Effects of beach slope breaks on nearshore groundwater dynamics

Abstract

Interactions between fresh groundwater and seawater affect significantly the nearshore pore water flow, which in turn influences the fate of nutrients and contaminants in coastal aquifers prior to discharge to the marine environment. Field investigations and numerical simulations were carried out to examine the groundwater dynamics in the intertidal zone of a carbonate sandy aquifer on the tropical island of Rarotonga, Cook Islands. The study site was featured by distinct cross‐shore slope breaks on the beach surface. Measured pore water salinities revealed different distributions under the influences of different beach profiles, inland heads, and tidal oscillations. Fresh groundwater was found to discharge around a beach slope break located in the middle area of the intertidal zone. The results indicate a strong interplay between the slope break beach morphology and tidal force in controlling the nearshore groundwater flow and solute transport. The fresh groundwater discharge location was largely determined by the beach morphology in combination with the tidal force. The nearshore groundwater flow can be very sensitive to beach slope breaks, which induce local circulation and flow instabilities. As slope breaks are a common feature of beaches around the world, these results have important, general implications for future studies of nutrients transport and transformations in nearshore aquifers and associated fluxes via submarine groundwater discharge.

Transformation in the Stability of Tide‐Induced Upper Saline Plume Driven by Transient External Forcing

“…(2020), Y. Zhang et al. (2017), and J. Zhang et al. (2021) confirmed the unstable phenomenon of USP and highlighted the influence of aquifer heterogeneity.…”

Effects of Unsaturated Flow on Salt Distributions in Tidally Influenced Coastal Unconfined Aquifers

“…The dimensions of field-scale models were 154 m (length) × 20 m (height) with a shoreline slope of 1:8 (Figure S1c in Supporting Information S1). The adopted slope and aquifer depth are similar to several published case studies of coastal settings (e.g., Heiss & Michael, 2014;Mao et al, 2006;Raubenheimer et al, 1999;Vos et al, 2020;Xiao et al, 2019;Zamrsky et al, 2018;Zhang et al, 2017). The aquifer was assumed to consist of fine sand (Rehovot sand; Mualem, 1976a) with n e = 0.4 and K s = 1.33 × 10 −4 m s −1 (Röper et al, 2013;Vandenbohede & Lebbe, 2006.…”

Visualizing the ‘iron curtain’ formation in the subterranean estuary using sand tank experiments

“…As field sites are often difficult to access, the replica of a beach aquifer under laboratory conditions can aid to better understand the fundamentals of the involved physical and biogeochemical processes (Stoeckl & Houben, 2023). For instance, Röper et al (2014), Shen et al (2015), Zhang et al (2017) or Delwar et al (2024) mimicked coastal aquifers in laboratory experiments and found the freshwater-seawater interface in the STE to be far less stable than previously thought. These findings are of great value to further interpret the complexity of groundwater flow and transport in tidal realworld STEs which were studied by, for example, Grünenbaum et al, 2023or Waska et al, 2019 In this study, we mimic the 'iron curtain' formation below a beach in a sand tank experiment to highlight the importance of the USP as well as the associated mixing zone at the interface between USP and underlying freshwater on mixing controlled reactions in tidal STEs.…”