A Defensible Approach to Flood Mitigation, Stream Restoration and Water Quality Kevin Corwin, PE, PLS, PMP Todd Williams, PE
City of St. Charles, MO River town Mississippi River Population of 68K 23 Sq Miles 9 Major Watersheds
City of St. Charles, MO Flooding Issues Stream Degradation MDNR WQ Permit up for Renewal
Comprehensive Storm Water Management Plan (SWMP) 9 Watersheds Mississippi Flooding Mitigation Stream Stabilization Sandfort Cole Boschert Missouri Water Quality Analysis Taylor Branch Webster Blanchette Crystal Springs
SWMP Challenges 1. Project Identification Dealing with Requests for Micro Fixes Processing Massive Amounts of Data 2. Flood Mitigation vs Stream Restoration Is one more important? How do you prioritize? 3. Identifying the Appropriate Level of Spending Which Projects are Worth Building? Maximize Return on Investment
SWMP Progressive Approach 1. City Treated Both Public and Private Stormwater Issues as Public Issues 2. Mitigate, not Necessarily Eliminate, Problems 3. Data Driven, Managed Risk Approach 4. Frequent Meetings/ Presentations (buy in) 5. Pilot Program
Summary of Storm Water System: 9 Major Watersheds 7051 Drainage Areas (6000+ Street Inlets) 2216 Overland Flow Paths 100+ Culverts/Bridges 111 Detention Basins 560 Natural Stream Reaches (33.2 Miles) 10000+ Pipes
Determine the Root Cause(s) of Flooding Converted USACE HEC-RAS to XP-SWMM Created XP-SWMM of Enclosed System 7 Models (9700 nodes, 12500 links) Modeled 2,5,10,15, 25,50 100 year events Models Calibrated to Historical Flooding
Modeling Revealed Root Cause(s) of Flooding: Lack of Inlet Capacity Undersized Piping Undersized Bridges/Culverts Structures Built in the Floodplain Combination of the Above
Flood Mitigation Potential Project Identification (50 Year Lifespan) Bridge/Culvert Replacement Regional Detention Buyouts Improve Enclosed System Flood Proofing Hundreds of Potential Solutions - Which Are the Right Ones?
Return Period Damage ($$) HEC-FDA type Analysis 100 Year Return Period Depth (ft) FFE D 10 Yr =$$ D 15 Yr = $$ D 25 Yr = $$ D 50 Yr = $$ D 100 Yr = $$ 15 Year 2 Year 25 Year 5 Year 50 Year 10 Year
Data Processing - Custom GIS Applications Model Results Processing Tools
Identify Projects to Reduce Damage Regional Detention Basin - $300K Flood Proofing- $50K House (x6) 5 Year - $0K Damage 2 Year - $0K Damage Widen Stream - $750K Widen Culvert - $250K
Benefits Calculation for 50 Year Project ($1,000) Return Period (Yrs) Damage Before Damage Before Damage After Damage After Project (Per House) Project for 6 Houses Project (Per House) Project for 6 Houses 2 $0 $0 $0 $0 5 $0 $0 $0 $0 10 $10 $60 $0 $0 15 $12 $72 $0 $0 25 $20 $120 $10 $60 50 $25 $150 $15 $90 100 $50 $300 $25 $150 TOTAL $702 $300 Damage Reduction (Benefit) Project Cost B/C Ratio $402 $250 1.6 Used Custom GIS App to Process Flood Mitigation Data for Thousands of Properties
90 Flood Mitigation Projects Were Identified
Identified Funding Level BENEFIT COST RATIO COST - ($1,000) Flood Reduction Benefit to Cost Analysis 12.00 10.00 B/C for Project Cumulative Benefit Costs $50,000 $45,000 $40,000 8.00 B/C Curve Fitted Through Data Point Where Cum. Benefit Costs = Cum. Project Costs $35,000 $30,000 6.00 $25,000 4.00 2.00 Cumulative Project Costs $20,000 $15,000 $10,000 $5,000 0.00 0 2 4 6 8 10 12 14 16 18 20 PROJECT RANK (HIGHEST TO LOWEST BENEFIT TO COST RATIO) $-
Identified Funding Level Data Driven, Comprehensive Analysis Projects Designed to Provide Regional Benefits Identified Logical and Defensible Investment Point
Assess the Extent of Stream Degradation Inspected Over 32 Miles of Stream Rapid Geomorphic Assessment Aggradation Degradation Widening Planform Adjustment
