Docket OW-2002-0049 DCN#: 6-50463



### I 1

/+ Bailev, D. J. Loos, A. Weannoutli, P. Lungley, andE 

# .*

? Per.&, Mirant Mid-Atlantic Effectiveness, Operation and Maintenance, and Costs of a Baiier Net System for Impingement Reduction at the Chalk Point Generatinn Station 

### EFFECTIVENESS, OPERATION AND MAINTENANCE, AND COSTS OF A 

### BARRIER NET SYSTEM 

### FOR 

### IMPINGEMENT REDUCTION 

### AT THE 

### CHALK POINT GENERATING STATION

David Bailey, Jules Loos, Ann Wearmouth, Pat Langley Mirant Mid-Atlantic Upper Marlboro 

### MD Elgin Perry Private StatisticalConsultant HuntingtownMD

Barrier nets at the Chalk Point Generating Station on the Patuxent River Estuary are deployedto reduce environmental and operational impacts of fish and blue crab impingement. A temporaryprototype net was deployed in 1981 ; a singlepermanent net, in 1982;and a second (additional)permanent net, in September 1984. The nets are constructed 

### of woven mesh netting (0.75-in. and 1.25-in mesh width on inner and outer nets, respectively) suspended on 40-ft pilings at the mouth ofthe station intake canal which has a maximum depth of 15 feet. The barrier nets cost $100,000 

### to install and $75,000to $88,000 for annual operation and maintenance. After deployment 

### ofthe second net, impingementliabilitywas evaluatedbased on size and number of organisms impinged as specified in the Maryland Code ofRegulation. There was no estimation of overall percent reduction in impingement. 

### In the current evaluation,percent reduction was estimatedby comparingnumbers impinged in 

### a 12month period in 1984and 1985 after the secondbarrier net was deployed with baseline numbers 

### in an 18-monthperiod in 1976and 1977before deployment. There were 78% and 18%reductions in fish 

### and crab impingementtotals, respectively. However these estimateswere confoundedby changes in riverpopulations. To alleviatethe confounding,the estimatedreductions were adjusted where this could be supported by finding of good correlations (R-square > 0.4) between impingement and relative abundance for seven species whichrepresented more 1% of impingementtotals both evaluationperiods. The relationshp wasjudged sufficient for adjustment of estimates for Atlantic menhaden, spot, white perch, hogchoker and blue crab and insufficient for bay anchovy and Atlantic silverside. Proportionally adjusted estimates ofreduction for the former ranged 

### fiom 82 to 98%.


### Introduction

The Chalk Point Generating Station is currently owned and operatedby Mirant Chalk Point, LLC. A barrier net was first deployed at the Chalk Point Station in the summer of 1981, in response to operational problems. The Station was experiencingfrequent outages during a period ofpeak energy demand due to condenserblockagesbyjuvenile blue crabs. Subsequentlythe barrier net was used to satisfyrequirements for Best Technology Available for impingementunder Maryland’s State 3 16(b)regulations 

### (Loos, 1986 and 1987,Bailey et. al., 1998). There has also been a retrospective evaluation of decisions related to reducing and mitigating entrainment and impingementimpacts at Chalk Point (Bailey et. al, 2000). Thispaper will discuss the current state of the barrier net system developmentin terms of its design and operation. The paper will also provide informationon the barrier-net capital and operation and maintenance 

### (O&M) costs. The paper then discusses the effectivenessof the net and it’s ability to satisfy the performance standards proposed by EPA in the proposed Phase 

### I1 regulations issued in April of 2002 (EPA 2002). Two approaches are used in the evaluation. The first method involves calculatingpercent reductionsfor total fish and crabs, as well as the major fish species,by comparing impingementlevels before and after the

1barrier net system was installed. The second method uses relative abundance data 

### from long-term seining and trawling studies to adjustperformance estimates for the most commonlyimpinged species based 

### on their inter-year relative abundance. 

### Design, Operation and Maintenance

The Chalk Point Generating Stationis located
on the Patuxent River tidal estuary at the confluence 

### of the Patuxent River and Swanson’s Creek (Figure 1). Units 1 and 

### 2 of the Stationuse once through cooling water for condenser cooling. Each Unit is equipped with 

### two 125,000 gpm circulating water pumps such that the Station uses 500,000 gpm with all pumps in operation. Each pump draws water through four travelling screens equipped with 0.375411. wire mesh panels.

