Lyttelton Harbour Sedimentation

The study focuses on sedimentation and sedimentary processes in Lyttelton Harbour, a distinctive rock-walled tidal inlet. Its uniqueness stems from a negligible freshwater input, unlike most inlets described in literature. Lateral grain size contours run parallel to current flow paths, a key observation. The harbour's sedimentation rates are significantly altered by a substantial maintenance and channel dredging program, far exceeding natural rates. The rigid rock boundaries laterally confine processes, influencing and controlling circulation patterns within the harbour. This creates a unique environment for sediment transport and deposition unlike typical estuaries.

Sediment transport of sand-sized material in Lyttelton Harbour is bidirectional along the harbour's length. Erosion of sandy sediments occurs in the harbour's center, while deposition takes place at the harbour head and entrance. In contrast, fine muddy sediments are predominantly transported towards the harbour entrance, accumulating in the channel and on the northern side. These fine sediments form fluid mud layers, particularly concentrated in areas of rotatory currents at the ends of the tidal gyre, where weaker currents allow sediment deposition. The cross-harbour transport of sand is negligible compared to the longitudinal movement.

The study highlights the significant influence of dredging on sediment dynamics in Lyttelton Harbour. The harbour experiences the removal of large quantities of sediment annually—estimates range from 16,000 to 250,000 tonnes—from the channel and port berthage areas. This dredged material (dredge spoil) is then dumped within the harbour along the northern perimeter. However, a temporal analysis reveals that once a dump site reaches capacity, the deposited spoil is rapidly removed. While catchment erosion contributes some sediment, estimated at less than 45,000 tonnes per annum, it is far less than the channel siltation rate. Therefore, the recirculation of dredge spoil is identified as the primary source of sediment causing channel siltation, significantly impacting the overall sedimentation patterns of the harbour.

A significant source of sediment in Lyttelton Harbour is identified as thick layers of aeolian loess. Transported from the Canterbury Plains during the later Pleistocene (as documented by Raeside, 1964), these deposits reach depths of 10 meters in areas at the harbour head. These loess deposits are considered the primary source of silt that has infilled the harbour basin to depths of up to 47 meters (Bushell and Teear, 1975). The erosion of these deposits, along with general catchment erosion, contributes to the ongoing sedimentation within the harbour. The geological history of the area, including several volcanic phases in the Miocene, is detailed by Liggett and Gregg (1965) and Speight (1917; 1944), providing context for the sediment composition and source materials. The study acknowledges and utilizes these previous geological findings to inform the current research on sediment sources and harbour morphology.

The study considers the broader regional sediment dynamics influencing Lyttelton Harbour. Net sediment movement on the Canterbury shelf is northward, with sediments primarily flowing into Pegasus Bay (Herzer, 1981). Coarser, sandy sediments are transported north or reworked around Banks Peninsula, depositing in a broad sand ribbon off the peninsula's eastern end. A modern mud facies, however, is deposited in quieter regions, reaching its greatest thickness in Pegasus Bay (Herzer, 1977, 1981). This regional pattern indicates that the marine sediments adjacent to the entrance of Lyttelton Harbour are composed of very fine muds, influencing the sediment composition at the harbour's mouth and contributing to the overall harbour morphology and sedimentation processes. This context helps explain the sediment characteristics found within the harbour itself.

Previous research highlights a unique morphological aspect of Lyttelton Harbour. Heath (1975) studied the stability of 20 tidal inlets, establishing a relationship between tidal compartments and entrance cross-sectional areas. Lyttelton Harbour was among four harbours that deviated significantly from this relationship, possessing a comparatively larger cross-sectional area than expected for its tidal entrance. This anomaly was later explained by Heath (1976a) as resulting from persistent swell at the harbour entrance combined with a reduced sediment supply. This unique morphological characteristic, distinct from the established relationships for other tidal inlets, is a central focus of this study's investigation into the harbour morphology and its effects on sedimentation.

Harmonic analysis of tidal records in Lyttelton Harbour reveals a dominant tidal amplitude with a frequency around 160 minutes. Significant power is also observed at 96 and 16-minute frequencies, corresponding to quarter and half-wavelength harbour resonator nodes respectively. However, the 160-minute peak is considerably larger than the quarter-wavelength resonance. Further analysis by Heath (1979, 1982) identified a 2-3 hour oscillation in other tidal records, attributed to continental shelf edge-wave effects. These findings illustrate the complex interplay of tidal forces and the harbour's hydrodynamics, influencing sediment transport and deposition. The study's hydrodynamic analysis considers both external forcing (e.g., tidal waves) and internal phenomena (e.g., dredging), impacting the overall harbour circulation patterns.

