Zone A Modeling (What Makes A Equal Approximate, Adequate, or Awesome) ASFPM 2016 GRAND RAPIDS CONFERENCE Kevin Donnelly, P.E., GISP, PMP, CFM June 23, 2016
Agenda 1 Introduction 2 Flood Hazard Analysis and Mapping Process 3 Study of Uncertainty 4 Benefit Cost Analysis 5 Conclusions
1 Introduction Great Minds Impact of Flood Hazard Modeling Parameters on Base Flood Elevation (BFE) and Floodplain Top Width (Including Calculation of Return on Investment (ROI) for Selected Input Variables), FEMA, February 29, 2016 Goals 1. Review uncertainty in flood hazard mapping 2. Analyze sensitivity to changes in hydraulic analysis variables 3. Analyze the Return On Investment (ROI) in refining hydraulic variables Summary of Findings plus added insights/thoughts
Credits STARR II Mike DePue (Atkins) Ferrin Affleck (Atkins) Soumya Sagarika (Atkins) FEMA Rick Sacbibit
Zone A Flood Hazard Mapping Approximate, Base Level Special Flood Hazard Area (SFHA) 1% Base Flood Elevations, not mapped flood profiles, floodways
Zone A - Musts Must be cost effective - ~900,000 miles Must be defendable (model backed) Must make choices that impact cost and accuracy
Zone A Choices Terrain Source Modeling Approach Hydrology Hydraulics 1D steady, 1D unsteady, 2D Assumptions & Parameters
2 - Flood Hazard Analysis and Mapping Process Elevation Data Terrain Data Bathymetry Data Hydrology Data Rainfall Runoff Models Flood Frequency Analysis Regression Equations Hydraulic Analysis Contraction/Expansion Friction Loss Cross Section Geometry Flood Hazard Map
Elevation and Terrain Data THE most important factor in determining BFEs and the extent of flooding accuracy of flood hazard maps in riverine areas found by multiple studies. Cook and Merwade (2009) found that the average width and area of inundation for a 30 m DEM are more that 25% higher compared to the that produced by using LIDAR. Solution terrain standards for regulatory mapping.
Hydrology Flood Frequency Analysis gage analysis Regression Equations flow function of drainage area and (maybe) other parameters, derived from gages Rainfall Runoff Models hydrologic simulations Solution 1%- and 1%+ flood profiles Upper and lower bounds of predictive error (84% limit) WSEL changes -3.38 to +1.10 in NC NRC study (2009)
Hydraulics Model choices Type 1D, steady or unsteady 2D Software HEC-RAS, Flo-2D, SWMM, ICPR, MIKE, etc. Parameters N values Expansion/Contraction Coefficients Approach Bridges Bathymetry Solution Study Types? Detailed, Limited Detailed, Approximate, FOA/LSAE
3 Study of Uncertainty Isolate single variable and test impact on 1% elevations and width Test 30 streams - all HEC-RAS 1D steady flow models
Terrain Divide country into 3 equal distributions of slope High Hilly Medium Rolling Low Flat Terrain categories are used to define vertical accuracy requirements per FEMA SID #43 Compute average slope for HUC-12 basins
Terrain
Slope-Based Terrain Map of US
List of Studied Streams Terrain Category Stream Name Length (miles) of structures Discharge (cfs ) FEMA Region Hilly Cedar 3.8 6 2,608 RX Deschutes (D2) 14.5 7 8,568 RX Rogue River 2 11.6 4 31,413 RX Sandy River 7.6 2 39,721 RX Stoneway Wash 0.3 5 259 RIX Washogal River 9.1 5 39,939 RX
List of Studied Streams Terrain Category Stream Name Length (miles) of structures Discharge (cfs ) FEMA Region Rolling American Wash 3.9 9 1,151 RIX Big Cotton Indian Creek 19.5 10 9,923 RIV Camp Creek 2.5 4 2,263 RIV Hazel Creek 6.1 5 2,504 RIV Indian Creek 3.0 20 3,827 RVII Kelly Creek 4.1 16 464 RX Little Butte Creek 9.1 10 11,680 RX Little Mud Creek 10.2 11 3,155 RIV Mint Wash 1.2 1 7,311 RIX Pumphouse Wash 4.5 20 6,963 RIX Reeves Creek 5.2 8 2,941 RIV Camp Creek 2.5 4 2,263 RIV Hazel Creek 6.1 5 2,504 RIV
