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

Abstract

AbstractThe 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, > 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 > 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.