Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone 703-385-6000

fax 703-385-6007

DATE: November 1, 2011

TO: Paul Shriner and Lisa Biddle, USEPA

FROM: John Sunda and Kelly Meadows, Tetra Tech

SUBJECT: Intake/Screen Velocity

Determination

Tetra Tech was tasked with contacting vendors 

compliance with intake velocity requirements

described below, there are devices or methods that 

none are likely to be practical for directly measuring 

measurement of velocities in open water 

conducted on a routine basis at existing water intake

and State fisheries criteria. Additionally, there are other technical factors that may affect the 

compliance options for intake velocity that may be worth considering for the final rule.

I. Direct Measurement of Water Velocity

Water velocity can be directly measured at specific locations in water using several types of 

devices including propeller-type, electr

propeller-type can only measure velocity in a single dimension (1D) and must be oriented 

properly. However, these types of 

conditions in the range of 0.5 fps 

significant impact on accuracy.

Electromagnetic (EM) velocimeters can measure velocity in 1D or two dimensions (2D)

depending on the probe type, and rely on measuring the voltage associated with the current that 

is induced in a conducting fluid as it passes through a magnetic field.

conduct electricity the probe must be kept a minimum distance of 3 inches 

or any metal components (U.S. DOI 2009).

 

1 The sampling volume for the 2D EM is spherical in shape and has a diameter of approximately 5 inches. It 

measures the average fluid velocity over the sample volume.

MEMORANDUM

, 2011

Paul Shriner and Lisa Biddle, USEPA

John Sunda and Kelly Meadows, Tetra Tech

Velocity Compliance Alternatives and Methods for Compliance 

Tetra Tech was tasked with contacting vendors to identify techniques (and costs) 

compliance with intake velocity requirements in the proposed rule for existing facilities

are devices or methods that can be used to measure water velocity, but 

practical for directly measuring the through-screen velocity

measurement of velocities in open water or close to a screen is not only possible but is currently

conducted on a routine basis at existing water intakes in determining compliance with Federal 

Additionally, there are other technical factors that may affect the 

ance options for intake velocity that may be worth considering for the final rule.

Direct Measurement of Water Velocity

Water velocity can be directly measured at specific locations in water using several types of 

type, electromagnetic, and acoustic Doppler velocimeters

type can only measure velocity in a single dimension (1D) and must be oriented 

types of devices are generally not recommended for low velocity 

0.5 fps because any change in resistance to rotation can have a 

Electromagnetic (EM) velocimeters can measure velocity in 1D or two dimensions (2D)

and rely on measuring the voltage associated with the current that 

is induced in a conducting fluid as it passes through a magnetic field.1 Since metal screens 

conduct electricity the probe must be kept a minimum distance of 3 inches away

or any metal components (U.S. DOI 2009).

The sampling volume for the 2D EM is spherical in shape and has a diameter of approximately 5 inches. It 

measures the average fluid velocity over the sample volume.

Methods for Compliance 

(and costs) for determining 

xisting facilities. As 

used to measure water velocity, but 

screen velocity. Direct 

close to a screen is not only possible but is currently

in determining compliance with Federal 

Additionally, there are other technical factors that may affect the 

ance options for intake velocity that may be worth considering for the final rule.

Water velocity can be directly measured at specific locations in water using several types of 

omagnetic, and acoustic Doppler velocimeters. The 

type can only measure velocity in a single dimension (1D) and must be oriented 

are generally not recommended for low velocity 

to rotation can have a 

Electromagnetic (EM) velocimeters can measure velocity in 1D or two dimensions (2D),

and rely on measuring the voltage associated with the current that 

Since metal screens 

away from the screen

The sampling volume for the 2D EM is spherical in shape and has a diameter of approximately 5 inches. It 
Acoustic Doppler velocimeters (ADVs) measure velocity by analyzing the Doppler shift of 

sound waves reflected off particulates suspended in the water at a fixed distance in front of the 

probe and rely on the assumption that the particulates are moving at the same velocity as the 

water. The device uses one sound transmitter and three offset receivers, allowing the probe to 

measure three velocity vectors at once (3D). Sample volume is relatively small (>1.0cc) and the 

device’s primary limitation is that it is not effective in extremely clear, particle-free water such 

as spring-fed rivers.

All three of the devices described above can be mounted on a support structure and then 

positioned at specific locations in front of the screens.

Measuring or Estimating Through-screen Velocity

Tetra Tech contacted a traveling screen vendor regarding methods for measuring through-screen 

velocity. The response was that there is really no practical method available for monitoring or 

direct measurement of through-screen velocities. While head loss across the screen provides a 

potential measure, the head loss at through-screen velocities near the proposed 0.5 fps standard 

would be on the order of several millimeters, which would be difficult to measure2 with 

sufficient precision to provide meaningful results.

