David Z. Zhu

Holmboe's instability in exchange flows

A laboratory study of the exchange of two fluids of different density through a constant-width channel with an underwater sill has enabled us to study Holmboes instability in greater detail than has been possible in mixing-layer experiments. The internal hydraulics of the exchange flow are such that we have been able to observe the initiation of instability, the development and behaviour of both symmetric and asymmetric Holmboe instabilities, and the suppression of the instability at bulk Richard-son numbers above about 0.7. A number of stability criteria resulting from previous numerical investigations have been verified experimentally. Our laboratory measurements are consistent with theoretical predictions of wave speed and wavenumber.

Non-hydrostatic effects in layered shallow water flows

This paper develops a one-dimensional extension to classical layered hydraulics that incorporates non-hydrostatic effects. General results for a homogeneous layer in a multi-layer steady flow are applied to single- and two-layer flow over a two-dimensional sill. The equation obtained for single-layer flows is the same as that obtained by Naghdi & Vongsarnpigoon (1986) using the direct theory of constrained fluid sheets, and compares very well with the laboratory measurements of Sivakumaran et al . (1983). The new equation derived for two-layer flows provides excellent agreement with the laboratory measurements of Lawrence (1993). Accurate solutions are obtained for a regime of two-layer flow whose behaviour cannot be explained, even qualitatively, using classical hydraulic theory.

Experimental Study of Sand and Slurry Jets in Water

This paper presents the results of an experimental study of turbulent sand jets and sand-water slurry jets impinging vertically into a stagnant water body. The jets contained silica sand with a median diameter D50 of 206 μm , and with an initial concentration 0.60 by volume for the sand jets, and 0.055–0.124 by volume for the slurry jets. The jets had densimetric Froude numbers between 2.0 and 5.94. The sand concentration and velocity profiles were measured simultaneously using a novel fiber optical probe, up to a distance of 130 do for sand jets, and 65 do for slurry jets, where do is the jet diameter at the water surface. The jets were found to have self-similar Gaussian profiles. The centerline sand concentration within the jets was found to decrease rapidly, following trends similar to single phase plumes. The centerline sand velocity profile decreased significantly before reaching a plateau region. The “terminal” centerline sand velocity within this region varies somewhat depending upon sand mass flu...

Mean Flow Characteristics in a Rock-Ramp-Type Fish Pass

AbstractDetailed mean flow characteristics generated by a staggered arrangement of boulders in a rock-ramp-type naturelike fish pass were investigated experimentally at three different channel slopes (5%, 3%, and 1.5%). The results showed that this type of fish pass can produce adequate water depth and favorable flow velocity suitable for fish passage. Three different characteristic velocity regions were identified, namely, downstream of boulders (wake region), intermediate region, and upstream of boulders. Some general correlations were developed for predicting the flow depth and velocity in a rock-ramp fish pass as a function of normalized discharge. Moreover, a flow-resistance analysis based on basic concepts for wake-interference flow regime in this fish pass has resulted in a general equation for the average velocity. This study improves the understanding of the complex flow characteristics in a rock-ramp fish pass.

Kernel Density–Based Algorithm for Despiking ADV Data

AbstractAcoustic doppler velocimeter (ADV) data can be contaminated by spikes from various sources. Available despiking methods were found to encounter difficulties in despiking ADV data from a turbulent jet flow. An iteration-free despiking algorithm was developed for highly contaminated ADV data by applying a bivariate kernel density function and its gradient to separate the data cluster from the spike clusters. It is shown that the new method overcomes some of the deficiencies of the existing despiking methods.

Model development for prediction and mitigation of dissolved oxygen sags in the Athabasca River, Canada.

