A field study on turbidity currents initiated from spring runoffs

Abstract

The hydrodynamics of turbidity currents from spring runoffs of an influent river is examined by measuring velocity, water temperature, and concentration of suspended sediment in a reservoir. Crosssectional measurement of turbidity currents, made from a bridge, shows the effect of centrifugal force on the downstream movement. This is due to considerable meandering of the old river channel. The turbidity currents, initiated at high sediment concentration of 1,700 mg/L, were bifurcated into an underflow and some interflows in the downstream reach, because of the relatively dense bottom water of the reservoir. The "lower layer" of turbidity currents exhibits a log linear relationship between dimensionless velocity and height under a hydraulically smooth condition. The velocity distribution in the upper and lower layers could be explained by overall Richardson number. Two drag coefficients, related with shear stresses at bottom bed and interface, generally decrease with increasing densimetric Froude number, but are independent of Reynolds number. The conditions of sediment deposition and erosion by turbidity currents can be explained by using the "extended Shields diagram." centration (25-2500 mg/L) and at low velocity (the order 10-• to 10 -2 m/s), while the former currents, called high-density turbidity currents, usually transport sand and gravel in high concentration (25-250 g/L) and at high velocity (the order 10 m/s) [Stow, 1986]. Actual events of turbidity currents have been noted by the sequential breakage of submarine cables (high density) [Heezen and Ewinq, 1952] and through field measurements (low density) in the sea [Shepard et al., 1977], reservoirs [Chikita and Okumura, 1987], and lakes (especially, proglacial lakes by Gustat•son [1975], Smith et al. [1980], and Weirich [ 1986]). Mathematical models of the dynamics of turbidity currents, with erosion and deposition, have recently been developed by Parker [1982], Akiyama and Stefan [1985], Fukushima et al. [1985], and Parker et al. [1986]. However, these models are based on a small amount of field data and few experimental results; they leave some doubts about reality in the field. Particular effects which are not taken into account are the topography on the downstream movement of turbidity currents, interaction among wind-driven currents and turbidity currents, and the grain size distribution of suspended and bottom sediments. Furthermore, the shear stress at an interface between turbidity currents and clear flows is negected, and the

How are subaqueous sediment density flows triggered, what is their internal structure and how does it evolve? Direct observations from monitoring of active flows

“…This allows the formation of plunging density flows in freshwater with low sediment concentrations. Lakes and reservoirs are often seasonally strongly density stratified (Talling, 1966), and incoming density flows commonly form very dilute surface flows or interflows along thermoclines, as well as bottom hugging density flows (Pharo and Carmack, 1979;Weirich, 1984Weirich, , 1986aChikita, 1989Chikita, , 1990De Cesare et al, 2001Umeda et al, 2006;Fernandez and Imberger, 2008;Marti et al, 2011). The dilute nature of many density flows in lakes and reservoirs may also make them prone to lifting off with reversing buoyancy, once they have started to deposit sediment (Sparks et al, 1993;Hurzeler et al, 1996).…”

“…This should be borne in mind when using the timing of cable breaks to infer frontal flow speeds. Flow speeds calculated from individual cable Chikita & Okumura, 1987, 1990Chikita, 1989Chikita, , 1990. Grover and Howard, 1938;Bell, 1942;Gould, 1960 breaks during this event show irregular down-flow changes from~12 tõ 1.5 m/s (Gavey, 2012), which may be related to delays in the cable breaking after the flow front arrived.…”

“…Turbidity currents can play an important role in the management and initial design of reservoirs (Chikita, 1989(Chikita, , 1990Fan and Morris, 1992;Umeda et al, 2000Umeda et al, , 2006De Cesare et al, 2001). Sediment deposition by turbidity currents affects their storage capacity, which can have significant strategic importance in locations affected by rapid siltation, such as the large reservoirs along the Yellow River in China (Hu et al, 2012;Wei et al, 2013).…”

The Use of Modified Annandale’s Method in the Estimation of the Sediment Distribution in Small Reservoirs—A Case Study

“…Sedimentation conditions can change when the muddy river water is heavier than the bottom water of the reservoir; it plunges just beyond the topset-foreset break to form a bottom turbidity current. This has been observed by many authors, e.g., Lane [10], Normark and Dickson [11], Lambert [12], Chikita [13] Fan and Morris [3].…”

Hydro-physical processes at the plunge point: an analysis using satellite and in situ data

“…Some studies regarding density currents have been carried out in the Feitsui Reservoir (Chen et al, 2006), Lake Ogawara (Dallimore et al, 2001) and the Katsyrasawa Reservoir (Chikita, 1989) in Japan, Lake Geneva in Switzerland (Lambert and Giovanoli, 1988), Lake Onachota and Fayetteville Green Lake in the United States (Ford, 1990), and the Alpine Luzzone Reservoir in Switzerland (De Cesare et al, 2001). To our knowledge, there have been no studies reporting the use of remote sensing to study the plunge point location in reservoirs.…”