316B RULE

TELEPHONE LOG

Name of Caller: John Sunda, Tetra Tech

Date: October 26, 2011

Company Name: Ovivo (formerly Eimco)

Person Contacted: Trent Gathright

Title: Sales Manager

Telephone #: 281-480-7955

Email: trent.gathright@ovivowater.com

Called to inquire about issues and methods for determining or monitoring through-screen velocity of 

traveling screens particularly at velocities near 0.5 fps. Mr. Gathright has extensive experience regarding 

traveling screen performance and installation.

He said there was no practical method available for monitoring or direct measurement of the throughscreen velocities. Screen velocities can be calculated mathematically but these are just computational 

estimates

He could not provide estimates of the degree of variation in velocity spatially across the face of the screen 

In a lab model flume and traveling screen they used a Marsh-McBirney electromagnetic flowmeter to 

establish approach velocities and determine if they had laminar flow before testing fish behavior around 

the screens. This provided velocity data in two dimensions which was suitable for this application.

Head Loss Across the screen

At a through-screen velocity of 0.5 fps the head loss for a clean screen will be in the few mm range and 

would not be a suitable measure for estimating screen velocity. Not until the debris blinding reaches 50% 

to 75% does the head loss increase to several inches. Head loss monitoring is performed to adjust screen 

operation on/off and speed in response to debris buildup. Water level measurements mounted on upstream 

and downstream of a traveling screen typically use ultrasonic and more recently radar instruments that 

bounce sound or light beams off the water surface to determine water level distance from the instrument 

and thus elevation. Because of variations due to wave action, the instrument readings are typically 

averaged over a period of time (e.g., 2 to 5 minutes) depending on site-specific conditions such as the 

speed at which blinding may occur. In some cases rapid blinding of the screens may occur such as when a 

large amount of jellyfish are encountered or after storm events when runoff delivers large amounts of 

vegetation or other debris. Conventional older through-flow screens may typically rotate at 10 fpm when 
on and will simply be turned on and off or operated at a higher speed as needed. For Ristroph screens 

which are operated continuously, lower operating speeds in the 2 to 3 fpm are used to reduce wear. The 

screens drives may be equipped with variable speed motors and the speed can be adjusted up to 10 fpm or

greater depending on the head loss instrument readings. He noted that differences in head loss will vary 

with water depth (e.g., tidal waters) as well since the velocity will increase as depth decreases. Accurately 

determining percent debris blinding based on head loss is nearly impossible due to the minor variations in 

head loss involved and the complex hydrodynamic variables involved.

The costs of these systems are approximately $20,000-$32,000 per screen including al instrumentation 

and motor control systems. The cost of the water elevation monitoring instruments alone are $7,500-

$8,000 for ultrasonic and $12,000-$13,000 for radar systems. The difference in costs is roughly $5,000. 

Spatial Velocity Variation

We briefly discussed spatial variations in velocity across the face of a screen and he said the variations are 

highly sight specific being dependent on such factors as the influence of pump swirl, physical 

configuration of the intake and the distance of different portions of the screen to the pump inlet. Besides 

direct measurement at multiple locations in front of the screen (approach velocity), the only other way to 

estimate this would be through performance of a computational fluid dynamics (CFD) analysis using

advanced computer software to model system flow which would require detailed system data and would 

typically cost about $60,000 per screen. Unlike open channel flow where certain proportional depths can 

be chosen to represent overall velocities, the velocity profile for each screen is site-specific and could 

only be reasonably estimated using CFD analyses or direct measurement at multiple locations close to the 

screen using velocity meters.

Retrofitting Existing Screens

He noted that for an older through-flow screen the design through-screen velocity was typically 2.5 fps

and that overall percent open area was typically 50% to 55% which meant that the approach velocity 

would be in the 1.0 to 1.5 fps range which is well above the 0.5 fps range and the only solution would be 

a larger intake.