Summary

The fate of sediment mobilized during dredging and placement operations will also be examined. The majority of modeling, field measurement, and data analysis work will be focused on the longer-term aspects of movement and fate of dredged material. In this study, long-term is defined as having a time scale of weeks to a year. A smaller portion of the resources will be devoted to quantifying the fate of sediments over shorter time scales (hours to days) during the dredging operation itself.

At Brunswick Harbor, placement of mixed sediments in shallow-water mounds, or berms, is planned for the very near future. Monitoring of these dredged material placement activities provides an unique opportunity for examining and quantifying the migration and dispersion of shallow water mounds and assessing the predictive skill of the models that have been used to simulate these physical processes at the Savannah River Entrance.  Valid models that predict water circulation, wave propagation and transformation, as well as transport and fate of sediment that is entrained and placed during the short-term dredging operation, and over the long term following placement, serve as valuable tools for planning future sediment management practices at both sites. Also, the models benefit other types of studies that are being conducted or are proposed for areas in and around these complex inlet entrances.  The work also provides great benefit to the Dredging Operations and Environmental Research (DOER) Program, which is investigating the practice and implications of dredged material placed in the shallow nearshore environment, particularly the behavior of mixed sediments comprised of silts, sands, and clays.

In addition to model set up and validation, the proposed work involves one year of field data collection at two mounds, one in water of about 3-5 m depth, and a second mound that is located in shallower water, in depths of 2-3 m.  In addition, a number of wave, current, and optical backscatter sensors (to estimate sediment concentration) that are owned by the DOER Program will be provided for use in this study.  At the conclusion of the study, and one year of data collection, District and CHL staff will jointly develop plans and recommendations for any future monitoring of berms at these sites.  Details of the proposed studies are provided below.

Detailed Scope of Work

The remainder of this proposal is divided into three tasks. The first will describe field instrumentation and a measurement strategy to measure wave, current, and suspended solids conditions at and near the mounds. The second task describes tracer, bathymetry, and sediment bed property studies to monitor the transport and deposition of fine-grained and sand-sized particles from the mounds.  The third task will discuss model set-up, development, input/calibration, and validation.

Task 1: Field Instrumentation
(POC: Carl Miller,CHL-Field Research Facility (FRF), Duck North Carolina)

The two monitored mounds would be at designated sites that are approximately 4000’ from the channel. The first mound for monitoring is the mid-depth mound (MD), which would be at sites A, B, or C in Figure 2. The second mound is a nearshore, shallow-water mound (SD) at the designated nearshore placement site (Figure 3). The SD mound would have a crest elevation just below MLLW. These mounds should be clearly separated from other, nearby placements to avoid contamination of monitoring efforts. The area around the mounds will be monitored using Acoustic Doppler Current Profilers (ADCP) for measuring wave conditions and the vertical distribution of currents throughout most of the water column above the sensor, Acoustic Doppler Velicometers (ADV) for measurements of current at a single position in the lower 1-2 m of the water column, Optical Backscatter Sensors (OBS) for measuring turbidity and Total Suspended Solids (TSS) at discrete positions in the water column, and pressure gages for monitoring water surface elevation. OBS require calibration to grain size distribution, which will be measured using a Laser In-Situ Sediment Transmisometer (LISST) system. Deployment of the LISST also allows processing of the backscatter intensity signals from the ADCP to examine sediment concentration through the water column.  Unlike the ADCP, the ADV and OBS make measurements only at single points in the water column.  Minimally intrusive, storm-resistant structures called bipods house the ADV/OBS/LISST instrumentations. Instrumentation on each bipod will include one ADV, three OBS at different heights, one LISST, and one pressure gauge. ADCPs will be deployed on separate instrument mounts.

