An ArcView GIS Application for Deriving Threshold Runoff Values to assist with Flash Flood Guidance Seann M. Reed Research Hydrologist NWS-HRL Silver 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-ThreshR with 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 Q f or “bankfull” flow TR = Q f /Q p Q f is the “flooding” or bankfull flow [cfs] Q p is the unit hydrograph peak flow [cfs/inch]

Estimators TR Q f : Flooding FlowInputs not Available from GIS database Manning’s Equation B b, D b, S c, n Flood Frequency (e.g. Q2)inputs now available for each U.S. state Q p : Unit Graph Peak Flow Synthetic unit graph Snyder’s UGC p, C t Geomorphological UGB b, S c, n

Flash-Flood Guidance TR Flash flood: “a flood which follows generally less than 6 hours of heavy or excessive rainfall.” Flash flood guidance: “average rain needed over an area during a specified period of time to initiate flooding on small streams” From NOAA Tech Memo NWS Hydro 44: “Modernized Areal Flash Flood Guidance”

How is threshold runoff used in FFG? TR Rainfall-runoff curve for current conditions in an RFC basin TR FFG Rainfall Runoff

Threshold Runoff : General Computational Steps 1. Define “small” subbasins and compute subbasin parameters. 2. Compute flooding flow (Q f ), unit hydrograph peak flow (Q p ), 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 Coverage TR 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 DEMs TR 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 subbasins 5 headwaters split)

TR Use Flood Frequency Equations to Approximate Q f USGS Water-Resources Investigations Report , 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, 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: Q t = aX b Y c... 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: Q 2B = 0.120A 0.78 S 0.42 (P-30) 0.55 E 0.75

TR GIS Database for Regression Equations In addition to basic topographic data sets, these layers are also required: 73 total

Q pR = f(A, L, L c, t pR, C t, C p ) Time Discharge Q pR tRtR t pR TR Unit Graph Peak Flow, Snyder Method t p =C t *(L*L c )^0.3 t r =t p /5.5 t pR =t p -((t r -t R )/4) Q pR =640*(C p /t pR )*A

TR AV-ThreshR Functions

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

Basic Parameters Computed for All Locations

4. Compute Subbasin Parameters TR

4. Compute Subbasin Parameters TR CHSL0 = ( ) = 30.9

4. Compute Subbasin Parameters TR Parcode Ftype Fwidth Fprecision Description Units Meastype Srcfile Srcdir Srctype Conversion

4. Compute Subbasin Parameters TR Meastype attribute: Generic grid average and polygon average functions for local calculations.

4. Compute Subbasin Parameters TR Which parameters must be computed for a given subbasin? Attribute Table for Statekey.shp

5. Compute Q2, Q5 etc TR State Stateabbr Region Regionnum Arealevel Retperiod Special Numterms Terms Coeffs Ulimit

5. Compute Q2, Q5 etc TR AR OK

6. Unit Graph Peak Flow TR

7. Subbasin Threshold Runoff TR

8. Interpolate to HRAP GRID TR What does “gridded” flash-flood guidance mean?

.. (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: Please send questions or comments to 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

TR Future 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

TR Terrain/Network Database Size Estimates (ABRFC*) 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 and No 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.