Aeolian sand transport thresholds in excavated foredune notches

Abstract

Excavation of 'notches' in foredunes aims to facilitate sand transport through the foredune zone to enhance biodiversity and increase foredune resilience. Recent research has examined notch morphodynamics, however, the underlying aeolian processes in relation to sand transport have not been examined. This study determines:(i) the critical incident wind conditions resulting in sand transport inside the notch; and (ii) the pattern of wind flow and sand deposition/erosion inside the notch.Secondary winds are recorded using 12 ultrasonic anemometers deployed at crossnotch stations and compared with incident winds measured on a mast 6.5 m above foredune crest. Instantaneous sand transport is recorded using 12 laser particle counters located on four masts across the notch. Sand deposition is measured using erosion pins and digital surface models derived from remotely piloted aerial system.Field data are used to validate Computational Fluid Dynamic wind flow simulations to provide three-dimensional wind direction and speed contours in the notch. The study area is

Dynamic restoration and the impact of native versus invasive vegetation on coastal foredune morphodynamics, Lanphere Dunes, California, USA

“…dune notching, cf. Laporte‐Fauret et al, 2021; Nguyen et al, 2021; Riksen et al, 2016; Ruessink et al, 2018). Furthermore, the resulting landform has exhibited characteristics of a more resilient landscape, with more gradual slopes and faster reconnection to the upper beach following scarping (Figures 6 and 9).…”

Wind flow dynamics and sand deposition behind excavated foredune notches on developed coasts

“…The simulated winds are presented using flow vectors, wind speed contours, streamlines and wall shear stress in the y ‐axis direction (wall shear Y in CFD‐post; Fluent, 2013). The first three are commonly used in CFD studies (Hart et al, 2012; Hesp & Smyth, 2017; Hesp et al, 2015; Hilton et al, 2017, 2019; Nguyen et al, 2021a, b; Parsons et al, 2004; Smyth & Hesp, 2015, 2016; Smyth et al, 2020; Wakes et al, 2005, 2010, 2021). In the computational domain, the y ‐axis direction is approximately onshore normal (Figure 3), so positive values of the wall shear Y indicate near‐surface onshore flow, and negative values indicate near‐surface offshore flow.…”

“…Multiple notches have recently been cut through a stable, narrow foredune on one developed coast to promote sand deposition behind the foredune (Nguyen et al, 2021a(Nguyen et al, , 2021b. In this case the goal is to achieve a wider and less vegetated foredune that has enhanced dynamism and the potential to migrate inland in response to shoreline erosion and vegetation disturbance.…”

“…This type of intervention could be an efficient nature-based solution to cope with expected erosion pressure related to eustatic sea level rise (Castelle et al, 2019;Laporte-Fauret et al, 2021). Previous studies have found that notches may promote onshore wind flow and sand transport (Nguyen et al, 2021a(Nguyen et al, , 2021b, extend the area of bare sand downwind of each notch (Pye & Blott, 2016) and increase rates of sand deposition in the backdune environment (Arens et al, 2013a(Arens et al, , 2013bMeerkerk et al, 2007;Riksen et al, 2016;Ruessink et al, 2018). The patterns of wind flow and sand deposition behind foredune notches, however, have not been examined, despite the potential insights into the morphodynamic evolution of notched foredunes and dune landforms in the lee or notches.…”

Skimming Flow and Sand Transport Within and Above Ammophila (Marram) Grass on a Foredune

“…In coastal regions, it is common to find that the highest sedimentation on foredunes occurs between the dune foot and the crest, particularly on moderately to well‐vegetated foredunes (e.g., Arens, 1996; Bauer et al., 2013, 2022; Hesp, 1983, 1988, 2002; Hesp et al., 2013; Keijsers et al., 2015; McLean & Shen, 2006; Ollerhead et al., 2013; Rotnicka, 2011, 2013a; Sarre, 1989; Schwarz et al., 2020). However, sand transport has been observed to occur at relatively significant distances past the foredune crest (Hesp et al., 2013; Keijsers et al., 2015), especially under conditions of high incident wind speeds (Hesp et al., 2009; Petersen et al., 2011), in the presence of steep vegetated stoss slopes (Arens, 1996; Arens et al., 2002; Hesp et al., 2009; Jackson et al., 2013), where unvegetated aeolian ramps (or scarp fills) are present (Bauer & Wakes, 2022; Christiansen & Davidson‐Arnott, 2004; Hesp et al., 2009; Walker et al., 2021), when jets occur (Hesp & Smyth, 2016), and via delivery of suspended sediments from blowouts (Arens et al., 2013; Hesp & Hyde, 1996; Hesp et al., 2017; Nguyen et al., 2022a, 2022b; Van Kuik et al., 2022). The aeolian sand transport may occur by creep and saltation under the vegetation in either high velocity incident winds and/or in lower percent vegetation densities (Hesp et al., 2009), modified saltation where grains bounce off the streamlined dense vegetation surface (Hesp et al., 2009; Petersen et al., 2011), and/or by suspension where grains are transported relatively long distances across and above the vegetation canopy (Petersen et al., 2011).…”