2~10

FP-P5-wJ

In: EA. 1978. Fourth National Workshop on

Entrainment and Impingement. December 5 , 1977.

Chicago, IL. L. D. Johnson (Ed.

A Practical Intake Screen which Substantially

Reduces the Entrainment and hnpingement of

Early Life Stages of Fish

Brian N. Hanson, William H. Bason

Barry E. Beitz, and Kevin E. Charles

Ichthyological Associates, Inc.

R. D. Box 286

Middletown, Delaware 19709

INTRODUCTION

The objective of this study was to develop a surface

water intake compatible with power plant operation and

capable of protecting the early life stages' of fishes. Review

of existing technology indicated profIle wire screen (Figure I) possessed the greatest potential to meet the desired

objectives. Screens were purchased from Johnson Division

of Universal Oil Products, and this report evaluates their

potential as a low-impact surface water intake screen.

Studies were initially directed toward the determination

of entrainment and impingement of striped bass eggs,

larvae, and young by a profile wire screen intake. Studies

. were expanded to include other fishes, egg mortality resulting from impingement, biological and detrital fouling

rates, in situ long-term testing, and cleaning techniques.

Because of the practical and applied nature of the work

all experiments were designed to provide a go/no-go result.

If the screen failed to meet any biological or engineering

requirement, experimentation would have ceased because

of the lack of improvement over existing technology.

MATERIALS AND METHODS

Test Apparatus

Most experiments were conducted in a 30-X- 15-ft oval

flume (Figure 2). The aluminum and plywood channel

was 33 in. wide and 48 in. deep. A 4-X- 4-ft sump

attached to the flume s inner wall served as the site for

!he model intake. Water was pumped from the sump by a

S-hp horizontal turbine pump, which initiated a gravity

flow from the flume channel through the test screen. The

maximum sustainable pump rate was 505 gpm. Pump rate

was measured with an Envirotech Sparling water meter

and controlled by butterfly valves. Discharge reentered

the flume opposite the sump box. Total volume of the

facility at maximum depth (40 in.) was 5,472 gallons.

A single 83.in. diameter paddle wheel with six

27- 40-in. blades was used to generate flume currents.

The wheel was driven by a 3-hp electric motor tluough a

variable speed hydraulic transmission coupled to a 20 : I

right angle gear reduction. Water velocity was measured

with a Marsh-McBimey Model 201 electromagnetic current

" meter.

Egg mortality studies were conducted in a IO-Xoval flume. The channel was 12 in. wide and 17 in. deep.

A 36-in. diameter paddle wheel with six 17- Il-in. blades

was used to generate current. The wheel was driven by a

1/2-hp variable speed motor coupled to a variable speed

hydraulic transmission. Total volume" of the facility was

239. 1 gallons..

A test cage (29. 22. 28. 5 in.) was used to keep

~est specimens in the proximity of the intake during

l ~xperimental runs in static mode (channel velocity = 0).

--rIle front panel was plexiglass; the sides and bottom were

500-micron mesh nytex fabric. The bottom was custom

fit to expose the specimens to only the top half of the

screen (Figure 6). The bottom portion of the screen was

frequently covered to reduce the effective open area and

increase the maximum intake velocity.

Since the exclusion of striped bass eggs and larvae was

the major objective, s-mriWtd.tm':of':o.~04Q:in.andlesswere

;~~r

,)

AlI cylindrical screens viet~" lTih"':'lff' diamet~r

393
394 Fourth National Workshop on Entrainment and Impingement

Figure 1. Schematic diagram of a cylindrical Johnson

screen with detailed enlargement showing

profile shape of wire. The screen is con.

structed wiLh a single continuous wrap of

wire (A = internal support rod, B = wire).

i \'('i'Im

Egg Mortality

. Striped bass eggs were supplied from an onsite hatchery.

All eggs were incubated in a solution of50,000 IU/I

penicillin-G and 50 mg/I streptomycin sulfate for at least

15 hours before testing. Each test consisted of a 30-, 60-

G -

Figure 2. Schematic diagram of the large flume

(A = channel, B = sump, C = screen

D = intake pump, E = paddle wheel

F = paddle wheel drive, G = hydraulic

backwash pump, H = air compressor).

and 120-second impingement trial of live eggs with one

replicate and one control for each duration. Any given test

used eggs from a single brood. Natural dieoff rates were

monitored with long-term controls, which were accomplished by holding live eggs in a gallon jar of antiseptic solution. Eggs were aerated and held until the brood's testing

was completed. Total dead eggs were counted and recorded

every 30 minutes. Eggs were then preserved in a formalinrose bengal solution for microscopic examination.

Impingement trials and replicates proceeded in the following manner. An unknown number oflive eggs were released a t ~q"4~p,!b.,jA!9f,~~rp~1(!i,i~~r~$if~~1bf

the test screen~ After impingement for the prescribed duration, Lhe current was stopped. Eggs were siphoned from

the screen into a gallon jar partially filled with antiseptic

solution. Samples were inspected for mortality at 5, 20,

, and 60 minutes for the first two tests; thereafter inspection was made at 5, 30, and 60 minutes. Samples were

preserved after I hour.
Trial controls were held in flume water for the appropriate

time period and then siphoned into a holding jar filled with

the antiseptic solution. Mortality was monitored for 1 hour

and the samples preserved. Preserved samples were examined With a Bausch & Lomb stereo zoom dissecting scope.

. Developmental stage was determined and the live and dead

ratio detennined. Eggs were termed dead when they contained abnormal or disintegrating embryonic material, an

emulsified oil globule, or translucent perivitelline space.

The smallest and largest eggs in each sample were measured.

