# Presentation: 524135

## An Operational, Spatially-Variable, Physically-Derived Gridded Flash Flood Guidance Model

**John Schmidt, Tony Anderson, **
**James Paul & Britt Westergard**
**Arkansas-Red Basin River Forecast Center**
**National Weather Service**

**Notes:**
The National Weather Service strives to provide timely, meaningful hydrologic warnings at a variety of spatial and temporal resolutions to mitigate the loss of life and property.

## Gridded Flash Flood Guidance

**Gridded Flash Flood Guidance**

**  ****WHY?  **Do we need a new FFG product for FFMP use?
-   **HOW?**  ABRFC GFFG Model.
**  **Customization
-   Verification (and the hurdles in getting any meaningful information).

## Legacy Basin, County and Gridded

**Legacy Basin, County and Gridded**
**Flash Flood Guidance**

**60-day precipitation accumulation in Pawnee County, KS ending December 21, 2006.**

**Notes:**
The legacy flash flood guidance system performs a “reverse run” of the basin-averaged hydrologic model that is used at NWS RFCs to produce river flood forecasts on larger order rivers.  Instead of estimating runoff from an input precipitation event, the model estimates what precipitation event would cause a predefined runoff event (ThreshR).  The RFC hydrologic models are conceptual models that do not directly account for inter or event intra-basin variability in slope, land use, soil types, etc.  Calculations are performed in a basin-averaged fashion, including the basin-averaging of precipitation, soil moisture and runoff.  ABRFC’s average basin size is approximately 300? square miles.  County guidance is simply calculated by performing a weighted average of all of that county’s constituent basins.  Gridded guidance is simply a gridded sampling of the basin-averaged FFG values.

## Issues of Scale and Spatial Variability

**Point A: Parunuweap Canyon (10 mi NW of Point B)**

**Point B: Sand dunes near Moquith Mountain**

**Courtesy of Greg Smith, CBRFC**

**Courtesy of Greg Smith, CBRFC**

**Courtesy of Greg Smith, CBRFC**

**Issues of Scale and Spatial Variability**
**Basin/County/FFMP**

**Notes:**
This example, from CBRFC, is a dramatic depiction of the intra-basin variability of landforms, topography, geology, etc that are both assigned the same FFG value using the legacy FFG system.

## Issues of Scale and Spatial Variability

**Issues of Scale and Spatial Variability**
**Basin/County/FFMP**

**Notes:**
With the introduction of AMBER and FFMP into WFO flash flood warning operations, some WFOs began analyzing radar estimates of rainfall accumulations on very small creeks.  They compared these precipitation accumulations to the legacy flash flood guidance products produced by RFCs at the larger, RFC-scale basins.  Analysis and comparison of FFG and precipitation accumulation at this scale started to expose the flaws in sampling basin-averaged FFG values down to the HRAP or FFMP scale.  The overlays of RFC-basin, county and FFMP-basin on top of LULC data graphically illustrated the potential problems in this method of analyzing small-scale flash flood threats.

## Goal of GFFG...

**Goal of GFFG...**

- Produce a flash flood guidance product at a resolution similar in scale to FFMP that:
- reflects the spatial variability of physical characteristics that impact the susceptibility of a location to flash flooding,
- uses freely available data (or easily manufactured data) with nation-wide coverage,
- requires minimal calibration effort and
- fits seamlessly into the existing RFC and WFO operational flash flood warning architecture.

## Calculating Soil Moisture Conditions Using a Distributed Hydrologic Model

**Arkansas-Red Basin River Forecast Center**

**Calculating Soil Moisture Conditions Using a Distributed Hydrologic Model**

- Sacramento
- Model
- Storages

- Upper Zone
- Saturation Ratio

**National Weather Service**
**Office of Hydrologic Development**
**Research Distributed Hydrologic Model**

**4km gridded precipitation**

**4km gridded soil moisture products**

_**UZ Sat**_

**Dry**

**Wet**

**Notes:**
Hourly 4-km HRAP gridded precipitation data is fed into the NWS Office of Hydrology’s Research Distributed Modeling System (RDMS).  This model utilizes the Sacramento Soil Moisture Accounting Model.  Upper Zone Saturation Ratios are calculated for each grid cell at the end of the RDMS run and used to modify the NRCS curve number.

## +

**+**

**30m NLCD Land Use Land Cover Data**

**1000m STATSGO HSG Data**

**Resultant NRCS Curve Number**

**NRCS Curve Number Rainfall-Runoff **
**Model for New Orleans, LA**

**Notes:**
The National Resources Conservation Service (NRCS) Curve Number model was originally developed on a set of chiefly agricultural basins.  The land use and soil properties of these basins were surveyed and a variety of rainfall-runoff events were analyzed.  Empirical relationships were derived from the test basins.  This method has been widely extrapolated and calibrated for specific basins where streamgage data is available.  The ABRFC GFFG model employs the NRCS CN model in its generic form.  It is not calibrated per se as the basins being modeled are ungaged.