Northern rivers exposed to high biochemical oxygen demand (BOD) loads are prone to dissolved oxygen (DO) sags in winter due to re-aeration occurring within limited open water leads. Additionally, photosynthesis is reduced by decreased daylight hours, inability of solar radiation to pass through ice, and slower algal growth in winter. The low volumetric flow decreases point-source dilution while their travel time increases. The Athabasca River in Alberta, Canada, has experienced these sags which may affect the aquatic ecosystem. A water quality model for an 800 km reach of this river was customized, calibrated, and validated specifically for DO and the factors that determine its concentration. After validation, the model was used to assess the assimilative capacity of the river and mitigation measures that could be deployed. The model reproduced the surface elevation and water temperature for the seven years simulated with mean absolute errors of <15 cm and <0.9 °C respectively. The ice cover was adequately predicted for all seven winters, and the simulation of nutrients and phytoplankton primary productivity were satisfactory. The DO concentration was very sensitive to the sediment oxygen demand (SOD), which represented about 50% of the DO sink in winter. The DO calibration was improved by implementing an annual SOD based on the BOD load. The model was used to estimate the capacity of the river to assimilate BOD loads in order to maintain a DO concentration of 7 mg/L, which represents the chronic provincial guideline plus a buffer of 0.5 mg/L. The results revealed the maximum assimilative BOD load of 8.9 ton/day at average flow conditions, which is lower than the maximum permitted load. In addition, the model predicted a minimum assimilative flow of about 52 m(3)/s at average BOD load. Climate change scenarios could increase the frequency of this low flow. A three-level warning-system is proposed to manage the BOD load proactively at different river discharges. Other mitigation options were explored such as upgrading the wastewater treatment of the major BOD point source and oxygen injection in the effluents. The model can be used as a management tool with updated SOD values to forecast the DO in low flow years and evaluate mitigation measures. As well, the methodology presented here can be applied to manage other ice-covered rivers.

Behaviour and Locomotor Activity of a Migratory Catostomid during Fishway Passage

Fishways have been developed to restore longitudinal connectivity in rivers. Despite their potential for aiding fish passage, fishways may represent a source of significant energetic expenditure for fish as they are highly turbulent environments. Nonetheless, our understanding of the physiological mechanisms underpinning fishway passage of fish is still limited. We examined swimming behaviour and activity of silver redhorse (Moxostoma anisurum) during its upriver spawning migration in a vertical slot fishway. We used an accelerometer-derived instantaneous activity metric (overall dynamic body acceleration) to estimate location-specific swimming activity. Silver redhorse demonstrated progressive increases in activity during upstream fishway passage. Moreover, location-specific passage duration decreased with an increasing number of passage attempts. Turning basins and the most upstream basin were found to delay fish passage. No relationship was found between basin-specific passage duration and activity and the respective values from previous basins. The results demonstrate that successful fishway passage requires periods of high activity. The resultant energetic expenditure may affect fitness, foraging behaviour and increase susceptibility to predation, compromising population sustainability. This study highlights the need to understand the physiological mechanisms underpinning fishway passage to improve future designs and interpretation of biological evaluations.

Turbulence Characteristics in a Rock-Ramp-Type Fish Pass

AbstractThis paper presents the results of an investigation on the turbulence characteristics of a rock-ramp-type, nature-like fish pass with a staggered arrangement of boulders for slopes of 1.5 and 3% for different discharges. This study showed some major differences between the flow with cluster of boulders (wake-interference flow) and the flow with isolated boulders (isolated-roughness flow) in the decay rate of turbulence and turbulence magnitude. General correlations were derived for the turbulence intensity and turbulent kinetic energy (TKE) as a function of normalized streamwise distance in a rock-ramp fish pass. Turbulence statistics (skewness and kurtosis), energy dissipation rate, and dissipative eddy size in this fish pass were also discussed in relation to fish passage.

Reattached Turbulent Submerged Offset Jets on Rough Beds with Shallow Tailwater

This study investigates the characteristics of reattached plane turbulent offset jets in channels with rough beds and shallow tailwater depths. The flow consists of a deflecting free jet caused by the Coanda effect and the evolving wall jet past reattachment. In limited tailwater conditions, the jet flow is affected by the relative tailwater depth or the submergence parameter, which is defined with respect to the maximum B-jump at a negative step. The results show that for an offset height larger than the jet thickness, the forward-flow momentum, local maximum velocity, and wall shear stress decrease faster in the longitudinal direction in the offset jet than in plane turbulent wall jets. The influence of roughness on reattached offset jets appears to be less than that in submerged wall jets on a similar rough bed. The presented results and the comparative analysis with respect to wall jets in hydraulic jumps are significant for controlling and implementing similar flows on rough beds with variable downst...

HYDRAULIC RESISTANCE OF SUBMERGED VEGETATION RELATED TO EFFECTIVE HEIGHT

Submerged vegetation has a significant impact on water flow velocity. Current investigations include the impact through adding drag resistance and increasing bottom roughness coefficient, which cannot elucidate the characters of real submerged vegetation. To evaluate the effects of submerged vegetation on water currents at different velocities, a laboratory experiment was conducted using three kinds of vegetations. The effective heights of these vegetations on varying flow velocities were evaluated. An equation describing the relationship between the normalized resistance of the submerged plants and the Reynolds number based on the plant effective height was then established and used to calculate the hydraulic resistance parameters of submerged plants in different stages of growth.