Egg-Screen Interaction

Egg impingement studies were conducted in the large

flume. Tests were designed to evaluate egg-screen interactions at various channe e.ocitie Approximately 300

preserved t!RGG!! .:1,

~~. C;W""

 

lfi~~~

!;,

~r~ ,0

release4JQJt..upstr~~ th~ '

_,,

1I1., s1oU~~~t\ri opet-

",.h,"r" ",,"..!Ij;j:,

 ,.. ._,,,,

"h"* 

 .

atedatameanintii.k~:yeH5cityof'O;5:Jp,-. 'T4ree experiments of one trial eac ~~re ron at\::hamiei'v~i&Hiesof

io:$d;:9.75tand r'2$'~~ Interactions were fli;:;'e-Jc;;nd'; 1.,

percen tage1~pffi'ge:(ffi~fed. 1D.Ei:O.04O-in.::Slot'sdeeriil

.  ., .. , ,

functioningjatO.s. fp~Was exposed to 15,000 preserve

e~;;.~acl=~ili€Wffect of various channel velocities

(0. 6 fps) on impingement. Changes oflocation and

number of impinged eggs were monitored for each of the

four test velocities.

The value of an air bubble curtain as an impingement

preventative and cleaning mechanism was investigated at

channel velocities of 0.5 and 2.0 ips. Compressed air was

released through a 12-in. long perforated nozzle at 10 psi.

The nozzle was positioned to allow the bubble curtain to

strike tl-J.e leading side of tlJ.e screen.

Approximately 500 ml of preserved eggs (2-3 mm

diameter) were dispersed in the large flume at a channel

velocity 0 , e.nt. Intake was

initiated throu~ a 12-m. ;s~~iAd hole at

500 gpm for 10 minutes. Entrained eggs were collected

in the sump with a SOD-micron mesh net, and their

volume was measured.

The O.04O-in. slot screen was installed and the above

procedure repeated. In addition, eggs still in the flume

were removed; those impinged on the screen were recovered in a 32- 60-in., 500-micron mesh net. The volume

ofimpinged eggs was measured and used to determine

the percentage impinged.

Striped Bass Larvae

Swimming ability and avoidance behavior of striped

bass larvae exposed to a functioning screen were studied

in a static mode. Specimens were obtained from the

onsi te ha tchery \.T.~g:j1~kSof ~sli,"1and 5 days old, were

held in separate 20-gaIIon aquaria and fed brine shrimp.

Daily tests of each stock were made for 16 days.

Hanson etal. : A Practical Intake Screen 395

Fish in groups of 50 or less were acclimated to flume

temperatures for up to 3 hours and released into the test

area formed by the screen and test cage. Mter a 2-minute

acclimation to

!l!e e~.

~~osure, a

:

~~f,~~jty:1

~: ~~f~:?O, fPJl)'cw~1Jlijtj,~~~;h~l,~~. ":i 9:4Wgi'~t1

'scr~~'i!hJ~ Swimming ability and behavior oflarvae were

. noted until all specimens were entrained (usually less

than 5 minutes). Specimens were recovered in a 500-

micron mesh net placed over the screen discharge pipe.

Total length of the largest and smallest specimens and of

all dead fish was recorded. Survivors of each test were

held in a separate 20.gallon aquarium for later experiments.

M~~r

s!~~$.~~ste;..,~;;:imens was depleted

,allStiJYlvo~(f~~1t~2y,~1~141\were' transferred to a 500-

ga1rori' lanksupplied with unf1ltered pond water and used

in subsequent tests. These fish exhibited greater resistance

to entrainment, and consequently tests were terminated

after 30 minutes. Inta:ke\,eJocitie~range(J.frOII1, ();5;tQ-,

ffiJlJ~, Impingeme~t&thr1~a~~~~flsh-~in impinged

, 

number entrained, behavior, and swimming ability were

recorded. Fish-min denotes t.i.e sum of t.i.e products 

the number of fish and the time exposed to any event.

~ry~~w.e.:.J;~fSCO~ x g;!,~e1js p~(;!J1ieriS' i~athe di. '

., ,

. Y.soK~ge':'(llPj!.~J1Jnl1,l., !.L).

Fish Impingement

Fish were collected by seine from the Bohemia River,

Chesapeake and Delaware (C&D) Canal, Delaware River

and nearby freshwater ponds. Striped bass were also

supplied by the onsite hatchery.

Test specimens were transported in insulated, aerated

containers and held in two 6- ft deep plywood

boxes continuously supplied with flume water. Water was

pumpedthrougb. a sand filter to the holding boxes and

drained throu standpipes back to the flume. Some \~~m~ ~:..s!lf~~~1t) were held in a 500-

gallon tank and 42S-gallon swimming pool. These

facilities were supplied with water from a nearby pond.

Except for striped bass, specimens were not fed prior 

testing.

Specimens acquired from brackish waters were

acclimated (4-16 hours depending on salinity) to flume

water in insulated, aerated containers prior to testing.

Flume salinity was raised to 4 ppt during the summer to

minimize holding and acclimation mortality. This limited

the osmotic stress on wea:.lcer and more sensitive species as

well as on migrant marine fishes. Before testing, specimens

held in flume water were allowed to adjust to the test cage

for 5-20 minutes; fish held in other facilities were acclimated for as long as 60 minutes. Tw~ differen re'used~~rif1g

iiiipift~mli1

, . .

pr~ ~

   , , ,,,, , ,..

ce: d ure ~' , 1. 'Ie, , p 1JWPN~'as st 1Ii;!~9; a,t l~,pt~ ~! ;~~t~ ,. iiPd ,tbg;;.~ lf~.c is
396 Fourth National Workshop on Entrainment and Impingement

~ noted. Intake velocity was t.~en increased approximately

6 0.2 fps at lO-minute intervals until the maximum rate was

reached. Starting intake velocity ranged between 0.0 and

i;; 1.3 fps. The second procedure was similar except the pump

; 

rate was held constant for 30 minutes. Three intake

velocities (0. , 1.0, and 1.5 fps) were tested. Specimens

(i. were continually monitored for impingement, entrainment,

and behavior. Upon test termination all specimens were

" measured (FL).

~ Far-field effects of the screen on seven species were

~ investigated by increasing the size of the test area to 6 ft

~, of flume channel centered around the O. OIO-in. slot screen.

