an arcview gis application for deriving threshold

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

FFG

Rainfall

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

TR

Subbasin 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.

B

A

Hydrologic regions in Arkansas:

Example:Q2B = 0.120A0.78S0.42(P-30)0.55E 0.75

TRGIS 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

cha

rge QpR

tR

tpR

TR

Unit 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

TR

AV-ThreshR Functions

12

43

567

98

TR

1. Setup

TR

2. Define Subbasins: Streamlink Method

5 Outputs from the this step

TR

3. Determine Connectivity

• Stream arcs have a direction

(subbasin)

(stream)

• One arc per subbasin (common grid-code)

TR

Cumulative 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 + 1.2716

R2 = 0.9124

0

100

200

300

400

500

600

0 200 400 600

CHSL0 (ft/mi)

CH

SL

(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

TR

Write 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 seann.reed@noaa.gov

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 System

Required 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