**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 no ne are likel y to be practical for directly measuring measurement of velocities in open water conducted on a routine basis at existing water intake and State f

isheries criteria. Additionally , there are other technical factors that may affect the compli ance 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 t ypes of devices including propeller

- ty pe, electr propeller

- ty

pe can onl y measure velocity in a single dimension (1D) and must be oriented properl y. However, these ty pes 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 rel y 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 an y 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 e xisting 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 intake s 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 t ypes of ty pe, electr omagnetic, and acoustic Doppler velocimeters ty pe can onl y measure velocity in a single dimension (1D) and must be oriented ty pes of devices are generall y not recommended for low velocity 0.5 fps because an y change in resistan ce to rotation can have a Electromagnetic (EM) velocimeters can measure velocity in 1D or two dimensions (2D) and rel y 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 an y 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 t ypes of omagnetic, and acoustic Doppler velocimeters

. The ty pe can onl y measure velocity in a single dimension (1D) and must be oriented are generall y not recommended for low velocity

to rotation can have a Electromagnetic (EM) velocimeters can measure velocity in 1D or two dimensions (2D) , and rel y 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. ItAcoustic Doppler velocimeters (ADVs) measure velocity by anal yz ing the Doppler shift of sound waves reflected off particulates suspended in the water at a fixed distance in front of the probe and rel y on the assumption that the particulates are moving at the same velocity as the water. The devi c e 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 reall y 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 measure 2 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 th e 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 t ypical Ristroph screen is actuall y 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 an y indication of spatial variations across the screen surface. Such spatial variations could be estimat ed through use of a computational fluid dy namics (CFD) anal ys is to model sy stem flow. However, this would require detailed sy stem design data and would ty picall y cost about $60,000 per s creen (one

- time cost) to perform (Gathright 2011 ). The results would still be estimates , however, and would be influenced b y 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 w ater surface are subject to short

–term variations due to w ave and ripple action. This is typically addressed by programing the instrume ntation 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 velocit y, velo cities 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 readil y available and necessary calculations can be performed in

- house at m inimal cost. If the standard requires consideration of spatial variations, then a CFD anal ys is 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 sy stem. Monitoring Approach Velocity or Velocity at Inlets without Screens T he velocit y in front of screens or at inlets without sc reens can be measured directly using several t ypes of velocit y measuring devi c es. 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 in strumentation would cost around $9,000 to $11,000 per unit and would last more 10 y ears (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 m onitoring c ould 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 L**  **ocation (Point of C**  **om pliance)**  There are two general approaches to creating an intake velocit y standard: setting a limit on the approach velocit y or setting a limit on the through

- screen velocit y. One f actor 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 dr awn 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 velocit y i s a concern since higher velocities are ass ociated with higher pressure differentials across the screen which could promote injury

. For situations where the screen openings are s maller than the fish of concern (incl uding all screens with ≤ 3/8

- in ch mesh ) ,
3 It may be useful to consult aquatic biologists regarding the screen avoidance behavior of the majority of species of concern , particularly with regar d to distance in front of screen that fish recognize the danger and begin to swim aw ay.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 ope nings , then the velocity of concern is the through

- screen velocit y. 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 through

screen velocit y helps prevent the adher ence of debris and non

- motile organisms to the screen face. One advantage of applying the standard to the through

- screen velocity is that i t ensures that the approach velocity will be equal to (if there is no scree n ) or lower than the standard. A disadvantage is that the replacement of a coarse screen that barel y meets the through

- screen standard with a fine r screen could result in the screen no longer meeting the standard 4 even though it is likely

that the impact with respect to organisms that were impin ged on the coarse mesh screen would be no worse. An addition al concern is the velocity in upstream channels and constrictions . While a standard based on through

- screen velocity will ensure that the approach velocit y 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 , potentiall y re sulting 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 su ch that average scree n 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 velocit y of the remaining open space will increase

. **Spatial V ariation**  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 velocit y , this is the only practical method possible ; as described above, there is no practical method t o measure the v elocity 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 hydrod yn amic conditi ons. 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 appl y 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 velocit y 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 evenl y across

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

- screen velocity. 5 Guidelines in sanitary se wer design recommend a minimum velocity of 2 fp s 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 by pass such as those commonly used in h ydro power installations may

also have velocit y criteria for the bypass velocit y vector perpendicular to the screen. For situations where debris blinding is significant , the degree of debris blinding will affect the through

- screen velocit y 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 fl ow (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 progressivel y collect debris that impedes passage of water. Thus, the approach velocity may vary considerabl y from the b ottom to the top of the screen. The presence of spatial velocity variations raises the issue as to whether the maximum through

screen velocit y 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 velocit y standard were based on approach velocity , compliance can be determined using direct measurement in most cases and the “maximum” velocity standard could apply to an y 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 i ssues include:

 There is no available method to directly measure through

- screen velocity ;  Can be estimated using a basic computational method using available data;  The m aximum velocity at maximum intake flow and minimum water level conditions can be estimated based on historical hydrology data ;  Unless a CFD anal ys is is performed , the estimated maximum screen velocity likel y will not take into consideration spatial variations of screen veloc ities 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 velocit y at inlets where there is no screen include:

 Can be measured directly and accuratel y 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 onl y be performed during low flow events. F or rivers and streams , measurement for compliance at the defined low flow condit ion may be delay ed until the condition occurs;  If the proposed numeric standard is applied as the approach velocit y instead of through

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

- screen velocities that are above 0.5 fps to pote ntially

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 velocit y criteria are alread y being moni tored 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 Screen ing Facilities,” p rovides guidance ( including p hotographs ) of applications regarding flow measurement technologies and methods

. Ty

pical methods involve mounting the probes on rods or poles in a manner that allows for them to be quickl y repositioned for me asurement 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 alway s a site

- specific challenge. Devising a workable mounting sy stem 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 sy stem is devised, and all equipment and mounting hardware is purcha sed and installed, the majority of the costs will be associate d 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 AD V probes at 3 to 5 locations spaced vertically across each screen and at depths of 2 0% and 8 0% 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 scre ens, 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 velocit y 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 laterall y at 0.5, 2, 4, 6, 8, 10, and 11.5 ft. from the end of a 12 ft. screen using an AD V probe (DeMo ye r 2004).

6 For a screen with through

- screen velocity of 0.5 fps, the equivalent approach velocity could be around 0.3 fps. How ever, the potential f

or 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 prototy pe 1/8

- in ch wedge wire fish diversion bar screen with 1/8

- in ch spaces , the probe w a s mounted on a modified screen - cleaning sweep bar that allowed for the probe to be selectivel y positioned at an y 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. Itwas 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 me ans of measuring velocity at multiple locations is possible and was justified because they

were performing an extensive test on a protot ype 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 variet y of conditions that provide an indication of trends with regard to intake flow, water depth, and screen cleanliness, the frequency of velocit y measurements c ould 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) , mo re routine velocity monitoring could be required.
**References** Chamness, Mickie, Scott Aberneth y, Cheryly n Tunnicliffe, "Walla Walla River Basin Fish Screens Evaluations", 2006 Annual Report, Project No. 199601100, 21 electronic pages, (BPA Report DOE/BP

- 00000652
- 36) DeMo yer, 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 vel ocities 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.