The impact of morphodynamics and storm floods on pore water flow and transport in the subterranean estuary

Subterranean estuaries are considered powerful biogeochemical reactors affecting elemental net fluxes to the sea (Anschutz et al., 2009; Charette & Sholkovitz, 2006; Moore, 1999, 2010; Santos 2008). The hydrobiogeochemical reactions and thus solute depend on dispersive mixing subsurface residence times (Anwar 2014; Robinson 2009). These in turn controlled by hydro(geo)logical boundary conditions, such as meteoric groundwater flow, tides, waves, storm floods beach morphology (Robinson 2018). classical view of a subterranean estuary comprises distinct rather position stable salinity stratification upper tide-induced seawater recirculation underlying freshwater tube pinching out near low tide mark 2006). In this study, we demonstrate numerical density-dependent flow transport modelling how transient regular that typical for high-energy beaches change picture estuary. model results suggest variable seasonal lead strong spatio-temporal variability hydrodynamic patterns reaching several 10th meters into subsurface, thereby distorting stratification. We believe these findings particularly relevant sandy which commonly present at global coastlines. A two-dimensional cross-sectional was set up with USGS-software SEAWAT (Langevin 2008) simulate processes along cross-shore transect It is based conditions barrier island Spiekeroog southern North Sea coast, Germany. respective north-facing meso-tidal (tidal range 2.7 m) exposed predominant wave directions from north–west mean significant heights 1.4 m (Herrling Winter, 2015). builds previous efforts site (Beck 2017; Grünenbaum 2020a). extends north–south direction dune base surface aquifer base. grid discretization 2 horizontal direction. Vertically, extents 3 above 40 below level (base aquifer), 0.5 except top layer has thickness 4.5 m, encompassing intertidal zone. This important avoid MODFLOW/SEAWAT-specific complications could arise during frequent drying re-wetting layers phreatic simulations when considerably varies time. Also, it done allow introducing effect dynamic without need step-wise/transient re-gridding domain (see below). Note vertical hydraulic gradients system very small compared lateral unresolved within thick saturated zone first have not much overall behaviour deeper zones. accounts topography variation over time, loosely linear interpolation available scans al. (2020b). Cauchy-Boundary (?S) (Figure 1 top) reflects tide-averaged heads depending time-varying 1, bottom). varied step-wise daily increments half year back second 20 years. applied grid. only used calculate were assigned ?S. calculated following approach Vandenbohede Lebbe (2007) (2020a), where head given (i) height actual inundates (ii) constant height, is, topographic time surface. More details condition ?S Supporting Information S1. addition, three annual incorporated Holt (2019), recharge flux equal amount water needed fill unsaturated flood applied. At ?F discharge m3/day per metre shoreline 1). bottom northern boundaries no-flow boundaries. beach, fresh 400 mm via Neumann-boundary (?U). salt non-dispersive enters or leaves domain. Seawater (salinity 35 g/L) infiltration ?U through Neumann flow-boundary combination boundary. During infiltrating g/L defined, while outflow, computed concentration cells assigned. Freshwater 0. Horizontal conductivities porosity (KH = 11 m/day, Kv 5.5 n 0.35) similar Longitudinal transverse dispersivities ?L ?VT 0.2 respectively. also illustrate ages, conducting so-called ‘direct age simulations’ Goode (1996), treated undergoing zero-order production. Along inflow ages increase linearly depth 4 51 years according dating islands' lens (Seibert When accounted for, simulated fundamentally differ There one but multiple saline plumes migrate 2, DYNAMOD). Consequently, salinities strongly saltwater-freshwater interfaces moving entire Flow paths becoming complex pore velocities constantly changing. Over zone, areas can quickly become vice-versa. areas, discharging does salinity. Rather, dynamically brackish salinity, field. Strikingly, snapshots distribution field reveal possible deduce vice versa. stark contrast affected estuary, lines aligned more less parallel salt-freshwater interfaces. feature lost model. order further explore model, time-series age, passed since parcel entered extracted fictive observation wells A, B C 3, 8, 15 30 locations (red lines) show values vary between whereby they generally decrease proportion increases 3). peaks towards closest high some extent an floods. frequency appears depth, whereas amplitude increases. displayed mixed (rather old) location, why mirroring extent. Though be relatively old shallow depths location becomes example visible B, almost approaches 10 content comparatively high. Hence, ranges weeks decades ground Deeper spectrum decreases decades. presented generic simplified, reality even than illustrated here. For instance, waves known affect (Geng Geng Boufadel, 2015; Xin 2014). models average setup typically introduced closed-form approximations (Nielsen, 2009) added tide-resolved sea-side (Wu However, mostly idealized, sloping undulating shapes including runnel ridge structures, which, however, meso- studied structures complicate wave-patterns likely shielding dampen amplitudes. Thus, presumably characterized highly difficult quantify priori. variably area swash complicates process 2017). though wave-setup neglected complicated demonstrated realistic sites. idea illustration evolved course BIME project (‘Assessment ground- water-derived nutrient German Sea—Is there a, Barrier Island Mass Effect?’ funded Ministry Science Art (lower Saxony), VW ZN 3184 MWK preparation follow-up project. would like thank our colleagues continuous fruitful discussions research, namely S. Ahmerkamp, J. Ahrens, M. Beck, P. Böning, H. Brumsack, Degenhardt, T. Dittmar, C. Ehlert, B. Engelen, N. Grünenbaum, Holt, Holtappels, K. Lettmann, Marchant, D. Meier, Müller-Petke, Niggemann, Pahnke, V. Post, A. Reckardt, Schnetger, Schwalfenberg, Seibert, Simon, Sültenfuß, Vandieken, Waska, O. Zielinski. All input files, python-scripts pre- post-processing provided Information. Open Access funding enabled organized Projekt DEAL Appendix S1: Detailed description side condition. S2: DYNAMOD – Dynamic Animation simulation results, Salinity (blue) (red). left domain, terrestrial serves red particles enter due pure line. blue indicate origin. beige i.e., line S3: Model post-processing: Beach data python-script well files python-based FLOPY-code generating them. Python scripts generate Figures S1 results. Please note: publisher responsible functionality any supporting information supplied authors. Any queries (other missing content) should directed corresponding author article.