Summer Destratification
What is Reservoir Destratification?
Reservoir destratification is the process of mixing or disrupting thermal layers in a reservoir or lake to maintain a uniform temperature and oxygen distribution throughout the water body. This helps prevent problems caused by thermal stratification, such as oxygen depletion in deeper layers.
Why Does Stratification Occur?
In warm months, reservoirs often develop layers:
- Epilimnion: Warm, oxygen-rich surface layer.
- Metalimnion (thermocline):Transition zone with rapid temperature change.
- Hypolimnion:Cold, often oxygen-poor bottom layer.
Importance of Destratification in Source Water Reservoirs
- Ensures consistent water quality by preventing oxygen depletion at the bottom.
- Oxidizing the sediment reduces the release of harmful substances like manganese, hydrogen sulphide, ammonia, and other metals from sediments.
- Inhibits harmful algal blooms by reducing internal loading of phosphorous from sediments.
- Shifts the algae species composition by negating the competitive advantage of cyanobacteria to regulate their own buoyancy to move toward light and nutrients.
- Helps water treatment plants by providing more stable water quality.
Methods of Reservoir Destratification
- Mechanical Destratifiers (Downflow): Push oxygen-rich water directly from the surface to the lake bottom, where it naturally spreads for long distances, blanketing the sediments with oxygen-rich water. Flow rates of 25,000 gallons per minute are typical.
- Solar Circulators: Pumps that move water from an intermediate depth to circulate the reservoir above the depth of the circulator intake. Flow rates of 3,000 gallons per minute are typical.
- Bubble Plume or Diffused Aeration Systems: Compressed air bubbles rise and draw water and bottom contaminants from the diffuser depth to the surface. Water from mid-depth sinks to replace this bottom water.
Benefits
- Improved dissolved oxygen levels throughout the reservoir.
- Minimized risk of taste and odor problems in drinking water.
- Enlarges habitat for fish and aquatic life survival.
Challenges and Considerations
- Public safety and boating culture must be considered.
- Energy consumption and operational costs of destratifiers.
- Design must fit reservoir size, depth, and usage.
- Monitoring water quality to adjust destratification processes effectively.
Original Research
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Garton & Rice, “Improving the Quality of Water Releases from Reservoirs by Means of a Large Diameter Pump”, Oklahoma State University OKLA C-5228 Agreement No. 14-31-0001-4215, 1976
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Punnett, “Design and Operation of Axial Flow Pumps for Reservoir Destratification”, US Army Corps of Engineers Water Quality Research Program Instruction Report W-91-1, 1991
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Raman & Evans, "Aeration-Destratification of Lake Eureka: Second Year Operation", Illinois Dept. of Energy and Natural Resources, ISWS/CIR-159/84, Circular 159