Mapping the Zone: Improving Flood Map Accuracy

BOX S.1

Committee Charge

The committee will

Examine the current methods of constructing FEMA flood maps and the relationship between the methods used to conduct a flood map study (detailed study, limited detailed study, automated approximate analysis, or redelineation of existing hazard information), the accuracy of the predicted flood elevations, and the accuracy of predicted flood inundation boundaries.
Examine the economic impacts of inaccuracies in the flood elevations and floodplain delineations in relation to the risk class of the area being mapped (based on the value of development and number of inhabitants in the risk zone).
Investigate the impact that various study components (i.e., variables) have on the mapping of flood inundation boundaries:
1. Riverine flooding
  - The accuracy of digital terrain information
  - Hydrologic uncertainties in determining the flood discharge
  - Hydraulic uncertainties in converting the discharge into a floodwater surface elevation
1. Coastal flooding
  - The accuracy of the digital terrain information
  - Uncertainties in the analysis of the coastal flood elevations
1. Interconnected ponds (e.g., Florida)
  - The accuracy of the digital terrain information
  - Uncertainties in the analysis of flood elevations

Provide recommendations for cost-effective improvements to FEMA’s flood study and mapping methods.
Provide recommendations as to how the accuracy of FEMA flood maps can be better quantified and communicated.
Provide recommendations on how to better manage the geospatial data produced by FEMA flood map studies and integrate these data with other national hydrologic information systems.

A study of sampling uncertainties in extreme stage heights at USGS stream gages in North Carolina and Florida found that for 30 of 31 gages, the average uncertainty is approximately 1 foot with a range of 0.3 feet to 2.4 feet. Uncertainties do not appear to vary with the size of the drainage basin or its topographic slope. It may thus be inferred that the lower bound on the uncertainty of the base flood elevation is approximately 1 foot. For the river reaches studied in North Carolina, a 1-foot change in flood elevation corresponds to a horizontal uncertainty in the floodplain boundary of 8 feet in the mountains, 10 feet in the rolling hills, and 40 feet in the coastal plain. This uncertainty has a significant impact on the delineation of inundated areas on flood maps.

The constriction of flood flow by bridges and culverts raises the base flood elevation in the three study areas. Such backwater effects are largest just upstream of the constriction and diminish progressively upstream. They are most pronounced in the coastal plain, extending an average of 1.1 miles and raising base flood elevations by up to 2.5 feet (average 0.9 foot). They are least pronounced in mountainous areas, raising the base flood elevation an average of 0.2 foot, which is not significant, given the sampling uncertainty noted above.

The largest effect by far on the accuracy of the base flood elevation is the accuracy of the topographic data. The USGS National Elevation Dataset (NED), developed from airborne and land surveys, is commonly used in flood map production, even though the elevation uncertainties of the NED are about 10 times greater than those defined by FEMA as acceptable for floodplain mapping. Data collected using high-resolution remote sensing methods such as lidar (light detection and ranging) can have absolute errors on the order of centimeters, consistent with FEMA requirements, but they are not available nationwide. A comparison of lidar data and the NED around three North Carolina streams revealed random and sometimes systematic differences in ground elevation of about 12 feet, which significantly affects predictions of the extent of flooding (e.g., Figure S.2). These large differences exceed FEMA’s stated error tolerances for terrain data by an

Table of Contents

Front Matter R1-R14

Summary 1-6

1 Introduction 7-12

2 Flood Mapping and Flood Insurance 13-24

3 Elevation and Height Data 25-40

4 Inland Flooding 41-66

5 Coastal Flooding 67-78

6 Benefits and Costs of Accurate Flood Mapping 79-88

7 Mapping and Risk Communication: Moving to the Future 89-100

References 101-104

Appendixes 105-106

Appendix A: Methods for Estimating Base Flood Elevations in Approximate Studies 107-108

Appendix B: Biographical Sketches of Committee Members 109-112

Appendix C: Glossary 113-120

Appendix D: Acronyms and Abbreviations 121-122