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Influence of Vegetation Cover on Coastal Aquifer Fluctuation and Sand Transport on Matakana Island

The Bay of Plenty beaches on the east coast of the North Island, New Zealand are of significant physical, ecological and economic importance. Over the previous century, anthropogenic development and the introduction of non-indigenous coastal plant species has lead to a degradation of many of the Bay of Plenty regions’ coastal dune environments. Restoring these sections of coastline to their natural state strengthens the barrier they provide between coastal developments and the coastal ocean, whilst also promoting the growth of native plant species and improving habitat for local macro-invertebrate species. The influence of vegetation on aquifer levels and aeolian sediment transport in the dune and foredune was investigated at Matakana Island in the Western Bay of Plenty. Monitoring of the water table between March and November 2010 was undertaken at two adjacent sample sites, with different dominant overlying vegetation, Ammophila arenaria and Spinifex sericeus. During this time, aeolian sediment transport rates were also monitored through the deployment of sediment traps and two small climate stations. Results showed that aquifer levels beneath the dune face were highly variable. Fluctuations occurred at a range of time scales, stemming from variations in tide, rainfall and profile shape. Short-term fluctuation was primarily linked to tidal forcing. Tidal fluctuations were observed in the aquifer, and differed from tidal fluctuations directly offshore in their shape and amplitude, with some lag between tide and aquifer fluctuations also evident. Aquifer fluctuation shape and lag, and differences between sample sites were linked to the beach drainage capability through aquifer porosity and permeability; hydraulic conductivity; and transmissivity. Long-term change in beach profile shape further influenced aquifer levels, with an accreting beach resulting in an elevating average aquifer level and an eroding beach resulting in a diminishing aquifer level. Aeolian sediment deposition varied greatly across the cross-shore profile. Transport rates were limited by a small beach width when high tides combined with storm surge and wave run-ups limiting the source area. Rainfall further reduced transport potential when coinciding with high wind speed events. Sediment deposition was evenly distributed in the Spinifex dominated dune system, whilst deposition in the Ammophila dune primarily occurred at the seaward limit of vegetation growth. This pattern of deposition is linked to the characteristics of each species, primarily their average height and growth density. Sediment deposition differences between sites explain variances in sediment compaction which alters dune porosity and permeability at each site. Greater porosity and permeability in the Spinifex dominated dune saw the aquifer draining more readily. Lower beach aquifer levels aid accretion and greaten the source for onshore aeolian sediment transport. Spinifex dominated dunes are therefore suggested to provide healthier beach states on Bay of Plenty beaches.
Type of thesis
Muller, J. A. (2011). Influence of Vegetation Cover on Coastal Aquifer Fluctuation and Sand Transport on Matakana Island (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/5760
University of Waikato
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