Deposition by seasonal wave- and current-supported sediment gravity flows interacting with spatially varying bathymetry: Waiapu shelf, New Zealand

The Waiapu River sedimentary system, New Zealand, provides a prototype for investigating the relative importance of wave- versus current-supported gravity flows on continental shelf deposition. A two-dimensional model was used to represent gravity-driven sediment transport and deposition on the Waiapu shelf over an annual cycle of storm events and associated Waiapu River floods. Model inputs of waves and wind-driven currents were derived from WAVEWATCH III hindcasts and constrained by benthic tripod data. The 12-month model run included a low-energy period (September 2003 to May 2004) with weak waves and currents and low river discharge, and a high-energy period (May to August 2004) with stronger waves and wind-driven currents and more frequent river floods. Model results suggested that during the low-energy period, riverine sediment was trapped between the 20- and 80-m isobaths. During the high-energy period, sediment was deposited obliquely across the shelf between the 60- and 120-m isobaths. The predicted deposit locations for the low- and high-energy periods, respectively, were consistent with short- and long-term observed accumulation patterns based on 7Be and 210Pb activity [Kniskern, T.A., Kuehl, S.A., Harris, C.K., Carter, L., 2010. Sediment accumulation patterns and fine-scale strata formation on the Waiapu River shelf, New Zealand. Marine Geology 270, 188–201]. Gravity flows were mainly wave-supported landward of the 60-m isobath, but became increasingly current-supported as wave orbital velocity attenuated in deeper water. Both analytical theory and numerical results indicated that wave-supported gravity currents were sensitive to local water depth and favored deposition parallel to isobaths as depth increased. In contrast, current-supported gravity currents were more sensitive to spatial variations in seabed slope, with seaward decreases in slope and along-shelf embayment of bathymetry favoring transport convergence and deposition. We conclude that the longer term (∼ 100 yr) shelf-oblique mud deposit on the Waiapu shelf mainly reflects current-supported gravity flows responding to local variations in seabed slope and curvature of isobaths.