How Lidar Elevation Data Help Manage Flood Risk: The USGS 3D Elevation Program

A 3D Elevation Program lidar point cloud, colored from National Agriculture Imagery Program photos, showing simulated flooding in Denver, Colorado. Jason M. Stoker, USGS.

From flood hazard to flood risk

Over the last 30 years, flooding has cost the United States nearly $8 billion a year on average and killed more than 80 people a year, according to NOAA's National Weather Service. It is the leading cause of presidential disaster declarations.

Reducing flood damage has long been a goal, and the national strategy has shifted from building dams and levees to three prongs:

1. better design and operation of those structures;
2. more accurate and accessible flood forecasting;
3. moving FEMA's National Flood Insurance Program (NFIP) to a more balanced, less costly footing.

High-quality three-dimensional elevation data from lidar (light detection and ranging) can sharpen all three. The U.S. Geological Survey's 3D Elevation Program (3DEP) has the infrastructure to supply it: partnerships that pool funding and avoid duplication, contracts with experienced private mapping firms for consistent, low-cost data, and the standards and expertise for seamless, edge-to-edge data anyone can use.

The National Enhanced Elevation Assessment named flood risk management one of 27 business uses needing better elevation data — for better flood maps (including Flood Insurance Rate Maps), dam and levee safety, hydrologic modeling and forecasting, state and local risk management and emergency response, floodplain and stormwater management, and the design of drainage and dams.

What a flood-inundation map shows

A flood-inundation map shows at least how far a flood spreads along a stretch of river, and often how deep. Extent and depth usually come from hydraulic models using streamflow, channel shape, floodplain terrain, and structures such as bridges and levees. The USGS has developed several methods — see its Flood Inundation Mapping Program.

USGS flood-inundation maps for 3.95 miles of the Saluda River near Greenville, South Carolina, starting about 150 feet below Saluda Lake Dam, built with a lidar elevation model. They show flooding at river stages of 20.0 feet (A) and 6.0 feet (B) at USGS streamgage 02162500 (yellow triangle); the red line marks the model's limit. USGS.

Streamflow and 3DEP in flood risk management

Flood defenses, forecasts and insurance all matter, but floods will still happen and some risk always remains. Disasters teach lessons only if each flood is understood and documented. FEMA and the USGS send USGS stream hydrographers to map flood elevations and extents after major floods — and 3DEP data are key to turning that into fast, usable information.

Flood control. Dams and levees greatly reduce flooding where they're built, within their design limits. Renewing aging infrastructure needs better estimates of flood risk — which means improving the USGS streamflow record and the flood equations in StreamStats, driven by longer records and detailed basin data. 3DEP data already feed more accurate equations where available; more of it could improve the design and renewal of flood structures and the hundreds of thousands of miles of storm sewers and culverts that carry floodwater away.

Flood forecasting. The National Weather Service has long forecast floods at about 4,000 locations, from rainfall forecasts and USGS streamflow — but as flow magnitudes and timings, hard for many people to use. USGS and NWS flood-inundation map tools turn them into maps of a flood's spread hour by hour, street by street, house by house — which needs the detailed terrain only lidar provides. The NWS National Water Model, using data from more than 8,100 USGS streamgages, forecasts flows for some 2.7 million stream reaches; pairing it with 3DEP-powered inundation maps nationwide could greatly improve forecasting at a large saving.

Flood insurance. Communities that enforce floodplain rules can buy NFIP insurance. But traditional Flood Insurance Rate Maps (FIRMs) mark the floodplain as the area of the 1-percent-chance flood, and nearly every property inside pays about the same regardless of its real risk — including properties flooded and rebuilt again and again.

• FEMA's Risk MAP program set out to reform this, and the Biggert-Waters Flood Insurance Reform Act of 2012 wrote many reforms into law and re-established FEMA's Technical Mapping Advisory Council (TMAC).
• TMAC's two key recommendations: make floodplain mapping fully digital, and base premiums on the flood risk of individual structures. Both depend on widespread 3DEP data — and could transform flood insurance, spurring communities and owners to reduce damage.

