Wave flume experiments to unravel saltmarsh cliff hydrodynamics

Abstract

<p>Nature-based flood defences receive increasing interest as a viable option for improving flood safety worldwide. A contemporary case is using the ability of saltmarshes to attenuate waves during storm conditions for strengthening coastal flood defences. To ensure a long-term reinforcement of flood protection, it is important to understand the erosion mechanisms of saltmarshes during storms. One of the critical locations for erosion is at the transition between the saltmarsh and the bare mudflat, often characterized by a vertical step or cliff. These cliffs vary between 0.2 to 2.0 m in height, depending on soil composition and local hydrodynamics. However, wave-induced hydrodynamics that controls the (mass) erosion at the saltmarsh cliff are not fully understood. Also the role of saltmarsh vegetation on these near-cliff hydrodynamics are not clearly quantified. In this research, a series of wave flume experiments are performed in a 40 m long, 0.8 m wide and 1.0 m deep wave flume. Generated waves traverse over a saltmarsh transect, consisting of 5 m foreshore section with a slope of 1:45, a varying cliff height to a maximum of 0.2 m, 7 m saltmarsh section and dike section with a slope of 1:3. The experiments are designed according to Froude scale of 1:10. Vegetation is modelled by elastic cylinders, scaled according to Cauchy similitude. Offshore irregular wave height will range between 0.1 and 0.2 m at a water depth varying between 0.3 and 0.6 m, while nearshore (at the saltmarsh) waves are mainly depth limited. Water levels are measured by several wave gauges and laser scanners. Wave-induced pressures at the saltmarsh cliff are measured by pressure transducers mounted around the cliff. Particle Image Velocimetry is used to quantify near-cliff velocities patterns in a non-intrusive way and validated with additional velocity measurements. Quantification of the wave-induced hydrodynamics and its dependency on vegetation will result in a valuable dataset for model development and validation, and will help in understanding erosion mechanisms at a saltmarsh cliff during storm events.</p>