CFD flow dynamics over model scarps and slopes

Abstract

As sea level rises, and during storm and surge events, coastal dunes may become cliffed or scarped by wave action. Knowledge of wind flow over dune scarps, and as scarps fill, their subsequent various slopes, is an essential first step to understanding sediment transport pathways from the beach to the dunes. In this study, flow over scarps (also termed forward facing steps) is reviewed, and the flow over a vertical scarp (90°) and three slopes of 45°, 24° and 14°, all 2 m in height, is examined via CFD modelling. The flow over three 90° scarps with heights of 1 m, 2 m and 4 m, and over a 2 m high vertical (90°) scarp for three increasingly oblique incident winds is also studied. The extent of wind flow deceleration, separation and recirculation becomes smaller with decreased slope, with maximum flow separation and reverse vortex development occurring in the front of the vertical scarp. The extent of crest wind flow separation and recirculation is greatest for the scarp (7.8 m in length), and is considerably less for the 45° slope (2.4 m in length). As scarp height increases, so too does the spatial extent of turbulent wind flow, wind speed, and extent of the flow separation region. For cases where the scarp slope varied but height remained constant, the extent of the flow separation region was greatest when the scarp was vertical. Wind flow separation was dramatically reduced below a scarp slope of 45°. As incident wind direction became more oblique over a vertical scarp, wind speed undergoes significantly less deceleration, and helicoidal vortices replace roller vortices. Our results demonstrate how scarp morphology and wind direction are likely to influence transport pathways.

Wind and Sand Transport Across a Vegetated Foredune Slope

“…For instance, during post‐storm recovery scarps fill up by wind‐blown sand deposited at the dune foot, building a so‐called dune ramp (e.g., Carter et al., 1990, Ollerhead et al., 2013), altering sediment transport and deposition patterns across the foredune. Previous studies used numerical models to compare flow deflection across a scarped and nonscarped foredune, with uncertain implications on dune recovery (Hesp & Smyth, 2016a, 2019; Piscioneri et al., 2019). Volumetric field observations led to conceptual insights, showing that a dune ramp has a major impact on sand transported to the dune crest and lee slope, where scarped dunes devoid of a dune ramp show significantly reduced crest directed transport (Castelle et al., 2017; Christiansen & Davidson‐Arnott, 2004; Donker et al., 2018; Ollerhead et al., 2013; Figure 1).…”

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

“…More recently, CFD has helped to substantially advance our understanding of airflow dynamics over dunes (Smyth, 2016; Walker et al, 2021). The formation of helicoidal flow has been examined using flow streamlines in CFD, utilizing post‐processing to provide three‐dimensional visualizations (Hesp & Smyth, 2019; Jackson et al, 2013; Wakes et al, 2021). To date, however, the characteristics of helicoidal flow, including the response to variations in wind speed, have barely been reported.…”

“…Previous work has shown that the k – ε RNG turbulence model modelled wind flow well, compared with field observations, over complex dune landforms such as a foredune (Hesp et al, 2015; Pattanapol et al, 2011; Wakes et al, 2010), blowouts (Smyth et al, 2012, 2019) and parabolic dunes (Delgado‐Fernandez et al, 2018; Smyth et al, 2020; Wakes et al, 2016). A second‐order spatial discretization scheme was employed to interpolate values between cell centres, and calculations were considered complete once the initial residual of each iteration was <0.0001 m s −1 for U X , U Y and U Z (Hesp & Smyth, 2019).…”

“…At location II (mast A, mid‐notch), CFD simulations predict a normalized wind speed that is greater than the field observations (the difference for U / U C is up to 0.1, Figure 4e), which is likely due to the slope of notch C in the CFD (modelled) terrain being greater than that of the real terrain, up to 5° different (Figure 5). Flows over steeper surfaces may experience stronger acceleration (Hesp & Smyth, 2019; Parsons et al, 2004).…”

Aeolian sand transport thresholds in excavated foredune notches

“…During the last decade, flow–form–sand transport interactions have been studied on various coastal dune landforms, including foredunes (de Winter et al, 2020; Delgado‐Fernandez et al, 2013; Hesp & Smyth, 2016; Hilton et al, 2016; Jackson et al, 2013, 2011; Lynch, Jackson, & Cooper, 2009; Lynch, Jackson, & Cooper, 2010; Petersen, Hilton, & Wakes, 2011; Schwarz et al, 2021; Wakes, Bauer, & Mayo, 2021; Wakes, Hilton, & Konlechner, 2016; Wakes et al, 2010; Walker et al, 2017), foredune scarps (Hesp & Smyth, 2019; Piscioneri, Smyth, & Hesp, 2019), blowouts (Garès & Pease, 2015; Hesp & Walker, 2012; Hugenholtz & Wolfe, 2009; Pease & Gares, 2013; Smyth, Jackson, & Cooper, 2012; Smyth, Jackson, & Cooper, 2013; Smyth et al, 2019; Smyth, Jackson, & Cooper, 2014), parabolic dunes (Anderson & Walker, 2006; Delgado‐Fernandez et al, 2018; Hansen et al, 2009; Smyth et al, 2020; Wakes, 2013), nebkha (Hesp & Smyth, 2017; Zhao et al, 2019, 2020, 2021), sand cays (Hilton et al, 2019), beaches with large woody debris (Grilliot, Walker, & Bauer, 2018; Grilliot, Walker, & Bauer, 2019), and beach scarped ridges (Smyth & Hesp, 2015). These studies have contributed to our understanding of flow–form–sand transport relationships in a wide range of aeolian situations.…”