american fisheries society - beaudrey study - bigbee et al

8/3/2019 American Fisheries Society - Beaudrey Study - Bigbee Et Al

http://slidepdf.com/reader/full/american-fisheries-society-beaudrey-study-bigbee-et-al 1/14

Survival of Fish Impinged on a Rotary Disk Screen

DANIEL L. BIGBEE,* RONALD G. KING, AND KENT M. DIXON

EA Engineering, Science, and Technology, Inc., 221 Sun Valley Boulevard, Suite D,

Lincoln, Nebraska 68528, USA

DOUGLAS A. DIXON

Electric Power Research Institute, 3420 Hillview Avenue, Palo Alto, California 94304, USA

ELGIN S. PERRY

2000 Kings Landing Road, Huntingtown, Maryland 20639, USA

Abstract. —An impingement survival study was conducted to determine 48-h survival of fish impinged on a

modified rotary disk screen equipped with fish protection features. The rotary disk screen was installed for a

technology evaluation at the cooling-water intake structure of the North Omaha Station located on theMissouri River in Omaha, Nebraska. Hatchery-raised fish and native fish collected from the Missouri River

were released in batches into the rotary disk screen bay and collected with a system that was constructed to

recover fish from the screen’s vacuum system. That system was designed to remove fish from the rotary disk

screen and return them to the river. Screen performance was assessed in April and August, representing spring

and summer environmental conditions. The 48-h survival rates of hatchery-raised fathead minnow

Pimephales promelas, channel catfish Ictalurus punctatus, and bluegills Lepomis macrochirus and a group

of mixed native species approached 100%. Survival rates were not statistically different between test groups

and controls, indicating that impingement did not contribute to the observed mortality. High survival rates of

impinged fish removed from the screen indicated that the rotary disk screen would reduce impingement losses

at the North Omaha Station, where losses due to impingement on the existing vertical traveling screens are

assumed by the U.S. Environmental Protection Agency to approach 100% because the screens lack fish

protection features. Our study results suggest that the rotary disk screen tested could be considered analternative technology under section 316(b) of the Clean Water Act, which requires power plants to install the

best technology available to reduce impingement. Use of hatchery-reared fish and native fish collected from

the river assured that an adequate number of fish were tested to provide statistically reliable results and

allowed the use of controls to account for mortality due to handling stresses experienced by test fish.

The use of surface waters for cooling water at

conventional power plants results in the impingement

of aquatic organisms at cooling water intake structures

that are screened to limit the size of particles passing

through condenser systems. The primary objective of

our study was to evaluate survival of impinged fish

removed by a rotary disk screen installed for evaluation

at Omaha Public Power District’s North Omaha Station

(NOS; Figures 1, 2) as an alternative technology to the

conventional vertical traveling screens used at the

facility. The Clean Water Act (CWA; CWA 1972)

requires that ‘‘the location, design, construction, and

capacity of cooling water intake structures reflect the

best technology available [BTA] for minimizing

adverse environmental impact.’’ Our study was planned

and implemented based on the U.S. Environmental

Protection Agency’s (EPA) Phase II Rule (U.S. Office

of the Federal Register 2004) and preceded the U.S.

Second Circuit Court of Appeals remanding of several

provisions on 25 January 2007 and the EPA’s

subsequent suspension of the rule in July 2007 (U.S.

Office of the Federal Register 2007). Until a new rule

is promulgated, states and EPA regions have reverted

to administration of section 316(b) on a ‘‘best

professional judgment ’’ basis. Under the Phase II Rule,

the EPA required reducing impingement losses by 80–

95% relative to a baseline. The baseline at NOS would

represent 100% mortality of the fish impinged on the

conventional vertical traveling screens used at the

facility because the cooling water intake structure does

not provide fish protection features.

A modified rotary disk screen manufactured by

Beaudrey USA was selected for evaluation by the

Omaha Public Power District as a potential engineering

option for meeting the section 316(b) BTA require-

ments. The modified rotary disk screen provides fish

protection and was expected to alleviate maintenance

issues associated with debris carryover, bearing wear,

* Corresponding author: [email protected]

Received April 17, 2009; accepted August 13, 2010Published online December 13, 2010

1420

North American Journal of Fisheries Management 30:1420–1433, 2010Ó Copyright by the American Fisheries Society 2010DOI: 10.1577/M09-059.1

[Article]



and plugged spray wash nozzles associated with the

existing vertical traveling screens, which meet EPA’s

definition of a baseline intake screening system that

does not provide fish protection features (e.g., fishbuckets, low-pressure screen washes, or a fish return

system). The existing vertical traveling screens at NOS

have 9.5-mm mesh and are equipped with pressure

spray washes (4.9–7.0 kg/cm2

) to remove accumulated

debris and impinged fish. These screens are typically

operated intermittently, based on pressure differentials

across the screens.

The modified rotary disk screen can be installed in

intake bays without the need for civil works, does not

require a pressurized screen wash system, and

eliminates debris carryover associated with verticaltraveling screens. Excessive maintenance of the

existing spray wash at the NOS was necessary because

the sediment load in the Missouri River consists

primarily of sand, which causes excessive wear to

spray pumps and spray nozzles. In addition, the

modified rotary disk screen has only one bearing at

the center of the wheel; this was expected to eliminate

wear of the pin joints connecting the vertical travelingscreens, which are pinned at the corners of each screen

panel. These pin joints wear due to sand in the source

water and eventually causes the chain to lose tension

leading to additional wear to other screen parts,

including the upper and lower sprockets on which the

screens are rotated.

The traditional rotary disk screen consists of a flat

disk (wheel) covered with screening material that

rotates on a horizontal axis perpendicular to the water

flow. As water flows through the submerged portion of

the disk, impinged organisms and debris are retainedon the screen. That material is removed by a spray

wash system when the disk rotates above the water

line. The modified rotary disk screen installed at NOS

consists of one rotating wheel that rotates within a

FIGURE 1.—Location of the North Omaha Station, where a rotary disk screen was installed for a technology evaluation at the

cooling water intake structure on the Missouri River.

