Rapid transition of sediment consolidation across an expanding mangrove fringe in the Firth of Thames New Zealand

Mangrove habitats are complex systems, which are subjected to both natural and human external forces such as tidal variations, sediment supply, deforestation, and climate change, which in many locations are causing mangroves to decline and even disappear. Global measurements of surface elevations have been conducted at many locations to understand if sedimentation rates in mangrove areas will keep pace with sea level rise. However, extending results to other areas requires a detailed understanding of subsidence processes in mangrove areas. Here, we provide a detailed geotechnical investigation (sediment cone resistance and friction, coefficient of consolidation, grain size, normalised tip resistance and friction ratio, etc.), critical for understanding surficial sediment dynamics and vertical sediment accretion rates, of the mangrove forest edge and its surrounding mud flat in the Firth of Thames, New Zealand. Eight in situ samples were collected and tested in oedometer experiments to evaluate the coefficient of consolidation. Furthermore, a kinematic penetrometer, NIMROD, was used to estimate the resistance forces of the mud flat and mangrove forest, from which soil properties were evaluated. Our results show that mangroves are able to change soil properties to enhance sediment resistance towards erosion. An increase of sediment strength correlated with an increase of mangrove tree density as well as a decrease in the coefficient of consolidation. Hence, the increase of tree density to a decrease of the coefficient of consolidation correlates as well. In this study, our results suggest that the Firth of Thames mangroves are able to accrete enough sediment to keep up with local sea level rises if sediment supplies remain similar in the future. The correlation of tree density to the coefficient of consolidation can be used to assess future mangrove-soil interaction and mud flat progression. Finally, the applicability of literature soil classifications for the kinematic penetrometer was realized for the first time and applied to mangrove and mud flat areas.