Internal hydraulic jumps in a long narrow lake

Abstract

High‐resolution three‐dimensional numerical modeling and field observations were used to describe the nonlinear response of Cayuga Lake to surface wind forcing. The degeneration of the basin‐scale internal wave field was characterized according to the composite Froude number (G2), Wedderburn number (WN), and Lake number (LN), which are measures of hydraulic control and bulk and integral wind disturbance force to the baroclinic restoring force, respectively. The typical Cayuga Lake response was a nonlinear surge when ∼ 1 < WN (or LN) < ∼ 2–12 and a surge with emergent nonlinear internal waves when WN or LN < ∼ 2, in agreement with published laboratory studies. An observed shock front was simulated to be an internal hydraulic jump, occurring at midbasin during strong winds when WN < 0.8. To our knowledge, this is the first simulation of a midbasin seiche‐induced hydraulic jump (supported by field data) due to supercritical conditions (G2 > 1) in a lake. The occurrence of the hydraulic jump was correctly predicted using scaling parameters. It was also shown that topographically induced internal hydraulic jumps form when the nonlinear surges interact with a sill‐contraction topographic feature. In contrast to published literature, the observed high‐frequency nonlinear internal waves were preferentially associated with internal jumps, as opposed to steepened internal surges. Computed vertical diffusivities showed mixing was enhanced by two orders of magnitude within both the surges and hydraulic jumps as they propagated through the basin and interacted with topography. Our results can be generalized to other lakes and fjords with similar long‐narrow geometry and topographically separate side basins.