SURVIVAL OF IMPINGED FISH 1421



frame at 2 revolutions/min and provides a fish

protection system. The modified rotary disk screen

system consists of a 2.438-m rotating screen wheel and

backwash pump (Figures 3, 4). The screen openings on

the modified rotary disk screen are 6.1 mm

2

, comparedwith 9.5-mm screen openings for the existing vertical

traveling screens (i.e., 36% larger). A hydraulic pump

and drive motor rotate the screen, which is divided into

pie-shaped sections by plates on the inlet side of the

screen. Impinged fish and debris are vacuumed from

the plates as they rotate under a stationary scoop

mounted over one section of the rotary disk screen. A

Hidrostal screw-centrifugal backwash pump used by

fish hatcheries to transfer live fish is used to vacuum

the screen. The backwash pump impeller is designed to

separate the water into pockets that provide a safe place

for the fish as they move through the pump. The

backwash pump causes a backwash flow that removes

fish, sand, and debris off the screen wheel. During

operation, the disk rotates at a constant 2 revolutions/

min that limits retention time on the screen to 30 s or

less, depending on position of the impinged object on

the screen. The angular velocity of the screen is 0.21

rad/s, so at the outside diameter the screen would be

moving at about 0.256 m/s. The backwash pump has a

variable speed drive that maintains a constant flow rate

through the pump in response to river stage and allows

the pump to remain at peak operating efficiency.

North Omaha Station is a coal-fired generating

station located in Omaha, Nebraska, on the bank of the

Missouri River at river kilometer 1,006 (Figure 1). It

has five generating units with a combined capacity of

663 MW that utilizes a once-through water cooling

system. Cooling water is withdrawn from the MissouriRiver through three onshore cooling water intake

structures. The study was performed at intake 3, which

has three circulating water pumps that provide

condenser cooling water for unit 5 (Figure 2). There

are two sets of trash racks, inlet bays, sluice gates, and

vertical traveling screens for each circulating water

pump. One of the six upstream vertical traveling

screens at intake 3 was replaced with a modified rotary

disk screen to evaluate its effectiveness (Figure 2).

Impingement survival studies have been conducted

since the mid-1970s in response to requirements of

section 316(b). Although study results have varied

widely, both among species and among the screen

systems tested, they have shown species-dependent

impingement survival rates of 70–80% or higher at

facilities with adequate screen design and operation

(EPRI 2003). That report reviewed 71 studies at 35

power plants with survival data for angled, dual-flow,

and single-flow traveling screens, most of which

evaluated modifications to conventional screen designs

(e.g., vertical traveling screens with the addition of

Ristroph-type modifications) or operational changes

(e.g., modifying screen wash operation from intermit-

tent to continuous) to improve impingement survival.

FIGURE 2.—Schematic layout of intake 3 at the North Omaha Station, showing the locations of the rotary screen, collection

tank, acclimation tanks, and control and test tanks.

1422 BIGBEE ET AL.



These studies generally showed that screen wash

frequency, screen travel time, screen modifications

for separating fish, and debris handling were important

factors influencing survival rates. Survival increased

with decreased time between screen washes, continu-

ous screen rotation providing the highest survival rates

(King et al. 1978; Tatham et al. 1978). Several studies

have shown that survival rates increase at higher screen

speeds because the faster screen travel time, the less

time impinged organism are retained on the screens

(Beak 1988).

Studies of the physical modifications to the rotary

disk screen evaluated to protect fish have not been

previously conducted. Results from field, laboratory,

prototype, and full-scale pilot studies of other screening

systems have shown that survival of impinged fish

depends on the species and site-specific factors (EPRI

2003), making it difficult to estimate survival of

untested screen designs. The primary objective of the

survival study at the NOS was to evaluate survival of

impinged fish removed by the rotary disk screen and its

ability to reduce impingement losses relative to the

existing vertical traveling screens, which do not

provide fish protection. The use of both hatchery-

raised and wild, native fish allowed separation of

handling and holding effects from impingement effectson survival through the use of controls, an approach

that also yielded adequate sample sizes amenable to

valid statistical analyses of the test results.

Methods

The study consisted of three components: a pilot

study, two impingement survival tests, and a collection

efficiency study. The pilot study was conducted to (1)

develop a method for introducing fish into the rotary

disk screen bay that would minimize handling stress,

(2) determine safe procedures for personnel while

performing study tasks, and (3) determine the recovery

rate of fish so that the number of released fish could be

estimated to yield statistically valid results. Impinge-

FIGURE 3.—Front view of the rotary screen installed in

intake 3 at the North Omaha Station, showing the hydraulic

pump, rotating screen wheel, and stationary scoop. FIGURE 4.—Side view of the rotary screen installed in intake

3 at the North Omaha Station, showing the hydraulic pump,

drive motor, rotating screen wheel, and backwash pump.




ment survival tests were conducted during the weeks of

28 April and 11 August 2008 to represent spring and

summer conditions when impinged fish are most

stressed due to warm water temperatures and lower

dissolved oxygen (DO). The collection efficiency studywas conducted to evaluate differences in recovery rates

observed during the pilot study and impingement

survival tests.

Source of test fish.—Our original intent was to use

wild, resident fish impinged during scheduled im-

pingement monitoring events as the test fish for the

impingement survival study; however, an impingement

monitoring study from April 2007 to July 2008 (D. L.

Bigbee, unpublished) reported impingement rates of

about 1.0 fish/h on the rotary disk screen, which were

too low to provide a statistically adequate number of fish for testing of survival following impingement.

Those low rates necessitated the use of hatchery-raised

and wild, native fish collected from the Missouri River

rather than using fish impinged during normal screen

operations. That approach provided statistically ade-

quate sample sizes and increased the ratio of

information to effort. We selected three hatchery-raised

species native to the Missouri River and readily

available from a local hatchery for testing: fathead

minnow Pimephales promelas, channel catfish Ictalu-

rus punctatus, and bluegills Lepomis macrochirus.These species were selected because they were

impinged on the rotary disk screen during the

impingement monitoring study (Bigbee, unpublished).

The selected species were purchased from a licensed

Nebraska hatchery and transported to NOS in an

aerated tank. Upon delivery of fish, Missouri River

source water was slowly mixed with hatchery water

until the water temperature equalized. Hatchery fish

were acclimated in three 750-L aerated tanks with a 4-h

exchange rate for 48 h prior to introduction to the

rotary disk screen bay. Only fish exhibiting normal

swimming behavior and free of external abnormalities

(e.g., signs of infection, wounds, or loss of scales) at

the end of the 48-h acclimation period were tested.

