Flood Hazard Inundation Mapping Verne Schneider, James Verdin, and JeradBales U.S. Geological Survey Reston, VA Presentation Flood Hazard Mapping Requirements Practice in the United States Real Time Inundation Mapping Flood Hazard Mapping Flood Hazard Mapping is a basic tool for flood preparedness and mitigation activities, including flood insurance programs Planners want to know inundation: Spatial extent Depth Flow velocity Probability of occurrence Format should facilitate integration with information on population and infrastructure 1
Flood Hazard Mapping in the U.S. Federal Emergency Management Agency (FEMA) National Flood Insurance Program Digital Flood Insurance Rate Maps (DFIRM) Floodway: 1% annual chance (100 yr) flood Flood risk: 0.2% annual chance (500 yr) flood Scales of mapping depend on density of population and infrastructure Scale 1:12,000 common, 1:6,000 for some urban settings, 1:24,000 in rural settings Sample DFIRM for Broomfield, Colorado Flood Hazard Mapping in the U.S. Flood estimation is based on USGS stream flow records and flood flow frequency analysis U.S. Water Resources Council Bulletin 17B, 1982, Guidelines for determining flood flow frequency Assemble annual maximum daily discharge rates for a 30+ yr time-series Fit a log Pearson Type III distribution Determine the discharge value that has 1/100 probability of occurrence in any given year 2
Flood Hazard Mapping in the U.S. Typically, locations of interest are ungauged If on a gauged river, discharge values can be simply scaled to ungauged locations by area ratio (if the value is near 1.0) More commonly, USGS regional regressions for flood frequency are used Each state s equations use basin descriptors (area, slope, precipitation) as independent variables Equations based on groups of stations with similar regional characteristics, for several return periods Flood Hazard Mapping in the U.S. Mapping spatial extent, depth, and velocity associated with the 100 yr flood is done using a hydraulic model US Army Corps of Engineers HEC-RAS is popular Field-surveyed cross sections of the river channel are required Water surface elevations between cross sections are interpolated with a DEM DFIRM Tools extension for ArcGIS Interactive websites to access DFIRMs 3
Flood Hazard Mapping in the U.S. Obstacles to global application of U.S. approach: Sparse stream gauge data in many regions Regional regressions often not available Overwhelming nature of river cross section survey requirement Inadequate elevation data available for 1:6,000 or 1:12,000 mapping Digital orthophoto quads (from air photos) not available Impossibly high cost Global Flood Hazard Mapping Global approach to flood hazard mapping is feasible Unprecedented availability of global data for basins, hydrography, elevation, land cover, soils, and climate Computer hardware and software are more powerful and easy to use than ever Compromise on scale will be required Identify potentially flooded villages or neighborhoods, but not individual houses Global Flood Hazard Mapping Exploratory analysis of Christian Herold, UNEP- Geneva, and Frédéric Mouton, University of Geneva and University of Grenoble Demonstrated that global scale basin descriptors can provide the basis for regional flood frequency analysis Further showed that globally available elevation data can be used to map inundation patterns associated with estimates of 100 yr flood 4
Global Flood Hazard Mapping Pilot studies done for North America (Eastern U.S.) and South America Global Runoff Data Center (GRDC) provided stream flow data from its worldwide collection Log-Pearson Type III distributions fit to individual stations using Bulletin 17B Peak discharge for 100 yr return period estimated for each gauge Eastern U.S. - Estimates of 100 yr Flood South America - Estimates of 100 yr Flood 5
Global Flood Hazard Mapping Stations were grouped by similarity of basin climate and vegetation characteristics as represented by the Holdridge Life Zones data set from IIASA Regression equations to estimate Q 100 were developed with candidate independent variables derived by GIS (descriptive of basin size and elevation, channel length, soils, precipitation, etc) Regions relevant to station groups identified Equations applied to estimate Q 100 at ungauged locations Global Flood Hazard Mapping To generate estimates of inundated area associated with 100 yr floods, Herold and Mouton used Shuttle Radar Topography Mission (SRTM) 90 m digital elevation data and GIS methods to solve the Manning equation (relating discharge to cross sectional area) South America Estimates of 100 yr Flood Inundation 6
Global Flood Hazard Mapping GIS DEM generation of inundated area estimates an area of active research The objective is to overcome the need for surveyed river cross sections while still respecting hydraulic principles Comparisons for a North Carolina river are encouraging With a reference gage stage of 101ft NC SD With a reference gage stage of 107ft NC SD 7
With a reference gage stage of 119.5ft NC SD GIS inundation mapping in Mozambique Real Time Flood Forecasting Routinely done using Precipitation Runoff Models with Flow Routing to Forecast Discharge or Water Surface Elevation at a point of interest Must be combined with Flood Inundation Maps to get flooded area. 8
Real Time Flood Inundation Mapping Research Topic Develop an a priori library of inundation maps referenced to specific gage elevations Flow/stage forecasts are used to describe spatial extent of flooding in near-real time. Result is spatially specific flood warnings http://nc.water.usgs.gov/finmap/ Global Flood Hazard Mapping A globally consistent approach to flood hazard characterization is feasible For regional regressions, key data are: Stream flow records from GRDC (locally augmented wherever possible) HYDRO1k elevation data and basin derivatives Global gridded precipitation data, e.g., CRU TS 2.1 For inundation mapping, key data are: SRTM 90 m elevation data SRTM 30 m elevation data, potentially Global Flood Hazard Mapping Participation of local experts will be essential: To benefit from previous efforts and existing products To identify highest priority areas for flood characterization To augment global data with local data To ensure products are of a useful scale and content To ensure local knowledge of natural systems is incorporated in the study 9
Global Flood Hazard Mapping Potential partner organizations: Regional Center for Mapping for Resource Development, Nairobi SADC Regional Remote Sensing Unit AGRHYMET Regional Center Mekong River Commission Agencia Nacional de Aguas, Brazil Central Water Commission, India More. 10