We have investigated beach stability against storm waves. The studies are done in relation to eroded beaches. We are testing a cobble-sand-gravel mixture as a means of using a soft method for coastal protection on nourished beaches. A physical model of an existing beach was built at scale 1:3. The cobble/sand grain size is in 1:3 scale while the gravel is 1:1.5 scale. The large scale experimental flume tests have been set-up in the new outdoor 120 m long flume in Québec city, Canada. The tests were conducted over two test seasons (2013–14). While we in the first test season studied impact on the beach due to incoming regular plunging breakers, the last season contained tests with incoming irregular plunging breakers on the beach with/without tidal variation.
Herein, we primarily report on the wave impact due to irregular plunging breakers on constant and tidal varying water depths. The wave-tide interactions were conducted with a tidal range of 1 m in relation to beaches with steep beach slopes (1:10, 1:5, 1:1). The model inlet significant wave height was 1.1–1.5 m corresponding to equivalent prototype waves in the range of max. wave heights of 6–8.5 m with dominant periods of 12 s in water depth of about 15 m and tidal range of 3 m.
In general, the Equilibrium Beach Profile (EBP) was reached after exposure to about 10,000 plunging breakers or the equivalent of five storms assuming each lasting 3 hours. A cobble berm was formed rapidly on the top of the beach, protecting the backshore against wave action and flooding while finer sediment was transported “offshore”. Beach width reduction was observed when the initial slope of the beach fill material exceeded the equilibrium beach slope. Sediment grain size sorting along the beach profile is discussed and compared to existing beach models, and EBP was compared to several EBP equations.
From a coastal management perspective, in terms of durability, the mixed cobble-sand-gravel material is showing promise as a material to use for coastal protection. It is highly absorbent and the beach tends to maintain its shape over long time when exposed to several storms. However, storm surges in the combination with high tides can results in excessive run-up and potential flood risks. The stabilized beach typically had slopes of 1:7–1:9 independent of the initial slope. We found that irregular seas result in a less pronounced trough in the beach profile in the swash zone than incoming regular plunging breakers. The tidal interaction was further advantageous, naturally shifting the material back and forth. However, other materials and other sensitivity studies are necessary in order to provide firm conclusions about the usage of the cobble-gravel-sand mixture for coastal protection.