Damage Assessment Forecast Streams Broken into Reaches Defined Stream Stability Index Forecasted Future Stream Configuration Established Risk Zones Loss of Buildings/Property/Utilities Customized GIS Tool to Process Damage Data
Stream Erosion Risk Zones LOW RISK ZONE
Benefit Cost Calculations Prevented Loss = Benefit $$ Cost of Project REACH = Cost $$ Determine B/C Ratio
250 Stream Project Reaches Identified
Identified Funding Level
Summary of Financial Findings Flooding 21 Flood Reduction Projects Project Cost of $54M Flood Damage Prevention of $55M Stream Erosion 11 Stream Projects Identified Project Cost of $9M Erosion Damages Prevention of $9.6M
Water Quality Assessment Operating on the Expired NPDES Phase II Permit No Streams Currently on the 303 (d) List Assessed the Existing Program vs Potential Future WQ Requirements Green Point Rating System Guide to incentivize sustainable development Stream Restoration Projects ARE Water Quality Improvement Projects
Balance Among Competing Interests: Regulatory compliance Business and Development Environmental responsibility Fiscal responsibility
Identify WQ Hot Spots Analysis Custom GIS Tool Based on EPA Guidance Estimates Annual Pollutant Loads for 10 Contaminants
Water Quality Recommendations Closely Monitor Regulatory Requirements Continue to Promote the Existing WQ Program Revitalize the Green Point Rating System Recognize that Biostabilization Projects ARE Water Quality Improvement Projects Identified 13 Flood Mitigation and Stream Stabilization Projects with WQ Benefits
Traditional Private WQ Approach 30 Watershed Public WQ Approach City adopts WQ Code City Permitting Development builds compliant BMPs Private Association takes over BMPs BMPs degrade over time City monitors and enforces private BMPs Private Association Resistance/Compliance
Traditional Private WQ Approach 31 Watershed Public WQ Approach City adopts WQ Code City integrates Watershed BMPs in Stormwater projects City permitting Development purchases credits or partners on Watershed BMPs City acceptance and O&M of BMPs Stormwater EPA-WQ revenue collected Watershed BMPs degrade City O&M of BMPs
32 Private WQ vs. Public WQ Costs The same work NEEDS to be done Only Difference? Individuals/Businesses pay annual dues to Association vs. Individuals/Businesses pay taxes and fees
CHALLENGE SOLUTION Project Identification Dealing with Requests for Micro Fixes Processing Massive Amounts of Data Need for Effective Solutions Comprehensive and Objective Economic Analysis Created Regional Solutions That Benefit the Community as a Whole. Custom GIS Tools Were Developed to Process Data and Manage Complex Analysis. GIS Mapping Used for Presentations. Data Driven Analysis to Identify Root Causes and Analyze Multiple Potential Solutions.
CHALLENGE SOLUTION Flood Mitigation vs Stream Restoration Is one more important? Economic Analysis Allows Objective Comparison. How do you prioritize? Resulting Benefit/Cost Ratios for Both Flood Mitigation and Stream Stabilization Allows Integration into a Single CIP.
CHALLENGE SOLUTION Identifying the Appropriate Level of Spending Which Projects are Worth Building? Maximize Return on Investment Cumulative Cost Meets Cumulative Benefit=Investment Point. Crossover Projects (Flood/Stream/WQ) were given higher priority. Comprehensive, Defensible and Complex Analysis, Objective Comparisons, B/C Ratios, Identify Appropriate Investment Point. Results: a Clear Path Forward for Stormwater Management
St. Charles Proposition P
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