Coastal change at Omaha and Great Barrier Island

Abstract

Since 1966, the relative greater degree of erosion at Omaha, on the east coast north of Auckland, near which substantial inshore dredging had occurred up until 1963, compared with that at Great Barrier Island where there has been scarcely any dredging, has been due to an offshore buildup of an ebb-tide delta at Omaha rather than to a continuing effect of the earlier period of dredging of sand for industrial use. Change in predominant wind direction has probably been a factor, particularly in the marked erosion at the sensitive northern end of Mangatawhiri Spit at Omaha. Five different orders of sea-level change and associated sea-floor and coastal change are recognised. Time is an important factor in determining whether or not full equilibrium is reached and in controlling the area of sea floor that is affected by sea-level change. The longer the period of time, the greater the area of sea floor affected. Another probable limiting factor is the offshore coarse belt.

Prograded coastal barriers provide paleoenvironmental records of storms and sea level during late Quaternary highstands

“…), essentially cut off sediment supply to the north‐east coast at a time when sea level was more than 100 m below present. The sediment that migrated landward from the continental shelf during the post‐glacial marine transgression (∼12–6 ka) is thought to have been reworked onshore to form barriers, such as Marsden Point, during the ensuing relative sea‐level ‘stillstand’ (Schofield, ). While sea level during the last 6,000 years is generally referred to as stable, curves published for New Zealand are not definitive for the Holocene (Gibb, ; Clement et al ., ).…”

Authors’ Final Response

“…The resulting coastal systems are called prograded barriers, and they have been studied for over a half-century to decipher their evolution and extract paleoenvironmental records (e.g. Bernard et al, 1962;Curray et al, 1969;Schofield, 1985;Thom et al, 1981). Over the past few decades, more traditional methods have been augmented by state-of-the-art remote sensing, geophysical, and geochronological techniques (e.g.…”