Sedimentation in a rock-walled inlet, Lyttelton Harbour, New Zealand

摘要

The principal concern of this study is the examination of sedimentation and sedimentary processes in a rock-walled tidal inlet, Lyttelton Harbour. The harbour is distinctive from other forms of inlets commonly discussed in the literature due primarily to a negligible freshwater input, lateral grain size contours which are parallel to current flow paths, and a maintenance, channel dredging programme which greatly exceeds the natural harbour sedimentation rates. A further unusual characteristic of the harbour is the lateral limitation imposed on processes within the harbour by the surrounding rigid, rock boundaries. These boundaries influence and control circulation patterns within the harbour through the interaction between processes and geometry. Thus Lyttelton Harbour is a structurally controlled tidal inlet. The only harbour boundary which is free to respond to changes in the system is the bed. For these reasons traditional inlet concepts, applicable to estuaries and inlets with unconsolidated boundaries, were found to be unsatisfactory for explaining how Lyttelton Harbour operates. The study approach involves fieldwork and analysis of both sedimentary and hydraulic processes within the harbour. A bed sediment survey showed that the harbour can be divided along its longitudinal axis with very fine mud sediments on the northern side, and coarser, sandier sediments on the southern side. All bed sediments are predominantly fine, and a survey of near-bed suspended sediment concentrations revealed fluid mud layers on the northern side of the harbour, at the harbour entrance, and within the channel. The primary hydraulic processes operating are tidal currents, although a mixed wave field, comprising ocean swell and locally generated wind waves, frequently penetrates the harbour. Tidal current velocities are on average around 0.22 ms-¹, with flood tide velocities stronger on the south side of the harbour and ebb tide velocities stronger on the north side, inducing a clockwise circulation pattern. External factors are an important component in the tidal driving forces, comprising coastal weather patterns and a continental shelf edge wave oscillation effect with a period of 2.5 - 3.5 hours. These external influences are the main cause of the duration of both ebb and flood tides varying from 5.0 to 8.25 hours. Interaction between tidal currents and the harbour geometry induces a large gyre in the outer harbour which varies in duration from being absent to operating for up to 50% of any given tidal cycle. The transport of sand sized material is bidirectional along the harbour length, with erosion of sandy sediments in the centre of the harbour, and deposition at the head of the harbour and at the harbour entrance. Fine muddy sediments are transported predominantly towards the harbour entrance and accumulate in the channel, on the northern side, and in the entrance, forming fluid mud layers. The most concentrated fluid mud regions coincide with the rotatory currents at both ends of the tidal gyre, where sediment is deposited from weaker currents. Transport of sand across the harbour is not apparent, although lateral movement of fine, suspended particles occurs by advection and diffusion in response to the sediment flux differentials and flux gradients within the harbour. This movement of fines results in the lateral gradation of grain size, from coarse to fine, across the harbour from south to north. The major sedimentary process within the harbour is the maintenance dredging programme which removes up to 1,000,000 tonnes of sediment annually from the channel and port berthage areas. The dredge spoil is subsequently dumped within the harbour along the northern perimeter, although a temporal analysis of dump sites indicated that once a site capacity has been attained, all the spoil dumped at that location is rapidly removed. Sediment input from other sources, primarily erosion of the catchment, has been estimated at less than 45,000 tonnes per annum, substantially less than the channel siltation rate, and the recirculation of dredge spoil was identified as the primary source of sediment causing channel siltation. Two processes induce spoil recirculation. Firstly the tidal gyre, and secondly the dynamic trap. The dynamic trap system provides a mechanism for the transport of fine grained sediments to regions of high sediment flux, and for the deposition of fine grained sediments under current regimes having both a high competence and a high capacity. The system provides an explanation for the lateral grain size gradation within the harbour, the maintenance of dredge spoil mounds at dump sites, the insensitivity of channel siltation rates to the location of spoil dumping sites around the harbour, and the quasi-equilibrium state of the harbour in spite of the extensive dredging operations. Little sediment is able to escape from the harbour to the open sea due to the flux gradients at the entrance, and the dynamic trap principles. Thus the long term stability of Lyttelton Harbour is maintained, under both natural and dredging conditions, by the redistribution of available sediment within the harbour as a function of internal harbour dynamics. Throughout the thesis the dynamics of Lyttelton Harbour are compared to existing inlet concepts and theories in order to identify those areas in which this type of inlet is significantly different and where other, poorly understood inlets may be comparable to Lyttelton. Finally, Lyttelton Harbour is defined and classified and a set of principles pertaining to this type of inlet are proposed.