Modeling Along Front Instabilities of a River Plume Modulated by a Cross Tidal Flow

Field observations of the Connecticut River plume revealed complex frontal zone dynamics as the plume interacts with a strong cross tidal flow. In this study, 3D nonhydrostatic numerical simulations, using NHWAVE, were carried out to simulate an idealized condition similar to Connecticut River plume. A buoyant river plume was sent into the model domain with a prescribed cross current. Here, we focused on the along front instabilities predicted by the numerical model at the main plume front. Model results showed that the along front instabilities exist and propagate following the direction of the cross current. Each instability billow consists of a pair of alternating positive and negative vorticity regions. The length scale and propagation speed of the instabilities are comparable to a recent observation via aerial drone images with a length scale of O(10) meter. Such along front instabilities have also been reported in several earlier studies near the river plume front, for instance, Trump and Marmarino (2003, Estuaries, 26, 878-884). The 3D flow structure of these instabilities resemble the "lobe" and "cleft" feature well-known from density current fronts over a no-slip boundary. Further analysis suggested that the generation mechanism and the intensity of the along front instabilities can be connected to a nondimensional parameter, which is a ratio of the timescale of horizontal shear to the time scale of gravity current, described by White and Helfrich (2013, J. Fluid Mech., 721, 86-117). Through several numerical experiments by varying the cross current intensity, we observe that the length scale and propagation speed of the instabilities are directly associated with the plume and the cross current dynamics.