List of Studied Streams Terrain Category Stream Name Length (miles) of structures Discharge (cfs) FEMA Region Flat 7th Avenue Creek 2.0 13 677 RV 7th Avenue Creek Trib 0.9 11 185 RV Deschutes (D1) 12.1 12 10,891 RX Mid Br Clinton River 8.4 22 1,808 RV rth Fork Teton 5.9 7 3,243 RX Paint Creek 1.8 13 1,714 RV Plumb Brook 2.4 4 152 RV Walnut Creek 4.7 4 29,246 RVII West Branch Walnut River 0.6 2 16,222 RVII West Branch Walnut River Overflow 0.6 3 1,345 RVII Whitewater Creek 6.7 4 74,951 RVII
Testing Approach Isolate single variable and test impact on 1% elevations and width Test 30 streams - all HEC-RAS 1D steady flow models Base models all prepared for Zone AE mapping Surveyed channels Surveyed structures Overbank from LiDAR/2-4 contours Variable Manning s N Values from NLCD or Aerial Imagery
Testing Approach - Parameters 1. Channel Bathymetry Surveyed DEM (LiDAR or 24 contours)
Testing Approach - Parameters 2. Structure Modeling Surveyed Assumed Structures as Weirs Structures
Testing Approach - Parameters 3. Manning s Roughness Horizontal Variation NLCD or Aerial Imagery Bank-Channel-Bank (BCB) NLCD Averaged over each bank and channel One Value Assumed or averaged from NLCD Banks at end of cross sections
Parameter Modifications Scenarios Run. Bathymetry Source Structure Approach Manning s Approach 1 (Base) Surveyed bathymetry Surveyed structures Horizontal Variation 2 DEM terrain only Surveyed structures Horizontal Variation 3 DEM terrain only Assumed structures Horizontal Variation 4 DEM terrain only Structures as weirs Horizontal Variation 5 DEM terrain only structures Horizontal Variation 6 DEM terrain only structures Bank-channel-bank 7 DEM terrain only structures One value 8 DEM terrain only Assumed structures Bank-channel-bank 9 DEM terrain only Assumed structures One value
General Results Summary of BFE Comparison to the Run 1 Terrain Difference (feet WSEL) Run 2 Bath. Surv. Struc. Horiz. Var. Run 3 Bath. Assm. Struc. Horiz. Var. Run 4 Bath. Weir Struc. Horiz. Var. Run 5 Bath. Struc. Horiz. Var. Run 6 Bath. Struc. Bank- Channel -Bank (BCB) Run 7 Bath. Struc. One n Run 8 Bath. Assm. Struc. Bank- Channel- Bank (BCB) Run 9 Bath. Assm. Struc. One n Average 1.4 2.1 1.2 1.3 1.3 4.4 2.2 5.1 Hilly Maximum 10.3 22.5 10.2 10.2 10.0 16.2 22.5 23.0 Std. Dev. 0.7 2.1 0.8 0.8 0.8 3.0 2.1 4.1 Average 0.4 0.7 1.3 1.0 1.2 1.5 0.9 1.3 Rolling Maximum 6.5 7.9 23.2 16.6 16.8 16.7 11.5 11.8 Std. Dev. 0.3 0.7 1.3 0.8 0.8 0.8 0.6 0.8 Average 0.4 0.7 1.0 0.7 0.7 0.9 0.7 0.9 Flat Maximum 4.0 8.3 13.2 13.2 13.2 13.3 8.3 8.4 Std. Dev. 0.3 0.6 1.2 1.0 1.0 1.0 0.6 0.6
General Results Summary of Top Width Comparison to the Run 1 Terrain Difference (feet top width) Run 2 Bath. Surv. Struc. Horiz. Var. Run 3 Bath. Assm. Struc. Horiz. Var. Run 4 Bath. Weir Struc. Horiz. Var. Run 5 Bath. Struc. Horiz. Var. Run 6 Bath. Struc. BCB Run 7 Bath. Struc. One n Run 8 Bath. Assm. Struc. BCB Run 9 Bath. Assm. Struc. One n Average 57.1 97.1 56.9 72.9 59.4 187.4 100.8 205.6 Hilly Maximum 816.3 1519.3 816.3 816.3 816.0 1565.3 1521.7 1615.0 Std. Dev. 31.7 74.4 29.8 34.5 27.4 101.9 114.6 118.3 Average 19.3 30.6 49.6 34.5 43.4 90.4 39.7 93.6 Rolling Maximum 1332.7 1346.0 838.2 860.6 986.1 1772.6 1344.5 2207.6 Std. Dev. 31.2 39.1 47.1 26.6 22.4 118.0 35.2 142.6 Average 93.0 110.0 111.2 101.7 118.9 185.0 127.6 184.0 Flat Maximum 2115.6 2117.9 2128.5 2088.7 2286.3 2408.5 2293.2 2405.5 Std. Dev. 131.3 132.1 121.3 111.2 154.6 231.4 165.7 238.1
