Delineating River Bottom Substrate using Very High-Resolution Digital Imagery derived from Large Scale Aerial Photography.

Abstract

Progress Energy is currently relicensing the Tillery and Blewett Falls developments (i.e., Yadkin-Pee Dee River Hydroelectric Project No. 2206) with the Federal Regulatory Commission (FERC). As part of the relicensing process, Progress Energy established Resource Work Groups (RWG) during May 2003 to identify environmental issues associated with Project operations and develop study plans, if necessary, specific to Project lands and associated lakes and tailwater area. The Water Resources Work Group identified the need to document existing gravel and cobble bars in the immediate tailwater areas below the Tillery and Blewett Falls Hydroelectric Plants and determine the stability or persistence of these substrate types through time (i.e., Progress Energy [2004] and Water RWG Issue No. 10, "Sediment Transport").

Characterizing substrates is important for three reasons. First, many fish and other aquatic species require specific substrate types for spawning and inhabitation. Gravel and cobble, in particular, provide a substantial amount of surface area for deposition of eggs into redds or adhering to exposed surfaces. These substrate types also provide the micro-habitat conditions and interstitial spaces needed by many fish and aquatic invertebrate species. Second, the substrate composition determines the roughness of stream channel, and roughness has a large influence on the channel hydraulics (water depth, width, and current velocity) of stream habitat. Third, substrate composition, including the degree of embeddedness, can indicate localized and broader watershed anthropogenic influences on stream habitat quality. For example, small particle composition may reflect land surface disturbances such as forestry and agriculture practices (Bain and Stevenson 1999).

The study objective was to assess the distribution and extent of the existing substrate types, including gravel and cobble bars, in the immediate tailwater areas below the Tillery and Blewett Falls Hydroelectric Plants. Each substrate type was classified and delineated from high-resolution aerial photography using Geographical Information System (GIS) software. The study also evaluated the spatial patterns of classified substrate types within each power plant tailwater study area.

Low altitude, high-resolution aerial photography was acquired with a fixed wing aircraft at each study area during February 2005. The aerial photographs were scanned, orthorectified with established ground elevation control data, and then composited to produce one georeferenced mosaic digital image for each study area. Substrates were delineated and classified in a digital environment using GIS software. A ground-truthing accuracy assessment was conducted by comparing known field substrate classifications from GPS established transects to the classified GIS electronic image.

A total of 1,261 substrate type areas encompassing 296 acres were mapped within the Tillery and Blewett Falls Hydroelectric Plant tailwater study areas. Bedrock outcrops were the most frequently mapped habitat category numerically (1,106 areas) but only comprised 16 percent of total mapped acreage. Smaller particle substrates or mixtures of small and large particle substrates (e.g., cobble⁄gravel, bedrock⁄boulder⁄cobble, and cobble⁄gravel⁄boulder) were the most prevalent on an area basis within both study areas. Collectively, they comprised 76 percent of the total study area.

Based on these results, cobble and gravel substrates or mixtures of these substrates with larger substrates such as boulder and bedrock were prevalent in the immediate tailwater areas below the Tillery and Blewett Falls Hydroelectric Plants. There was no evidence of substantial scouring or armoring of the river channel given that areas with smaller substrate particles or mixtures of smaller and coarser particle substrates were frequently mapped within each power plant tailwater area.