~; Intake velocity was held at 1.0 fps for 10 minutes then inf. creased to 1.25 fps for IS minutes. After 25 minutes , par-

~ titions were removed and fish were allowed to swim freely

~: about the flume for 13 minutes. A 1.00fps channel current

~ was then initiated and maintained for 20 minutes. Velocity

~ was increased to 1.35 fps for an additional 8 minutes, after

, which the test was t~lminated. 

Irt;.!l~amiCIJ:r , fIsh were acclimated for 15-

minut~fif":"

plastic bag suspended in the

channel 10 ft upstream of the intake. Current of a

selected velocity was generated in the flume and the pump

started. Specimens were released and their behavior noted,

especially on the first pass b)' t.lJ.e intake. . ~uifi.,i~\y~~9

(, /,

~ ~4~ row 

J~~, J:!~ J:il;x ~1 Q(; tM t~J,~h ~~~i? 

'J",'.j;

?

oo.J ~1;;ID taf((b:ve Q.ci~"L~cqI1~JI!!lJzt;:tfte r 10. minute .

exposure at 2.00 ips, current in the flume was reduced to

zero. Fish were exposed to Lite functioning inta.1ce for 

additional 5 minutes.

An index was devised to compare the relative susceptibility of each species to a functioning Johnson screen and

is given by the formula:

Su bilit In

fIO- r FM seep y ex - L 10 

LTFM

where

1:1 10 is the number of impingement occurrences

ES is LlJ.e number of escapes

FM is fish-min impinged

TFM is total fish-min exposed.

The index can range from 0 to 2 and is sensitive to differences in behavior and swimming ability.

Detrital Fouling

Detrital fouling studies have been conducted in the large

flume from October 1976 through the present. Effects and

interrelationships of channel current, intake velocity,

detrital load, and screen slot size have been examined.

Effectiveness of hydraulic backwash, compressed air

(1)

backwash, and continuous cleaning with bubbles also have

been investigated. Two types of detrital material, peat

moss and detritus from C&D Canal ichthyoplankton

samples, have been used. Detritus has been renewed

periodically because particle size was eventually reduced

by pumps and cleaning activities.

All tests have been run at flume levels of 30-35 in. Insofar as possible, flume volume has been held constant for

each series of experiments. Th~fflcfs-gf'firt1llCe~vel~

\ 

(();41j7f( ~~IWfiie1&~10C1~~Oi2S~1i7 S' fi ' s Y"aff(file tri t81

I~~dn~5'~~3'7~'3' I!!;:l3;'l~r!t3)~f)~1l~s~r~~~i p~r-r;)1ifi'fuce

have been evaluated.

A selected wet volume of test material was dispersed in

the flume before testing began. Channel current was maintained at a predetermined velocity. The screen was cleaned

by backwashing and/or brushing immediately prior to the

test. Time-to-clog (TIC), head differential versus time, and

visual observations have been recorded for each run. 

head differential of 12 in. in the sump has been considered

the endpoint of each test. Tests have usually terminated

in 4 hours if substantial head differential has not been

generated. When the volume of detritus required to clog

the screen has equaled a substantial part of the test load,

an additional 16 oz of material has been added 10 minutes

after initiation of the test.

An air bubble curtain identical to thatused in egg impingement studies has been evaluated as a fouling preventative. The nozzle is placed so that air bubbles sweep up

across the leading surface of the screen. TIC and fouling

pattems have been compared to tests conducted under the

same parameters but without the bubbler system.

Hydraulic backwash normally has been used to clean

the screen prior to testing. A 610-gpmHomelite trash

pump supplies a reverse flow of 500 gpm through th.e test

screen (Figure 2). A1t.~oug.~ not strictly documented,

hydraulic backwash efficiency has been observed and noted.

Oeaning effectiveness of air/hydraulic backwash has been

investigated. Each test consisted of a comparison of TTC

after thorough manual cleaning and after air/hydraulic backwash. Manual cleaning has been performed with a stiff

bristled brush and is considered the basic cleaning technique. Compressed air was accum'ulated in a 2- fe vessel

equipped with a 4-in. butterfly valve. Backwash has been

accomplished by closing the 12-in. intake butterfly valve and

instantaneously releasing air into the screen through a 6-

length of 4-in. PVC pipe. Two concentrations of detritus

were used in the tests.

RESULTS AND DISCUSSION

Dynamics

The initial phase of thestudy consisted of refining the

experimental apparatus and determining the hydrodynamics
$"I'IP c h4Md

Sc,...",

, . ., ,

""" ........ c"

...

t Ve/oclt.'J

UrTOCe.S

Figure 3. Schematic diagram of flow patterns

associated with a Johnson screen

(provided by Lee Cook, Universal

Oil Products).

of the test screens. Information provided by Johnson

Division of Universal Oil Products, Inc. (personal communication, lee Cook) indicated angle and velocity of

approach should decrease from the proximal to the distal

end of the cylindrical screen (Figure 3). Current measurement and behavior of test specimens confirmed this.

As expected, measured approach velocity was always

less than calculated intake velocity. Actual intake velocity

could not be measured for comparison with calculated

values since the available current meters could not operate

at the screen surface. Theoretica!1y, approach velocity

at one wire width from the screen surface is equal to the

product of intake velocity and the open area ratio.

Approach velocity then decreases as a function of the

square of the radius. Approach velocity 3 in. from the

screen surface at 455 gpm was 0.25, 0. 27, and 0.34 fps

measured equidistant from the distal to the proximal end.

The calculated mean intake velocity for this rate was

1.5 fps. The presence of higher approach velocity at the

proximal end was verified by egg and detrital impingement patterns.

