Gridded Flash Flood Guidance
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Transcript Gridded Flash Flood Guidance
An Overview of Gridded Flash Flood
Guidance; A Spatially Distributed Runoff and
Threshold-Runoff Based Approach
Erick Boehmler
NERFC
6/3/2010
ER FFG Conference
Eastern Region Flash Flood Guidance
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Goals of Gridded FFG Development
Produce a flash flood guidance product at a
resolution closer in scale to basins in FFMP
that:
Reflects the spatial variability of the physical
characteristics that impact the susceptibility of a
location to flash flooding
Uses freely available (or easily derived) data with
nationwide coverage
Requires minimal calibration effort
Fits seamlessly into the existing RFC and WFO
operational flash flood warning infrastructure
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Gridded Flash Flood Guidance
Advantages of GFFG approach are:
Improves the spatial resolution of soil-moisture
conditions and flash flood thresholds
Applies a spatially consistent soil-moisture
accounting model across RFC areas and
independent of RFC forecast basins
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Gridded Flash Flood Guidance
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Gridded Flash Flood Guidance
Gridded FFG has a significant precedence
It is an accepted and operationally utilized method for FFG grid
displays from Southern Region RFCs beginning in 2007
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Gridded Flash Flood Guidance Model
Length to
divide
Slope
OHD RDHM
Soil Moisture
Area
Static CN
Gridded Runoff
Duration
2-yr, 3-hr DDF
Dynamic NRCS
Curve Number
Gridded
FFG
How much rain to
produce runoff
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Variable
Threshold
Runoff
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Arkansas-Red Basin River Forecast Center
How much runoff
to produce flash
flooding
Soil Moisture Accounting with a Distributed
Hydrologic Model
4km gridded temperature
4km gridded precipitation
National Weather Service
Office of Hydrologic Development
Research Distributed Hydrologic Model
Sacramento
Model
Storages
4km gridded soil moisture products
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Arkansas-Red Basin River Forecast Center
UZFWC
LZTWC
LZFSC
LZFPC
UZTWC
Upper Zone
Saturation Ratio
Upper Zone Saturation
Upper zone saturation, 5/1/2009
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Upper zone saturation, 6/1/2009
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Runoff Computation
Development of Dynamic Curve
Number
Combined to create an average (ARCII)
Curve Number grid at HRAP scale
30m National Land Cover Dataset
1000m NRCS Hydrologic Soil Groups
Utilized upper zone saturation ratio from
RDHM output with NRCS curve number
model (NEH Part 630, chap. 10) to formulate
an automated curve number adjustment
Convex of average for wet conditions
Concave of average for dry conditions
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Soil Moisture Adjusted Curve Number
Varying Curve Numbers by
Antecedent Soil Moisture Conditions
100
90
75% Upper Zone Saturation
80
70
*
60
WET
NORMAL
50
DRY
40
30
20
10
0
0
10
20
30
40
50
60
Average Curve Number
6/3/2010
ER FFG Conference
Arkansas-Red Basin River Forecast Center
70
80
90
100
NRCS Curve Number Graphic
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ER FFG Conference
Arkansas-Red Basin River Forecast Center
Threshold Runoff Computation
Development of Variable Threshold
Runoff (Thresh-r)
Thresh-r is estimated at the HRAP scale
Ratio of flood flow, Qs to peak flow, Qp of the unit
hydrograph.
Qs is approximated by the annual return
period flow, Q2
Qp is derived through the use of NRCS’
triangular unit hydrograph method
Adjustment for runoff anticipated within the
next 6-hours from rainfall in progress through
latest available MPE grid
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NRCS Triangular Unit-graph Variables
Known Variables
CN = NRCS Curve Number
S = Abstraction =
(1000/CN)-10
l = length to divide
tp
l 0 .8 ( S 1)0.7
1900 y 0.5
y = average watershed slope
TR D / 2 t p
A = drainage area
D = duration of rainfall ( 1 Hour for unit
Hydrograph)
Calculated Variables
tp= lag time (time from center of
mass of rainfall to Qp)
TR = time to Qp from beginning of rainfall
Qp = peak discharge from unit hydrograph
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Arkansas-Red Basin River Forecast Center
484* A
Qp
TR
NERFC Area Computed Thresh-r
Qs = f (design storm,
slope, curve number)
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=
Qp =
f
(slope, curve number)
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Thresh-r
GFFG System FFG Calculation
0.4S Q Q 2 4QS
FFG
2
S = (1000/CN)-10
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Q = ThreshR
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Arkansas-Red Basin River Forecast Center
Limitations of the GFFG system
GFFG system limitations include:
No projection of snow-melt runoff within applicable
duration of FFG value
RDHM modeled SWE 1/25 12Z
MPE 12 hours ending 1/25 12Z
SCHOHARIE COUNTY
SWE about 3 inches
Approaching rainfall
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Limitations of the GFFG system
GFFG system limitations include:
No projection of snow-melt runoff within applicable duration of
FFG value
GFFG system 1/25/2010 00Z
GFFG system 1/25/2010 12Z
SCHOHARIE COUNTY
00Z 3-hr FFG = 1.9
12Z 3-hr FFG = 1.8
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Limitations of the GFFG system
GFFG system limitations include:
No projection of snow-melt runoff within applicable duration of
FFG value
Legacy system 1/25/2010 00Z
Legacy system 1/25/2010 12Z
SCHOHARIE COUNTY
00Z 3-hr FFG = 2.0
12Z 3-hr FFG = 1.4
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Limitations of the GFFG system
GFFG system limitations include:
CN adjustment for runoff response is
consistently overestimated during cool season
months
RDHM models upper zone tension water at
capacity
Upper soil zone contents to capacity ratio remains
high
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Questions / Contacts
John Schmidt
(918) 832-4109
[email protected]
Tony Anderson
(918) 832-4109
[email protected]
James Paul
(918) 832-4109
[email protected]
Erick Boehmler
(508) 824-5116
[email protected]
6/3/2010
ER FFG Conference