EVALUATION OF BENEFITS TO PEPCO OF IMPROVEMENTS

IN THE BARRIER NET AND INTAKE SCREENS

AT CP~LK POINT STATION BETWEEN 1984 AND 1985

Prepared by Jules L. Loos

June, 1986

Environmental Affairs Group

Water and Land Use Department

Potomac Electric Power Company

Washington, DC

20068
1'"

1 : CONTENTS

OBJECTIVES OF THIS REPORT. . . 1

INTRODUCTION .. . 2

, 

BACKGROUND . 5

GENERAL LAYOUT OF INTAKE AREA. . 

I "

. 

h-i

DESC~IPTION OF BARRIER NET AND ITS MODIFICATIONS . 9

DESCRIPTION OF INTAKE SCREEN AND

BACKWASH SPRAY MODIFICATIONS

. .

METHODS FOR EVALUATION OF BARRIER NET

AND INTAKE SCREEN MODIFICATIONS. . 19

EVALUATION OF BARRIER NET ~~D

INTAKE SCREEN MODIFICATIONS. . 

. .

RESULTS OF SCUBA DIVER INSPECTIONS

OF THE BARRIER NET. 

! . .

COMPARISON OF BLUE CRAB ABUNDANCE

IN THE PATUXENT RIVER VERSUS

IMPINGEMENT ON INTAKE SCREENS

EXPECTED IMPINGEMENT OF CRABS IN

1985 BASED ON 'NUMBERS IN PATUXENT RIVER

VERSUS OBSERVED IMPINGEMENT--IMPROVED

BARRIER NET PERFORMANCE . 30

l '

! 0

IMPROVED SAMPLING PROCEDURES INDICATE

FEWER FISH AND CRABS IMPINGED THAN

PREVIOUSLY ESTI~~TED. 

.. . . .. .

f "

LOWER IMPINGEMENT LIABILITY ESTIMATES

. .

IMPINGEMENT VERSUS CA.~Y-OVER--IMPROVED

IN SCREEN BACKWASH SPRAY SYSTEM PERFORMANCE

. .

VOLUME OF MATERIAL IN WATER BOX--IMPROVED

SEALS AROUND INTAKE SCREENS

.. . . . .

0 . 

,; . 

. 39

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ELIMINATION OF OUTAGES DUE TO CONDENSER

BACK PRESSURE IN 1985 

.. . . .

COSTS 1u~D BENEFITS

.. . . .

RECOMMENDATIONS. . 

. .

REFERENCES

ill
OBJECTIVES OF THIS REPORT

To evaluate the effectiveness of modifications in the barrier

( ,

net design and operation.

.

To evaluate the effectiveness of modifications in the intake

screens backwash spray system.

The criteria used to make these evaluations were:

1.. Elimination of outages due to high condenser back pressure and

Lo. I.""

decreases in the volume of debris in the condenser tube water

box.

Reduction of liability associated with impingement of fish and

crabs.

Evaluation of the performance of the sponge ball cleaning

system for the condenser tubes was beyond the scope of this

report. It should be noted that this system is designed to

control microfouling and according to Norair, the contractor which

installed the system, its successful operation is dependent on

, 0 control of macrofouling and prevention of blockage of condenser

tubes at the tube sheet face.

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! ; . 

INTRODUCTION

Chalk Point station has had outages due to condenser tube

blockage since it first started operation. In 1981 the problem

reached its peak with 10 outages due to condenser tube blockage by

i I blue crabs. In response the company deployed a barrier net at the

mouth of the intake canal in July 1981 to help control the

problem. Since its first deployment the net has evolved to the

( .

double net system now in place. In addition, the travelling

screen system for Unit 2 was upgraded in 1985 to provide tighter

seals around the screens, improve the screen spray backwash system

and upgrade and reactivate the sponge ball system for cleaning the

condenser tubes which had been inoperative. The goal of both

proj ects was to reduce the number of outages caused by tube

blockage. The barrier net proj ect had a second goal to reduce the

impingement liability at Chalk Point , which was originally

estimated to be over $2, 000, 000 and to satisfy Best Available

i i Technology (BAT) requirements for intake structures. This

liability is the amount of money that could be required for intake

modifications to reduce the number of fish and crabs killed at the

intake. Maryland' s BAT liability is equal to the dollar value of

impingeable organisms estimated to be killed over a 5-year period

(COMAR 10. 50. 01. 13F and 08. 02. 09. 01) .

As little as 100 gallons of crabs in the intake water box can

cause an outage. Each outage is estimated to cost the company

approximately $130 000 in lost production.

- 2 -
Chalk Point is located on the Patuxent River in a rich

estuarine environment approximately 20 miles from the Chesapeake

Bay. Blue crab eggs hatch in the lower portions of the Chesapeake

Bay and then the larvae and juveniles migrate up-estuary. By the

time they reach the vicinity of the plant most are about 0.

inches wide or larger. Once the crabs reach a width of about 1.

inches they are large enough to block the condenser tubes.

must first circumvent the travelling screens by passing

They

1 .

underneath, around or over the screens when they are rotated for

cleaning purposes.

 ! .

The commercial harvest of blue crabs in the Patuxent River

has been increasing in recent years (Figure 1) which helps explain

;;;

why the outage problem has been increasing. The year (1985) was

projected to be a banner year for blue crab harvest. To make a

quantitative evaluation of barrier net effectiveness it is necessary to take into account the size of the blue crab population in

the vicinity of the Chalk Point Station. This was done by using

data on their abundance from PEPCO I S trawling program. The double

barrier net was evaluated by comparing impingement in 1984 (single

coarse-meshed net) with impingement in 1985 (double barrier net

finer mesh) based on the statistical relationship between

impingement and crab abundance. Comparisons were also made of the

amount of material found in the condenser water box after each

; .

outage in 1984 and 1985.

I 1

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(

L....... ..

.. 

_

t.5

-Po

'U) 1.2

::)Ii.

II) 0.

~ 0.

1883 urn t87&

YEAR

t878 t8ee t888

Figure 1. 

