Hydrodynamic impacts of tectonics in prehistoric Ohiwa Harbour, North Island, New Zealand

Abstract

Ohiwa Harbour is an estuarine lagoon located in the eastern Bay of Plenty, North Island, New Zealand. Ohiwa Harbour is bounded by two sand spits, Ohope and Ohiwa Spits. This study assessed the likelihood of a resistant barrier underlying Ohiwa Spit, which would control the inlet and spit locations. The observed depths of layers unable to be penetrated by a vibrocorer on Ohiwa Spit supported this idea, along with the discovery of a rock outcrop on the eastern harbour entrance using sidescan SONAR. Vibrocoring on Ohiwa Spit added to the knowledge of the prehistoric evolution of Ohiwa Harbour, and an attempt to infer various subsidence events on Ohiwa Spit were made. Fining upwards coarse sand sequences with dominant shell material were found in the cores. These sequences could be related to a change in harbour hydrodynamics, or recent subsidence events in Ohiwa Harbour, such as the 0.6m subsidence of the Waimana Fault 636 to 575cal yrs BP. A sharp change in the core profile was observed at 1.4m in core C and 1.7m in core C2, marked by increased grain size and an abundance of shell material, mostly Austrovenus stutchburyi. This could be related to a change in wave energy in the harbour or an erosional contact associated with subsidence of Ohiwa Spit. Comparison of radiocarbon dated shells in this study with Murdoch (2005) on Ohope Spit suggests that more subsidence has occurred at Ohiwa Spit than Ohope Spit; this may be associated with a fault through the harbour entrance. Subsidence associated with earthquakes, and erosion associated with at least four tsunami events in the last 6000 years has increased the depth and extent of Ohiwa Harbour, increasing its volume. This study used numerical modeling to determine the hydrodynamic impacts of past catastrophic events (earthquake related subsidence and volcanic eruptions) on Ohiwa Harbour. Ten scenarios along with present conditions were modelled. • In scenarios where sea level was modified only (scenarios 1-5 and 8), tidal range and surface elevation increased within the harbour, but not at the harbour entrance. The entrance hydrodynamics were also influenced by its width; as Ohope Spit accreted, the surface elevation and flow velocities increased within the tidal inlet • Flow speeds increased at the harbour entrance by up to 50% following subsidence in scenarios 9 and 10 • A drop in sea level during subsidence (scenario 6) resulted in a smaller tidal prism and hence reduced flow velocities at the inlet, despite the subsidence • Scenario 7 involved subsidence and the breaching of Ohope Spit, which would divert flow away from the entrance and result in reduced flow velocities at the entrance