Table of Contents
This page outlines and compares efforts used to identify a hydrologic, geomorphic, and geospatially sound method to delineate floodplain extent. The goal of this effort is to map floodplains for RSQA sites, and eventually expanding these efforts to a CONUS scale. Smith Creek near New Market, Virginia is used as a test basin for each method. Each method is outlined below followed by a comparison of the results.
Methods explored include:
USGS Stream Channel and Floodplain Metric Toolbox (Beta Version 1.3) by Lamont et al.
An adaptation of the methods outlined in Geospatial Assessment of Ecological Functions and Flood-Related Risks on Floodplains along Major Rivers in the Puget Sound Basin, Washington by Christopher P. Konrad
USFS Riparian Buffer Delineation Tool by Sinan Abood
This tool was created by the Natural Resource Analysis Center (NRAC) at West Virginia University (WVU) to demonstrate the feasibility of mapping fluvial geomorphic features from multi-resolution bare-earth elevation data. A Python toolbox for ArcGIS was built to calculate key metrics describing channel and floodplain geometry based on published works such as GeoNet (Passalacqua, 2012), the Riparian Topography Toolbox (Dilts and Yang, 2010), and the River Bathymetry Toolkit (McKean, et al, 2009), among others. This toolkit provides the abilty to calculate specific channel and floodplain geometry metrics on a watershed scale. The tool works using TauDEM, a free terrain analysis software package which can be obtained here. TauDEM is open source and has the ability to run on regional datasets.
This toolbox was designed for regional use, specifically in the Chesapeake Bay watershed. The processing time for DEM acquisition, pre-processing, post-processing and analysis of a HUC 10 watershed is approximately 90 minutes using mostly default parameter settings. Processing and run-time statistics would be subject to change with higher or lower resolution DEMs and larger or smaller watersheds.
The output raster can be seen in Figure 3.
Figure 1. 1/9 arc second bare-earth Digital Elevation Model (DEM) of Smith Creek
Figure 2. Default parameters for Bank Detection and Floodplain Analysis Tool
Figure 3. Close-up view of Floodplain extent raster (green) overlaying 1/3 arc second DEM (using hillshade effect).
This tool was created by Dr. Sinan Abood from the U.S. Forest Service in an effort to create a more robust, hydrologically, and geomorphologically significant approach to delineating riparian ecotones. This approach was designed to enhance the fixed width buffer that has commonly been used for historical ecological riparian analysis. Fixed width buffers have been proven to be inadequate, as they do not emulate natural riparian corridors since they have no functional relationship to the naturally varying watercourse. In Skally and Sagor (2001), natural riparian ecotone boundaries were proven to be an average 2.5 time farther from the stream than what was mapped using a fixed width buffer. This tool hydrologically defines the riparian ecotone area by incorporating calculated 50 year flood heights with a DEM and the National Hydrography Data set using an ArcGIS toolbox.
The tool utilizes a Digital Elevation Model (DEM). For Smith Creek, a 1/3 arc second (~10 meters) DEM was downloaded from the National Map.
This tool was designed by the UFS to create a national context inventory of riparian areas and their condition within national forests and rangelands, but can be used for simple hydrologic estimation. The output of the tool is shown in Figure 4.
Figure 1. Second order polynomial equation fitted to 50 year flood heights vs. stream order
Figure 2. RBDM tool inputs
Figure 3. Floodplain extent using Riparian Buffer Delineation Tool
This is an adaptation of the flood risk assessment created by Christopher Konrad in the Puget Sound Basin. It extracts floodplain delineation techniques used in the assessment of flood risk and hazard in the Puget Sound Basin in Washington. The original methods address five ecological functions, five components of flood-related risks at two spatial resolutions—fine and coarse. The fine-resolution assessment compiled spatial attributes of floodplains from existing, publicly available sources and integrated the attributes into 10-meter rasters for each function, hazard, or exposure. The raster values generally represent different types of floodplains with regard to each function, hazard, or exposure rather than the degree of function, hazard, or exposure. The coarse-resolution assessment tabulates attributes from the fine-resolution assessment for larger floodplain units, which are floodplains associated with 0.1 to 21-kilometer long segments of major rivers. The coarse-resolution assessment also derives indices that can be used to compare function or risk among different floodplain units and to develop normative (based on observed distributions) standards.
This tool was adapted from a regional flood risk analysis in Washington. A 50-year flood height was used for comparison purposes, but different flood heights could be tested. The output raster is shown in Figure 3.
Figure 1. Stream surface elevation IDW interpolation
Figure 2. Conditional statement to extract floodplain extent
Figure 3. Floodplain extent raster