, , 

1985 ? t88t t884 Yearly commercial harvest of hlue crabs from the Pattlxent River

fr'olll 1%1 In 19134 anti projections for 10S5.
! , , 

BACKGROUND

! i In 1981, when outages were occurring frequently, the barrier

net was first used since it was the only technology which could be

quickly installed at an economical cost. The net was supported by

( 

gill net poles put into the bottom by hand. The net did not work

well that year but demonstrated that deployment and operation of

this technology was feasible. The most serious problems were

fouling of the net by colonial hydro ids and bryozoa, and clogging

by jellyfish which resulted in a collapse of the net after a few

weeks of deployment. In 1982 the gill net poles were replaced

with 12-to-13-inch pilings driven into the substrate. The net

(,

worked well in 1982 with no outages due to crabs. Fouling and

clogging were controlled by changing the net panels once or twice

a week. Fouling organisms were killed by letting the net panels

dry out after the change.

Based on 1982 data PEPCO submitted an estimate of impingement

liability to Maryland I s Office of Environmental Programs (OEP).

Up until that time there had been no formal impingement liability

estimates but a draft document by the Academy of Natural Sciences

(ANSP) had been submitted which estimated a two million dollar

impingement liability based on data for 1976-77 before the barrier

net was installed with no tempering pump liability included. The

estimate based on 1982 data was reduced by about 75% to $500, 000

including tempering pump liability.

In most of 1982 only unit 1 was operating along with one tempering pump; unit 2 being down for a maj or overhaul. When both

Units came back on-line net performance declined. It was hard to

- 5 '
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remove jellyfish rapidly enough to prevent clogging which caused

the net to lift off the bottom. It was also found that small

1 ; crabs could pass through the 1. 25- inch stretch mesh of the net.

In 1983 detailed impingement data were not collected and

there were outages due to condenser tube blockage. Divers

inspected the net and identified large gaps at the bottom as the

cause. In 1984 the Water and Land Use Department began its

studies to evaluate the barrier net. It was clear that outage

problems were continuing so in September 1984 a second barrier net

was deployed 100 feet outside the original net. The outside net

had 1. 25-inch stretch mesh while the inner net was changed to

75-inch netting.

In 1985 additional improvements to the net were made and the

original contractor used to deploy the net in 1982 was rehired.

The deployment procedures used by this contractor were superior to

those used in 1983 and 1984.

Also in early 1985 there was an overhaul of the intake

screens of Unit 2. The screen backwash spray system was improved

and seals were installed underneath and at the sides of the

screens. A similar upgrade for unit 1 was completed during the

fourth quarter of 1985.

- 6 -
r .

\ 

GENERAL LAYOUT OF INTAKE AREA

! 

r ' Chalk Point has two Units with once-through cooling (Figure

2). Each unit draws approximately 225, 000 gallons per minute (gpm)

from an intake canal (450 ft long; 200 ft long; 15 ft deep) .

intake of each Unit has two travelling screens with 3/8-inch

square mesh. Three auxiliary tempering pumps discharge water

directly from intake to d~scharqe canal with a combined flow of

225, 000 gpm. Water flowing into the tempering pumps is not

screened. The total flow including the tempering pumps is

approximately 675, 000 gpm.

! 

bed

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The
i. '

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, 19ure '-.

" DISCHARGE

CIRCULATING PUMPS

WATER BOX

CONDE NSER 

k0 

,.:

INTAKE

/0 

'0 , I

, CI, INJECTION POINTS 

=jI 

/ 

TRAVELING SCREEN 

TRASH RAKE/STOP LOG XILIARY

COOLING

PUMPS

. .

PILI N G &./1

.::,?

11 !? NET .

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

Sch~mati~ diagram of t1e ~ halk Point Station intake.
l ..

DESCRIPTION OF BARRIER NET

AND ITS MODIFICATIONS

! ;

There are two barrier nets. The outside net catches most of

\ .

the debris and jellyfish which allow a finer mesh to be used for

the inner net. The outside net was first deployed September 

1984.

The barrier net pilings are arrayed in two rows at the mouth

of the intake canal as indicated diagrammatically in Figure 2 and

to scale in Figure The distances from shore-to-shore along

these rows are 533 and 671 feet, respectively, for inner and outer

rows. There are 40 pilings in the inner row and 51 in the outer

L",

row; the pilings are usually 12 to 13 feet apart. A 2 a-foot gap

in outer row allows large boats to enter the area between rows for

maintenance work. Between shore and the first pilings on each end

of these rows there is a fence of a. 5-inch wire mesh. The pilings

are driven 10 or more feet into the substrate and extend about 5

feet above the water. They are 12 to 13 inches in diameter and in

1 ,

the channel where the water depth is 15 feet deep they are

typically 30 feet long (Figure 4).

 ,

For the inner barrier net,

ft deep are deployed at a time.

eight panels 150 ft long and 25

Panels are suspended from hooks

on the piling about 3 ft above the water surface at high tide.

There is a heavy chain attached to the bottom of the net and

, 

floats attached to the top. When deployed the chain line of the

net rests on the bottom more than 5 feet upstream from the poles.

Each panel overlaps the previous panel by about 60 ft and is in

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CHAIN LINK FENCE

, ,

WOVEN WIRE FENCES 

/

NNER ROW aF BARRIER 

,

(a. INCH SO 11ESH)

, 

NET PILINGS 

/, . ::,

" (Oo7S- INCH STRETCH ~1ESH 

:. ,

f l )"'

NETTING USED ALONG THIS ROWJ' 

.... : ;

. , , , t.

;;,; 

;;l~;E~~E~ ~::s

. :. . . . . 

, . PILINGS , ' FOR

. , 

BOAT ENTRY

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

OUTER ROW OF BARRIER NET PILINGS

(1. 2S- INCH STRETCH MESH HETTING USED

ALONG TH IS ROW)

i':';'

,. ,

CHALK POINT INT AKE

BARRIER NET

! ,

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.,.

100 FEET

Figure 3. Map showing details of Darr~er net dep1oyrnent.

