ALTERNATIVE INTAKE DESIGNS

FOR REDUCING FISH LOSSES

MYSTIC STATION - UNIT 

BOSTON EDI SON COMPANY

JULY 1979

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I hereby certi fy that thi 

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that I am a duly registered

professional engineer under the

laws of the State of Massachusetts

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Date JJ.

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Reg. No. 

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STONE & WEBSTER ENGINEERING CORPORATION

BOSTON, MASSACHUSETTS
Section

3. 1

4. 1

TABLE OF CONTENTS

Ti tIe Page

SUMMARY AND CONCLUSIONS

INTRODUCTION . 1-1

PRESENT INTAKE DESIGN AND OPERATION . 2-1

SUMMARY OF FISH IMPINGEMENT EXPERIENCE. 

.. . 

. 3-1

INTRODUCTION. . . 3-1

RESULTS OF IMPINGEMENT MONITORING STUDIES . 3-1

CONCLUSIONS

.. . 

. 3-1

EVALUATION OF ALTERNATIVE INTAKE DESIGNS FOR THE 

MYSTIC STATION - UNIT 

.. . .. . 

. 4-1

INTRODUCTION. . 

.. . 

. 4-1

Assessment of Fish Impingement Losses on the

Population

.. . 

. 4-1

Divers ion Devices . 4-2

Behavioral Barriers

.. . 

. 4-2

Fish Collection Systems . 4-3

Physical Barriers

.. . 

. 4-3

Summa ry . 4 - 4

VELOCITY REDUCTION . 4-4

Biological Evaluation . 4-4

Engineering Description of Alternatives for

Reducing Velocities. . 4-

Removing the Bottom Sill at Certain Times of

the Year

.. . .. . .. . 

. 4-4

Utilizing the Units 1-3 Intake Bays for

Supplemental Cooling Water for Unit 7. . 4-5

Reducing Unit 7 Operation to One Circulating

Water Pump

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

. 4-7

Installing Variable Speed Control on Circulating

Water Pumps to Discharge Temperature

.. . 

. 4-8

Installing the Screens Flush with the Front

of the Intake Bays

.. . 

. 4-10

Constructing Additional Intake Bays . 4-11

Deepening the Area Immediately in Front of

the Existing Intake Bays . 4-13

PHYSICAL BARRIERS - BARRIER NET IN FRONT OF THE

EXISTING INTAKE BAYS. . 

.. . 

. 4-14

111
TABLE OF CONTENTS (Cant)

Section Ti tle Page

FISH COLLECTION AND REMOVAL SCHEME - MODIFIED

TRAVELING SCREENS

. 4-14

Biological Evaluation

. 4-14

Engineering Description . 4-18

Evaluation af the Modified Screen Alternative

. 4-19

ESTIMATED COSTS AND CONSTRUCTION SCHEDULE

. 5-1

REFERENCES. . 

. R-1
LIST OF TABLES

Table Ti tle

After

Page

3-1 Mortality of Winter Flounder at

Several Screenwash Intervals with

95 Percent Confidence Intervals. . 3-3

3-2 Summary of an Analysis of Variance

of Differences Observed in the

Mortality of Winter Flounder at

Several Screenwash Intervals and

During the 96-Hr Holding Period. . 3-3

4. 4-1 96-Hr Impingement Survival Results

Pulliam Power Plant , 1977. . 4-17

5-1 Estimated Costs. 

.. . .. . 

5-1

5-2 Total Proj ect and Capitalized Costs

(in $1 000). . 5-1

5-3 Construction Periods

.. . 

.. 5-2
LIST OF FIGURES

Figure Ti tle

2-1 Plot Plan

.. . 

2-2 Circulating Water Intake. 

2-1 Flow Transport From Units 1 , 2 & 3 - Plan View

2-2 Flow Transport From Units 1 , 2 & 3 Section View.

2-3 Plan - Flush-Mounted Screens

2-4 Section A-A - Flush-Mounted Screens. . 

2-5 Additional TWS Scheme - Plan below Operating Deck.

2-6 Additional TWS Scheme - Plan above EI. 17 ft 6 in.

2-7 Additonal TWS Scheme - Scheme Views. 

2-8 Proposed Dredging and Flounder Sill. 

4-1 Screen Detail - Modified Through - Flow Screens -

Front Wash

. . . .

4-2 Screen Detail - Modified Through - Flow Screens -

Back Wash

After

Page

2-1

2-2

4-5

4-6

4-10

4-10

4-11

4-11

4-11

4-13

4-15

4-18
SUMMARY AND CONCLUSION

Fish impingement monitoring studies have been ongoing at Boston Edison

Company s Mystic Station since 1971. On the basis of a review of the study

resul ts , the U. S. Environmental Protection Agency (EPA) and the Massachusetts

Division of Marine Fisheries requested that a feasibility study of alternative

intake designs be conducted even though Boston Edison Company has taken a

number of measures to reduce fish impingement at the station.

At the request of EPA , Boston Edison Company investigated the feasibility

of utilizing various intake modifications which would minimize the numbers

and/or mortality of fish at Mystic Station.

This effort consisted of (1) reviewing the existing intake design at Mystic

and the impingement losses associated with its present operation , (2)

evaluating available fish protection systems that might be applied at this

site , and (3) developing engineering designs and costs for those systems

which appeared to be feasible , cost- effective , and to offer potentially

effective methods for minimizing fish entrapment at Mystic.

The alternative designs evaluated can be grouped into four categories

depending on their mode of operation: diversion devices , behavioral barriers , collection systems , and physical barriers. Available fish protection

systems in each of these categories were reviewed relative to their potential

biological effectiveness , engineering practicability, and operational

reliability as applied to the Mystic Station. A number of these alternative

designs and/or modified operating schemes were specifically suggested by

EPA. On the basis of this review , nine al ternati ve intake designs or

operating schemes were selected for detailed evaluation and conceptual

design as described briefly below. Estimated costs were developed for

those designs which appeared feasible.

1 . Removing the Bottom Sill at Certain Times of the Year

Impingement studies at Mystic have shown that a bottom sill located in

the Unit 7 screenwell reduces flounder impingement but may cause an

increase in the numbers of pelagic species collected , presumably due

to the increase in velocities caused by the sill. If benthic and

pelagic species were present at different periods of the year , the

sill could be installed only during the period of flounder impingement.

At other times of the year , there would be no sill , thus reducing

veloci ties and , consequently, the impingement potential of pelagic

species. Unfortunately, both types of species fish generally occur

simultaneously and removal of the sill would be of little or no benefit.

Utilizing the Units 1-3 Intake Bays for SuEPlemental Cooling Water

for Unit 7

Another potential method of reducing impingement losses would involve

reducing the flow, and therefore velocity, into Unit 7 by utilizing

the Units 1-3 intakes to supplement the existing Unit 7 intake flow.
The scheme would consist of modifying the Units 1-3 pump bays , replacing the existing pumps with vertical axial flow pumps , and manifolding

the pump discharge pipes into a supply pipe that would traverse the

front of the bays and would discharge into the Unit 7 intake. The

Unit 7 screens would be relocated farther upstream toward the front of

the bays. A new wharf with protective dolphins would also be required

as an unloading facility for oil barges.

The total capitalized cost of this scheme is $6 674 000.

While this action would reduce velocities at Unit 7 , data are not

available to quantify the possible degree of fish protection which

would be achieved. In fact , studies on the Hudson River indicated

that reducing approach velocities to 0. 5 fps at a power plant did not

significantly reduce impingement. Considering the costs associated

with this scheme , and the uncertain influence of velocity reductions

on fish impingement , it is not believed that this concept is the most

cost- effective option for reducing fish losses.

Reducing Unit 7 Operation to One Circulating Water Pump

The feasibility of reducing velocities at Unit 7 by operating one

circulating water pump was also investigated. The results , however

indicated that operating Unit 7 with one circulating water pump would

result in loss of plant generating capacity, violation of thermal rise

and discharge temperature limitations , reductions in condenser reliability

and pump life , and an increase in velocity in the operating screen

bay.

Because of the operational problems of this option and because the

operation of one pump only would actually increase velocity in the

operating bay, it is believed that this alternative is impractical.

Installing Variable Speed Control on Circulating Water Pumps

Another option for reducing velocities at Unit 7 would be to incorporate variable speed control on the circulating water pumps to permit

flow reductions during periods of low demand. Unfortunately, the use

of variable speed control would not reduce the velocity in the Unit 

screenwell substantially due to loss of pump discharge pressure and

would create reliability problems. Operating problems which would

affect reliability include lower pump discharge pressures , condenser

tube fouling, increased pump motor maintenance , and potential violation

of thermal discharge limits. Therefore , this scheme is not considered

viable at Mystic Station.

Installing the Screens Flush With the Front of the Intake Bays

Moving the traveling screens forward in the existing screenwells such

that they would be flush with the front of the intake bays would

create a more uniform velocity distribution through the screens and

would eliminate embayment areas that might entrap fish. However

al though these improvements would probably reduce impingement to some
extent , velocities would still be rela ti vely high. Therefore , the

potential biological effectiveness of this design is uncertain.

The total capitalized cost of this scheme is $1 168 000.

Since velocities would not be reduced appreciably and potential biological effectiveness is unknown , it is believed that pursuing this

alternative at this time is unwarranted.

Constructing Additional Intake Bays

Another method for reducing velocities at Unit 7 would consist of

constructing two additional intake bays west of the existing intake.

A new screenhouse would contain four new screens and two low-head

supplemental pumps that would feed water into the existing Unit 7

bays. The existing traveling screens would be moved to the front of

the Unit 7 bays to be flush with the piers. Lateral fish passageways

would also be provided.

The total capitalized cost of this scheme is $7 383 000.

Because of the uncertainty of velocity reductions on biological effectiveness and the high cost , it is not believed that such a major modification

offers significant benefits over other more cost- effective alternati ves .

7 . Deepening the Area Immediately in Front of the Existing Intake Bays

Deepening the area in front of the Unit 7 intake through dredging

would create a new bottom sill consisting of the vertical foundation

wall of the screenhouse , thereby allowing removal of the existing

bottom sill. This action could maintain the reduced level of flounder

impingement achieved by the existing sill and potentially reduce

impingement of pelagic species because of lower intake velocities

throughout the water column. However , actual reductions in impingement

cannot be quantified.

Dredging would be confined to the immediate area in front of Unit 7 to

maintain structural integrity of the screenhouse. Since the dredged

area would fill with sediment , maintenance dredging would have to be

carried out periodically. This would require obtaining permits for

dredging and spoil disposal on a periodic basis.

The total capitalized cost of this scheme is $393 000.

Because of the limited areal extent of dredging possible , the necessity

to periodically dredge , and the unknown benefits of this scheme , it is

believed that this alternative provides only limited potential advantages.

Barrier Net In Front of the Existing Intake Bays

Physical barriers , such as barrier nets , have been evaluated as a

method for preventing fish impingement at power plant intakes; results

have been variable
Barrier nets have been at least partially effective in reducing fish

losses at several power plants. However , a net would be difficult to

maintain at Mystic because of strong tidal currents and heavy debris

loading, particularly with jellyfish and mussels. For these reasons

this scheme is believed to be impractical as a long-term solution at

Mystic StatioQ.

Modified Traveling Screens

Fish collection and removal systems , specifically various types of

traveling screens , have been used at several power plants with good

results. Recently, modifications to conventional traveling screens

have been developed which have greatly improved the survival potential

of impinged fish. Modifications include the addition of fish lifting

buckets , a low pressure spray wash for gently removing fish , and the

capability for continuous operation. Such modifications could be made

at Mystic at a total capitalized cost of $596 000. Available data

indicate that modified screens could act to reduce losses of fish

occurring at this site.