Visual observation in the dynanrl,c mode indicated the

screen s influence was restricted to that portion of the

water column that would have passed through or within

2 in. of the screen. Impingement or entrainment of small

suspended particles was most common along the leading

edge of t.lIe screen. This phenomenon was caused by

channel current and enhanced by pump rate. Particles

that approached the screen more obliquely were generally

Hanson et ai. : A Practical Intake Screen 397

deflected by a boundary current. Deflection was augmented by the cylindrical shape of the screen. The

buildup of impinged material was greatest on the proximal

t'Iird of tfJ.e upstream edge where intake velocity was

hig.hest. Materia! impinged elsewhere on t.lIe screen was

not held as tightly and tended to move to the downstream side. Eddy and channel currents in this area

produced a washing effect, which increased with increasing

c.hannel velocity.

Egg-Screen Interaction

Egg-screen interaction tests revealed two distinct areas

of impingement on the screen. Screen orientation to flow

exposed eggs to direct impact and possible impingement

on the leading edge of the screen; eddy currents resulted.

in a secondary area of impingement in t.l-J.e proximal onethird of the trailing edge.

Impingement on the leading edge was a function of

channel velocity but was self-limiting above 2.0 fps. At

velocities greater than 2.0 fps , all available surface area

was rapidly occupied; thereafter, a slow rate of interchange,

approximately 1% per minute, accounted for all new impingement. A very low percentage of the eggs exposed

impinged on the leading edge. This portion of the screen

was the only area where entrainment and wedging in the

slots occurred. Entrainment and wedging in slots is

apparently caused by pressure generated by channel

velocity rather than intake current. .

Impingement on the trailing edge was maximum when

channel velocity was equal to or less than intake velocity.

Eggs in this area were loosely held as evidenced by their

movements on the screen surface. As much as 20% of

the passing eggs impinged on the leading edge, but most

(95%) rolled over the screen and gathered in the proximal

portion of tfJ.e trailing edge. . At higher channel velocities

the number of eggs present in this area approached zero.

Entrainment or wedging in the slots was never observed

in this region.

Incorporation of a bubble curtain was effective in reducing egg impingement. Eggs did not impinge on areas

of the screen that were struck by bubbles. The bubble

curtain also removed previously impinged eggs. Entrained

bubbles were vented in the sump and prevented pump

cavitation. Results indicate impingement of eggs can be

virtually eliminated by a bubble curtain. Reorienting the

screen so that the long axis was parallel to the current

would also have virtually eliminated impingement and

entrainment. In addition, hydraulic problems associated

with the leading and trailing edges would be eliminated

and the screen should effectively exclude particles as small

as one-half the slot size because of the coanda effect.

Entrainment was primarily associated with the upstream

side of the screen. Less than 0. 1 % of all eggs exposed
..

..

. 

.

Table 1 Summary of Striped Bass Egg Mortality Resulting from Impingement on a 0.020-in.

Slot Johnson Screen at a 0. fps Approach Velocity

Age Diameter Temperature

Developmental Stage (hr) (mm). (C)

...s::sr.

Late gastrula-early embryo (LG-EE) 18.0-31. 1.8- 15. 22.

Tailbud.free (TBF) 21.5-22. 22.

F1,1l1y developed embryo (FDE) 36. 38. 20. 21.

Total 18. 38. 1.8- 15. 22.

:::to

Devel. 30-Second 60-Second 120.Second Totals Long.term

Stage Test Control Differ. Test Control Differ. Test Control Differ. Control Test Control Differ. All ;::s

La- EE

Trials

;;!

Dead n 123 202

...

% Dead 4.3 1.6 c:o

Total n 813 478 763 498 669 431 296 245 1,407 948

't;j

TBF

Trials 

;;!

Dead n

...

% Dead 1.1 1.1

Total n 188 131 116 118 435 226 779

FDE

Trials 

Dead n .

% Dead 0.4 0.1 0.4

Total n 283 376 254 127 225 302 651 762 805 218

Totals

Trials

.

Dead n 129 

212

% Dead l,3 0.1 1.4

Total n 284 930 148 713 010 795 065 442 438

.

945

(a) These eggs were held from 2. 0 to 6. 5 hours; percentages are on a per hour basis for comparison.

(bJT'I~M 

1" -'In 1 nn~ +Q~ nnnhn
:7"\

4.0

..g,

c: 

2.0

5 I

LG - EE

c Test

Contro

TBF FDE

oS 

Tota

Figure 4. Mean mortalities of striped bass eggs

impinged on a 0.020-in. slotJohnson

screen for 0. , 1.0, and 2.0 minutes.

entrained in 2 hours of testing. In tests run without the

screen (pumping through the 12-in. mounting orifice), 40%

of all eggs were entrained during each of two IO-minute trials.

Only 1% entrained and 11 % impinged during two similar

trials with the screen in place, yielding a 97.5% reduction

in entrainment.

Egg Mortality

A total of 26 tests was conducted with 6 945 striped

bass eggs of six different stocks. Eggs ranged from 18 to

38 hours old with diameters of 1.8 to 3.2 rom (after

preservation). Developmental stages tested were late

gastrula (LG), early embryo (EE), tail bud free (TBF), and

fully developed embryo (FDE). Mean mortalities of tests

Hanson et al. " A Pr(Ji:tical Intake Screen 399

and controls at three different impingement durations are

summarized in Table 1 and Figure 4. Statistical analyses

for differences in percentage mortality (student s t-test

after arcsine transformation; Sokal and Rohlf 1969)

yielded significant (p"::::: 0.05) differences between test and

control for 30-second impingement duration (stages combined), lG-EE stages (durations combined), and all data

combined. In general, LG.EE eggs suffered greater

mortality than other stages in both tests and controls.

Higher mortality of the earliest stages tested may reflect

the greater fragility of these stages.

Mortality resulting from impingement (test.control

difference) ranged from 0.0% to 11.9%. Mean mortality of

developmental stages versus impingement duration ranged

from 0.0% to 2.0%. Overall, mean mortality owing to

impingement was 1.4% (Table 1). The low average

mortality oflong-term controls probably resulted from less

handling stress. Visual inspection of eggs during the 60-

minute postimpingement holding time revealed most

mortality occurred in t.he first 30 minutes.

