### 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 

### 200681'" 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 \ "



### ELIMINATION OF OUTAGES DUE TO CONDENSER



### BACK PRESSURE IN 1985 

# .. . . .

COSTS 1u~D BENEFITS 

# .. . 

# . .

RECOMMENDATIONS. . 

# . .

REFERENCES



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

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.

- 1 -! ; . "

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 aquantitative 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 I . - -

- 3 -: -"(-----L.........--..~----_"'t.5-Po'U)1.2::)Ii.II) 0.~ 0.1883urnt87&YEARt878t8eet888Figure 1.-------,, -----1985 ?t88tt884Yearly commercial harvest of hlue crabs from thePattlxent Riverfr'olll 1%1 In19134 antiprojections 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 tem

### pering 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 '\ ,

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

;.. I r ; ; , ' . ;..,



### ( i

i ; 1 I 'c- ( ;

- 7 - 

### Thei. ' 

### i i \ ,

i ,

. 0 

### , '

, ,

I ; 

# !:'

, 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

- 9 -CHAIN LINK FENCE

, ,

WOVEN WIRE FEN CES 

# /'

NNER ROW aF BARRIER 

# " '
' ,

(a. INCH SO 11ESH) 

### , '



# NET PILINGS 

# . " ' "::, 

## /" ~,



# " (Oo 7S- INCH STRETCH ~1ESH ~:. ' ,

f l )"' NETTING USED ALONG THIS RO 

# WJ' ' : '~; 

# ....

. , , , 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 

### ! ,



# c:: .,.

100 FEET

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

1:) -t' '



### r . L(;' 

### : '

r' , 

### \ '

I. . " . ( : 

# ~.

,.,

. 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 thebarrier 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 . ,



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

# ft. 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 

### ! '

special care is taken so that a minimal



## clean panels (Figure 5) amount of debris and crabs passes through any gaps during the 

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

- 12 -CLEAN PANELCURRENTDIRTY PANELSPIUN GS,;,---'-'----First panel beingrelooved; cupping ofthispanel prevents debris from drifting onto cleanrepl acement panel being deployed behind it.Gap between panels is closed while removalof di rty panel isc6mpl eted......Net replacement panel being deployed about 15 ft from pilings to allow chain line to reachbottom well away from pilings beforeupper net drifts back to pilings."'-Final net configuration showing overlapping patterntypical of inner barrier net.Figure 5.Diag,"am of proceduresIIspd to~xchangp fouler!harrier ret panelsl'ril) r.1'~dl1 IHlt"Once a week this check is

is on the bottom all the way 

## across. 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.

- 14 -

### 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



### i i r ' ! .

- 15 -

### TABLE 2. COMPARISONS OF BARRIER NET DEPLOYMENT PROCEDURES IN 1984 AND 1985 , ;



### 1984 

### 1985 . '

( J 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 . ,

r' ,

- 16 -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.

- 18 -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
- 19 -
, J , I

### 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 IMPINGE

### MENT 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 MODIFICA

## TIONS. 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

- 20 -

### " I

i ; ' . \ : , ,

1;;'

l ,

L.i ! . ~

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
- 22 -! 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

DAILY APR. 16 TO JUN. 15, 1984 

### 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) DAILY MAY 28 TO SEP. 6, 1985 *SCREENS WERE NOT ROTATED FOR CLEANING ON WEEKENDS DURING THIS 

### PERIOD. :;j

- 23 -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.

- 24 - 

### I ', '

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 occa

### sions 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. 

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

### 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 The new outer net was found to 

### between March 30 and October 21. 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:::::=?--....J0:::....:2 80:z:::E 10Figure 7.-, .r=-~:~:-..140100MEAN NUMBEROF" CRABSIN TRAWL COLLECTIONS:19M1985120FEBWAR19M/85APRWAYJUNJUlAUGSEPOCTNOVComparison of the mean number of blue crabs per station on eachsampling date in 1984 with the mean number in1985.--'-DECL-.'ffi 80ffi0.. :10(IJ, c,__no__-NUMBER OF CRABS. 1984 OBSERVED VAlUES1984REGRESSION UNE. 1985 OBSERVED VAlUES1985 REGRF.SSIONUNE. -I) .204Q8080tOO120ABUNDANCE (MEAN NUMBER PER TRAWLCOLLECTION)Fi gureComparison of the relationship between blue crab abundance in trawlcollections 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 withvariable slopes. The r-squared value of 0.80 is the measure ofoverall fit; this was good (l.OO=best fit).L~_'-:;" .,,_.,.. .I.,. ,, -~~,ffi0.. 50(I)NUMBER OFaWE CRABSI MPlNGfD:1984 OBSERVED1984- EXPEc;.!. EQ..1985 OBSERVED1985- EXPE9!FfBMAR1984 /852ie"lI:::E 20.APRUAYJUNJULAUGSEPocrNOVDECFigure 9.Comparison of observed impingement nf blue crabs in 1984 withexpected impingement computed from the 1984 regression equationfor estimating impingement from trawl data, and a similar comparisonfor 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 effec

