Prodanovic, Pat

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Conference Paper, Published VersionProdanovic, PatDispersion of Suspended Sediment from DredgingOperations in the St. Clair RiverZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:TELEMAC-MASCARET Core GroupVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/100439Vorgeschlagene Zitierweise/Suggested citation:Prodanovic, Pat (2013): Dispersion of Suspended Sediment from Dredging Operations in theSt. Clair River. In: Kopmann, Rebekka; Goll, Annalena (Hg.): XXth TELEMAC-MASCARET.User Conference 2013. Karlsruhe: Bundesanstalt für Wasserbau. S. 121-125.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Dispersion of suspended sediment from dredging operations in the St. Clair River  P. Prodanovic Water Resources and Marine Engineer Riggs Engineering Ltd. London, Ontario, Canada pprodanovic@riggsengineering.com    Abstract—Focus of this paper is on dispersion of suspended sediment resulting from maintenance dredging operations in the St. Clair River, located between the Province of Ontario (Canada) and the State of Michigan (United States). A portion of the dredging efforts are required near the river's outlet at Lake St. Clair, where combined effects of upstream river flows and lake circulation govern the sediment transport processes. Generally, as the distance of the navigation channel into the lake increases, the sediments become finer from the sudden drop of river velocity. Reduced river velocities and wind driven circulation and waves developing in Lake St. Clair are responsible for creating deposits of fine materials within the downstream navigation channels. Removal of these sediments is required through dredging operations. Coupling of TELEMAC-2D and TOMAWAC numerical models is used to capture hydrodynamics of both marine and riverine influences of the study area. The CDFATE model is used to quantify near-field mixing and dispersion, while the PSED particle tracking model is used to assess far-field suspended sediment concentration and its eventual deposition.  I. INTRODUCTION AND BACKGROUND The outlet of the St. Clair River, located between the borders of Canada and Unites States, serves as a key part of the shipping route for vessels sailing between the upper and lower Great Lakes. A portion of the eroded sediment carried by the St. Clair River is deposited within limits of the navigation channels near the outlet at Lake St. Clair. Dredge areas near the outlet and within the limits of the South Channel are the focus of this study (see Fig. 1). Sediment accumulating in these areas poses navigation hazards for commercial vessels that require grade depths of 8.6 m below chart datum for safe passage. Regular dredging operations are thus required to maintain navigation depths. The dredge areas within the limits of the South Channel are referred to as Upstream Shoal area, and the Cut-off Channel. The Upstream Shoal area (located between Basset and Harsen islands) generally accumulates sediment from reduced currents as the flow in the South Channel separates in two directions causing fine and medium sand fractions to fall out of suspension. The Cut-off Channel is a man-made navigation channel 8.5 km long and 200 m wide that runs between Seaway and Basset islands and is entirely within Canadian waters. The target dredging area within the Cut-off Channel is the lower third reach where deposition of sediment and proportion of fines generally increase with channel distance towards the lake. Deposition within the Cut-off Channel results from reduced river velocities that make the sediments settle (or drop out of suspension). During wind events, lake circulation and wave effects can move lake sediments and deposit it within the limits of the navigation channel thus increasing sediment deposition.  Figure 1.  Study area The focus of this paper relates to potential water quality impacts associated with dredging operations. Dredging operations have the potential to release sediments into the water column that could cause adverse water quality impacts on aquatic life within the waterway. Specifically, the main focus of this paper is to formulate a methodology that can quantify impacts from different dredging equipment and thus assess impacts in terms of total particulate matter or total suspended solids concentrations. Secondary objective is to assess the fate of the particulate matter released into the water column (i.e., where will be resuspended sediment eventually deposit). For water quality impacts, the Canadian TUC 2013, 16-18/10/2013, Karlsruhe - Kopmann, Goll (eds.) - © 2013 Bundesanstalt für Wasserbau ISBN 978-3-939230-07-6121XXth TELEMAC-MASCARET User Conference Karlsruhe, October 16–18, 2013   Water Quality Guideline for the Protection of Aquatic Life [1] for total particulate matter suggests that a maximum increase of 25 mg/L above background levels should not be exceeded for any short-term exposure. Similar guidelines are used elsewhere in the world.  Current government regulations require near- and far-field analyses associated with discharges of sediment laden waters to watercourses. Near-field mixing is defined as a region of the receiving water body where initial momentum, buoyancy and outfall geometry influence the process of mixing. The mixing in this zone occurs in the immediate vicinity of the discharge and is dependent solely on the characteristics of the effluent and its discharge mechanism (outfall pipe, overflow weir, etc.). The area where the momentum, buoyancy and the outfall geometry govern the shape and size of the plume is defined as the near-field. After the plume rises or falls (depending on its characteristics), the initial momentum and buoyancy effects are dissipated, and effects of the receiving water velocity, density and temperature become more pronounced. The plume may then bend and disperse due the hydrodynamic characteristics of the ambient waters. The region where the plume is dominated by receiving water characteristics is defined as the far-field zone.  