The 1000m Hydrologic Soil Group (HSG) data is resampled down to 30m resolution of the LULC data to perform the calculation of Curve Number at a common resolution.  The resultant 30m grid is then aggregated up to approximately a 4000m grid for operational GFFG production.

The very specific definitions of runoff potential that are evident in the 30m product are lost to some extent during the aggregation up to the 4km HRAP scale.

## *

**Varying Curve Numbers by **
**Antecedent Soil Moisture Conditions**

**Average Curve Number**

**Soil Moisture Adjusted Curve Number**

*****

**75% Upper Zone Saturation**

**Notes:**
The original NRCS Curve Number model allows for the varying of the curve number based on antecedent soil moisture conditions (AMCs).  The NRCS developed adjustment equations based on a limited set of basins/events to serve as general bounding envelopes to these curve numbers.  During wetter AMCs, the curve number is increased.  During drier AMCs, the curve number is decreased.  We have modified this approach by calculating an “upper zone saturation” value from the NWS Office of Hydrology’s Research Distributed Modeling System (RDMS).  At the time of GFFG issuance, the state of the upper zone of the RDMS is captured. The upper zone is made up of free water and tension water components.  50% saturation is considered to be “normal”.  The curve number is linearly interpolated to the “wet” AMC equation that we define to be 100% upper zone saturation.  Likewise, the curve number is linearly interpolated to the “dry” AMC equation that we define to be 0% saturation.   The result is a rainfall/runoff relationship that is unique for each grid cell and unique for each time step. 

## Threshold Runoff (ThreshR) Calculation

**Threshold Runoff (ThreshR) Calculation**

**Flood Flow = 800 cfs**

**Peak Unit Flow = 1300 cfs**

**ThreshR is an *****estimate*** **of the amount of runoff required in a given basin area to produce flooding flows.**

**ThreshR**** = ****Flood Flow**** / ****Peak Unit Flow**

**ThreshR**** = ****800**** / ****1300**** = ****0.62**** **

**Notes:**
The legacy definition of Threshold Runoff (ThreshR) is used in GFFG, just calculated at the HRAP grid cell resolution.  ThreshR is a ratio of critical flow to unit peak flow.  Critical flow (bankfull flow, flash flood flow, etc) is left constant and the peak unit flows are varied by event duration.  Therefore, 1-hr ThreshR will be lower than 6-hr ThreshR because the peak flow of the 1-hour unit hydrograph is higher.

Qp is calculated using the NRCS synthetic unit hydrograph method whose variables include basin slope and basin curve number.  Since we are calculating Qp values on a grid, the remaining basin geometry variables are constant.  Estimating the “flood flow” for each grid cell is more problematic since these “basins” are ungaged.  Literature suggest that generally a 2-year return flow event represents a bankfull condition.  We are estimating this 2-year return flow by inputting the 5-yr, 3-hr design precipitation event into the curve number model to calculate runoff.  The runoff value can then be multiplied by the peak unit flow to calculated the associated flow level for that runoff.  This is only an estimate of ThreshR.  Ideally, these estimates would be replaced with measured values where appropriate.

## ThreshR Calculation

**ThreshR Calculation**

**Flood Flow (Q****s****)**

**Peak Unit Flow (Q****p****)**

**ThreshR**

**.**

**.**

**|**

**Legacy ThreshR**

**Q****s ****=**** ****f **** ****(design storm, slope, curve number)**

**Q****p ****=**** ****f ***** *****(slope, curve number)**

**Notes:**
At a finite number of river forecast points, hydrologists can physically measure the stage at which flooding begins to occur.  The NWS defines this stage as “flood stage”.  It is not feasible to visit each of the millions of FFMP basins to determine a “flash flood stage”.  Therefore, we must determine a method of estimating “bankfull flow” at these small headwater streams.  A rule-of-thumb description of bankfull flow is the 2-year return interval flow.  Localized research has shown that this value may be too high or too low in some instances.  Additionally, estimating 2-year return flows at ungaged basins requires some sort of modeling technique.  The USGS publishes regression equations for different return interval flows at different basin sizes, but these equations are generally based on political aggregations of gaged basins.  Applying these relationships at the RFC-scale produces data in which these political boundaries are evident.  ABRFC tested several different methods of estimating “bankfull flow” before deciding to input a “design precipitation event” into the NRCS curve number model and using the output peak flow as the “flash flood flow”.  This value is only an estimate and further refinement of this model would include user defined “flash flood flow” estimates at low water crossings, urban areas and other flash flood prone basins that violate the broader assumptions of “bankfull”.

