Andrew Lane

Suspended sediment modelling in a shelf sea (North Sea)

This paper extends the modelling of suspended particulate matter (SPM) on the local coastal scale (described in preceding papers) to SPM modelling on the scale of the North Sea, focusing on representing SPM patterns and their seasonal distribution. The modelling includes a sensitivity study, in which model results are assessed using surface SPM concentration patterns extracted from NOAA reflectance imagery, as well as North Sea Project in situ data. Over the past decade or so, first-order estimates of the net suspended load and its associated sources and sinks have been available and are generally substantiated. However, developments in the simulation of large-scale SPM behaviour are still severely restricted by the available descriptions of available sediment sources and sediment erosion and deposition processes. This paper indicates how remotely sensed reflectance images can provide additional information on the spatial distribution of (sea surface) suspended sediments. A primary objective of this paper is to examine sensitivities of SPM simulations in 2D (vertically averaged) and 3D models. A boundary-fitted coordinate modelling approach with intra-tidal resolution and synoptic meteorology is applied, as well as more schematic approaches. A related objective is to examine how both limited in situ observational data and reflectance imagery can be used to assess and improve such simulations. An integrated modelling-monitoring approach, using inverse and ‘Goodness-of-Fit’ (GoF) approaches applied to remotely sensed reflectance imagery, is used to derive a structured sensitivity analysis providing a quantified assessment of the strengths and weaknesses of modelling and input data. It is shown that, especially in the coastal zone where salinity stratification may occur, 3D modelling is required while much of the sensitivity analysis can be based on a 2D modelling approach. This quantification of the effects of uncertainties of inputs and erosion/deposition parameters improves understanding of the sediment distribution and budgets on the North Sea scale. It is concluded that whilst process studies are likely to contribute to improving erosion/deposition algorithms, and model developments will provide enhanced dynamical descriptions, accurate overall simulation will remain dependent on some (inverse) processes to reduce the uncertainty in sediment sources.

Tide, wave and suspended sediment modelling on an open coast — Holderness

Abstract An intensive series of observations off the Holderness coast was followed by a related set of modelling applications. Observations included: aircraft and satellite remote sensing, H.F. and X-band radar, ship surveys and in situ instruments on the sea bed and at the sea surface. These observations aimed to monitor, over three successive winter periods, the dynamics and sediment distributions in the vicinity of this rapidly eroding coastline. Associated modelling applications included components simulating: (i) tides and surge currents; (ii) wave evolution; (iii) vertical distributions of turbulence and SPM (suspended particulate matter) and (iv) resulting spatial patterns of sediment transport in the region. Simulations of tidal currents confirmed the accuracy of such models, given accurate fine-resolution bathymetry and appropriate boundary conditions. New developments of WAM, the spectral wave model required for fine-resolution applications in shallow water (described by Monbaliu et al. [Monbaliu, J., Padilla-Hernandez, R., Hargreaves, J.C., Carretero Albiach, J.C., Luo, W., Sclavo, M., Gunther, H., 2000. The spectral wave model WAM adapted for applications with high spatial resolution. This volume.]) are tested here. A number of additional features pertaining to shallow water are revealed including the sensitivity to specification of wind directions and the excessive temporal spreading of short-lived distant events. Likewise, the application of the generic single-point models for vertical profiles of turbulence and SPM (described by Baumert et al. [Baumert, H., Chapalain, G., Smaoui, H., McManus, J.P., Yagi, H., Regener, M., Sundermann, J., Szilagy, B., 2000. Modelling and numerical simulation of turbulence, waves and suspended sediment for pre-operational use in coastal seas. This volume]), are tested and also shown to be appropriate for simulating localised resuspension of SPM. This simulation also illustrates how, in shallow water ( Some preliminary simulations of net sediment movement are included, involving an integration of the above effects. These simulations emphasise how, in all but the shallowest water, the mobility of coarse grain sediments is limited to occasions of extreme waves. By contrast, the movement of fine sediments follows that of the residual tidal current streamlines, i.e., primarily longshore with attendant cross-shore dispersion. However, significant variation between closely-spaced observations indicates the irregularity and complexity of such distributions. It is concluded that because of the inability to prescribe the spatial distribution of available surficial sediments (including size distributions) such simulations can only be expected to reproduce the essential statistical characteristics of SPM concentrations. The availability of extensive remote sensing or in situ data can help to circumvent this problem.

Current measurement technology for near-shore waters

Abstract This paper summarises recent experiences (in SCAWVEX and elsewhere) in measuring near-shore currents, together with subsequent techniques of data analyses. In situ instrumentation described includes: mechanical rotors, electromagnetic and ADCP (Acoustic Doppler Current Profiler) deployed on sea bed frames, in towed mode, and mooring lines. Surface observations from the Ocean Surface Current Radar (OSCR) H.F. radar system are also included. Deployments are in estuaries, along open coasts and in tidal straits—all involving strongly tidal regimes around the UK coasts. The characteristics, including specific capabilities and limitations of each instrument are listed. Synoptic comparisons are used to illustrate those components such as the depth mean principal tidal constituents which show good agreement and to highlight other components, e.g., vertical profiles of residuals, most sensitive to the system adopted. A coordinate system suitable for analysing net fluxes in shallow estuary with large tides is described. Associated aspects, which impact on planning current meter deployments, are outlined.

Observational data sets for model development

Abstract The requirements of ‘comprehensive’ or ‘compatible’ observational data sets for developing and verifying models are examined. ‘Compatibility’ over the range of key parameters involves accuracy, spatial and temporal extent, and resolution. The importance of documentation is emphasised on all aspects from experimental strategy to sensor calibration. Likewise, maximising accessibility involves listing in international directories, quick-view summary facilities as well as detailed data listings. Such accessibility generally includes: multi-media dissemination involving the Internet; printed papers and reports; CD ROMs. Experiences from two coastal observational experiments are reviewed: Holderness on the UK east coast and Sylt-Romo in the German Bight. These examples provide particular illustrations of the generalised principles. They extend to usage of satellite, aircraft, radar, ship, surface and sea bed moorings, and piles as platforms. Specific capabilities, limitations and idiosyncrasies of a range of instruments are described. Effective monitoring strategies must aim to exploit the associated synergies between this full range of platforms and instrumentation.

Effects of freshwater inflow on sediment transport

This study investigates the effect that freshwater inflow has on the transport of dredged material in Liverpool Bay. The sediment dispersal is first simulated using constant density throughout the domain, and then density gradients and stratification will be allowed to develop from riverine inputs. The first case shows the sediments slowly being transported east-westwards away from the estuary; while the simulation with freshwater shows the sediment being rapidly transported south-eastward towards the estuary. Model simulations such as these may inform decisions of where to put the dredging spoil to minimise the likelihood of it returning directly to the estuary.