Skinner (1974) reported mortality of striped bass eggs

impinged on 16 mesh/inch screen (0.044-in.) was generally

less than 10% at approach velocities less than 0.9 fps and

less than 30% at 1.0 fps. At 0. 5 fps approach velocity,

immediate mortality was approximately 8% and 12% at

I and 2 minutes, respectively. In our study, comparable

mean mortality (test only) was 3.7% and 2. 9% for 1- and

minute impingement durations, respectively. The

differences between studies may be due to screen construction, slot size (0.044-in. versus 0.020-in.), procedural

variation, egg stage, or egg origin (wild versuS hatcheryreared in our study).

Striped Bass Larvae

A total of 42 tests were conducted with more than 1,000

hatchery-reared striped bass larvae. Larvae 4-33 days old

(5. 20.0 rom FL) were exposed to mean intake velocities

of 0. 13- 1.50 fps at temperatures that ranged from 13 to

24 C.

During one set of experiments, more than 930 larvae

(aquaria-held) were tested daily from age 4 to 20 days

(5. 2 rom TL) at mean intake velocities of 0. 13-

fps and temperatures of 13- 19 C. Ability to resist entrainment was rated subjectively on a group and individual

basis (Table 2). Performance was rated on general swimming

ability, resistance efforts, and burst ability compared with

the previous day s testing.

Avoidance behavior was observed in all experiments

with aquaria-held larvae. Although most specimens entrained within 1 minute, many exhibited positive rheotaxis and actively resisted entrainment upon contact with

the screen; those that did not touch the screen were entrained more or less passively.
c=~=~-= 

...::. 

~~?C: ~

::

? 

.

Table2 Performance of Hatchery-Reared Striped Bass Larvae Exposed

.;.

to a 0.040-in. Slot Johnson Screen 

Min to Relative Resistance to Intake

;::

Age x Intake Est. % Ent. 100%

...

TL(mm) (days) Vel. (fps) inlmin Entrained Group fudividualc

::to

1.43

1.33

0-6. 0'1 100

0.13 D'T 1.25

6.3 35 ,

6.3- 0.35 .

35 . 

;::

0.35 1.50

6.4 35 .

6.4 0.35 20.

;::

0-6. 0.35 10.

... ;::

I:.

6.4 . 10 0.35

5.3-6.3 0.35 '\:s

6.4

0-6. 0.35

;:: ...

0.350.35 10.0.35

6.4. 0.35

50 , I'J 1.50

5- 7.3 0.35 . 15.

0- 7. 50 . 10,

7.3

6.3-

100 1.00

7.4 100 1.00

0.50 22.00

12.

fa) 

Ambient water temperature ranged from 13 to 19 

b)p = Poor; F = Fair; G = Good; E = Excellent.

(c) Rating of 10% of total.
Most larvae were en trained in the proximal half of the

screen where intake velocity was greatest. The percentage

that actively resisted entrainment increased with age, but

100% still were entrained (Table 2). Mean intake velocity

was increased to 0. 5 fps after 11 tests because of improvement in swimming ability; results were similar to tests

at 0.35 fps (Table 2).

The extent of escape resulting from a washing current

or movement away from the screen could not be ascertained since all tests were in the static mode with a test

cage. However, in a situation with current, a "burst" from

t.he screen after contact should greatly decrease the potential of impingement or entrainment.

Tests were originally designed to monitor the increase

in avoidance capability as larvae increased in age and size.

However, by test day 10 t..lJ.e performance of the aquariaheld larvae was more or less constant. Comparison with

specimens of the same age that were fed natural food and

held in larger tanks or rearing ponds revealed substantial

differences in size and swimming ability. Mansueti (1958)

noted stunting may result from factors such as overcrowding,

lack of proper nutrients, variable effects of metabolites in

the aquaria, and hormonal imbalances owing to an artificial

environment. These factors may explain the rapid change

in growth rate '!Ild swimming ability oflarvae transferred

to larger tanks supplied with natural food.

In another set of tests, 100 larvae held in a SOD-gallon

tank and a 425-gallon pool were tested at mean intake

velocities of 0.35- 1.50 ips. Ages and totallengt.'1s ranged

from 10 to 33 days and 8 to 17 rom. Water temperature

ranged from 19 to 24 C. Many of these specimens were

survivors of earlier experiments. Larger size and greater

ability enabled testing at higher intake velocities and resulted in frequent but usually temporary impingements.

The oldest specimens tested (13- 17 rom TL) were easily

able to resist intake velocities up to 1.5 fps. Most temporary impingements occurred near the top of the screen

where intake currents were vertical.

Overall, there were 93 impingement occurrences, 75

escapes, and 16 entrainments (Table 3). Mortality resulting

from impingement was not investigated. Most prolonged

impingements were wedged tail first between the wires and

held by their opercles; the damage incurred was probably

fatal. Avoidance (no entrainment or prolonged impingement) ranged from 40% to 100% for 30-minute tests. ill

tests with currents 'of 1.0-1.5 frs, 92.7% of the larvae (9.

17.0 mm) swam for 30 minutes wiQ1out entrainment or

prolonged impingement.

,C'".

FISb. Impingement

A total of 1 387 specimens of20species were tested

(Table 4). Relative susceptibility indices for species tested

in numbers of 50 or more are presented in Figure 5,

:;Q1;t

Hanson et al. : A Practical Intake &reen 401

,,~, 4""c"

,,:?,?,

V(':Y~'l!, :r:~:1r'?~" '~?""i"'Y' i"'.

.,,,.,., , . ,.

O:04O-iIh' s!()fScreen waHested at mean intake velocities \

1.""