Of the alternatives investigated , it is believed that the modified traveling

screen concept presently represents the most cost- effective system for

further reducing fish mortality.

If additional measures for protecting fish are required at Mystic Station

Unit 7, it is proposed that a study be conducted which would be designed to

quantify survival rates of selected , important species and to identify

optimal survival conditions for each species on a modified screen. A

preliminary study plan for such an acti vi ty is presented in this report.
SECTION 

INTRODUCTION

Impingement monitoring studies were conducted at Boston Edison Company

Mystic Station from August 1971 to December 1977. The purpose of the

studies was to determine the level of fish losses occurring at Units 1-6

(1971 to 1975) and at Units 1-7 (1975 to December 1977).

Initially, review of the earlier studies by the U. S. Environmental Protection Agency (EPA) and the Massachusetts Division of Marine Fisheries indicated

that the level of winter flounder impingement was relatively high at Unit 6

and that measures should therefore be undertaken to reduce impingement of

this species at Unit 6 and Unit 7. In response to this concern, Boston

Edison Company initiated several design modifications (i. , bottom sills

reduced curtain walls , etc) for incorporation into Units 6 and 7. As a

result of these modifications , flounder impingement was reduced dramatically

(approximately 50 percent). Upon subsequent review of the later monitoring

study reports by EPA and other agencies , concern was expressed that , although

flounder impingement was reduced , total numbers of fish impinged were high.

At the request of the EPA , Boston Edison Company initiated a study to

investigate the feasibility of utilizing various intake modifications which

would minimize the numbers and/or mortality of fish presently being impinged

at the station. The EPA suggested specific options to be investigated.

The engineering options included (1) modification of existing screens for

continuous operation during times of the year when impingement is most

serious , (2) reduction of intake velocities by modifying the existing

intake structure or adding new structures to reduce flows , and (3) removing

the bottom sill and dredging the area immediately in front of the intake.

In addition , the EPA requested information regarding the feasibility of

determining the population levels of key species within the Mystic River 

Boston Harbor estuarine complex for the purpose of assessing station losses

on the selected species population. It was also suggested that a program

be described to evaluate survivability of fish impinged on modified screens

during continuous operation. The EPA also requested any additional information on other alternatives which may be practical for reducing fish impingement

or mortality.

This report evaluates the status of existing intake technologies for

minimizing fish losses and determines the engineering feasibility, potential biological effectiveness , and order- of-magni tude costs of the options

presented by EPA. In addition, information is provided on other potentially

cost- effective means for reducing fish losses at Mystic Station.

1-1
SECTION 2

PRESENT INTAKE DESIGN AND OPERATION

General

Mystic Station Unit 7, owned and operated by Boston Edison Company, is

located on the north bank of the Mystic River. Most of the 42- acre site is

in Everett , Massachusetts. A small portion along the western edge is in

Boston. The station is in an area zoned for industrial development. The

site is bounded by a refinery on the northeast , a tank farm on the east

the Mystic River on the south , Alford Street on the west , and a playground

and residential area on the north.

At this location , the Mystic River is approximately 1 100 ft wide and its

depths vary from 0 ft to 30 ft measured from mean low water. Amelia Earhart

Lock and Dam is about 0. 5 mi upstream of the site. This dam effectively

bounds the salt water portion of the river. Relatively small amounts of

fresh water pass the dam , making tidal exchange the only important natural

flow at the station. Mean tidal flow is approximately 1 000 cfs. The

highest known tide was at el 15. 78 ft. The lowest tide level was el -2. 72 ft.

Mean high water is at el 10. 45 ft , mean water level is el 5. 65 ft , and mean

low water is el O. 78 ft. Tide levels are referenced to Boston City Base

(eI O).

The Mystic

corner has

refineries

bank. The

12. 5 ft in

River is used for commercial shipping. Everett' s southwest

docking facilities for oil tankers and cargo ships. Two sugar

are located southeast of the station on the river s southern

United States Pier and Bulkhead Line is located approximately

front of the existing screenwells.

Mystic Station has four operating and three retired units. Units 1 , 2 , and

3, now retired , each had a capacity of 50 MW. Units 4 , 5 , and 6 each have

a capacity of approximately 150 MW. Unit 7 is rated at 600 MW. The six

older units were built between 1943 and 1961 and were originally coal-fired.

They were converted to oil-fired units by May 1966. Unit 7 was constructed

as an oil-fired station and started commercial operation in 1975.

All units have once-through circulating water systems. They draw water

through individual screenwells located in the southwest corner of the site

and have a common discharge east of the intakes. The discharge was designed

as a surface jet to provide improved mixing of the effluent with the cooler

river water. Discharge velocities vary from 4. 5 fps at high tide to 7. 0 fps

at low tide. Velocity decreases rapidly as river water is entrained. At

200 ft from the outfall , velocities are 2. 9 and 2. 3 fps for low and high

tide , respectively.

The general site plan for the station is shown in Figure 2-1.

Units 1 , 2 , and 3 at Mystic Station utilized four circulating water pumps

each in its own bay. Normally, three pumps operated and one was used as a

spare. All four bays are 10 ft wide at the entrances and have a bottom

elevation of -27 ft - 3 in. The curtain walls are at el -13 ft - 3 in.

2-1
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FIGURE 2-1

PLOT PLAN

ALTERNATIVE INTAKE STUDY

MYSTIC STATION - UNIT 7BOSTON ED I SON COMP ANY

STONE & WEBSTER ENG I NEER I NG CORPORA T I 
Each bay is

Uni ts 1 and

screen bay.

resul ting 

gross area.

equipped with trash racks located in front of the curtain wall.

2 have 6- ft-wide screen bays; Unit 3 has a 6 ft-8 in. -wide

When operating, each unit used 91 cfs of circulating water

a mean velocity at the curtain wall of 0. 65 fps , based on the

Units 4 and 5 each have one 10-ft-wide screenwell. The curtain walls are

at el -13 ft - 3 in. and the floors are at el -27 ft - 3 in. These screenbays have trash racks in front of their curtain walls and 9 ft - 0 in.

screens behind the walls. Each unit uses 169 cfs of circulating water

resul ting in a gross area mean velocity of 1. 21 fps under the curtain wall.

Unit 6 uses a single 9-ft-wide bay. A curtain wall extends to el -7. 25 ft

and the floor is at el -27 ft - 3 in. The trash rack and screen arrangement

are similar to those of the other five units. With its circulating water

flow rate of 169 cfs , the gross area mean velocity under the curtain wall

is 2. 35 fps.

Intake Design

The circulating water system for Mystic Station Unit 7 pumps 646 cfs at a

total dynamic head of 21. 6 ft when the river is at mean sea level. The

circulating water system was designed to remove 2. 77 x 109 Btu/hr from the

main station condensers with a maximum temperature rise of 19. 9 F. After

the Unit 7 discharge is combined with the discharge from the other units

the resulting temperature rise is approximately 19

The main features of the Unit 7 screenwell , shown in Figure 2-2 , are trash

racks , curtain walls , traveling water screens , and circulating water pumps.

There are two screenbays; each is 9 ft wide with a bottom elevation of

-27 ft - 3 in. Curtain walls extend to el -7 ft - 3 in. and are located

11. 5 ft behind the entrance to the screenbays. Trash racks consisting of

sloping steel bars are located upstream of the curtain walls. The racks

are manually raked to remove debris. An 8-ft-high bottom sill is installed

at the entrance of each screenwell to exclude flounder and other bottom

dwelling organisms.

The upstream faces of the traveling water screens are 25. 5 ft behind the

entrances of the screenbays. Screen panels are made from 3/8-in. wire mesh. The screens are equipped with low-pressure (30 psi) and high-pressure

(90 psi) spray headers. The purpose of the low-pressure spray is to wash

fish from the face of the screen and the trash lips. The high-pressure

spray, located above the low-pressure header , removes debris from the

screens. Fish and debris are washed into a common 16-in. semicircular

trough and are sluiced back to the Mystic River. I The Unit 6 screen also

washes into the Unit 7 trough. A minimum water depth of 6 in. is maintained in the trough when the screens are operating. Approximate flows

from each screen are 16 gpm for the low-pressure wash and 234 gpm for the

high-pressure wash , for a total of 250 gpm.

Behind the traveling water screens are two vertical mixed flow pumps. Each

is rated at 323 cfs. A specially designed suction elbow conducts water

from the screenwell to the pump impellers. Calculated velocities at mean

low water el +0. 78 ft in the screenwell are 2. 6 fps over the sill , 2. 9 fps

2-2
- 6

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SECTION A-A

10 

SCALE - FEET FIGURE 2-2

CIRCULATING WATER INTAKE

ALTERNATIVE INTAKE STUDY

MYSTIC STATION - UNIT 7

BOSTON ED I SON COMPANY

STONE & WEBSTER ENG I NEER I NG CORPORA T ION
under the curtain wall , and 3. 2 fps approaching the screen based on flow

streamlines.

Mystic River water quality is poor but improving. Trash loading of the

screens can be high at some times during the year. Also , jelly fish are

periodically impinged on the screens in large numbers creating operating

problems. Normally the screens are rotated once per 8-hr shift.

2-3
SECTION 3

SUMMARY OF FISH IMPINGEMENT EXPERIENCE

3. 1 INTRODUCTION

Impingement monitoring studies were conducted at Boston Edison Company I s

Mystic Generating Station from August 1971 to November 1977. The studies

consisted of (1) an impingement monitoring program for Units 1-5 combined

and Unit 6 from 1971 to 1975 , including a bottom sill evaluation for Unit 

(2) an impingement monitoring program for Units 4 , 5 , 6 , and 7 from 1975 to

August 1976; and 3) an impingement monitoring program for Units 6 , 71, and

72 from December 1976 through December 1977 , including a fish recirculation

and reimpingement study, a bottom sill evaluation for Unit 71, and a winter

flounder mortality study. A summary of these studies is presented below.

RESULTS OF IMPINGEMENT MONITORING STUDIES

Fish impingement monitoring began at Mystic Station in August 1971 and was

centered around Units 1-5 and Unit 6. Field data indicated that flounder

Atlantic herring, Alosa spp. (alewife and blueback herring), and rainbow

smelt were the most frequently impinged species. From 1971 through 1973

Unit 6 consistently impinged more fish (two to three times as many) than

Units 1-5. The higher rate of impingement at Unit 6 was believed to be

attributable to a higher intake velocity (Stone & Webster Engineering

Corporation (S&W), 1976). In December 1973, at the request of the U.

Environmental Protection Agency (EPA) and the Massachusetts Division of

Marine Fisheries , a bottom sill , consisting of an 8-ft stop log gate , was

installed in the Unit 6 intake bay in an attempt to reduce flounder impingement. In addition , 12 ft - 2 in of the curtain wall were removed , raising

it from el- 19 ft - 5 in. to el - 7 ft - 3 in. and thereby reducing the

velocity in this area. During the 1974 monitoring program , the bottom sill

successfully reduced flounder impingement by 53 percent and reduced impingement of all other species by 27 percent (S&W , 1976).

In 1975 , with the addition of Unit 7 to the Mystic Station , total impingement of fish increased. Unit 7 accounted for approximately 87 percent of

the fish impinged that year. The higher impingement rate for Unit 7 was

tentatively attributed to the higher flow rate and intake velocitiesDuring the 1976 monitoring program , Units 6 and 7 , operating individually

or together , accounted for 96 percent of all fish impinged. Herring and

flounder were the two most abundant species impinged. Flounder was more

abundant in the spring and fall collections than in the winter. Fish

length data indicated that the majority of flounder , herring, and alewives

impinged were juveniles. Impinged smelt appeared to be adults , at least

2 yr old.