For the August tests, wild, native fish were collected

along the shoreline of the Missouri River near NOS via

seining (6.1 m, 4.8-mm mesh) in shallow areas with

firm substrate and low water velocity. Fish were

transferred from the seine to a holding tank and

subsequently transferred to an acclimation tank at

intake 3 near the rotary disk screen bay (Figure 2).

Native fish were not segregated by species prior to

testing to minimize handling stress and were observed

for 24 h prior to introduction to the rotary disk screen

bay. Only fish exhibiting normal swimming behavior

and free of external abnormalities (e.g., signs of

infection, wounds, or loss of scales) were introduced

into the rotary disk screen bay.

Pilot study.—Bluegills and channel catfish were

introduced as separate 500-fish batches into the rotary

disk screen bay during the pilot study by lowering thefish into the screen bay. A tether was used to invert the

buckets so that fish were released below the water

surface. The rotary disk screen was shut down at 30-

min, 60-min, and 120-min following release of each

species in order to remove recovered fish from the

collection tank. Released bluegills averaged 52 mm

total length (TL) and channel catfish averaged 76 mm

TL. Recovery results from the pilot study were used to

estimate the number of test fish to release for the

impingement survival tests with the goal of recovering

50 fish for each survival test.

Impingement survival tests.—The impingement sur-

vival study consisted of 48-h tests conducted in April

and August 2008. Fish that were stunned and failed to

recover upon recovery from the collection tank were

considered dead, as were damaged fish. This assump-

tion potentially underestimated survival rates; however,

only 7.0% of 1,134 test fish were dead or died during

the impingement survival tests, indicating few fish

were stunned or damaged. Only minor damage was

observed on fish classified as alive, which was

consistent with results from the NOS impingement

monitoring study classifying that 2.4% of the fishremoved from the rotary disk screen were damaged

(Bigbee, unpublished). Acclimation of both hatchery-

raised and wild, native fish captured for this study

reduced the likelihood of diseased or injured fish

influencing survival through the selection of healthy

fish.

Collection efficiency study.—Tests using live and

freshly killed fish were conducted to evaluate the fate

of fish released into the rotary disk screen bay that were

not recovered. Those tests compared recovery rates

between live fish that could actively avoid the rotarydisk screen and freshly killed fish that were passively

impinged. The same hatchery-raised species used

during the pilot study and impingement survival tests

(fathead minnow, bluegills, and channel catfish) were

used for the collection efficiency study. Fresh-killed

fish received five different fin clips (anal fin, dorsal fin,

left pectoral, right pectoral, and the bottom portion of

the tail fin) that allowed tracking of the released

batches. One group of live fish with anal fin clips was

also introduced so that comparisons could be made

between percent recovery of freshly killed and live fish.Marked fish were introduced to the rotary disk screen

bay at 15-min intervals and were collected from the

screen and processed after each 15-min interval. Live

test fish were enumerated on-site, and dead fish were

1424 BIGBEE ET AL.



taken to the laboratory for processing. The collection

efficiency test was terminated 105 min after fish were

first released.

Collection and test tanks.—A collection tank (2.443

1.98 m, 0.67 m deep) was constructed to receive fishremoved from the rotary disk screen during the pilot

study, survival tests, and collection efficiency study.

The collection tank contained a screen in front of a weir

upstream from the outlet. The weir served to reduce

velocity through the screen to less than 0.15 m/s.

Screen openings in front of the weir matched the 6.1-

mm2 mesh of the rotary disk screen. The collection tank

had a bypass outlet with a removable screen and gate.

Test tanks for bluegills, fathead minnow, and native

fish were 80 L, whereas 120-L tanks were used for

channel catfish (only used for the impingement

survival tests). All test tanks were continuously

supplied with source water from the Missouri River

and monitored during each event to assure that

environmental conditions were adequate to support

the species used in the tests and to minimize external

factors that could affect survival. Water temperature

and dissolved oxygen (DO) was monitored throughout

each 48-h test. Test date and time, species, the number

of fish placed in each tank were recorded. Water

temperature, DO, and flow were recorded at the

beginning of each test. Observation times, water

quality readings, and flow to the tanks were recordedevery 8 h, along with general observations of the

aeration system. Dead fish (those with no opercular

movement and no response to stimuli) were removed at

8-h intervals.

The test tanks included two control groups and four

test groups for each species. The test tanks, which were

set near the collection tank (Figure 2), held approxi-

mately 50 fish, with a goal of limiting fish biomass to

less than 5 g/L of source water to prevent overcrowd-

ing and to ensure adequate DO levels. The test tanks

were conditioned and water temperatures stabilized byregulating source water flow at an exchange rate of at

least one volume every 4 h. Each test tank had an air

stone as a DO source.

Introduction of fish.—Hatchery-raised fish for the

pilot study, two impingement survival tests, and a

collection efficiency study were introduced to the

rotary disk screen bay one species at a time after

grouping the test species fish in a 19-L bucket. The

bucket was attached to a rope and lowered through an

open floor grate. A tether attached to the bottom of the

bucket was used to invert the bucket so that fish werereleased below the water surface. Native fish collected

from the Missouri River for the August impingement

survival test were similarly introduced, but as a batch

of mixed species in order to reduce handling stresses

that would have occurred if the fish had been separated

by species. Impingement rates at the NOS were low

(Bigbee, unpublished), and we assumed that only

released fish were recovered. That assumption was

supported by the fact that only the three hatchery-raisedspecies were collected during each impingement

survival test.

Upon initiation of each test, the rotary disk screen

was shut off and the gate to the bypass outlet was

removed to allow the collection tank to drain. Once the

collection tank was drained, a net was used to remove

fish and debris that had accumulated in the collection

tank. The gate was replaced and the rotary disk screen

restarted. A net was placed in front of the inlet for 2

min, allowing the screen to make four revolutions so

that fish or debris that may have accumulated on the

screen when it was shut down could be removed and

discarded. The collection tank was monitored to

prevent debris from accumulating on the collection

tank screen.