Numerical modeling is used in this study to evaluate impact of dredging activities on water quality within a waterway. A hydrodynamic model is used to quantify velocity magnitudes and directions, which are then used as inputs to water quality and sediment transport modules. This paper will present a methodology that was applied to study water quality impacts of dredging operations within the lower reaches of St. Clair River. The rest of this paper is organized as follows: Section II presents results of a literature review on previous numerical modeling efforts within St. Clair River and Lake St. Clair. The methodology developed to assess water quality impacts associated with dredging operations is presented in Section III, and provides a brief description of the models used. Application of the numerical modeling related to dispersion of suspended sediment is given in Section IV. Results are presented in a qualitative manner in Section V (since the project is currently ongoing). Concluding remarks are offered in Section VI along with recommendations for future work. II. LITERATURE REVIEW Hydraulics of the St. Clair River and hydrodynamics of Lake St. Clair have been extensively studied by Canadian and American researchers. In a study by Environment Canada [2], an RMA-2 hydrodynamic model was used to study encroachment effects on upstream water levels. Reference [3] also used RMA-2 to develop and calibrate a hydrodynamic model of St. Clair River, Lake St. Clair and Detroit Rivers as part of a source water assessment program for public water intakes in the State of Michigan. References [4] and [5] describe application of the FVCOM model to develop a real time Huron to Erie Connecting Waterways Forecasting System (HECWFS). The forecasting system provides real-time information on current and water levels for the entire St. Clair River, Lake St. Clair and Detroit River waterway.  Multiple independent studies were recently completed that focused on erosion, conveyance, and the ability of the St. Clair River to regulate water levels of the upper Great Lakes. Numerical models were developed and calibrated for use in assessments of water level impacts. As part of that study Canadian Hydraulics Centre [6] has developed a TELEMAC-2D model of the St. Clair River and Lake St. Clair.  Since the study objectives were related to water levels of the upper reaches of the St. Clair River near Lake Huron, marine influences of Lake St. Clair were not necessary and therefore not included in their modelling work.   Hydrodynamics of Lake St. Clair are presented in reference [5], where mixing and relationship between hydraulic and wind-driven currents was studied. Their observations and simulations show that there exist distinct regions in the lake that are forced by either hydraulic flow from the river, or wind stress over the lake. During wind storms these regions can shift, and significantly influence currents. Measurements of sediment resuspension in Lake St. Clair have been studied in [7], which shows wave action as the major cause of sediment resuspension.  Reference [8] investigates wave-current interaction in Lake St. Clair and show effects of wave propagation against strong current gradients. III. METHODOLOGY In order to study the impact of sediment released into the water column from dredging operations a number of different models were used and linked. Each of the models is briefly described below. The TELEMAC numerical modelling system is used in this work to estimate hydrodynamic forcing of the study area. Specifically, the TELEMAC-2D model is used to capture hydraulic and wind-driven currents, while the TOMAWAC model is used to represent wave generation and propagation from open lake to the dredge areas. TELEMAC-2D solves numerically the non-linear Saint-Venant equations using the method of finite elements. TOMAWAC is a third generation phase-averaged wave model which captures the evolution of the two dimensional wave energy spectrum from winds, currents and bathymetry also using the finite element method. In this study, TELEMAC-2D and TOMAWAC were coupled such that each model influences, and is influenced by, the other. The coupling allows the process of wave-current interaction to be captured. Near-field mixing calculations have been completed using the CDFATE mixing model, developed by the U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Mississippi [9]. CDFATE was developed for the purpose assessing impact on receiving waters from continuous discharge of dredged material into the water column. The CDFATE model is based on the CORMIX model, which is used for evaluating near-field effects of municipal outfalls discharged into receiving waters.  Far-field TSS concentrations are simulated by using the PSED particle tracking model [10]. PSED model was 122XXth TELEMAC-MASCARET User Conference Karlsruhe, October 16–18, 2013   that are lowered to the bottom of the dredged area to loosen the bottom materials. The sediments along with water are suction lifted and placed onto onboard hoppers. The heavier sediment particles inside the hoppers settle out and remain on the bottom of the hoppers, while the finer fractions remain in suspension within the water column in the hopper. Once the capacity of the on board hoppers is reached water, along with finer material fractions, is discharged back into the watercourse. Decant water discharge is assumed to occur at a rate of 1 m3/s (3600 m3/hr), with assumed concentrations of 200, 400 and 1000 mg/L of total suspended solids.  