The benefits of the Sediview Method are greatest where a large amount of data are required to study dynamic sediment transport processes, particularly over large areas.
Discharge and Sediment Flux studies, Thames Estuary, UK
Several surveys have recently been undertaken in the Thames estuary with HR Wallingford. The surveys have been carried out for various clients to provide design data for port developments and the construction of water intakes. In addition, the results of the surveys are being used to compile a database of discharge and solids transport in the estuary. The figure shows the variation of discharge and solids flux observed during a survey at Coalhouse Point, downstream of Gravesend. This work is providing a new understanding of the importance of secondary current flows on the distribution of sediment in the river and is revealing complexities in sediment transport patterns that, until recently, were incompletely understood.
Harwich Haven, UK
Our sister company, Dredging Research Ltd (DRL) undertook two large Sediview surveys in the UK port of Harwich. Project leaders HR Wallingford, working for Harwich Haven Authority, were calibrating a large numerical model of the complex estuary of the Rivers Stour and Orwell and required detailed data on currents, discharge and sediment flux at three key transects and five subsidiary transects. Two surveys, each of three days duration, were undertaken to obtain the required data.
of Sediview concentration estimates with
contemporaneous water samples obtained in up to 16 metres of
water during the Harwich survey
The estuary is characterised by steep vertical and horizontal concentration gradients and a variable sediment population that differs between the two rivers and changes during the tidal cycle. Relatively coarse sediment enters the estuary from the rivers during the ebb tide to be replaced by relatively fine marine sediment during the flood tide. In order to address these changing conditions, we used the Mersey Method of data collection which was originally devised during our work on the River Mersey for the UK Environment Agency in 1997 and ensures that calibration data are obtained systematically throughout the work. Time-variable calibrations were developed for each of the three key areas which resulted in a very detailed and coherent data set.
Cape Fear, NC, USA
In 2001, we visited Cape Fear in North Carolina where the US Army Corps of Engineers is undertaking detailed monitoring of an experimental dredged material disposal mound (right). Working with oceanographers Evans Hamilton Inc, the Virginia Institute of Marine Sciences and Sequoia Scientific Inc, we calibrated an array of bed-mounted ADCPs that have been installed to monitor currents and waves (using RDIs WavesMon software) in and around the disposal site. This work was unique because it is the first time that continuous particle size data have been obtained (using Sequoias LISST 25 instrument) during a Sediview calibration and because it involved a detailed comparison of concentration data obtained using four independent methods, water sampling, OBS turbidity sensors, LISST and Sediview.
View of the experimental disposal mound
We are pleased to report that all four methods yielded data that were in close agreement and that Sediview was clearly able to accommodate the significant variation of particle size that was observed during the experiment. An account of this work is to be published soon by the Corps To learn more about the experimental mound, visit www.frf.usace.army.mil/mound.
Comparisons of water sample OBS and ADCP / Sediview concentration data against LISST concentration data
Cleveland Bay, Australia
Marine and Coastal Solutions used Sediview to establish boundary and calibration conditions for a 3-D hydrodynamic model of Cleveland Bay near Townsville in North Queensland, Australia. The hydrodynamic modeling was undertaken by GHD, a large Australian consulting engineering firm, and utilized the Delft 3D package to examine berth area development options that could result in a reduction in sedimentation in the channel and berth pocket.
The results of the Sediview / ADCP surveys enabled identification of conditions resulting in sediment mobilisation, as well as the current flow patterns, for use in calibration of the model. An item that proved of more interest than originally envisaged was the establishment of sediment concentrations at the model boundaries. Localised effects such as surface sediment plumes (see picture at right) and eddies, and reductions of sediment concentrations with increasing distance from shore, provided a situation where individual water samples or siltmeter results could produce an incorrect assessment of the sediment supply conditions at the boundaries. Model calibration required maintaining material in suspension at the observed levels in various areas of the bay combined with sedimentation at rates measured by survey.
|Surface sediment plume observed during the Cleveland Bay Sediview survey||Filtering water samples for calibration of Sediview|
Visualization of results over an area was enhanced by production of hourly snapshots of flow patterns and sediment concentrations by interpolation of results from a repeated vessel sailing pattern. This required optimization of ADCP settings to maximize data acquisition at reasonable levels of accuracy with a relatively high vessel speed required to complete the pattern in less than an hour. The sound assessment of the suspended sediment conditions used for model calibration has provided the Townsville Port Authority with an increased level of confidence in the options that have been shown by the modeling to achieve a reduction in overall sedimentation in the berth area, particularly in areas that are more difficult to access by the dredger used for annual maintenance.
The Sediview package provided by HR Wallingford includes: