An ArcView GIS Application for Deriving ThresholdRunoff Values to assist with Flash Flood Guidance
Seann M. ReedResearch Hydrologist
NWS-HRLSilver Spring, MD
Outline
• Definitions (ThreshR/FFG)
• Input data sets
• AV-ThreshR Software (Methods/GUI)
• Status/Future Plans
TR
What is AV-ThreshR?
• A GIS database and tool set used to objectively derive threshold runoff values.
TR
• Uses ArcView and Spatial Analyst with a customized GUI
• Uses the same theory as the previous GRASS-ThreshRwith enhanced features:
› delivered with a custom database› ArcView GUI › computes more than 30 different subbasin characteristics
What is threshold runoff?TR
Threshold runoff (TR) = average runoff in inches over an area required to fill a channel
Qf or “bankfull” flow
TR = Qf/Qp
Qf is the “flooding” or bankfull flow [cfs]
Qp is the unit hydrograph peak flow [cfs/inch]
EstimatorsTR
Qf : Flooding Flow Inputs not Available from GIS database
• Manning’s Equation Bb, Db, Sc, n
• Flood Frequency (e.g. Q2) inputs now available for
each U.S. stateQp : Unit Graph Peak Flow
Synthetic unit graph• Snyder’s UG Cp, Ct
• Geomorphological UG Bb, Sc, n
Flash-Flood GuidanceTR
• Flash flood: “a flood which follows generally lessthan 6 hours of heavy or excessive rainfall.”
• Flash flood guidance: “average rain needed overan area during a specified period of time to initiateflooding on small streams”
From NOAA Tech Memo NWS Hydro 44: “ModernizedAreal Flash Flood Guidance”
How is threshold runoff used in FFG?TR
Rainfall-runoff curve for currentconditions in an RFC basin
TR
FFGRainfall
Run
off
Threshold Runoff : General Computational Steps
1. Define “small” subbasins and compute subbasin parameters.
2. Compute flooding flow (Qf), unit hydrograph peak flow (Qp), and threshold runoff for each subbasin of interest.
3. Interpolate to the HRAP grid for the “gridded” flash-flood guidance option.
TR
DEMs with National CoverageTR
DEMs OtherwiseKnown As:
~ Minimum BasinSize:
ApproximateSource Map Scale
3 arc-second 1 DegreeDEMs
5 km2 1:250,000
15 arc-second IHABBS DEM 60 km2
1 arc-second NED < 1 km2 1:24,000
Known Issues:• 15 arc-second DEM not accurate enough to resolve flash flood scale basins; 3 arc-second DEM may not be much better, esp. in areas of low relief
• NED is the best available data -- now nationally available
Solutions:• NSSL national basin delineation using NED• NED-H
Example of Problem(s) with Coarser DEMsTR
15 arc-second corrected with RF1 (gray)
3 arc-second (red)
RF1 (dark blue)
RF3 (light blue)Enlarged Area
NED boundary
TRSubbasin Delineation Methods
c. Define subbasins(catchment reaches)
a. Define stream cells
b. Define stream reaches (links)
d. Add headwaters of uniform size(optional)
(9 subbasins)(14 subbasins5 headwaters split)
TRUse Flood Frequency Equations to Approximate Qf
USGS Water-Resources Investigations Report 94-4002, Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., "Nationwide Summary of U.S. Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods for Ungaged Sites, 1993," Reston, VA, 1994.
• States may be divided into different regions (in the 50 U.S. states and Puerto Rico, there are 210 Regions)
• Most (but not all) regression equations are of the form:
Qt = aXbYc . . .
• Equations for Q2, Q5, Q10, Q25, Q50, and Q100 are typically available in each region; however, equations for certain return periods are not available in some regions.
BA
Hydrologic regions in Arkansas:
Example:Q2B = 0.120A0.78S0.42(P-30)0.55E 0.75
TR GIS Database for Regression Equations
Data Description File Type # of Statesrequiringthis
Surface Water Storage Shapefile-Polygon
16
Mean Annual Precipitation[in]
Grid 19
Minimum mean JanuaryTemperature [degrees F]
Grid 4
Rainfall amount for a givenduration [in]
Grid 14
Forest Cover [%] Grid 8Soils Characteristics Shapefile-
Polygon3
Mean Annual Snowfall [in] Grid 2Area of Stratified Drift [%] Shapefile-
Polygon1
Runoff Coefficient Grid 1Drainage Frequency (numberof first order streams persquare mile)
Shapefile-Line 1
Mean Annual Runoff [in] Grid 1Normal Daily May-MarchTemp [degrees F]
Grid 1
Impervious Cover [%] Shapefile-Polygon
1
Annual PET [in] Grid 1
In addition to basic topographic data sets, these layersare also required:
73 total
QpR = f(A, L, Lc, tpR, Ct, Cp)
Time
Dis
char
ge
QpR
tR
tpR
TRUnit Graph Peak Flow, Snyder Method
tp=Ct*(L*Lc)^0.3
tr=tp/5.5
tpR=tp-((tr-tR)/4)
QpR=640*(Cp/tpR)*A
TRAV-ThreshR Functions
12
43
567
98
TR1. Setup
TR2. Define Subbasins: Streamlink Method
5 Outputs from the this step
TR3. Determine Connectivity
• Stream arcs have a direction
(subbasin)
(stream)
• One arc per subbasin (common grid-code)
TRCumulative vs. Local Parameter Calculations
Soil permeability example
Description Field NameCumulative Drainage Area (mi2) Arm
Slope of the Longest Flowpath (100%) (ft/mi) Chsl0
Slope of the Longest Flowpath (85%-10%) (ft/mi) Chsl
Length of the Longest Flowpath (mi) Chln
Length to the Point on the Longest Flowpath Oppositethe Basin Centroid
Chcn
Cumulative Mean Slope BASL
Code indicating whether a subbasin is a headwater(1=headwater; 0=non-headwater)
Shdishead
Code indicating whether a subbasin is and outlet. Shdisout
Two letter state code indicating which state containsthe subbasin centroid. For non-headwater subbasins,this is computed using the cumulative subbasin shape.
