Two basins separated by a strait often have different densities due to environmental factors, resulting in a situation in the strait where fluids of different densities are essentially side-by-side, causing an exchange flow due to gravitational forces. Dense fluid is pulled below light fluid and the light fluid is pushed above the dense, creating an opposing flow in the two layers. This exchange is often “controlled” at the point in the strait where cross-sectional area is minimized due to a constriction, either horizontal or vertical. Exchange in the strait can control the dynamics, and in turn energy, nutrient, pollutant and biological transport between the basins. Since strait dynamics are often not resolved in regional or global models, it is useful to parameterize the exchange based on external variables such as the density difference in the basins, the level of the dense water in the dense basin, and the tidal forcing. Exchange flow can be “maximal” or “sub-maximal”. The flow is “maximal” if raising the interface in the dense basin (presumably by modifying light water to be dense) does not further increase the exchange flow through the strait. While many ocean straits are usually “maximal”, there are also many that are “sub-maximal,” and thus require separate theoretical treatment. Time-dependent external barotropic forcing (i.e. the tide) modifies the time-averaged exchange flow in a strait. The relationship between tidal forcing and the average exchange flow in a channel has been examined for maximal exchange (Helfrich, 1995). In the present study, that effort is extended to include tidal forcing on a sub-maximal exchange flow. A strait with a sill is simulated numerically, using a two layer hydrostatic approximation. Time-averaged exchange flow increases with tidal amplitude depending on three factors: the physical dimensions of the problem, the tidal amplitude, and the relative strength of flow of the density layers. Results show that all exchange flows increase at a similar rate with tidal forcing, after being normalized by a parameter relating physical dimensions of the strait to the interfacial wave speed. This result quantifies the exchange increase due to tidal forcing for all degrees of “maximality” in this simple sill-only geometry. This relates time-dependent sub-maximal flows to the maximal case that has already been studied in depth. / Graduate / 0415 / ryanpc@uvic.ca

The need to consider groundwater and surface water as a single resource has fostered the interest of the scientific community on the interactions between surface water and groundwater. The region below and alongside rivers where surface hydrology and subsurface hydrology concur is the hyporheic zone. This is the region where water exchange determines many biogeochemical and ecological processes of great impact on the functioning of rivers. However, the complex processes taking place in the hyporheic zone require a multidisciplinary approach. The combination of innovative point and distributed techniques originally developed in separated disciplines is of great advantage for the indirect identification of water exchange in the hyporheic zone. Distributed techniques using temperature as a tracer such as fiber-optic distributed temperature sensing can identify the different components of groundwater-surface water interactions based on their spatial and temporal thermal patterns at the sediment-water interface. In particular, groundwater, interflow discharge and local hyporheic exchange flows can be differentiated based on the distinct size, duration and sign of the temperature anomalies. The scale range and resolution of fiber-optic distributed temperature sensing are well complemented by geophysics providing subsurface structures with a similar resolution and scale. Thus, the use of fiber-optic distributed temperature sensing to trace flux patterns supported by the exploration of subsurface structures with geophysics enables spatial and temporal investigation of groundwater-surface water interactions with an unprecedented level of accuracy and resolution. In contrast to the aforementioned methods that can be used for pattern identification at the interface, other methods such as point techniques are required to quantify hyporheic exchange fluxes. In the present PhD thesis, point methods based on hydraulic gradients and thermal profiles are used to quantify hyporheic exchange flows. However, both methods are one-dimensional methods and assume that only vertical flow occurs while the reality is much more complex. The study evaluates the accuracy of the available methods and the factors that impact their reliability. The applied methods allow not only to quantify hyporheic exchange flows but they are also the basis for an interpretation of the sediment layering in the hyporheic zone. For upscaling of the previous results three-dimensional modelling of flow and heat transport in the hyporheic zone combines pattern identification and quantification of fluxes into a single framework. Modelling can evaluate the influence of factors governing groundwater-surface water interactions as well as assess the impact of multiple aspects of model design and calibration of high impact on the reliability of the simulations. But more importantly, this modelling approach enables accurate estimation of water exchange at any location of the domain with unparalleled resolution. Despite the challenges in 3D modelling of the hyporheic zone and in the integration of point and distributed data in models, the benefits should encourage the hyporheic community to adopt an integrative approach comprising from the measurement to the upscaling of hyporheic processes.