The hydrodynamics, sediment characteristics and inferred sediment dynamics of Mangawhai estuary, Northland

Abstract

Mangawhai estuary is a 4 km long, barrier-enclosed estuary in Northland, New Zealand, comprised of a bifurcating channel and intertidal flats with mangrove stands in the middle to upper reaches. The expansion of mangrove habitat in the estuary over the last 60 years is a contentious issue for the Mangawhai community, and debate over the effects of removing the mangroves suffers from a paucity of scientific data pertaining to the estuary specifically. This study has investigated the hydrodynamic and sediment dynamic processes operating in the Mangawhai estuary based on collected field data and inferences from sedimentological and morphological observations. Hydrodynamic data and sediment samples were collected at Mangawhai during two, two week-long deployments in summer and winter, and during summer a bathymetric survey of the lower and middle estuary was conducted. Sandy conditions prevail throughout the estuary, and even in areas where mangroves are present silt content was low (5 – 16%). Sediment is interpreted to be sourced from the coastal foreshore and barrier, and transported by aeolian and tidal mechanisms. There was no discernible catchment-derived sediment identified. Mangawhai estuary is notable for its shallowness, and this appears to be the dominant control on hydrodynamics, influencing mixing and tidal asymmetry. Ebb durations increase moving away from the mouth; the most upper estuary measuring site recorded ebbs that were more than twice as long as flood. Tidal duration asymmetries suggest that Mangawhai is a flood-dominant system, but velocity asymmetries were not locked with those of duration and showed variation between sites and seasons. Patterns of velocity asymmetry are proposed to be controlled by specific morphological features and bathymetric changes, and possibly show an equilibrium response due to net sediment transport produced by the asymmetries. The results from this study show that classifying an estuary as either flood- or ebb-dominant, and the inference of the estuary as a sediment source or sink from such a classification, is not accurate. The low density and low elevation of the mangrove forest at Mangawhai means that Mangawhai hydrodynamics are not typical of other mangrove systems in tropical areas. The small area recently approved for mangrove removal is not expected to significantly alter the hydrodynamics.