Foredune restoration and dune system development, Mason Bay, Rakiura/Stewart Island, New Zealand

Author: Hankey, Megan Kate

Date: 2021

Publisher: University of Otago

Type: Thesis

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Transgressive dune systems are naturally dynamic environments that have been actively stabilised through the 20th century. Awareness of the biodiversity of these systems has increased in recent years, globally, and in New Zealand. This has led to the Rakiura (Stewart Island) Dune Restoration Programme, one of the largest dune restoration projects in the world. Restoration of the largest dune system on Rakiura commenced at Mason Bay in 2000. Since 2010, work has focussed on removing Ammophila arenaria (marram grass) and destabilizing a Type I, A. arenaria foredune. The impacts of deliberately devegetating a foredune of this magnitude have not been previously examined, including the consequence of releasing large volumes of sand for downwind plant communities. This research aims to understand the impacts of foredune devegetation on (i) dune system morphology; (ii) rates and processes of aeolian sediment transport and landscape evolution; and (iii) deflation surface plants and plant communities at Mason Bay, downwind of this eroding foredune. The study area includes three adjacent long-walled parabolic dunes (P4-P6). The foredune upwind of P6 was treated with herbicide to eradicate A. arenaria in 2010; whereas the foredune upwind of P4 and P5 was treated in 2015. This situation, coupled with a long-term vegetation monitoring programme, has provided a unique opportunity to address the objectives. Changes in the morphology of the foredune-parabolic dune complex were quantified by comparing digital surface models derived from UAV photogrammetry obtained in 2015 and 2020, topographical profiles and soil pits. Estimates of aeolian sand flux within the parabolic dunes were gained using sonic anemometers and sand traps during strong (typical) onshore wind events. Vegetation in six plots located in the deflation surfaces of each parabolic dune were monitored between 2015 and 2020. Changes in plant community composition were analysed to determine the influence of burial on deflation surface plant communities. The morphology of the foredune (P6 section) changed slowly between 2010 and 2017. After 2017, the rate of change accelerated and the devegetated foredune became lower and wider and the crest shifted landward. About half of the sediment eroded from the foredune since 2017 has been deposited locally, in the lee of the foredune and across the seaward half of the P6 deflation surface. Most of the remaining sediment has primarily been deposited in the depositional lobe of the parabolic dune, in conjunction with Ficinia spiralis, which has also extended inland. Less than 2% of the sediment eroded from the foredune and depositional lobe has been transported beyond the parabolic dunes, into the wider dune system. Estimates of sand flux were made in P5 and P6 to compare sand input to the parabolic dunes from the eroding and largely intact sections of foredune. Sand flux rates recorded during a strong (15-20 ms-1) onshore wind event downwind of the devegetated foredune in the P6 deflation surface were 575 times greater than downwind of the vegetated foredune in the P5 deflation surface. A second experiment examined shore-normal trends in sand transport between the beach and the depositional lobe. Sand transport increased with distance inland through the devegetated parabolic dune. Sediment transport recorded in the deflation surface and depositional lobes (0.90-1.65 kg/m2/min) was almost three times greater than sediment transport recorded on the foredune crest (0.48- 0.51 kg/m2/min). A two-stage model of sedimentation is proposed. During low to moderate wind events (8-12 ms-1) sediment is eroded from the stoss face of the foredune and deposited in the lee of the foredune. During stronger wind events (>15 ms-1), this sediment is transported further inland, across the deflation surface and into the depositional lobe. Foredune erosion and downwind sedimentation has implications for downwind plant communities. The species richness and abundance of plants in the seaward plot in the P6 deflation surface decreased between 2015 and 2020 (17 species in 2015 and 5 species in 2020). Most species were not able to keep up with the rate of sediment deposition (24- 93 cm of accretion in the quadrats in the seaward P6 plot between 2015 and 2020). Prostrate, herbaceous species experienced the greatest decline. In contrast, the plant communities in the P4 and P5 deflation surfaces, where there has been less sand accumulation, showed little change between 2015 and 2020. A. arenaria seedlings were not recorded in the plots in the deflation surfaces, despite the existence of a large foredune seedbank. This indicates that as the foredune has eroded and seeds released, A. arenaria seeds are either not viable, are being transported further inland, beyond the parabolic dunes or the deflation surface environment is not conducive to germination or recruitment. Foredune devegetation has restored dynamic geomorphic processes at Mason Bay. It is anticipated the rate of foredune erosion will slow and the former deflation surfaces associated with the parabolic dunes will be transformed into an undulating, hummocky, landscape, composed of F. spiralis nabkha and associated sand dune-specific species. It is expected the increase in dunal species within the foredune and deflation surface environment will enhance the deposition of sediment, as has occurred in the P6 deflation surface. Therefore, minimal sediment will be transported further inland, beyond the parabolic dunes. Subsequently, it is expected the area downwind of the restored foredune will become representative of the pre-A. arenaria landscape, resulting in enhanced biodiversity within the dune system.

Subjects: Dynamic dune restoration, Marram grass, Foredune destabilization, Stewart Island, Transgressive coastal dunes

Citation: ["Hankey, M. K. (2021). Foredune restoration and dune system development, Mason Bay, Rakiura/Stewart Island, New Zealand (Thesis, Master of Science). University of Otago. Retrieved from"]

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