Four instrument locations are suggested for long-term deployment and are denoted as packages I1-I4 in figures 2 and 3. Package I1, a bipod, will be placed atop the MD mound to provide current and suspended solids information which can be used to examine processes and evolution on the mound. Package I2 will provide deeper-water wave and current conditions using an ADCP in a trawler resistant pod, seaward of the influence of the mounds.  The measurements will be used to characterize the ambient conditions, and provide data for validating the wave and current models that will then serve as a basis for using model results at locations where measurements are not available.  Package I3, a bipod, is located near the SD mound, between the mound and the Jekyll Island shoreline; but it cannot be located atop the mound because of wetting and drying issues. The approximate position was selected to examine what is moving off the mound toward shore. The exact position will be selected once the mound is created. Package I4, a bipod and an ADCP, is located on the shoreward side of the MD mound to examine what is moving off the mound toward shore. It is proposed that data are downloaded and instruments serviced four times during the first year of monitoring. These data will be used to validate STWAVE, ADCIRC, and LTFATE models. Long-term deployment of the packages (on the order of one year) is planned.

To address stripping and dispersion during dredging and placement operations, some of the ADCP and bipods will be placed outside the channel and pipeline disposal area for short-term monitoring on the order of 1-2 days. The short-term monitoring will occur prior to initiation of the long-term monitoring plan.  At the conclusion of the short-term monitoring, the bipods and ADCPs will be re-positioned to their long-term deployment locations. These data will be used to validate model estimates of the stripping and dispersion processes. However, these instruments only provide time histories of TSS at single points. Therefore, in addition to these measurements, single-day boat cruises will be performed during dredging and placement operations. One cruise will follow the dredge and one will circle the pipeline placement location. These cruises will collect vertical profiles of TSS and velocity data at multiple locations around the dredge and pipeline. Instrumentation will include vessel-mounted ADCP, LISST, bottle samplers, OBS, and RTK-GPS. It is critical that SAS install an RTK-GPS on the dredge and pipeline as well. These data will be used to validate SSFATE and DCORMIX modeling efforts.

Task 2: Sediment Tracer/Sediment Properties/Bathymetry
(POC: Jarrell Smith, CHL, Vicksburg Mississippi)

This study includes three components to analyze different, but related, mound and sediment transport properties. An additional fourth component is described, but is not included in the cost estimate. Each component analysis will be performed 3-4 times during the year to monitor evolution of mound properties as well as mound migration.

Task 2.1.  Sediment Tracer Studies.

The first component is sediment tracer studies, which involve seeding of environmentally safe, fluorescent tracer particles in the SD and MD mounds to track transport direction and magnitude for various particle size classes. Evidence of tracer material will not be detectable by eye except at the mounds. Post-processing of the tracer data quantifies the transport magnitude through the nearshore region for various size classes (sand and fines) and placement locations. To conduct this component of the study, fluorescent particles manufactured to specific sizes and densities are placed on or within the dredged material mounds.  The particles are transported similarly to the dredged material, but are easily distinguished from native sediments when sampled from the surrounding sediments.  To quantify movement of the sediment tracer, periodic surface grab samples are collected from multiple locations within the domain and tested for particular wavelengths of fluorescence. Sediment tracers will be manufactured with distinct fluorescent signatures for two mounds and two sediment sizes (sand and fines), resulting in the capability of distinguishing both source and grain size for tracer sampled from the surrounding seabed. These data can then be used to estimate the amount of fine and coarse material from each mound at a given location in the domain and ultimately, the direction and magnitude of transport from the mound. The technology is relatively new, but has been successfully applied to assess sediment pathways from a nearshore bank in the UK ( ABP Research & Consultancy LTD, 1999. Final Report: Offshore – Onshore Sediment Exchange; Helwick Bank, Gower Peninsula)

Grab samples will be collected at the same time and from the same vessel as the bathymetric data and grain size samples.

Task 2.2. Grain Size Distribution and Bed-Erosion Characteristics.

The second component of Task 2 is analysis of the mound grain size distribution and sediment bed erosion characteristics. Nine cores 0.5-1 m in depth will be extracted from each mound four times during the year. In addition, three or four deep cores 1-2 m will be extracted from each mound. The short core extraction will be accomplished using a minimally intrusive method that requires SAS divers. The deep core extraction will be performed using SAS or contractor vibra-core capabilities. The cores will be collected at three sites on each mound, three cores at each site. For each site, one short core and the deep core will be frozen and then sliced for grain size distribution and bulk density (short core only) analysis for various layers in the mound. Grain size distribution will be measured using a combination of sieve analysis (for material > 1 mm) and Coulter counter or Malvern particle sizer for the fine material.