Stateabbr
List of regions intersected by the cumulative subbasinshape.
Regions
Fraction of subbasin (within the state of interest) thatfalls in each region.
Reg_fract
Basic Parameters Computed for All Locations
4. Compute Subbasin ParametersTR
4. Compute Subbasin ParametersTR
y = 0.7764x + 1.2716R2 = 0.9124
0
100
200
300
400
500
600
0 200 400 600
CHSL0 (ft/mi)
CHS
L (ft
/mi)
CHSL0 =(649 - 97)
94386= 30.9
4. Compute Subbasin ParametersTR
ParcodeFtypeFwidthFprecision
DescriptionUnitsMeastypeSrcfile
SrcdirSrctypeConversion
4. Compute Subbasin ParametersTR
Meastype attribute: Generic grid average and polygon average functions for local calculations.
4. Compute Subbasin ParametersTR
Which parameters must be computed for a given subbasin?
Attribute Table for Statekey.shp
5. Compute Q2, Q5 etcTR
StateStateabbrRegionRegionnum
ArealevelRetperiodSpecialNumterms
TermsCoeffsUlimit
5. Compute Q2, Q5 etcTR
AR
OK
6. Unit Graph Peak FlowTR
7. Subbasin Threshold RunoffTR
8. Interpolate to HRAP GRIDTR
What does “gridded” flash-flood guidance mean?
6 ||| 355 600 1 1.13 6 ||| 355 601 1 1.13 6 ||| 355 602 1 1.06 6 ||| 355 603 1 0.10 6 ||| 355 604 1 0.00 6 ||| 355 605 1 0.00 6 ||| 356 600 1 1.13 6 ||| 356 601 1 1.13 6 ||| 356 602 1 1.04 6 ||| 356 603 1 0.12
. . . .
. . . .
. . . .
(hrapx)(hrapy) (ThR)(Dur)
FFG ASCII File
TRWrite FFG ASCII File
AV-ThreshR Status
• AV-ThreshR Version 1 of programs is complete
• Qx regression equation data layers delivered by MTU (Dec. 1999)
• Flood frequency regression data and flow direction grids for ABRFC and MBRFC checked, corrected, and pre-processed (requires 2-3 days per RFC)
• Draft documentation is online:http://hsp.nws.noaa.gov/oh/hrl/gis/threshrhome.html
Please send questions or comments to [email protected]
TR
TR
• AV-ThreshR Computational steps are modular so that » time intensive calculations are separated from quick calculations that may be repeated» users can look at subbasin parameter values in tables to understand where threshold runoff values come from
• It is important to recognize database limitations. Database improvements are needed.
• ThreshR is only one part of FFG -- the other major part is the underlying rainfall-runoff model.
Review of Main Points
TRFuture Plans
• Possible Database Improvements:» National database of unit graph coefficients (or regression equations to derive these coefficients).
» Use basins and parameters derived from NED (NSSL work)
» Update USGS flood frequency equations
• Validation and feedback
TRTerrain/Network Database Size Estimates (ABRFC*)
RF1, 15 arc-second sizes(GB)
RF3/NED, 1 arc-second sizes (GB)
Edited StreamlineData
0.005 0.5
DEM 0.005 1.2
Flow Direction 0.004 0.86
Stream Grid 0.002 0.29
Flow Accumulation 0.026 5.9
Flow Length 0.026 5.9
Slope 0.005 1.2
Vector Streams 0.002 0.27
Vector Subbasins 0.013 1.6
Grand Total 0.088 17.7
RF1, 15 arc-second sizes(GB)
RF3/NED, 1 arc-second sizes (GB)
Edited StreamlineData
0.005 0.5
DEM 0.005 1.2
Flow Direction 0.004 0.86
Stream Grid 0.002 0.29
Vector Streams 0.002 0.27
Vector Subbasins 0.013 1.6
Grand Total 0.031 4.7
Input/Output Terrain Data** for Current SystemRequired Data w/ Pre-computed Parameters
(Retaining the Ability to Re-delineate Basins)
Required Data w/ Pre-computed Parameters andNo Opportunity to Re-delineate Basins
* To approximate sizes for the largess RFC (MBRFC) multiply these values for ABRFC by a factor of 2.5. ** Does not include flood frequency layers. The size of these data layers will not change when using a higher resolution DEM.
RF1, 15arc-secondsizes (GB)
NHD, 1 arc-second sizes(GB)
Vector Streams 0.002 0.27Vector Subbasins 0.013 1.6 Grand Total 0.015 1.9