An experimental study on the drag and inertia forces due to mussel growth on vertical poles exposed to waves

Abstract

Mangroves trap sediment and are the first coastal defence line for many coastal areas. Unfortunately, mangrove coasts worldwide are eroding due to deforestation, exposing local communities to flood hazards. In Demak, Indonesia, a Building with Nature approach was chosen to counteract erosion. Permeable dams were built on the foreshore to attenuate waves and restore the sediment balance. The dams have increased the bed level locally and mangroves expand after construction. A new project called MuMaCo (Mussels as Mangrove facilitators for Coastal defence) is launched to integrate mussel aquaculture into the mangrove rehabilitation project in Demak. Mussels increase the roughness and diameter of the bamboo poles, which could increase their wave attenuation capacity. Moreover, mussel culture gives the local communities an economic incentive to sustain the structures. This thesis describes and quantifies how mussels' growth influences the forces acting on bamboo poles. Small-scale experiments are performed in the wave flume of the Delft University of Technology on a scale of 1:3 and 1:6. Physical models were used for experiments with steady flow and waves. The steady flow experiments cover a range of Reynolds (Re) numbers between 10^3 and 10^4 to seek the drag crisis. Single mussel covered cylinders for both scales are compared to smooth cylinders and cylinders with an equivalent diameter. For the wave experiments, cnoidal waves with Keulegan-Carpenter ($KC$) numbers ranging from 3 to 113 that correspond with measured waves in Demak are chosen. Moreover, only mussel covered cylinders are compared to a smooth cylinder for a scale of 1:6. During the experiments, the flow velocity, force, and surface elevation are measured, and the drag and inertia coefficients are obtained with the help of the Morison equation. The drag coefficient shows a clear relationship with the Reynolds numbers for steady flow. Although the drag coefficients display the same qualitative behavior as in other studies in the literature, the drag coefficients of the smooth cylinder are more than 40 % larger than in the literature. This is probably due to large variations in the offset of the velocity meters (EMS) during the experiments. The large mussels show higher drag coefficients than the smooth cylinder. For the wave experiments, the drag coefficients show good correspondence to other studies in the literature, but the inertia coefficients are a factor two larger than expected for smooth cylinders. Treating the peak and trough of the cnoidal wave as individual sinusoidal waves shows similar results and suggest that cnoidal waves can be represented by KC numbers, as it is done for linear waves. The best fit to the literature values was found by solving the Morison equation with the measured force with the depth-integrated velocity squared and acceleration from the Fenton theory. This suggests that the results of the wave experiments have been uncertain due to inaccuracies in the EMS recordings. This thesis presents a method to schematize mussels for laboratory experiments and provides a qualitative comparison of the forces acting on poles with and without mussels