The through-screen velocity can be calculated using flow data (measured or estimated), total 

screen panel face area at some specified depth of submergence (water depth), and the percent 

open area of the screen mesh. The total screen face area is not the product of the dimensions of 

the screen but rather takes into consideration the portions of the screen face that are not available 

for flow. For example, only about 85% of the area in front of a typical Ristroph screen is 

actually screen material; the remainder includes the structural components at the sides the screen 

basket edges, the fish buckets and panel joints, and the foot structure at the bottom.

The simple computational method provides an average velocity over the total open pore space 

and does not provide any indication of spatial variations across the screen surface. Such spatial 

variations could be estimated through use of a computational fluid dynamics (CFD) analysis to 

model system flow. However, this would require detailed system design data and would 

typically cost about $60,000 per screen (one-time cost) to perform (Gathright 2011). The results 

would still be estimates, however, and would be influenced by accuracy of the input data and the 

model itself.

Monitoring Costs

Monitoring Through-screen Velocity

 

2

Several methods are available to measure water depth. Even those that are quite accurate such as ultrasonic and 

radar-based systems that bounce sound or light beams off the water surface are subject to short–term variations due 

to wave and ripple action. This is typically addressed by programing the instrumentation to average the 

instantaneous depth readings over a period of several minutes (Gathright 2011). 
Since there is no readily available method for directly measuring through-screen velocity, 

velocities would instead be estimated through mathematical calculation as described above. If 

the standard involves a simple calculation that does not consider spatial variations (see below for 

additional discussion), then the costs involved are simply those for engineering services and data 

gathering. If these flow estimating methods are deemed sufficient, all of the data necessary

should be readily available and necessary calculations can be performed in-house at minimal 

cost. If the standard requires consideration of spatial variations, then a CFD analysis may be 

necessary for each unique screen configuration at a cost of around $60,000 per screen. In 

general, there will be no ongoing operations and maintenance (O&M) costs since re-evaluation 

and additional computations would only occur when significant modifications are made to the 

system.

Monitoring Approach Velocity or Velocity at Inlets without Screens

The velocity in front of screens or at inlets without screens can be measured directly using 

several types of velocity measuring devices. The acoustic Doppler velocimeters appear to be the 

instrument most commonly used in fish screen evaluations. An ADV vendor was contacted and 

indicated that the probe and instrumentation would cost around $9,000 to $11,000 per unit and 

would last more 10 years (Edelman 2011). The number of probes and the mounting equipment 

needed will vary depending on the monitoring requirements and methodology specified. 

Additional costs for installing the probe mounting structures will be site-specific but will involve 

mostly initial costs for labor and installation. Unless hydrological conditions that vary outside of 

those during the initial velocity monitoring are expected, follow-up monitoring could be limited 

in scope.

II. Technical Considerations

As noted above, there are several technical aspects of velocity monitoring that may be worth 

considering in crafting the requirements for the final rule. These are discussed below.

Measurement Location (Point of Compliance)

There are two general approaches to creating an intake velocity standard: setting a limit on the 

approach velocity or setting a limit on the through-screen velocity. 

One factor to consider in choosing one or the other is the underlying basis of the standard and the 

behavior of the organisms to be protected. If free swimming fish and their swim speed is the 

basis and the response of the fish is to avoid the screen as they are drawn close to it, then the 

compliance location should be the location where this response occurs.

3

If the concern is 

potential injury to fish and non-motile organisms impinged and held against the screen, then the 

through-screen velocity is a concern since higher velocities are associated with higher pressure 

differentials across the screen which could promote injury. For situations where the screen 

openings are smaller than the fish of concern (including all screens with ≤3/8-inch mesh),

 

3

It may be useful to consult aquatic biologists regarding the screen avoidance behavior of the majority of species of 

concern, particularly with regard to distance in front of screen that fish recognize the danger and begin to swim 

away.
because the fish are not able to swim within the openings, the velocity of concern is the velocity

of the water in front of the screen or the approach velocity. For screens or devices where the 

openings are large enough for the fish of concern to swim within the openings, then the velocity

of concern is the through-screen velocity. For intake technologies that rely on a sweeping flow 

to carry fish and debris across the screen face and away from the screen, limiting the throughscreen velocity helps prevent the adherence of debris and non-motile organisms to the screen 

face. One advantage of applying the standard to the through-screen velocity is that it ensures 

that the approach velocity will be equal to (if there is no screen) or lower than the standard. A 

disadvantage is that the replacement of a coarse screen that barely meets the through-screen 

standard with a finer screen could result in the screen no longer meeting the standard4even 

though it is likely that the impact with respect to organisms that were impinged on the coarse

mesh screen would be no worse.

