Coastal erosion and sedimentation of Pukehina Beach and Waihi Estuary

The Pukehina coastal sector is subject to severe frontal dune erosion. This has led to concern from local residents who reside in frontal houses. Accordingly, this investigation was initiated to assess the sedimentary and physical processes within the Pukehina coastal sector, with the aim of identifying causes of enhanced frontal dune erosion. To investigate this problem the following processes were investigated: wave focusing from offshore bathymetric lenses, sediment lost due to the Waihi Estuary acting as a sediment sink, and/or a change in sediment volume or sediment transport processes within the Pukehina coastal sector. Methods to investigate these processes included: collection and analysis of surficial sediment samples from Pukehina Beach and Waihi Estuary, construction of a digital terrain model, side scan sonar surveying, calculation of offshore suspended sediment concentrations, numerical modelling of wave refraction and sediment transport processes, and collection and analysis of wave and estuarine current, depth, and direction data. Wave data were collected at eastern and western ends of the study location, during two separate deployments during July 2000 and March 2001. Outcomes depict differing incoming wave directions at each site, due to offshore undulations in bathymetry. Wave energy focusing was observed along the Pukehina coastal sector (specifically at benchmarks 30, 29, 27 and 26a) during average wave conditions recorded, and enhanced as wave period and wave height increased (approximately 0.2-2.0 m in each simulation, respectively). Wave focusing is significantly influenced by the ebb tidal delta offshore from Waihi Estuary, leading to larger wave heights present in eroding regions of Pukehina Beach. Potential sediment transport rates were investigated, utilising collected wave data. Outcomes illustrate a potential sediment transport to the east. A negative littoral drift gradient, was observed between a) Okurei Point and Waihi Estuary inlet, and a positive littoral drift gradient between b) benchmark 29 and 26. This implies a potential accumulation of sediments near Waihi Estuary inlet (some of which, would be lost to infilling of Waihi Estuary). However, a long-term deficit (-12 m3/m per year) of sediment is lost along Pukehina Beach, due to the apparent positive littoral drift gradient. This outcome is identified as a major component of the long-term 'erosion' problem at Pukehina Beach. Tidal and current gauges were deployed within Waihi Estuary. Data revealed strong evidence that the estuary is acting as a sediment sink. Tidal asymmetry, attenuation of the tidal signal and a dominance of sediment transport into the estuary, depict that Waihi Estuary is an influential component of the erosion occurring at Pukehina Beach. The Waihi Estuary is calculated by geomorphic and hydraulic analysis to be tending towards deposition in the inlet, leading to a general instability of the Waihi Estuary inlet. Nearshore sediment transport rates within the region are significantly influenced by localised wind patterns within the Pukehina coastal sector. The Pukehina coastal sector was identified as having a net deficit of sediment, approximately 4,500 m³/year or -0.5 m³/m per year. However, due to errors involved in estimate calculations, this value may be larger. Sediment textural distributions have altered since previous surveys conducted in 1993 by PHIZACKLEA. Particularly, an influx of sediment to the Okurei Point region, reducing the area of exposed bedrock, suggests a change of sediment transport patterns within this region. Texturally, offshore sediments vary from fine sand sized sediments to very coarse sand, while beach sediments are predominantly medium sand sized particles, with a decrease in sediment size towards Okurei Point (fine sand sized particles), indicating a sediment transport direction to the west. Causes of significant long-term erosion to the Pukehina coastal sector, have been identified as: a) beach-nearshore sediment being lost or 'sucked out' of the coastal sector, due to the combination of a positive littoral drift gradient along Pukehina Beach, and wave energy focusing; and b) infilling of the Waihi Estuary. Other processes, such as aeolian transport etc., do reduce the volume of available sediment to Pukehina Beach, but are not implicated to the long-term 'erosion' problem occurring. Possible remedial solutions, influenced by data observations, are suggested to reduce current frontal dune erosion. However, further research is recommended to identify what coastal management options are required for the Pukehina Beach shoreline