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. 19ure .

ROPE TIE

FOR SJ(JRT

.;:::::;

12 TO 13 INCH PlUNG

HOOK

HEADLESS NAIL 

usm TO SUIPDID NET NET 3 FEET DURlN8 CHAM" 

ABOVE WATER

WATER SURFACE

.:::

CURRENT I

MAXIMUM DEPTH

ABOUT 15 FEET

OVERLAPPIN G

NET PANElS

Don INCH mtlCH MESH

CHAJ N

UNES BOTTOM

P!UNG DRIVEN 10 FEET INTO BOTTOM

Cross-sectianal 1iagram)f ~vpical b~rrier-net piling, overlapping

harrier-net p~~e1~ ~nd ~k;rt.

- 11 -
turn overlapped by the next panel by 60 ft (Figure 5). The mesh

size for the inner barrier net is 0. 75 inch stretch mesh.

starting in 1985 a skirt 4 ft deep and 380 ft long (Figure 

was attached to the pilings of the inner barrier net across the

mid -channel area. The skirt serves to prevent gaps between the

barrier net chain line and the river bottom should the panel temporarily hang up on the piling during deployment. The skirt was

treated with a copper anti-fouling dip and was not changed during

the year as were the barrier net panels.

The outside barrier net has been sewn into panels 200 ft long

by 25 ft deep. The maximum water depth along this net is about 

. ,

ft. At the present time there are no floats on the top of the net

which makes deployment more difficult. When' new nets are ordered

we recommend that they be included. The mesh width of the outer

barrier net is 1. 25 inch stretch mesh.

,..

The barrier net panels are changed once or twice a week

during the spring and summer to control fouling and clogging.

. ,

the fall the frequency is reduced to once every two weeks because

of reduced abundances of jellyfish and fouling organisms. The

procedures used to change net panels is shown in Figure First

I. ;

the fouled panels are unhooked and suspended on the headless nails

just below the hooks (Figure 5) which allows the net to be pulled

away from the pilings quickly with a boat. The dirty panels are

pulled from the pilings one at a time and immediately replaced by

I I

! 

clean panels (Figure 5) special care is taken so that a minimal

amount of debris and crabs passes through any gaps during the

changing procedures. A SCUBA diver makes sure that the chain line

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CLEAN PANEL

CURRENT

DIRTY PANELS

PIUN GS

,;, 

First panel being relooved; cupping of this

panel prevents debris from drifting onto clean

repl acement panel bei ng deployed behi nd it.

Gap between panels is closed while removalof di rty panel is c6mpl eted.

.....

Net replacement panel being deployed about 15 ft from pilings to allow chain line to reach

bottom well away from pilings before upper net drifts back to pilings.

Final net configuration showing overlapping pattern typical of inner barrier net.

Figure 5. Diag," am of procedures IIspd to ~xchangp fouler! harrier ret panels

l'ril ) r.1'~dl1 IHlt"
is on the bottom all the way across. Once a week this check is

repeated by a PEPCO diver to make sure the seal is still good.

The present net design and changing procedures differ from

those used in 1984 (Tables 1 and 2). In 1984 the net was dropped

down just in front of the pilings instead of about 15 ft in front

of the pilings as is now the practice. That often caused the net

. to hang up on the poles before it reached the bottom. The net was

pulled down with a rope attached to the bottom of the net which

ran through a pulley on the pole at the river bottom. Pulling the

net down tight caused it to bridge over depressions in the river

bottom which left gaps under the net especially near the pilings.

Even with a SCUBA diver it was not possible to completely close

these gaps. Deployment was easier in 1985 because the net panels

for the inner barrier net were 5 ft deeper in 1985 than those used

in 1984. In 1985 there were no gaps found under the inner net.

The barrier nets are not deployed in the winter when ice

could damage the netting.

redeployed in early April.

They are removed in early December and

Crabs remain buried in the mud during

the winter so the nets are not needed during this period from an

operational standpoint and historically winter finfish impingement

has been low.

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TABLE 1. COMPARISONS OF BARRIER NET SPECIFICATIONS

IN 1984 AND 1985

1984 1985

NUMBER OF NETS

SINGLE NET UNTIL SEPT. 7 WHEN

OUTER BARRIER NET WAS INSTALLED.

MOST CRAB IMPINGEMENT OCCURRED

BEFORE SEPT. 7 .

TWO BARRIER NETS

( ,

NET PANEL DIMENSIONS

INNER AND OUTER BARRIER NET

PA-~ELS THE SA-ME:

100 FT LONG X 20 FT DEEP

INNER NET:

150 FT LONG X 25 FT DEEP

OUTER NET:

200 FT LONG X 20 FT DEEP

NETTING

BOTH NETS:

25-INCH STRETCH MESH

INNER NET:

75-INCH STRETCH MESH

OUTER NET:

25-INCH STRETCH

) ,

t -

AUXILIARY NET

NONE SKIRT ALONG BOTTOM OF PILINGS

IN CHANNEL PORTION OF INNER

NET

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TABLE 2. COMPARISONS OF BARRIER NET DEPLOYMENT PROCEDURES

IN 1984 AND 1985

, 

1984 1985

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DEPLOYMENT

PULLEY SYSTEM USED TO PULL NET

TO BOTTOM INSTALLED IN JULY

CHAIN PULLED TIGHT TO BOTTOM OF

POLE .

NET P&~ELS DROPPED ABOUT 15 FT

IN FRONT OF PILING THEN ALLOWED

TO DRIFT BACK TO PILING. CHAIN

LINE RE~~INS 5 OR MORE FEET IN

FRONT OF PILING WITH PLENTY OF

SLACK.

