S.B. Mitchell

Prediction of nearshore wave energy distribution by analysis of numerical wave model output: East Sussex coastline, UK.

Abstract Prediction of wave energy distribution in coastal areas is necessary if an assessment of the likelihood of cliff collapse is to be undertaken. Use has been made of numerical modelling to predict relative wave heights along the chalk cliff coastline of East Sussex between Brighton and Eastbourne, UK. In this study, wave modelling has been undertaken using the University of Delaware REFDIF-1 software with a 100m mesh size to predict nearshore wave heights for boundary unit wave height conditions from a range of different incident directions. The results from this wave modelling have been combined with the frequency distribution of incident waves obtained from analysis of time series of 12 years of hindcast wave data in the English Channel, obtained from the UK Meteorological Office. The resulting distribution of nearshore wave heights is presented as a surrogate for the distribution of wave energy over the 12-year period. Some concern exists about the quality of the output data, in particular of the effect of the relatively coarse bathymetry grid used for the model. Some wave focusing is evident from the model output, caused by the presence of local shoals in the model grid, leading to a ‘banding’ effect in the model output. Some suggestions are made for the improvement of the modelling scheme, including the use of finer mesh size, bathymetric smoothing and the use of a spectral model such as REFDIF-S.

Influence of fresh water discharge on nutrient distribution in a macrotidal lagoon, West Sussex, UK

Results of nitrate and phosphate concentrations measured using hand-held ‘Hach’ monitors are presented, both over individual tidal cycles and over longer term deployments at Pagham Harbour, West Sussex, UK. This macrotidal lagoon (offshore tidal range 3.0 m neaps–6.5 m springs) is a site of key importance as a nature reserve and a home for several rare species of plants and animals. In particular, the effects of fresh water-salt water stratification over 4 tidal cycles at two tidal-fresh water boundaries is presented. It is shown that obtaining periodic vertical profile measurements during individual tidal cycles helps to quantify the transport mechanisms of nutrients from the tidal limits into the main body of the lagoon. Of key interest is the interaction between sediment-bound nutrients with the surrounding water in which the sediment is suspended during parts of the tidal cycle. Synthesis of these results with existing knowledge about sediment-water-nutrient interactions reveals how it is possible for nutrients to become trapped at the muddy tidal limits of the lagoon. In certain cases it is shown that nutrient-rich water from fresh water streams only gradually mixes with the denser, salt water of the incoming tide. Whilst a degree of salinity-induced stratification may be expected during the flood tide, these observations suggest that the water column is stratified with respect to both N and P, even well into the ebb tide. Thus at sites where stratification is important, there is a tendency for nutrients to remain preferentially near the water surface, and thus come into contact with fine, less mobile sediments near the surface of inter-tidal zones, which are themselves, in general, accreting. Since the overlying water is generally slow-moving during high water, it is postulated that saline-induced vertical stratification of estuarine water is an important mechanism in promoting nutrient build-up in muddy inter-tidal areas of this kind.

The Influence Of Tides And Wind Speed On Fine-sediment Transport In A Semi-enclosed Natural Harbour (Pagham Harbour, UK)

Preliminary analysis of results from a study of tine sediment transport in a macrotidai harbour (Pagham Harbour, West Sussex, U.K.), has revealed patterns of fine sediment transport which may be related to the tidal range and the wind speed. This paper will present preliminary analysis of regular and fkequent monitoring of turbidity, salinity, water level, wind speed and wind direction over a continuous 70 days period. Analysis of sediment transport over individual tidal cycles has also been carried out, in order to ascertain a possible mechanism for the flux of fme sediment to and from the intertidal mud areas. Shorter, faster flood-tide currents lead to concentrations of suspended sediment that are normally highest during the first 90 minutes of the flood tide. These concentrations decrease rapidly due to settling, during the relatively low velocities at, and immediately following, high slack water. Peak flood-tide suspended sediment concentrations appear to be correlated more with tidal range, than with wind speed, suggesting that there is a greater sediment mobility during spring tides.