GPS elevation surveys a key to proactive flood plain management

Transcription

1 Destructive Water: Water-Caused Natural Disasters, their Abatement and Control (Proceedings of the Conference held at Anaheim, California, June 1996). IAHS Publ. no. 239, GPS elevation surveys a key to proactive flood plain management DAVID F. MAUNE Dewberry & Davis, 8401 Arlington Boulevard, Fairfax, Virginia , USA Abstract For the Federal Emergency Management Agency (FEMA), Dewberry & Davis (D&D) surveyed thousands of homes with the Global Positioning System (GPS) accurate to ±2 inches (5 cm). D&D demonstrated that accurate and inexpensive GPS elevation surveys, mass-produced for all buildings in or near Special Flood Hazard Areas (SFHAs), are a key to proactive flood plain management. Without elevation data, flood plain managers are generally restricted to reactive measures. With elevation data, flood plain managers can: (a) perform reliable hazard identification and risk assessments; (b) take proactive measures to actually reduce flood risks; (c) produce GPS Elevation Certificates which help home owners recognize their true flood risk and obtain best-value flood insurance to reduce their financial vulnerability; and (d) help accelerate Federal disaster assistance funding when flooding actually occurs. The benefits to a flood-prone community appear to greatly outweigh the low, mass-produced cost to the community in obtaining the highly accurate GPS elevation surveys that make proactive flood plain management possible in the first place. INTRODUCTION In 1994, in response to severe flooding in Georgia, Florida, Alabama and Texas, Dewberry & Davis surveyed nearly 8000 flooded buildings to collect flood inventory data for FEMA's Individual Assistance Program. Concurrently, certified flood adjusters made "windshield survey" damage estimates for these buildings; these estimates were subsequently found to be in error by %. Damage estimates can be vital for timely and correct rebuild/buy-out decisions. These decisions often depend on whether estimated repair costs exceed 50% of the replacement value of the flooded building. In 1994, FEMA determined that existing computer models could more accurately estimate flood damages by knowing three things about each flooded building: (a) What is the approximate area of the building's "footprint"? (b) What is the building's estimated replacement value? (c) How deep was the interior flooding, to the nearest whole foot (30 cm)? The answers to (a) and (b) could be collected in advance for all flood-prone buildings in a community; but FEMA needed a way to quickly obtain the answers to (c) for each building actually flooded. Additionally, D&D sought other means to help flood plain managers to be truly proactive. Central to this need was the means to better perform flood hazard identification and risk assessment, vital for flood mitigation initiatives.

2 332 David F. Maune GPS COMPARISON In 1995, in cooperation with the Louisville and Jefferson County, Kentucky, Metropolitan Sewer District, FEMA sponsored a comparison of two GPS technologies on the basis of vertical accuracy and cost/productivity. We refer to these technologies as "GPS Backpack" operated by Larry N. Scartz, Ltd, and "GPS TruckMAP" operated by John E. Chance & Associates. Both utilized Trimble 4000SSe receivers with real-time kinematic (RTK) and on-the-fly (OTF) reinitialization. They used alternative techniques for surveying the 3-D coordinates (latitude, longitude, and elevation) of survey target points on buildings without intruding on private property. D&D calls this "stand-off surveying". For the productivity portion of the test, nearly 1300 high-density houses were surveyed to determine if Elevation Certificates could be mass produced for US$30 per house, as opposed to the typical price of US$250 per house. For the accuracy portion of the test, 62 of the 1300 total houses were selected to be independently surveyed by both methods because they presented technical difficulties in one or more of the following ways: (a) they were located along tree-lined streets where tree canopy cover would interfere with GPS signals and where D&D could test the OTF capabilities when satellite lock was lost; (b) they were located on the opposite side of hills from the GPS base station, where RTK radio corrections would have difficulty reaching the GPS rover units; and/or (c) they were located up to 200 feet (61 m) off the road so that elevations would be "cantilevered" by significant distances. These three technical challenges were considered essential to test the true capabilities and limitations of stand-off GPS survey techniques. BackPack and TruckMAP would independently survey these 62 houses, and correct for local variations in gravity. D&D would then compare the two elevation data sets and determine if FEMA's 6-inch (15 cm) vertical accuracy requirement for this test was satisfied. If the GPS technologies performed well under these difficult conditions, they could be relied upon also to perform well under simpler conditions. When the two elevation data sets were laid side-by-side for the 62 difficult houses, the results were amazing! The elevations all agreed within about one inch (2.5 cm). The r.m.s. difference was two-thirds of an inch (1.7 cm), and the maximum error was less than +2 inches (5 cm) at the 95% confidence level. In highdensity housing areas, both methods proved that highly-accurate Elevation Certificates could be mass produced for less than US$30 per house. Both BackPack and TruckMAP gave comparable results and were considered useable. FEMA subsequently sponsored GPS elevation surveys of thousands of other homes in 61 counties in eight states nationwide. With the best geodetic-grade GPS receivers and exacting procedures, D&D found that survey control points and benchmarks are typically in error by 6 to 12 inches (15 to 30 cm), and sometimes by several feet or more. D&D found some new homes nationwide had been constructed at elevations that make them vulnerable to predicted floods in the future. And D&D found that approximately one-third of conventional Elevation Certificates, that establish the cost of flood insurance for post-firm homes, were in error by more than 1 foot (30 cm) when checked by more-accurate survey methods.

