Deposition in Biologically Cohesive Sediment within Emergent Canopies

Extracelluar Polymetric Substances (EPS) produced by microorganisms are ubiquitous in aquatic vegetated channels, playing a crucial role in geophysical processes. Biofilms formed by EPS bind sediments together, enhancing the stability of the channel bed and reducing erosion. The change in sediment porosity and cohesiveness due to biofilm can also affect deposition processes within the vegetated canopies, but previous studies on vegetated conditions have focused on abiotic (non-cohesive) sediment. Therefore, this study investigated the role of biofilm in deposition process within a vegetated channel through laboratory experiments. Mangrove pneumatophores were modelled using cylinder dowels, and a biologically cohesive sediment bed was created by mixing non-cohesive sand with commercial EPS (Xanthan gum), ranging from 0 to 0.5 % of dry sand weight. A fixed amount of fluorescent particles were released in suspension, and the deposition rate was measured by counting the number of particles deposited to the bed over a fixed time. The deposition rate was inversely proportional to the velocity. At the same velocity, the deposition rate decreased with increasing cohesion (increasing EPS). This reduction in deposition rate was attributed to the decrease in porosity caused by increased EPS, which weakened the hiding effect and enhanced exposure effect. The critical Shields parameter for the deposited particles decreased with increasing EPS, which was used to develop a new deposition probability model.