UNDERWATER INSPECTION AND

ADJUSTMENT IMMEDIATELY

AFTER DEPLOYMENT

STARTED IN JULY STARTED WITH INITIAL DEPLOYMENT

,

REMOVAL OF MASSES OF

CLOGGING MATERIAL

~.A.WAL WASHED FROM NET BY PULLING NET

AWAY FROM PILINGS BY BOAT

r :

PERIOD THAT OPENING OCCURS

BETWEEN EACH PAIR OF PANELS

DURING NET CHANGES

ABOUT 10 MINUTES ABOUT 2 MINUTES

. ,

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DESCRIPTION OF INTAKE SCREEN AND

BACKWASH SPRAY MODIFICATIONS

The following changes were made to the intake screens and

backwash spray systems as indicated in the purchase order and

descriptive documents from Norair Engineering corporation, the

! 

vendor selected for this work by the Engineering Construction

Group:

*Refurbishing the travelling screens to allow continuous

operation. (In 1985 the screens were not operated

continuously but apparently that is possible if required.

*Installation of Nortex sealing strips on side of screens.

. ,

The specifications indicate that gaps on the sides should

be reduced to 1/32 inch after this installation.

l:;,::

*Installation of non-corrosive boot plate at the bottom of

the screens.

*Installation of second spray header.

*Replacement of existing spray nozzles with flat spray type

nozzles.

*Modification of spray wash piping using 4-inch pipes instead

of 3-inch pipes and installation of in-line strainer for

each screen.

*Installation of isolation valves for use with the spray wash

headers to allow operation flexibility i. e., to turn off

, 

one of the headers during periods of low impingement.

addition to this work modifications were made to the water

box and ball recircu~ator piping, strainers, collectors

and controls so that the ball cleaning system is

operational for Unit 2.

- 17 -
ond quarter of 1985.

These modifications were made to unit 2 in the first and secWork for Unit 1 is in progress in the fourth

quarter of 1985.

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l :

METHODS FOR EVALUATION OF BARRIER NET

&~D INT&~ SCREEN MODIFICATIONS

Methods used in these evaluations are summarized in Table 

Once a week starting in July 1984, PEPCO divers inspected the

barrier nets to determine the size and number of any gaps under

the net. This information was used to determine how well the net

was being deployed.

Since the magnitude of problems related to crab impingement

and condenser tube blockage is dependent on the number of crabs in

the area it was necessary to obtain information on their

abundance. Data from PEPCO'S ongoing trawling program with 24

trawling stations in the patuxent River and Swanson Creek (Figure

6) was used for this purpose. Collections were made by towing a

l6-ft trawl for 1 minute at each station for approximately 560

feet once a week in May through October. All crabs were measured

and counts were tallied for large, medium and small-sized individ-

 

uals (2. 4-inch size classes). The relative abundance was defined

as the mean number of crabs per collection over the 24 stations.

To monitor how many fish and crabs were getting through the

barrier net a complete count of all impinged organisms was made

from March 1, 1984 through September 6, 1985. The crabs were

counted in 2. inch size classes. The second barrier net was

 

deployed on September 7, 1984 so the study period includes two

crab seasons (1984 and 1985) to compare performance before and

after modifications. In addition the 1985 study provides a full

year with the double net, the minimum necessary for computation of

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TABLE 3. SUMMARY OF METHODS USED TO MAKE EVALUATIONS OF

BARRIER NET AND INTAKE SCREEN MODIFICATIONS

i .

PROCEDURE

l :

SCUBA DIVER INSPECTION OF

BARRIER NET

! :

MEASUREMENT OF ABUNDANCE OF

CRABS IN THE PATUXENT RIVER

WITH TRAWL

COMPLETE COUNTS OF IMPINGED

FISH A.~D CRABS

I;;,~ CONTINUATION OF OLD IMPINGEMENT SAMPLING- PROGRAM BASED

ON HALF~HOUR COLLECTIONS

COMPLETE COUNT OF CARRY-OVER

FOR UNITS 1 AND 2

..,

MEASUREMENT OF VOLUME OF

CLOGGING MATERIAL IN WATER

BOX AND NUMBER OF OUTAGES

TO MONITOR NUMBER AND

CAUSES OF OUTAGES

PURPOSE

TO COMPARE GAPS UNDER BARRIER

NET BEFORE AND AFTER CHANGES

IN NET SPECIFICATIONS AND

DEPLOYMENT PROCEDURES

TO ADD RELIABILITY TO EVALUATIONS

OF BARRIER NET PERFORMANCE BY

DOCUMENTING PRESENCE OF CRABS

NEAR THE INTAKE

TO COMPARE IMPINGEMENT BEFORE

AND AFTER BARRIER NET MODIFICATIONS. IMPINGEMENT USED AS MEASURE OF BARRIER NET PERFORMANCE.

TO RECOMPUTE OF IMPINGEMENT

LIABILITY

TO CLARIFY PREVIOUS SAMPLING BIAS

TO COMPARE PERFORMANCE OF UPGRADED

BACKWASH SPRAY SYSTEM BEFORE AND

AFTER INSTALLATION ON UNIT 

ADDITIONAL COMPARISONS TO UNIT 1

WHICH HAD NOT BEEN MODIFIED YET

EVALUATION OF SEALS AROUND INTAKE

SCREENS

OVERALL EVALUATION OF ALL

MODIFICATIONS

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1..

BARRIER

NETS

NORTH

PATUXENT RIVER

TRAWLING

STATIONS

16- FT TRAWL

TONED FO"'.

ABOUT 560 FT

I I) 5000

FEET

I ( HUNTING

CREEK

I j 

CHALK POINT STATION

DISCHARGE CANAL

I NTAKE CANAL

SWANSON

CREEK

OVERHEAD

POWER LINE

AND 5UPPO"'.TlNG

PIERS

Figure 6. Map of the Patuxent River near Chalk Point Station $~owing

trawling stations where hlue crab abundance was determined

in 1984 an~ 1985. These abundance measures were used to hel p

eval uate the effectiveness of the barri~r nets rel at lve to

the number of crabs in the i'irea. 

- 21 -
impingement liability; i. e., the one year value is multiplied by

five to obtain the 5-year liability value. Crab impingement

r ' occurs primarily from May through August and outages due to

condenser tube blockage by crabs occur primarily from June through

August. Thus the period when crabs cause operational problems to

PEPCO was sampled in 1984 and 1985.