3 GPS elevation surveys a key to proactive flood plain management 333 CHALLENGES PREVIOUSLY UNSOLVABLE See Table 1 for a summary of nine common challenges that can be solved with GPS elevation surveys. Elevation errors. Challenges 1 and 2 pertain to elevation errors. Errors in survey control points, benchmarks, elevation reference marks (ERMs), etc. can undermine the accuracy and intended utility of products from the National Flood Insurance Program (NFIP). Flood Insurance Studies (FISs), Flood Insurance Rate Maps (FIRMs) and conventional Elevation Certificates can all have undetected errors if they result from poor survey control. Rigorous GPS elevation surveys can resolve control point/benchmark discrepancies and identify the best control in each county for NFIP use. National Geodetic Survey (NGS) control points, regularly updated on NGS' electronic bulletin board ( ) are the most reliable. Table 1 Challenge solutions from GPS elevation surveys. CHALLENGES currently facing flood plain managers 1. Benchmarks nationwide have cm errors; some errors >1.0m. 2. Large % of conventional elevation certificates have elevation errors > 30 cm. 3. Cannot quantify hazards and risks from 500-, 100-, 50- and 10- year floods. NOTE: Hazard identifications and risk assessments are key to all mitigation efforts. 4. Difficult to justify drainage improvement projects. 5. Conventional Elevation Certificates: costly (typically US$250), less accurate. 6. Pre-FIRM buildings currently don't require Elevation Certificates to identify actual flood risks. Subsidy is expensive; Congress directed 1996 subsidy restudy. 7. Difficult to predict candidate buildings for retrofit/floodproofing. 8. Post-flood "windshield" estimate errors of %. 9. Lengthy delay for disaster inventories and "rebuild/buy-out" decisions for buildings substantially damaged. SOLUTIONS with pre-flood GPS elevation surveys to predict depth of interior flooding of flood-prone buildings Use best control from National Geodetic Survey (NGS) in each county for all NFIP products. Strictly follow NGS Guidelines for GPS Elevation Surveys [5 centimetre accuracy] to survey all flood-prone buildings. Correct GPS surveys rigorously for local gravity variations. Apply FEMA/USACE computer models to reliably estimate flood damages. These models require (1) predicted flood depths, (2) "footprint" areas, and (3) replacement values. Quantify legitimate flood risks for individual buildings and for the entire community as a basis for mitigation initiatives. Computer models determine expected damages from 100-year and other floods without drainage improvements (higher BFEs) and with drainage improvements (lower BFEs). Determine benefits of project in terms of damages avoided. Produce GPS Elevation Certificates: - Highly accurate and affordable when mass produced: * elevation accuracy: +5 cm, * < US$30 per building in high density urban areas, * < US$70 per building in low density rural areas; - BFE interpolated to +3 cm. Community eliminates excuses for not buying insurance by providing certificates free to Pre-FIRM and Post-FIRM homeowners and encouraging purchase of flood insurance. Apply for CRS credits to reduce rates and offset costs. Use GPS elevations to run computer models for 500-, 100-, 50- and 10-year floods. Identify candidates for relocation, elevation in place, flood walls, levees, flood-proofing (wet and dry). Perform benefit-cost analyses; take proactive steps. Survey post-flood elevations of several high water marks; then, calibrate H&H models to flood event. Estimate damages to individual buildings and communities. Accelerate rebuild-buy-out decisions; expedite receipt of Individual/Family and Hazard Mitigation Grant moneys.

