Effects of wave forcing on a subterranean estuary

Wave and tide are important forcing factors that typically co-exist in coastal environments. A numerical study was conducted to investigate individual and combined effects of these forces on flow and mixing processes in a near-shore subterranean estuary. A hydrodynamic model based on the shallow water equations was used to simulate dynamic sea level oscillations driven by wave and tide. The oscillating sea levels determined the seaward boundary condition of the coastal aquifer, where variably-saturated, variable-density flow was modeled. The simulation results showed that waves induced an onshore upward tilt in the phase-averaged sea level (wave set-up). The resulting hydraulic gradient generated pore water circulations in the near-shore zone of the coastal aquifer, which led to formation of an upper saline plume (USP) similar to that due to tides. However, mixing of recirculating seawater in the USP with underlying fresh groundwater was less intensive under the high-frequency wave oscillations. In the case of combined forcing, wave-induced circulations coupled with the intra-tidal flows strengthened the averaged, circulating pore water flows in the near-shore zone over the tidal period. The circulating flows increased exchange between the subterranean estuary and ocean, contributing 61% of the total submarine groundwater discharge for the simulated condition in comparison with the 40% and 49% proportions caused by the same but separate tidal and wave forcing, respectively. The combined forces also created a more extensive USP with the freshwater discharge zone shifted further seaward. The freshwater flow paths in the intertidal subterranean estuary were altered with a significant increase of associated transit times. The interplay of wave and tide led to increased mixing between discharging fresh groundwater and recirculating seawater. These results demonstrated the complexity of near-shore groundwater systems and have implications for future investigations on the fate of land-sourced chemicals in the subterranean estuary prior to discharge to the ocean.

Tidal influence on BTEX biodegradation in sandy coastal aquifers

David Andrew Barry, Alessandro Brovelli, Clare Robinson

A numerical study was conducted to investigate the influence of tides on the fate of terrestrially-derived BTEX discharging through an unconfined aquifer to coastal waters. Previous studies have revealed that tide-induced seawater circulations create an active salt-freshwater mixing zone in the near-shore aquifer and alter the specific subsurface pathway for contaminants discharging to the coastal environment. Here the coupled density- dependent flow and multi-species reactive transport code PHWAT was used to examine the impact of these tidal effects on the aerobic biodegradation of BTEX released in a coastal aquifer and its subsequent loading to coastal waters. Simulations indicated that tides significantly enhance BTEX attenuation in the near-shore aquifer. They also reduce the rate of chemical transfer from the aquifer to the ocean and exit concentrations at the beach face. For the base case consisting of toluene transport and biodegradation, 79% of toluene initially released in the aquifer was attenuated prior to discharge with tides present, compared to only 1.8% for the non-tidal case. The magnitude of tidal forcing relative to the fresh groundwater flow rate was shown to influence significantly the extent of biodegradation as it controls the intensity of salt-freshwater mixing, period of exposure of the contaminant to the mixing zone and rate of oxygen delivery to the aquifer. The oxygen available for biode-gradation also depends on the rate at which oxygen is consumed by natural processes such as organic matter decomposition. While simulations conducted with heterogeneous conductivity fields highlighted the uncertainties associated with predicting contaminant loadings, the study revealed overall that BTEX may undergo significant attenuation in tidally-influenced aquifers prior to discharge.

2009

Influence of tides and waves on the fate of nutrients in a nearshore aquifer: Numerical simulations

David Andrew Barry, Clare Robinson

A numerical investigation is presented that demonstrates the influence of tides and waves on the transport and transformation of nutrients (NO3-; NH4+; PO43-) in a homogeneous unconfined nearshore aquifer and subsequent fluxes to the sea. Simulations of an aquifer subject to semi-diurnal tides and constant waves acting on a sloping beach face were conducted using SEAWAT-2005 combined with PHT3D v2.10. Tidal amplitude (A) and wave height (H-rms) varying from 0.25 to 0.75 m and 1 to 2 m, respectively, were examined. Results show that tides and waves modify the subsurface discharge pathway of land-derived nutrients by changing the nearshore groundwater flow dynamics. More importantly, the oceanic forcing impacts nutrient cycling as it causes significant seawater exchange (along with dissolved O-2 and organic matter) across the aquifer-ocean interface. Although steady wave forcing caused higher seawater influx, tides led to greater seawater-freshwater mixing in the nearshore aquifer and subsequently greater transformation of land-derived nutrients. Nutrient processing was strongly controlled by the availability and reactivity of marine dissolved organic matter (DOM) as its degradation consumed O-2, released inorganic N and P, and altered redox conditions in the salt-freshwater mixing zones. For the conditions and reaction network simulated, nutrient regeneration by marine DOM degradation was independent of the seawater-freshwater mixing intensity, and therefore was greatest for the wave case due to the high seawater influx. For simulations without marine DOM considered, NO3- discharge to the sea increased by 32% for the tidal case (A = 0.5 m) compared to only 13% and 8% for the wave (H-rms = 1 m) and no oceanic forcing cases. With labile marine DOM considered, the NO3- discharge decreased by 90% relative to the land-derived flux for the tidal case (A = 0.5 m). For all simulations PO43- removal was high due to its adsorption to Fe oxide minerals. The model enables evaluation of the complex coupled physical-biogeochemical processes controlling nutrient loading to the sea via submarine groundwater discharge in dynamic coastal environments. (C) 2014 Elsevier Ltd. All rights reserved.

Elsevier2014

Impacts of wave and tidal forcing on 3D nearshore processes on natural beaches. Part II. Sediment transport

Roham Bakhtyar, David Andrew Barry

This is the second of two papers on the 3D numerical modeling of nearshore hydro- and morphodynamics. In Part I, the focus was on surf and swash zone hydrodynamics in the cross-shore and longshore directions. Here, we consider nearshore processes with an emphasis on the effects of oceanic forcing and beach characteristics on sediment transport in the cross- and longshore directions, as well as on foreshore bathymetry changes. The Delft3D and XBeach models were used with four turbulence closures (viz., k-ε, k-L, ATM and H-LES) to solve the 3D Navier-Stokes equations for incompressible flow as well as the beach morphology. The sediment transport module simulates both bed load and suspended load transport of non-cohesive sediments. Twenty sets of numerical experiments combining nine control parameters under a range of bed characteristics and incident wave and tidal conditions were simulated. For each case, the general morphological response in shore-normal and shore-parallel directions was presented. Numerical results showed that the k-ε and H-LES closure models yield similar results that are in better agreement with existing morphodynamic observations than the results of the other turbulence models. The simulations showed that wave forcing drives a sediment circulation pattern that results in bar and berm formation. However, together with wave forcing, tides modulate the predicted nearshore sediment dynamics. The combination of tides and wave action has a notable effect on longshore suspended sediment transport fluxes, relative to wave action alone. The model’s ability to predict sediment transport under propagation of obliquely incident wave conditions underscores its potential for understanding the evolution of beach morphology at field scale. For example, the results of the model confirmed that the wave characteristics have a considerable effect on the cumulative erosion/deposition, cross-shore distribution of longshore sediment transport and transport rate across and along the beach face. In addition, for the same type of oceanic forcing, the beach morphology exhibits different erosive characteristics depending on grain size (e.g., foreshore profile evolution is erosive or accretive on fine or coarse sand beaches, respectively). Decreasing wave height increases the proportion of onshore to offshore fluxes, almost reaching a neutral net balance.