An Overview of Gridded Flash Flood Guidance; A Spatially Distributed Runoff and Threshold-Runoff Based Approach

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An Overview of Gridded Flash Flood Guidance A
Spatially Distributed Runoff and Threshold-Runoff
Based Approach
Erick BoehmlerNERFC
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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
Variable Threshold Runoff
Gridded FFG
How much runoff to produce flash flooding
How much rain to produce runoff
Arkansas-Red Basin River Forecast Center
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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
Upper Zone Saturation Ratio
4km gridded soil moisture products
Arkansas-Red Basin River Forecast Center
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Upper Zone Saturation
Upper zone saturation, 5/1/2009
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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Varying Curve Numbers by Antecedent Soil
Moisture Conditions
75 Upper Zone Saturation

Soil Moisture Adjusted Curve Number
Average Curve Number
Arkansas-Red Basin River Forecast Center
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NRCS Curve Number Graphic
Arkansas-Red Basin River Forecast Center
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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
y average watershed slope
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
Arkansas-Red Basin River Forecast Center
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NERFC Area Computed Thresh-r
?

Qs f (design storm, slope, curve number)
Qp f (slope, curve number)
Thresh-r
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GFFG System FFG Calculation
S (1000/CN)-10
Q ThreshR
Arkansas-Red Basin River Forecast Center
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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

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 [phone] john.schmidt_at_noaa.gov
Tony Anderson [phone] tony.anderson_at_noaa.go
v
James Paul [phone] james.paul_at_noaa.gov
Erick Boehmler [phone] erick.boehmler_at_noaa.
gov