Impinged fish and crabs were obtained by rotating the screens

once or twice a day as indicated in the schedule in Table 

Impinged organisms were collected by placing a net in the screen

wash sluiceway while the screens were rotated for cleaning. The

,

net fit the sluiceway exactly and had a O. 25 - inch stretch

mesh-width which is smaller than that of the travelling screens

. 

(0. 375-inch square mesh). A back-up net was installed at the end

of the sluiceway to collect organisms if the screens were cleaned

when monitoring personnel were not present; this rarely occurred.

In the past there have been problems in obtaining accurate

estimates of impingement from half-hour samples. These problems

were clarified by comparing the complete counts with estimates

based on the old methods. The old procedure was to take two

! .

half-hour collections per week: one collection at night and one in

the daytime. The number of crabs impinged in that hour was

essentially multiplied by the number of hours per week. There was

a correction included to reflect variable impingement rates in

daylight and nighttime hours but these were not very important

since during the months when crab impingement was high these time

periods were of nearly equal duration.

Crabs that were not dislodged by screen backwash and carried

over the screens were picked from the screens by hand and counted

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! i TABLE 4. SAMPLING FREQUENCY FOR IMPINGEMENT CENSUS PROGRAM.

, .

I i DATES FREQUENCY

DAILY MONDAY THROUGH FRIDAY EXCEPT

HOLIDAYS * (MONDAY COUNTS INCLUDE

SATURDAY AND SUNDAY)

MAR 1 TO APR 13, 1984

APR. 16 TO JUN. 15, 1984 DAILY

JUN. 16 TO JUL. 24, 1984 TWICE A DAY

JUL. 25 TO SEP. 14, 1984 DAILY

SEP. 17 , 1984

TO MAY 24, 1985

DAILY MONDAY THROUGH FRIDAY EXCEPT

HOLIDAYS* (MONDAY COUNTS INCLUDE

SATURDAY AND SUNDAY)

MAY 28 TO SEP. 6, 1985 DAILY

*SCREENS WERE NOT ROTATED FOR CLEANING ON WEEKENDS DURING THIS

PERIOD.

:;j

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separately. Each sampling set was comprised of 4 counts: sluiceway

counts for Units 1 and 2 and "carry-over" counts for each unit.

Whenever there was an outage , regardless of cause, the

monitoring group was notified and were present when the water box

was opened. The volume of the material in the box was measured

and an estimate was made of percentages of the maj or constituents.

The barrier net evaluation was based on analysis of

covariance with variable slopes. The weekly totals for crab

impingement in 1984 (single net) were compared to that in 1985

(double net) with the mean weekly trawl count as the covariate.

Similarly the improved backwash system was evaluated by comparing

the weekly carry-over totals in 1984 with those in 1985 using

r ' impingement counts as the covariate. For 19 a 4 the weekly

impingement counts were also compared to the estimates based on

one hour of sampling per week; e., the count times the number of

hours per week (i. e., 168). R-squared measures the degree to which

these models fit the data; its value is printed on the regression

plots generated from the model parameters. It ranges from zero

(worst fit) to 1 (best fit). Fits were improved when individual

size classes were treated separately. So, to predict overall

impingement from regression lines , we added predictions for

individual classes. However to simplify this report most of the

graphic analyses were done with size classes pooled.

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EVALUATION OF BARRIER NET AND

! 

INTAKE SCREEN MODIFICATIONS

, .

RESULTS OF SCUBA DIVER INSPECTIONS OF THE BARRIER NET.

In 1984 there were 20 SCUBA inspections of the inner barrier

net between July 17 and November 13. In July there were two occasions before the contractor started using the pulley system to

pull the net down when the net was found to be several feet off

the bottom near several pilings. After the pulley system and

diver deployment was initiated 11 out of 18 inspections indicated

that the net was at least several inches off the bottom near one

,

or more pilings. It was noted that the chain line was never

or ~

completely snug with the bottom across its whole length because

.....:

tension on the chain line caused it to bridge over depressions 

the bottom. The pulley system brought the net very close to the

,

bottom but did not eliminate gaps.

I i In 1985 use of the pulley system was discontinued in favor of

using a deeper net, installing a skirt, and deploying the net away

, 

from the poles so the chain line would settle to the bottom about

5 feet in front of the pilings.

1 .

I ; In 1985 there were 29 SCUBA inspections of the barrier net

between March 30 and October 21. The new outer net was found to

be off the bottom about 6 inches near several pilings on eight

occasions. This net collected very large masses of jellyfish

which did not pass through to the inner net. There were three

- 25 -
\ 

t, ,

occasions in 29 inspections when one or more inner barrier net

panels were not on the bottom. On each occasion the seal was

( !

preserved because the net was resting on the skirt. Therefore in

1985 there were never any detectable gaps under the inner barrier

net.

COMPARISON OF BLUE CRAB ABLTND1u~CE IN THE PATUXENT RIVER VERSUS

IMPINGEMENT ON INTAKE SCREENS.

  ,

While seasonal changes in crab population curves for the Patuxent River between 1984 and 1985 were similar crab population was

higher in the Patuxent River in 1985 than 1984 (Figure 7).

. , j ,

Analysis of covariance indicated that year, relative abundance of

crabs in the river, and interaction of year and relative abundance

 .

had significant effects on impingement at 0. 01 probability level.

There were significant differences in slopes between the

, .

impingement-versus-abundance regression lines for 1984 and 1985.

This difference indicates that there was change in the

I ;

relationship between impingement and abundance, i. e., a much

smaller portion of the crabs in the river were impinged (F igure

8) . This change was attributed to modifications in the barrier

~ 1

net and improvements in operating procedures.

parameters estimated by the covariance model were used to

predict impingement from the trawl and these were compared in

time-series plots to the observed impingement (Figure 9). There

j ,

L. ,

was a good fit between observed and predicted values except for a

peak in June 1985. This peak occurred when the tempering pumps

- 26 -
c:::::=?-

-....J

....

