River Plume Liftoff Dynamics and Surface Expressions

Abstract

The water surface expression of liftoff and its dependence on discharge are examined using numerical simulations with the Regional Ocean Modeling System (ROMS). Liftoff is the process by which buoyant river water separates from the bed and flows over denser saltwater. During low‐discharge conditions liftoff occurs in the river and is accompanied by a change in the surface slope. During high‐discharge conditions liftoff occurs outside the mouth and generates a ridge on the water surface. The location and height of the ridge can be described by analytical equations in terms of discharge, shelf slope, and river mouth aspect ratio. The offshore distance and height of the ridge are proportional to the river discharge and vary inversely with river mouth aspect ratio. For steep shelf slopes liftoff occurs close to the river mouth and generates a large ridge. The ridge is modified, but not eliminated, by the presence of tides. The water surface slope change at the ridge peak is large enough to be detected by the upcoming Surface Water and Ocean Topography (SWOT) altimeter and can be used to identify the liftoff location during high discharge. However, during low discharge the water surface slope change at the liftoff location is too small to be detected by SWOT. These results indicate that remote measurements of the presence or absence of the ridge may be useful to distinguish between low and high flows, and remote measurements of the ridge location or height could be used to estimate freshwater discharge.

Are Periodic Oscillations of Sea Surface Height Inherent to River Plumes?

“…where ρ is the water density, P is the pressure, and x is the horizontal coordinate across the plume [hereinafter, we adopt the notation used by Branch et al (2020)]. Differentiating Eq.…”

“…increases with the plume size. The amplitude of the oscillations will depend on the initial conditions, i.e., the initial liftoff of the plume "ridge" (Branch et al, 2020) with respect to the mean level. It seems reasonable to assume that this liftoff is proportional to the river discharge rate Q.…”

“…(6) represent the advection of momentum and the vertical friction, respectively, C D is the dimensionless bottom drag coefficient. This form of momentum equation was used in Branch et al (2020), cf. Eq.…”

“…The author is not aware of any previously published works reporting on oscillatory motion of the plume's surface nor elaborating on plausible physical mechanisms of such motion. In a recent study by Branch et al (2020), the authors obtained analytical and numerical solutions describing the shape of a «plume ridge» assuming its stationary state. They defined the ridge as a result of liftoff process by which buoyant river water separates from the bottom and flows over saltier and denser seawater generating a ridge on the water surface.…”

Behaviors of the Yukon River Sediment Plume in the Bering Sea: Relations to Glacier-Melt Discharge and Sediment Load

“…In the coastal area receiving the river inflow, the river water is mixed with the marine water, accompanied by the dissipation of flow energy. Thereafter, the mixed water (brackish water) departs from the bottom bed to the surface layer, and moves offshore as a slow hypopycnal (buoyant) surface plume (flow speed of the 0.1 m order) [1]. The liftoff of the mixed water up to the surface layer is based on the fact that the mixed water density  (e.g.,  =1,001-1,012 kg/m 3 at 25 C in temperature and 5-20 ‰ in salinity at water surface) is smaller the marine water density (~1,023 kg/m 3 at 25 C and ca.…”