A sediment budget for the east coast between Oamaru and Banks Peninsula, South Island, New Zealand

Abstract

Two beach systems, the Waitaki (84 km) and Canterbury (136 km), separated by Timaru Harbour breakwater, are considered. Established input (all units in Mt/year; 1 Mt = 10⁶ t) to the Waitaki is 0.82 bedload and 2.0 suspended load; to the Canterbury,. 1.9 and 15.0. Additional unmeasured abnormal load from intense storms and earthquakes may double the suspended load estimates. Inputs from rivers and eroding seacliffs account almost equally for bedload, but most suspended load is from the rivers. Sediment supplied to both beach systems is moved predominantly northeastward in the Nearshore Transport Zone (NTZ). The inner part includes the steep, coarse-grained, reflective beach and surf zone, the latter being generally narrow or nonexistent. Here, almost all the coarse beach sediment is transported in the swash-backwash zone where about 95% is abraded to mud, the remainder accumulating along Kaitorete Barrier. The outer part of the NTZ extends seaward from the surf zone, for 3–15 km across the inner continental shelf, to depths of 3D-60 m. Here, the very fine sand is moved as bedload, and mud as suspended load, in a net northeasterly direction, by waves and currents. Banks Peninsula traps 0.26 Mt/year of the suspended load and bedload in the inlets and 5.5 Mt/year on the shelf around the peninsula. The remaining 11.0 Mt/year is lost to the middle and outer Canterbury Shelf or carried northward into both the Conway and Hikurangi Troughs.