0:::

:2 80

:z:::E 10

Figure 7.

, .

r=-~:~:

..

140

100

MEAN NUMBER OF" CRABSIN TRAWL COLLECTIONS:

19M

1985

120

FEB WAR

19M /85

APR WAY JUN JUl AUG SEP OCT NOV

Comparison of the mean number of blue crabs per station on each

sampling date in 1984 with the mean number in 1985.

DEC
L-.'

ffi 80

ffi0.. :10

(IJ

, c,__no__-

NUMBER OF CRABS

. 1984 OBSERVED VAlUES

1984 REGRESSION UNE

. 1985 OBSERVED VAlUES

1985 REGRF.SSION UNE

. 

I) . 20 4Q 80 80 tOO 120 ABUNDANCE (MEAN NUMBER PER TRAWL COLLECTION)

Fi gure Comparison of the relationship between blue crab abundance in trawl

collections and the number impinged in 1984 (single barrfer netcoarser mesh) with that in 1985 (two barrier nets , finer mesh).

Regression lines were fit to data ~y cna1ysis of covariance with

variable slopes. The r-squared value of 0. 80 is the measure of

overall fit; this was good (l. OO=best fit).
L~_

: 

. ,,_

,

I.,. ,

, 

,

ffi0.. 50(I)

NUMBER OFaWE CRABS

I MPlNGfD:

1984 OBSERVED

1984- EXPEc;.!. EQ..1985 OBSERVED

1985- EXPE9! 

FfB MAR

1984 /85

2i e" 

lI:

::E 20.

APR UAY JUN JUL AUG SEP ocr NOV DEC

Figure 9. Comparison of observed impingement nf blue crabs in 1984 with

expected impingement computed from the 1984 regression equationfor estimating impingement from trawl data , and a similar comparison

for 1985 using the 1985 regression equation.
were turned on. There was a sharp decrease when the pumps were

turned off. It appeared that when the pumps were in use that the

. . ,

barrier net did not work as well. While the timing of the

impingement peak and tempering pump usage were similar there was

not necessarily a causal relationship. Underwater inspection of

. ,

the barrier net during this period did not indicate that the

increased flow was causing gaps under the inner net or between net

panels. There is a possibility that these higher impingement

rates were caused by stretching the mesh of the net so that small

young juvenile crabs could pass through.

,

EXPECTED IMPINGEMENT OF CRABS IN 1985 BASED ON NUMBERS IN PATUXENT

RIVER VERSUS OBSERVED IMPINGEMENT--IMPROVED BARRIER NET PERFORMANCE.

 

A comparison of expected impingement using the 1984 regression line with the 1985 trawl data was used to compute the

i- i expected impingement if there had been no change in the barrier

I \ net and its method of deployment (Figure 10). The overall effecti veness was computed by comparing the expected total computed in

this way for the period from May through August with the actual

 !

total. The expected total was 381, 000 crabs, while the actual

 ,

total was 63, 000 crabs; these totals indicate an 82% reduction in

crab impingement after the second barrier net was deployed.

On.

- 30 -

..J
.

~~ C:'

.. ,

1-'

._

t~" =J- - ..

.

.

:

::J .

FEe UAR

1984 JeS

\ I

1 \ 

r \ 

1 V 

I 1

NUMBER OF

eWE CRABS

IMAINGm:

1985 OBSERVED

1984 OBSERVED

J98~ PREDICTED

120

8' 100

D.-en eo

c,)

.fa

:::E

APR MAY JUN JUL AUG SEP OCT NOV DEC

Figure 10. Comparison of observed impingement of blue crabs in 1985 (after

deployment of second barrier net) with observed impingement in

1984 (before deployment) and with predicted impingement in 1985

assuming the same relationship between impingement and crab abundance as in 1984. 
IMPROVED SAMPLING PROCEDURES INDICATE FEWER FISH AND CRABS

IMPINGED THAN PREVIOUSLY ESTIMATED.

I ,

One of the things that was learned in doing the intensive

studies needed to evaluate the barrier net was that the methods

previously used to estimate impingement were biased. PEPCO estimated impingement liability to the State of Maryland based on such

biased data collected in 1982 (PEPCO 1 1983). In 1982 two

half-hour impingement collections were collected each week, one in

the day and the other at night or one hour of sampling from both

Units each week. In 1984 we continued that type of sampling but

also counted all the crabs impinged during the week.

Theoretically one would expect the number of crabs impinged per

.,

week to be equal to the number collected in an hour times the

number of hours in a week or 168. When plotted on a graph this

relationship is represented by the green dashed line with a slope

of 168 in Figure 11. The actual slope indicated by regression

analysis for small crabs is less than 1/4 that value or 38 (red

line in Figure 11). Larger crabs showed an even more dramatic

difference. The overall reduction for all size crabs was about

84%. Thus the evaluation based on the improved counting

procedures indicates that we greatly overestimated the number of

fish and crabs impinged in our report to the state.

- 32 -
L--e80 

lIJ

. 

ffi a. 

:SO

CJi

,

.., 

--"--1

NUMBER OF SMALL 

BWE CRABS IMPINGED:

. OBSERVED VAWIES

REGRESSION EQUATION

EXPEC I t:.U EQUATION

REGRESSION EQUATION:

NUMBER PER WEEK ,- 38.7 X 1-HOUR COUNT

EXPECTED EQUATION:

NUMBER PER WEEK - 168 X 1,-HOUR COUNT

Numbers of small crabs (0. 25 to 2. 4 inches) impinged per week at

Chalk Point Station in 1984 as determined ~y census versus numbers

in biased I- hour samples. The line fit by linear regression of

observed values is compared to the line rlrawn from ~he expected equation for I- hour samples t i . , the I- hour count times the number

of hours in a week (168).

1.15

0 -

0.4 1.2

HOUR COUNT (THOUSANDS)

Figure 11.
LOWER IMPINGEMENT LIABILITY ESTIMATES.

