Evaluation of an Inner Shelf Site Off Tauranga Harbour, New Zealand, for Disposal of Muddy-Sandy Dredged Sediments
Keywords:Disposal, cohesive dredge spoil, bed shear stress, threshold velocity.
A planned extension of the Port of Tauranga requires capital dredging of material containing significant amounts of silt and clay. The existing disposal ground located about 4 km offshore in water depths of 15 to 25 m, is unsuitable as it was designed for slow migration of predominantly sandy materials onshore to nourish the adjacent beaches.
Investigation for a new disposal site involved consideration of alternatives, but the "best practical option" selected was offshore of the existing ground in water depths of 28-33 m. Site research included side scan sonar imagery of the sea floor, sediment sampling by SCUBA diving, and deployment of a current meter for several weeks during the spring season to obtain background hydrodynamic data.Analysis of the data indicated that motion of medium to coarse sands occurs during periods of high swell conditions. Calculations of potential transport of discrete mud "clasts" suggest that small units (≤2 cm) may move under large waves (Ts = 11 s, Hs = 1.6 m), but larger mud "clasts" would be stable. It is expected that the existing high disposal mound immediately shoreward of the proposed new disposal ground would hinder onshore migration of muddy clasts.