ALBERT

Description: This paper presents a vertically averaged model for studying water and solute exchanges between a large river and its adjacent alluvial aquifer. The hydraulic model couples horizontal 2D Saint Venant equations for river flow and a 2D Dupuit equation for aquifer flow. The dynamic coupling between river and aquifer is provided by continuity of fluxes and water level elevation between the two domains. Equations are solved simultaneously by linking the two hydrological system matrices in a single global matrix in order to ensure the continuity conditions between river and aquifer and to accurately model two-way coupling between these two domains. The model is applied to a large reach (about 36 km2) of the Garonne River (south-western France) and its floodplain, including an instrumented site in a meander. Simulated hydraulic heads are compared with experimental measurements on the Garonne River and aquifer in the floodplain. Model verification includes comparisons for one point sampling date (27 piezometers, 30 March 2000) and for hydraulic heads variations measured continuously over 5 months (5 piezometers, 1 January to 1 June 2000). The model accurately reproduces the strong hydraulic connections between the Garonne River and its aquifer, which are confirmed by the simultaneous variation of the water level in the river and in piezometers located near the river bank. The simulations also confirmed that the model is able to reproduce groundwater flow dynamics during flood events. Given these results, the hydraulic model was coupled with a solute-transport component, based on advection-dispersion equations, to investigate the theoretical dynamics of a conservative tracer over 5 years throughout the 36 km2 reach studied. Meanders were shown to favour exchanges between river and aquifer, and although the tracer was diluted in the river, the contamination moved downstream from the injection plots and affected both river banks. Copyright © 2008 John Wiley & Sons, Ltd.

Description: Hydrological interaction between surface and subsurface water systems has a significant impact on water quality, ecosystems and biogeochemistry cycling of both systems. Distributed models have been developed to simulate this function, but they require detailed spatial inputs and extensive computation time. The soil and water assessment tool (SWAT) model is a semi-distributed model that has been successfully applied around the world. However, it has not been able to simulate the two-way exchanges between surface water and groundwater. In this study, the SWAT-landscape unit (LU) model - based on a catena method that routes flow across three LUs (the divide, the hillslope and the valley) - was modified and applied in the floodplain of the Garonne River. The modified model was called SWAT-LUD. Darcy's equation was applied to simulate groundwater flow. The algorithm for surface water-level simulation during flooding periods was modified, and the influence of flooding on groundwater levels was added to the model. Chloride was chosen as a conservative tracer to test simulated water exchanges. The simulated water exchange quantity from SWAT-LUD was compared with the output of a two-dimensional distributed model, surface-subsurface water exchange model. The results showed that simulated groundwater levels in the LU adjoining the river matched the observed data very well. Additionally, SWAT-LUD model was able to reflect the actual water exchange between the river and the aquifer. It showed that river water discharge has a significant influence on the surface-groundwater exchanges. The main water flow direction in the river/groundwater interface was from groundwater to river; water that flowed in this direction accounted for 65% of the total exchanged water volume. The water mixing occurs mainly during high hydraulic periods. Flooded water was important for the surface-subsurface water exchange process; it accounted for 69% of total water that flowed from the river to the aquifer. The new module also provides the option of simulating pollution transfer occurring at the river/groundwater interface at the catchment scale. © 2016 John Wiley & Sons, Ltd.

Description: For 2 years, water flow-patterns in the Garonne floodplain of south-western France were studied in the field and through hydrodynamic modelling (MARTHE Hydrodynamic Software developed by BRGM). Water flow-paths and the transport of dissolved elements between river and aquifer have been investigated and modelled. In order to quantify the buffer function of the alluvial floodplain, we focused our work on the effect of a major flood on the water flow-direction, and on nitrate transport. Thus, we showed that the effect of a large flood in the river was rapidly lost with increasing distance from the river. During the observation period, a hydrologically active strip only 300 m wide on either side of the riverbed played a buffering role in absorbing the flood crest. It was also found that meanders favour the exchange between river and alluvial aquifer, shown by the creation of bypasses between the upstream and downstream parts of meanders. This, in turn contributes to a dilution of nitrates in the phreatic aquifer, which here has higher nitrate content than the surface water; such dilution may result in an overestimation of the denitrification process in the wooded riverbanks. The coupling of chemical measurements-especially of chlorides and nitrate-with modelling of the dissolved-element transport allows us to establish the water balance for the riparian wetland, and to separate the effect of dilution and denitrification on nitrate concentration. This indicated the existence of areas in the riparian wetlands where denitrification is particularly strong, leading to reductions in nitrate concentrations of 10 to 30 mg/l NO3- during the flood. Copyright © 2003 John Wiley & Sons, Ltd.