Near-field dispersion and mixing analysis was evaluated for the dredge areas at Upstream Shoal and the Cut-off Channel. Velocities were extracted from the coupled TELEMAC-2D TOMAWAC simulations and used in the near-field calculations. Geometry of a typical THSD was used assuming that the overflow discharge takes place via single outlet pipe submerged 1 m below the water surface. In order to assess far-field impact of the proposed THSD overflow discharges, the hydrodynamics generated by TELEMAC-2D and TOMAWAC models was used as input into the PSED particle tracking model. Characteristics of the native sediments within the dredge areas are obtained via grain size curves collected during the 2012 sampling program and are used to define properties of both bed and source sediments within the study area. (Source sediments are referred as those that are released into the water column during dredging operations). The PSED model is used to simulate the fate of sediments introduced into the water column, specifically focusing on total suspended solids concentrations away from dredging activities. PSED was also used to identify locations where the source sediment falls out of suspension (or settles on the bottom). Discharges associated with the THSD overflow operations were simulated in PSED using the same parameters as in the near-field calculations. Far-field simulations were carried for Upstream Shoal and Cut-off Channel dredge area. A range of simulation scenarios were used to assess impact (no wind, light breeze, and storm conditions). Discrete particles were released for a typical 12 hrs dredging operation, while the simulations were carried out for a total of 36 hrs. Longer simulations were required to identify fate of the suspended sediments. For each scenario, averaged path of the plume were extracted, and concentrations at 100, 500, 1000, and 1500 m away from the dredge areas were computed. Depth-averaged total suspended solid concentrations were computed on a 25 m by 25 m grid by counting the number of particles in each cell and dividing it by the cell volume. (Mass of each particle in the simulations was set at 1 kg.) Total suspended solids concentrations at 100 m downstream predicted by the PSED far-field model have been compared with the CDFATE near-field model for consistency. V. RESULTS Since the project is currently in the stages of environmental approvals, only qualitative results shall be presented. Velocities and streamtraces established by carrying out the coupled TELEMAC-2D TOMAWAC simulations are shown in Fig. 3  and Fig. 4 for the cases of calm winds and light breeze conditions, respectively. It is evident that wind-induced circulation plays significant roles in establishing velocity for the dredge areas adjacent to the Cut-off Channel. A plot showing significant wave heights generated using light breeze southwest winds is shown in Fig. 5, where it is evident that wave propagation is heavily influenced by strong currents within the limits of the navigation channel.  Currents at the Upstream Shoal dredge area are not affected by wind-driven circulation and wave effects (as they are more than 8 km upstream from the lake), but can be affected by backwater influence. Backwater influence is particularly important during periods of strong winds when seiche effects generate standing waves in the lake. The dredge areas of the lower third of the Cut-off channel are however affected by lake influences, as evidenced by a gyre that is present just south of Seaway Island during southwest conditions. Capturing hydrodynamics of upstream river and downstream lake influences is absolutely necessary for water quality assessments. Near-field simulations carried out using CDFATE were able to demonstrate that the Upstream Shoal dredge area has a significantly more dilution capacity then downstream dredge areas. Generally high rates of dilution at the Upstream Shoal area results from high river velocities and limited lake influence. The dredge areas in the Cut-off Channel, having lower flow velocities and being under marine influences of the lake have shown lower dilution rates of the released sediment. Far-field calculations using the PSED particle tracking model were able to validate the near-field results when comparing concentrations of total suspended sediment at a distance 100 m away from the discharge. Having two completely different methods produce nearly identical results only adds to the confidence in the selected methodology. Simulations show that the Upstream Shoal dredge area has a high far-field dilution potential (and relatively low total suspended solids concentrations downstream) since it is located in a zone of relatively high river velocity.  The downstream dredge areas are characterized by much lower channel velocities and finer sediment fractions. The interaction between lake circulation, wind generated waves and upstream river flows all play a role in determining suspended sediment concentrations before eventually settling out in the lake. Simulations have shown smaller far-field dilution potential and generally higher total suspended solids concentrations. Higher concentrations were encountered when the plume travels from initially deep waters within the navigation channel to shallower portions of the lake. VI. CONCLUSIONS The open source TELEMAC-2D hydrodynamic and TOMAWAC wave modules were applied in a study investigating water quality impacts associated with dredging operations in the lower St. Clair River. The TELEMAC system and its modules were able to seamlessly capture the interaction of both riverine (upstream flows) and marine (wind induced currents, wave generation and propagation) 124

Dispersion of Suspended Sediment from Dredging Operations in the St. Clair River

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Dispersion of Suspended Sediment from Dredging Operations in the St. Clair River

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