Intake liability is based on impingement and tempering-pumpentrainment mortality, not on the total number impinged or

entrained. The original estimates of such mortalities from 1982

I ; i '

data were 34 thousand crabs killed by impingement and 40 thousand

crabs killed by tempering pump entrainment or 74 thousand crabs

total (mortality indicated by crosshatching in Figure l2--blue for

impingement and red for tempering pump entrainment). The ANS 

estimated 15% mortality for impinged crabs returned to the discharge canal (ANSP 1983) and 42% mortality for crabs entrained by

the tempering pumps (Hirshfield, et ale 1982). If a correction is

made to impingement estimates as indicated by the previous regression analysis then the estimate for impingement mortalities is

reduced from 34 to 6 and the estimate for total mortalities from

74 to 46. Tempering pump entrainment mortal i ty increased to 87%

of the total mortality. Cadman and Holland (1985), in a report

prepared for Maryland' s Power Plant siting Program, recommended

that the tempering pump usage be discontinued or that screens be

installed to prevent entrainment of impingeable organisms. They

further recommended that a study be done to determine which of

these choices would be the most cost effective. In 1985 PEPCO

discontinued use of the tempering pumps on an experimental basis

which eliminated that part of the liability problem ($379, 470)

(Figure 13). There was no evidence of an increase in impingement

when the tempering pumps were turned off which was a concern

expressed by Cadman and Holland; in fact impingement decreased.

- 34 -
I :

. . 

i '

1982 ORIGINAL

34K

7.4%

40K

. .

1982 REVISED

NUMBER OF CRABS

J MPINGEMENT MORTAUTY

IZ2J I MPINGEMENT SURVIVAL

ENTRAINMENT MORTAUTY

rz2J ENTRAINMENT SURVIVAL

Figure 12. Original estimates of numbers of blue ~rabs impinged ~nd entrained

through tempering pumps are compared to a revision in which

impi ngement was reduced hy 84% to compensate for bi as observed in

lq74. Sizes of pies are proportional to total numbers impinged ?nd

entrained. Lfability, is associated with mortality not total numbers.

' 35 -
! i

! 

i .'

\ . ( , , , .

l ;

.

PREDICTED

. 

0 BSERVED

20K

~ 15.

115K 

85.

NUMBER OF CRABS

IMPINGEMENT MORTAUTY

rz2! I MPINGEMENT SURVIVAL

Figure 13. Predicted impingement (based on trawl data and the 198~ relationship

hetween blue crab abundance ond impingement) ;s cGmpared to the

numbers actually impingerl from May 14 to August 27, 1985. The 11se

of tempering pumps was discontinued in 1985. The size of pies is

proportional tn numbers ~ntrained. Liability is associated with

mortal i ty not total nu~bers. 

. 36 -
To summarize the corrected counting procedures resulted in a

38% reduction in estimates of the number of crabs killed during

impingement and tempering pump entrainment. Elimination of the

use of tempering pumps caused an 87% reduction in the new liabili ty Figure. The improved barrier net caused an 82% reduction in

the remainder. Overall there was a 98. 5% reduction in crab mortalty assuming a constant crab population as a result of the study

program and modifications to the barrier net. Computation of a

new liability Figure is not complete but is expected to be less

than $60, 000, a considerable reduction from the original estimate

of 2 million dollars for 1976-77 by ANSP (1983) which did not

include tempering pump liability.

IMPINGEMENT VERSUS CARRY-OVER--IMPROVED SCREEN BACKWASH SPRAY SYSTEM PERFORMANCE.

One of the ways that crabs get past the travelling screens

and from there into the condenser water box by clinging to the

travelling screens and riding over the top when the screens are

rotated for cleaning. As one might expect carry-over' of crabs

is significantly correlated with impingement. The effectiveness

of improvements to the Unit 2 screen backwash system was evaluated

by comparing carry-over in 1984 before the improvements with

carry-over in 1985 after the improvements with impingement as the

covariate (Figure 14). The analysis is made more difficult

because impingement was so much less in 1985 than in 1984;

nevertheless the data strongly suggest that improvements in the

- 37 -
L__-

(X)

2.4

::::J

1.(1

;:..ffi

1.2

Q::C3 D.

Figure 14.

.

.

--:J

:5.2

2.8

NUMBER OF CRABS

AT UNIT 2 SCREENS:

. 1984 OBSERVED VAWES

~984 REGRESSION UNE

. 1985 OBSERVED VAWES

1985 REGRESSION UNE

a -

18 24 J2 

IMPINGEMENT (THOUSANDS PIER WEEK)

Comparison for unit 2 of the relationship of crab carry-over and

impingement for 1984 (before installation of double spray header)

with that for 1985 (after installation).

Regression lines were fit to data by analysis of covariance with

variable slopes. The r-squared value of 0. 96 is the measure of

overall fit; this was excellent (l. OO=best fitl.
backwash system and decreased impingement related to barrier net

improvements were both important for reducing carry-over.

A similar analysis for Unit 1 where the screen-wash system

i .

was not modified also indicated reduced carry-over (Figure 15).

In this case part of the improvement can probably be attributed to

cleaning of the spray wash nozzles. The data indicated that the

wash system for Unit 2 was superior and more consistent than for

unit 1 (Figure 16). The low R-square value is the result of the

relatively high variability for Unit 1 when impingement was low.

VOLUME OF MATERIAL IN WATER BOX--IMPROVED SEALS AROUND INTAKE

SCREENS.

In 1984 and 1985 all carry-over crabs were picked from the

screens and there were still crabs entering the condenser water

box~ thus it is obvious that crabs were getting around the sides

or underneath the screens. In 1985 these seals were improved for

unit 2. As a result there was a dramatic decline in the number of

crabs in the water box for Unit 2, but not for Unit 1 where no

modifications were made (Table 5).

L_.-

ELIMINATION OF OUTAGES DUE TO CONDENSER BACKPRESSURE IN 1985.

The benefit to PEPCO from improvements to the travelling

screens and the barrier net is that there were no outages in 1985

attributed to condenser tube blockage while in 1984 there were

five such outages (Table 5).

- 39 -
L..-.

