Site Characterisation and Assessment of Sediments For Beach Renourishment

Marsden Point, Northland, New Zealand, is a Holocene, prograded barrier spit at the tidal inlet to Whangarei Harbour. The development of the Northport timber export port at Marsden Point in late 2000 modified the tidal inlet dynamic equilibrium due to a large reclamation (32.6 ha) and dredged turning basin (31.8 ha to 13 m below Chart Datum). The effects of the Northport development on the inlet morphodynamics and the potential use of dredge sediments as beach nourishment fill are the foci of this study. Regional sediment transport patterns adjacent to the Northport development were inferred and documented through the interpretation of sediment grain size distributions, geomorphic analysis of historical shoreline and bathymetry data, analysis of beach profiles, identification of seabed sediment transport pathways, and calculation of potential sediment transport vectors from hindcast wind-wave and current meter data. Sediments found in the lower harbour are consistent with Schofield's (1975) Hauraki (B) facies. Shelly lag sediments, which armour and stabilise the bed, were observed in similar locations to those present in 1983. Sediment transport directions inferred from sediment textural analyses were generally in agreement with directions found from beach profile analysis, morphologic interpretation and calculation of potential sediment transport vectors. Analysis of thirteen rectified and geo-referenced vertical aerial photographs of the Marsden Point region over the period 1984 to 2001, indicated a general reduction in dry beach widths within the harbour, and an increase in beach width at Marsden Point, where opposing sediment transport vectors meet and the ebb tidal delta of Mair Bank became 'welded' to the shoreline. This area exhibits highly variable beach profiles and offshore bathymetry, indicative of the relatively large potential sediment vectors meeting at this location. Comparison of digital bathymetric surveys, sediment transport pathways and bedform locations both pre and post-development, indicates changes have been limited to the local reclamation and dredge basin areas. Reduced post-development sediment transport potentials were found in the northern areas of the dredge basin. There is potential for scour and the development of a shell lag at the dredge basin's eastern edge. Small-scale (-1,000m³yr¹} accretion has been observed against both western and eastern edges of the Northport reclamation. Accretion is occurring due to dominant longshore sediment transport patterns (western edge) and the creation of a 'current transport shadow zone' (eastern edge). Dredge basin sedimentation is expected to increase over the next 5-10 years to reach 15-20,000 m³ yr¹ as the beach profile against Northport's western edge 'fills' with sediment. Sediment grain size comparisons and sediment transport patterns have been used to develop an effective beach nourishment plan for Marsden Bay (West and Central) using sediments sourced from Northport's dredge basin. The Northport development has caused only local modifications to sediment transport processes operating within the lower Whangarei Harbour. There have been no largescale changes in sediment transport patterns or morphology observed postdevelopment