Flash Flood (FEMA Photo Library) Jump to contentNational Basin Delineation Project
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To develop, enhance and maintain a national flash-flood-scale basin dataset in support of the NWS Flash Flood Monitoring and Prediction (FFMP) system, and to provide technical assistance for dataset customization efforts at local Weather Forecast Offices.

The National Basin Delineation Project was undertaken to centralize the consistent definition of flash-flood-scale basins for the conterminous U.S., Puerto Rico, and Guam.  FFMP basin datasets consisting of synthetic streams, basin boundaries, radar bin center points, and other reference data were assembled for 143 WSR-88D coverage areas and delivered to 115 Weather Forecast Offices.

NMQ schematic

Digital elevation data from the NED is used to derive the stream network and define flash-flood-scale basin boundaries

Project Overview

This project involves the delineation of flash-flood-scale basins for use in the NWS FFMP system. Implemented in the Advanced Weather Interactive Processing System (AWIPS), FFMP is used to assist in flash flood warning decisions at 121 Weather Forecast Offices (WFOs) throughout the country. FFMP is based on the Areal Mean Basin Estimated Rainfall Offsite link warning (AMBER) Program developed by Bob Davis and Paul Jendrowski. It computes basin average precipitation rates and accumulations from the WSR-88D Digital Hybrid Scan Reflectivity product, and updates these values with each volume scan of the radar. The accumulations are then compared to NWS Flash Flood Guidance (FFG) values or to user-defined threshold values to determine flash flooding potential.

The basin delineation was performed using ESRI's ArcView and ARC/INFO geographic information system (GIS) software, and was based on the National Elevation Dataset Offsite link warning (NED). Produced at the EROS Data Center Offsite link warning (EDC), the NED is a seamless mosaic of the highest-resolution, best-quality elevation data for the United States. It was available for use in this project through a Memorandum of Agreement (MOA) with the EDC.

Delineated basins were assigned unique IDs using the Pfafstetter system of basin codification (Verdin, 1997 Offsite link warning). This system defines basins according to the natural topographic control of drainage and the topology of the river network. Hydrologic ordering and relationships can be easily extracted from the Pfafstetter IDs, adding versatility to the basin dataset.

The initial FFMP basin datasets were completed and delivered via CD to each CONUS WFO during the period from September 2001 to July 2002. The initial datasets include the following shapefiles:

  • Aggregated Basin Shapefile (xxxx_aggr_basins)
  • Radar Bin Center Point Shapefile (xxxx_bins)
  • National Hydrography Dataset (NHD Offsite link warning) Streams Shapefile (xxxx_nhd_rch)
  • National Hydrography Dataset (NHD Offsite link warning) Waterbody Shapefile (xxxx_reg_nhd)
  • Original Basin Shapefile (xxxx_orig_basins)
  • Synthetic Streams Shapefile (xxxx_str_gen)

Enhanced FFMP basin datasets with hydrologic connectivity attributes were completed and delivered via CD to all WFOs during the period from April 2003 to September 2004. The enhanced datasets include the shapefiles listed above plus the following two:

  • Basin Shapefile (non-aggregated) with Hydrologic Connectivity Attributes (xxxx_ref_basins)
  • Synthetic Streamlink Shapefile with Hydrologic Connectivity Attributes (xxxx_ref_sl)

The hydrologic connectivity attributes enable the use of the shapefiles with the ArcView Basin Customization Extension Offsite link warning. The Basin Customization Extension contains helpful tools for editing and customizing the FFMP basin and stream shapefiles. In addition, the extension's tools for tracing downstream flow paths and defining upstream drainage help dataset users to better understand the hydrologic network.