, . , ...

iiiSm~'aSm5 ,. S'; t aJ.er:te,mp~rClt\1r~, 1:3Jlgedfrom3 to

"~~hTj~~i11' . e

~ete

;

tf6~ji~~~~d~.

caiire'$fftlie:COns1anfex"osu~"'an(tTh:cf " . ri'" ' 0 'cu e~

t ;, 'i ~~ ~jf~ ntifuti #fefit7~?1 ~ Pl!i &~d;~p~;; n

. pr6ve YV:!IIi ~ih

: ,

J~en interaction was virtually n ~m-

,ilit~nfintheayJ cIrtOde.

illtake velocity, fish size , and behavior were the major

factors that influenced1 impingement. Effect of low temperature on performance was not investigated thoroughly.

Bibko et aI. (1974) noted tllat long-term swimming endurance decreased with decreasing temperature. Although

some impingement occurred among the 70 striped bass

(105- 141 mm FL) tested at temperatures below 10 C,

the data were inadequate to determine effects of temperature on performance. Observations indicated specimens

were under no stress.

Frequency of impingement varied between species and

in some cases was a function of behavior. Mummichog and

banded killifish occasionally' rested on the screen surface

and left at will regardless of intake velocity. Some species

appeared to play with the functioning screen while others

exhibited feeding behavior on and around the screen.

direct relationship between intake velocit andim ing;

ment was assumed but was not clearly exhibited. In at

least two cases the inverse was true. Atlantic menhad

and spot sustained hig.lJ.er impingement rates at lowe velocities, 

A total of 1,318 specimens of 19 species were tested

in the static mode. Only 20% of the 261 impingement

occurrences failed to escape. The majority of screen

interactions occurred at intake velocities in excess of I fps

or after more than 10 minutes of exposure. Both values

exceed the expected velocity (0.5 fps) and exposure time

(1 minute) for an intake. Mortality usually resulted after

prolonged impingement and was primarily associated with

weaker or more sensitive fishes. Handling stress was

probably the most important factor in causing prolonged

impingement. A total of 34 specimens died as a result of

testing; most were in poor condition before the tests

began.

Although impingement-induced mortality should be

investigated, it is expected to be a minor factor in impact

assessment for fish larger than 20 mm FL. Little or no

loss of sca.ies or other physical damage was incurred by

specimeriS impinged for long durations. The only instances

of screen-induced injury occurred during runs of striped

bass larvae when specimens were wedged in the slot to the

level of the opercles.

A total of 69 specimens of seven species (Atlantic

menhaden, banded killifish, mummichog, tidewater silverside, Atlantic silverside, bluefish, and spot) were tested

in the dynamic mode, Channel and mean intake velocity
._

. H

...

Table 3 Performance of Hatchery-Reared Striped Bass Larvae Exposed to

04O-in. Slot Johnson Screens 

x Intake Est. % Relative Resistance

to Intake Age Velocity Entrained Impingement

...

TL (mm) (days) (fps) in First Min Entrained Occurrences Escapes Fish-min Group Individual 

:::to 10. 10-14 1O0d

10. 24.

11.2 1.0 107.

;:s

8.3- 12.4 1.0 160.

;:;:s

13.0-15.4 19- 53.

...

10. 10.

'1::3

8.3- 12. Ji' 

...

13. 29.

14. 1.0

12. 16. 1.5 45.

13.0-17. 1.0

13.0-17. 32- 1.5

(a) Ambient water temperature ranged from 19 to 24 C. Fish were held in a 500-gallon Bishop tank ~d a 425-gallon pool and fed freshwater invertebrates. Test duration was 30 minutes.

(b)p = Poor; F = Fair; G = Good; E = Excellent.

(c) Rated 10% of total.

