Hydrodynamic Zone of Influence Due to a Floating Structure in a Fjordal Estuary—Hood Canal Bridge Impact Assessment

Floating structures such as barges and ships affect near-field hydrodynamics and create a zone of influence (ZOI). Extent of the ZOI is of particular interest due to potential obstruction to and impact on out-migrating juvenile fish. Here, we present an assessment of ZOI from Hood Canal (Floating) Bridge, located within the 110-km-long fjord-like Hood Canal sub-basin in the Salish Sea, Washington. A field data collection program allowed near-field validation of a three-dimensional hydrodynamic model of Hood Canal with the floating bridge section embedded. The results confirm that Hood Canal Bridge, with a draft of 4.6 m covering ~85% of the width of Hood Canal, obstructs the brackish outflow surface layer. This induces increased local mixing near the bridge, causes pooling of water (up-current) during ebb and flood, and results in shadow/sheltering of water (down-current). The change in ambient currents, salinity, and temperature is highest at the bridge location and reduces to background levels with distance from the bridge. The ZOI extends ~20 m below the surface and varies from 2–3 km for currents, from 2–4 km for salinity, and from 2–5 km for temperature before the deviations with the bridge drop to <10% relative to simulated background conditions without the bridge present.

Potential Vorticity Dynamics of Coastal Outflows

A simple quasigeostrophic model is used to examine the outflow from a river, estuary, or strait into a coastal ocean. As shown by Johnson et al., these quasigeostrophic outflows are accurately described by analytical long-wave solutions. This paper first uses these solutions and contour dynamics simulations to discuss the behavior of coastal outflows. Second, it extends the model and the long-wave theory to consider the effects of ambient currents, tides, winds, or a variable source flux. Third, consideration of the momentum flux at the source is used to understand the turning of the current, showing that steady solutions conserve momentum, hence resolving the momentum imbalance paradox of Pichevin and Nof. Finally, a new numerical scheme to compute steady outflow boundaries is developed. The model focuses on the key dynamics driven by the source velocity and the generation of vorticity as the buoyant fluid adjusts. The simplicity of the model, and insight given by the long-wave solutions, enables a full understanding of the dynamics. The outflows display a range of behaviors, including indefinitely growing near-source bulges, steady boundary profiles with varying offshore width, bidirectional currents, and rarefying or eddy-like leading heads, all of which can be understood with the long-wave theory. Despite the simplicity of the model, the results show good agreement in comparison with observations, experiments, and numerical models.

Minimum return dilution method to regulate discharge of brine from desalination plants

The mixing zone approach in regulating the discharge of brine and other toxic dense discharges has many limitations when applied in environmentally sensitive areas. A well-defined minimum return dilution is advocated in this study as an alternative method to regulate the disposal of brine and other toxic dense discharges. This study examined experimentally the development and dilution of turbulent vertical dense jets (or fountains) at small Froude numbers. The study complements an earlier larger Froude number investigation. The mean and fluctuating temperature fields were measured with fast responding thermocouples, and an emphasis was given to the minimum return dilution, which occurred just outside the edge of the discharge pipe. The study has revealed that at small Froude numbers (Fr < 5) the normalized minimum dilution, μmin/Fr, decreased linearly with the Froude number and it became constant only at larger Froude numbers (Fr > 7). Simple design equations for the calculations of minimum return dilution and maximum excess temperature and salinity at the level of the source are provided for small and large Froude number regimes. This study also recognized the advantage of using a vertical discharge configuration (inclination θ = 90o with horizontal) as opposed to an inclined configuration (0o ≤ θ < 90o) when discharging brine into water environments. The inclined discharge configuration has the potential of producing higher concentrations of brine and temperature near the source when ambient currents are in a direction opposite to the discharge.

EFFECTS OF TIDE ON WAVES IN THE OUTER SEINE ESTUARY AND THE HARBOR OF LE HAVRE

The present study examines the influences of time-varying tide-induced water depths and currents on waves in the outer Seine estuary (southern central English Channel) and their penetration in the harbor of Le Havre and its new infrastructures Port 2000. The investigation is based on a numerical procedure which links regional phase-averaged wave modules with a local phase-resolving wave module within Port 2000 harbor. Required spatio-temporal evolutions of tidal free-surface elevation and current are computed by circulation modules. Numerical results of wave height are compared with field data collected at three wave buoys in the access harbor channel and its inner basin. Predictions exhibit a local increase (up to 30 %) of wave height induced by current refraction at slack tide in the access harbor channel. Respective mappings of the wave height modified by the tide, the water levels alone and the currents alone confirm this finding. The effect of currents on waves are pronounced along the southern breakwater of Port 2000 harbor and in the vicinity of coastal topographic features of the outer Seine estuary. Ultimate predictions of wave propagation within Port 2000 basin exhibit, however, the negligible direct influence of local ambient currents on wave height. Observed-semidiurnal wave-height variations in the inner basin are thus mainly associated with the propagation of the outer tide-induced modulation. Mappings of maximum wave-height within harbor basin reveal an increased exposition of the northern wharves at high tide and the southern western breakwater at low tide in relation to current-induced changes in the approaching-waves direction.

The Effects of Fish Cages on Ambient Currents

Experiments were carried out to measure forces on and wake characteristics downstream from fish cages. Cylinders made from metal mesh with porosities of 0%, 30%, 60%, 75%, 82%, and 90% were tested in a towing tank. The drag force was measured with strain gauges, and the flow field downstream from the models was analyzed using particle image velocimetry. The Reynolds numbers ranged from 1000–20,000 based on the model diameter and 15–300 based on the diameter of the strings of the mesh as an independent obstacle. High porosities (here, 82% and 90%) lead to low water blockage and allow a substantial amount of water to flow through the model. The data indicate that the wake characteristics change toward the wake characteristics of a solid cylinder at a porosity just below 75%. The drag force is highly dependent on the porosity for high porosities of a cylinder.

Extended time-dependent mild-slope and wave-action equations for wave–bottom and wave–current interactions

Extended mild-slope (MS) and wave-action equations (WAEs) are derived by taking into account high-order derivatives of the bottom profile and the depth-averaged current that were previously neglected. As a first step for this derivation, a time-dependent MS-type equation in the presence of ambient currents that consists of these high-order components is constructed. This mild-slope equation is used as a basis to form a wave-action balance equation that retains high-order refraction and diffraction terms of varying depths and currents. The derivation accurately accounts for the effects of the currents on the Doppler shift. This results in an ‘effective’ intrinsic frequency and wavenumber that differ from the ones of wave ray theory. Finally, the new WAE is derived for the phase-averaged frequency-direction spectrum in order to allow its use in stochastic wave-forecasting models.

MODELING OF WAVE-CURRENT INTERACTION USING A MULTIDIRECTIONAL WAVE-ACTION BALANCE EQUATION

This study presents an integrated numerical model to simulate wave deformation/transformation in tidal inlets or river mouths with ambient currents (e.g. tidal currents, river inflows) by carefully modeling the effect of wave-current interaction. A multidirectional wave-action balance equation is used to compute random/directional wave processes such as diffraction, refraction, shoaling, wave breaking, as well as wave-current interaction. This wave action model is coupled with a two-dimensional hydrodynamic model, the feedback effect of wave and current can be effectively simulated. This model is validated by simulating wave laboratory experiments in an inlet entrance, and waves and tidal currents in Grays Harbor, WA by using available field observation data in 1999. The capabilities of the wave model for simulating wave-current interaction and the corresponding breaking effect are confirmed in the study.