CBRFC Flash Flood Support

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CBRFC Flash Flood Support
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Call Outline

• CBRFC Flash Flood Guidance (credit Greg Smith)
• CBRFC Distributed Model Project (credit Ed
  Clark)
• FFMP Enhancements (OCWWS Mary Mullusky)
• WFO Best Practices

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WR produced training 2007 training module
describing FFG from each of the three WR
RFCs www.wrh.noaa.gov/wrh/hydro_science/wr_ffg/
This training augmented FFG module produced by
COMET
WR RFC FFG Generation Methods
Empirically based legacy method skewed and
given spatial variability based on the GIS
generated flash flood potential index (FFPI).
Colorado Basin
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Western Region Flash Flood Factors . . .

• Flash floods are generally localized,
  short-duration events associated with heavy rain.

• Flash floods frequently occur in small basins
  generally less than 20 square miles in size.

• Flash floods occur rapidly, on at time scale
  less than 1 hour.

• SAC-SMA calibrated basins are much larger in
  size than flash flood basins.

• SAC-SMA operates on time scales much greater
  than the flash flood time scale.

• It is very difficult to determine a critical
  flash flood flow/stage level required by ThreshR.

• SAC-SMA calibrations do not exist across the
  entire RFC area.

• Due to the limitations listed resulting FFG
  using SAC-SMA requires significant modification.

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The Flash Flood Potential Index uses information
such as land-use, forest density, slope, and soil
type to characterize an areas potential response
to heavy rainfall
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FFPI Method Creating a single FFPI layer
Hydrologic response
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Slope
Greater response and potential
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8
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Forest Density
Lesser response and potential
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6
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Land Use
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9
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Soil
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Flash Flood Potential Index
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FFPI in a Gridded Format
East Fork of the Virgin River Southwest Utah
FFPI
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10
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CBRFC FFG Method
Combination of an empirically based legacy method
and the GIS based flash flood potential index.
Empirical In Nature Based On Historical
Observations
Palmer Drought Index (PDI) Used to skew FFG
values (soil moisture effect). The PDI
application has minimal influence resulting in
FFG changes by approximately /- .10 inch.
Gridded FFPI - Used to adjust FFG on a grid-cell
by grid-cell basis to create greater spatial
resolution in FFG and account for physiographic
characteristics.
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Palmer Drought Effect
While the PDI isnt ideally suited for this
application it is a hold-over from an older
method and was an attempt to account for soil
moisture. Within the CBRFC application the PDI
accounts for less than a .20 change in the
original FFG value.
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CBRFC FFPI Multiplication Factors
The FFPI values range from 1 to 9 with a mean
value for the CBRFC near 4

• if FFPI 1 FFG multiplication factor is 1.50
• if FFPI 2 FFG multiplication factor is 1.30
• if FFPI 3 FFG multiplication factor is 1.20
• If FFPI 4 FFG multiplication factor is 1.0
• If FFPI 5 FFG multiplication factor is .90
• If FFPI 6 FFG multiplication factor is . 85
• If FFPI 7 FFG multiplication factor is .75
• if FFPI 8 FFG multiplication factor is .70
• if FFPI 9 FFG multiplication factor is .65

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CBRFC Flash Flood Guidance Influenced by the
Flash Flood Potential Index
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What about wildfire burn scars?
The Oxnard and Sand Diego WFOs within the CNRFC
area in California receive threshold FFG values
from the USGS. These values are then input into
FFMP using the forced-FFG option
The CBRFC has modified FFPI values within
wildfire burn areas in a test mode. Upon regular
receipt of burn area information it is
anticipated this will become an operational
practice.
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Distributed Model and Flash Floods

• Ultimately we want to model flash floods
  directly.
• Long term effort beginning in Tucson area this
  and next summer

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Proposed Methodology Hydrologic Threshold
Frequency Response

• Run the historical observed precipitation
  through the Hydrologic Lab Research Distributed
  Hydrologic Model (HL-RDHM) saving grids of
  discharge and surface flow for each hour.
• Establish the hourly distribution of discharge
  and surface flow for each cell.
• Run the model in real-time, sub-hourly, with
  quality controlled QPE and very-short-term QPF
  (e.g. NMQ or WFO generated) to produce the
  discharge and surface flow grids.
• For each grid cell, compare the value to the
  distribution of values and assign the exceedence
  category.
• Identify critical runoff values regionally and
  locally to determine flash flood occurrence.

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Operational Flood Threshold Frequency Concept
For Each Cell
Critical Threshold
Frequency
Runoff
Grid of Discharge
Discharge Exceedence values
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Improvements over existing FF Programs

• Explicit, real time, modeling of flash flood
  events.
• Continuous model maintains soil moisture states.
  
• Model is connected flows routed from cell to
  cell to show impacts in areas downstream from the
  precipitation event.
• Indicates where significant flows (e.g. flash
  floods) are occurring or could occur based on
  each cells hydrologic distribution.
• In time, WFO forecaster short-term storm
  projections can be used to derive quantified
  stream response.
• Begins to bridge the gap between traditional RFC
  river forecasts and the Flashflood program.

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Coming Soon to FFMP(Mary Mullusky)
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WFO Best Practices
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CBRFC WFO Meeting

• September 4
• Salt Lake
• Attendees
• SH or HFP (CBRFC AHPS can cover travel)
• One (or more) other WFO rep (e.g. WCM, SOO, or
  MIC)
• Agenda (to be fleshed out this summer)
• Highlight new CBRFC developments
• Meet new personnel
• Discuss service concerns
• Etc
• Please send any input to me