ABSTRACT: Urbanization, deforestation, channelization, and other man made changes have caused the natural state of many river systems to be disturbed, often to the point of channel degradation. Grade control structures can be implemented to mitigate channel degradation by controlling the level of a stream. Through years of research and development, grade control structures have been constructed utilizing numerous grade-change geometries and construction materials. General design guidelines for grade control have been previously outlined; however, to date the design guidelines have not made an attempt to quantify and compare pertinent design parameters of different grade-change geometries, especially for porous structures. To develop a decision-making process for selection and design of porous low-drop structures, a multiphase testing program was initiated at the Colorado State University Hydraulics Laboratory. For this study, grade control structures were constructed utilizing a 1:4-scale concrete armor unit, namely A-Jacks®, manufactured by Armortec Erosion Control Solutions, Inc.

An extensive literature review was conducted which identified: the mechanics of channel instability, five basic grade-change geometries of grade control, and numerous design criteria. Three design parameters were determined imperative for structure design: 1) flow depth and water surface elevation upstream of a structure, 2) energy loss through a grade change, and 3) local scour depth downstream of a grade control structure. Two test facilities were utilized to physically conduct a series of eighty-eight separate tests incorporating nine test configurations of variant structure geometry. Data collection included: discharge measurements, detailed bed and water surface profiles, 1-D velocities, and cross-sectional surveys upstream and downstream of each structure.

The test program resulted in developed relationships of each of the three design parameters, and a discussion of additional considerations including: A-Jacks structure stability, aquatic habitat development, fish passage through grade control, and a cost benefit analysis of each test configuration. A hydraulic control relationship of a porous weir was parameterized by expressing flow depth upstream of a structure as a function of the length of the control section, porosity of the structure, tailwater depth, and discharge. Additionally, energy loss relationships across a grade change were parameterized as a function of discharge, structure geometry, and Froude number upstream of the grade change for each test configuration. At the location of maximum scour depth, relationships were developed relating scour depth to tailwater elevation, discharge, scour-hole geometry, and velocity at the onset of motion of the sediment. Utilizing the developed relationships and additional considerations, a decision-making process for the selection and design of porous low-drop grade control structures was developed.

GRADUATED: Spring 2003

RESUME (pdf)