Ray Kostaschuk

Macroturbulence generated by dunes: Fraser River, Canada

Abstract A selectable frequency acoustic profiler and electromagnetic current meter were deployed in the lower Fraser River, Canada, to examine large turbulent structures. Features were generated on the lower stoss sides of dunes and appeared at the surface tens of metres downstream as strong upwellings (‘boils’). They were identified with acoustic ‘clouds’ in the acoustic profile. Event recurrence periods measured from acoustic time series and from ‘ejection’ events identified in the current meter record had similar two-component frequency distributions, but the acoustic values lacked the highest frequencies. A comparison of measured periods with predictions from the Strouhal law indicate that both dune amplitude and the internal boundary layer developed over the dunes are significant length scales associated with the generation of the macroturbulence. Our data give no support to the notion that boils develop from microturbulent bursts.

Whole flow field dynamics and velocity pulsing within natural sediment-laden underflows

Sediment-laden density underflows are important agents of erosion and deposition and are especially significant in the management of human-made reservoirs, pollutant dispersal, and sediment deposition in the world9s oceans. Quantification of continuous, sediment-laden underflows in Lillooet Lake, British Columbia, shows that the underflows descend along a distinct plunge line but, although the input from the source is constant, adopt a distinct pulsing in their velocity structure. Such velocity pulsing will produce temporally and spatially varying bed shear stresses, sediment erosion and/or deposition, and fluid mixing, and represents a central property of underflows that must be incorporated into models of density current behavior.

Rates of floodplain accretion in a tropical island river system impacted by cyclones and large floods

Abstract Fluvial processes, especially rates of floodplain accretion, are less well understood in the wet tropics than in other environments. In this study, the caesium-137 (137Cs) method was used to examine the recent historical sedimentation rate on the floodplain of the Wainimala River, in the basin of the Rewa River, the largest fluvial system in Fiji and the tropical South Pacific Islands. 137Cs activity in the floodplain stratigraphy showed a well-defined profile, with a clear peak at 115 cm depth. Our measured accretion rate of 3.2 cm year−1 over the last ca. 45 years exceeds rates recorded in humid regions elsewhere. This is explained by the high frequency of tropical cyclones near Fiji (40 since 1970) which can produce extreme rainfalls and large magnitude floods. Since the beginning of hydrological records, large overbank floods have occurred every 2 years on average at the study site. The biggest floods attained peak flows over 7000 m3 s−1, or six times the bankfull discharge. Concentrations of suspended sediments are very high (max. 200–500 g l−1), delivered mainly by channel bank erosion. In the future, climatic change in the tropical South Pacific region may be associated with greater tropical cyclone intensities, which will probably increase the size of floods in the Rewa Basin and rates of floodplain sedimentation.

Initiation and evolution of gullies along the shoreline of Lake Huron

Over one hundred gullies are deeply incised into Algonquin Bluff along the eastern shoreline of Lake Huron. Aerial photographs between 1930 and 1992 were used to measure temporal changes in area and length of a sample of forty-four gullies. Regression analysis shows that gully area and length have increased in a linear fashion, and that gully initiation occurred in the nineteenth century. Gully initiation coincides with deforestation and settlement of the area. Surface drainage was redirected into drainage ditches, concentrating the flow and enhancing gully erosion. Natural processes associated with climatic fluctuations are probably insufficient to cause gully initiation, but may contribute to ongoing gully expansion.

Patterns and controls of gully growth along the shoreline of Lake Huron

An empirical approach was used to examine the morphology and behaviour of gullies along the eastern shoreline of Lake Huron, Canada. Gully and watershed dimensions and percentage vegetation cover of a sample of 44 gullies were measured from aerial photographs between 1930 and 1992. Gullies with larger watersheds had higher area growth rates. Larger gullies continue to expand over time while small gullies are more likely to become stable. Growth rates increased between 1955 and 1978 because of increased snowfall, extreme flow events, the extension of municipal drains, and the use of subsurface drainage. After 1970, efforts to rehabilitate gullies using drain pipes and check dams contributed to a decrease in gully growth rates.

Suspended sediment hysteresis in a salt-wedge estuary: Fraser River, Canada

Abstract Suspended sediment transport in the Fraser River estuary, British Columbia, Canada, is affected by fluvial and tidal time lag, or hysteresis. Fluvial hysteresis is evident after mobile sediment is exhausted early in the spring freshet and the peak in daily sediment concentration precedes the peak in river discharge. Tidal hysteresis is apparent over tidal cycles as the peak in sediment concentration generally follows the peak in velocity. This is a consequence of settling lag resulting from enhanced shear velocities during decelerating currents.

