Temporal variability of hydrodynamics, sediment concentration and sediment settling velocity in a tidal creek

Measurements of flow, turbulence and suspended sediment concentration were carried out in a tidal creek that acts as a conduit for water and sediment to an intertidal salt marsh. A laser diffraction system (LISST), optical backscatter sensors (OBS) and acoustic Doppler velocimeters (ADV) were used to measure suspended sediment concentration and particle settling velocity variability throughout various tidal cycles for spring and neap conditions. Results suggest that ADV is a very useful system capable of measuring both sediment concentration and settling velocities. During neap tides, sediment in the tidal creeks is primarily in the form of flocs with a settling velocity ranging between 0.02 and 0.20 mm/s and a mean particle size of 25–75 μm. This population of sediment is enriched by both larger flocs and larger individual grain particles, during spring tides. The results indicate that in the tidal creek, erosion can occur only during the ebb stage of the spring tides, while during the flood tides the tidal creeks do not experience any significant erosion. On the contrary, they might act as temporary depositories for the small flocs. Sedimentation on the marsh surface occurs predominantly in the form of flocs, which seems to have the same settling velocities (0.24 mm/s) both during neap–spring tides. This indicates that the neap–spring tidal cycle is controlling marsh sedimentation through sediment availability for deposition and inundation time. Spring tides provide more sediment and more time for the flocs to be deposited and thus contribute more to marsh accumulation.