4 334 David F. Maune Inability to quantify flood hazards and risks. Challenge 3 indicates the dilemma in being unable to accurately quantify hazards and risks from 500-year, 100-year, 50-year, and 10-year floods. By pre-flood surveying the elevation of the reference level of each building in or hear a Special Flood Hazard Area (SFHA), the community can estimate, on a house-by-house basis, the depth of interior flooding that would be caused by the standard flood events. The computer models, cited above, can then compute the estimated damages to each building, and to the community as a whole, as a result of the standard flood events (500-year, 100-year, 50-year, and 10-year floods). Such hazard identifications and risk assessments are the key to all mitigation efforts, and the community can then be aggressive and proactive in taking mitigation initiatives to reduce future flood losses, and in promoting flood insurance to owners of at-risk homes. Difficulty in justifying drainage improvement projects. Challenge 4 indicates that it is difficult to justify drainage improvement projects without detailed elevation data on individual buildings in the drainage basin. For example, how does one prove whether or not it is worth US$2 million to construct a drainage improvement project that will lower the base flood elevation (BFE) by 2 feet for an area that includes 400 flood-prone homes? By knowing the elevation of the lowest floor of each home, its "footprint" area, and its replacement value, computer models can accurately estimate expected damages from standard flood events prior to drainage improvements, and then recompute the expected damages with drainage improvements that lower the BFEs. The drainage improvement project benefits can be determined in terms of the damage avoided. Limitations in conventional Elevation Certificates. Challenges 5 and 6 pertain to conventional Elevation Certificates, which are sometimes considered to be an impediment to the sale of flood insurance. All Elevation Certificates (conventional or GPS) are expensive when not mass-produced. Although Elevation Certificates aren't required for pre-firm buildings (constructed prior to publication of Flood Insurance Rate Maps of the area), the US Congress has directed a 1996 study of the current subsidy for pre-firm homes. Without Elevation Certificates, it is difficult to identify candidate buildings for retrofit/flood-proofing. Challenges 5 and 6 can be solved by producing highly-accurate GPS Elevation Certificates, mass-produced and quality-controlled, for all buildings in or near flood plains, providing them free to pre-firm and post-firm homeowners, and encouraging them to purchase flood insurance. A sample GPS Elevation Certificate is at Fig. 1. In addition to the individualized photograph of the building in question, the background map pinpoints the building's geographic location centred on the base map road network and also its position in or near the SFHA shown in blue. The BFE is interpolated to the nearest 0.1 foot (3 cm), and the elevation of the "target point" surveyed on the house is also shown to the nearest 0.1 foot. Target points are most typically the bottom of front door (BFD) or the top of foundation (TOF). Offsets to below-ground floors are estimated, based on standard 8-foot (244 cm) basement foundations, or 9-foot (274 cm) standard offsets between floors. Corrections can be made by the insurance agent and owner if the offset distance error is significant for insurance rating purposes. The estimated depth of interior flooding from the 100-year

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6 336 David F. Maune base flood is also provided on the GPS Elevation Certificate. GPS Elevation Certificates, free to all, would clearly be important in the event Congress decides to eliminate the subsidy for pre-firm homes. In fact, they would probably be the key to success or failure in getting pre-firm homeowners to purchase flood insurance at actuarial rates. Challenge 7 can be solved by using GPS elevation data to run the computer models for standard flood events to identify candidates for relocation, elevation in place, flood walls, levees, dry or wet flood-proofing. Benefit-cost analyses indicate the viability of retrofitting/flood-proofing of selected buildings. Limited response to actual flood events. Challenges 8 and 9 pertain to current problems in estimating actual flood damages and in expediting Federal moneys to assist flooded homeowners and affected communities. The solution is actually quite simple. By already knowing the elevation of the lowest floor of each flood-prone building, its area and replacement value, the community would merely need to survey the post-flood elevation of several high water marks (e.g. 14th, 12th, and 9th Street bridges) in order to calibrate the hydrologie and hydraulic (H&H) models to the actual flood event. Then, flood plain managers could quickly and accurately estimate the depth of actual interior flooding, estimate the damages to individual buildings and communities, accelerate rebuild/buy-out decisions, and expedite the receipt of Individual and Family Grant (IFG) and Hazard Mitigation Grant Program (MHGP) moneys. For more information, contact Dr David Maune, D&D's Director of Mapping, Remote Sensing and GPS/GIS Services, at Acknowledgements The author wishes to acknowledge the assistance of the Federal Emergency Management Agency, especially Michael Buckley and Matthew Miller of the Hazard Identification and Risk Assessment Division, and Howard Leikin of the Federal Insurance Administration, who were instrumental in providing support for the innovative GPS survey projects described in this paper.