Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Wind flow and sediment transport through foredune notches during oblique-onshore incident winds Thesis

Simons-Smith, T.
Journal / Source
University of Otago
Coastal managers have often promoted the stabilisation of coastal dunes to protect coastal lands and infrastructure. In fact, some coasts might be better protected by encouraging the development of broader, active dune systems allowing coastal sands to be shifted inland, away from the impact of storm waves and sea level rise. In recent years notching has been examined as a method that might be used to re-establish dune dynamics for ecological restoration. This thesis examines the viability of notching as a strategy to achieve foredune ‘roll back’ (broadening). The thesis addresses four research questions: (i) What influence does notch morphology have on beach-backdune sand flux? (ii) In what incident wind conditions does significant onshore sand flux occur, and how much sediment might accumulate inland of the notches as a result of these winds? (iii) What are the barriers to aeolian sand transport through the notches? (iv) What effect does backdune topography and wind flow have on patterns of sand deposition inland of the notches? The ability of notches to encourage downwind sand deposition and facilitate landward movement of the foredune was assessed by manipulating plant cover and foredune morphology at St. Kilda Beach, Dunedin, New Zealand. Three notches, each with different morphology and orientation, were excavated in the St. Kilda foredune in April 2016. Patterns of wind flow, sand transport and deposition were examined for a period of nine months (April 2016 - January 2017). Wind flow evolution was examined from beach to notch to backdune. Steering, acceleration and deceleration of wind flow were recorded in each of the notches during a range of incident wind conditions, from alongshore to onshore (primarily, oblique-onshore). Simultaneous estimates of sand transport were obtained using self-orienting swing traps and Wenglor particle counters during incident wind speeds of 5 ms-1 to 20 ms-1. Patterns of depositional lobe development inland of each of the notches were monitored using RTK-GPS surveys, UAV imagery, photographs and ground-based measurements. The total amounts of sand shifted through each notch were quantified using RTK-GPS, while estimates of sand flux and measurements of wind flow allowed depositional development to be modelled. Notch morphology was found to influence rates of sand transport. Estimates of sand flux suggest that the rates of transport were two times higher in Notch B than Notch C (research question 1). The steepest notch, Notch B experienced the most frequent flow acceleration (relative to the beach) during the widest range of incident wind conditions. Notch B, which was better aligned with (oriented towards) the prevailing highly oblique winds at St. Kilda, experienced the most effective steering, with limited flow deceleration occurring even when the incident wind direction was oblique to the long axis of the notch (> 40° oblique). The steep lateral walls of Notch C (~80°) seemed to increase the complexity of flow within the notch, while the more gently sloping lateral walls of Notch B (~35°) may have allowed air flow to more readily escape the notch, providing for decreased air pressure and sustained aeolian sand transport in the notch. Significant onshore sand flux was recorded in all the notches when the incident wind direction was ~40° oblique to the long axis of notches and the incident wind speed was > 10 ms-1 (research question 2). When the incident wind direction was highly oblique to the notches (> 40°), deceleration occurred in all the notches irrespective of wind speed. Between 20 and 30 cubic metres of sediment accumulated inland of each of the three notches during the study period (derived from RTK-GPS surveys), with 57% (Notch B) and 29% (Notch C) of this sediment (derived from modelling) being shifted through the notches by the south-westerly winds examined in this thesis. The primary barriers to aeolian sand transport were incident wind speed and direction (research question 3). Winds that were highly oblique (> 40° oblique to notches) to alongshore decelerated through the notches, and these winds only rarely contributed to significant onshore sand flux and then only in Notch B. When the incident wind speed was < 7.5 ms-1 acceleration rarely occurred in the notches. Moisture conditions, associated with precipitation during and preceding sand transport events and tidal conditions (which relate to wind fetch) were only addressed briefly in this thesis. However, their influence on rates of sand transport were negligible, given the incident wind speed was > 10 ms-1. Backdune topography and wind conditions influenced the patterns of sand deposition inland of each of the notches (research question 4). Each depositional lobe was skewed to the east (alongshore). Most sand deposition occurred on the lee slope of the foredune. This pattern of deposition resulted from flow deceleration as wind exited the notches and encountered highly modified backdune flow conditions. The backdune winds were typically of low wind speed and often oriented alongshore as a result of topographic steering along John Wilson Drive. Sand deposition across, and landward of the swale only occurred when the incident wind speed was > 10 ms-1 and when the wind direction was aligned approximately with the orientation of the notches. Notches constructed in an otherwise stable foredune can act as conduits for wind flow and sand transport, encouraging beach-backdune sand transport. Excavating notches in a stable foredune is a viable strategy for enhancing coastal protection, allowing significant amounts of sand to be added to an otherwise stable foredune This addition of sand allowed the foredune to ‘roll back’ (broaden). The steep, ramp-like Notch B which is oriented towards the prevailing winds (at St. Kilda) was the most effective notch at encouraging onshore sand flux. By maintaining the steep gradient of the foredune this notch experienced frequent landward flow acceleration and high rates of sand flux during a wide range of incident wind conditions. Similarly, its orientation was important in enabling highly oblique winds to become aligned with the notch. Locally, this study has contributed to an understanding of how the St. Kilda foredune might be managed, in order to prolong its role in protecting the South Dunedin area.