OHD/HL - Threshold Runoff: Introduction

Background

For flash flood guidance purposes, threshold runoff for a basin is the stream flow at "flood stage" divided by the unit hydrograph peak flow for that basin as shown in Equation 1.

        Equation 1                          R = Qf / qpR* A

In Equation 1, R is threshold runoff [inches], Qf is the flood discharge in cubic feet per second [cfs], qp is the unit hydrograph peak flow in cfs per unit area in square miles [cfs/sq mi], and A is the subbasin drainage area [mi2].

The task at hand is to determine threshold runoff values for thousands of small watersheds across the country.  Nationally available GIS databases can be used to define basins and compute basin characteristics that will help in the estimation of Qf and qp.
 

• USGS digital elevation models can be used to define basin boundaries and to derive topographic attributes such as drainage area, longest flow path length, and longest flow path slope.  3 arc-second digital elevation models have been available nationally for many years.  The National Operational Hydrologic Remote Sensing Center resampled a 3 arc-second DEM to produce a 15 arc-second dataset of the United States that is more manageable than the complete 3 arc-second data set on a national basis.  For the AV-ThreshR project, these 15 arc-second data were projected and resampled to produce a 400 m DEM for each RFCs.

• The 2 year or 5 year return period peak flows (Q2 or Q5) can be used as a surrogate for flooding flow.  Regression equations to estimate these quantities in all 50 U.S. states and Puerto Rico have been published by the U.S. Geological Survey in Water Resources Investigations Report 94-4002.  To provide software that will estimate these flood flows for thousands of basins in the United States, an extensive underlying GIS parameter database is required.  A contractor, Michigan Technological University, has collected GIS layers that can be used to derive the required parameter information and provided this data to HRL.

An ArcView application has been devloped to work with these datasets and automatically calculate the required basin parameters.  The major improvements in this system over the initial GRASS-based threshold runoff software are that a national terrain and regression parameter database has been prepared and the software functionality has been increased to estimate many more parameters.  Using the popular and user friendly ArcView interface will make it easy for RFCs to understand, update, and correct derived parameter values as necessary.  As with the earlier GRASS-based system, there is no national database to support selection of synthetic unit graph coefficients (e.g. Snyder's Cp and Ct) in the initial release of AV-ThreshR, so estimation of these parameters still requires local knowledge.

Documentation Overview

The philosophy in developing this system was to complete the one-time, data preparation tasks that require extensive GIS experience before delivering the system to the RFCs.  With the database in place, the software functionality can be thought of as consisting of three major components:  (1) defining subbasins and computing subbasin topographic and climatic parameters (25 parameters are required for MBRFC), (2) for each subbasin, calculating 2 year or 5 year return period flood flows (Q2 or Q5) (uses as surrogates for bankfull flow) from regression equations, calculating the unit hydrograph peak Qp, and computing threshold runoff, and (3) interpolating results to the HRAP grid and exporting to FFG.

In addition to this overview, the AV-ThreshR documentation includes "User Documentation," "System Documentation," and "Workshop Instructions."  The user documentation is the most extensive, describing the ThreshR database, methods, and user options in enough detail so that the user can understand the computational procedures without needing to understand the actual code.  The user documentation also provides information about the limitations of the input data and software.  With knowledge of how the programs work from the user documentation,  the system documentation provides more detail about the codes and database so that subsequent developers can add to or modify the codes and update the database.