22-27 September 2019
Trade Fairs and Congress Center (FYCMA)
Europe/Madrid timezone

Experimental investigations of enhanced solute dispersion in beach aquifers due to wave action

27 Sep 2019, 09:40
15m
Conference room 2.1 ()

Conference room 2.1

Oral Topic 8 - Groundwater quality and pollution processes Parallel

Speaker

Dr Martin S Andersen (Connected Waters Initiative Research Centre, School of Civil and Environmental Engineering, UNSW Sydney, Australia )

Description

The interface between coastal aquifers and the marine environment is an important zone controlling fluxes and processing of nutrients and carbon. To quantify these fluxes and the processes that may modify them, it is important to have a good understanding of the physical processes controlling transport and mixing at the land-ocean interface. Currently there is little experimental research on the quantitative effect of wave forcing on solute dispersion and mixing in beach aquifers. In this study we used a very large-scale (1:1) physical model experiment undertaken with a ~250 m long and 5 m wide wave flume as part of the BARDEX-II experiment (Delta flume, Deltares in Vollenhove, Netherlands) to investigate subsurface solute transport and mixing processes due to wave action within a coastal sand barrier. Specifically, relationships between ocean wave forcing and hydrodynamic solute dispersion within beach sediments were examined. The sand barrier (L=75m; H=4.5m; W=5m) was instrumented with 19 pressure transducers to measure the groundwater levels and their temporal variations. Custom designed and constructed tracer injection rigs were buried in the beach profile at two locations (2.5 and 10 meters horizontally from the mean sea level mark) to study transport and mixing processes on injected solute slugs in response to wave forcing of the beach profile. The wave forcing was controlled by a wave paddle creating waves with significant wave heights of 0.6-0.8 m and peak wave period of 8 and 12 s. Freshwater was used in the flume which eliminated the added complexity of density dependent flow and transport. Due to the stochastic nature of the wave forcing the advection and dispersion of the breakthrough curves of individual injected solute slug transport could not be analysed using analytical solutions but had to be analysed statistically. The results show that transient conditions in the beach re-circulation zone due to wave run-up and beach face infiltration and hydraulic head oscillations caused by the wave forcing strongly disperse and mix subsurface solute plumes. The apparent solute dispersion due to wave forcing increased by an order of magnitude near the beach compared to the dispersion without waves (steady state and uniform flow). Our results show that beach aquifer solute transport models need to consider the enhanced dispersion to correctly quantify mixing and biogeochemical processes in this highly dynamic zone. The findings have implication for quantifying contaminant dispersion and natural attenuation as well as the natural processing and resultant fluxes of greenhouse gasses such as CO2, CH4, etc.

Primary authors

Dr Martin S Andersen (Connected Waters Initiative Research Centre, School of Civil and Environmental Engineering, UNSW Sydney, Australia ) Dr Gabriel C Rau (Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany) Prof. Ian T Turner (Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Sydney, Australia )

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