Title: CBRFC Flash Flood Support
1CBRFC Flash Flood Support
2Call Outline
- CBRFC Flash Flood Guidance (credit Greg Smith)
- CBRFC Distributed Model Project (credit Ed
Clark) - FFMP Enhancements (OCWWS Mary Mullusky)
- WFO Best Practices
3WR 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
4Western 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.
5The 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
6FFPI Method Creating a single FFPI layer
Hydrologic response
3
7
Slope
Greater response and potential
7
8
3
Forest Density
Lesser response and potential
3
6
4
Land Use
4
9
6
Soil
9
Flash Flood Potential Index
7FFPI in a Gridded Format
East Fork of the Virgin River Southwest Utah
FFPI
1
10
8CBRFC 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.
9Palmer 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.
10CBRFC 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
11CBRFC Flash Flood Guidance Influenced by the
Flash Flood Potential Index
12What 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.
13Distributed Model and Flash Floods
- Ultimately we want to model flash floods
directly. - Long term effort beginning in Tucson area this
and next summer
14Proposed 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.
15Operational Flood Threshold Frequency Concept
For Each Cell
Critical Threshold
Frequency
Runoff
Grid of Discharge
Discharge Exceedence values
16Improvements 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.
17Coming Soon to FFMP(Mary Mullusky)
18WFO Best Practices
19CBRFC 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