Removal of fish from collection tank .—Fish recov-

ered during the pilot study, survival tests, and

collection efficiency study were removed from the

collection tank with a net that was swept through the

collection tank. The net was kept submerged while fish

were removed with a dip cup. After multiple net

sweeps yielded no fish, screens were placed in front of

both the inlet and bypass outlet and the rotary diskscreen was shut off. The gate to the bypass outlet was

then removed to lower the water level in the collection

tank to approximately 10 cm, at which point remaining

fish were recovered. A dip-cup was used to keep the

fish submerged at all times to reduce stress while

transferring fish from the collection tank to the test

tanks (Figure 2).

Sample processing.—Control and test tanks for the

impingement survival tests were observed approxi-

mately every 4 h during the 48-h impingement survival

tests. During each observation, dead specimens wereremoved, identified, weighed, and measured. At the

conclusion of the 48-h tests, a small submersible pump

was used to withdraw water from the test tanks. When

approximately 4 L of water remained in the test tanks,

all fish were transferred to a smaller bucket for a final

count. The number of surviving fish was then recorded.

Twenty specimens of each species were weighed and

measured (TL) to obtain an average size of the test

population. Counts and batch weights of the remaining

test fish were recorded for the subsequent counts.

Native species that could not be easily identified werepreserved for identification in the laboratory.

Controls.—Fish used as controls for the impinge-

ment survival tests were subjected to the same handling

procedures used for the impinged fish except for




introduction into the screen bay and subsequent

collection from the rotary disk screen. Control fish

were removed from the acclimation tanks and placed in

19-L buckets covered with a fine-mesh screen. Control

fish buckets were then submerged in the screen bay,inverted, and retrieved without releasing the fish. The

control buckets were then submerged in the collection

tank and emptied. Control fish were subsequently

retrieved from the collection tank and transferred to

covered control tanks where they were held for the 48-

h test period. Each control group consisted of

approximately 50 fish. The native controls consisted

of 112 fish composed of six species.

Tests.—Test groups of each species and the native

fish group for the impingement survival tests were

populated with approximately 50 fish/test (range, 49–

78) when recovery yielded sufficient numbers of

impinged fish. Survival of 229 channel catfish, 199

bluegills, and 208 fathead minnow in four batches were

tested during the April tests. The average total lengths

of fish used in the April tests (96 mm for channel

catfish, 76 mm for bluegills, and 56 mm for fathead

minnow) were representative of the size of fish

collected during impingement monitoring at the NOS

(Bigbee, unpublished).

Test fish for the August survival tests included both

hatchery-raised fish and wild, native fish collected from

the Missouri River. Fathead minnow obtained from thehatchery exhibited poor survival during the August

acclimation period and, therefore, were not tested. The

August survival tests of channel catfish, bluegills, and

the native species group were conducted with 37–59

fish/test, except one of the three bluegill tests used just

a single bluegill. The average total lengths of the fish

tested in August (72 mm for channel catfish, 57 mm for

bluegills, and 45 mm for the native fish) were

representative of the size of fish collected during

impingement monitoring at the NOS (Bigbee, unpub-

lished).Test conditions.—The pilot study, impingement

survival tests, and collection efficiency study were

conducted with the cooling water pumps fully

operational at river levels that ranged from 294.7 to

295.9 m above mean sea level. The top of the rotary

disk screen is at 295.0 m. The rotary disk screen was

fully submerged during the pilot study (295.8 m) and

the April (295.7 m) and August (295.9 m) impinge-

ment survival tests, whereas river level was lower

(294.7 m) during the collection efficiency study (water

surface was 0.3 m below the top of the rotary diskscreen). Water temperatures were lowest during the

collection efficiency study (4.58C) compared with

13.58C for the pilot study, 14.08C for the April survival

tests, and 27.08C for the August survival tests. The DO

concentrations in the source water averaged 12.8 mg/L

during the April survival tests (range, 10.4–11.7 mg/L)

and 7.9 mg/L for the August tests (7.4–8.2 mg/L).

Statistical analysis.—Survival data were analyzed

using logistic regression (McCullagh and Nelder 1989)as a function of the treatment (impinged versus control)

and the date of the survival tests. Logistic regression is

a statistical technique that makes the output of a

generalized linear regression model suitable for

modeling survival probabilities. The logistic regression

model used in this study was

logit ð pÞ ¼ logeð p=1 À pÞ¼ b0 þ b1 3 treatment þ b2 3 date;

p ¼ survival probability;

treatment ¼ coding variable for treatment (0 ¼control, 1 ¼ impinged);

date ¼ coding variable for date of test;

b0,b1,b2¼ model coefficients.

Since survival probabilities follow a binomial

distribution, the estimates for the model coefficients

and their associated standard errors were obtained

using a goodness-of-fit test called quasi-likelihood

estimation that allows for overdispersion (i.e., covari-

ance) among survival trials relative to the binomial

distribution (McCullagh and Nelder 1989).Pairwise comparisons among levels of treatment

factors were computed based on asymptotic z-scores of

the model coefficients and their associated estimated

covariances. All analyses were performed using the R

statistical programming language (R Development

Core Team 2007).

Results

Pilot Study

During the pilot study, 433 of the 500 bluegills

(86.6%) were collected within 30 min after they werereleased into the rotary screen bay, and 12 additional

bluegills were recovered after a total of 60 min (89.0%

accumulative); no additional bluegills were recovered

60–120 min after release. For channel catfish, 285 were

recovered within 30 min (57.0%), 12 within 60 min

(59.4%), and 6 additional channel catfish were

collected after 120 min (60.6%); no additional channel

catfish were recovered 120 min after release.

Impingement Survival Tests

Recovery rates (i.e., the number of tested fishcollected after treatment and then held for 48-h survival

observation) for the April survival tests were 49.8% for

bluegills, 23.1% for fathead minnow, and 20.6% for

channel catfish, yielding a sample size of at least 49

1426 BIGBEE ET AL.



fish per species for each test (Table 1). For the August

tests, 19.7% of the channel catfish, 9.6% of the

bluegills, and 15.9% of the native fish that were

released were recovered, yielding at least 37 fish per species for each test except for two bluegill tests (Table

2). Tested fish were recovered within 60–120 min of

release into the rotary disk screen bay for both the April

and August tests.

