Sediment Flux on the High Energy Taranaki Coast, New Zealand

The Taranaki headland in New Zealand protrudes into a high energy wave climate system with a potential for strong littoral transport. The shoreline is typically comprised of a narrow cobble to boulder reflective beach, surmounting a rugged wave-cut shore platform which is carved into lahar deposits from the nearby andesitic composite cone of Mount Taranaki. Historically along this coast, pocket sandy beaches have endured within embayments and adjacent to headland features. The Taranaki littoral system has been described as sand-starved. However, in 1998 persistent heavy rainfall resulted in the collapse of scoriaceous sand and gravel on the side of Mount Taranaki, leading to massive injection of sand and gravel directly into the Hangatahua (Stony) River catchment. Since the initial collapse in 1998, the adjacent coastal shoreline has experienced a continuous influx of dense 'black' titanomagnetite-rich volcanic sands from the Stony River. These sediments are being rapidly transported to the north-east by the energetic wave climate, creating upper-shore sandy beaches on what is normally a rocky boulder coast devoid of sand. This study focuses on onshore geomorphology and sediment characteristics of this coast in June, September and November 2008. Between June and September 2008, results indicate that there has been a decrease in the beach sediment volume and mean grain size with distance north-east of the Stony River. This "sand lens" ejected from the Stony River mouth in 1998 sits on the upper shore, being transported only when high tides coincide with energetic wave condtions.