Nearshore-Surfzone System Limits and the Impacts of Sand Extraction

Abstract

Nearshore coastal sand mining may adversely affect coastal development where extractions occur within the active beach-nearshore sand system. In this paper the outer limits of the beach-nearshore system are reviewed, with the inclusion of new data from Pakiri, New Zealand. These include interpretation of nearshore sedimentology, morphology, bedforms and benthic fauna, theoretical estimates of sediment movement, historic evidence of beach-nearshore morphodynamics, measurement of the seaward extent of riphcad plumes during or following storm wave conditions, and stratigraphic indications of seabed erosion and deposition. There is convergent data that for east coast Australian and New Zealand moderate to high energy beaches, including Pakiri, the maximum limit of the modern beach-nearshore sand system occurs around the 25 m isobath.

Recent interpretations of the geomorphology of the Pakiri-Mangawhai sand system, within which shallow marine mining takes place, are re-examined in light of the results of this review. The conclusion that substantial onshore sediment exchange occurs between inner shelf and nearshore environments, and that the Pakiri-Mangawhai sand system is therefore open to sediment inputs, is not sustained. The results of investigations of bedforms, subtidal facies, historic morphodynamics and theoretical estimates of sediment transport thresholds, indicates that the nearshore-inner shelf boundary approximates the 25 m isobath at Pakiri. Significant sediment transport does not occur beyond this depth over the relevant time scales. A re-analysis of the beach profile records, as well as interpretation of barrier morphostratigraphy, indicates that, contrary to most previous interpretations, the Pakiri-Mangawhai coast shows no strong accretionary trend, is at best stable and possibly erosional. We hypothesize that the weak recovery of the Pakiri-Mangawhai coast following severe erosion in 1978 may be a consequence of sand mining.

In the Pakiri-Mangawhai context nearshore coastal sand mining entails a high risk of adversely affecting coastal processes and landforms.