(d) Entrained in 25 minutes.
~~~;'~;;;rii\~;;" e" "

.......,

.............

. .

,.

Table 4 Summary of Performance of Fishes Exposed to Johnson Screens

in Static and Dynamic Modes

Species

. Alewife

JAtl~tic menhaden

.....

' Bay. anchovy

CarpSilvery minnow

Golden shinerSpottail shinerBanded killifish

- M~g~~

TIdewater silversideAtlantic silversideThreespine stickleback

FL (mm) Temp. (C)

X Intake

Velocity

(fps)

50- 1.50

50-1.50

50,1.50

00-

50-1.50

21- 1.42

50-1.50

96- 1.4298-1.42

0.41.

50- 1.50

1.00

I. O.

White perch 96 21-41 17. 24.0 0.50-1.50 

- griRed f)!iSs 

.

648 8-151:6,;;; .,;: "3. 0-25. h~0.3H.50 . Pumpkinseed 70-91 23. 1.50 Bluegill 30 25-98 11.0-23.0 0.20-1.50 Yellow perch 18 34-40 17.0 0. 50-1.50 

~~Ii~'& 4~);~J~+' 1.1i

.~~~~;2'(~r

;

,t::f:Yi"

. . 

.

Spot 64 20. 23.5 0. 50.1.50 

Total

37- 17. 24.

'77 38. 145j): ":~22.Oc24.Q,,

~6$ 25. 7L~1t, i';22~0. 27.

17- 19. 23.

30-31 17. 21.0

35- 12.23- 17. 24.

34-89 22. 24.

, 

Z; '

;;:

L:, ~?:I?YfJl..;.1i i&;;i~Ltt;~L'

44 27-81 20.0-24.

136 34-95 22. 26.

24.

00-1.$0 , 263

Mean

Impingement Suscept.

Escapes Fish-rnin Dur. (min) Index

0000

34. . 0.0l6~

214 53 

 

,

OX202~

26. 0118

00000000

186.D1 37. 2989

0000

'.c,. QQQO

0001

00130017

::!

996.96, 12. g~3.~'l

,:.,

:;c:!,;"""".,,,

...C\\

0000

00000000

:::t.

qPjJ :: W'0004 i:t

, 

71.65 3.12 0427

212 535. 0250

.;:.,

387 151 0-27.

(all. 00 = Impingement Occurrence.

(blT.c 00005.
404 Fourth National Workshop on Entrainment and Impingement

,.... .....

r-1

01'

0"1 5/,01 ~N~ ~.....I,c alh,ft ~I, .I1th",/"

//AJ(!/,ovy &rss '-~.!.J -'bv, .5;~dc

25- 71""" '&'-98 8-1S1 .J9-lfIs .Z'-~""" .31-'l:1 31'-

.u=68 AI=?,! AJ=t'/B,d= 77 "'=S'd: AJ=o$.3 11.1=/;5'

\ (

Figure 5. Relative susceptibility indices for

species tested (n ? 50).

ranged from 0. 0 to 2.0 fps and 0.41 to 1.25 fps, respectively. Only three impingements occurred; the total time

impinged was 5.0 seconds. Results and observations indicate impingement oflonger than 5 seconds is not expected

to Occur.

Striped bass was the most extensively studied fish. The

following discussion is based on its performance. Results

of the other 18 species tested are also presented in Table 4.

A total of 648 specimens (8-151 rom FL) were exposed

to mean intake velocities of 0.31- 1.50 fps in the static

mode. The 77 impingement occurrences resulted in a

relatively low susceptibility index of 0.0335.

In 1976 533 specimens (8- 151 mm FL) were exposed

to intake velocities of 0.3 1- 1.43 fps. Of the 58 impingements, 30 escaped. Most impingements (72.4%) occurred

in three of the 22 tests at intake velocities greater than

90 fps. Two of these tests used specimens of 8- 17 mm

TL All impingements of specimens greater than 17 mm

FL occurred after 7 minutes of exposure time.

Refinement of procedure and technique during 1977

studies yielded more realistic results. Of the 115 specimens (14-46 rom FL) tested, 60 were hatchery-reared

and had undergone earlier testing. Exposure to mean

intake velocities of 0. 1.5 fps resulted in a total of 19 impingement occurrences with 19 escapes. Most impingements lasted less than I second; the longest duration was

4 seconds (0.07 minutes). Hatchery-reared specimens

suffered impingement at all velocities tested; however

most (69%) occurred at 1.0 fps. All impingements of

wild specimens occurred at an intake velocity of 1.5 fps

after 6 minutes of exposure.

Performance of test specimens was consisten t with

published swimming speed data. Skinner (1974) found

90% of the 40-50 mm striped bass tested could swim

as long as 6 minutes in a current of 0.8 fps. Tatham

(1970) reported 0. 9 fps was the maximum swimming

speed for 30-41 rom bass, 0. 1.3 fps the maximum for

43-50 mm fish, and 1.1-1.2 fps the maximum for 50-

63 mm fish. Tests by Bibko et aI. (1974) indicated a

sustained swim speed of 0. 0 fps for 88. 137.5 mm

striped bass and 1. 7- 5 fps for 150. 212. mill fish.

Exposure to the screen elicited a distinct and consistent

behavior pattern. Upon introduction to the test cage, one or

two schools formed and milled about the lower half of the

enclosure. When the pump started, specimens quickly fanned out along the midsection of the screen. Most remained

3 in. off the screen surface and oriented into the approach

vector. As intake velocity was increased, specimens tended

to move toward the distal end and farther from the screen

surface (Figure 6). Observation of fish activity and measurement of approach velocity indicated striped bass appeared

to prefer velocities of 0. 15- 25 fps.

Most bass appeared unstressed during tests, as evidenced

by feeding behavior. Avoidance behavior consisted of rapid

movement to areas of reduced current; some specimens eventually returned to the school, others remained in areas of reduced current. Impingement occurred when physical control

was lost; impact usually occurred in the proximal half of the

screen. Most impinged individuals escaped the screen at least

once. Some specimens escaped the screen on the first attempt; others moved to the distal end and escaped. Less

than 3.0% of the impinged bass suffered immediate mortality or equilibrium loss.

Tests in the dynamic mode indicated young striped bass

were essentially invulnerable to screen interactions because

of strong orientation to the bottom.
5 fps

..

0 fps

\ 

Jo-.

5 fps

Figure 6. General location and orientation of

striped bass at three intake velocities.

Detrital Fouling

A total of29 trials were conducted with three concentrations of C&D Canal detritus and peat moss. A O.04O-in.

slot, O. 120-in. wire screen was operated at a mean intake

velocity of 0.4 fps; channel velocity ranged from 0.25 to

1. 7 5 fps. Table 5 gives the mean TIC of the three

concentrations tested at various channel currents. Values

preceded by the symbol), indicate only 0.75- 1.25 in. head

loss at the time of termination.

Hanson et al: A Practical Intake Screen 405

Screens clogged relatively quickly in tests at 77 times

normal Canal concentration. TIC generally increased as

channel velocity increased owing to washing by the

current. The greater TIC for tests at a 0. 25-fps current

compared to tests at 0.50 fps probably resulted from

detritus settling to the bottom at the lower channel

velocity.

Screen clogging patterns were similar at all channel

velocities. The leading or upstream side of the screen impinged a 3-4 in. band of detritus along the length of the

screen. because of channel current. As the test progressed

detritus built up and the band spread in the proximal high

intake velocity areas. At the same time, a 1-2 in. band

formed on the trailing edge. Both bands continued to

widen, particularly in proximal areas, until they met on the

top and bottom of the screen. Spreading continued on the