Impinged fish tested in April exhibited 48-h strong

survival rates similar to control fish: 99% for bluegills

versus 100.0% for controls, 96.1% for channel catfish

versus 90.0% for the controls, and 79.3% for fathead

minnow versus 81.0% for controls (Table 1). Bluegills

had the highest survival rates, and fathead minnow had

the lowest survival rates.

Impinged fish tested in August exhibited 48-h

survival rates similar to or better than control fish:

99.5% for channel catfish versus 100.0% for controls,

91.5% for bluegills versus 89.0% for the controls, and

84.6% for the native group versus 82.1% for the

controls (Table 2). Channel catfish experienced the

highest 48-h survival rate, and the native fish group had

the lowest survival rate. Of the native species tested,

emerald shiner Notropis atherinoides accounted for

96.3% (26 of 27 fish) of the test fish that did not

survive the 48-h test (Table 3). The overall survival rate

of impinged native fish (84.6%) tested in August was

similar to the survival of impinged fathead minnow

(79.3%) in April.

Collection Efficiency Tests

Results from the pilot study and impingement

survival tests raised questions regarding the fate of

unrecovered fish introduced into the rotary disk screen

bay, although recovery rates were generally adequate to

meet the targeted sample sizes for the survival tests.

Tests using both live and freshly killed fish were

conducted to evaluate collection efficiency. We

assumed that freshly killed fish, which were unable

to avoid the rotary disk screen, would have higher

recovery rates than live fish that presumably could

avoid impingement on the rotary disk screen.

A total of 250 channel catfish, 194 fathead minnow,

and 91 bluegills (all freshly killed) were introduced in

groups of 15–50 fish at 15-min time intervals over a

90-min period for the collection efficiency test. The

average total lengths of the released fathead minnow

(55 mm), bluegills (73 mm), and channel catfish (53

mm) were representative of the fish collected during

impingement monitoring at the NOS (Bigbee, unpub-

lished). All of the recovered fathead minnow and

bluegills were collected within 30 min of release, and

all except one channel catfish were collected 45 min

following release. The recovery rate for bluegills

(95.6%) was the highest of the three species: 87

recovered of 91 introduced. The recovery rates for

channel catfish (93.6%) and fathead minnow (94.3%)

were only slightly lower than the recovery rate for

bluegills.

TABLE 1.—Percent of hatchery-reared channel catfish, bluegills, and fathead minnow in control and test tanks that were alive

48 h after being impinged on a rotary disk screen installed in intake 3 at the North Omaha Station during April 2008.

Species

Test type

and tank

Sample

size

Number

alive at 48 h

Survival

at 48 h (%)

Channel catfish Control 1 50 41 82.0Control 2 50 49 98.0Total 100 90 90.0

Test 3 50 49 98.0

Test 4 51 50 98.0Test 5 78 74 94.9

Test 6 50 47 94.0

Total 229 220 96.1Bluegills Control 1 51 51 100.0

Control 2 50 50 100.0Total 101 101 100.0

Test 3 50 48 96.0Test 4 50 50 100.0

Test 5 49 49 100.0

Test 6 50 50 100.0Total 199 197 99.0

Fathead minnow Control 1 50 42 84.0Control 2 50 39 78.0

Total 100 81 81.0

Test 3 53 28 52.8Test 4 51 42 82.4

Test 5 51 47 92.2Test 6 53 48 90.6

Total 208 165 79.3




Discussion

The impingement survival study conducted at the

NOS showed that fish impinged, removed, and

recovered from the rotary disk screen exhibited

survival rates that were not statistically different from

those observed for the controls, suggesting that

impingement on the rotary disk screen did not

contribute to the observed mortality. Study results

were consistent with field (EPRI 2003) and pilot-scale

(EPRI 2006; EPRI 2007) impingement survival studies

of other screen configurations or modifications de-

signed to improve impingement survival. Survival rates

of the species we tested were generally higher thandocumented in field studies (Table 5). The average

survival rate for channel catfish for the only extended

survival study (96 h) reviewed by EPRI (2003) was

93.0%, compared with the average 48-h survival rates

of 96.1% and 99.5% we observed, rates that wouldapproach 100.0% if adjusted for control survival rates.

Although our 48-h rates may not be directly compa-

rable to the 96-h rates of EPRI (2003), delayed

(‘‘extended’’) survival has been traditionally studied

at 24-h, 48-h, or 96-h intervals. The EPRI (2003) report

shows that latent effects of impingement appears to be

greatest between 24 and 48 h after impingement and

levels off rapidly after 48 h.

Ictalurids as a group had an average survival rate of

74.3% for the reviewed field studies (Table 5). None of

the 33 records for ictalurid survival tests were controladjusted, including the 96-h survival rate for channel

catfish. Although survival rates for our study were not

TABLE 4.—Summary of point estimates and 95% confidence intervals for survival of impinged and control fish in 48-h

postimpingement survival tests of fish removed from a rotary disk screen installed in intake 3 at the North Omaha Station, 2008.

Species Treatment

Survival

Estimate Lower bound Upper bound

Channel catfish Control 0.984 0.920 0.997

Impinged 0.990 0.950 0.998

Bluegill Control 0.911 0.810 0.961

Impinged 0.971 0.917 0.990Fathead minnow Control 0.780 0.213 0.979

Impinged 0.795 0.509 0.935Native (wild) fish Control 0.821 0.630 0.925

Impinged 0.850 0.728 0.923

TABLE 3.—Percent of native fish in control and test tanks that were alive 48 h after being impinged on a rotary disk screen

installed in intake 3 at the North Omaha Station during August 2008. The percent of total population is the number impinged

divided by the number of all species impinged.

