title: Flocculation controls in a hypertidal estuary
Creators: name: Ramirez Mendoza, Rafael
Publisher: name: University of Liverpool Repository
Year: 2015

description: Estuaries are ephemeral geological formations in constant change, which is being accelerated by human intervention. Fine sediments are an important characteristic of estuarine systems upon which anthropogenic and natural activities depend. An important feature of fine sediments is their cohesiveness, following which individual particles aggregate into the so-called flocs or break-up due to turbulent motions. Throughout the flocculation process (aggregation and break-up), flocs change their properties of size, density and settling velocity with consequences on suspended sediment transport, primary productivity and contamination. The prediction of possible changes in flocculation represents an important tool to help in decision making. Although some knowledge has been gained from laboratory and field investigations, the behaviour of the flocs in real conditions is still not well understood. Even though formulations to describe changes in floc dynamics have been proposed, there is a wide disparity between studies. The aim of this research is to contribute to the knowledge of the flocculation process and propose novel methods for the implementation in numerical models, via a case study in the Dee Estuary. The Dee is a hypertidal Estuary located in Liverpool Bay, with surface tidal currents over 1 m/s. The effects of turbulence generated by the strong tidal currents and waves on floc properties are of particular interest. The possibility of a simple formulation for the flocculation process is also investigated in order to be implemented in a state of the art coupling of hydrodynamic (POLCOMS), turbulence (GOTM) and waves (WAM) numerical models. To achieve these objectives, hourly data of grain size, volume concentration and current velocity from a mooring deployed in the Welsh Channel and water samples from a research vessel taken from 12 February to 9 March on 2008 have been used. The high sampling rate for the currentv elocities enables the calculation of turbulent stress, turbulent kinetic energy, shear rate, Kolmogorov microscale and dissipation. Mass concentrations were obtained froma series of water samples collected from a research vessel during the 12-14th February 2008, which were used to convert volume concentration into mass concentration. Three hydrodynamic regimes have been distinguished from the observations: “current only” (negligible effect of waves), “combined currents-waves” (important effects from both forcings) and “wave dominant”. Quarter-diurnal variability of floc size was present during the first two regimes. Observations showed aggregation of flocs during periods of low turbulence with higher magnitude during low water slack than during high water slack. Break-up occurred concurrently with high turbulence periods during flood and ebb phases with higher magnitudes after ebb. Differences are likely due to turbulent stress asymmetries related with mixing and periodic stratification even though freshwater input was low. The “current only” regime was used to investigate the changes in floc settling velocities in relation with turbulent stress. A simple semi-empirical formulation was proposed and implemented in the numerical models. This expression depends on a single variable, which can be obtained from the turbulence model, and is both physically and mathematically correct. Model results qualitatively reproduced the neap-spring variability and the quarter-diurnal variability of floc settling velocities and suspended sediment concentration. During the “combined currents-waves” regime, waves were tidally modulated and led to enhanced aggregation and break-up, with higher floc size range than during the “current only” regime. Wave tidal modulation and quarter-diurnal variability of floc size were lost when waves were dominant. Flocs sizes exhibited a low range related to wave height. Inverse relationships between turbulent properties and median grain size were found for the three regimes, with higher scatter of data for the Kolmogorov microscale and shear rate due to different floc behaviour during flood and ebb phases. Turbulent kinetic energy showed a better relationship with floc size, which suggests its use as a floc size predictor instead of turbulent stresses.