An additional concern is the velocity in upstream channels and constrictions. While a standard 

based on through-screen velocity will ensure that the approach velocity near the screen will be 

the same or lower, there is no guarantee that the velocity further upstream in a channel or canal 

may not be higher. In fact, many intake screen forebay channels have skimmer or baffle walls, 

gated openings, or bar racks that may constrict the opening, potentially resulting in velocities 

greater than the standard. Sedimentation on the intake channels may also form constrictions.5 

Thus, in areas where suspended silt and sand are present, even though the channel may have 

been built or dredged such that average screen velocities are <0.5 fps, solids may be deposited 

over time upstream of the screen such that the channel velocity may be well above 0.5 fps.

Applying the standard as the through-screen velocity presents an issue regarding whether the 

application of standard must consider the fact that, as a screen becomes clogged with debris, the 

through-screen velocity of the remaining open space will increase.

Spatial Variation

In general, the common practice regarding compliance with a through-screen standard has been 

to apply it as an average across the entire face of the screen or device based on engineering 

calculations. For through-screen velocity, this is the only practical method possible; as described 

above, there is no practical method to measure the velocity within the small spaces of the screen 

openings. Whether the point of compliance is within the pores of the screen or at some point 

upstream, there may be considerable variation in velocity at different locations due to varying 

hydrodynamic conditions. The influences of channel walls, pump swirl, obstructions, eddy

currents, inlet currents, and distance from different portions of the screen to the pump suction 

can create significant variations in the velocity at different locations across the screen face. Fish 

screening and diversion criteria based on approach velocity often apply the standard to the 

velocity vector perpendicular to the screen at a distance of 3 inches in front of the screen and

may also include a requirement that the velocity be uniform across the face of the screen (U.S.

DOI 2009). If spatial differences are encountered and are a concern, then it may be possible to 

install baffles downstream of the screen that would help distribute the flow more evenly across 

 

4 A finer mesh screen would have less open area, resulting in a higher through-screen velocity.

5 Guidelines in sanitary sewer design recommend a minimum velocity of 2 fps to prevent deposition of solids but a 

velocity of 1.0 fps is considered sufficient to prevent serious deposition.
the screen face. Screens with a bypass such as those commonly used in hydro power 

installations may also have velocity criteria for the bypass velocity vector perpendicular to the 

screen.

For situations where debris blinding is significant, the degree of debris blinding will affect the 

through-screen velocity and may also affect the distribution of flow and approach velocity across 

the face of the screen. For traveling screens, the cleaned screen panels that are introduced to the 

through flow (at the bottom for conventional through flow screens) will allow passage of a larger 

amount flow which will diminish as the screen panels ascend upward and progressively collect 

debris that impedes passage of water. Thus, the approach velocity may vary considerably from 

the bottom to the top of the screen.

The presence of spatial velocity variations raises the issue as to whether the maximum throughscreen velocity should consider spatial variations as well. However, as described above, even if 

EPA were to establish such a requirement, there would be no direct way to monitor compliance.

If the velocity standard were based on approach velocity, compliance can be determined using 

direct measurement in most cases and the “maximum” velocity standard could apply to any 

location in front of the screen.

Potential Concerns with Using Through-Screen Velocity as the Standard

Commenters have raised a number of similar issues, but the list below summarizes several 

potential issues with the use of through-screen velocity as the basis for existing intakes. These 

issues include:

 There is no available method to directly measure through-screen velocity;

 Can be estimated using a basic computational method using available data;

 The maximum velocity at maximum intake flow and minimum water level conditions can be 

estimated based on historical hydrology data;

 Unless a CFD analysis is performed, the estimated maximum screen velocity likely will not take 

into consideration spatial variations of screen velocities at different locations across the screen 

face and may allow for higher velocities at “hot spots” on the screen.

Potential Concerns with Measuring Velocity at Intakes Without Screens

Issues regarding direct monitoring of approach velocity or the velocity at inlets where there is no 

screen include:

 Can be measured directly and accurately using reasonably priced instrumentation;

 Velocity can be measured in multiple locations and it is possible to measure the true maximum 

velocity, including those at hot spots;

 Eliminates the velocity “penalty” associated with the increase in through-screen velocities that 

occurs when coarse mesh screens are replaced with finer mesh. The velocity standard should not 

dissuade use of fine mesh;
 Is consistent with the rationale that the standard is based on fish swim speed and ability to 

escape;

 The evaluation of velocities at minimum low flow can only be performed during low flow 

events. For rivers and streams, measurement for compliance at the defined low flow condition 

may be delayed until the condition occurs;

 If the proposed numeric standard is applied as the approach velocity instead of through-screen 

velocity, it would allow some intakes that currently have through-screen velocities that are above 

0.5 fps to potentially become compliant since overall approach velocities will be lower than 

through-screen velocities;

6

 A wealth of existing guidance, technology, and monitoring resources are available since fish 

screen approach velocity criteria are already being monitored and enforced at many existing fish 

screens throughout the U.S.