General Results Distribution of BFE Change vs. Base Condition (Run 1) Hilly Terrain 25.00 Change in Base Flood Elebvation (ft) 20.00 15.00 10.00 5.00 0.00 Channel Geometry Representation Assumed structures Structures as weirs structures structures and BCB mannings structures, one mannings Assumed structures, BCB Manning Assumed structures, One Mannings Hydraulic Model Type Average
General Results Distribution of BFE Change vs. Base Condition (Run 1) Rolling Terrain 25.00 Change in Base Flood Elebvation (ft) 20.00 15.00 10.00 5.00 0.00 Channel Geometry Representation Assumed structures Structures as weirs structures structures and BCB mannings structures, one mannings Assumed structures, BCB Manning Assumed structures, One Mannings Hydraulic Model Type
General Results Distribution of BFE Change vs. Base Condition (Run 1) Flat Terrain 25.00 Change in Base Flood Elebvation (ft) 20.00 15.00 10.00 5.00 0.00 Channel Geometry Representation Assumed structures Structures as weirs structures structures and BCB mannings structures, one mannings Assumed structures, BCB Manning Assumed structures, One Mannings Hydraulic Model Type
Parameter-Specific Results Channel Bathymetry Terrain Difference caused by removing bathymetry BFE change (ft) Top Width Change (ft) Average 1.4 57.1 Hilly Maximum 10.3 816.3 Std. Dev. 0.7 31.7 Average 0.4 19.3 Rolling Maximum 6.5 1332.7 Std. Dev. 0.3 31.2 Average 0.4 93.0 Flat Maximum 4.0 2115.6 Std. Dev. 0.3 131.3
Parameter-Specific Results Structures Terrain Hilly Difference Assumed structures BFE Change (ft) Structures as weirs structures Assumed structures Top Width Change (ft) Structures as weirs Average 1.2 0.4 0.4 39.6 11.2 25.3 Maximum 18.0 6.4 6.4 1,435.0 930.2 933.5 Std. Dev. 1.4 0.5 0.5 51.8 8.2 39.0 structures Very Stream Specific Taller structures caused higher variability Average 0.5 1.0 0.8 13.9 33.3 20.9 Rolling Maximum 8.3 18.5 16.7 917.1 1,281.3 1,280.1 Std. Dev. 0.6 1.1 0.8 15.5 42.5 21.8 Average 0.4 0.7 0.5 40.0 55.7 48.2 Flat Maximum 8.7 13.2 13.2 815.4 1,883.5 1,295.2 Std. Dev. 0.6 1.1 1.0 37.6 52.0 41.4
Parameter-Specific Results Manning s N Value Terrain Hilly Rolling Flat Difference BFE Change (ft) Bank-Channel- One Value Bank (BCB) Struc. Run Assm.. Struc. Manning s Approach Struc. Structure Assm. Approach Struc. Struc. Top Width Change (ft) Bank-Channel- Bank (BCB) Comparable Horizontal Assm. Variation Struc. RunStruc. One Value Assm. Struc. Bank-channelbank structures 5 6 Average 0.1 0.1 3.4 3.1 26.5 59.2 164.4 163.5 7 One value Maximum 1.5 1.3 Bank-channelbank 2.3 2.1 46.4 52.2 3 77.6 93.6 11.3 13.0 788.8 1439.0 1420.6 1459.7 8 Assumed Std. Dev. 0.1 0.0 structures Average 0.4 0.5 9 One 0.8 value 0.9 12.9 12.5 55.1 58.4 Maximum 6.1 12.5 7.3 12.8 972.1 972.1 1767.6 1916.5 Std. Dev. 0.4 0.6 0.7 0.7 15.0 14.6 112.1 117.0 Average 0.1 0.1 0.4 0.7 34.1 28.8 149.6 150.5 Maximum 1.0 2.3 2.5 10.9 1802.2 1545.6 2994.6 3016.6 Std. Dev. 0.1 0.1 0.4 0.5 78.2 63.5 277.2 297.0
4 Benefit Cost Analysis Relative Return On Investment (ROI) Process Determine Total Cost Length * Unit Cost Determine Total Loss Unit Loss = Total Loss in Census Block/Block Area Total Loss = Unit Loss * Floodplain Area Determine the ROI Index Identify two scenarios to compare Calculate the change in total loss and total cost between the 2 scenarios ROI Index = ABS(Change in Total Loss/Change in Total Cost)
ROI Index Intended to help decision making Indicates which change in approach yields the largest change in estimated losses per dollar spent. Higher ROI Index = Greater ROI for given stream Value of ROI Index less important than comparison with that of another parameter on the same stream.
