Coastal hydrodynamics and shoreline change at Buffalo Beach

Coastal hazards such as beach erosion, coastal flooding and tsunamis have become an important issue at Whitianga, particularly since coastal subdivision in the early 1960s. A Masters thesis focussed on patterns of shoreline change, nearshore sediments, and patterns of sediment transport within Mercury Bay to better understand the processes driving patterns of erosion and accretion on Buffalo Beach. Aerial photographs and historical beach profiles indicate that the beach has been in a state of dynamic equilibrium since the early 1940s. However, erosion after 1995 has placed beachfront properties at significant risk. Buffalo Beach, and the adjacent nearshore zone to a depth of around 5 m, is dominated by fine sand, with a gradual fining of sediment offshore. Side-scan sonar imagery revealed an essentially fine featureless seabed that dominated the majority of Mercury Bay. However, coarse grained mega-rippled deposits were found extensively along the northern periphery and central parts of Mercury Bay. Sediment is unlikely to be active for significant periods of time at a depth greater than approximately 13 m. It is likely that the prominent headlands at either end of the beach restrict longshore sediment transport to Buffalo Beach and past the Whitianga Estuary. A small scale bathymetric survey was undertaken to assess any detrimental effects of a recently constructed seawall. Wave interactions with the seawall were evident as offshore scour troughs. It is clear that the seawall also accelerates erosion at either end, and this is likely to be exacerbated by the hooked nature of the seawall at is southern extremity. Almost 67 days of wave and current information were collected over two deployment periods. Results indicate a highly variable hydrodynamic regime in Buffalo Bay. Wave heights varied considerably along the beach and between each deployment. Maximum significant wave heights of 1.4 m were recorded at a site near the centre of the beach. A site further along Buffalo Beach, but influenced by wave refraction around a headland, recorded waves typically half the height at the centre. Currents measured near the seabed in 3 m water depth reached 35 cm.s-1 associated with the maximum recorded wave heights. These wave induced currents exceed the threshold speed for entraining and transporting sandy sediment. The tides are responsible for over 99% of the sea level variability in Buffalo Bay. During periods of strong offshore winds, a residual current analysis identified onshore directed bottom return currents up to 10 cm.s-1. Diabathic sediment transport dominates nearshore exchanges of sediment, particularly during storm events. Furthermore, seasonal influences appear to be the dominate signature evident in the cut and fill cycle of sediment from Buffalo Beach. An anticlockwise circulation cell was identified through a vector current analysis and is driven by the strong tidal asymmetry in Mercury Bay. This cell is likely to contribute to the slow southwards migration of sand along Buffalo Beach towards the estuary. Erosion at Ohuka Beach appears to be related to sediment starvation from a beach at the head of a local littoral drift system. It is likely that any renourishment will ultimately be more beneficial to central and southern parts of Buffalo Beach compared to Ohuka Beach. However, short-term benefits will accrue to Ohuka Beach due to the increased volume. Retaining these will require periodic maintenance renourishment. Therefore emphasis in monitoring of this renourishment should be placed on the onshore-offshore exchanges of sediment and the extent to which sediment is periodically removed from the beach face. Furthermore, the monitoring should establish the rate of loss in order to plan future renourishments. To reduce the rate of loss, the sediment used should be sufficiently coarse to ensure that it is not lost to depths where the seafloor is essentially inactive.

Data collected included: Weather station - rainfall, atmospheric pressure, wind speed and direction. Old bathymetric charts digitised and compared - 1852 & 1979. 36 sediment samples from Buffalo Beach and 9 offshore samples in Mercury Bay - analysed for grain size. Sidescan sonar imagery of Mercury Bay sea bed throughout Mercury Bay. Bathymetric survey data fronting and adjacent to seawall at northern end of Buffalo Beach. 67 days of wave and current information were collected over two deployment periods.

Buffalo Beach and Mercury Bay

Sediment samples: 11/08/2002 (beach) and 05/09/2002 (offshore). Hydrographic survey 05/09/2002. Sidescan sonar survey: 25-26/06/2002. Instruments for waves and currents two deployment periods: 27/02/2002 - 03/04/2002 and 07/08/2002 - ?? Weather station: 10/02/2002 - 25/06/2002 and 06/08/2002 - 18/11/2002.

Hydrographic survey undertaken using RTK-GPS. Coarser sediments from beach analysed with RSA, finer from offshore with Malvern. 4 x S4ADW current meters. One-off study.

Data presented in hard copy thesis. Digital Format: Appendices 4,5,6 on CD in back of thesis.

Thesis and CD available for loan from University of Waikato Library. Sensitivity/Confidentiality: Not confidential.

Data set closed/completed.