Research and Development Office

Improving Coanda-Effect Screen Technology

Project ID: 6295
Principal Investigator: Tony Wahl
Research Topic: Agriculture Water Supplies
Funded Fiscal Years: 2015, 2016 and 2017
Keywords: hydropower, renewable energy, screening, debris removal, fish, water conservation, safety, o&m

Research Question

The objective of this research is to improve the ability to predict the flow capacity and debris-handling performance of Coanda-effect screens, a hydraulically self-cleaning overflow screen that removes fine debris from diverted water.

BACKGROUND
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S&T-funded research in 2000-2001 led to equations for predicting clean-water flow capacity of Coanda-effect screens, and the results of that work are now widely relied upon. However, questions about screen capacity have arisen in recent years, with anecdotal reports of installations that have not achieved expected flow capacities. Reasons are uncertain, perhaps including poor installation, out-of-spec screen materials, and sites that experience significant debris clogging, although most screens effectively self-clean. Due to these anecdotal reports, there is concern among potential users that predictions of screen capacity are inaccurate.

In 2012 the USBR hydraulics lab performed tests for a private client developing a small hydropower intake in British Columbia. These tests showed that the equations developed in 2001 could be significantly improved, and that the result for many applications was an increase in flow capacity of 20-30%. The majority of this testing was focused on high Froude numbers that were appropriate for the client's application, but do not fully span the range of Reclamation's potential applications. The test facility constructed with private funding is still available in the hydraulics lab.

Although a primary motivation for the use of Coanda-effect screens is their ability to self-clean, the specific hydraulic conditions, screen parameters, and debris properties that are needed to achieve self-cleaning have never been scientifically determined. Experience-based rules of thumb have generally been used by screen designers without solid justification. There is anecdotal evidence that some debris types are problematic.

Need and Benefit

Screening of trash, plant materials, and fish is often required on Reclamation projects to facilitate efficient water operations, prevent the spread of non-native plant and fish species, and preserve fish resources.

Potential applications of this technology are widespread, including small hydropower intakes, municipal water intakes, stormwater intakes, and agricultural water diversions where fine debris or weed seed removal is needed. These screens are especially useful in remote locations, since they require minimal manual cleaning and no power for a mechanical cleaning system. Coanda-effect screens have been used by USBR on salinity control projects where flood irrigation systems are being converted to more efficient application methods that require cleaner water (e.g., gated pipe, sprinkler).

Fish screening is possible using Coanda-effect screens, both for exclusion of undesirable fish and for protection of fish resources. For fish protection, fisheries agencies are sometimes reluctant to consider this type of screen because it is a radical departure from established technology. The work proposed by the Canadian group would help to address this concern.

Coanda-effect screens have been widely applied and are being considered for many more applications where self-cleaning screening is needed. Questions about screen capacity need to be resolved, and a better understanding is needed of the effects of debris and the ability of the screens to self-clean in a range of situations. If additional testing could confirm the findings from the privately-funded testing in 2012 (20-30% more flow capacity), this could make Coanda-effect screens technically feasible or economically viable for an even wider variety of applications.

Contributing Partners

Contact the Principal Investigator for information about partners.

Research Products

Bureau of Reclamation Review

The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.

Improving Coanda-Effect Screen Technology (final, PDF, 1.7MB)
By Tony L. Wahl
Research Product completed on September 30, 2017

Improving Coanda-Effect Screen Technology (final, PDF, 1.7MB)
By Tony L. Wahl
Research Product completed on September 30, 2017

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