A study of the siltation of marinas with particular reference to Port Hamble

Tosswell, Paul (1983) A study of the siltation of marinas with particular reference to Port Hamble. University of Southampton, Doctoral Thesis.

Abstract

Marinas in estuaries have commonly been created by dredging the inter-tidal zone at the margins of a deep water channel. These areas are prone to siltation and consequently present particular problems of maintaining adequate water depths at minimum cost. The siltation characteristics have been investigated using a typical marina, Port Hamble, as a case study. This marina is located in the lower reaches of the River Hamble estuary, which in view of the small freshwater inflow has little stratification. An optical suspended solids monitor, electromagnetic current meter and pneumatic tide level recorder were installed and operated continuously (apart from malfunctions) over a period of 19 months. The data, recorded on chart paper, was converted to digital form by sonic digitiser, and then processed for computer storage with 15 min. time intervals. The total number of data values was approximately 150,000. Bed and suspended samples were obtained, the 32 bed samples being analysed by Coulter Counter and wet sieving to derive the particle size distribution. The deposited material was found to have a higher than expected sand content. A comprehensive tracer experiment using a fluorometer and fluorescent dye (Rhodamine W.T.) was performed and enabled flow patterns to be established. The results assisted in explaining the distribution of bed material. The marina was the subject of three detailed hydrographic surveys, enabling siltation regimes to be identified. Deposition was found to be of the order of 0.1 to 0.3m/year, according to location. The suspended solids concentration was found to be generally low (<50mg/1), but short bursts of up to 750mg/I did occasionally occur, mainly during the flood tide. This and other evidence suggested that the majority of sedimentary material was entering from outside the estuary. The time history of suspended sediment was examined together with that of possible influencing factors. It was found that the highest values occurred during April and September suggesting that tidal height was a principal factor, although no significant correlation was found. The correlation with rainfall was stronger, suggesting that turbid runoff from the tertiary portion of the Solent drainage basin and local cliff erosion might be significant, although the receding mudflats of the West Solent may well make a greater contribution. It is concluded that siltation in this type of marina is a very complex process, with no single factor being attributable. Suggestions are made for improving the tidal flush and more effective dredging techniques. Further research is needed and this should include the development of a suspended solids monitor capable of accurately recording low concentrations (<100mg/1), since, in quiescent conditions these may give rise to quite appreciable deposition as the present study has shown.

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