in the Year 2100
5.3.1 Flood Increases Because of Land Use Changes
5.3.2 Flood Increases Caused by Climate Change
5.4 Uncertainty in Future Flood Hazard Projections
5.5 Summary - Future Flood Resiliency
Section 6 – Potential Flood Risk Reduction Measures
6.1 Non-structural Flood Risk Reduction Measures
6.1.1 Cluster Subdivision
6.1.2 Overlay Zoning
6.1.3 Voluntary Buyout Program
6.1.4 Setbacks and Riparian Preservation
6.1.5 Low-Impact Development Regulations
6.1.6 Higher Floodplain Management Standards
6.1.7 Summary of Non-structural Flood Risk Measures
6.2 Structural Flood Risk Reduction Measures
6.2.1 Existing Hydraulic Model
6.2.2 Existing Conditions Flows
6.2.3 Future Conditions Flows
6.2.4 Existing, Updated, and Future Impacts
6.2.5 Current Flood Management Measures
6.2.6 Potential Flood Management Measures
6.2.7 Results with Structural Flood Management Measures
6.3 Summary - Future Flood Resiliency
Section 7 - Funding Source Analysis
Section 8 - Recommendations
List of Figures
Figure 1. City of Shawnee Floodplain Map
Figure 2. Website from Watershed Management – City of Lincoln, Nebraska
Figure 3. No Net Rise
Figure 4. Compensatory Storage
Figure 5. Minimum Flood Corridor
Figure 6. Wilderness Park – An Example of Open Space Preservation
Figure 7. Branched Oak Lake – One of 10 Large Flood Management Dams Built by the U.S. Army Corps of Engineers (USACE)
Figure 8. Salt Creek Flood Risk Reduction Project
Figure 9. Union Plaza
Figure 10. Deadmans Run Flood Risk Reduction Project
Figure 11. Salt Creek Subbasins
Figure 12. Atmospheric Model Schematic (image source: National Oceanic and Atmospheric Administration [NOAA])
Figure 13. Greenhouse Gas Concentrations of Representative Concentration Pathways (RCPs) (image source: Intergovernmental Panel on Climate Change [IPCC])
Figure 14. Ratio of 1 percent Annual Chance (2100) Future Conditions to Updated Conditions Peak Precipitation by Subbasin
Figure 15. City of Lincoln Growth Tiers
Figure 16. Salt Creek HEC-RAS Model - Reaches and Cross-Sections
Figure 17. Existing Conditions (TP40), Updated Conditions (NOAA Atlas 14), and Future Conditions 1 percent Annual Chance Flood Profiles Without Structural Flood Management Measures
Figure 18. Existing Conditions (TP40), Updated Conditions (NOAA Atlas 14), and Future Conditions 0.2 percent Annual Chance Flood Profiles Without Structural Flood Management Measures
Figure 19. Contributing Drainage Areas Managed by the Modeled Dams
Figure 20. Existing (TP40), Updated (NOAA Atlas 14), and Future 1 percent Annual Chance Flood Profiles with Structural Flood Management Measures
Figure 21. Existing (TP40), Existing (TP40), Updated (NOAA Atlas 14), and Future 0.2-percent Annual Chance Flood Profiles With Structural Flood Management Measures
List of Tables
Table 1. Comparison of Corresponding Precipitation Values from the City of Lincoln’s Drainage Criteria Manual and NOAA Atlas 14 for Five Annual Chance Events
Table 2. Organizations and Topics Surveyed as National Best Management Practices
Table 3. Evaluation of Best Practice/Guidance for Providing Outreach to Community Members Impacted by the Salt Creek Levees
Table 4. Evaluation of Best Practice/Guidance for Practicing Effective Levee Operation and Maintenance
Table 5. Evaluation of Best Practice/Guidance for Preserving Open Spaces or Converting Developed Areas to Green Spaces
Table 6. Evaluation of Best Practice/Guidance for Considering Participation in a National Network to Improve Preparedness and Response to Extreme Weather Events
Table 7. Evaluation of Best Practice/Guidance for Establishing a Culture of Preparedness and a Well-informed Public
Table 8. Evaluation of Best Practice/Guidance for Leveraging the Resources of the Floodprepared Communities Project
Table 9. Evaluation of Best Practice/Guidance for Leveraging the Flood-ready Infrastructure Statement of Principles
Table 10. Evaluation of Best Practice/Guidance to Consider a Network for Resilience
Table 11. Evaluation of Best Practice/Guidance for Mapping the Residual Risk Behind Levees and Other Flood Management Structures
Table 12. Evaluation of Best Practice/Guidance for Providing Future Conditions Flood Risk Products and Information
Table 13. Evaluation of Best Practice/Guidance for Moving Away From the 1-percent Annual Chance Flood Event
Table 14. Evaluation of Best Practice/Guidance for Investing in Interventions to Mitigate the Impacts of Flooding to Socially Vulnerable Populations
Table 15. Evaluation of Best Practice/Guidance for Considering Innovative Solutions in Identifying and Communicating Urban Flood Risk