Since the impingement of fish at Unit 6 had been effectively reduced by

reduction of the curtain wall and the installation of a bottom sill , it was

recommended that a similar sill be installed in one of the two forebays of

Unit 71 and that impingement rates of Unit 71 (with the sill) and Unit 72

3-1
(without a sill) be compared. At the request of the EPA and the Massachusetts Water Resources Commission, a 52-week impingement monitoring program

was conducted from December 1976 through November 1977 at Units 6 , 71, and

72, The purpose of the program was to determine the effectiveness of the

bottom sill in reducing flounder impingement at Unit 7 and to examine

several additional aspects relating to impingement , including recirculation

reimpingement , and flounder mortality, as discussed later.

With respect to winter flounder , the results of the bottom sill evaluation

in 1974 were consistent with the results obtained for Unit 6. Impingement

of flounder in Unit 71 was reduced by 53 percent. However , impingement of

pelagic species was variable and species-dependent. An increase in the

impingement of Atlantic herring, rainbow smelt , and other species was

observed in Unit 71 when compared to Unit 72'

Al though the da ta collected indicated an increase in the impingement of

pelagic species , the reduction in flounder impingement resulted in the

recommendation that bottom sills be installed in Unit 72 as a deterrent to

flounder. A bottom sill was subsequently installed in Unit 72 in November 1977.

Based on the rate of impingement calculated during the impingement monitoring program from December 1976 to November 1977 , it was estimated that

000 flounder , 16 000 Atlantic herring, and 43 000 Alosa spp. would be

lost annually. This conservatively assumes 100 percent mortality of fish

which have been impinged. As discussed later , mean survival of flounder

under existing operating conditions approaches 50 percent. Therefore , the

estimated loss of this species due to impingement is conservatively high.

These numbers represented O. 14 , 0. , and O. 16 percent of the average

1970-1977 Massachusetts annual commercial catch for these species , respectively (S&W 1978). In terms of the dollar value of the fish impinged at

the Mystic Station (assuming 100 percent mortality), approximately $1 747

of flounder , $161 of herring, and $22 of Alosa spp. would be lost annually.

Based on the results obtained during impingement monitoring studies from

1971 to 1976 , it became evident that the mortality associated with impingement at Mystic was not 100 percent , and that , for some species , survival

may actually be relatively high. Accordingly, various studies began in

late 1976 to investigate the survival potential of fish and to determine

whether certain design and operational features of the screenwell might be

contributing to mortality. Specifically, these studies were designed to

(1) determine the physical condition of fish being impinged on the Units 6

and 7 screens , (2) determine whether fish returned to the Mystic River via

the sluiceway are subject to recirculation , (3) establish whether fish tend

to fall off of the screen s lifting lips during screenwashing and are

thereby subj ect to reimpingement , and (4) evaluate the effect of reducing

the time interval between screenwashings on flounder survival.

The physical condition of impinged fish was investigated from December 1976

to May 1977. Sampling consisted of collecting all fish impinged on the

Units 6 and 7 screens over 9-hr periods , determining initial survival , and

checking the fish for screen markings , scale loss , and other damage. The

data indicated that the majority of the impinged fish were dead (88 percent)

and consisted primarily of the fragile clupeids and smelt. Winter flounder

3-2
cunner , and threespine stickback survival was higher (36 percent for all

three species). Mummichog exhibited the highest survival (40 percent) of

all species collected. Considering that these fish were impinged for up to

9 hr prior to removal , it would appear that higher survival could be achieved

if the screens were washed more frequently. This conclusion is substantiated

by results of the flounder mortality study discussed later.

The recirculation , reimpingement , and winter flounder mortality studies

were conducted between October and December 1977. During the recirculation

study, marked live and dead fish were released from the screenwash sluiceway

servicing Units 6 and 7 on the high and ebbing tide (maximum potential for

recirculation) Following release , the traveling screens of all operating

units were monitored daily for 72 hr. In four releases of 259 fish (total),

only 11 fish were recirculated in 72 hr , most of which had been dead when

released. Therefore , recirculation of live fish is not considered a problem

at Mystic.

During the flounder reimpingement study, live and dead flounder were tagged

and released in the Unit 71 screenwell between the curtain wall and traveling screen. The screens were then operated continuously for 8 hr and the

time of recapture was noted for each fish. The results of this study

indicated that flounder reimpingement does occur , particularly among live

fish. Of the dead test fish recaptured , all but one were collected within

the first 15 minutes of the screenwashing (one screen revolution). 

three tests , a maximum of 40 percent of the live fish were recaptured

during the same period. Recapture of live fish continued at a low level

throughout the 8-hr collections. It was noted that , although all fish

removed from the screens were still alive even after 8 hr , the condition of

those fish recaptured toward the end of the period was poorer than those

recaptured during the first hour. Therefore , it appears that live fish are

flipping off the screen lips as the screen clears the water surface and are

dropping back into the screenwell. This process certainly contributes to

mortality and probably accounts for the fact that survival of flounder

observed during the impingement mortality study (discussed below) was lower

than expected for such a hardy species.

The flounder mortality study involved the collection of fish from the

Units 71 and 72 screens under the following operating modes: continuous

2-hr , 4-hr , and 8-hr washes. Fish were collected over an 8-hr period which

encompassed low tide (when impinged fish would experience higher intake

velocities). Initial mortality was recorded and the fish were then held

for 96 hr. Mortality was recorded daily.

In general , results of this study showed a trend of increasing mortality

with an increase in impingement duration. Initial mortality (Table 3-1) of

impinged flounder for the continuous , 2-hr , 4-hr , and 8-hr screenwash

intervals ranged from 2. 6 percent (continuous) to 41. 4 percent (4-hr interval).

Analysis of variance CANOVA) demonstrated that the differences in initial

mortality between screenwash intervals were not statistically significant

at the p ( 0. 05 level (Table 3-2). However , flounder mortality after 96 hr

ranged fr 10. 3 percent (continuous) to 58. 2 percent (4-hr interval), and

ANOVA and least- squares differences demonstrated that survival under the

continuous wash was significantly higher than at the 4-hr screenwash interval.

Other wash intervals were not significantly different.

3-3
TABLE 3-1

MORTALITY OF WINTER FLOUNDER AT SEVERAL SCREENWASH INTERVALS

WITH 95 PERCENT CONFIDENCE INTERVALS

Percent Mortali ty

Initial 96-Hr Total Screenwash Interval

Continuous 6 (05 _16. 10. 3 (0. 4-30. 14. 6 (1. 0-39.

22. 3 (5. 6-45. 29. 3 (9. 3-54. 44. 2 (17. 9- 72.

41. 4 (18. 6-66. 58. 2 (32. 4-81. 66. 2 (37. 6-89.

35. 6 (14. 2-60. 38. 6 (15. 7-64. 54. 4 (26. 3-81.

2-Hr

4-Hr

8-Hr

NOTES:

1 . Calculated from arcsine transformation.

Percentage of flounder found dead on screens.Percentage of flounder which died during the 96-hr test period. Number dead on screen plus number died during 96-hr test perioddivided by total number of flounder collected.

Confidence interval less than zero.

Source: Stone & Webster Engineering Corporation , July 1978
TABLE 3-2

SUMMARY OF AN ANALYSIS OF VARIANCE OF DIFFERENCES OBSERVED IN THE MORTALITY OF WINTER FLOUNDER

AT SEVERAL SCREENWASH INTERVALS AND DURING THE 96-HOUR HOLDING PERIOD

Initial

Mortali ty

F-Ratio

96-hour

Mortali ty

Total

Mortali ty Source of Variation

Degrees ofFreedom

Screenwash Interval1 (A) 294 5. 741;~ 196;

96-hour Test Period2 (B) N/A 289; N/A

A x B N/A 430 N/ALeast Square Differences

(significantly different

screenwash intervals

at p 05 level)

Cont. vs. 4-hour Cont. vs. 4- hour

NOTES:

1 . Continuous , 2-hour , 4-hour , and 8-hour screenwashes.

Four 24-hour periods within 96-hour test period.

*Significant at p 05 level.

Source: Stone & Webster Engineering Corporation

July 1978
Total mortality (initial plus 96-hr) ranged from 14. 6 percent (continuous

wash) to 66. 2 percent (4-hr screenwash interval). Again , least- squares

differences demonstrated that the continuous wash survival was significantly

different only from the 4-hr screenwash.

Mortali ty among flounder held as controls was low , averaging 2. 5 percent

for the four 96-hr monitoring periods and indicating that mortality among

the test fish was most likely the result of impingement.

CONCLUSIONS

On the basis of studies conducted at Mystic to date , several conclusions

can be drawn:

1 . The bottom sill is effective in reducing flounder impingement;

however , it may increase the impingement of pelagic species.

Survival of pelagic species under the present operating conditions (8-hr wash cycle) is relatively low.

On the average , survival of flounder is relatively high and tends

to be highest under a continuous screenwash operating mode.

Reimpingement of flounder , particularly live individuals , is

occurring; survival might be enhanced if reimpingement were

eliminated.

Recirculation of fish from the screenwash sluiceway to the intake

screens does not appear to be a problem.

These results indicate that modifications to the existing traveling screens

which would reduce impingement time and eliminate reimpingement might

greatly enhance survival of flounder as well as pelagic species. An alternative scheme incorporating such modifications is presented in Section 

3-4
SECTION 4

EVALUATION OF ALTERNATIVE INTAKE

DESIGNS FOR THE MYSTIC STATION - UNIT 

4. 1 INTRODUCTION

As discussed in Section 1 , there are two basic approaches to resolving the

question of impacts related to fish impingement losses at power plants.

One approach is to gather biological field data in the area of plant influence and develop an impact assessment based on data analyses. For the

reasons discussed below , this approach is not deemed cost-effective for the

Mystic Station. The second approach is to modify an existing intake design

or alter operating conditions , such that the number of fish impinged is

actually minimized. This approach is preferred for the Mystic Station , and

descriptions of various alternative designs or operational modifications

are presented in this section.

Assessment of Fish Impingement Losses on the Population

Fish impingement represents a loss of individuals to their respective

populations and can therefore be characterized as an additional source of

exploitation to species populations. Considerable experience has been

gained in the assessment of exploitation effects on a population

, primarily

in the area of fisheries management. Therefore , one useful method of

evaluating impingement effects is the analysis of this exploitation on the

persistence of the species population.

This type of assessment has been applied at many power plant sites with

varying degrees of sophistication. The type of information available on

the size and dynamics of the population of the exploited species 

is the

primary limiting factor in the level of sophistication of the analysis.

Basic population dynamics information on length of life , age of sexual maturity, sex ratio , fecundity, and survivorship can frequently be obtained

from scientific literature. Information on the local population is usually

more difficult to obtain and , if not presently available , is quite expensive

to gather. If , however , the population is presently being exploited for a

commercial fishery, this type of information can usually be developed from

reported landings.

The information of interest for the local population is its geographic

distribution and its size by age. Such information is not available for

the local populations of fish found in the Mystic River. Without this

information , the only assessment that can be made is the static loss due to

impingement. This static loss is helpful , as can be seen in the comparison

discussed in Section 3 of this report.

In summary, proceeding from the present level of analysis to a dynamic

assessment of the effect of impingement on the respective species ' populations would require extensive information on the local populations of these

species. The information would need to focus on the size and distribution

of the population with attention to secondary characteristics such as age

structure. Due to the location of Mystic Station and the nature of the

4-1
species involved , primarily herring, alewife , flounder , and smelt , a considerable field effort over several seasons throughout the harbor and possibly

adj acent areas would be required to study these populations. Such an

effort is not believed to be practical since alternate design modifications

exist to minimize impingement losses.