The other cores from each site will be used for erosion analysis. The mound may include a sufficient amount of fine sediment such that the erosion rates cannot be described by the known erosion rates for sand. The fine sediments will result in cohesive sediment bed behavior. This behavior can not be quantified by grain size distribution alone because it is a function of multiple parameters, including bulk density, mineralogy, organic content, and chemistry. The cohesive bed behavior can only be determined by performing erosion experiments on the sediment of interest. CHL maintains Sedflume, a flume specifically designed to measure cohesive, mixed sediment erosion rates and the variation of these rates with depth. Core samples will be eroded using Sedflume. The first goal of the Sedflume experiments is to determine if the sediment bed behaves in a cohesive on non-cohesive manner. If evidence of cohesive bed erosion is observed, Sedflume is used to develop site-specific algorithms for erosion rate (as a function of applied shear stress) and the variation of these rates with depth (i.e., variation with increasing bulk density or due to bed stratification). If the bed behaves in a cohesive manner, these algorithms are critical for model predictions of storm and ambient condition mound erosion and migration.

The flume was used to analyze slurries of material that were dredged at Savannah. At Brunswick, the material will be extracted directly from the mound. This means that there is a higher degree of uncertainty in the Savannah erosion predictions because the dredging and placement process alters the grain size distribution. The Savannah mound grain size distribution was assumed to be the same as the maintenance material. To quantify the change in size distribution between maintenance material and the mound, it is proposed that the district collect core samples from the stretch of channel that will be placed at the MD mound just prior to dredging. Grain size distribution from these samples will then be compared to mound sediments from the deep cores to analyze change during the dredging and placement process. In addition, Sedflume tests will be performed on slurried material and compared to erosion rates for the relatively undisturbed cores extracted from the mound.

Task 2.3.  Bathymetric Surveys.

CHL has learned that SAS presently has or is in the process of acquiring multi-beam survey and side-scan sonar capabilities. The horizontal and vertical accuracy available from these systems, when coupled with RTK-GPS and a motion sensor, is 5‑10 cm. The district would collect pre- and post-dredging bathymetries of the mounds and nearby regions. Surveys would be conducted again at 1 month and then every three months thereafter. The bathymetry, coupled with proposed bulk density measurements in Task 2.4, would provide information on consolidation and migration of the mound as well as erosion from the mound. In addition, side-scan sonar can be used for measurement of surface bedforms, providing information for more accurate prediction of sediment transport during non-storm conditions.

Task 2.4. Mound Consolidation.

This task relates to mound bulk density, which is critical when differentiating between mound consolidation and mound erosion.  Bathymetric data measure elevation change, but with a significant fine-grained fraction, observed change can be due to either erosion or consolidation.  In addition, if the mound sediments do behave as a cohesive bed, bulk density is a critical component for estimating erosion rates. The method proposed for this effort will use resistivity probes that are placed at the site prior to mound placement. This process is experimental for offshore sites and will be demonstrated only at one mound. The method is frequently used to determine the consolidation of marsh areas. The system provides a non-intrusive method for measuring the density profile of the entire mound and the consolidation with time. Sediments around the probe will consolidate naturally and no error will be introduced by core-tube or instrument insertion.

A series of electronic pulse transmitters are placed in a vertical, non-metal u-beam and the u-beam is filled with epoxy to create a watertight seal. Wires from the transmitters extrude from the bottom of the vertical beam and are incorporated into a watertight cable. Four vertical beams are created and are kept upright, at the corners of a 3.3x3.3 m. square base formation by a series of horizontal, cement-filled, PVC pipes. Cables holding the transmitter wires run trough the PVC pipe. The system is placed on the ocean bed, tested and calibrated prior to dredged material placement. The cables are laid such that the end is outside the dredged material disposal site. The end of the cables is marked with a buoy. The dredged material then covers the system to within a few inches of the top of the vertical probes. Bulk density is measured as a function of the resistivity between any two transmitters in a single probe. Using this method, bulk density profiles of the mound are measured at the vertical probe locations. Data are collected at the same frequency and same time as bathymetric surveys. In this manner bulk density profile and consolidation times are measured. Data will be downloaded during the sediment sampling/core extraction cruises. SAS will provide a platform boat, winch, and crew from which to install the approximately 8x8 m (including extensions to the base) resistivity probe system prior to dredged material placement operations. The system can be built as components and installed by divers to aid installation logistics.  In addition, SAS will be responsible for installing a buoy or fixed structure for the end of the cable (Alternatively, the cable can be secured to the near-mound bipod at I4). Brunswick is not considered by CHL staff to be the best implementation site for this instrument. Chances are that due to the high sand fraction, this mound will consolidate in a matter of hours, and vertical profile of density will be relatively constant. Non-cohesive bed erosion is not influenced by bulk density variation.  Therefore, the cost/benefit ratio of this study may be marginal unless the placed material is much finer than the maintenance material presently in the channel.  Savannah, which includes a larger fine fraction, may be a more suitable application of these probes.

