Field characterization of the negatively buoyant inflow of the Rhône River into Lake Geneva

<p>River inflows have a major influence on lake and reservoir water quality through their input of momentum, heat, oxygen, sediment, nutrients and contaminants. The fate of these components is controlled by the hydrodynamic processes at the interface between the inflowing river and the receiving lake or reservoir. The inflow can be positively buoyant, leading to a near surface inflow current (overflow), or negatively buoyant, causing it to plunge and form a gravity-driven current near the bed (density current) and/or intermediate current (interflow). In the case of a plunging inflow, the plunging process provides upstream boundary conditions for density currents, which can continue for significant distances along the lakebed. It is therefore important to understand the mixing processes involving entrainment of ambient water into the plunging flow. The hydrodynamics of the plunging process are still poorly understood, especially in laterally unconfined configurations.</p><p>Field measurements of a laterally unconfined plunging flow of the Rhône River into Lake Geneva are presented. A vessel-mounted ADCP was used to measure the three-dimensional velocity field of the plunge region. Remote sensing images of the lake surface in the plunge region were captured with a static camera system set up on a nearby mountain overlooking the inflow. Additionally, a mobile camera system attached to a balloon was operated above the inflow to capture high-resolution videos of the inflow. Both camera systems were equipped with RGB and IR cameras. The ADCP measurements and remote sensing images were combined to detect mixing processes in three dimensions.</p><p>The remote sensing images show that the incoming river flow forms a distinct plume of sediment-rich water with a triangular shape leading away from the river mouth in downstream direction towards a sharp tip. Horizontal vortical structures visible at the surface, range from larger gyres, over vortex shedding and dipole formation downstream of the plume, to smaller scale structures such as Kelvin-Helmholtz instabilities at the plume edges. The ADCP measurements show the presence of vertical secondary circulation cells in transects perpendicular and parallel to the plume centerline. In addition, intermittent ‘boils’ of sediment rich water up to more than 200 meter downstream of the plume were observed in the images.</p>

Analysis of Vortex Pool-and-Chute Fishway

Fishways are constructed in riverine habitats where structures such as culverts, dams, and flood channels have negatively impacted flow conditions suitable for the movement of native and migratory fish species. These auxiliary channels are engineered to resist gravitational force with frictional force, resulting in sustained depth and reduced velocity over a range of design flow rates. The Chézy hydraulic resistance coefficient accounts for such forces and provides a metric useful for determining the effectiveness of a fishway to alter flow conditions prohibitive to the passage of fish. The objective of this analysis was to use a scale model of an innovative vortex pool-and-chute fishway, that operates with both plunging and streaming flow simultaneously, designed by Michael Love and Associates, to determine the Chézy resistance coefficients over a range of flow rates under controlled hydraulic conditions. Using dimensional analysis to ensure proper scaling allowed laboratory measurements of the model to be translated into a real-world prototype design. The conceptual prototype fishway is a 144-foot-long by 30-foot-wide channel with an 8% slope. A 1:15 scale model was constructed to evaluate the design at prototype equivalent flow rates between 58 and 283 cubic feet per second (cfs). Chézy coefficients were estimated by two different calculation methods; the streaming flow method and the streaming and plunging flow method. Coefficients ranging between 22.3 and 39.2 ft1/2/s were determined by the streaming flow calculation method, whereas the streaming and plunging flow calculation method yielded estimates from 18.9 to 25.0 ft1/2/s at corresponding flow rates. For flows that were exclusively plunging, values of 32.2 to 41.9 ft1/2/s were found. In general, Chézy coefficient estimates were observed to decrease with increasing discharge and values were found to be comparable to those calculated for fishways implemented at similar slopes. The preliminary model fishway results indicated that implementation of a prototype fishway could effectively alter flows for adequate fish passage under the given conditions. KEYWORDS: Hydraulics; Fish Passage; Fishway; Chézy Coefficient; Geometric Scaling; Froude Scaling; Streaming Flow; Plunging Flow; Dimensional Analysis; Similitude

Choking conditions inside plunging flow dropshafts

Severe rain events can subject our existing drainage systems to capacity. While the conveyance capacity of sewers can be increased by temporarily tolerating a full pipe flow condition, the complex dynamics of water–air mixture occurring in vertical dropshafts can largely restrict the overall conveyance capacity of drainage systems. In this study, the hydraulic performance of plunging flow dropshafts is investigated experimentally with respect to their conveyance capacity. The model dropshaft consisted of an upstream horizontal inflow pipe, a vertical circular shaft, and an outflow pipe discharging to the atmosphere. Five setups were built with Ds/Di from 1.0 to 3.0 for the range of drop heights H/Ds = 3.0 to 21.0, where Di is the inlet pipe diameter, Ds the shaft diameter, and H the drop height. A wide range of discharges up to Q* = 25 was tested, where Q* = Q/(gDi5)(1/2). Flow patterns recognized within the shaft (free flow, surface roller, plug flow, slug flow, and full pipe flow) help to assess the two stages of choking: the incipient and fully choked state. Criteria to predict choking occurrences were developed experimentally. The flow is considered to reach its conveyance capacity when an incipient choking state is visible. For a typical dropshaft with Ds/Di = 2, Q* was found to be approximately 5.

The Dynamics of Ascent-Forced Orographic Convection in the Tropics: Results from Dominica*

The mountainous Caribbean island of Dominica was chosen as a natural laboratory for studying orographic convection in the tropics. Here, the authors focus on a prototypical case study, taken from the Dominica Experiment (DOMEX) field campaign in the spring of 2011. Airborne measurements and high-resolution numerical experiments are used to examine the mesoscale dynamics of moist airflow over Dominica and its relationship to convection, turbulence, and rainfall. Upwind of the island, there is minimal lateral deflection or lifting of the flow, largely because of latent heat release in the overisland convection. Over the terrain, forced ascent leads to rapid development of moist convection, buoyancy-generated turbulence, and rainfall. Although this convection produces sporadic bursts of heavy rainfall, it does not appear to enhance the time-mean rainfall. Over the lee slopes, a dry plunging flow produces anisotropic shear-generated turbulence and strong low-level winds while quickly dissipating convection and rainfall. In the wake, low-level air is decelerated, both by turbulence in the plunging flow and by frictional drag over the island. Low-level wake air is also dried and warmed, primarily by turbulent vertical mixing and regional descent, both associated with the downslope flow. Rainfall and latent heating play only a secondary role in warming and drying the wake.

Two-layer hydraulics with comparable internal wave speeds

Two-layer hydraulics is developed for problems in which the moving layers can have stagnant layers above and below, the two internal wave modes can have comparable speeds and the total depth of the moving layers may vary. The general development allows both Boussinesq and non-Boussinesq problems to be studied. Solutions are presented in the Froude-number plane and the effect of different layer densities on the form of the solution space is shown. The theory is applied to two-layer plunging flows and a variety of controlled solutions are found. Solutions for the 2½-layer theory and the plunging flow theory are demonstrated experimentally. Shear instability is often observed in the divergent section of the channel.