Observations and modeling of the impacts of mangrove extent on hydrodynamics and sediment transport in Waikaraka Estuary, New Zealand

Abstract

Mangroves are often referred to as ecosystem engineers because of their influence on estuarine sediment dynamics and morphology. Yet, mangrove may also respond passively to morphologic change, and the impacts of mangrove extent on sediment- and hydro-dynamic are still poorly understood. A field study was conducted in Waikaraka Estuary, Tauranga Harbor, New Zealand, to investigate the impact of mangrove coverage on estuarine sediment dynamics. Mangroves expanded in the estuary from 1940 until a removal program began in 2005 with the goal of flushing fine sediment out of the estuary. Water velocity, wave height, and turbidity were measured in June 2019 along with sediment grain size. The bed sediment was predominantly sandy in the lower estuary and muddy in the upper estuary. Flow in the lower estuary was ebb dominant resulting in net sediment export, while flow in the upper estuary was weakly flood dominant resulting in sediment retention. A Delft3D numerical model calibrated with this in-situ data was used to further examine the impact of varying mangrove coverage on hydrodynamics. Tidal asymmetry, velocity skew, and peak ebb-tide shear stress were not significantly altered by varying the mangrove extent between the 2005 maximum and complete removal. Therefore, mangroves were not a primary control on flow in this system; instead, the hydrodynamics and net sediment transport were controlled by bathymetric interactions with the tide. Fine sediment is unlikely to be flushed out of the upper estuary despite mangrove removal. When mangroves were expanded to cover all intertidal areas, the water velocity on the intertidal flats decreased while the peak ebb-tide velocity in the main channel increased. This indicates that fine-sediment export may actually decrease with mangrove removal. These results support recent findings that mangroves act as opportunistic colonizers not ecosystem engineers.