Assessment of Economic Benefits of the North Carolina Floodplain Mapping Program Hydrologic and Hydraulic Case Studies Adapted from a Presentation to NRC.

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Presentation transcript:

Assessment of Economic Benefits of the North Carolina Floodplain Mapping Program Hydrologic and Hydraulic Case Studies Adapted from a Presentation to NRC Committee on FEMA Flood Maps January 14, 2008 by Thomas Langan, P.E., CFM North Carolina Floodplain Mapping Program

Purpose Assess impacts to 100-yr water surface elevations (WSELs) and special flood hazard areas boundaries by changing:  Physiographic region  Hydrologic modeling methodology  Hydraulic study type (Primary Goal)  Topographic data source

Site Selection Criteria Physiographic Regions – Coastal, Piedmont, Mountains  Should be in or near County w/ GIS building footprints Hydrologic Methodology  All locations should have stream gage  One County with a rainfall runoff model  Suburban/Rural land use Hydraulic Study Type  Existing detail study (DS) HEC-RAS model 20 years of peak and average daily discharge record Topography  NC LiDAR or Equivalent Accuracy (2 ½ -ft contour)

Length (mi)7.1 XSs80 Average XS Spacing472 Structures6 US Drainage Area (mi 2 )60 DS Drainage Area (mi 2 )136 (7545 cfs)

Length (mi)5.7 XSs66 Average XS Spacing459 Structures4 US Drainage Area (mi 2 )8 DS Drainage Area (mi 2 )32 (4394 cfs)

Length (mi)4.8 XSs59 Average XS Spacing496 Structures9 US Drainage Area (mi 2 )108 DS Drainage Area (mi 2 )133 (19710 cfs)

Analytical Approach Hydrologic Methodology Hydraulic Study Type Topography Statistics Hydraulics Hydrology Floodplain Mapping Physiographic Region Variables Analyses

Hydrologic Methodology Flow estimate methods  Rainfall-runoff (RR) – HEC-1 or HEC-HMS  LPIII Weighted Adjusted Regional Regression (ADJREG)  Rural Regional Regression (REG) Upper and Lower Limit of the Standard Error of Prediction of Regional Regression (REGUP & REGLOW) (+/- 42% Coast, Piedmont, Mountains - +/- 47%) Parameters varied for only DS model runs and for 100-yr return period

USGS Regional Regression Equations For a 100 square mile drainage area, the 100- year flood discharge estimate is 13,250 cfs in the Blue Ridge- Piedmont, 6340 cfs in the coastal plain, 3400 cfs in the Sand Hills. Blue Ridge - Piedmont Sand Hills Coastal Plain 95% Confidence interval is +/ % of the estimated flow

14-ft WSEL Downstream Bottom of Bridge Deck

15-ft

19-ft

Conclusions All hydrologic methods are calibrated to results from frequency analysis at USGS gages so gage analysis drives results Main effect is produced by stretching USGS regression equation results out to their limits (not realistic actually) Varying hydrologic methods changes elevation by 1-2 ft

Analytical Results Water Surface Elevation Profiles Hydraulic Study Type

Changes SFHA

Conclusions Uncertainty induced by changing the terrain data dominates other hydraulic model uncertainties There is a huge effect of terrain data uncertainty in the in the Long Creek example in Charlotte, NC  Floodplain derived from the LIDAR data is underground when viewed on the National Elevation Dataset!