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NWS Flash Flood Guidance System


Modernized Flash Flood Guidance

Timothy L. Sweeney, HRL Thomas F. Baumgardner, MARFC Last Modified 6/23/1999 modflash_ffg1.jpg - 63120 Bytes Flash flood guidance is the amount of rainfall needed in a specified period of time to initiate flooding on small streams. Modernized flash flood guidance provides both standard methodology and finer resolution for computing flash flood guidance. Flash flood guidance is computed for small, ungaged streams in areas (grids, zones, and counties) and for gaged streams (primarily headwaters) for 1-, 3-, and 6-hour durations, (12- and 24-hour are optional). Rainfall estimates from the WSR-88D radars are derived on the Hydrologic Research Analysis Project (HRAP) grid, nominally 4 km on a side. To more effectively use the radar rainfall estimates in the flash flood program, flash flood guidance is needed on the same spatial grid scale. USE The Weather Forecast Offices (WFO) use zone/county and headwater flash flood guidance as criteria for issuing flash flood watches and warnings. For instance, the WFO application Flash Flood Monitoring and Prediction system compares gridded flash flood guidance and rainfall estimates from the radars. If the estimated or projected rainfall by the radar equals or exceeds the flash flood guidance for a grid(s), the grid(s) would be depicted as red on a graphical display and the WFO would issue a flash flood warning for the flood-prone area represented by the grid(s). If observed or projected rainfall from the radar were slightly less than gridded flash flood guidance, the grids would be shown as yellow and a flash flood watch may be issued. If the estimated rainfall were much less than the gridded flash flood guidance, the grids would be green and the WFO would not issue any watch or warning but continue to monitor the rainfall. METHODOLOGY Flash flood guidance is a derived amount of rainfall that is controlled by soil moisture state and threshold runoff. Soil moisture state changes continuously depending on precipitation (or the lack thereof) and is maintained in the river forecast models used at the RFCs. When using a rainfall-runoff model, rainfall and moisture state are input to give runoff. Computing flash flood guidance works in the opposite direction. Needed runoff and current moisture state are input to give rainfall. The resulting rainfall is the flash flood guidance. A typical rainfall and runoff relationship is shown graphically in the following diagram: modflash_ffg2.jpg - 5872 Bytes Threshold runoff is the runoff needed to initiate flooding. It is a fixed value based on the geographic and hydrologic features of the stream channel and the basin. Threshold runoff for a headwater is the flow at flood stage divided by the unit hydrograph peak for a specified duration. The unit hydrograph relates one inch of runoff over a specified basin to the volume of runoff at specified time intervals. The flow at flood stage is determined from the rating curve for the stream gage. The rating curve relates the vertical depth of water in the stream to flow (volume per unit time). Computing threshold runoff for areas is less direct. Since these are ungaged streams, there are no stream gages, no flood stages, and no rating curves to simply determine the flows at flood stages. In place of flood stage a bankful stage can be determined from field surveys of several ungaged streams. The bankful stage is the depth of water in the channel at which flooding begins. (The following figure shows flood stage/bankfull and the threshold runoff with water in the stream.) The unit hydrograph peak must be determined empirically using physical characteristics of the ungaged basins. modflash_ffg3.jpg - 10232 Bytes Without the use of computers this is not practical for hundreds of locations. A reasonable approach for deriving areal runoffs that has some merit and is used by a few RFCs is to plot and analyze threshold runoffs for the headwaters. Threshold runoffs for areas (zones/ counties) can be read from the plot of the runoff isolines. The Flash Flood Guidance System (FFGS) was designed to be independent of any rainfall-runoff model. The FFGS obtains all soil moisture conditions as rainfall-runoff curves generated in the RFC forecast system where the rainfall-runoff models reside. Depending on the availability of precipitation data, the forecast system can update soil moisture conditions every six hours and likewise, the FFGS can compute flash flood guidance every six hours. PHASE 1: FFGS IMPLEMENTATION The FFGS was designed to be implemented in phases to minimize the impact on operations at the weather offices and external users. Most county/zone and headwater products have been retained with FFGS. Some RFCs have added products to cover more of their forecast areas. INITIAL GRIDDED RUNOFF Current threshold runoffs for zones/counties were derived many years ago by various methods used at the RFCs. Runoffs varied little from county to county and in some states the same runoff was used over the entire state. Some RFCs based county guidance on soil moisture conditions of RFC forecast basins in the county as depicted in the following graphics (the headwater guidance method) while others simply defined counties as basins. modflash_ffg4.jpg - 12575 Bytes If gridded guidance is available, a county boundary can be overlaid on the grid and the county guidance can be the average of all the gridded values inside the county boundary as shown below. modflash_ffg5.jpg - 9336 Bytes Prior to implementing FFGS at the first RFC, initial gridded runoffs were defined from the existing county runoffs. The county runoffs were assigned to each HRAP bin in the respective counties. Not only is county guidance being computed in this manner but all the gridded algorithms are thoroughly exercised before the GIS-based runoffs are available. In the meantime, there is no degradation of the county flash flood guidance. The initial gridded implementation is a stepping stone to using the GIS-based threshold runoffs. PHASE 2: GIS-BASED THRESHOLD RUNOFF In phase 2, threshold runoffs will be derived in a multi-step process using an objective method based on GIS algorithms. As the first step of Phase 2, digital elevation model (DEM), and river reach data are processed. Next, using stream junctions as reference points (see the following graphics), watersheds are delineated for areas ranging in size from about 2 square miles (5 sq. km) to 770 square miles (2000 sq. km). modflash_ffg6.jpg - 15504 Bytes The physical characteristics of these watersheds, e.g., area, slope, stream length, are derived, too. Then, threshold runoffs are computed for both local and accumulated watershed areas at the stream junctions. In the fourth step the runoffs at the stream junctions (a non-uniform grid) are interpolated to the HRAP grid as depicted in the following graphics. Finally, the gridded runoffs are defined in the FFGS where gridded flash flood guidance is computed. modflash_ffg7.jpg - 18924 Bytes The multi-step process to derive gridded runoffs requires considerable resources. Fortunately, the process needs to be executed only once. If the raw data and/or algorithms are changed, the process may be repeated to derive updated threshold runoffs. SNOW ISSUES Frequently, rainfall occurs on a snow pack, and it would be possible that significant headwater or even flash flood events could result. The FFGS was designed and developed to include snowmelt in computing flash flood guidance. With the snow model flash flood guidance will have a much wider range of values than with no snow model as in the past. High flash flood guidance in the middle of winter and with a significant snow pack should not alarm a forecaster. On the other hand, low flash flood guidance accompanied by warm temperatures and a riper snow pack should alert the forecast to a potential flood situation. Add rain, and most likely there will be a flood. PRODUCTS Headwater, zone/county, and flash flood guidance are generated operationally at the RFCs to support the current flash flood watch and warning program at the WFOs. modflash_ffg8.jpg - 3046 Bytes

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