Cresta Reservoir Historical Case Study

For the EDDY Pump demonstration, approximately 7,833 cubic meters of sandy sediment deposits near Cresta Dam were dredged and returned to the reservoir bottom 600 meters upstream. Turbidity, suspended solids, dissolved oxygen, flow, the density of the slurry, and production rate were monitored. It was concluded that the dredge performance exceeded the manufacturer’s production specifications and easily complied with restrictive water quality standards. Slurry densities greater than 70 percent solids by volume were sustained, while peaks over 90 percent were achieved. Production rates of over 230 cubic meters per hour were observed. The maximum turbidity of the discharge plume was only 12 Nephelometric Turbidity Units (NTU) over ambient.

The new EDDY Pump technology promised: 1) little or no turbidity or resuspension at the dredge suction, 2) ability to dredge to the full depth of the reservoirs, 3) ability to handle a wide range of materials from clay to cobbles and organic debris, 4) ability to transport sediments as high-density slurry, and minimal effluent to be treated at the disposal site. In addition, the high density of the slurry suggested the feasibility of depositing the slurry underwater without excessive re-suspension, with the potential for saving the reservoir owner millions of dollars compared to landfill disposal.

To verify the feasibility of EDDY Pump dredging and in-reservoir deposition, the owner entered an agreement with “PB-MK Consultants and Engineers,” an affiliate of the EDDY Pump Corporation, to perform project study. a full-scale demonstration of the technology at Cresta Reservoir. The demonstration called for approximately 7,646 cubic meters to be dredged near the dam and re­deposited on the reservoir bottom 610 meters upstream. Environmental parameters of turbidity, suspended solids, and dissolved oxygen was to be monitored in addition to dredge performance. The RWQCB imposed the same restrictive limitations on water quality for the demonstration as it had proposed for production dredging.

The dredges main barge platform supporting the power and control cabin modules, and the pumping platform from which the dredge pump and drive motor was suspended. The two platforms were connected by a boom with a pivot connection at the main platform. An EDDY Pump with 0.254-meter diameter discharge driven by a variable speed submersible 224 kW electric motor was the heart of the system. Main electrical power was supplied by a 500 kW diesel generator. Pump speed control was achieved by varying the power frequency using a solid-state frequency driver. A 7.6 cubic meter/minute diesel-driven flushing water pump was mounted on the main platform and operated in standby mode to guard against pipeline plugs in the event of a malfunction. A flexible pump discharge hose, approximately 60 meters in length, was run from the pump platform through the boom and under the main platform on a system of rollers, allowing the pump to be raised and lowered as necessary. The equipment was set-up to dredge up to 30 meters deep, Electric drives were used for most powered equipment to minimize any chance of oil spills.

An induction type flow meter and a nuclear densitometer were mounted on a small instrument­ barge at the end of the discharge hose behind the main platform to monitor flow and density. Other instrumentation included an underwater video camera to observe the · dredge suction area, side-scan sonar, continuous reading turbidimeters, power demand meters, and position indicators. An on-board computer-assisted with system control computed the production rate and recorded production and water quality data

A 0.254-meter diameter high-density polyethylene pipeline 550 meters long with thermal-welded joints was extended from the instrument barge to the discharge platform. Floats were strapped to the pipeline at 4.5-meter intervals. At the end of the pipeline, a floating platform was designed to suspend a sediment curtain and to support a ported diffuser within the curtain. For the discharge distance of 610 meters, no booster pumps were required. If needed for longer pipelines, an EDDY Pump could also serve as an efficient booster pump.

Unfortunately, the design of the discharge sediment curtain proved unsuccessful and the curtain was removed for most of the demonstration following initial start-up. Even so, the water quality • parameters remained well within the agency requirements. It is believed by the authors that an improved curtain design could have achieved even lower levels of turbidity and TSS and may be necessary for finer-grained sediments than were encountered at Cresta Reservoir.

