Transient river flow into a fjord and its control of plume energy partitioning

Abstract

The influence of variable inflows on near‐field plume dynamics and energy partitioning was examined using observations of a controlled flow into Doubtful Sound, New Zealand. The high temporal changes in flows passing through the Manapouri hydroelectric power station mimic the magnitude and variability seen in small mountainous river systems (SMRS) globally. The variable flow coupled with strong vertical density gradients akin to ambient conditions in coastal systems enabled plume behavior to be characterized for differing flow, wind, and tidal inputs in a quasi‐idealized “laboratory” system. Comparisons of the frequency distributions of energy for different forcing conditions showed that baroclinic and barotropic processes were closely intertwined for transient forcing. The periodicity of density due to tidal oscillations was initially absent; headwaters of the fjord absorbed the momentum when inflows were substantially increased from the mean of ∼420 m3 s−1. From the buoyancy frequency squared N2, six events were identified when N2 was greater than 0.07 s−2. Seven occurrences of supercritical flow (Froude number, Fri > 1) and associated transitions to subcritical flow were observed over the duration of mooring deployment. Transient inflows induced internal hydraulic jumps in the near‐field region which lead to a rapid breakdown of vertical stratification. The horizontal length scale of an internal hydraulic jump is O (1 km). Not all transitions from Fri > 1 to <1 are explained by transient forcing and the role of strong, up‐fjord wind stress is likely to initiate supercritical flows via setup at the head of the fjord.

Turbulent Scales Observed in a River Plume Entering a Fjord

“…This is in contrast to a number of plume and estuarine studies, which show that variability in is primarily driven by tides, with the strongest turbulence occurring during the ebb phase (Kay & Jay, 2003;Nash et al, 2009;Peters & Bokhorst, 2000). However, O'Callaghan and Stevens (2015) noted that the headwaters of the fjord absorbed the momentum of tidal oscillations, which would in turn reduce the impact of tides on in the near field, as observed here.…”

“…At the head of the fjord (upstream of DCA), a maximum s = 7.8 × 10 −2 W/kg was measured by the VMP when the tailrace inflow was near its upper limit of Q = 534 m 3 /s. While O'Callaghan and Stevens (2015) observed internal hydraulic jumps in the near-field plume region caused by variable discharge rates, the maximum measurements of within the near-field plume region did not occur when tailrace discharge rates varied (Figure 2c). Therefore, the high observed was not enhanced by the increased turbulence within a hydraulic jump (Nash & Moum, 2001) and is instead representative of the intense shear-dominated mixing in the near-field plume region.…”

“…Therefore, knowledge about fjord‐river interactions can be directly applied to coastal plume systems. Such a comparison was made by O'Callaghan and Stevens () who examined the near‐field energetics of a transient freshwater plume flowing into the head of a fjord, namely, the same system which is studied in this paper.…”

The role of turbulence and internal waves in the structure and evolution of a near-field river plume

“…3were estimated using a control volume method. As the majority of energy was found in the surface layer (O'Callaghan and Stevens, 2015;McPherson et al, 2019), the upper and lower integration limits were defined by the depth 0 ≤ z ≤ 10 m and along-axis distance 0 < x ≤ 3 km. Across-fjord limits (in the y-direction) were defined by the relative plume width (b(x)) determined by the freshwater conservation equation,…”

“…When a riverine inflow is discharged into the head of the fjord (Pickard and Stanton, 1980), this provides an idealised domain in which to isolate specific aspects of near-field plume dynamics. While the deep bathymetry minimises tidal exchange and the inner basin is sheltered from large ocean swell, fjord-river interactions can been directly applied to coastal plumes as the two systems share many common features: freshwater inflow to the fjord produces similar density gradients observed in major rivers such as the Hudson and Columbia Rivers (MacDonald and Geyer, 2004;Hunter et al, 2010); comparable estimates of occur in each of the near-field plume regions (MacDonald et al, 2007;McPherson et al, 2019); and timevariable discharge rates, common to both systems, similarly impact the plume structure and energetics (Yankovsky, 2001;O'Callaghan and Stevens, 2015).…”