Sedimentology and evolution of Ohiwa Harbour, a barrier‐impounded estuarine lagoon in Bay of Plenty

Abstract

Ohiwa Harbour is a 24 km² estuarine lagoon impounded by the 6 km long Ohope spit in the west and the 0.7 km long Ohiwa spit in the east. These barrier sand spits are presently separated by a c. 340 m‐wide inlet channel where the maximum harbour depth of 14 m occurs. Seventy percent of the harbour consists of tidal flats supporting a rich shelly benthos and diversified by stands of mangrove and backed locally by salt marsh. Lower harbour sediments in barrier beach, dune, and entrance shoal and channel environments are well sorted, negatively‐skewed, medium to fine sands. In contrast, upper harbour sediments are poorly to very poorly sorted, positively‐skewed, medium to very fine (silty) sands, the coarser of these deposits occurring in channel and restricted harbour beach environments, and the finer in inter‐tidal flat, creek, and channel bank areas. The terrigenous mineralogy is consistent with a dominantly acid volcanic provenance, directly from the tephra mantle of the catchment and, most importantly, indirectly from the oceanic littoral zone. Sediment dispersal is dominated by tidal currents. Speeds decrease systematically up‐harbour from maximum values of 100–150 cm s^‐1 at the inlet channel to 5–10 cms^‐1 in upper‐harbour reaches. Current ripples, megaripples, and sand waves characterise the higher energy, current‐dominated lower harbour deposits, hereas small‐scale current and wave ripples, together with biogenic markings and burrows, characterise the lower energy, inner harbour deposits. Small amplitude, wind‐forced waves are important for resuspending and moving sediment in intertidal areas, particularly on the upper tidal flats. Most areas of the harbour are affected by biological processes. Rapid growth of Ohope barrier spit resulted from increased eastwards transport of littoral sands around Whakatane Heads, west of Ohiwa Harbour. The probable cause of this was the infilling of the up‐drift Rangitaiki Plains embayment which was essentially completed soon after the time of the Taupo Pumice eruption (AD 131). Over the last 2000 years Ohope spit has accreted laterally eastwards at an average rate of about 3 m y ‘, Ohiwa spit has concomitantly eroded, and there has been accelerated infilling of Ohiwa Harbour.

Key words:

• Ohiwa Harbour
• Bay of Plenty
• Ohope spit
• Ohiwa spit
• estuarine lagoon
• tidal flats
• tidal channels
• barrier sand spits
• sediments
• bed‐forms
• biological communities
• sedimentation
• spit erosion
• inlet channel migration
• barrier evolution
• Holocene

Notes

Present address: United States Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, United States of America