Typical Unit Costs used in ROI Index Run Channel Bathymetry Run Description Structures Approach Manning s Approach Comparable Type of Study Estimated average cost per mile 1 Surveyed Surveyed Structures Horizontal Variation Detailed $9,333 2 DEM only Surveyed Structures Horizontal Variation Limited Detail w/ Structures $4,366 3 DEM only Assumed Structures Horizontal Variation Approximate (High) $1,000 4 DEM only Structures as weirs Horizontal Variation Approx. (Medium/Low) 5 DEM only structures Horizontal Variation Approximate (Low) $250 6 DEM only structures Bank-channel-bank Approximate (Low) $150 7 DEM only structures One value FOA/LSAE $32 8 DEM only Assumed Structures Bank-channel-bank 9 DEM only Assumed Structures One value Approximate (Medium) Approximate (Medium/Low) $500 $667 $550
ROI Index Hilly Terrain Parameter Runs Compared Parameter Change ROI Index Bathymetry 2 to 1 Upgrade from DEM terrain only to surveyed bathymetry Structure 3 to 2 Upgrade from assumed structures to surveyed structures 4 to 3 Upgrade from structures as weir to assumed structures 5 to 4 Upgrade from no structures to structures as weirs Manning s 6 to 5 Upgrade from bank-channelbank to horizontal variation 7 to 6 Upgrade from one manning s n value to bank-channel-bank Average = 1.9 Std. Dev. = 2.2 Average = 5.9 Std. Dev. = 12.6 Average = 42.9 Std. Dev. = 92.2 Average = 6.3 Std. Dev. = 15.0 Average = 8.1 Std. Dev. = 12.8 Average = 201.7 Std. Dev. = 242.4
ROI Index Rolling Terrain Parameter Runs Compared Parameter Change ROI Index Bathymetry 2 to 1 Upgrade from DEM terrain only to surveyed bathymetry Structure 3 to 2 Upgrade from assumed structures to surveyed structures 4 to 3 Upgrade from structures as weir to assumed structures 5 to 4 Upgrade from no structures to structures as weirs Manning s 6 to 5 Upgrade from bank-channelbank to horizontal variation 7 to 6 Upgrade from one manning s n value to bank-channel-bank Average = 0.1 Std. Dev. = 0.2 Average = 0.1 Std. Dev. = 0.3 Average = 1.7 Std. Dev. = 3.8 Average = 3.5 Std. Dev. = 7.3 Average = 3.7 Std. Dev. = 3.6 Average = 72.2 Std. Dev. = 153.3
ROI Index Flat Terrain Parameter Runs Compared Parameter Change ROI Index Bathymetry 2 to 1 Upgrade from DEM terrain only to surveyed bathymetry Structure 3 to 2 Upgrade from assumed structures to surveyed structures 4 to 3 Upgrade from structures as weir to assumed structures 5 to 4 Upgrade from no structures to structures as weirs Manning s 6 to 5 Upgrade from bank-channelbank to horizontal variation 7 to 6 Upgrade from one manning s n value to bank-channel-bank Average = 0.4 Std. Dev. = 0.3 Average = 0.4 Std. Dev. = 0.5 Average = 12.1 Std. Dev. = 31.6 Average = 23.4 Std. Dev. = 64.7 Average = 4.5 Std. Dev. = 4.8 Average = 26.0 Std. Dev. = 27.3
5 - Conclusions Results show sensitivity of the BFE and top width to model parameters should be considered when selecting a modeling method Type of terrain affects the uncertainty of a model Sensitivity of the BFE is more prominent in hilly areas Sensitivity of the top width is more prominent in flat areas Upgrading from single Manning s N method provides high ROI, larger impact than adding structures Benefits of improving structure modeling depends on terrain
5 - Conclusions Method of modeling should ideally consider: the present and future characteristics of the area being mapped topographical factors hydraulic factors population growth land available for development economic value of structures As conditions change, different variables dominate the potential for variation in BFE and top width, and indirectly influence the ROI of modeling investments.
Decision chart by terrain and variable type
Observations Decision guides can add logic to modeling choices Need to decide what is more important: BFE or Flood Zone ROI Index may not be the best way to evaluate improvements in quality but factors in inherent risk of the streams location
References Cook, A., Merwade, V., (2009), Effect of topographic data, geometric configuration and modeling approach on flood inundation mapping, Journal of Hydrology, Volume 377, Issues 1 2, 20 October 2009, Pages 131-142, ISSN 0022-1694, http://dx.doi.org/10.1016/j.jhydrol.2009.08.015. FEMA, (2014b), Guidance for Flood Risk Analysis and Mapping, First Order Approximation, Issued vember 14, 27 pp. FEMA, (2015), Guidance: General Hydraulic Analysis, Issued vember 2016 FEMA (2016) Impact of Flood Hazard Modeling Parameters on Base Flood Elevation (BFE) and Floodplain Top Width, Issued February 29, 2016. Jung, Y., & Merwade, V. (2015). Estimation of uncertainty propagation in flood inundation mapping using a 1 D hydraulic model. Hydrological Processes, 29(4), 624-640.
Questions?