Determining the Limits of Beach-Nearshore Sand Systems and the Impact of Offshore Coastal Sand Mining

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 riphead plumes during or following storm wave conditions, and stratigraphic indications of seabed erosion and deposition. Convergent data indicate that the maximum limit of the modern beach-nearshore sand system occurs around the 25 m isobath on moderate to high energy beaches, on the east coast of Australia and New Zealand including Pakiri. The 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, indicate 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 and 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, there is a high risk that nearshore coastal sand mining will adversely affect coastal processes and landforms.