Diversion Devices

Diversion devices are not considered to represent primary alternatives for

minimizing impingement losses at Mystic Station for a number of reasons.

Inclined plane screens are not commercially available at present. Large

revolving drum screens require more costly support structures than vertical

traveling screen structures and have not been evaluated biologically (U. S.

EPA , 1976); thus , they are not considered to offer an advantage over the

existing screen structures at Mystic. The horizontal traveling screen has

been shown to be an effective device for guiding juvenile and adult fish;

however , application of this screen system at power plant intakes is presently

impractical since the screen can only be used when water depth does not

exceed 10 ft and is relatively constant (Rexnord , 1973). Furthermore , the

horizontal traveling screen is not commercially availabl~.

Although louver systems have been shown to have high guidance efficiencies

with respect to juvenile and adult fish (Taft and Mussalli , 1978), stationary

louvers at Mystic could create reliability problems due to their potential

for clogging. Traveling louver systems minimize clogging problems but are

costly and would require maj or structural modifications. In addition

since louvers have wide spacings (1 in. or greater), backup screens are

required to collect unscreened debris and fish. Thus , the overall cost of

a louver system is relatively high. Therefore , louvers are not considered

to offer an operationally reliable or cost- effective means of reducing fish

losses at Mystic.

Angled traveling screens operate on the same guidance principle as louvers.

However , since the screens utilize 3/8-in. mesh , they act as both a diversion

device and a screening medium (Taft and Mussalli , 1978). Installation of

angled traveling screens at Mystic would require maj or modifications to the

existing intake which are not warranted at this time considering the availability of other less costly and effective systems as presented later in

this section.

Behavioral Barriers

A number of devices have been developed which are designed to alter , or

take advantage of , the natural behavioral patterns of fish so that they

will avoid , or be deterred , from an intake flow. These devices include

electrical screens , air bubble curtains , hanging chains , lights , sound

water jet curtains , and chemicals. However , the potential effectiveness of

any of these devices at Mystic Station is questionable , based on experience

at other power plants and hydroelectric facilities (U. S. EPA , 1976).

Therefore , behavioral barriers are not considered to offer a viable al ternative for minimizing fish impingement at the Mystic Station.

4-2
Fish Collection Systems

Collection systems are devices which are designed to actively collect

organisms entrapped within a screenwell. These include fish pumps , netting

devices , and various types of traveling water screens.

Fish pump systems are generally used where velocities are relatively low

and uniformly distributed. They operate on the principle that fish entering a screenwell will congregate in low velocity areas at the surface and

therefore , will be susceptible to the suction of the pump (Eisele and

Malaric , 1978). However , bottom-dwelling fish , such as flounder , would

probably not be successfully collected. Thus , the fish pump systems are

not considered to offer a viable means of collecting and removing fish from

the Mystic screenwells. Similarly, fish netting systems , although they

have been shown to be effective interim measures for reducing fish losses

at several power plant intakes (Detroit Edison Co. , 1975), are not considered

to offer an effective long-term solution to impingement losses.

The center-flow vertical traveling screen is also not considered viable for

Mystic , since use of this system in the existing screen bays would result

in unacceptably high intake velocities (throat velocities ranging from 4.

to 9. 9 fps , corresponding to locating the screen at the front of the bay or

at the present location) and would adversely affect pump performance due to

uneven flow distribution downstream of the screens.

Modified , through-flow traveling water screens are presently considered to

offer a potentially effective and practical means of reducing fish losses

at Mystic. The modified through-flow screens incorporate a low pressure

spraywash system , with fish lifting buckets and frequent or continuous

operation. This system is discussed in detail in Section 4.

Physical Barriers

The concept of using low screen velocities for reducing fish losses at

power plants has been considered for many years and has been recently

incorporated into the design of various new intake structures. By reducing

velocities at the traveling screens , fish are better able to avoid impingement , and the screen , therefore , acts as a physical barrier (U. S. EPA

1976). The EPA has generally recommended velocities of 0. 5 fps or less for

reducing impingement losses at new intakes. However , the actual reduction

in impingement which can be achieved through low velocities has not been

quantified. At Mystic Station , there are a number of structural or operational modifications which could be considered for achieving lower intake

velocities than presently occurring. These modifications are discussed in

detail in Section 4.

Physical barriers , such as infiltration intakes (radial wells or artificial

filter beds), are not feasible at Mystic since they require an adequate

aquifer and other geological conditions not found at Mystic. Barrier nets

however , have been shown to be effective under certain conditions and have

been considered for application at Mystic Station. Such nets are described

in detail in Section 4.

4-3
Summary

On the basis of the available information , it is believed that three alternative fish protection concepts offer the potential for reducing fish losses

at Mystic Station and therefore warrant further consideration: velocity

reduction , barrier nets , and modified traveling screens. Accordingly,

various methods of implementing these concepts at Mystic are presented in

detail in Sections 4. , 4. , and 4. , respectively. Total costs for each

engineering design are given individually; a complete breakdown of these

costs is presented in Section 

VELOCITY REDUCTION

Biological Evaluation

The concept of designing intake structures to incorporate low velocities at

the traveling screens has recently received much attention as a method for

minimizing fish impingement. In fact , the use of velocities of 0. 5 fps (or

less), coupled with mounting the screens flush with the shoreline (no

embayments) and creating lateral fish passageways , has currently been

accepted as best available technology at some power plant sites. Unfortunately,

since few intakes of this design are in operation , few studies have been

conducted to quantify the level of protection which can be expected.

Resul ts of one study conducted at a Hudson River power plant indicated that

reducing approach velocities from 1. 0 to 0. 5 fps did not significantly

reduce the impingement of fish (Texas Instruments , 1974).

Until further research has been conducted at power plants with low intake

velocities , it will not be possible to draw general conclusions regarding

acceptable velocities at specific sites. It is likely that velocity effects

are species- specific , age-dependent , and influenced by seasonal variations

in water quality and other environment~l parameters (particularly temperature),

since all of these factors influence the swimming speed of fish. Thus

while various design modifications for reducing velocities are possible at

Mystic (as discussed below), it is not possible to estimate quantitatively

the potential reductions in impingement which might be achieved through

their implementation. It is reasonable to assume that reducing the approach

velocities at Mystic would not have a negative effect , and , in some cases

may reduce impingement. However , considering the cost (Section 5) of

implementing most of the alternative designs and the uncertainty regarding

their potential effectiveness , it is not believed that the concept of

reducing velocities is the most cost-effective option for reducing fish

losses.

This general discussion applies to all of the alternatives described below.

However , each design modification has additional biological advantages

and/or disadvantages which are addressed further in the following discussions.

Engineering Descriptions of Alternatives for Reducing Velocities

Removing the Bottom Sill at Certain Times of the Year

As discussed in Section 3 , a bottom sill placed in the Unit 6 intake bay at

Mystic Station was successful in reducing flounder impingement by 53 percent.

4-4
Impingement of all other species was reduced by 27 percent. A similar

bottom sill , installed in one of the two forebays of Unit 7 , also reduced

flounder impingement by 53 percent; however , impingement of other species

increased , presumably due to the increased velocities created by the presence

of the sill.

It has been suggested that removing the bottom sill at Unit 7 during certain

times of the year might reduce impingement of pelagic species. This suggestion

was based on the idea that there may be differences between the seasonal

abundance of pelagic species and demersal species. If this were the case

the sill could be installed only during the period of flounder impingement

while during the remainder of the year the sill would be removed , thereby

eliminating the potential for increased impingement of pelagic species.

Although seasonal variations are observed in the total numbers of fish

impinged at Unit 7 , (generally fish are more abundant in the spring and/or

fall than during the winter , except for of Atlantic herring, which was more

abundant during the winter and spring), data indicate that the seasonal

pattern of impingement of flounder is similar to the pattern for other

species. Therefore , periodic removal of the bottom sill in Unit 7 would

probably not reduce impingement of pelagic species and might cause an

increase in total flounder impingement.

Utilizing the Units 1-3 Intake Bays for Supplemental Cooling

Water for Unit 

Another concept for reducing velocities of Unit 7 would involve the use of

the decommissioned Units 1-3 screenwells and pump houses to supply additional

screening area for the Unit 7 flow. Of the total 646 cfs required for

Unit 7 , 320 cfs would be pumped from the Unit I , 2 , and 3 screenwells

conveyed by a new pipe network, and discharged into each of the two bays at

Unit 7. The existing Unit 7 screens would be relocated farther upstream in

the bays to provide sufficient space for the 320-cfs flow to be discharged

downstream of the screens. The remaining 326 cfs required would pass

through the Unit 7 screens. The approach velocity at the face of the

screen mesh for Unit 7 would be 0. 87 fps at mean low water el 0. 78 ft.

As shown in Figure 4. 2-1 , the forebays of Units I , 2 , and 3 are each 10 ft

wide at the entrance where the trash racks are located. The bays then

transition down to a 6-ft width at the face of the traveling screens.

Prior to decommissioning, Units 1 , 2 , and 3 had a normal rated flow of

000 gpm (80 cfs) at mean low water (el 0. 78 ft). This flow is based on

three pumps operating, with one pump idle as a spare. At this flow , the

approach velocity at the face of the screen mesh is 1. 57 fps. The 36 000-

gpm flow rate per bay was selected as the quantity to be pumped to Unit 

while maintaining the previous maximum appproach velocity of 1. 57 fps.

It would be necessary to replace the four existing vertical volute dry pi 

pumps at Units 1 , 2 , and 3 with vertical column axial flow pumps (Figure 4. 2-2)

each rated at 36 000 gpm at approximately 5 ft of head. The pump suction

bells would be connected directly to the existing suction tubes located at

pump pit floor level. A hydraulic model study would be required to determine

if any modifications of the suction tubes would be needed to provide suitable

flow patterns for the axial flow pumps , since such pumps are more sensitive

4-5
EXISTING 5 '11 PIPES TO UNITS

L ~-

21TS

4 - 6

'- - I , -J 

C. W. PUMPS

-t-- 

I'--

SUPERSTRUCTUREPENETRA TE

PI PES GU I DE 

EXISTING SCREEN

INSTALL BAFFLE IN

SLOT

( TYP)

DECK

PENETPI PES

INSTALL REMOVABLE I

TRASH RACKS

BFV

CZJ

TR V. 

RELOCATED!

WATER

SCREENS

RACKS

TRASH

FACE tTYP)

SlOPI~ 

I I 

....

1

-

EL. . J I I I EXISTINq STOP lOG SLOT

...

... , 
to inlet flow conditions than the existing volute pumps. The four new

pumps would have an additional installed power requirement of 373 kW. The

screens at Units 1 , 2 , and 3 would require additional installed power of

45 kW. The additional annual energy required for this scheme would be

294 MWh.

The four pump discharge pipes would be located above the operating deck

level and would manifold into one 7. 5-ft-dia pipe. This pipe would be

supported on a pile and steel frame structure constructed along the eastern

side of the Unit 1 screenwell and then along the southern (harbor) side of

the screenwells of Units I through 7. The pipe would then bifurcate into

two 5-ft-dia pipes on the western side of Unit 7. These pipes would penetrate the Unit 7 screenwell superstructure , and one pipe would penetrate

the deck above each of the two Unit 7 bays. Each pipe would be equipped

with a butterfly valve for use in flow balancing or shutoff in the event

that one of the associated Unit 7 pumps were shut down.

