M.Sc Thesis
M.Sc Student Asher Shai Vertical Variations of Coral Reef Drag Forces Department of Civil and Environmental Engineering Professor Uri Shavit

Abstract

Modeling flow in a coral reef requires a closure model that links the local drag force to the local mean velocity. However, the inner geometry of the space between the coral branches and the spatial flow variations make it difficult to predict the distribution of the local drag. Here we report on vertical profiles of measured drag and velocity in a laboratory reef that was made of 81 Pocillopora Meandrina colony skeletons, densely arranged along a tilted flume. Two of these corals were CT-scanned, sliced horizontally and then printed using a 3D printer. Drag in the reef was measured as a function of height above the bottom by connecting the horizontal slices to drag force sensors (DFS). Vertical profiles of velocity were measured using a Laser Doppler Anemometer (LDA), both in-between the coral branches and in the free stream above the reef. Measured drag of whole coral colonies shows an excellent agreement with previous field and laboratory studies; however these studies never showed how drag varies vertically. The vertical distribution of drag is reported here as a function of flow rate and three water-depth-to-reef-height ratios. When the water level is the same as the coral reef height, Reynolds stresses are negligible and the drag force per unit fluid mass is nearly constant. However, when the water depth is larger, Reynolds stress gradients become significant and drag increases with height above the bottom. An excellent agreement was found between the drag calculated by momentum budget calculations and the measured drag of the individual printed slices. The drag coefficient derived from the measured drag and velocities varies greatly as a function of height above the bottom and as a function of water level. In order to reduce these variations, we test a modification of the drag force formulation. This modification includes the normal dispersive stress term and results in reduced variations of the drag coefficient at the cost of introducing an additional coefficient.