Hydrodynamics in mangrove prop roots and their physical properties

The circulation of water in riverine mangrove swamps is expected to be influenced by mangrove roots, which in turn affect the transport of nutrients, pollutants and sediments in these systems. Geometric properties of mangrove (Rhizophora sp.) prop roots in the field were studied through the use of photogrammetric methods and samples of such roots were harvested to determine their material properties in the laboratory. The field studies were carried out in the mangrove areas along some of the coastlines of Singapore where the Rhizophora genus are located. We found that the prop roots under tidal hydrodynamic loadings in a mangrove environment can be regarded as fairly rigid on account of a large Young's modulus of about 15 GPa. Physical prop root models were fabricated from downscaling based on field observations with porosity values ranging from 0.96 to 0.98. Flume experiments were performed and measurements of mean flow velocities, Reynolds stress and turbulent kinetic energy (TKE) were made. Our results indicate that the prop roots provide blockage effect on the flow and cause complex secondary flow. The turbulence energy can be generated by both wake and shear, with shear-generated turbulence being dominant in the upper open area between prop roots. A force balance analysis was performed in the nearly uniform flow region to investigate the flow resistance caused by the prop root models. The Chézy roughness coefficient C was found to be 10 and the drag coefficient CD was found to be 1.2 in the fully developed flow, which agrees with reported field studies in mangrove swamps.