A detailed hydraulic model study would be required to assure uniform flow

patterns approaching the Unit 7 pumps. Pending results of such a study, it

might be necessary to include diffusers at the discharge of each pipe to

spread the flow in the screen bays and a vertical baffle array between the

diffusers and the pump inlet conduits to straighten the flow (Figure 4. 2-2).

Baffles may also be required between the relocated traveling water screens

and the diffusers to assure that flow from the diffusers does not disturb

the flow patterns at the screen faces. A reduction in flow through the

Unit 7 pumps may result if the baffles required have significant head loss.

Based on an estimated 1-ft head loss , the flow would be reduced about

3 percent.

New trash racks would be installed in the existing Unit 7 stop log slots

upstream of the screens. The rack structure would be designed such that

stop gates could be placed over the rack face for dewatering.

Most of the new pipe network would be operated under siphon. Therefore , a

vacuum priming system would be installed to fill the pipes and then maintain

the siphon by continuous air removal.

The existing traveling water screens at Units I , 2 , and 3 would be replaced

the screen wash systems would be rebuilt , and the trash racks would be

repaired , if necessary. A bearing lubrication and seal water system would

be required for the new pumps. Electrical systems would be rebuilt as

necessary to meet current codes and system needs , and new control systems

would be installed for all operating equipment. The screenwells and pump

house would be renovated , as required.

The existing Units 1 , 2 , and 3 screenwells are presently used as a dock and

unloading facility for oil barges. These barges have a 22-ft draft and

would obstruct or distort flow patterns at the screen bay entrances if used

as a supplemental intake. Therefore , it would be necessary to construct a

new wharf east of the screenwell with protective dolphins and an access

ramp to shore to avoid this problem.

The total capitalized cost of implementing this design for reducing intake

velocities would be $6 674 000.

4-6
RELOCATED TRAVEL ING

WAT ER SCREEN

EL, 17-6

BAFFLE

INSTALL

II (WALL NSTALL 

10-

RACKS

TR~SH

, I

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...

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AX.

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MEAN H. W.

(-) 2.

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CURTAIN WALL TO EL .

INSTALL REMOVAB LE

(-)3_

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,

 .

DIFFUSER

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9 '- 0

....

.... 

..

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9 '- 0

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. 

. 

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MOTOR ,EXISTING PUMP

REMOV E

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: 1

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INSTALL N

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EX ISTINBOLT SU
Reducing Unit 7 Operation to One Circulating Water Pump

Another method of reducing intake velocities at Unit 7 would involve reducing the flow of circulating water to the plant at selected times by shutting

down one of the two circulating water pumps. Unfortunately, this mode of

operation can have serious impact on plant load and heat rate.

The use of a single circulating water pump would not be feasible , even at

low demands , for several reasons.

Reducing the flow may result in increased bio-fouling of interior tube

surfaces since velocities would be substantially reduced (6 fps to 3. 6 fps).

It would be possible to limit biofouling by isolation of condenser sections;

however , the ability of Unit 7 to respond to sudden system demands would be

seriously impaired.

As a result of reduced flow , system pressures would drop, which may cause

increased corrosion of tube surfaces since tube surfaces would be exposed

at low tidal conditions. Loss of system pressure would also necessitate

throttling of the condenser outlet valves to maintain sufficient flow in

the cooling water heat exchangers. This would cause a further loss of

cooling efficiency.

With a two-pump parallel system , shutting down one pump will cause the

second pump to run out to 60 percent of the two-pump flow. This would

reduce pump efficiency from 84 to below 40 percent , with a corresponding

increase in motor load and subsequent shortening of pump motor life. Since

the operating pump will actually pick up 60 percent of the two-pump flow

veloci ty into the operating bay will increase by 20 percent.

If single-pump operation was implemented during peak load periods several

addi tional problems would occur. In order to go from two-pump to one-pump

operation, load would have to be decreased gradually to avoid unnecessary

stresses to plant equipment and subsequent reduction of equipment life.

Flow reduction and subsequent loss in pressure would reduce plant output by

as much as 280 MWe at an equivalent cost of $10 750/hr for replacement

power. Thermal criteria would also be violated at high load conditions. A

temperature condenser rise of up to 35 of could occur with one-pump opera tion;

consequently, during times of the year when ambient water temperatures are

high (above 60 F), maximum allowable outfall temperatures of 93 F would be

exceeded.

In summary, reducing the circulating water system flow by operating with

one of two circulating pumps would have the following results:

Condenser tube fouling due to tube water velocity reduction from

0 fps to 3. 6 fps;

A severe loss of plant generating capacity, approaching 280 MW/hr

during peak demand periods;

Violation of thermal rise and/ or discharge temperature limitations will occur at loads low enough to allow operation of the

4-7
condenser below turbine back pressure limits (5 in. ) and above

the plant load necessary to violate the temperature limitations;

A reduction in pump life;

A reduction in condenser reliability and availability due to

increased corrosion problems; and

A 20 percent increase in approach velocity to the operating

screen.

Accordingly, although one-pump operation would limit impingement to one of

the two Unit 7 screens , increased velocities could actually increase impingement on the operating screen. Such operation would also create serious

reliabili ty problems which could jeopardize plant output. Therefore , it is

not believed that potential environmental benefits of this operational mode

justify the risks and costs.

Installing Variable Speed Control On Circulating Water Pumps to

Control Discharge Temperature

Another method for reducing the flows and velocities at the Mystic Unit 

intake structure would involve installing variable speed capability on the

circulating water pumps to control flow based on discharge temperature.

Theoretically, by reducing the speed of the pump, a reduction in flow rate

directly proportional to the speed could be realized. However , as the

speed is reduced , the pump discharge pressure would fall off in proportion

to the square of the speed reduction. This would be done in order to

maintain the discharge channel temperature at below 93 F and to prevent the

thermal rise from intake structure to outfall channel from exceeding 2s

The required speed reduction at a 60 F inlet water temperature with only

Unit 7 operating would be approximately 33 rpm , which represents a 13 percent

reduction. This would result in a decrease in velocity approaching the

intake screens to approximately 2. 8 fps , at mean low water el. 0. 78 ft.

During the warmer months when river temperatures are high , flow reductions

would not be allowable at all. In order to achieve this reduction in

veloci ty by pump speed reduction , the following options are available:

1 . Two- speed Motor Drives - By analyzing the discharge flow rate

tidal level , intake water temperature , and Units 4 through 

operating loads , it might be possible to find a single lower

speed which would provide some reduction in flow rate at constant

second speed. A two- speed motor drive arrangement would be the

least expensive and least complex speed control method available.

The key factor in applying two- speed motors is in the coincidence

of the circulating pump system operating point with an available

second speed on the motor. Two- speed , single-winding motors are

limi ted to a 2-to-l speed ratio. Since the second operating

point at Mystic is not one-half of normal speed , these motors

were not considered. Two- speed , two-winding motors , not limi ted

to 2-to-1 speed ratios , may have a second speed at any synchronous

subset of base speed; for example , 1 200/900 rpm , 1 200/720 rpm

900/720 rpm , etc.

4-8
Finding a second speed which would be matched to the entire plant

circulating water system and the motor is a complex procedure

which would be unlikely to provide significant flow reductions at

Mystic. Also , operating the circulating water pumps with two- speed

motors would require timed load reductions which would further

complicate the operating process.

Dc Motor Drive - Direct current motors (with electronic ac to

adjustable voltage Dc power) have become a most common means of

providing adjustability in speed and torque. A limiting factor

in dc motor application is the fact that energy is supplied to

the motor rotor through a series of copper bars around the periphery of the rotor , called the commutator. Large amounts of

driving power must pass from the dc line to the rotor through

stationary carbon brushes rubbing against rotating commutator

bars. This energy transfer may reduce the practical base speed

of a high horsepower dc motor drive to a point below the speed

required for the circulating water pumps. Constant maintenance

requirements of the commutator and brushes also discourages the

application of dc drives in this case.

Electronic ac Motor Drives - There are two types of electronics

drive systems available. The first of these ac power systems

utilizes wound rotor motors to drive the pumps. Simply stated

the speed of a wound rotor motor can be adjusted by changing the

power flow through its rotor circuit. This system is referred to

as the wound rotor slip recovery drive.

The second of these ac power electronic drives is known as the

synchronous motor adjustable frequency system. This system works

on the principle that the synchronous speed of the motor 

directly proportional to the frequency applied to the motor

terminals and inversely proportional to the number of its field

poles. This system is about 2 percent more efficient than the

wound rotor slip recovery system , making it the most efficient

electrical system available over wide speed ranges. Utilizing

brushless excitation to supply rotor field current , the system

has no commutator or slip ring with their attendant maintenance

problems. Other benefits of this system are equal acceleration

and shutdown times and low electrical line power draw.

The cost of the most flexible variable speed conversion , including

motors and controls , is approximately $925, 000. The practical

benefits of a variable speed conversion are minimal. Screen

approach velocity would be reduced from 3. 2 fps to approximately

8 fps (mlw) or lower (depending on the various uni t loads).

However , operating problems would result , including lower pump

discharge pressures , tube fouling, increased pump motor maintenance

and potential thermal violations , since the speed changes necessary

to avoid thermal limitations are limited by time versus load

pickup restraints. Changes in thermal discharge temperatures

could occur quickly during unit trips , necessi tating increased

flow to the remaining condensers in operation.

4-9
In view of the minimal reduction in velocity which could be

achieved with a variable speed control on the circulating water

pumps , the operational problems which would result , and the cost

of implementation , this scheme for reducing velocities is not

considered viable at Mystic Station.

Installing the Screens Flush with the Front of the Intake Bays

As discussed previously, the flush mounting of screens , combined with

lateral fish passageways and low velocities , has been suggested as a possible means of reducing fish losses at power plants. There are , however , no

data which would indicate the degree of reduction in impingement which

might be expected to occur at different sites. Installing screens flush

with the front of the intake at Mystic would result in a more uniform

velocity distribution through the screens than presently exists due to the

increased distance between the circulating water pumps and the screens. In

addition , the existing screenwell embayment , which may act to entrap fish

would be eliminated. However , although these improvements would probably

reduce impingement to some extent , the total flow through the screens would

not change and the velocities would still be relatively high (approximately

7 fps at mlw). Therefore , the potential biological effectiveness of this

design is uncertain.

This scheme would consist of installing the traveling screens flush with

the piers at the front of the Unit 7 intake bays , as shown in Figures 4. 2-3

and 4. 2-4. The existing traveling screens , curtain walls , and racks would

be removed. New screens , with either a back-wash or front-wash system (as

discussed in Section 4. 2), would be installed flush with the front of the

intake bays. The screens would be designed for continuous operation , if

required. However , the additional annual energy requirements of 4 MWh are

based on intermittent operation. New vertical racks and curtain wall would

be located in front of the existing structures. The racks and piers and

the curtain wall would be separated from the screen by 5 ft to provide a

lateral fish passageway. The curtain wall would extend down to el -3. 0 to

exclude cold air in winter and to block large floating debris. The trash

racks would be cleaned by hoist-operated mechanical rakes traveling in

guides down the sides of the racks. Each traveling screen could be isolated

by stop logs and dewatered for repair. A superstructure for the screenwell

addi tion would enclose all equipment.

During scheduled downtime , the area in front of the Unit 7 screenwell could

be excavated , piles driven , and a tremie concrete mat poured supported on

the piles. During normal operation , the piers and curtain walls of the

rack structure would be fabricated as a unit from steel plate on shore and

partially filled with concrete as ballast. This unit would then be floated

into position at the face of the Unit 7 screenwell and anchored into place.

