Parallel Walls as an Abutment Scour Countermeasure

Scour at bridge abutments can cause damage or failure of bridges and result in excessive repairs, loss of accessibility, or even death. To mitigate abutment scour, both clear-water and live-bed laboratory experiments in a compound channel were performed using parallel walls. Two types of parallel walls were tested: the first was made of a solid thin wood plate and the second was made of piled rocks. For solid parallel walls, a series of vertically oriented, rectangular, straight plates of different lengths attached to the upstream end of a wing wall abutment parallel to the flow direction were employed. Three velocities of 0.9, 1.5, and 2.3 times the incipient motion value for bed sediment movement were used. The bed material was sand with a mean diameter of $0.8\mathrm{mm}$ and a standard deviation of 1.37. All the plates were seated at the bottom of the compound channel bank slope and were even with the abutment face. It was found that straight plates thus situated are able to move the scour hole away from the upstream corner of the abutment. As the length of the plate increased, the scour at the abutment declined. It was found that a length of $1.6L$ , with $L$ being the length of the abutment perpendicular to the flow, caused the scour to be eliminated at the abutment for a velocity ratio $(U∕U_c)$ of 0.9 (clear-water scour). Similarly, a $1.6L$ long wall can reduce the time-averaged scour depth at the abutment by 100% for a velocity ratio of 1.5, and 70% for a velocity ratio of 2.3. If the upstream end of the wall is anchored below the scour depth, this countermeasure would likely be feasible for situations where rock is expensive. For parallel rock walls, various values of wall length and protrusion length into the main channel were tested. It was found that a wall that does not protrude into the main channel and having a length of $0.5L$ minimizes scour at the abutment for all three different velocity ratios (0.9, 1.5, and 2.3).