Nonlinear Dynamics of the Nearshore Boundary Layer of a Large Lake (Lake Geneva)

We examine nearshore and pelagic current variability in Lake Geneva, a large and deep lake in western Europe, using observations from several measurement locations and a three-dimensional numerical model for the period 2014-2016. Linear internal seiche modes excited by wind forcing clearly appear as peaks in the energy spectra for measurements in offshore locations. In contrast, spectra from the nearshore data, where currents interact with the lake bed, reveal a negligible contribution of internal seiches to the total kinetic energy. A similar contrast is seen in the spectra obtained from the numerical model at the same locations. Comparing the contribution of the different terms in the vertically averaged momentum equation from the modeling results shows that the nonlinear advective term dominates in the nearshore boundary layer. Its contribution decays with distance from shore. The width of this nearshore boundary layer, which may extend for several kilometers, seems to be mainly determined by local topography. Both field measurements and modeling results indicate that nonlinear dynamics are of primary importance in the nearshore boundary layer. Plain Language Summary Motions in large lakes are often interpreted as a combination of linear waves, akin to the vibrations of a guitar string from a mathematical point of view. In a large lake, these linear motions are constrained by the Earth's rotation to follow lines of constant depth, thus preventing cross-shore transport. We studied the variability of currents in Lake Geneva, a large lake in Western Europe, using both field observations and a computer model. We found that, while the linear interpretation is satisfying in the central, deep part of the lake, it is on the other hand incomplete if nearshore motions are considered. Near the shore, more complex, possibly turbulent motions are observed. This nearshore richer variability seems to be largely due to the interaction of linear offshore motions with the nearshore bottom topography. In order to better understand transport processes from and toward the shore, these complex nearshore motions should be studied in more detail.