Monitoring the Coastal Sand Wedge Outbreak from the Stony River, Taranaki

Abstract

The Taranaki headland protrudes into a high energy wave climate system with a potential for strong littoral transport. This 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; otherwise the the Taranaki littoral system is sand-starved. 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 Stony River, from which the adjacent coastal shoreline has experienced a continuous flux of dense 'black' titanomagnetite-rich volcanic sands. These sediments are rapidly transported to the north-east by the energetic wave climate, creating sandy beaches on what is normally a rocky boulder coast. Coastline changes between Cape Egmont and New Plymouth were analysed by the comparison of aerial photographs dated 1995, 2001, and 2007. These photos illustrated a marked increase in sandy sediment in 2001 which has diminished rapidly by 2007. Sub-aerial beach profiles were undertaken, over the duration of this study, at 12 locations between Komene Road, just south of the Stony River mouth, and Back Beach in New Plymouth. The beaches including and south of Ahuahu Beach are characterised by dunes which reach greater elevations than those further north where the dune elevation decreases. This evidence along with visual observations suggest that the sediment derived from the Stony River sits on the upper beach forming a berm and high dunes. Sediment textural analysis was conducted on samples collected at the 12 beach profiling locations. This analysis was undertaken every three months over the course of this study and showed (1) a decrease in mean grain size with distance north of the Stony River mouth; and (2) sorting generally improves with distance north-east in the direction of the dominant littoral drift. Mineralogical analysis illustrated that beach sediments are dominated by the heavy mineral titanomagnetite and the opaque minerals of augite, hornblende and plagioclase feldspar. The longshore sediment transport flux between Rahotu Road, south of Cape Egmont) and Back Beach in New Plymouth was examined. Wave climate parameters were generated at 55 locations between 1998 and 2007 using the SWAN third generation numerical model. The CERC (1973) formula was used to calculate the potential longshore sediment transport flux, at each of these locations using the significant wave height and angle of incidence. Analysis of the wave data illustrate a wave energy gradient extends from the Cape toward New Plymouth, caused by: (1) the exposed nature of the coast near Cape Egmont to the dominant south-westerly swells; (2) the greater seabed gradient near Cape Egmont, and (3) the refraction shadowing that occurs with distance to the northeast of the Cape. The potential longshore sediment transport results indicate low potential sediment transport fluxes south of Cape Egmont, which increase, reaching a maximum potential flux slightly south of the Stony River mouth. These potential longshore sediment transport fluxes then decrease towards Back Beach as a result of refraction in the vicinity of the Sugar Loaf Islands. The size of the 'slug' of sand derived from the headwaters of the Stony River is likely to have diminished in size substantially when it has been transported as far north as Back Beach, and these results indicate that there will be insufficient energy for any substantial volumes to be transported around the Paritutu Headland. A sediment budget has been ascertained from the volume of sediment that has recently been eroded from the scarp on Little Pyramid on Mount Taranaki, and the volume of sediment that can be accounted for today on the beaches, and in the Stony River channel as aggradation deposits. Up to 14.4 M m3 of sediment was estimated to have been deposited in the Stony River system directly from the scarp. Of this, ~ 3.5 M m3 has accumulated on the sub-aerial beach over a distance of 14 km to east from just south of the river mouth, and ~3.3 M m3 has been deposited along the Stony River channel. The remaining ~7.6M m3 of sediment is likely to be deposited offshore, overlying the wave-swept boulder platform and in the interstitial space of the sediments that form these shore platforms. Significant quantities of this sediment are also likely to have been transported in the north-east directed littoral transport.