The relation between shear velocity and suspended sediment concentration over dunes: Fraser Estuary, Canada

Abstract Two types of dunes develop in the Fraser River Estuary, Canada. Symmetric dunes have stoss and lee sides of similar length, gentle lee-side slope angles and rounded crests, while asymmetric dunes have superimposed small dunes on their stoss sides, sharp crests and steeper lee sides. Symmetric dunes appear to be near-equilibrium features at higher flows. Asymmetric dunes are transitional between symmetric dunes and the small superimposed dunes that are in equilibrium with the lower velocity and sediment transport conditions. Spatially averaged velocity profiles over dunes consist of upper and lower log-linear segments. Upper segments of profiles reflect the total stress of the flow. The lower segment on symmetric dunes reflects skin friction from sand particles, but for asymmetric dunes it is skin friction plus form roughness from the superimposed dunes. Predictions from the Rouse equation indicate that sediment suspension is controlled by total stress for symmetric dunes, whereas for asymmetric dunes sediment suspension is related to stress associated with skin friction plus form roughness. It is the equilibrium bedform — the large symmetric dunes, and the small superimposed dunes in the asymmetric case — that controls sediment suspension over dunes.

Quantitative Visualization of Flow Fields Associated with Alluvial Sand Dunes: Results from the Laboratory and Field Using Ultrasonic and Acoustic Doppler Anemometry

This paper presents results detailing the quantitative visualization of flow fields associated with natural sand dunes, Fraser River Estuary, Canada, using the complementary approaches of laboratory modelling and field instrumentation. Ultrasonic Doppler velocity profiling is used in the laboratory to elucidate the mean flow fields of low-angle dunes (leeside slope angle ≈14°) that are typical of many large natural rivers. These dunes do not possess a zone of permanent flow separation in the dune leeside and have a velocity structure that is dominated by the effects of flow acceleration and deceleration generated by topographic forcing of flow over the dune form. Turbulence associated with these dunes appears linked to both longer-period shear layer flapping and eddy generation along the shear layer. The field study uses acoustic Doppler profiling to reveal similar mean flow patterns and shows that flow is dominated by deceleration in the leeside without the presence of a region of permanent separated flow.

Monitoring Suspended Sediment Dynamics Using MBES

This technical note describes use of a multibeam echosounder (MBES) to quantify the dynamics of suspended sediment in a large open channel. A methodology is detailed that uses the backscatter magnitude from the MBES water-column data to adjust the magnitude of sonar returns for the various sonar settings, spatially and temporally average the data to account for the random nature of acoustic backscatter from the suspended sediment, and calibrate the processed data with direct samples. A case study of flow at the confluence of the Rio Parana and Rio Paraguay, Argentina, where there is a distinct turbidity difference along the mixing interface of the two flows, is used to demonstrate the unique capabilities of MBES to quantify sediment concentrations and dynamics within the water column.

Tidal currents, bed sediments, and bedforms at the South Branch and the South Channel of the Changjiang (Yangtze) estuary, China: Implications for the ripple-dune transition

Measurements were made of tidal currents, bed sediment particle sizes, and bedform dimensions at the South Branch and the South Channel of the Changjiang estuary, China, during the dry season in 1997 and the flood season in 1998. The near bottom current speed and direction were measured by a mechanical current meter for 10 h in 1997. The near surface current speed and direction were measured by a Current Meter of Endeco/YSI Inc. 174 SSM for 14 h in 1997 and 1998. Nine bed sediment samples were taken and their particle sizes were analyzed with sieves and siphons. The bedforms were nautically detected by an echo sounder and a side scan sonar. Results show that the ebb tides had larger near-bottom and near-surface current speeds and longer durations than the flood tides, in which the former occurred during the flood season in 1998 and not in the dry season in 1997. The bed sediments were composed of coarse silts and very fine sands during the dry season but of fine sands during the flood season. Bedforms were dominated by ebb tidal currents, the height∶length ratios of dunes and lee face angles were low, and heights and lengths were larger during the flood season in 1998. The ebb and flood tidal currents, bed sediment sizes, and dune morphology were largely controlled by the seasonal runoff variations. A new tentative boundary might be proposed for natural dunes in very fine sand with the availability of additional field data in the future.