All joints would be grouted to provide an effective seal and the remainder

of the steel plate unit would be filled with concrete.

After the rack structure was completed , the area surrounding each new

screen would be graded to allow installation on the screen in the dry.

This would require partial decrease in generation due to one pump shut-down

during screen installation.

4-10
.

,

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The total capitalized cost for this alternative intake design will be

168 000.

Considering the cost associated with moving the screens , the fact that

velocity reductions are minimal and that the potential biological effectiveness is unknown , it is not believed that such modification is warranted.

Constructing Additional Intake Bays

The purpose of this concept would be to provide flush-mounted traveling

screens while achieving a maximum velocity at the screen mesh of 0.

5 fps

(at mean low water el O. 78 ft). This concept would also provide a lateral

fish passageway along the face of the screens.

The flow approaching the existing Unit 7 screens would have to be reduced

in order to lower the velocity. The design flow of Unit 7 is 646 cfs. The

maximum allowable flow through the Unit 7 screens to meet a 0. 5 fps criterion

would be 180 cfs. Therefore , the flow through the existing screens would

have to be reduced by 466 cfs. This would be achieved by constructing two

additional intake bays containing two supplemental pumps

' and associated

screening equipment.

The additional screenwell would be constructed on the western side of the

Unit 7 screenwell and would be connected to the screenwell wall 

(Figure 4. 2-5).

Each pump would discharge into a separate s-ft-dia pipe which would

traverse above the operating deck and penetrate the Unit 

7 screenwell

superstructure wall. Each pipe would then penetrate the deck above a

screenbay and discharge downstream of the flush-mounted Unit 7 screens.

The screenwell addition would have two pump bays , each served by two 10-ftwide traveling screens mounted in Il. 2s-ft-wide bays and oriented flush

with the front of the structure. The screenwell addition , as shown in

Figures 4. 2-5 , 4. 2-6 , and 4. 2-7 , has been conceptually designed to meet

Hydraulic Institute Standards. However , model testing would be required to

finalize the bay configuration to ensure satisfactory pump performance.

Vertical trash racks would be located along the face of the new screenwell

structure. The racks and piers would be separated from the screens and

screen bay piers by 5 ft to provide a lateral fish passageway. Curtain

walls upstream of the trash racks would extend down to el -3.

0 ft (extreme

low water elevation = -2. 72 ft) to exclude cold air in winter and to block

large floating debris.

The trash rack and curtain wall structure would also extend across the face

of the existing Unit 7 screenwell. In addition , trash racks and curtain

walls would be installed at the ends of the lateral fish passageways. The

trash racks would be cleaned by a hoist- operated mechanical rake traveling

in guides along the sides of the racks. Each traveling screen could be

isolated by stop logs and dewatered for repair. The superstructure for the

screenwell addition and for Unit 7 would enclose all equipment.

As with the screenwell addition , a hydraulic model study would be required

to ensure uniform flow patterns approaching the existing Unit 

7 pumps.

Diffusers might be required at the outlets of the 5-ft-dia pipes to distribute

the flow in the screen bays. A vertical baffle across the screen bay might

4-11
EXISTING UNITS

PROPOSED UNIT 7 ADDITION

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also be required downstream of the discharge pipes. In addition , baffles

might be required between the flush-mounted traveling screens and the

discharge pipes to ensure that the flow approaching the screens is not

disturbed by backflow from the discharge pipes.

One or both supplemental pumps could be shut down without affecting the

existing Unit 7 pumps. In this case , the total flow would be drawn through

the existing Unit 7 screens.

The upper part of the supplemental pump columns and most of the discharge

piping would operate under siphon. Therefore , a vacuum priming system

would be installed to fill the system and to maintain the siphon during

operation by continuous air removal.

The two supplemental pumps would require additional installed power of

448 kW. The screens and screenwash system in the new screenwell would

require additional installed power of 158 kW. The additional annual energy

required for this scheme is 3 945 MWh.

The screenwell addition would be constructed in a braced sheet pile cofferdam.

Sheet piling would form the perimeter walls except for the east wall , which

is common to Unit 7. The sheeting would be braced , mud would be removed

and the cofferdam would then be dewatered in a manner similar to that used

in construction of the existing screenwells. The screenwell addition would

be constructed inside the cofferdam to the point where it was watertight.

The sheet piling at the flow entry openings below the curtain walls would

then be removed. The new screenwell would become operational prior to

initiation of work on the existing Unit 7 screenwell to minimize downtime.

One existing Unit 7 pump bay would be kept in operation during construction

in the other bay. The non- operating bay would be dewatered , the traveling

screen removed , baffles installed , and the discharge pipe and diffuser

installed. The other bay would then be modified in the same manner.

The rack structure for Unit 7 would be constructed in a manner different

from that used for the new screenwell addition in order to minimize Unit 

downtime. During scheduled downtime , the area in front of the Unit 7

screenwell could be excavated , piles driven , and a tremie concrete mat

poured supported by the piles. During normal operation , the piers and

curtain walls of the rack structure would be fabricated onshore as a unit

from steel plate and partially filled wi th concrete as ballast. This unit

would then be floated into position at the face of the Unit 7 screenwell

and anchored into place. All joints would be grouted to provide an effective

seal and the remainder of the steel plate unit would be filled with concrete.

Stop logs would be installed in the rack structure and upstream of the

discharge pipe in one bay such that the new screen area could be dewatered

and the screen installed in the dry. The affected Unit 7 circulating water

pump would remain operational at a reduced flow rate by utilizing the

respective supplemental pump. The remaining screen would be installed in 

similar manner. Upon removal of the stop logs , the scheme would become

opera tional.

The total capitalized cost for this alternative intake design would be

$ 7 , 383, 000.

4-12
Since this alternative incorporates all of the suggested features for

reducing impingement (low velocity, flush-mounted screens , and lateral fish

passageways), it would probably have the best potential for biological

effectiveness at Mystic. However , as previously discussed , there is no

assurance that it would reduce impingement to a level which would be considered

acceptable. Considering the cost of backfitting additional intake bays

(Section 5) and the uncertainties regarding potential biological effectiveness

it is not believed that such a major modification is warranted.

Deepening the Area Immediately in Front of the Existing Intake

Bays

The concept of removing the existing bottom sill at certain times of the

year was discussed in Section 4. 2. 1 and was eliminated as a viable option

for reducing impingement. An alternative to this option would be to create

a new bottom sill by excavating the area upstream of the Unit 7 screenhouse

to a depth of 8 ft and thereby exposing the vertical support wall of the

screenhouse to act as a new bottom sill. This action would then allow

removal of the existing sill without eliminating the barrier concept for

blocking the movement of flounder into the screenwells. . Biologically, this would appear to be a viable al ternati ve , assuming that removal of the

existing sill would actually reduce the impingement of pelagic species.

There is some question as to whether this reduction would actually occur

since data from two different studies yielded conflicting evidence.

Figure 4. 2-8 shows the proposed dredging plan. The area immediately in

front of the Unit 7 intake bays would be dredged to el -35 to provide an

8-ft-high bottom sill. Existing sheet piling at the entrance to the intake

bays will prevent structure undermining. The dredging to el -35 would

extend about SO ft from the bays in conformance with the existing sill/

bottom topography relationship. Dredging slopes are shallow to provide a

smooth transition from the existing topography. Existing rip rap would be

retained with the proposed dredging scheme , with the addition of a small

rip rap area to provide stability in an area of steeper dredging slopes.

Entrance velocity to the structure would be reduced to 1. 64 fps while

screen approach velocity would be 2. 83 fps at mlw.

Dredging would be confined to the immediate area in front of Unit 7 in

order to protect the structural integrity of the screenhouse. The anticipated volume of dredged material would be 3 400 cu yd , and some existing

rip rap might have to be stockpiled temporarily. It is anticipated that

all existing rip rap would be utilized in the proposed dredging scheme.

The total capitalized cost of this scheme is $393, 000.

Silt deposition near the intakes in the dredged area would gradually reduce

the effectiveness of the sill. Therefore , periodic dredging would be

required to maintain an adequate sill height. Permits for dredging and

spoil disposal would be required. For estimating purposes , it is assumed

that maintenance dredging would be performed once every 2 yr.

4-13
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EXISTING DREDGED AREA
PHYSICAL BARRIERS - BARRIER NET IN FRONT OF THE EXISTING INTAKE BAYS

Barrier nets have been installed at several power plants and results have

varied. At Detroit Edison 

I s Monroe 

Plant on Lake Erie , a net was placed

across the intake canal in an effort to exclude fish , primarily yellow

perch and gizzard shad. Unfortunately, because of high intake velocities

() 1. 3 fps), the net clogged wi th fish and collapsed.

At Orange & Rockland Utilities ' Bowline Point Generating Station on the

Hudson River , good results have been obtained with a barrier net placed in

a V- arrangement around the intake structure (Lawler , Matusky, and Skelly,

1978). At this site , it was possible to achieve velocities which are

generally well below 0. 5 fps. Also , because the intake is situated in a

relatively stagnant pond , it is not subjected to the current forces of the

river. Nylon mesh nets with 0. 95- and 1. 27-

2 openings have been under

investigation since the spring of 1976. Impingement monitoring and tagging

studies have indicated that the nets effectively reduce impingement of

whi te perch and striped bass , the two species evaluated. Al though the

plant is on a major river , clogging of the net by debris or detritus has

not been a problem.

Similar results have been obtained at Carolina Power and Light Company

Brunswick Nuclear Power Station. The intake structure at this station 

located at the end of a long canal and has , in the past , impinged relatively

large numbers of fish (primarily menhaden). In January 1979, a barrier

screen (1. 27 cm mesh) was placed in a V- shape in front of the intake

similar to the arrangement at Bowline Point. As of March 1979 , the barrier

was still in place and has substantially reduced impingement at the intake

during the entire period. Debris has not created any problems to date;

however , the periods of heavy debris loading and biofouling have not yet

occurred.

Although extensive information on barrier nets is not yet available , it

appears that under the proper conditions , such a design can be effective in

reducing impingement at power plants. The most important factors in effectiveness appear to be velocity and debris.

At Mystic , such a net would be difficult , if not impossible , to maintain

due to strong tidal currents and heavy debris loading, particularly with

mussels , jelly fish , and other aquatic organisms which have created operational problems in the past. The influx of such debris has been so great

in the past that even the traveling screens have experienced difficulty in

operation. Should a barrier net clog in a similar fashion , it would restrict

plant flow and might actually be drawn into the intake , possibly resul ting

in shutdown. It is believed , therefore , that this alternative is not

practical as a long-term solution for reducing fish losses at Unit 

FISH COLLECTION AND REMOVAL SCHEME - MODIFIED TRAVELING SCREENS

Biological Evaluation

Many types of traveling water screens are commercially available for power

plant application. Most power plants in the United States presently utilize

through-flow traveling screens similar to those at Mystic , as described in

4-14
Section 2. However , with attention being focused on fish protection , new

screen types and modifications to old screens are being developed. 

stated in Section 4. , the modified through-flow screen is considered to

offer the most potentially effective and practical means of reducing fish

losses at Mystic. Information supporting this design concept is presented

in the following discussion.

Conventional traveling screens can injure fish because of the multiple

sources of stress to which they are exposed. First , where screen rotation

is intermittent , fish may be subj ect to suffocation while impinged. Second

as fish are lifted free of the water surface , they are no longer held

against the screen cloth and can , therefore , fall back into the water and

reimpinge. As discussed in Section 3 , it appears that reimpingement is

occurring at Mystic to some extent. Third , conventional screenwash systems

operate at high pressure (80 to 100 psi). The high velocity jets which

clean the screens of debris can cause injury. Finally, where attempts have

been made at power plants to return live fish to their natural environment

after impingement and removal , the discharge point is frequently close to

the intake structure , a situation which can result in recirculation and

consequently, reimpingement. However , studies at Mystic ' indicate that

recirculation is not a problem.