The study reveals a prominent large-scale clockwise harbour circulation pattern in Lyttelton Harbour. This pattern involves the development of a major gyre in the lower harbour during ebb tides. A counter-rotating gyre develops during flood tides, likely due to the oblique angle of flood currents entering the harbour mouth, inducing rotatory currents around Godley Head. The formation of these gyres is particularly noticeable towards the end of each tide, potentially reaching a critical threshold. Locally induced eddies also contribute to the complex circulation, particularly a persistent eddy observed near the breakwater, extending flood currents. These complex circulation patterns, including rotatory currents at both ends of the tidal gyre, significantly affect sediment deposition and distribution. The study also observes evidence of layered flow, indicating both vertical and horizontal components to the harbour's hydrodynamics.

The study emphasizes the significant role of topography in shaping Lyttelton Harbour's hydrodynamics and circulation. An initial eddy around Gollans Bay develops into the large ebb-tide gyre, while a similar counter-rotating gyre forms during flood tides near Godley Head. The study infers that these patterns are driven by topographical influences on tidal currents and their oblique angle to the harbour mouth. Rotatory currents are observed at the harbour entrance and near the breakwater, but no evidence suggests a circulation system in the upper harbour. This indicates a division between upper and lower harbour circulation, with the transition zone located in the narrow central region. The presence of the breakwater, although considered incidental, likely exacerbates existing eddies, further complicating the overall harbour hydrodynamics and influencing sediment transport.

Analysis of sediment grain size reveals distinct spatial patterns within Lyttelton Harbour. Sandy-mud (10-50% sand) is prevalent at the harbour head, while mud (less than 10% sand) dominates at the entrance. Between Quail Island and Little Port Cooper, the harbour is longitudinally divided into zones of mud and sandy-mud. A significant lateral variation is also observed: the northern side consistently displays predominantly fine muds, while the southern side shows coarser sandy-muds and muddy-sands, with a sharp demarcation between the two. This distinct lateral pattern is only disrupted at the harbour entrance where mud extends uniformly across the harbour. The study acknowledges that traditional grain size analysis offers only an indirect relationship with the physical processes of erosion, transportation, and deposition. The importance of grain surface area to volume ratio in sediment transport is noted (Winkelmolen, 1982).

The study employed a variety of sampling methods to characterize sediment characteristics and spatial distribution. A total of 86 samples were collected: 75 from the harbour bed, 3 from stream beds (Governor's Bay, Head of the Bay, and Purau Bay), and 8 from beaches. Different techniques were used depending on the location: hand collection for stream beds and beaches, short cores (up to 1m long) for intertidal zone samples, and a small pipe dredge for disturbed surface samples from the sea bed. Precise locations were determined using sextant and compass bearings to harbour and channel markers. Laboratory analysis involved air-drying, sieving (through a 0.0625 mm sieve), and removal of shells. This rigorous sampling approach ensures a comprehensive dataset for analysis of sediment characteristics and spatial distribution within the harbour.

The study specifically investigates the location and transport of fluid mud deposits within the harbour. A custom-designed suspended sediment sampler collected 12 simultaneous samples between the bed and 1 meter above, at 8.5 cm intervals. This sampler, diver-operated, allowed for targeted sampling within fluid mud regions. The study notes that although fluid mud areas are relatively few, they are concentrated in and around the channel and near the harbour entrance. This distribution is linked to the identified rotatory currents and weaker currents at both ends of the tidal gyre, which facilitate the deposition of fine-grained sediments. The study also attempted tracer studies to further investigate sediment transport, but no tracer was found at the sampled sites, highlighting the complexity of sediment movement within the harbour.

The study incorporates wave data collected by the Lyttelton Harbour Board between 1955 and 1959. This data, collected from a piezo-electric pressure unit located on the seabed (9.5m depth), provides information on wave amplitude and wave period. The recording device measured waves with amplitudes exceeding 0.3m, either continuously or at 17-minute intervals every two hours. Because the instrument was a pressure-type, seabed recorder, calculations of wave amplitude had to account for hydrostatic effects of water depth, using a formula adapted from Hastie (1983). The data analysis includes determining significant wave amplitude (H3) and wave period (T3), key parameters used in subsequent calculations. The absence of a breakwater during this data collection period eliminates potential reflection issues near the instrument. The initial data showed that long period waves were infrequent and related to a single storm event.

Further analysis of wave records, this time from the port wharf frontage, was conducted by the Wallingford Hydraulics Research Station (Report No. EX862, 1979). Using spectral analysis, this research showed strong peaks for long-period waves (T > 50 seconds) and swell (10s < T < 20s). Importantly, this analysis indicates that the amount of long-wave energy is larger than the swell energy. It's crucial to note, however, that only the highest energy wave conditions were examined in this secondary analysis (E.C. Bowers, WHRS, pers. comm., 1983), representing only a small portion of the overall wave climate. This highlights the importance of considering the full range of wave conditions in understanding their impact on the harbour's hydrodynamics and sedimentation processes. The limited scope of this secondary analysis necessitates further investigation into the complete wave climate of Lyttelton Harbour.