Species

Number

impinged

Number

alive at 48 h

Survival

at 48 h (%)

Percent of total

population

Controls

Emerald shiner Notropis atherinoides 86 66 76.7 76.8Red shiner Cyprinella lutrensis 19 19 100.0 17.0

River shiner Notropis blennius 4 4 100.0 3.5Temperate bass Morone spp. 1 1 100.0 0.9

Gizzard shad Dorosoma cepedianum 1 1 100.0 0.9

White crappie Pomoxis annularis 1 1 100.0 0.9All species combined 112 92 82.1

Tests

Emerald shiner 115 89 77.4 65.7River shiner 27 27 100.0 15.4

Silver chub Macrhybopsis storeiana 10 10 100.0 5.7

Buffaloes Ictiobus spp. 6 6 100.0 3.4White bass Morone chrysops 5 5 100.0 2.9

Red shiner 4 4 100.0 2.3

Channel catfish Ictalurus punctatus 2 1 50.0 1.1Plains minnow Hybognathus placitus 2 2 100.0 1.1

Gizzard shad 1 1 100.0 0.6

Goldeye Hiodon alosoides 1 1 100.0 0.6Orangespotted sunfish Lepomis humilis 1 1 100.0 0.6

Sand shiner Notropis stramineus 1 1 100.0 0.6

All species combined 175 148 84.6




control-adjusted, statistical analyses suggest that the

observed mortality of channel catfish impinged on the

rotary disk screen was not the result of impingement.

Survival of bluegills from seven extended (24-h and96-h) survival studies ranged from 79% to 100% and

averaged 99.3%; as a family, centrarchids had an

average extended survival rate of 71.1% (Table 5). The

bluegills and centrarchid tests included in the review,

similar to ictalurids, were not control adjusted.

The EPRI (2003) review did not include tests for

fathead minnow, which was the primary cyprinid tested

during the current study, but did include results for

emerald shiners, which had an average 24-h survival

rate of 94.2% (Table 5), compared with the 77.4%

survival rate of emerald shiners in the native fish testsfor the current study. However, the results from our

study indicated that impingement on the rotary disk

screen did not contribute to the observed mortality

because there was no statistical difference between

control and test groups.

Survival rates of fish removed from the rotary disk

screen were compared with results from another 2-year

pilot-scale study of a Geiger multi-disk screen

(hereafter, Geiger screen; another modified screen

system with fish protection features), where fish

impingement rates and subsequent survival were tested.The Geiger screen is composed of circulating sickle-

shaped mesh panels connected to a revolving chain

installed across an intake (EPRI 2007). Prior to its

installation at the Potomac River Generating Station,

the Geiger screen had only been used for debris

handling at the D.C. Cook Nuclear Station on Lake

Michigan (Peltier 2004). Ten Geiger screens were

installed at the Potomac River facility to improve

removal of debris and aquatic weed loads. One of the

Geiger screens was provided with fish protection

features so that impingement survival could beevaluated. Fish protection features included a fish

bucket for carrying submersed fish to a return trough, a

low-pressure through-screen wash, and external wash

to transfer impinged fish to the fish bucket. The screen

panels were drilled plastic with 9.5-mm openings,

which were larger than the 6.1-mm2

screen openings

on the rotary disk screen we evaluated.

The 2-year study of the modified Geiger screen

demonstrated the difficulty in obtaining reliablesamples for survival tests when relying on ambient

impingement rates. Most species were impinged at low

rates that limited the number of fish available for

survival testing. Only 4 of 20 impinged species were

collected in numbers greater than 100 during the 2-year

study. Bluegills and channel catfish were collected in

sufficient numbers during the Geiger screen study to

evaluate impingement survival. Bluegills exhibited

annual survival rates of 94% and 95%, and channel

catfish had annual survival rates of 50.0% and 100.0%.

Because almost all fish were collected during and

immediately following major runoff events, reduced

water quality during these events may have compro-

mised survival results during the study (EPRI 2007).

Survival rates on rotary disk screen tested during our

study were higher than survival rates of fish impinged

on the Geiger screen.

We also compared the survival rates from our study

to those from a recent laboratory evaluation of

modified Ristroph vertical traveling screens (hereafter,

Ristroph screens) because the study also used hatchery

fish. Ristroph screens are conventional vertical travel-

ing screens equipped with fish buckets attached to thescreens. They have low-pressure spray washes to

remove fish, separate debris and fish returns, and a

continuous screen operation. The 48-h survival of

fathead minnow, channel catfish, and bluegills had

survival rates of at least 95% at three different approach

velocities following removal from the Ristroph screens

(EPRI 2006). As in our study, test fish for the Ristroph

screen were obtained from hatcheries and therefore

may have been in better health than wild fish from the

source water, based on a recent impingement study that

incorporated a health assessment of both impinged fishand fish collected from the source water (Baker 2007;

Knight 2008). That study showed that the majority of

impinged fish were in poorer health than nonimpinged

fish. Based on those results and the assumption that

site-specific environmental factors (e.g., water quality,

water temperature, and turbulence) could reduce

impingement survival, the use of hatchery fish may

overestimate survival. However, a comparison of

results from our study with results from field studies

indicate that the survival estimates achieved using

hatchery-raised fish are within the range of survivalobserved in the field and may more accurately estimate

the ability of the rotary disk screen to return fish to the

Missouri River.

Monitoring of DO and water temperature during the

TABLE 5.—Summary of extended (24-h to 96-h) post-

impingement survival rates (%) from historical field studies

conducted at other facilities for those fish taxa that were

impinged on the rotary disk screen installed in intake 3 at the

North Omaha Station. Source: Appendix B of EPRI (2003).

Taxon Average Minimum Maximum SD

Ictaluridae 74.3 20.0 100.0 10.8

Channel catfish 93.0Centrarchidae 71.1 0.00 100.0 35.4

Bluegill 99.3 79.0 100.0 8.2

Cyprinidae 67.2 0.00 100.0 34.2Emerald shiner 94.1 7.7 97.5 40.5

1430 BIGBEE ET AL.



survival tests showed that environmental conditions in

the control and test tanks were stable and probably did

not affect 48-h survival rates in either the control or test

groups. Although variations in river levels and water

temperatures observed during the study potentiallyinfluenced recovery of released fish, they did not

explain the lower recovery rates during both the April

and August impingement survival tests compared with

recovery rates during the pilot study.

The survival rates documented in our study may also

be due to the continuous screen rotation that reduces

the retention time of fish on the rotary disk screen and

the use of a vacuum to remove impinged fish from the

screen. Retention time on the rotary disk screen is 30 s

or less, depending on position of the impinged object

on the screen. In comparison, retention time on vertical

traveling screens can range from a few seconds to

several minutes, assuming continuous screen operation

and depending on where on the screen fish are

impinged. Retention time for the Ristroph laboratory

study was about 40 s, based on a rotation speed of 2.4

m/s and an assumption that fish were impinged at the

deepest point of the screen (EPRI 2006). Retention

time was not provided for the Geiger screen study.

