Emory River Dredging and Dewatering
Significant Project Features
Phase 1 – Time Critical Removal Action:
- Hydraulic dredging of 1,500,000 CY of fly ash at a production rate of 12,000 CY per day to meet the schedule
- Emergency mobilization of dredge and ash recovery system equipment including 20 inch, 16 inch and 14 inch cutterhead dredges; dredging support equipment (barges, scows, pumps, pipe, process equipment); and conventional earthwork and excavation equipment (backhoes, loaders, off road dump trucks)
- Installation of 1,000 lf by 25 ft deep impermeable turbidity curtain with integral floats
- Installation of 8,000 lf 20 inch HDPE piping and 18,000 lf 16 inch HDPE piping to support the dredging operation
- Installation of 1,800 lf of sheet pile wall for structural support
- Installation of 1,800 lf of matted roadway for excavators and support equipment.
Phase 2 – Additional Work Order:
- Hydraulic dredging of 1,000,000 CY of sediments at a rate of 10,000 CY per day. Four (4) month completion schedule
- Design, installation, operation and maintenance of a bank of 12 filter presses capable of processing more than 4,000 CY/per day insitu
- Design, installation, operation and maintenance of a 1 acre engineered pad and service roads to house the dewatering system in unseasonably cold winter conditions
History & Location Details
The Kingston Fossil Plant (KIF) is located at the confluence of the Emory and Clinch Rivers on Watts Bar Reservoir near Kingston, Tennessee. Kingston is one of TVA’s larger fossil fuel plants. It generates 10 billion kilowatt-hours of electricity a year. Plant construction began in 1951 and was completed in 1955. Kingston has nine coal-fired generating units.
The Emory River borders the KIF ash cells to the east. Fly ash and bottom ash from the plant is hydraulically transported wet at a rate of approximately 2,400 gpm per operating generating unit through an ash sluicing trench, an ash settling pond, and a stilling basin to remove solids prior to discharging the transport water into the intake channel of the power plant. During normal plant operations, the coarser bottom ash is removed from the ash sluicing channel by mechanical excavator and the finer fly ash is removed from the ash settling pond by hydraulic dredge.
On December 22, 2008 the storage cell dike failed and over 5,400,000 CY of fly ash was released into Swan Pond Hollow and from there into the main river channel of the Emory River. Prior to the ash release, the fly ash which accumulated in the settling pond was removed and deposited into a wet storage cell immediately west of the fly ash settling pond using a hydraulic dredge. The storage cell was constructed by raising the embankment of the cell as it filled using the upstream dike construction method, which decreases the size of the storage cell as the cell fills.
The Tennessee Valley Authority (TVA) recovered fly ash released into the Emory River utilizing both hydraulic dredges and mechanical excavators.
Phase 1 – Time Critical Removal Action
Sevenson provided dredging, ash recovery, processing and dewatering services. Phase 1 was performed as a Time Critical Removal Action. Phase 1 included the dredging, recovery, and processing of 1,500,000 CY of fly ash from the Emory River. Phase 1 tasks included the following.
Sevenson mobilized dredging, processing and construction equipment from its Niagara Falls, New York central equipment yard to the project site in 30 days. This included process components, material handling systems, pipelines and process equipment, barges, tugboats, consumables and supplies. Equipment staging and set up at the site to become operational took 24 days with a crew of 150 skilled trades working 24 hours per day, 6 or 7 days per week.
Prior to initiating dredging operations Sevenson performed a debris removal operation. This operation used two barges equipped with hydraulic clamshells and debris rakes mounted on excavators and two material barges to stage debris. A tugboat moved the debris barges as necessary to a shore based mechanical unloading system. A 1,000 linear feet by 25 feet deep impermeable turbidity curtain with integral floats was deployed and anchored to control the movement of suspended sediments away from the debris removal and sediment removal areas. Debris removal was an ongoing operation. Debris removed included boulders, cobbles, and trees.
Sevenson’s primary dredge fleet for this project included large cutterhead dredges (20 inch, 16 inch, and 14 inch), mechanical excavators, a clam bucket, crane, and deck barge. Dredging operations were performed on a 24 hours/day 6 or 7 days/week effort. Sevenson dredging production averaged 10,000 CY per day to meet schedule requirement. Dredging was performed utilizing a DREDGEPACK® and verified using a bathymetric survey system. Dredges were fitted with a sound abatement system and engine enclosures. This minimized noise levels and disruptions to nearby residences. All dredging and dredging support equipment used bio-degradable vegetable oil in the hydraulic systems. Recovered ash was pumped 7,000 feet from the 20 inch dredge through a 20 inch HDPE pipeline powered by a 2,000 HP pump and placed into a temporary storage/stockpile area. Ash dredged by the 18 inch dredge was pumped 5,500 feet through 14 inch HDPE pipeline powered by a 570 HP pump and placed in the temporary storage/stockpile area.
Ash Recovery Storage and T+D
When proper moisture level was reached, the stored ash was loaded into rail cars or dump trailers for offsite transportation and disposal. The temporary storage/stockpile area contained up to 80,000 CY of material and up to 18,000 CY per day. Sevenson used a series of excavators: one PC800 with 60 feet of reach; 4 – PC600’s; 8 – PC400’s; 8 – D56 dozers; 6 – 30 ton articulating off road haulers; two water trucks; one fuel truck; and additional miscellaneous service vehicles and equipment to recover and deliver ash to the storage areas. Dust control/suppression was a continuous operation. Water was sprayed as mist in active work areas to control dust. Water trucks were used on all roads.
Phase 2 – Additional Work Order
The Phase 2 initiative was an extension to the work tasks required in Phase 1. An additional 1,000,000 cy of ash was hydraulically dredged. Phase 2 dredging consisted of the precision dredging using three (3) – 14 inch and two (2) – 16 inch hydraulic cutterhead dredges and several booster pumps to deliver the dredged slurry up to three miles. These dredges were fitted with high precision RTK GPS equipment. The dredges were also automated to remove ash to a pre-determined elevation. This automated system did not require the operator to manually raise and lower the ladder. This minimized the potential for over dredging.
During Phase 1 and 2 operations the settling pond at the Kingston plant was filling up with very fine ash and sediments. This necessitated the incorporation of mechanical dewatering with recessed chamber filter presses. Sevenson pumped dredged materials to a V-bottom tank system with hydrocyclones. The material was processed and transferred to feed tanks. The addition of polymer agents was not planned but could be added as a contingency if necessary. The filter presses separated the solids at an estimated 32% insitu solids and processed to an average solid content of 70 -72%. The filter press system consisted of twelve (12) 219 cf units with a total capacity of 2,628 cf operating 24 hours per day, 6 days per week. All filtrate water was returned to the rim ditch and then back to the settling pond.