Numerical modeling of the hydraulic and thermal exchanges between river and aquifer
In the plain, rivers are generally connected to an aquifer, with water exchanges occurring between the two in both directions. In order to quantify the water and heat exchanges a model based on a cross-section of the alluvial aquifer was designed. A parameter study was conducted with this model to draw nomograms pointing to a broad range of application of the results. These results are expressed in terms of the following parameters : - hydraulic conductivity of the aquifer, - slope of the hydraulic head in the aquifer, - geometric ratios describing the river-aquifer contact and thicknesses of the saturated and unsaturated parts of the aquifer. First the water discharges and corresponding heat transfers from the aquifer toward the river are estimated. This factor of the thermal balance of the river, usually neglected, is important in order to avoid systematic errors when modeling the thermal. behaviour of the river. Restricting the energy balance of a river to the exchanges through the water-atmosphere interface, thus neglecting the inputs of water from the aquifer which is colder than the river in summer and warmer in winter, lead to a biased calibration of the river thermal modes. On the other hand, the river affects the thermal behaviour of the aquifer, since the annual fluctuations of this temperature are greater important than the temperature fluctuations in the aquifer. When river and aquifer only exchange heat by conduction, the river influences the aquifer in a very restricted zone, only a few metres away from the bank. As soon as the river water enters the aquifer however, the thermal influence of the river may extend to several hundreds metres from the bank. The effect on the aquifer of an artificial increase in the river temperature was then simulated to produce situation at steady state for one or two years. The resulting increase in the aquifer temperature is calculated for a cross section. Results are gathered on monograms giving the distance between the river bank and the site where the temperature increase in the aquifer equals 50 % of the temperature increase of the river. This distance depends on the following main variables : water velocity in the aquifer, thickness of the saturated and unsaturated parts of the aquifer. Finally we have shown that the thermal balance of a river shoutd include heat exchanges with the alluvial aquifer, when noticeable inputs from the aquifer exist. Thermal modifications in the alluvial aquifer, due to water advection from the river are also reported. The search for wide applications of the results led us to simplify the actual configurations of the shape of the system and its hydraulic and thermal boundary conditions. At a given site the direction of the water fluxes between river and aquifer may change several times a year and transient hydraulic conditions will have to be considered in some real cases. Applications of these findings may be made, for example, in the fields of water supply or for a heat pump using water from this type of river-aquifer system.