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Lateral Variability of an ETM
 The estuarine turbidity maximum (ETM) is a feature
common to many estuaries, where sediments of marine and/or terrestrial origin
converge to form locally increased turbidity. The importance of the ETM arises
from its role in sedimentary and biological processes: local deposition is
enhanced near the ETM (e.g. Grabemann et al., 1997; Woodruff et al., 2001;
Ganju et al., 2004), while biological activity may be focused near the ETM as
well (e.g. Kimmerer et al., 1998; Islam et al., 2005). The
erosional/depositional nature of the local sediment bed is known to affect
benthic community composition (Aller and Stupakoff, 1996), therefore lateral
gradients of deposition may create gradients in the benthos.
The mechanism of ETM formation varies depending on the geometry, tidal
dynamics, and freshwater flow of an estuary. Some studies cite topographical
effects (Jay and Musiak, 1994; Schoellhamer, 2001), tidal asymmetry (Sanford et
al., 2001), and cyclical resuspension of temporarily deposited sediment
(Grabemann et al., 1997; Ganju et al., 2004), to name a few. One notable study
of lateral variability of the ETM was presented by Geyer et al. (1998), where
frontal convergence of ebb waters over distinct topography is a major
mechanism. In addition, a pool of erodible material is preferentially deposited
on one side of the estuary, resulting in ETM creation near that location. The
ETM can be thought of as a zone of locally increased turbidity, which has a
distinct center of mass, longitudinally, laterally, and vertically. If one
assumes that the ETM is longitudinally fixed, then the lateral and vertical
variability in a cross-section can be evaluated by monitoring the cross-section
laterally and vertically at several points. The lateral and vertical location
of this center over differing timescales provides insight into ETM formation
and maintenance, and may potentially link with studies of biological activity,
benthic community structure, and deposition patterns.
ETMs may become longitudinally fixed due to bathymetric constraints, and thus
the lateral position may vary significantly on differing timescales. Lateral
dynamics of the ETM may affect contaminant uptake in biologically active
regions, while local deposition patterns may be affected the dominant lateral
position. A longitudinally fixed ETM in Carquinez Strait, California was
studied to specifically investigate the dynamics of lateral ETM variability
during April, 2004. An abrupt topographical control on the north side restricts
gravitational circulation resulting in convergence and particle trapping. The
south side has no topographical control, and therefore no particle trapping
mechanism exists on the south side. The cross-section was continuously
monitored with two upward-looking velocity profilers and four optical
backscatterance sensors. In addition, cross-sectional measurements over one
tidal cycle were performed during a spring tide with boat-mounted velocity and
water quality profilers. The lateral and vertical position of the ETM
center-of-mass varied by a maximum of 250 m and 5 m respectively (20% of width
and 17% of depth) over the tidal timescale, while tidally averaged lateral and
vertical position varied substantially less (50 m and 1 m, respectively). ETM
position responded to tidal energy (Urms), with higher vertical position
resulting from increased mixing during spring tides, a northerly lateral
position during neap tides, and a laterally centered position during spring
tides. Hydrodynamic and sediment transport modeling of this period reproduces
the lateral and vertical movement of the ETM center-of-mass. Modeling results
indicate increased gravitational circulation in the Strait and enhanced
particle trapping on the north side during neap tides, thus displacing the ETM
center-of-mass to the north. Secondary circulation is strengthened on spring
tides, distributing near-bed sediment towards the south. The field and modeling
results are in agreement with previous work in Carquinez Strait, and further
elucidate the strong lateral component to the ETM, even in narrow, energetic
tidal straits.
Publication
Ganju, N.K., and Schoellhamer, D.H., in press, Lateral
variability of the estuarine turbidity maximum in a tidal strait. In: Kusuda,
T., Yamanishi, H., Spearman, J., and Gailani, J.Z., Eds., Sediment and
Ecohydraulics: INTERCOH 2005, Elsevier, Amsterdam, Netherlands.
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