Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 8/7/1999
Publication Date: N/A
Interpretive Summary: Many streams in agricultural watersheds are characterized by unstable channel boundaries. Erosion in these unstable channels not only destroys valuable agricultural land, but also degrades the habitat for fish and other aquatic organisms. Bank protection measures have the potential to preserve valuable agricultural lands and enhance aquatic habitats, however, abutments and spur dikes (structures commonly used to protect stream banks) also produce erosion of the stream bottom in the vicinity of the structure. The extent of this erosion must be accurately predicted and managed to prevent other problems. A laboratory study was conducted to improve the prediction of the shape and size of the local erosion associated with abutments. Accurate prediction of the size of the local erosion caused by the structure is critical for design and impact of the abutment on the habitat. Managing the location and size of pools can be an important part of improving aquatic habitats in unstable stream channels.
Technical Abstract: The present study was aimed at measuring the scour hole profiles with time at bridge abutments under steady flow conditions, and under simulated hydrographs. Experiments were conducted in a 30.5 m long, 1.22 m wide, and 0.61 m deep laboratory flume. Scour hole profiles in the vicinity of a model abutment were measured over prolonged periods of time. The experimental setup enabled the undisturbed scour profiles to be measured continuously with time. A total of 19 tests under steady flow conditions were conducted. Each test was a unique combination of flow depth, shear velocity, and flow rate. Data from this study demonstrated that semi-logarithmic equations better represented the temporal variation of maximum scour depth than power equations. Six experiments with simulated hydrographs were conducted. Based on the results from the steady flow experiments the scour depth under simulated hydrographs was calculated using the method of superposition. The measured scour depths were compared with the calculated scour depths and an acceptable agreement was obtained. The maximum scour depths measured under steady flow conditions and under simulated hydrographs were considerably less than the values calculated from FHWA recommended equations. For experiments under both steady flow conditions and simulated hydrographs, scour hole contour plots, in conjunction with the longitudinal and sectional profiles, demonstrated the geometric similarity of the scour holes. Scour hole extent was related to the maximum scour depth.