Influence of Permeability on the Local Scour in an ELJ`S Groyne

A series of 5 experiments was conducted to study the influence of groyne permeability on the local scour at the groyne tip. One permeable groyne, whose length (perpendicular to the flow direction) was 0.475 m (1.5 ft.) and whose width was 0.26 m (0.857 ft) was placed on the right side of a rectangular channel. Groynes were made up of cylindrical wood pieces that represented tree trunks. They were interlocked to form a rectangular prism that was a scaled down Engineered Log Jam (ELJ) groyne. Permeabilities of these structures ranged from 28 % to 51 %.

Tests where carried out in a 12 m (39.4 ft) long flume whose inner width was 1.9 m (6.23 ft). Maximum discharge was 155 L/s. Initial bed slope was 0.008 m/m. The streambed was composed by sand whose average diameter, D₅₀, was 0.204 mm. An Acoustic Doppler Velocimeter (ADV) was used to measure and map 3D velocities at selected cross sections and around the groyne. A 3D laser scanner was used to capture bed topography before and after the tests.

As a result of the experiments it was observed that local scour at the groyne tip decreases when groyne permeability increases. ELJ groyne permeabilities were 28%, 36%, 42.5%, 45.5% and 51%. For these permeabilities local maximum local scour, recorded at the groyne tip were 0.225 m, 0.092 m, 0.090 m, 0.088 m and 0.087 m, respectively.. A practical lower limit for maximum local scour was found when the groyne permeability was between 42.5 and 51%. ELJ groyne permeability may be included as a design criterion when local scour is to be reduced in an erosion control project.

Keywords: Engineering Log Jams, Permeable Groynes, Local Scour, Tip Groyne.