2D Large Scale Automated Engineering for FEMA Floodplain Development in South Dakota. Eli Gruber, PE Brooke Conner, PE

Transcription

1 2D Large Scale Automated Engineering for FEMA Floodplain Development in South Dakota Eli Gruber, PE Brooke Conner, PE

2 Project Acknowledgments FEMA Region 8 Staff: Brooke Conner, PE Casey Zuzak, GISP Ryan Pietramali Terrain Data Sources: USGS National Elevation Dataset South Dakota Department of Environment and Natural Resources May 15,

3 Overview 2D LSAE Defined South Dakota Project Background Technical Approach Calibration and Results Challenges and Key Takeaways *Terminology Note: Large Scale Automated Engineering (LSAE) is now referred to as Large Scale Base Level Engineering (LSBLE) May 15,

4 2D LSAE Conceptual Overview Terrain Rain-on- Grid Hydro 2D Grid Mesh Model Execution Floodplain Mapping May 15,

5 Why 2D LSAE? HEC-RAS 5.0 Data Availability Benefits of approach Accuracy & Efficiency Cost & Time May 15,

6 South Dakota 2D LSAE Application Perform 2D LSAE for 27 eastern South Dakota Counties Increase and enhance the flood risk products in South Dakota Flood damage in Brown County, S.D. in Apr Image: South Dakota Public Broadcasting May 15, Flooding on the James River in S.D. in Mar Image: Mitchell Republic

7 Project Scope Hydrologic and hydraulic modeling 10% through 0.2% annual chance exceedance (ACE) events Mapping FIRM database ready S_FLD_HAZ_AR with Zone A (1%) and Shaded X (0.2%) Scalable products Can be leveraged throughout the Risk MAP program, eventual production of regulatory and non-regulatory products Annual Chance Exceedance H&H Modeling Floodplain Mapping 10% R -- 4% R -- 2% R -- 1%-minus R -- 1%-plus R -- 1% R R 0.20% R R May 15,

8 Eastern South Dakota James River, Vermillion River, Big Sioux River Complex hydraulics May 15,

9 Terrain Data Overview High resolution 1-meter LiDAR available for entire project area (except half of Clark County) Non-model backed Zone A s on 10m DEM 2D HEC-RAS on high-res LiDAR May 15,

10 Model Area Delineation Main considerations for model areas: 1. SIZE: Max model area ~ 1,300 sq mi 2. DRAINAGE: Account for all contributing basin area (Rain-on- Grid + external inflows) 3. DATA: Leverage gage data for inflows and calibration 26 total model areas Basis for Hydrology May 15,

11 Rain-on-Grid Hydrology Applied directly to 2D Mesh No hydrologic losses in HEC-RAS 5.0 Simple HMS model SCS CN Method 24-hour storm NOAA Atlas 14 precip raster NRCS Soils + NLCD = Average CN Excess precipitation May 15,

12 Upstream Basin Contributions Applied at upstream inflow points Three methods: 1. Unit hydrograph approach where gage data available 2. Flow transfer from upstream 2D LSAE model 3. Simple HMS runoff model May 15,

13 Hydrologic Challenges Data availability Variable period of record record extension methods employed Snowmelt Two-station extension Rain-on-grid model not representative Flow regulation POR: 15 years POR: 62 years May 15,

14 Hydraulic Parameters Manning s n NLCD 2011 spatial coverage Boundary Conditions Upstream: Inflow hydrographs Downstream: Normal depth Breaklines May 15,

15 Mesh Refinements Breaklines define topographic features within 2D mesh Flow impediments defined to prevent water from passing across high points Offset breaklines to allow water to pass across unprocessed high points (i.e., structures) No Breakline With Breakline May 15,

16 Computational Options Computational options Diffusion Wave Equation 200-foot cell 30-second timestep Accuracy Run Times Stability May 15,

17 NUMBER OF GAGES Calibration Methods Peak flow targets at USGS gage locations for 1% ACE event Adjustments to Curve Numbers and/or hydrograph timing 38 total calibration points 0 T O 5 % 5 % T O 1 0 % 1 0 % T O 2 0 % 2 0 % T O 3 0 % > 3 0 % PERCENT DIFFERENCE May 15,

18 Calibration Challenges Single CN value Accounting for subbasin variability Period of Record (yr) Gage Analysis Flow (cfs) Modeled Percent Gage Name Flow (cfs) Difference Difference Big Sioux River at North Cliff Ave at Sioux Falls, SD 45 31,930 30,195-1,735-5% Big Sioux River near Dell Rapids, SD 67 30,730 35,133 4,403 14% Flandreau Creek above Flandreau, SD 34 5,939 15,985 10, % Skunk Creek at Sioux Falls, SD 65 18,020 17, % May 15,

19 Other Challenges Model run times Urban flooding Total deliverable ~7 TB! May 15,

20 Model Output Floodplain Mapping May 15,

21 Key Takeaways 2D LSAE methodology provides efficient, accurate option for floodplain hazard identification There are still engineering challenges Process will continue to evolve and improve May 15,

22 Questions May 15, Photo from FEMA

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