The screens evaluated for the Ristroph and Geiger

screening systems were operated continuously, as in

our study, whereas conventional vertical traveling

screens are typically operated intermittently based onpressure differentials across the screen, which increases

the retention time of impinged fish on the screens. Fish

impinged on the rotary disk screen remain immersed in

water prior to removal from the screen so they are not

exposed to air, an obvious source of stress, as is the use

of spray washes, especially high pressure washes

typical of the conventional traveling screens currently

at the NOS.

Survival tests of fish removed from the rotary disk

screen demonstrated that introduction of fish in front of

the screen was an effective alternative to relying onimpingement rates to provide adequate sample sizes for

survival tests, especially when impingement rates are

low and unpredictable, as they were at the NOS. It

allowed for scheduling tests and reduced the number of

visits that would have been required if the tests were

based solely on ‘‘ambient ’’ impingement rates. The

pilot study provided a reliable method for introducing

fish to the screen bay that yielded an adequate number

of fish for valid statistical analyses of the test results.

Percent recovery of introduced fish for the April and

August survival tests was lower than that observed inboth the pilot study and collection efficiency tests. All

fish used in the pilot study, survival tests, and

collection efficiency study were similar in size and

the same procedures were used to introduce test fish

into the rotary disk screen bay. The lowest recovery

rate occurred during the August tests when swimming

performance should have improved due to water

temperatures that were 13.18C warmer than during

the April tests.Fish could have avoided impingement on the rotary

disk screen, based on the calculated approach velocities

that range from 0.06 m/s at a high-river stage (304 m)

to 0.46 m/s at a low-river stage (294 m). The calculated

approach velocity at the normal river stage (296 m) is

0.16 m/s, which is close to the 0.15 m/s through-screen

velocity recommended by the EPA in its Phase II

316(b) rule (U.S. Office of the Federal Register 2004).

The calculated approach velocities are the average

water velocity found at 5–8 cm in front of the screen

taken in the same direction as the general flow (EPRI

2000). They are representative of the area between the

screen and intake bay inlet and are the velocities

experienced by fish as they swim near the screen,

which lower than through-screen velocities (i.e., the

velocity of water as it passes through the screen; EPRI

2000). Through-screen velocities would be experienced

only by fish when they are at the face of the screen and

are probably not as important a factor as approach

velocity, but may affect whether impinged fish can

remove themselves from the screen once impinged

(EPRI 2000). A review of swimming capabilities of

freshwater fish indicate fish can avoid impingement at approach velocities of 0.15–0.30 m/s (EPRI 2000),

particularly when exposed for short periods (120 min),

such as during our tests. Approach velocities during the

pilot study and both impingement survival tests ranged

from 0.179 to 0.191 m/s. Higher approach velocities

(0.265 m/s) occurred during the collection efficiency

study, when river level was 1.2–1.3 m lower than

during the pilot study and survival tests.

The high recovery rate of dead fish during the

collection efficiency study indicates that unrecovered

live fish probably sought refuge in the screen bay awayfrom the rotary disk screen or moved out of the screen

bay into the Missouri River. The small mesh (6.1-mm2

screen openings) of the rotary disk screen should have

retained all test fish; thus, it is not likely that fish were

extruded through the screen and its installation

prevented fish from passing between the screen and

bay walls without being removed by the screen. In

addition, the bay was blocked to prevent fish from

finding refuge in an adjacent intake bay.

Factors contributing to differences in recovery rates

of released fish during the pilot study, impingement survival tests, and collection efficiency study are

unclear. Only live fish were released during the pilot

study and survival tests, whereas both live and freshly

killed fish were released during the collection efficien-




cy study. As shown by the collection efficiency study,

recovery rates of freshly killed fish approached 100%,

as anticipated, and recovery was better than for live

fish, which could avoid the rotary disk screen. All fish

were released consistently during each test, followingthe same procedures by the same personnel. The

release depths relative to the top of the rotary disk

screen (295.0 m) for the April and August survival tests

varied by about 0.1 m, and fish were released at depths

that were 0.6–0.9 m above the top of the rotary disk

screen. Although fish were released closer to the top of

the rotary disk screen during the April tests (approx-

imately 0.6 m), the resultant recovery was lower than

for the pilot study. Recovery rates in August were even

lower than in both the pilot study and April survival

tests when fish were released about 0.9 m above the top

of the rotary disk screen.

Water temperatures during the tests were also

examined to evaluate whether differences among tests

could have affected swimming performance, possibly

accounting for the apparent avoidance of the rotary

disk screen in April and August compared with the

pilot study. Water temperatures were similar during the

pilot study (13.58C) and April (14.08C) survival tests,

much higher during the August tests (27.08C), and

much lower during the collection efficiency tests

(4.58C). Differences in recovery between the pilot

study and the April survival tests were probably not due to water temperature, which varied by 0.58C. More

fish may have avoided the rotary disk screen because

of the warmer temperature in August and because fish

were released further above the top of the rotary disk

screen than during the pilot study and the April

survival tests. Cooler water temperatures during the

collection efficiency study probably reduced swim-

ming performance and thereby contributed to the

generally higher recovery rates of live fish compared

with recoveries during the pilot study and survival

tests. Additionally, the river level was about 0.4 mbelow the top of the rotary disk screen during the

efficiency study, indicating higher approach velocities

than during the pilot study and survival tests, albeit the

volume of cooling water pumped through the screen

was the same during all three studies.

The impingement survival studies were not per-

formed during the winter season since impingement

rates are either low (which is the case on the Missouri

River) or if high, have been found to be primarily

associated with cold-stressed or moribund clupeids

(EPRI 2000).We also examined the size of the fish tested to see

whether recovery rates could be related to swimming

performance. However, even though mean total lengths

were greater for channel catfish (97 mm) and bluegills

(76 mm) in April than during the pilot study (76 and 52

mm, respectively), it is not likely that those size

differences (20–24 mm) would account for recoveries

that were 35–39% lower, especially because they were

released closer to the rotary disk screen than during thepilot study. The mean length of fish released during the

August survival tests were 21–24 mm shorter than in

April and were similar to the mean lengths of test fish

used in the pilot study. However, the August fish were

released about 0.9 m above the top of the rotary disk

screen and the low recovery rates suggest fish were

able to avoid impingement.