distal portion until the entire surface was covered. Although tests were terminated at a 12-in. head differential

the screen was still passing from 60% to 90% of its initial

capacity. Incorporation of a bubble curtain positioned to

sweep the leading side of the screen increased TTC up to

000%. Many tests were terminated with essentially no

increase in head.

As detrital concentration decreased, TTC generally

increased. The suspended detrital concentration normally

found in the C&D Canal is 9.46 cm3 /m . This load is

not expected to clog a screen of this type under actual

operating conditions. A model intake with a30-in.

diameter, 24-in. Iong screen (O.04O-in. slot and 0. 1 58-in.

wire) installed in the Canal in March 1977 has operated

continuously at a 0. fps intake velocity with no head

loss resulting from detrital clogging and fouling. The

screen did clog (12-in. head loss) on several occasions

during the summer months as a result of heavy biofouling.

Clogged screens were hydraulically backwashed as a

matter of course before a fouling test was initiated. Water

was pumped through the screen in a reverse flow at

rates of 200-400 gpm. All flow rates tested were equally

effective in removing material from the screen. This screen

was generally free of debris after 15 seconds of backwashing.

. Air/hydraulic backwash was investigated in greater

de~ail. Six experiments were conducted with a mean intake

velocity of 0. 5 fps and three channel velocities (0. 5, 1.0,

and 1.5 fps). Air pressures of 10, 25, and 50 psi were

tested. Comparisons of TIC for manual and air backwash

are presented in Table 6.

Approximately 23 gallons of water were contained in

parts of the air backwash system below water level. Release

of compressed air flushed this water through the screen slot

at a velocity of approximately 2 fps. The air blast reached

the slots at 36. 6% of initial pressure.

This system also cleaned the screen effectively. Cleaning

action appeared to result from the water pushed out ahead
iil

il'i

::i

liil

406 Fourth National Workshop on Entrainment and Impingement

Table 5 Mean Time-to-Clog (minutes) of a O.04O-in. Slot Johnson &reen

Exposed to Three Concentrations of Detritus

Channel

Velocity Mean Time-to-Clog

(fps) II 22X 77 X

30.

).240. 48.41 26.33

200. 43.

1.00 ),300. 82. 74.

1.25 ),315. 310. 315.

1.50 :::.420. ).240.

1.75 ),110.

(a)Mean intake velocity was 0. 4 fps.

(b) Ratio of detrital concentration to normal C&D Canal levels.

Table 6 Comparison of Manual and Air Backwash

Cleaning. Efficiency

Velocity

Channel Detrital Air Pressure Number Mean Concentration of Backwash

(fps) TIC (cm (psi) Trials (min)

105. Manual 28. 77

105. 30.33

1.0 105. Manual 25. 1.0 105. 25.38

209. Manual 16.33 1.5 209. 14.48

1.5 209. Manual 14.47 1.5 209.

13.

1.5 209. Manual 16. 1.5 209. 14.32

1.5 209. Manual 16. 1.5 209. 15.

(a)Based on time-to-clog (ITC) at a mean intake velocity 

of 0. 5 fps for a 0. O40-in. slot Johnson Screen.
of the pressurized air blast. Material was blown from all

areas of the screen and although a small amount reimpinged

on the leading edge, most was carried away by the channel

current. Head differential consistently returned to initial

levels after backwash.

CONCLUSIONS

Research has demonstrated profile wire screens have

great potential as surface water intakes. Impact on the

aquatic environment is expected to be many times less

than that of a comparable intake protected by traveling

screens. The ability to reduce entrainment and impingement results from the infinite number of escape routes

available; the flow dynamics, which enable a fish to easily

determine the direction of escape; the rapid decline in

approach velocity as a fish leaves the screen; t.he sma!!

slot size; and the ambient washing currents, which assist

escape and avoidance. Impingement of fishes larger

than 20 rom FL is virtually eliminated. The small slot

wid th transfonns most potential entrainment to impingement. Contrary to impingement on a traveling screen,

fish never leave the water, are supported by a greater

surface area, and can escape with less difficulty because

of the low intake velocities and self-cleaning characteristics of the screen. Planktonic organisms are offered greater

protection because of the small slot size and boundary

currents. Detrital fouling is greatly reduced and the screen

. can be quickly and easily cleaned. This system also

eliminates the problem of disposing of impinged de bris.

Biofouling is a major operational problem in brackish

and marine situations. Fouling organisms attach. to and

proliferate on the screen surface and ultimately require

removal. The use of metals with antifouling properties

to construct the screen should minimize this problem.

Prome wire screens are highly versatile and specialized

installations can be engineered for most locations and

conditions. Our studies were directed toward the protection of early life stages (eggs, larvae, and young) of

striped bass in the C&D Canal. Results to date indicate

Hanson et al. : A Practical Intake Screen 407

this protection can be accomplished with little or no

constraint on intake operation.

ACKNOWLEDGMENTS

The aut.~ors would like to express appreciation to the

entire staff of t.he Delmarva Ecological Study, Ichthyological Associates, Inc., for their contributions during construction of test apparatus and collections of test materials.

Special thanks are extended to Steven E. Allison, who

directed early experimentation; Michael R Headrick

and his hatchery crew, who supplied striped bass eggs

and larvae; and Ronnie J. Kernehan for editorial assistance.

Research. was funded by Delmarva Power and light Company, Isc. The manuscript was typed by Arlene H. Hanson

and R Lynn Cox.

BIBLIOGRAPHY

Bibko, P. N., L. Wirtenan, and P. E. Kueser. 1974. Preliminary studies on the effects of air bubbles and

intense illumination on the swimming behavior of the

striped bass (Morone saxatilis) and the gizzard shad

(Dorosoma cepedianum). Pages 293-304 in Jensen

L. D. , ed., Second Workshop on Entrainment and

Intake Screening. Edison Electric Institute Report 15.

Mansueti, R J. 1958. Eggs, larvae and young of the striped

bass Roccus saxatilis. Ches. BioI. Lab. Contr. No. 112.

35 pp.

Skinner, J. E. 1974. A functional evaluation of a large

louver screen installation and fish facilities research on

California water diversion projects. Pages 225- 249 

Jensen, L. D., ed. , Second Workshop on Entrainment

and Intake Screening. Edison Electric Institute Report 15.

Sokal, R. R., and F. J. Rohlf. 1969. Biometry. W. H. Freeman & Co., San Francisco. 776 pp.

Tatham, T. R. 1970. Swimming speed of the white perch,

Morone americana, striped bass Mororze saxatilis and

other estuarine fishes. Final Report No. 0-26156 Con.

Ed. Co. N. Y.; Inc. Ichthyological Associates, Ithaca

N. Y. 48 pp.