The Atacama desert, in the North of Chile, is the driest desert in the world. In some weather stations, rain has never been recorded (Wiki). However, fog is very common in this region, and for example it allows the plant Tillandsia landbeckii to survive. A way to provide water to local populations thus consists in collecting fog water.
My PhD focused on the relationship between the discharge of a river and the surrounding precipitations. More precisely, I studied how precipitations affect groundwater flow, and consequently the discharge of a river. Here is a small introduction of the problem.
Below is a typical "hydrograph", that is the evolution of the discharge of the Capesterre River (Guadeloupe, data on ObsErA website). When it rains, the discharge increases; when it stops raining, the discharge decreases.
If the ground is porous, rainwater infiltrates into the ground. Within the ground, a zone saturated in water forms. This zone accumulates water and rises.
We observed the behaviour of this saturated zone along a small river in Guadeloupe. Three wells spread at different distances of the river (see graph below). At the bottom of the wells, pressure sensors measure the level of water within the ground. We estimate this corresponds to the top surface (called water table) of the zone saturated in water. The water table is inclined towards the river. This induces a pressure gradient that pushes water towards the river.
We precisely study this process in a better constrained environment, that is in our laboratory. You can see the experiment on the picture below. It is a 143x40x5 cm tank, full of glass beads. When it rains, water infiltrates downwards until it reaches the impervious bottom. A zone saturated in water thus forms and rises (dark zone on the picture). A typical experiment consists in imposing rainfall for a few minutes.On the right, a permeable grid allows water to flow out. With a scale, we measure the discharge of water flowing out.
River Deltas Morphology
During a 3 months internship in Doug Jerolmack's group, at the University of Pennsylvannia, Philadelphia, I studied how a river builds its delta when it enters the ocean.
When it enters the ocean, a river deposits sediments. Little by little, these sediments build a delta. According to its sediment and water discharges, the river deposits its sediments in the middle of the stream or along the riverside. This leads to two opposite characteristic patterns. On the one hand, the river builds a levee in the middle of the bed and the channels end up bifurcating to create "tree-like" deltas (see for example the Wax Lake Delta, in Louisiana). On the other hand, the river builds levees along the sides of the stream, and the channel progrades without bifurcating, forming a "single channel delta" (see the Mississippi Bird's Foot, where a single channel progrades into the sea for dozens of kilometers). During this internship, I participated in the design and construction of a laboratory experiment to understand the physical processes which bring to these two opposite patterns.
adrien.guerin2 * gmail.com
Laboratoire Matière et Systèmes Complexes
UMR 7057 CNRS
Université Paris 7 Diderot
10 rue Alice Domon et Léonie Duquet
75205 Paris cedex 13