Straight Rock Vane Induced Scour in Central Pennsylvania Lower Order, Gravel Bed Streams

Open Access
Author:
McKay, Michael David
Graduate Program:
Agricultural and Biological Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 02, 2010
Committee Members:
  • Dr A R Jarrett, Thesis Advisor
  • Albert R Jarrett, Thesis Advisor
Keywords:
  • incipient motion
  • stream restoration
  • rock vane
  • scour
  • fill
  • bedload transport
  • scour chains
  • shear stress
Abstract:
Stream restoration is a rapidly growing field with an estimate of over a billion dollars being spent on thousands of projects every year in the United States. However, this type of restoration is a relatively new science and the depth of knowledge surrounding this type of endeavor is still somewhat limited. In the few cases where projects are systematically studied to evaluate their performance, many structures were found to be either ineffective or outright failures. One source of these failures is a lack of understanding of how these structures affect bedload transport within the reach. The purpose of this study was to determine how a straight rock vane affects scour within three lower order gravel bed stream channels in north central Pennsylvania. Scour chains were used to measure the scour and fill associated with flow events of sufficient size as to initiate bedload transport. Networks of 18 to 26 scour chains were installed in a grid that extended across the channel and from above to below the rock vane and were used to determine the amount of aggradation and degradation occurring at these specific sites. Bulk core samples of the bed material were taken in order to determine the size of particles that were moving during these flows. Each of the three sites experienced two events that initiated bedload movement. The amount of scour and fill occurring at each site was then plotted spatially within the channel to determine the effect of the rock vane. Analysis of scour patterns indicated that flow constriction caused by the vane and the slow velocity areas upstream from the structure and the dissipation of kinetic energy resulting from the drop as the flow passes over the face of the rock vane resulted in greater scour depths through these areas. A zone was developed in an effort to define better these areas and analysis concluded that the depth of scour occurring within this zone, which averaged 104 mm, was significantly greater than the 77 mm of average scour that occurred in other channel locations. These results will help to further the understanding of how rock vanes affect bedload transport within gravel bed channels.