Session: 12-03-01 Coastal Processes I

Paper Number: 121036

121036 - Regular Wave Attenuation in a Partially Vegetated Wave Flume 

Coastal vegetation plays a crucial role in damping wave energy and the impact of incoming waves, which has implications for shoreline protection, ecosystem health, and coastal management. Extensive research has been conducted on wave attenuation of diverse coastal vegetation types, such as seagrass and mangroves. Most of this research has concentrated on complete vegetation coverage environments. Recently, there has been a growing body of research focusing on flow development over partially vegetated waters. This research aims to pinpoint optimal vegetated areas for effective coastal erosion mitigation and improved shoreline stability without extensive vegetation. Nevertheless, wave attenuation in partially vegetated flumes remains an unexplored domain. 

 

As the vegetation is partially covered, the interaction between waves and vegetation includes not just standard wave damping but also diffraction processes. We have developed an analytical solution based on the principle of energy balance following Dalrymple et al. (1984), incorporating these diffraction processes. The wave attenuation in both the wave propagation and cross-sectional directions has been examined. Additionally, the mixing and turbulence at the interface between the vegetated and non-vegetated areas are assumed to be linearly distributed. The shear coefficient in the cross-section will be determined empirically. 

 

Experiments were conducted at the NUS Hydraulic Laboratory using a ferrocement wave flume with a piston-type wavemaker. The dimensions of the wave flume are 36 m in length, 2 m in width, and 1.3 m in depth. The rigid wooden stick is used to mimic mangroves. The diameter of the rigid wooden stick is 5mm, and the shoot density is 325.5 stem/m². Artificial flexible vegetation was used to mimic the seagrasses. All these vegetation models were mounted on perforated PVC boards that are 5 mm thick with a density of 1.4 g/m³. 3 mm aluminum boards were affixed beneath them to counter the buoyancy of the PVC boards. The vegetated section is located at the flume's center, 13m away from the wavemaker. This vegetated area spans 6 m in length and 1m in width.  Seven capacitance wave gauges were all mounted on a mobile towing carriage and were uniformly positioned in the wave propagation directions. These gauges are used to measure surface level across the flume's cross-section, such that differences in wave height can be directly measured. For the 6 m vegetated stretch, measurements were taken at 31 locations in the wave propagation direction with an interval of 0.2 m. The experiments were conducted with wave heights ranging from 0.04 m to 0.12 m and wave periods between 0.8 and 2 seconds. Initially, the experiments were conducted with empty PVC boards to measure wave damping caused by bed and side-wall friction. The discrepancy in results obtained with and without the presence of vegetation was used to validate our analytical model.

 

The analytical solution matched experimental data well in the wave propagation direction, although the beat pattern cannot predicted by the analytical prediction. Variation in wave height across the cross-section at various points along the wave propagation direction is shown in the experimental data. It is obvious that the wave attenuation in the cross-section direction intensifies when the wave propagation distance is increased. Notably, the presence of vegetation has a substantial impact on wave height amplitudes, even in the non-vegetated area. In our forthcoming research, we aim to determine the shear coefficient empirically. The subsequent step will involve a systematic exploration of different vegetation densities. We will investigate different vegetation types, particularly seagrass, and identify the optimal vegetation density to obtain the best outcomes. 

 

Reference: 

Dalrymple, Kirby &  Hwang (1984): Wave diffraction due to areas of energy dissipation, Journal of Waterway, Port, Coastal, and Ocean Engineering 110 (1), 67–79.

Presenting Author: Yufei Wang National University of Singapore

Presenting Author Biography: Yufei Wang is currently a research fellow in National University of Singapore. She did her PhD in NUS, MSc in Imperial College London, and BEng in Hohai University. Her research interests are natural based solutions, solitary waves, and HSPIV.

Authors:

Yufei Wang National University of Singapore
Jiarui Lei National University of Singapore