Flat Collar - A Faulty Scour Countermeasure
Flat collar or concrete armor around the vertical-wall abutment -- tests in AUR's Flume Facility
The figures below present the bed elevation contours around a vertical-wall abutment with and without a flat collar. The tests are completed under the incipient scour condition. The vertical-wall abutments are identical and the ratio of the protrusion abutment length to the water depth is 2.88. The abutment on the right side is surrounded by an additional flat apron, which is implemented to control the local scour around the abutment.
The flume test results below demonstrate that the flat collar surrounding the vertical-wall abutment fails as a scour countermeasure. The scour hole at the upstream corner is eliminated, but the downstream scour hole due to the wake vortex shedding becomes more severe and the maximum scour depth is about 0.09L, which is almost 30% deeper than the case without the collar. This is mainly because that more energy is dissipated at the upstream scour hole without the flat collar. The flat collar does not change the scour-causing complex vortical flow structures around the abutment and fails as a scour countermeasure.
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Figure 1. Bed elevation contours around a vertical-wall abutment with and without a flat collar. (L is the protrusion length and y is the bed elevation)