Modified screen and spray-wash systems have been recently developed which

eliminate some of the problems inherent in conventional screen designs

(Figure 4. 4-1). Since these modified systems can be backfitted to existing

screens , they have obvious advantages over systems that require construction

of new structures. However , because the modified screens rely on the

impingement of fish , such systems can be employed only after it has been

determined that the species of concern can withstand this type of handling.

The following briefly describes the operating experience of modified traveling screens relative to their biological effectiveness.

There is only one presently operating power plant with a modified traveling

screen and spraywash system which has been biologically evaluated (White

and Brehmer , 1975). The Surry Power Station is situated on the James River

and withdraws 3 740 cfs of water for condenser cooling. Prior to installation of the modified screens , unscreened water was pumped from the river to

a canal in which it flowed by gravity to the plant. Early operating experience

showed that fish were being pumped into the canal and were ultimately

impinged on the traveling screens located at the plant. To remedy this

situation , eight modified screens were installed at the river to collect

fish upstream from the pumps.

Screen modification involved bolting steel "fish lifting buckets " on the

trash lips of each conventional screen basket. The troughs extend approximately 5 in. from the screen face and are capable of maintaining a minimum

water depth of 2 in. during screen rotation. This arrangement prevents

fish from flipping off the screen and becoming reimpinged and also ensures

that the fish are in water as they are lifted to the point of release.

Collected fish are carried over the headshaft sprocket and either fall into

a collection trough or are gently washed into the trough with a low pressure

jet (15-20 psi) on the back side of the screen. The low pressure wash

feature of the modified screen prevents the damage caused by conventional

spray-wash systems.

4-15
RETURN 

TROUGH

I I 

U '

. . 

, 0 

... ,

. ~ 0 

. .

AVERAGE

WATER ELEVATION

WATER FLOW

 \\    . / ./ \  ::

FIGURE 4. 4-1

co .

 , , .

SCREEN DETA I L

MOD I F I ED THROUGH- F LOW

SCREENS - FRONT WASH

ALTERNATIVE INTAKE STUDY

MYSTIC STATION - UNIT 7

BOSTON EDISON COMPANY

STONE & WEBSTER ENGINEERING CORPORATION
To ensure maximum survival of impinged fish , the modified screens are

operated continuously at a speed of 10 fpm. Consequently, fish are not

impinged on the screen mesh for more than 2 minutes. Results of an 18-

month evaluation demonstrated that the modified screens can achieve a high

degree of success in collecting fish while maintaining low mortality (White

and Brehmer , 1975). Short-term holding studies (approximately 15 minutes)

showed that , on the average , survival of 58 fish species was 93. 3 percent.

Average survival of 13 Atlantic and 26 Atlantic-freshwater species (American

Fisheries Society designation) was 94. 3 and 92. 3 percent , respectively. A

number of species collected at Surry also occur at Mystic , as listed below:

Common Name

American eel

Blueback herring

Alewife

Atlantic menhaden

Carp

Mummichog

Striped killifish

Atlantic silverside

Threespine stickleback

Whi te perch

Bluefish

Weakfish

Scientific Name Percent Survival

Anguilla rostrata

Alosa aestivalis

Alosa pseudoharengus

Brevoortia tyrannus

Cyrinus carpio

Fundulus heterocli tus

Fundulus majalis

Menidia menidia

Gasterosteus aculeatus

Morone americana

Pomotomus saltatrix

Cynoscion regalis

98.

90.

90.

94.

92.

100.

100.

94.

100.

99.

85.

59.

Seven species of the family Clupeidae (which includes blueback herring,

Atlantic menhaden , and alewife), representing 58. 1 percent of the 18-month

collection total , showed a percent survival range from 82. 3 to 94. 3 with an

average survival of 93. 3 percent. These initial survival values are encouraging considering the fragile nature of these species.

On the basis of these short-term survival data , it appears that the modified

traveling screens have provided an effective means for minimizing fish

impingement mortality at Virginia Electric Power Company s Surry site.

Various utilities (among them Public Service Electric & Gas , Northeast

Utili ties Service Company, Long Island Lighting Company, and Wisconsin

Power and Light Company) are presently modifying existing screens or incorporating modified screens into the design of new power plants. Due to

this interest and the apparent potential for effective operation , several

screen manufacturers now offer modified screens and spray-wash systems as

standard items for new and existing stations.

Although modified screens appear to have good potential for alleviating

impingement mortality, it should be pointed out that short-term mortality

studies alone may not adequately reflect possible longer term mortality

which may result from injury incurred during the impingement and removal

process , particularly among less hardy species. On the basis of impingement survival data collected at a power plant located on the Hudson River

it has been suggested that immediate survival may be a poor indication of

impingement mortality for some species and that consideration of latent

mortality is required for adequate assessment of impingement damage (Ecological Analysts , Inc. , 1976). For example , at the Bowline Point Generating

4-16
Station , initial survival of young- of-the-year white perch and striped bass

impinged on continuously operated screens was high , ranging from 81-98 percent

for white perch and 89-98 percent for striped bass. Mortality increased

however , during the holding period , with survival after 96 hr ranging from

55-56 percent for white perch and 52-81 percent for striped bass. Similar

trends were noted at the Roseton and Danskammer Point Generating Stations

(King, Hutchison , and Huggins , 1978).

There are several reasons why the Hudson River data should not be considered contradictory to the results obtained at the Surry Power Station.

First , the Hudson River studies were conducted at plants which do not

incorporate modified screens (i. e. , lifting buckets). Therefore , there was

a good chance that fish could drop off the flat debris lip as it cleared

the water surface and be reimpinged one or more times , as occurred at

Mystic (Section 3). Second , those fish which were retained by the lip were

out of water for a period of time and , in some cases , were exposed to

relatively high pressure sprays. Finally, standard shallow , rough concrete

sluiceways which can cause injury by abrasion were generally used to convey

fish to a collection area. All these factors may have biased the results

toward higher mortality than would be expected with a modified screen

system.

Therefore , it is probable that modification of the traveling screens (incorporation of fish lifting buckets , low pressure sprays , and a return

system) and a more extensive evaluation would show substantially higher

survival rates.

Survival potential also appears to be species- specific. For certain species

initial survival may be a valid indication of impingement damage. For

example , 96-hr impingement mortality studies with yellow perch were conducted

at Wisconsin Public Service Corporation s (WPSC) J. P. Pulliam Power Plant

during 1977 (WPSC , 1978). Initial survival of yellow perch was high under

the two test conditions evaluated: (1) impingement only and (2) impingement

plus high pressure screenwashing. The screens at Pulliam are conventional

with no modifications for fish protection.

During the study, impinged fish were handpicked off the screens and placed

in holding tanks. Initial survival was high , ranging from 84 to 100 percent.

Survival after 96 hr did not drop appreciably. The lowest survival rate

(79. 2 percent) was believed to be the result of overcrowding in the holding

tank and warm water temperatures (77 F) rather than impingement stress.

Mean 96-hr survival (total of four tests) was 82. 1 percent (WPSC , 1978)

(Table 4. 4-1).

During the impingement- screenwash study, the use of the high pressure wash

(70 psi) did not increase mortality. Survival rates of impinged fish after

96 hr ranged from 73. 3 to 100 percent (Table 4. 4-1) with a mean of 87. 8 percent

(WPSC , 1978).

Thus , it is apparent that , although latent survival is probably the best

indication of impingement damage for fragile species such as clupeids

initial survival may be an appropriate measure of impingement impact for

some of the hardier species. As discussed in Section 3, mean initial

survival of flounder at Mystic was 97. 4 percent under the continuous screen4-17
TABLE 4. 4-1

96-HR IMPINGEMENT SURVIVAL RESULTS

PULLIAM POWER PLANT , 1977

No. Fish No. Fish Alive Percent

Test No. Placed in Tank After 96 Hr Survival

Phase I - Impingement only

100.

100.

743 589 79.

352 305 86.

Total 123 922 82.

Phase II - Impingement - Screenwash

100.

87.

89.

96.

78.

73.

Total

Source:

329 289 87.

Wisconsin Public Service

Corporation , 1978
wash cycle. Total survival at 96 hr (85. 4 percent) was only 12 percent

lower than the initial value. Initial mortality generally increased with

an increase in screenwash interval. However , total mortality (96 hr) was

never more than 25 percent greater than initial mortality. It should be

noted that , even though the Mys tic survival studies were conducted with

conventional traveling water screens , winter flounder survival rates were

relatively high. It would appear that modifying the existing screens with

lifting buckets and low pressure spraywashes would further enhance survival

of flounder. Results of survival studies at Surry further indicate that

the potential exists for collecting other , more fragile , species at Mystic

with relatively high survival. However , it is not possible to predict the

actual effectiveness of such a system without additional studies at the

plant. Therefore , since this scheme is believed to offer a cost- effective

means for minimizing fish losses at Mystic , a study program to evaluate

this system has been developed , as presented in Section 4.

Engineering Description

Two types of modified traveling screens with fish lifting buckets are

commercially available for collecting fish and debris: ' one with a frontwash spray system and one with a back-wash spray system.

The back-wash system , similar to that installed at Surry (shown in Figure 4. 4-2),

consists of screens equipped with lifting buckets that hold 2 in. of water.

The fish and debris are washed on the back side of the screen into separate

troughs. An overhead , 2-psi external spray washes fish into the fish

trough while a second high pressure spray (80 psi) internal spray washes

debris into the debris trough. At Mystic , this system would require raising

the screen head assembly about 2 to 4 ft above that of the existing installation to provide space for the second collection trough.

Front-wash spray systems are being installed at power plants on Long Island

Sound and are being evaluated in Florida and on the Hudson River for possible

application. This system consists of screens equipped with lifting buckets

that can hold 1. 75 in. of water. At Mystic , the fish and debris would be

washed from the front of the screen into a common trough. An external

spray with a 10-psi pressure would wash fish from the lifting buckets , and

a second high pressure (80 psi) spray (located above the low pressure

spray) would remove debris. Lateral sprays would also be incorporated to

wash remaining debris from the lifting buckets , thereby preventing carry- over.

Utilization of this system at Mystic would not require raising of the head

assembly.

Collected fish would be returned to the Mystic River in a trough carrying

approximately 6 in. of water at a velocity of about 3 fps. Augmentation

flow would be provided in the return trough to achieve these velocities and

water levels. An additional low head pump would be required to provide

this flow as well as the low pressure spray-wash flow.

Smooth-top, oblong (1/8 by 1 in. ) screen mesh material could be incorporated

which might enhance fish survival. In addition , a debris deflector could

be added to the back of the screen to seal the opening between the screen

and the concrete deck. A second high pressure debris spray could be added

4-18
l"USE 

'i"".i~"f'~'

TROUGH 

~~v

.;

, _'f~"'

../ 

LOW PRESSURE

FISH WASHING

SYSTEM

v8\J

FISH

BUCKET

CONVENTIONAL

HIGH PRESSURE

SPRAY

..,

SIDE ELEVATION

FISH SCREEN \ FLOW

FIGURE 4. 4-2

SCREEN DETAIL

MOD I F I ED THROUGH- F LOW

SCREENS - BAC K WASH

AL TERNA T I VE I NT AKE STUDY

MYSTIC STATION - UNIT 7

BOSTON EDISON COMPANY

STONE & WERBSTER ENGINEERING CORPORATION
to ensure the removal of debris , and a filter could be installed in the

spray nozzle supply lines to prevent nozzle clogging.