Table 16. Best Management Practices Reviewed Listed by Community
Table 17. Flood-prone Property Acquisitions Evaluation
Table 18. Cumulative Substantial Improvement Calculation and Tracking Evaluation
Table 19. Approximate Water Surface Elevation Difference Between the 1-percent and 0.2-percent Annual Chance Floods for Different Flooding Sources in the Salt Creek Basin
Table 20. Higher Floodplain Management Standards: 0.2-percent Annual Chance Regulation Best Management Practice
Table 21. New Lot Prohibition Evaluation
Table 22. Community Outreach and Education Evaluation
Table 23. Low-impact Development (LID) Evaluation
Table 24. Public Education and Outreach Evaluation
Table 25. Floodplain Buyout Program and Floodplain Restoration Evaluation
Table 26. Floodplain Buyout Program Evaluation
Table 27. Stormwater Grant Program Evaluation
Table 28. Future Floodplains (Urbanized Conditions) Evaluation
Table 29. Freeboard Evaluation
Table 30. Setback and Riparian Preservation Evaluation
Table 31. Subbasin HEC-HMS Model Summary
Table 32. Comparison of Existing Conditions Precipitation Estimates to Atlas 14 Precipitation Estimates
Table 33. Comparison of Existing Conditions Discharge Estimates to Updated Conditions Discharge Estimates
Table 34. Percentage of Change in Discharge Between Existing Conditions and Updated Conditions Models
Table 35. Comparison of Updated Conditions Flood Hazards (Discharge and Water Surface Elevation) to Existing Conditions Flood Hazards (Discharge and Water Surface Elevation)
Table 36. RCP4.5 Modeled Ratios of Future to Present-day Daily Precipitation
Table 37. RCP8.5 Modeled Ratios of Future to Present-day Daily Precipitation
Table 38. Comparison of Updated Conditions and Future Conditions Precipitation Frequency Estimates
Table 39. Comparison of Corresponding 24-hour Point Precipitation Values from Different Sources
Table 40. Projected Increase in Flood Discharges Caused by Projected Development (Modeled)
Table 41. Projected Increase in Flood Discharges Caused by Projected Development (Extrapolated)
Table 42. 1 percent Annual Chance Existing, Updated, and Future Conditions Discharges by Subbasin (Median General Circulation Model [GCM] and RCP8.5)
Table 43. Comparison of Future Conditions Flood Hazards (Discharge and Water Surface Elevation) to Existing Conditions Flood Hazards (Discharge and Water Surface Elevation) for Median GCM and RCP8.5
Table 44. Low, Median, and High Future Conditions Flood Hazards on Salt Creek Compared to Existing Conditions Flood Hazards (RCP8.5)
Table 45. 420 Open Space Preservation Points Schedule (Applicable Categories)
Table 46. Potential Applicable Community Rating System (CRS) Activities for Overlay Zoning
Table 47. Potential Applicable CRS Activities for Setback and Riparian Preservation
Table 48. Salt Creek HEC-RAS Model Summary
Table 49. Percentage of Change in Discharge Between Existing and Future Conditions (Median GCM and RCP 8.5)
Table 50. Details from Conceptual Design of Seven Dams
Table 51. Details From All 16 Conceptually Designed Dams and Manually Estimated Dams
List of Appendices
Appendix A. A Matrix of Community Best Management Practices
Table A1. A Matrix of Community Best Management Practices
Appendix B. Rainfall/Frequency Table by Subbasin
Table B1. The following table details the precipitation depths used in each subbasin with available HEC-HMS models
Appendix C. Future Conditions Land Use Loss Parameters
SEUSC
Middle
Oak
South Salt
Antelope
Little Salt
Cardwell
Appendix D. Relationship Between Percent of Subbasin to be Developed in the Future vs. Increase in Discharge
Figure D1. Percentage of Subbasin to be Developed in Future vs. Increase in Discharge
Appendix E. Relationship Between Ratio of Future to Updated Precipitation vs. Ratio of Future to Updated Discharge
Figure E1. Ratio of Future to Updated Precipitation vs. Ratio of Future to Updated Discharge
Appendix F. Description of the Manual Method Used to Estimate Dam Costs
Figure F1. Preliminary Opinion of Probable Cost vs. Contributing Drainage
Appendix G. Methodology Used for Adjusting Input Hydrographs to the HEC-RAS Model
Table G1. Comparison of Contributing Drainage Areas and Flood Reduction Factors for Dams Analyzed – 1 Percent Annual Chance Flood Event
Table G2. Comparison of Contributing Drainage Areas and Hydrograph Coefficients for Dams Analyzed – 0.2 Percent Annual Chance Flood Events
References
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