Tracer studies (Task 2.1) will be used to validate LTFATE modeling, but will also provide information to quantify when and where coarse and fine sediment move away from the mound, including in the surf zone. Sediment property studies (Task 2.2) will also be used for LTFATE model calibration and validation. In addition this component will address questions concerning composition of mounds versus composition of the maintenance material. Task 2.3, bathymetric surveys, will be used for LTFATE and PDFATE model validation as well as other methods to estimate long-term fate of sediment. Task 2.4 (bulk density), if performed, will also be used for LTFATE validation and will address questions concerning consolidation versus erosion of a new mound. 

Task 3: Sediment Transport Modeling
(POC: Joseph Z. Gailani, CHL, Vicksburg Mississippi)

Models that are being applied at Savannah River Entrance will be applied and validated at Brunswick Harbor. The various types of field data will be processed and analyzed in preparation for the model validation studies.  Model accuracy will be assessed based on comparisons between model predictions and measurements, and the models will be applied to examine the movement and fate of dredged material.  The models, and their role in predicting the entrainment, transport, and fate of dredged material are briefly described below.

Hydrodynamic Models. Advanced Circulation Model (ADCIRC) provides a means for accurately simulating tidal- , river-, and wind-driven circulation in areas characterized by the complex geometry of the Brunswick and Savannah Harbors, including the complex network of islands, channels and rivers, shoals, and the complexities associated with modeling extensive shallow marsh lands that alternately flood and dry. The model will be used to characterize the currents at and near the placement sites, in areas where measurements are not being made.  The ADCIRC model will be validated with water level and current data (from ADVs and ADCPs) that are collected as part of the field data collection effort, as well as any water level data that are collected concurrently by other agencies.  The steady-state wave model, STWAVE, will be used to simulate irregular wind wave transformation over the complex ebb tidal shoal topography and in the vicinity of the placement sites, at locations where measurements will not be available. The wave model will be validated to nondirectional wave information derived from pressure sensors and directional wave information derived from the ADCPs.

Dredged Material Fate Models The LTFATE model will be set up and applied to investigate longer-term movement and fate of the dredged material mounds.  The model will be validated using measured wave, current, water level data, surveys that capture mound evolution, sediment concentration measurements, results of sediment cores and consolidation tests, and results of the tracer studies. The Sedflume erosion tests will provide information on sediment bed characteristics for LTFATE modeling. If the sediments behave as cohesive materials, Sedflume will provide the appropriate algorithms to describe erosion rates. The D-CORMIX model will be used to estimate the total suspended sediment (TSS) concentrations in the water column due to stripping of fines during pipeline placement operations. Measured current data, and sediment concentration measurements during placement operations (collected from bottom mounted bipods and boat cruises) will be used to validate this model.  The PDFATE model will be used to simulate the configuration of the mounds developed during placement of material by pipeline.  The pre- and post-placement survey data will be used to validate the PDFATE model. The SSFATE model will be used to simulate the movement and fate of sediment that is entrained during the dredging and placement operations.  Current and concentration data from the bipods, ADCPS, and boat cruises will be used to validate the model.

This "operational research" at Brunswick, GA will provide needed confidence for model application at Savannah.   While this comprehensive model program is directly applicable for Savannah, some differences between the sites exist. For example, Savannah dredged material contains a higher percentage of fines. The FY03 data collection will provide guidance for continued monitoring at Brunswick and monitoring at Savannah. DOER funding may be greater than indicated in Table 1, but this cannot be assessed until the federal budget is completed.