Assessing, documenting and communicating flood risk

High-quality elevation data help both responders and planners.

A flood-risk product enhanced with lidar: where high-quality elevation models exist, the vulnerability of people and infrastructure to rising seas can be mapped. Modified from Thatcher and others (2013); USGS.

• Flood Insurance Rate Maps: FEMA's Risk MAP program identifies hazards, assesses risk and works with states and communities to cut it; FIRMs underpin the NFIP's floodplain rules and insurance requirements.
• Flood documentation studies: maps recording where a flood reached, to focus response, guide future floodplain planning, and support insurance claims.
• Flood-inundation map libraries: series of maps at rising water levels, typically every one to two feet of river stage, tied to USGS real-time streamgages and NWS forecasts, so officials can act hours or days ahead, block by block.

Examples:

• North Carolina began collecting lidar in 2000, making more flood studies possible at little extra cost. From 2000 to 2008 it studied nearly 27,000 miles of streams, rivers and coast, setting 305,530 base flood elevations and producing 8,609 FIRMs. From 2014, higher-quality (quality level 2) lidar let it move from identifying hazards to assessing risk for individual structures — including their building footprints and floor elevations (fris.nc.gov).
• Hurricane Sandy, October 2012: USGS crews gathered high-water marks that, combined with lidar, produced documentation maps letting FEMA judge damage to specific homes and pay out aid faster than ever. The USGS and FEMA have used such maps since, including in the August 2016 flood in Baton Rouge. New York's Rising Community Reconstruction Program treats lidar as critical for managing disasters.
• The Saline River, a tributary of the Ohio in southern Illinois, had a big flood problem and no lidar. A USGS pilot map library from 30-meter elevation data was rough but useful enough that the community bought lidar the next year, and the USGS remapped at 10-meter resolution for better planning and response.

A National Academy of Public Administration study for FEMA found that the 3DEP plan to collect elevation data nationwide would be far more cost-effective than piecemeal efforts, benefiting FEMA's flood mapping and users across government, and recommended it guide the President's budget.

The payoff

Agencies in the National Enhanced Elevation Assessment conservatively found $295 million a year in new benefits from better elevation data for flood hazard mapping, mostly from reduced property losses. Across the economy, 3DEP could conservatively provide $690 million a year in new benefits, with potential for $13 billion.

About 3DEP

3DEP, managed by the USGS, is acquiring high-resolution elevation data for the nation — lidar point clouds, bare-earth digital elevation models (DEMs) and more.

• Goal: full quality level 2 lidar coverage of the conterminous United States, Hawaii and the U.S. territories, and IfSAR radar data for Alaska, by the end of 2023.
• Quality: quality level 2 lidar has a nominal pulse spacing of at most 0.7 meters and vertical error of 10 centimeters; Alaska's quality level 5 IfSAR, 185 centimeters. Data must be less than eight years old (Lidar Base Specification).
• Products: classified point clouds and bare-earth DEMs; seamless national DEMs at 1/3, 1 and 2 arc-seconds; a standard 1-meter DEM; and for Alaska, surface models, intensity images and 5-meter DEMs — all part of The National Map.
• A national program:
• • cuts acquisition costs about 25 percent through economies of scale;
  • gives federal, state, tribal, territorial and local partners predictable, flexible investment — with the option to buy up to higher quality;
  • delivers consistent national coverage that crosses project and jurisdiction boundaries;
  • offers contracts, published specifications and quality assurance, so partners can focus on their own work.
• For users: data may already exist through 3DEP, ready to use; where new data are needed, 3DEP offers pooled funding, qualified contractors, contract management and inspection, and cost-share funding.

Sources

Based on The 3D Elevation Program—Flood Risk Management, USGS Fact Sheet 2017–3081 (January 2018), by William J. Carswell, Jr., and Vicki Lukas, U.S. Geological Survey; a work of the United States government in the public domain. Figures 1–3 are taken from the fact sheet's PDF; photographs and an image credited to a university and two state agencies are not reproduced.