Acoustic Determination of Sediment Loss Terms for Mechanical Dredging Operations At Providence, Ri, Usa
Type:
Presented during:
WODCON XVIII - 2007 Orlando, Florida, USA
Authors:
Land
<p>
<strong>Abstract:</strong>
Elevated suspended sediment concentrations or sedimentation rates during dredging operations have been a
persistent cause of concern among regulatory agencies charged with protection of the environment. Assessments of
potential detrimental effects on exposed organisms are hindered by a lack of predictive tools that can accommodate
the diverse ways in which mechanical and hydraulic dredges process and release sediment. Exposures also depend
heavily on site-specific factors including sediment properties, hydrodynamics, and bathymetry. Recent advances in
dredging simulation capabilities based on far-field particle tracking models driven by sophisticated hydrodynamic
models offer promising means to calculate exposures but all models require accurate input parameters. In common
among the current generation of far-field particle tracking models are input parameters that capture the properties of
the sediment, how it is injected into the water column, and the rate of release into the water column. The latter is
referred to as the loss term which is derived from dredging process models that require calibration and validation,
steps which in turn require field measurements for comparison of observed and simulated data sets. Field
measurements of the magnitudes of sediment loss to the water column and subsequent re-deposition have been
attempted for decades and have proven to be technically challenging. Reported losses for mechanical dredges have
ranged from less than 1% of the mass of sediment dredged to over 5%. Discrepancies have been difficult to attribute
to variation of soil type, variation of dredge plant (e.g., open versus closed bucket), or operational measures (e.g.,
bucket hoist speed). In this study an acoustic Doppler current profiler was used to estimate sediment loss during
closed bucket mechanical dredging of maintenance sediments in the Providence River, Rhode Island. Acoustic
backscatter was converted to total suspended solids concentration and the net flux of sediment across transects
perpendicular to the movement of the plume was determined at varying distances (or elapsed times equivalent to
plume "age" at a given distance) from the source. Repetitive transects were occupied at specified down-current
distances/times to capture variation in net flux due to the spatially heterogeneous distribution of sediment
comprising a fully developed plume. Loss terms for dredging during flooding tides at two locations were derived by
extrapolation of flux back to the source at time zero. Using this approach we estimated losses of 5.4% and 9.6%.
These losses are at the high end of those previously reported in the literature. We attribute this to a combination of
factors, including extremely soft soil, aggressive dredging by a contractor attempting to achieve maximum
production and possibly by differences in measurement methodologies. These findings are discussed with respect to
implications for future applications of dredge simulation models.
</p>
<p>
<strong>Keywords: </strong>
Suspended sediment, plume, bucket dredge, particle tracking model
</p>