Document ID: EPA-HQ-OW-2008-0667-2879
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2012-06-12T04:00Z

Velocity Caps as Preapproved Technology for Impingement Mortality Control
A velocity cap is a device that is placed over a vertical inlet at an offshore cooling water intake.  This cover converts vertical flow into horizontal flow at the entrance into the intake.  The device works on the premise that fish will avoid rapid changes in horizontal flow, thus reducing overall impingement and entrainment of aquatic organisms.
Historically, velocity caps have been shown to reduce impingement by 68 to 99 percent.  All velocity caps include certain characteristics that are critical to the success of this technology:
   1. Velocity cap(s) must be designed to ensure that fish are subjected to horizontal rather than vertical flow from the intake structure.
   2. Velocity cap intake flow should be at least 0.5 fps or greater to ensure that fish detect the horizontal flow in time to escape impingement.
   3. The velocity cap must be placed in an optimum location offshore and in the middle of the water column to reduce impingement and entrainment of aquatic organisms.
   4. The determination of optimum location and flow characteristics are site-specific and should be approved by the permitting director.

There are more than a dozen velocity caps in operation at various facilities throughout the U.S.  These structures provide intake water flow to nuclear, coal and natural gas-fired facilities located on the Pacific Ocean, Atlantic Ocean, and the Great Lakes.
Several studies in California have concluded that velocity caps significantly reduce impingement.  Studies at the Huntington Beach and El Segundo Generating Stations showed overall reductions in entrapment (equated to impingement mortality losses) of 80 to 90 percent. Two additional studies at the Scattergood Generating Station included measurements conducted pre‐ and post-installation of velocity caps, as well as more recent flow reversal studies (EPA 2011; LADWP 2007).  The results from the Scattergood study showed an overall reduction of 83 percent.  The Ormond Beach Generating Station studies conducted concurrently with the Huntington Beach studies demonstrated overall reductions in entrapment of 61 percent during the night and 87 percent during the day.
In comparative studies of offshore velocity cap intakes as compared to onshore intake structures, studies from 1992-1998 and 2002-2006 indicate that the Seabrook Nuclear Generating Plant in New Hampshire, with offshore velocity caps, reduced impingement impacts by 68 and 83 percent, respectively, as compared to the onshore intake structure of the Pilgrim Plant located in Massachusetts.
Based on the successful history of operating velocity caps at several facilities, this technology should be continued to be considered Best Technology Available for reducing impingement mortality.  Future installations of velocity cap structures should be designed to meet the general criteria described above.