Using 1D/2D Modeling to Analyze Stormwater Flooding Issues in La Crosse

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1 Using 1D/2D Modeling to Analyze Stormwater Flooding Issues in La Crosse Michael Schwar, Ph.D. PE CFM (MARS) Bernard Lenz, PE (City of La Crosse)

2 Using 1D/2D Modeling to Analyze Stormwater Flooding Issues in La Crosse Stormwater issues in the City of La Crosse Intended study outcomes Selected approach Results Prioritization Where we go from here

3 Stormwater Issues in the City Topography of La Crosse State Historical Society of WI Steep coulees draining to flat floodplain terrace

4 Stormwater Issues in the City History of La Crosse Newer development contributes flow to older systems closer to the Mississippi River Library of Congress

5 Stormwater Issues in the City of La Crosse Complex, interconnected storm systems developed over time

6 Stormwater Issues in the City of La Crosse Overflows from system to system Contributing areas often change over the course of events

7 Stormwater Issues in the City of La Crosse Sandy, well-infiltrating soils and subsoils Implementing green infrastructure (GI) approaches through stormwater utility

8 Intended Study Outcomes Incorporate overland flows into stormwater system assessments Understand limitations of existing drainage systems (pipes, pumps, overland flow) Identify potential GI benefits, support implementation Prioritize projects and develop an objective list of next steps Hydraulic models that are usable/ modifiable for design

9 Selected Approach FLO-2D/EPA-SWMM combination

10 Selected Approach 2-D surface flow modeling using digital elevation data provided by the City at a 15-ft grid size roads represented within DEM, buildings added as a separate layer

11 Selected Approach FLO-2D flow equations solved quasi-2d due to 1-D solutions in eight directions at each grid cell

12 Selected Approach FLO-2D Designed for alluvial fans changing directions, flat topography Very stable in our experience Public domain EPA-SWMM storm system hydraulics module

13 Products City-wide analysis (large scale model) Five watershed plans (detailed models) Summary report 10-year project plan/ list Five executable models capable of use for final design

14 Products Evaluated 4-6 alternative projects per watershed + three levels of GI implementation

15 Products Assessment of GI Effectiveness to Reduce Flood Impacts by Watershed Modify CN to represent percentage infiltration of runoff from impervious surfaces

16 Results Basin maps, compare to observations

17 Results - Movies Large-Scale Project Scale

18 Results Flow Vectors

19 Results Data output Numbers of properties Street closures Duration of intersection closures

20 Results Effectiveness of GI

21 Prioritization Ratings Objects Public Property Construction Impacts Alternative Maximum Points A B C Private Property Construction Impacts Street Flooding Benefits Private Property Flooding Benefits Intersection Closure Benefits Coordination with other CIP Projects Sustainability Rating Total Used model output to quantify flooding benefits Construction Cost ($100,000) Cost Effectiveness Coordinate with reconstruction of aging streets and pipes

22 Prioritization - Example Maintaining passage at intersections is a significant public safety concern during flood events. Using City performance standards, intersection closure is assumed to occur when flood levels exceed 0.5 feet. The proposed scale was developed to rate the benefits from reduced intersection closure during the 25-year recurrence event: 5 = Intersection not closed 4 = Intersection closure reduced by 60+ minutes 3 = Intersection closure reduced by minutes 2 = Intersection closure reduced by minutes 1 = Intersection closure reduced by 1-15 minutes 0 = No intersection closure prior to the project or no change in duration

23 Where Do We Go From Here? Finalize Watershed Plans Develop List of Projects Prioritized by Effectiveness Discuss Results with City Council in December Incorporate into Capital Budgeting Model Training for City Staff

24 Where Do We Go From Here? Develop recommendations regarding targeting and monitoring GI implementation Prioritization scheme yet to be finalized More detailed modeling will be required for final design, especially with regard to local inlets

25 Questions?