Sediment resuspension in a shallow lake.
The prevalence of sediment resuspension in shallow, wind-exposed lakes has resulted in the development of many parametric models attempting to describe the relation between the hydrodynamics of water bodies and sediment resuspension rates [Sheng and Lick, 1979; Aalderink et al., 1985; Luettich et al., 1990; Hamilton and Mitchell, 1996; Admiraal et al., 2000; Sanford and Maa, 2001; Mian and Yanful, 2004; Cozar et al., 2005]. The rate of sediment resuspension (or entrainment rate of sediment into suspension) is quantified by ΦE, the vertical flux of solid particles close to the bottom. Dividing the entrainment rate by ws, the terminal particle fall velocity in quiescent fluid, yields Es, the nondimensional coefficient for entrainment of bed sediment into suspension [Garcia and Parker, 1991, 1993; Parker, 2004; F. A. Bombardelli and M. H. Garcia, Numerical simulation of wind-induced resuspension of bed sediment in sallow lakes, paper presented at the International Water Resources Engineering Conference, American Society of Civil Engineers, Seattle, Washington, 1999]. When the entrainment process is in equilibrium [Admiraal et al., 2000; Parker, 2004], Es = cae, where cae is the equilibrium near-bed sediment concentration (generally expressed in volume of sediment per unit volume of fluid, and measured at a certain distance a from the bottom).
Most of the available models to compute Es or ΦE agree upon establishing a dependence of the entrainment rate on the bottom shear stress induced by the motion of the attendant fluid, and also on the local sediment characteristics, usually through a given measure of the sediment size [Sanford and Maa, 2001]. Aalderink et al.  put forward the following relation for sediment resuspension: ΦE ~ wK, where w indicates the wind speed at 10 m above the surface and K is an exponent which varies with the wind speed. Mehta et al. , Raudkivi  and Sanford and Maa  reviewed a large number of formulae for the erosion rate of cohesive sediments, which can be subsumed under general expressions shown in Appendix A (see equation (A1)). There is also a vast body of formulations to compute the entrainment rate of sediment in open-channel flows [Garcıia and Parker, 1991; Garcıa, 1999]. Most of the relations for Es are of the following type:
Es ~ ΦE ~ tPb ~ u*2P ~ wK
where tb and u* are the bed shear stress and the wall friction (shear) velocity due to skin friction, respectively, and P and K are empirical exponents. These relations for open-channel flows have been obtained under steady state, equilibrium conditions (or, moderate disequilibrium conditions), for essentially unimodal, noncohesive sediment particles, and for a uniform distribution of the shear stress in space. One such relation is the expression by Garcia and Parker [1991, 1993] for noncohesive sediment, also presented in Appendix A (see equation (A2)).
On the basis of a thorough review of the literature on the subject of sediment resuspension, the relations coming from open-channel flows have not been tested in lakes. In spite of the inherent differences between the boundary layers in open-channel and lake flows, this is somewhat surprising, especially considering the fact that these expressions have been derived mainly using dimensional analysis. Moreover, it turns out that the formulations for the computation of the sediment entrainment rate can produce widely disparate results under the same set of conditions, which adds to the inherent complexity of understanding sediment-related processes in lakes.
On the other hand, to observe sediment resuspension, sediment concentrations are usually measured close to the bottom, under the hypothesis that quasi-equilibrium conditions are prevalent. This measurement is undertaken either continuously, by using beam transmissometers or optical backscatter (OBS) sensors [Gloor et al., 1994; James et al., 1997; Jin and Wang, 1998; Weyhenmeyer et al., 1995], or in an integrated way by using sediment traps [Evans, 1994; Jurg, 1996; Kozerski, 1994; Lindstrom et al., 1999; Rosa et al., 1983].
In this paper, the results of a 4-month
field experiment on sediment resuspension in a large, shallow, hypereutrophic
lake, the Salton Sea in southern California, are presented. The field
measurement program included the observation of currents and waves
using a Nortek acoustic wave and current (AWAC) profiler, the observation
of water temperatures using a thermistor chain, the use of OBS sensors
for the measurement of turbidity, a surrogate for suspended sediment
concentration [Gippel, 1989; Suk et al., 1998; Cozar et al., 2005],
and the use of meteorological data. The paper reports one of the first
field campaigns in lakes using the AWAC, whose signal strength has
not been investigated in detail to date, and the correlation of a set
of variables coming from different sources during a relatively long
period of time (section 2.1; cf. Table 1). In section 3.1, the contribution
of different forcing mechanisms (waves, currents and surface seiches)
to the sediment resuspension in the lake is quantified, and the signal
strength of the AWAC is used to address the vertical distribution
of sediment in the water column. Section 3.1 also puts forward novel
relations between the AWAC’s backscatter intensity and turbidity.
Finally, section 3.2 presents the modeling of sediment entrainment
into suspension in the Salton Sea by introducing an extension of the
Garcia and Parker formula.
Department of Civil & Environmental Engineering - University of California, Davis
2001 Ghausi Hall, One Shields Ave., Davis, CA 95616