. Ii::-

I.&.J

2.4

(/)ffi

1.8

:;:,.ffi

1.2 . I

if:~ 0.

3.2

2.8 

I) -

Figure 15.

NUMBER OF CRABS

AT UNIT 1 SCREENS:

. 1984 OBSERVED VAWES

984 REGRESSION UNE

. 1985 OBSERVED VAWES

1985 REGRESSION UNE

R == 

UI 24 32 

IMPINGEMENT (fHOUSANDS PER WEEK)

:sa

Comparison for unit 1 of the relationship of crab carry-over and

impingement for 1984 (when bashwash spray nozzels were partially

clogge~) with that for 1985 (when nozzels were clean). Regression 1 ines were fit to data by analysis of covariance with

variable slopes. The r-squared value of 88 is the measure 

overall fit; this was good (1. 00=best fit).
L.J

0.24

8' 

0.20

ffi0..

I-'

ffi

~ 0.

. 

. ...

.Figur~ 16.

.. 

--O 

NUMBER OF CRABS

ON SCREENS IN 1985:

. UNIT 1 OBSERVED VAWES

~NIT 1 REGRESSION UNE

. UNIT 2 OBSERVED VAWES

UNIT 2 REGRESSION UNE

12 18 

I MPtNGEMENT (THOUSANDS PER WEEK)

Comparison for 1985 of the relationship of crab carry-over and

impingement for unit 1 (single hackwash spray header) with that

for unit 2 (new rlouble ~eader). 

Regression lines were fit to data by analysis of cova~iance with

variable slopes. The r-squared value of 53 is the measure 

overall fit; this was poor (1. 00=best fit).
L .

. 

'o" . 

,.,.. 

.. _..

l -'U

,. , 

.

.,

TABLE 5.

CHALK POINT STAllON OUTAGES IN 1984 AND 1985JUNE - AUGUST

OUTAGES

TOTAL DUE VOWME OF

NUMBER OF CONDENSOR: MATERIAL IN

OUTAGES BLDCKAGE WATER BOX

1984 3010

UNIT 1

1985 190

1984 749

UNIT 2

1985
COSTS AND BENEFITS

Only the pilings are included in the capital costs for the

barrier net; the netting is included in the operating costs. The

pilings cost $15, 000 for the inner net and $30, 000 for the outer

net. Thus the total capital cost was only $45, 000. This is a

relatively low cost compared to other barrier net or screen

systems used by other utilities at their generating stations.

In contrast, the average operating costs for the barrier net

are $76, 000 annually: $26, 000 per year to replace nets (nets are

replaced every two years) and $50, 000 to exchange net panels for

cleaning. These costs are high compared to an automated system in

use by Wisconsin Power and Light (WP&L) which costs only $15, 000

per year to operate. The WP&L station is located in freshwater

where little fouling occurs compared to the Chalk Point station

where high operating costs are primarily due to high clogging

rates by jellyfish and fouling.

Capital costs for the intake screen improvements are $89 500.

No increase in screen maintenance costs are expected. capital

costs for the ball condenser tube cleaning system upgrade were

$549, 939. Both capital costs were for modification to units 1 and

2 .

The above costs do not include the cost of biological studies

to design and evaluate these systems nor do they include the costs

of ongoing low level impingement studies and SCUBA inspections to

provide quality assurance for the system.

In 1984 before improvements to the barrier net and to the

intake screens there were five outages due to condenser tube

- 43 -
blockage by blue crabs. Each outage has been estimated by PEPCO to

cost approximately $130, 000 in lost revenues. At that rate

,

outages in 1984 would have cost $650, 000. There were no outages

in 1985 despite higher crab populations. Further impingement

liability was reduced from $2, 000, 000 to approximately $60, 000.

, 

- 44 -
f :C'

RECOMMENDATIONS

Have floats attached to top line of new barrier net panels to

facilitate net changes.

Extend the operating period of the barrier net as the Best

Available Technology to reduce impingement of fish as well as

crabs. Net deployment could safely be continued through

December and redeployment could be initiated in late February.

This extension would increase operating costs by about $5, 000.

Continue water box inspections.

continue twice weekly counts of impinged organisms and weekly

SCUBA inspections during the period when crab abundance is

high to provide quality assurance verification of effective

net operation. This should be combined with daily qualitative

observation of impinged material when screens are rotated and

removal of any crabs carried over. During periods when crabs

impingement is not high weekly counts of impinged organisms

would be sufficient.

Discontinue use of the tempering pumps to eliminate

tempering-pump entrainment liability.

Develop detailed procedures of barrier and net operation

methods.

- 45 -
, .,

I .

,.,

L "

, . , 

r'.

! ,, ., , 

Production (Chalk Point station Personnel) should continue to

issue and manage the contract for barrier net installation and

operation each year with the Water and Land Use Department

providing technical assistance in development of bid packages

and proposal evaluations.

- 46 -
t .

REFERENCES

! i

Academy of Natural Sciences of Philadelphia (&~sP). 1983.

Impingement. In: Chalk Point station 316 Demonstration.

Technical Reports. Vol. IV, Part I. ANSP, Philadelphia,

Pennsylvania. 74.

J .

t : Cadman, L. R. and A. F. Holland. 1985. Evaluation of Auxiliary

Tempering Pump Effectiveness at Chalk Point steam Electric station. Martin Marietta Environmental Systems , Columbia

Maryland. 68 p.

., ,

Hirshfield, M. F., J. H. Hixon, III and E. S. Perry. 1982. Chalk

Point Auxiliary Pump study 1981. Report No. 82-3F. Academy

of Natural Sciences of Philadelphia, Philadelphia Pennsylvania. 56 p. (Maryland PPSP-CP-80-2) .

Potomac Electric Power Company (PEPCO). 1983. Estimate of

Impingement and Auxiliary Tempering Pump Entrainment at the

Chalk Point station. PEPCO, Environmental Affairs Group,

washington, D. C. 63 p.

t .'

,; ; ,;;;, 

r. .,

r '

;..  

- 47 -