As discussed in Section 4. , the modified screens might have to operate

continuously during the period of greatest fish impingement. Such operation

would require upgrading of the screen components , including the use of

heavy duty chains , head shaft roller bearings , foot shaft stoody bushings

and liners , and proper slack tensioning. Based on continuous operation of

the screens , the additional annual energy required for this scheme is

463 MWh.

This scheme lends itself to a phased approach to evaluating its biological

effectiveness prior to complete installation at Mystic Unit 7. First , one

screen could be backfitted with a front-wash spray system and fish buckets

only. This design would require minimum modifications. A study plan for

conducting a biological evaluation of such a system is described in Section 4.

If the system were found to be acceptable , the other screen would be modified

and both screens would be upgraded for continuous operation. It is possible

that the condition of the existing screens could be such that the installation

of new screens would be more practical.

Construction of the modified screens would be carried out by isolating one

screen bay at a time to allow upgrading of the existing screens or removal

of the screens and installation of new screens. Alternatively, this activity

could be scheduled to occur during a plant maintenance shutdown to minimize

downtime.

The total capitalized cost for modifying the screens at Unit 7 is $596 000.

Evaluation of the Modified Screen Alternative

As previously discussed , results of studies at Mystic Station and other

power plants indicate that traveling water screens can successfully and

safely collect and remove fish given certain design and operational modifications. Additional studies could be conducted at Mystic to determine the

actual effectiveness of modified traveling screens in returning important

species at this site to their natural environment in a viable condition.

The following is a brief description of the type of study that could be

conducted to achieve this obj ecti ve.

On the basis of discussions with EPA , it would appear that winter flounder

rainbow smelt , and Alosa spp. (herring and alewife) are the primary species

of concern at Mystic Station. It has already been shown that flounder are

capable of withstanding impingement and removal from the existing traveling

screens with good survival (Section 3). It is probable that a modified

screen would enhance survival of this species further. Smelt and Alosa

spp. have not been evaluated at this site , but available data indicate that

adequate survival may be achieved under certain conditions. Therefore , the

intent of a modified screen survival study would be to examine fish mortality

at different operating modes to identify those conditions which yield the

greatest efficiency.

For the purpose of this study, one traveling screen at Unit 7 would be

modified to incorporate fish lifting buckets (on all screen panels) and a

4-19
low pressure spray wash system. Provisions for continuous long-term operation would not be included at this time. The screenwash sluiceway and

collection area would be modified , as necessary, to allow for the removal

of fish without additional injury.

The test program would consist of collecting fish from the modified screen

under different operational and environmental conditions and , subsequently,

holding them for up to 96 hr for observation of latent mortality. Prior to

each test , the traveling screen would be rotated and cleared of all fish

and debris. The screen would then be maintained in a stationary position

for the specific length of time under investigation. Subsequently, the

screen would be rotated and washed , and all fish would be collected and

transferred to a holding pool. While in the pool , all fish would be identified and recorded as initially alive or dead. Live individuals of those

species selected for detailed study would then be placed in holding tanks

for 96 hr; all other species would be returned to the river.

The primary operational variable of interest would be the frequency of

screenwashing. Since the purpose of the study would be to determine the

screenwash interval which yields the highest survival of fish , appropriate

test conditions cannot be firmly established at this time. However , initial

impingement durations would range from those resulting from continuous

screen rotation up to an 8-hr interval. Pending early results , intervals

would be shortened or lengthened , as appropriate.

To gather survival data with all species of concern , it is likely that

studies would be conducted periodically in the summer , fall , and winter months. Since environmental parameters (particularly water temperature)

vary widely over this period , correlations between these parameters and

survival would be possible. Therefore , all pertinent environmental parameters would be recorded during each study period. Plant operational

conditions which would influence the hydraulic characteristics of the

screenwell containing the modified screen , and would therefore influence

fish survival , would also be recorded.

At the completion of the study, it would be possible to identify optimal

screenwash modes for maximizing survival of each species and to evaluate

whether survival potential varies as a result of environmental and/ or plant

operational factors. The results would then serve as a basis for determining the practicability of modifying the other screen at Mystic Unit 7 and

confirming optimal operational conditions.

It is expected that all data required to achieve the objectives of this

proposed study program could be gathered over a maximum period of 1 yr

with actual sampling occurring seasonally over several 2-to-4 week periods.

Should this option be selected as the most feasible and found necessary to

implement , a detailed study plan would be developed.

4-20
SECTION 5

ESTIMATED COSTS AND CONSTRUCTION SCHEDULE

Order-of-magnitude costs were developed for each of the proposed alternative intakes for the Mystic Station - Unit 7. The order-of-magnitude cost

is defined as the cost that requires the minimum amount of time and has the

minimum degree of accuracy in its results. It is usually based on historical

data from other proj ects and is adjusted for identifiable differences and

size on a case-by- case basis. These costs allow a valid comparison of the

cost difference between alternatives as a reasonable estimate of the probable

cost of any scheme.

The estimated costs , listed in Tables 5-1 and 5-2 , are based on the follow1ng:

Present-day prices and fully contracted labor rates as of July

1979

Forty-hour work week , single- shift operation

Allowance for indeterminates to cover uncertainties in design and

construction at this preliminary stage of work

All construction performed on a fully contracted basis

Allowance for overhead to cover distributable construction cost

indirect costs , and general and administrative costs

Allowances for escalation , allowance for funds used during construction , taxes , contingency, and other owner costs.

The total evaluated costs for the various schemes are presented in Table 5-1.

Also included in this table are the estimated additional power and energy

requirements and the 1979 maintenance costs for each scheme. The energy

requirements are based on continuous operation of the screens for Scheme 

intermittent operation of the screens for Schemes 1 , 2 , and 3, and operating

the supplemental pumps continuously for Schemes 1 and 3. The maintenance

costs for the continuously operating traveling screens (Scheme 5) are based

on experience with similar installations. The maintenance costs for the

screenwash pumps are based on 5 percent of the capital cost of the pumps

and for the supplementary flow pumps on 1. 5 percent of the capital costs.

The maintenance cost for deepening the area immediately in front of the

existing screens is based on dredging every 2 yr.

Total project costs , capitalized costs , and levelized annual costs for the

various schemes are listed in Table 5-2. The total project costs consist

of the 1979 direct , distributables , and engineering costs , escalation to

proj ect completion , contingencies , and allowance for funds used during

construction. A common project completion date for all schemes has been

taken for the purpose of comparison.

5-1
TABLE 5-1

ESTIMATED COSTS

Scheme

Additional 1979 Installed Addi tional Addi tional

Direct Cost Power Required Annual Energy Maintenance

(in $1 000) MWh Costs (in $1 000:

428 542 294 1 . Utilizing Intake Bay of

Uni ts 1- 3 for Supplemental

Cooling Water for Unit 7

(Not including new wharf)

Installing Screens Flush withfront of Intake Bays

701

Cons tructing Additional Intake

Bays , 752 610 945

Deepening Area Immediately

in Front of Existing Intake

Bays

Modified Traveling Screens 259 463

Modifying 1 Screen
TABLE 5-2

TOTAL PROJECT AND CAPITALIZED COSTS*

(IN $1 000)

Item

Scheme 1

Utilizing IntakeBays of Units 1-3

for Supplemental

Cooling Water for

Uni t 7

Direct Cost

Distributables

Engineering

428

285

Subtotal 805

Escalation

Contingencies

Allowance for Funds

Used During Construction

331

470

108

Subtotal 714

Add Cost of New Wharf 472

186 Total Project Cost

Capitalized Performance Penalties 289Capitalized Operationand Maintenance Cost 199

Total Capitalized Cost 674Levelized Annual Cost 422

NOTES:

Q979 based data

~kRepresents equivalent capital and includes:

Replacement energy

Replacement capacityHeat rate charge

. Scheme 2

Installing

Screens Flush

wi th Front of

the Intake

Bays

Scheme 3Constructing Additional

Intake Bays

701

129

752

168

308

901 228

110116

431

699

290

, 155 648

155 648

526

209

168 7 , 383

249 573

Scheme 4

Deepening

Area Immediately

In Front of

Existing Intake

Bays

Scheme 5

Modified Traveling

Screens

259304

397397

153

303393 596

127
The capitalized costs are calculated by multiplying each year s cost by the

appropriate present-worth factor , summing the present-worth values , multiplying

the result by the capital recovery factor , and dividing by the fixed charge

rate. The total capitalized costs consist of the total proj ect cost , the

capitalized performance and auxiliary power penalty, and the capitalized

operation and maintenance costs. The levelized annual costs are calculated

by multiplying the capitalized costs by the fixed charge rate.

The construction periods required for each scheme are presented in Table 5-3.

These periods do not include engineering and design, equipment procurement

or permits.

5-2
TABLE 5..,3

CONSTRUCTION PERIODS

Scheme Mon ths

1 . Utilizing Units 1-3 Intake Bays

for Supplemental Cooling Water for Unit 

Installing Screens Flush with Front

of Intake Bays

Constructing Additional Intake Bays 16.

Deepening Area Immediately in Front

of Existing Intake Bays

Modified Existing Screens

Modifying 1 Screen
REFERENCES CITED

Ecological Analysts , Inc. 1976. Bowline Point Generating Station

Impingement Recirculation and Survival Studies , November 1975 - April

1976. Prepared for Orange & Rockland Utilities , Inc.

Eisele , P. J. , and Malaric , J. F. 1978. Summary Report on the Prototype

Fish Pumping Studies at the Monroe Power Plant. The Detroit Edison

Company, Technical Report.

King, L. R. ; Hutchinson , J. B.; and Huggins , T. G. 1978. Impingement

Survival Studies on White Perch , Striped Bass , and Atlantic Tomcod at

Three Hudson River Power Plants.

Lawler , Matusky, and Skelly Engineers , 1978. 1977 Hudson River Aquatic

Ecology Studies at the Bowline Point Generating Station. Prepared for

Orange & Rockland Utili ties , Inc.

Rexnord Company. 1973. Traveling Screens to Protect Fish in Water

Intake Systems. Envirex , Inc. Bulletin , 316-300.

Stone & Webster Engineering Corporation (S&W) , 1976. NPDES Permit

MAOO04740 , Fish Impingement Monitoring Program , Mystic Station - Unit

No. , Boston Edison Company (October).

1978. Fish Impingement Monitoring, Mystic Station -

No. , Boston Edison Company (July).

Unit

Taft , E. P. , and Mussalli , Y. G. 1978. Angled Screens and Louvers for

Diverting Fish at Power Plants. Proceedings of the American Society of

Civil Engineers , Journal of the Hydraulics Division

Vol. 104 , No. HYs (May).

Texas Instruments , Inc. 1974. Indian Point Impingement Study Report

for the Period June 15 , 1972 through December 31 , 1973. Prepared for

Consolidated Edison Company of New York , Inc.

White , J. C. , and Brehmer , M. L. 1976. Eighteen-Month Evaluation of

the Ristroph Traveling Fish Screens. In: Jensen , L. D. (Editor),

Proceedings of the Third National Workshop on Entrainment and Impingement

New York City, February 24 , 1976.

Wisconsin Public Service Corporation (WPSC). 1978. J. P. Pulliam Power

Plant , Fish Impingement , April 1977-December 1977 (March).

U. S. Environmental Protection Agency (EPA). 1976. Development Document

for Best Technology Available for the Location , Design , Construction

and Capacity of Cooling Water Intake Structures for Minimizing Adverse

Environmental Impacts.

R-1