Sediment Transport In Estuaries Research Articles

Field studies of velocity, salinity and suspended solids concentration in a shallow tidal channel near tidal flap gates

The design and operation of mathematical models of solute mixing and sediment transport in estuaries rely heavily on the provision of good-quality field data. We present some observations of salinity, suspended sediment concentration and velocity at one of the tidal limits of a semi-enclosed tidal lagoon in Southern England (Pagham Harbour, West Sussex, UK) where the natural processes of tidal incursion and solute mixing have been heavily modified as a result of the construction of sea walls dating back to the 18th Century. These observations, made immediately downstream of two parallel tidal flap gates by conductivity-temperature-depth (CTD) profiler, and also using bed-mounted sensor frames to measure velocity at 2 fixed depths, have yielded a set of results covering 11 tidal cycles over the period 2002–04. It is clear from the results obtained that over a typical tidal cycle, the greatest vertical salinity gradients occur in the 1–2 h immediately after the onset of the flood tide, and that subsequently, energetic mixing acts to rapidly break down this stratification. Under moderate-to-high fresh water flows (>0.5 m 3/s), the break-down in vertical salinity gradient is more gradual, while under low fresh water flows (<0.2 m 3/s), the vertical salinity gradient is generally less pronounced. Estimates of Richardson number during the early flood-tide period reveal values that vary rapidly between <1 and about 20, with lower values occurring after around 1.5–2 h after low water. Observations of suspended sediment concentration vary widely even for similar tidal and fresh water flow conditions, revealing the possible influence of wind speed, the storage effects of the water in the lagoon downstream of the observation site, and the complexity of the hydrodynamics downstream of tidal flap gates. The data also show that most of the sediment transport is landward, and occurs during flood tides, with estimated total tidal landward flood tide flux of fine sediment of the order of 50–120 kg under low fresh water flow conditions. These observations, which reinforce the results presented in Warner et al. (2004) and elsewhere, can help to provide information about the appropriate techniques for managing sediments and pollutants, including nutrients from sewage effluent waters, in estuaries where hydraulic flap gates are used to control the entry of fresh water over the tidal cycle.

Bathymetric sidescan investigation of sedimentary features in the Tay Estuary, Scotland

A comparison of results from mapping using a swath bathymetric sonar system in the Tay Estuary, Scotland, and remote sensing data on the position of frontal systems, indicates that the frontal systems are controlling the distribution of sedimentary features in the estuary. The boundaries between zones, defined by the advancing flood fronts and exiting fresh water, are sharply defined both by the front and the bottom bedforms. Static fronts, usually axial fronts, exist at well-defined bathymetric changes and result in relatively stable bedforms. However, measurements of current velocities at migrating fronts suggest that the vertical component of velocity accelerated at the front boundary will cause erosion at the bed and migrating bed features. Predictions of sediment movement and sediment feature migration rates across areas where these fronts migrate are confirmed by the bathymetric sidescan sonar results that show asymmetry of the sedimentary dune features. The use of new high-resolution sonar, together with its repeatable precision in locating sedimentary bedforms, has significant implications for long-term modelling of sediment transport in estuaries and other similar areas.

Tidal asymmetry of the sediment transport in estuaries and its amplification by the weir.

Vertical profiles of the velocity, turbidity and salinity were observed over one tidal cycle in the Hattae estuary in the inner part of Ariake Bay which contains wide tidal flat. The vertical pro-files are more uniform in the flood stage than those in the ebb stage. This tidal asymmetry causes net upstream transport of the suspended sediment. Closing the gate of the weir weakens the ebb currents but not the early flood current, thus will intensify the tidal asymmetry further.

Solute Dispersion and Sediment Transport in Estuaries

Synoptic measurements of velocity and salinity at several points on up to five vertical profiles at a cross-section in the Conwy show that solute dispersion coefficients are affected by secondary flow induced by the interaction of transverse shear and the salinity induced longitudinal density gradient. It is concluded that dispersion coefficients are temporally and spatially dependent and a simple empirical formula is suggested. Some data from the initial stages of a similar study of sediment transport show that techniques exist which permit the study of the turbulent fluctuations of suspended solids concentration and the evaluation of the tidal dependence of suspended solids transport phenomena.