Abstract
In recent decades, coastal foredunes on developed coasts have been managed to maximise vegetation cover and stability; and to increase dune height and uniformity, with the goal of reducing coastal flooding. Consequently, many foredunes are now relatively densely vegetated, high and narrow, ridges of sand compared with their previous pre-management form. Such foredunes may protect the hinterland from marine flooding, but they also block sand exchange between the beach and the backdune. The suppression of sand reaching the lee slope of the foredune or the back dunes might result in a foredune which is less able to migrate landward in response to eustatic sea-level rise. Depending on the circumstances, such foredunes might be both more vulnerable to erosion and susceptible to sea-level rise. Excavation of ‘notches’ in foredunes – which are essentially artificial blowouts - aims to facilitate sand transport through the foredune zone to achieve a range of management objectives. In the current study at St Kilda, Dunedin, New Zealand, notches were excavated by local government to produce a wider foredune by enhancing sand transport between the beach and the swale.
Recent research has described the morphodynamic development of excavated foredune notches, however, the underlying aeolian processes, namely wind flow dynamics and sand transport through the notches, is poorly understood. Developing an understanding of these processes is important to assess and increase the efficacy of excavated notches. Flow acceleration might result in notch erosion and sand deposition in the swale behind the foredune. In the long term, this might maintain the open, mobile, bare or semi-vegetated, sand environment behind the notches. Flow deceleration, however, might result in notch deposition and rapid in-filling of the notch. The current study examines aeolian sand transport in a notched foredune system to improve our understanding of flow – form – transport interactions in the notches; and to assist dune managers to design optimal foredune morphologies. It examines: (1a) how wind speed and direction change across the beach to the notch; (1b) the relationships between incident wind direction and secondary flows inside a notch; (2a) the thresholds of aeolian sand transport inside a notch; (2b) the patterns of wind flow and sand transport inside a notch; the patterns of wind flow (3a) and sand deposition (3b) behind notches and behind a foredune with multiple notches; and how patterns of wind flow (4a) and sand transport (4b) vary within different notch morphologies.
The study was conducted on a number of excavated notches with a range of morphologies: where the long axis of the notches is oriented between 53o and 67o to the shoreline (where 0o is the shoreline); 7 to 17 m in length; 4 to 7 m in width; 0.8 to 2.7 m in depth; and 9o to 14o floor slope. Aeolian field experiments, involving arrays of instruments and Computational Fluid Dynamics (CFD) are employed to address the research questions. Secondary wind speed and direction was recorded in the notch by 12 ultrasonic anemometers and compared with incident wind speed and direction measured at 6.5 m above the foredune crest. The anemometers in the notch are mounted on three masts erected along the long and across the short axes of the notch. Each mast contained four Gill 2-D Windsonic anemometers mounted at 0.20 m, 0.46 m, 1.05 m and 2.51 m from the bed. Instantaneous sand transport was recorded using Wenglor Laser Particle Counters (LPCs) and sand traps. Four LPC masts were erected across the notch. Each mast consisted of three LPCs at 0.02 m, 0.04 m and 0.1 m from the bed. Sand erosion/deposition inside and behind the notches was measured using erosion pins, sand traps (at event-scale) and, over months, digital surface models derived from a remotely piloted aerial system (RPAS) and RTK-GPS. Field data are used to validate Computational Fluid Dynamic (CFD) simulations of wind flow. CFD is then used to examine the flow dynamics at larger spatial scales, across the beach through and behind the notch in a range of incident wind conditions. ANSYS FluentTM is used for the CFD simulations using the Renormalised Group (RNG) k-e turbulence model.
The normalised wind speed inside the excavated notch is strongly correlated with incident wind approach angle (RQ 1a-b). This was the focus of the research in one notch, (notch C: 67o in orientation; 14.6 m in length; 6.8 m in width; 2.7 m in depth; and 8.6o in slope). Flow streamline compression, flow realignment with notch orientation and flow acceleration occurs when the incident wind angle is less than 27o relative to the long axis. Flow separation and deceleration occurs when incident wind angle is greater than 27o to the long axis. Wind flow from the beach through the notch can be divided into three flow zones: (1) back beach; (2) upper beach to notch throat; and (3) inside the notch. Flow steering occurs at higher deflection angles in zone 2 than observed in the intact vegetated (un-notched) foredune.
Sand transport inside the excavated foredune notch is related to incident wind speed and direction (RQ 2a-b). Sand transport occurs in notch C when the incident wind speed and direction are: 15 m s–1, 17 m s–1 and 23 m s–1 and 40o, 48o and 55o, respectively, relative to notch orientation (i.e. 37o, 29o and 22o relative to the shoreline, where 0o indicates the wind is alongshore). These incident wind angles are greater than those reported in published studies of sedimentation in vegetated foredunes. For example, a number of studies reported that when the incident wind is approximately parallel to the shoreline (< 30o relative to the shoreline), sand is transported along the toe of the vegetated foredune. Therefore, this indicates that during highly oblique onshore incident wind conditions (< 30o relative to the shoreline), alongshore sand transport may be steered towards and through excavated notches of a certain orientation (e.g. notch C with orientation of 67o relative to the shoreline).
Secondary wind flow in the swale, behind the notches (RQ 3a-b), is alongshore when the incident wind angle varies from -15o to 10o relative to the shoreline (where 0o indicates the wind is alongshore). When the incident wind direction varies from 10o to 30o and from 30o to 72o, secondary wind direction at the swale is offshore and onshore, respectively. Flow reversal occurs behind notch C when the incident wind direction is greater than 72o (that is, when the wind is aligned approximately with the long axis of the notch). Wind deceleration occurs between the depositional lobe and the swale when the incident wind varies between 20o and 80o relative to the shoreline. Most sand deposition occurs within 5 m of the lobe. Up to 2.5m depth of sand accumulated over 4 years downwind of notch C. Less than 0.5 m of sand accreted in the swale downwind the notch.
Notch morphology has a significant influence on wind flow dynamics and sand transport inside the notch (RQ 4a-b). Wind speed in a notch, which is orientated 52o relative to the shoreline (where 0o indicates the wind is alongshore), is greater than a notch oriented at 61o; when the incident wind direction varies from -9o to 40o. Wind speed decreases in the shallower notch (1.7 m), but increases in the deeper notch (2.7 m), when the incident wind direction varies from 65o to 132o relative to the shoreline. The highest sand transport activity parameter was recorded in the notch that is most parallel to the incident wind direction during experimental period. Similar average rates of sand deposition occur downwind of the foredune sections with multiple notches, including longer and shorter notches. However, very low rates of sand deposition were measured behind the intact section of foredunes. The notch with the orientation that is most closely aligned to the prevailing wind direction, and which has the steepest windward face, appears to be the most effective notch, as indicated by the depth and of accretion at the depositional lobe and distance of sand transported landward of the lobe.
This study contributes to our understanding of the efficacy of foredune notching as a coastal management technique on developed coasts. Sand transport commonly occurs alongshore in front of highly managed foredunes when the prevailing incident wind directions are alongshore to oblique onshore, restricting sand flux and sand deposition to the upper beach and stoss slope of the foredune. In such cases excavated foredune notches act as an effective intervention to create a conduit for sand transport to the backdune. It is suggested that follow-up management, such as removal of vegetation inside the notches, re-excavation of sand inside the notch, is required after foredune notching to maintain the designed morphology.