## GFFG Calculation

**Arkansas-Red Basin River Forecast Center**

**GFFG Calculation**

**S = (1000/CN)-10**

**Q = ThreshR**

**Notes:**
Given an estimate of the amount of runoff required to produce flash flooding (ThreshR) and the current level of hydrologic sensitivity of each HRAP “basin” due to soil moisture, land use and soil type, we can estimate the amount of rainfall required to produce that runoff (flash flooding).  We solve the NRCS CN runoff equation for the precipitation variable to calculate this FFG value.  The S term is variable from run to run based on soil moisture.  Currently, the Q term is a fixed value, although we are currently testing a method of estimating the current fullness of the channel that would produce a variable ThreshR term immediately following a period of heavy rainfall.

## 3-Hr Basin-Averaged FFG 09/17-18/2006 12z Keystone Reservoir, OK

**3-Hr Basin-Averaged FFG 09/17-18/2006 12z**** ****Keystone Reservoir, OK**

**Basin-averaged FFG product from September 18, 2006**

**Basin-averaged FFG product from September 17, 2006**

**Arkansas-Red Basin River Forecast Center**

**24-hour rainfall ending September 18, 2006 12z**

**Notes:**
Legacy method of calculating FFG from a September 2006 event.  Although ABRFC does use gridded estimates of precipitation operationally, the first step in producing a river forecast is averaging those gridded estimates of precipitation to a basin.  The high level of variability in precipitation and subsequent soil moisture and FFG values are lost in this averaging process.

## 3-Hr Gridded FFG 09/17-18/2006  12z Keystone Reservoir, OK

**3-Hr Gridded FFG 09/17-18/2006  12z**** ****Keystone Reservoir, OK**

**Gridded FFG product from September 18, 2006**

**Gridded FFG product from September 17, 2006**

**Arkansas-Red Basin River Forecast Center**

**24-hour rainfall ending September 18, 2006 12z**

**Notes:**
The same September 2006 event using the GFFG methodology.  The high resolution is preserved resulting in FFG values in the areas receiving heavy rain to drop significantly while the areas receiving little rain remain relatively unchanged.

## March 23-24, 2007 Precipitation (6-hour Update Interval)

**March 23-24, 2007 Precipitation ****(6-hour Update Interval)**

## March 23-24, 2007 GFFG (6-hour Update Interval)

**March 23-24, 2007 GFFG ****(6-hour Update Interval)**

## June 15, 2006 3-hr Operational FFG

**June 15, 2006 3-hr Operational FFG**

## June 15, 2006 3-hr Experimental FFG

**June 15, 2006 3-hr Experimental FFG**

## Thresh-R Customization Technique and GUI

**Thresh-R Customization Technique and GUI**

-   Allows WFOs (or RFCs) to overwrite estimated Thresh-R values at FFMP basins or HRAP grid cells where the assumptions of bankfull flow are not close to reflecting the local reality.
-   Consists of a simplified surveying exercise, the calculation of velocity and “flash flood flow” from that survey. The final step is a re-writing of the Thresh-R with a “burned-in” value at the HRAP grid cell that represents that particular flash flood location.

## Spunky Creek near Catoosa, OK

**Spunky Creek near Catoosa, OK**

## Spunky Creek, OK @ E. 11th St.

**Spunky Creek, OK @ E. 11****th**** St.**

## Thresh-R Customization Data 

**Thresh-R Customization Data **

**Roughness = Manning’s n estimation**

**Notes:**
In addition to Hydraulic Radius and the Manning’s roughness calculation, the peak unit flow at the point of interest is required to calculate the customized Thresh-R value.

## V = 2.71 fps

**V = 2.71 fps**

**Q****s**** = 217 cfs**

**If Q****p**** = 558 cfs, **

**Thresh-R****meas**** = **_**0.39**_

**vs.**

**Thresh-R****est ****= 0.62**

**or**

**FFG****meas**** = 2.03**

**vs.**

**FFG****est**** = 2.45**

**Notes:**
The GUI user can use the guidelines of the application to calculate the needed variables in Manning’s equation.  A more advanced user could make a more accurate measurement of the A, R and n values through more thorough surveying and enter those values directly into the appropriate boxes.  The final step of the process is for the user to send the updated ThreshR value and lat/long to the RFC where software will burn that value into the Thresh-R xmrg for the corresponding HRAP grid cell.

## Verification and Evaluation

**Verification and Evaluation**

## Questions/Contacts

**Questions/Contacts**

**John Schmidt**
**(918) 832-4109**
**john.schmidt@noaa.gov**

**Tony Anderson**
**(918) 832-4109**
**tony.anderson@noaa.gov**

**James Paul**
**(918) 832-4109**
**james.paul@noaa.gov**

**Britt Westergard**
**(918) 832-4109**
**britt.westergard@noaa.gov**