Modernized Flash Flood Guidance
Timothy L. Sweeney, HRL
Thomas F. Baumgardner, MARFC
Last Modified 6/23/1999
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.
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.
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:
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.
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
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
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.
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).
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.
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.
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.
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.