III. Examples of Field Measurements of Intake Velocity

Several publications are available that describe the methods used to perform field evaluations of 

fish screening facilities that include evaluation of approach velocities. Attachment 1, 

“Guidelines for Performing Hydraulic Field Evaluations at Fish Screening Facilities,” provides 

guidance (including photographs) of applications regarding flow measurement technologies and 

methods. Typical methods involve mounting the probes on rods or poles in a manner that allows 

for them to be quickly repositioned for measurement at multiple depths. Measurements are also 

taken at multiple locations across the width of the screen. As noted in Attachment 1, “Mounting 

instruments in and around structures is always a site-specific challenge. Devising a workable 

mounting system for the instrument can take a great deal of time and effort, but it is a worthwhile 

investment in order to improve the efficiency and accuracy of data collection.” However, once a 

system is devised, and all equipment and mounting hardware is purchased and installed, the 

majority of the costs will be associated with the labor required to reposition the probes and 

collect and analyze the data. In-house staff can be trained to perform the measurements.

In one evaluation at the Walla Walla River Basin fish screens, water diversion screen 

measurements were taken using ADV probes at 3 to 5 locations spaced vertically across each 

screen and at depths of 20% and 80% of the forebay depth (Chamness 2006). The probes were 

positioned 3 inches in front of the screens and samples were collected for 30 seconds at a rate of 

2 Hz (2 samples per second) for each location. While many fish approach velocity criteria 

require measurements to be performed 3 inches in front of the screens, for Ristroph traveling 

screens with fish buckets, probe positioning must take into consideration the protruding fish 

bucket and the localized influence on velocity as the bucket passes close to the probe.

In another evaluation at the Roza Fish Screens Facility, approach and sweeping velocity 

measurements were taken within 3 inches of the screen vertically at 0.05, 0.2, 0.5, 0.8, and 0.9 

times the water depth and laterally at 0.5, 2, 4, 6, 8, 10, and 11.5 ft. from the end of a 12 ft.

screen using an ADV probe (DeMoyer 2004). 

 

6

For a screen with through-screen velocity of 0.5 fps, the equivalent approach velocity could be around 0.3 fps. 

However, the potential for spatial variations may result in a localized maximum velocity somewhat higher than 0.3 

fps and it may be possible that the maximum velocity could exceed 0.5 fps if flow velocity is not distributed evenly.
In another investigation at a hydropower plant of the performance of a prototype 1/8-inch wedge 

wire fish diversion bar screen with 1/8-inch spaces, the probe was mounted on a modified 

screen-cleaning sweep bar that allowed for the probe to be selectively positioned at any location 

both horizontally and vertically across the face of the bar screen (U.S. DOI 2001). Such an 

application took advantage of the existence of a cleaning sweep bar and drive track which would 

not be present at the screens on most facilities. It was selected because of the screen’s position 

deep within the turbine inlet in a difficult-to-access location. This situation demonstrates that, if 

necessary, a sophisticated means of measuring velocity at multiple locations is possible and was 

justified because they were performing an extensive test on a prototype technology.

At any given time, the screen velocity profile will primarily be dependent on hydraulic 

conditions such as intake flow, water depth, and screen cleanliness. Once initial velocity

measurements have been collected over a variety of conditions that provide an indication of 

trends with regard to intake flow, water depth, and screen cleanliness, the frequency of velocity 

measurements could be reduced. In cases where significant variations in conditions that are 

outside of the range that occurred during previous monitoring events can be expected (e.g., 

intake forebay is prone to sedimentation), more routine velocity monitoring could be required.

References

Chamness, Mickie, Scott Abernethy, Cherylyn Tunnicliffe, "Walla Walla River Basin Fish 

Screens Evaluations", 2006 Annual Report, Project No. 199601100, 21 electronic pages, (BPA 

Report DOE/BP-00000652-36)

DeMoyer, Connie. “Roza Fish Screens Facility: Velocity Measurements at a High Canal Flow 

Rate.” Report No. HL-2004-06, Bureau of Reclamation, Water Resources Research Laboratory, 

Denver, CO. December 2004.

Edelman, Joel, Sontek/YSI. Telephone Contact Report regarding cost and application of 

acoustic Doppler velocimeter equipment for measuring water velocity near water screens. 

October 27, 2011.

Gathright, Trent. Ovivo. Telephone Contact Report regarding estimating through-screen 

velocities for traveling screens. October 26, 2011.

U. S. Department of Interior (U. S. DOI). Bureau of Reclamation. ”Guidelines for Performing 

Hydraulic Field Evaluations at Fish Screening Facilities.” April 2009.