## ti 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 .FEeUAR1984 JeS\ I1 \r \1 VI 1NUMBER OFeWE CRABSIMAINGm:1985 OBSERVED1984 OBSERVEDJ98~PREDICTED1208' 100D.-en eoc,).fa:::EAPRMAYJUNJULAUGSEPOCTNOVDECFigure 10.Comparison of observed impingement of blue crabsin 1985 (afterdeployment of second barrier net) with observed impingement in1984 (before deployment) and with predicted impingement in 1985assuming the same relationship between impingement and crababundance 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 methodspreviously 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--e80lIJ. ==ffia.:SOCJi,-'..,-----"--1NUMBER OF SMALLBWE CRABSIMPINGED:. OBSERVED VAWIESREGRESSION EQUATIONEXPEC I t:.U EQUATIONREGRESSION EQUATION:NUMBER PER WEEK ,- 38.7 X 1-HOURCOUNTEXPECTED EQUATION:NUMBER PER WEEK -168 X 1,-HOUR COUNTNumbers of small crabs (0.25 to 2.4 inches) impinged per week atChalk Point Station in 1984 as determined ~y census versus numbersin biased I-hour samples. The linefit by linear regression ofobserved values is compared to the linerlrawn from ~he expectedequation for I-hour samplest i ., the I-hour count times the numberof hours in a week(168).1.150 -0.41.2HOUR 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 ANSestimated 15% mortality for impinged crabs returned to the dis

### charge 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 theuse of tempering pumps caused an 87% reduction in the new liabil

## i 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::::J1.(1ffi;:..1.2Q::C3 D.Figure 14..'.--:J:5.22.8NUMBER OF CRABSAT UNIT 2SCREENS:. 1984OBSERVED VAWES~984REGRESSION UNE. 1985OBSERVED VAWES1985REGRESSION UNEa -1824J2IMPINGEMENT (THOUSANDSPIERWEEK)Comparison for unit 2 of the relationship of crab carry-over andimpingement for 1984 (before installation of double spray header)with that for 1985 (afterinstallation).Regression lines were fit to data by analysis of covariance withvariable slopes. The r-squared value of 0.96 is the measure ofoverall 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.&.J2.4ffi(/)1.8ffi:;:,.1.2. Iif:~ 0.3.22.8I) -Figure 15.NUMBER OF CRABSAT UNIT 1 SCREENS:. 1984OBSERVED VAWES984 REGRESSIONUNE. 1985 OBSERVEDVAWES1985REGRESSION UNER ==UI2432IMPINGEMENT (fHOUSANDS PER WEEK):saComparison for unit 1 of the relationship of crab carry-over andimpingement for 1984 (when bashwash spray nozzels were partiallyclogge~) with that for 1985 (whennozzels were clean).Regression1 ineswere fit to data by analysis of covariance withvariable slopes. The r-squared value of88is the measureoverall fit; this was good (1.00=best fit).L.J0.248'0.20ffi0.."""I-'ffi~ 0.. -.....Figur~ 16...'-----O---NUMBER OF CRABSON SCREENS IN 1985:. UNIT1 OBSERVED VAWES~NIT 1REGRESSION UNE. UNIT2 OBSERVED VAWESUNIT 2 REGRESSION UNE1218I MPtNGEMENT (THOUSANDS PER WEEK)Comparison for 1985ofthe relationshipofcrab carry-over andimpingementforunit 1 (single hackwash spray header) with thatfor unit 2 (newrlouble ~eader).Regression lines were fit to data by analysisofcova~iance withvariable slopes. The r-squared valueof53 is the measureoverall fit; this was poor (1.00=best fit).L .. "'o" ." ,.,..~.._..'-~ """'-l -'U",., --" -.--.,TABLE 5.CHALK POINT STAllONOUTAGES IN 1984AND 1985JUNE -AUGUSTOUTAGESTOTALDUEVOWME OFNUMBER OFCONDENSOR:MATERIAL INOUTAGESBLDCKAGEWATER BOX19843010UNIT 119851901984749UNIT 21985COSTS 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 -