The collection efficiency study results support the

hypothesis that unrecovered fish avoided impingement,

based on the 94–96% recovery rates of the freshly

killed fish, but those results were not particularly useful

for explaining the lower recovery during the survival

tests. The relatively good recovery of live fish during

the collection efficiency study was probably due to the

cold water (4.58C) and low river levels that were about

0.4 m below the top of the rotary disk screen. The

recovery rate for live channel catfish during the

collection efficiency study (82.0%) was much better

than for the other tests and their smaller mean total

length (53 mm) may have contributed to that recovery

rate. In contrast, the recovery rate of live bluegills

(70.0%) was lower during the collection efficiency

study than during the pilot test; those fish were larger (73 mm) than those in the pilot study (52 mm), which

could have contributed to the different recoveries.

Overall, the results suggest that, when introducing fish

in front of screens to examine impingement survival,

the release point should be adjusted with water level so

that fish are released at a consistent depth relative to the

center of the screen; this would assure good recovery of

test fish.

The use of hatchery-raised fish and wild, native fish

collected from the source water for the survivability

tests allowed clear interpretation of the results becausetargeted sample sizes for the survival tests and controls

were met and allowed valid statistical analyses. Study

results demonstrated that the absence of controls in

survivability tests can overestimate mortality or

underestimate survival. Use of controls allowed

evaluation of factors that could overestimate impinge-

ment mortality, including handling mortality, and

thereby provided accounting for prior moribund fish

and interactions of sampling stress and holding times

(EPRI 2005). Controls allowed for an adequate

evaluation of the screen’s ability to collect and returnfish to the river. Impinged wild fish probably reflect

site-specific environmental stress factors that poten-

tially increase impingement mortality; however, a

comparison of these pilot-scale results with observed

1432 BIGBEE ET AL.



field results indicate that the nominal survival rates

observed in the rotary disk screen pilot-scale study is

within the range of results from field studies that were

not adjusted for control mortality. The lack of statistical

differences between impingement and control survivalrates indicate that fish removed from the rotary disk

screen were unaffected by impingement.

Acknowledgments

The authors acknowledge the support provided by

Omaha Public Power District (OPPD), the corporate

sponsors and their technical staff, as well as personnel

at the North Omaha Station that provided site access

and security. They include Ron Stohlmann, Karen

Belek, Russ Baker, Igor Cherko, Nate Staroscik, and

Eric Pohl. We also acknowledge the technical and

managerial support provided by Greg Seegert of EA

Engineering, Science, and Technology, Inc. Field and

office staff that supported this effort included Mitch

Wallman, Jamie Suing, and Ben Carlson. We also

acknowledge the support provided by reviewers of the

draft paper including Dave Michaud (We Energy), Bob

Reider (DTE Energy), Casey Knight (Alabama Power),

Ron Stohlmann (OPPD), Ray Tuttle and Jon Black

(Alden Research Laboratory).

References

Baker, J. 2007. Health of fish impinged on cooling-water

intake screens. Master’s thesis. Auburn University,

Auburn, Alabama.

Beak (Beak Consultants). 1988. Dunkirk station biological

studies. Final report (January–December) prepared for

Niagara Mohawk, Lancaster, New York.

CWA (Clean Water Act). 1972. U.S. Code, volume 33,

section 1326(b).

EPRI (Electric Power Research Institute). 2000. Technical

evaluation of the utility of intake approach velocity as an

indicator of potential adverse environmental impact

under Clean Water Act section 316(b). EPRI, Report

1000731, Palo Alto, California. Available: epri.com.(April 2009).

EPRI (Electric Power Research Institute). 2003. Evaluating

the effects of power plant operations on aquatic

communities: summary of impingement survival studies.

EPRI, Report 1007821, Palo Alto, California. Available:

epri.com. (April 2009).

EPRI (Electric Power Research Institute). 2005. Impingement

and entrainment survival studies technical support

document. EPRI, Report 1011278, Palo Alto, California.

Available: epri.com. (April 2009).EPRI (Electric Power Research Institute). 2006. Laboratory

evaluation of modified Ristroph traveling screens for

protecting fish at cooling water intakes. EPRI, Report

1013238, Palo Alto, California. Available: epri.com.

(April 2009).

EPRI (Electric Power Research Institute). 2007. Latent

impingement mortality assessment of the Geiger Multi-

DiscTM screening system at the Potomac River generat-

ing station. EPRI, Report 1013065, Palo Alto, California.

Available: epri.com. (April 2009).

King, L. R., J. B. Hutchison, and T. G. Huggins. 1978.

Impingement survival studies on white perch, striped

bass, and Atlantic tomcod at three Hudson River power plants. Pages 217–223 in L. Jensen, editor. Fourth

national workshop on entrainment and impingement.

Ecological Analysts, Melville, New York.

Knight, A. C. 2008. General fish health assessment and age

evaluation of impinged fish at steam generating power

plants. Master’s thesis. Auburn University, Auburn,

Alabama.

McCullagh, P., and J. A. Nelder. 1989. Generalized linear

models, 2nd edition. Chapman and Hall, New York.

Peltier, R. 2004. Multidisc passes screen test. POWER (May):

32–36.

R Development Core Team. 2007. R: a language and

environment for statistical computing. R Foundation for Statistical Computing, Vienna. Available: R-project.org.

(November 2008).

Tatham, T. R., D. L. Thomas, and G. J. Miller. 1978. Survival

of fishes and macroinvertebrates impinged at Oyster

Creek generating station. Pages 235–243 in L. Jensen,

editor. Fourth national workshop on entrainment and

impingement. Ecological Analysts, Melville, New York.

U.S. Office of the Federal Register. 2004. National pollutant

discharge elimination system—final regulations to estab-

lish requirements for cooling water intake structures at

phase II existing facilities. Federal Register 69:131(9 July

2004): 41576–41624.

U.S. Office of the Federal Register. 2007. National pollutant discharge elimination system—suspension of regulations

establishing requirements for cooling water intake

structures at phase II existing facilities. Federal Register

72:130(9 July 2007):37107–37109.


american fisheries society - beaudrey study - bigbee et al

Documents