Premium
Burst‐like sediment suspension events in a sand bed river
Author(s) -
Lapointe Michel
Publication year - 1992
Publication title -
earth surface processes and landforms
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.294
H-Index - 127
eISSN - 1096-9837
pISSN - 0197-9337
DOI - 10.1002/esp.3290170305
Subject(s) - geology , turbulence , hydrology (agriculture) , sediment , suspended load , bedform , sediment transport , intermittency , suspension (topology) , bed load , flume , channel (broadcasting) , geomorphology , flow (mathematics) , geotechnical engineering , meteorology , mechanics , physics , pure mathematics , mathematics , electrical engineering , engineering , homotopy
In a much quoted paper, Jackson (1976) hypothesized that turbulent [bursting] motions such as those documented in laboratory boundary layers play a major role in alluvial sediment suspension. To date, the hypothesis remained largely untested, due to difficulties in monitoring turbulent suspension in rivers. This study provides field data documenting burst‐like turbulent motions over a sandy bed channel and quantifying the role of these motions in sand suspension. The data were collected in a 10 m deep channel of the Fraser River near Mission, British Columbia, Canada. Turbulent fluctuations of both flow components, downstream and normal to the bed, along with the output of an optical suspended sediment sensor, were monitored 1 m above the river bed. Typical flow velocities averaged 0·9 ms −1 at the sensors, where mean suspended sediment concentrations were 500 mgl −1 ; decimetre height small dunes on the backs of larger, half‐metre amplitude dunes covered the channel bed in the area. Brief but intense, burst‐like [ejection and inrush] events were identified in the flow records, where they are responsible for a high degree of [intermittency] in shear stress over the dunes: 80 per cent of the turbulent momentum exchange across the 1 m level can be ascribed to such brief (3‐8 s duration) events, active under 12 per cent of the time. In addition, the record of fluctuating sediment concentrations reveals these burst‐like motions to be highly effective in vertically mixing suspended sediment and thus, ultimately, in maintaining suspended sediment transport above the dune bed. The bulk (60 and 90 per cent in two deployments) of the vertical sediment mixing was accomplished by intense events active some 10 per cent of the time. No discrete recurrence timescale for these ‘burst‐like’ mixing events is evident, however. Rather, a continuous variation of return periods was observed as a function of the magnitude of vertical mixing event considered. To that extent, conceptual models of sediment transport in terms of burst events with a predictable recurrence such as proposed by Jackson (1976) may be misleading.