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Cresta Reservoir
Environmentally Friendly Slurry Dredging With a Submersible EDDY Pump
When strict water-quality limits ruled out conventional dredging, a 1994 demonstration at Cresta Reservoir used a purpose-built submersible EDDY Pump dredge to move sediment at over 230 m³/hr — holding turbidity to just 12 NTU over ambient, well under the 25 NTU limit.
HISTORICAL CASE STUDY
A 1994 demonstration proved EDDY Pump slurry dredging could remove reservoir sediment at high production rates while meeting some of the strictest water-quality limits in the industry.
A demonstration test of an environmentally friendly slurry-dredging technology — the patented EDDY Pump — was performed in 1994 at Cresta Reservoir on the North Fork Feather River in Plumas County, California. In service since 1949, the reservoir had accumulated roughly 2.3 million cubic meters of sediment — nearly half its capacity — threatening the dam and the 70,000 kW Cresta Powerhouse by obstructing its outlets.
230+ m³/hr
SUSTAINED PRODUCTION RATE (PEAKS TO 271)
SUSTAINED SLURRY DENSITY BY VOLUME — PEAKS OVER 90%
MAX TURBIDITY OVER AMBIENT — WELL UNDER THE 25 NTU LIMIT
The Challenge: Meeting Some of the Industry's Strictest Water-Quality Limits
Since the mid-1980s, the reservoir’s owner had evaluated ways to manage sediment at Cresta and the upstream Rock Creek Reservoir. Conventional options — clamshell and hydraulic suction dredging with disposal by truck, rail, or slurry pipeline to distant landfills — all carried environmental impacts that concerned regulators.
For production dredging, the Regional Water Quality Control Board (RWQCB) set strict limits: turbidity no more than 25 NTU above background, and total suspended solids capped at 80 mg/L regardless of background level. Doubtful that conventional equipment could meet those limits while economically moving large volumes of sediment, the owner identified EDDY Pump technology as the alternative to test.
The Solution: A Purpose-Built Submersible EDDY Pump Dredge
- EDDY Pump with 0.254 m (10 in) discharge, driven by a variable-speed 224 kW submersible electric motor
- 500 kW diesel generator with a solid-state frequency drive; electric drives used throughout to minimize any chance of oil spills
- Pump suspended from a boom on four winch lines — raised, lowered, and angled on all three axes for surgical-precision dredging
- Swung through a 120° arc and articulated by a pneumatic ram to cut a 2.4 m swath per pass; rated to dredge 30 m deep
- 0.254 m HDPE pipeline, 550 m long on floats, discharging 610 m upstream — with no booster pumps required
- Flow meter, nuclear densitometer, underwater video, side-scan sonar, and on-board computer monitored production and water quality in real time
Slurry densities above 70% solids by volume were sustained — with peaks over 90% — at production rates exceeding 230 cubic meters per hour, while the discharge plume stayed just 12 NTU over ambient.
1994 EDDY PUMP DEMONSTRATION · CRESTA RESERVOIR (PG&E, 1995)
How Eddy Pump Technology Works
The EDDY Pump creates a synchronized eddy current: a rotor spinning inside a volute sets the water column turning, forming a tornado-like vortex that draws in water and solids and loosens sediment as it goes. Because that vortex forms high-density slurry without water jets or a cutterhead, and the rotor sits out of the flow stream, gravel, cobbles, rags, vegetation, and woody debris pass straight through without clogging.
That design let the Cresta dredge deliver on four promises: little or no turbidity at the suction, the reach to dredge to the reservoir’s full depth, the versatility to handle everything from clay to cobbles and organic debris, and the ability to move sediment as high-density slurry, minimizing effluent to treat and making in-reservoir deposition feasible, with the potential to save the owner millions versus landfill disposal.
The Results: 230+ m³/hr at Just 12 NTU Over Ambient
- Sustained slurry densities over 70% solids by volume, with peaks above 90%
- Sustained production over 230 m³/hr — one 6.5-hour run moved about 1,681 m³
- Passed sunken driftwood, organic debris, and even large wood and concrete chunks without clogging
- Dredged to 15 m with no trouble; the 30 m design depth was not tested, to stay within the U.S. Army Corps of Engineers permit volume
- Turbidity near the pump intake averaged just 1.2 NTU over background — max increase 1.9 NTU, max TSS 9.0 mg/L
- Downstream plume peaked at only 12 NTU over ambient, and dissolved oxygen was unaffected — results that held even after the sediment curtain was removed
Selected Production Data (PG&E, 1995)
| Date | Dredging Time (hrs) | % of Operating Time Dredging | Total Cubic Meters | Rate (m³/hr) |
|---|---|---|---|---|
| 12/14/94 | 6.5 | 87 | 1,479 | 228 |
| 12/16/94 | 6.5 | 100 | 1,681 | 258 |
| 12/19/94 | 5.5 | 76 | 1,263 | 229 |
| 12/20/94 | 4.25 | 81 | 1,154 | 271 |
| 12/21/94 | 2.25 | 50 | 462 | 205 |
Key Conclusions
- EDDY Pump slurry suction dredging is an environmentally superior method for meeting strict water-quality standards
- Very little turbidity or re-suspension of sediment at the suction head
- High production rates — comparable to much larger conventional suction dredges
- Passes large volumes of woody debris and foreign material without clogging
- High slurry density minimizes the volume of effluent to treat and dispose of
- Dense slurry pumped over long distances offers a pipeline alternative to distant disposal sites
- After early start-up issues were solved, the prototype ran reliably — meeting the limits even without a sediment containment curtain
About Cresta Reservoir & PG&E
Cresta Reservoir sits on the North Fork Feather River in Plumas County, California, impounding water for the 70,000 kW Cresta Powerhouse. Placed in service in 1949 and owned by Pacific Gas and Electric Company (PG&E), it had by the 1990s lost nearly half its capacity to accumulated sediment — prompting a long-running search for an environmentally responsible way to desilt it and protect the dam’s outlets.
About Eddy Pump Corporation
Founded in 1984, EDDY Pump Corporation delivers breakthrough technology and service to industries including the U.S. Navy, oil & gas, mining, wastewater, paper/pulp, dredging, fracking, and chemical. At its core is the patented EDDY Pump — not a centrifugal, vortex, or positive-displacement pump, but a geometrically designed spinning rotor that harnesses the power of a tornado into fluid dynamics, handling material once deemed impossible or cost-prohibitive.