Abstract
Great Sound is a 6 km
2 shallow (average depth = 0.6 m) lagoon fringed by salt marsh and connected directly to the ocean by two large (5 and 10 km long) flood-dominated tidal channels which enter at opposite ends. The lagoon can be divided into three subenvironments: flood tidal delta and channel, transition area, and basin. Tidal deltas containing several distributary channels are deposited from expanding jet flow as tidal currents enter the lagoon. A physical process study (velocity profiles, salinity, temperature, suspended load and box cores) indicated that sedimentation patterns are determined by: (1) proximity to the point sources of sediment, the tidal channels, (2) spatial changes in tidal current strength and abundance of benthic organisms (primarily sand-trapping macroalgae), (3) dispersion of particulates by tidal and wave-generated currents, and (4) postdepositional processes including bioturbation and resuspension by waves.
Tidal currents ranged from
U
max > 40 cm/s (
U
∗
max
= 4.0
cm/s
) in the tidal delta channels, to
U
max < 20 cm/s (
U
∗
max
= 2.6
cm/s
) in transition areas to
U
max < 10 cm/s (
U
∗
max
< 1.0
cm/s
) in the basins.
U
max
, U
∗
max
=
maximum
current velocity and maximum shear velocity respectively. Typical suspended sediment loads were 10–50 mg/l near the bottom; however, sediment transport was enhanced by wind-generated waves (up to 0.5 m wave height) which eroded the bottom creating concentration spikes of greater than 300 mg/l. Bottom sediments ranged from well-sorted fine sand in most delta and channel areas to basins composed of 90% silt and clay. Factors affecting sediment supply, transportation, deposition and resuspension interact in complex ways so that wide variability in the textural characteristics of deposited sediment occurs and no single factor (e.g., velocity) is well correlated with sediment characteristics. A conceptual model illustrating these spatial relations between hydrodynamics, benthos and sedimentation in the Great Sound system, was developed. The model should be useful in other tide-dominated coastal lagoons as a guide for design of benthic and sedimentary studies.