Aeolian dynamics of beach scraped ridge and dyke structures

Abstract

Where urban areas are situated close to a beach, sand dunes act as protection from flooding and erosion. When a dune has been removed or damaged by erosion, dune, ridge or dyke re-building using heavy machinery, a process known as beach scraping, is a common method of restoration. Following construction, natural accretion of sediment on the backshore is preferable as it facilitates sustained natural dune building, growth of vegetation, habitat creation and reduces the need for further beach scraping.This study investigates the near surface flow and transport potential for three artificial structure designs: a single ridge, a double ridge and a dyke. The three shapes contained an identical volume of sand and were preceded by 50 m of beach at an angle of 3°. A computational fluid dynamic model (CFD) was created for each scenario to calculate wind flow and shear velocity from 4 different wind directions at 22.5° intervals from 0° (onshore) to 67.5°. From this data sediment flux was predicted along a two dimensional transect for each of the scenarios.For all structures, shear velocity on the beach and stoss slope decreased as incident wind direction became more oblique, conversely shear velocity in the lee of the crest increased. A reduction in shear velocity at the foot of each structure also occurred and appears related to stoss slope, with the reduction greatest reduction at the toe of the dyke structure (stoss slope 34°) and the least before the single ridge (stoss slope 17°).Specifically the results suggest that the double ridge structure is the most resilient to aeolian erosion. Shear velocity reduction on the back beach is comparable to the dyke and sediment flux from the stoss slope of the double ridge structure may become trapped in the swale between the two ridges encouraging sediment deposition, thus reducing sediment transport beyond the dunes and backshore. Although the dyke structure underwent the greatest reduction in shear velocity on the back beach it experienced substantial sediment flux at the crest and along the top of the structure, making it susceptible to erosion during a strong wind event. The highest sediment transport rate was calculated at the crest of the single ridge, and the single ridge structure also created the smallest reduction of shear velocity on the back beach, thus making it less desirable than the double ridge.

Flow dynamics over a foredune scarp

“…The flow dynamics at and over the foredune scarp crest are not dissimilar to the flow described at the crest of some non‐scarped relatively steep foredunes (Rasmussen, ; Arens et al, ; Arens, ; Walker et al, ; ; Hesp et al, , , ; Hesp and Smyth, ). Flow acceleration occurs as a result of topographic flow compression as the incoming flow is forced up the scarp wall and then over the scarp (Bowen and Lindley, ; Hsu, ; Oke, ; Hattori and Nagano, ; Smyth and Hesp, ). This acceleration is evident in the percentage velocity profile (2D‐5) at the scarp crest (Figure ).…”

“…Wind flow dynamics observed over scarped foredunes and scarps differ compared with wind flow observations over non‐scarped foredunes (Arens, ; Hesp et al, , ; Walker et al, , ; Bauer et al, ). Investigations of the wind flow over non‐scarped foredunes has been extensive with factors such as vegetation density and morphology (Hesp, ; Arens et al, ; Hesp et al, ; Houser et al ., ; Walker et al, ; Keijsers et al, ), the incident wind direction (Mikkelsen, ; Arens, ; Walker et al, ; Bauer et al, ; Jackson et al, ; Hesp et al, ) and the gradient and morphology of the foredune stoss slope (Bowen and Lindley, , ; Sarre, ; Arens, ; Hesp et al, , ; Smyth and Hesp, ) determining the nature and structure of wind flow (Hesp and Walker, ). In comparison, while the research on flow dynamics over ‘forward facing steps’, cliffs and scarps is significant, studies of flow over foredune scarps has been minimal (Hesp et al, ; Smyth and Hesp, ; Hesp and Smyth, ).…”

Surfzone-Beach-Dune interactions: Flow and Sediment Transport across the Intertidal Beach and Backshore

“…The SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm was used to solve the Navier-Stokes equations (cf. Smyth and Hesp, 2015). This method produces a steady-state, averaged solution of flow.…”

Topographic change and numerically modelled near surface wind flow in a bowl blowout

“…This is particularly the case within trough blowouts, which have narrow and topographically constrained 'Vand U-shaped' morphologies. McKenna Neuman et al, 1997;Walker, 1999;Hesp et al, 2015;Smyth and Hesp, 2015) and turbulent kinetic energy (Smyth et al, 2014), these measurements alone are not direct indicators or predictors of whether sediment is being eroded, transported or deposited, which, in turn, limits their value for understanding the evolutionary dynamics of the system. However, the precise measurement of flow dynamics and resulting patterns of topographic change have not yet been performed in detail.…”

“…Although aeolian sediment transport over a number of aeolian landforms has been demonstrated to increase with wind speed (e.g. McKenna Neuman et al, 1997;Walker, 1999;Hesp et al, 2015;Smyth and Hesp, 2015) and turbulent kinetic energy (Smyth et al, 2014), these measurements alone are not direct indicators or predictors of whether sediment is being eroded, transported or deposited, which, in turn, limits their value for understanding the evolutionary dynamics of the system. This paper moves beyond providing spatial and temporal snapshots of wind flow and sediment transport, and elucidates a meso-scale (33 days) association between flow dynamics and resulting landform change.…”