The pump was raised or lowered to dredging depth by four winch lines, one attached to each comer of the pump support frame. By varying the length of the suspender cables independently, the pump and suction nozzle was angled to optimize suction efficiency. The ability to control. the nozzle attitude and location on all three axes allowed material to be dredged with surgical precision compared to other dredging methods. In addition, the pump was mounted on trunnions •. in its support frame so it could be articulated back and forth by a pneumatic ram at the same time it was swung through the arc to dredge a 2.4-meter wide swath. After each swing, the dredge was repositioned for the next pass.

An induction type flow meter and a nuclear densitometer were mounted on a small instrument­ barge at the end of the discharge hose behind the main platform to monitor flow and density. Other instrumentation included an underwater video camera to observe the dredge suction area, side-scan sonar, continuous reading turbidimeters, power demand meters, and position indicators. An on-board computer-assisted with system control computed the production rate and recorded production and water quality data

A 0.254-meter diameter high-density polyethylene pipeline 550 meters long with thermal-welded joints was extended from the instrument barge to the discharge platform. Floats were strapped to the pipeline at 4.5-meter intervals. At the end of the pipeline, a floating platform was designed to suspend a sediment curtain and to support a ported diffuser within the curtain. For the discharge distance of 610 meters, no booster pumps were required. If needed for longer pipelines, an EDDY Pump could also serve as an efficient booster pump.

Turbidity, TSS, and dissolved oxygen (DO) were measured at the upstream control station, near the dredge suction, in the plume downstream of the discharge, and at the compliance station established at the power tunnel intake. On average, turbidity around the pump intake. exceeded background levels by only 1.2 NTU. At no time did turbidity or suspended solids concentrations measured at the intake structure compliance station approach the limits set by the agencies. The maximum turbidity increase was 1.9 NTU and maximum suspended solids measurement was 9.0 mg/1. Downstream of the discharge platform, turbidity was far below the limits specified for the compliance station. The maximum increase over ambient was 12 NTU. Dissolved oxygen concentrations were not measurably affected by the dredging. (Creek, PG&E, 1995)

A second generation dredge is now under construction that will improve upon the prototype dredge used for the Cresta demonstration. Portability and rapid mobilization capability are primary features of the new design.

• EDDY Pump slurry suction dredging provides an environmentally superior dredging method to meet strict water quality standards.
• Very little turbidity or re-suspension of sediments at the suction head.
• The EDDY Pump is capable of high production rates. Comparable to much larger conventional suction dredges.
• The EDDY Pump dredge is capable of passing large volumes of woody debris and other foreign material in sediments without clogging.
• High slurry density minimizes the volume of effluent to be treated and disposed of upland deposition of sediments.
• The capability of the technology to pump dense slurries long distances provides a pipeline alternative for transporting sediment to distant disposal sites.
• Following solution of initial start-up problems, the prototype dredge proved capable of sustained reliable operation.
• The discharge of sediment slurry into the water column resulted in low levels of turbidity and TSS, well within the limits of stringent water quality standards, without the use of a sediment containment curtain.

Harrison, Larry L., Wing H.Lee and ScottTu, (1995), “Sediment Pass-Through, An
Alternative to Reservoir Dredging,” ASCE Waterpower ’95 Conference, p.2236-2245.

• PG&E (Pacific Gas and Electric Co. Technical and Ecological Services Department) (1995) “Results of Water Quality Monitoring During the 1994 EDDY Pump Demonstration on Cresta Reservoir,” Report No. 402.331-95.23.REFERENCES

Creek, Korbin D. and Timothy H.Sagraves, (1995), “EDDY Pump Dredging: Does It Produce
Water Quality Impacts?,” ASCE Waterpower ’95 Conference, p. 2246-2252

Harrison, Larry L., Wing H.Lee and ScottTu, (1995), “Sediment Pass-Through, An
Alternative to Reservoir Dredging,” ASCE Waterpower ’95 Conference, p.2236-2245.

• PG&E (Pacific Gas and Electric Co. Technical and Ecological Services Department) (1995) “Results of Water Quality Monitoring During the 1994 EDDY Pump Demonstration on Cresta Reservoir,” Report No. 402.331-95.23.