The firstbarrier net ofthe current net system 

### (a prototype was temporarilyinstalled on hand placed gill net poles from August

- October of 1981)was deployed at the entranceto the intake canal in Swanson’s Creek 

### in April of 1982. It is currently made up of two (2) 300 

### ft. longby 27-ft. deep panels, supported on forty 

### (40) 

### 1O-in. pilings. The woven netting mesh width is 0.75-in. stretch. A second barrier net located approximately 100 A. outside ofthe first net was deployed in Septemberof 1984to relieve pressure on the inner net. This net currently consists ofthree (3) panels, 300 ft. long by 27 

### ft. deep, supportedby fifty-one 

### (51) 10 0 10 20 Miles 

# -

InsetScale

0.5 0 0.5 1 Mile P
Figure 1. Map showing locationof Chalk Point Generating Station and River Population Monitoring Stations 10-in.pilings. The woven netting mesh width is 1.5411. stretch. Both sets ofpilings now have a 4-ft. skirt attached along the bottom in the deepest part of the chmelto help ensure a good seal during times when there is a lot 

### ofpressure on the nets. The skirt ismade up of 0.75411. stretch mesh. There is 0.5-in. wire mesh fencing located on supportsbetween the shore and the first pilings. The top of each net panel is hung from hooks attachedto the supportpilings such that there is always several feet of net above the waterline at
mean high tide. Each net panel has a chain of 0.19-in. galvanized steel to hold the net on the bottom.

The net is located in an area 

### ofthe Patuxent where severalbiofouling organisms are found. The dominant species duringpeak fouling season are a colonial hydroid 

### (G~ruia~franciscana) and a bryozoan 

### (Victorella pavida). 

### In addition, debris (primarily leaves in the fall) andjellyfish (in the summer) can accumulate on the outer net. 

### To control fouling growth and remove debris andjellyfish, the barrier net panels are changed
on a regular basis. All net panels are changed once every two weeks except in the summer, when net changes can take place once or even twice per week at the peak of the biofouling season. Thenet is changed in a manner suchthat as panels are unhooked from pilings and pealed away with one boat, another boat comes behind with the replacement net. This minimizes the opportunity for fish or crabs to by-pass the



### 2 -net. The nets have a line of floats attached to the top line which keeps them buoyant duringthis process. The new net 

### is deployed about 15 ft. in fi-ontof the pilings and relies on the currentto cany it against the pilings. About 

### 6 to 

### 8 ft. of net lies on the substratein front of the pilings to
ensure a good seal. It takes approximately

### 40 minutes to complete a change ofthe innernet. After the change, divers inspect and adjustthe bottom of the nets to ensure there is a good seal with the river substrate.
Each fall around mid-Novemberthe barrier nets are removed for ten days to 

### two weeks. 

### Thsis done to prevent impingement ofmenhaden in the fall. It is believedthat smalljuvenile or late larval stagemenhaden go through the net in the spring or early summer and take advantage of the continuous flow 

### of cooling water and associated food supply in the intake canal. They 

# la 

# \* 

# \: Intake Canal N\

.I1E-------) Bridge 100 Feet

Chain Link Fence

-.*mma.*p. Outer Row Barrier Net Pilings (1.25411. stretch mesh netting)

grow quickly,reaching a size of 4 to 

### 6 in. by the Figure 2. Barrier net system as deployed in 1985. fall, and are too large to pass back through the net in order to migrate downstream in the fall. Small impingement incidents can occw in late November if the nets are not removed and the fish allowed to escape.
Table 1. Primary changes in barrier net deployment from 1982 to 2002. Change Before Sept. 1984 After Sept. 1984 1996 to 2002 # Barrier Nets Qne Net Two Nets Same as 1985 Mesh Size 1.5-in. Stretch Mesh Outer net
- 1.5-in. stretch mesh Same as 1985 Inner net 

### - 0.75-in. stretch mesh Sealing Skirt None Inner net only Inner net and outer net Sealing Net Deployment None Net deployed 15 ft. 

### in front of Same as 1985 pilings. 6-8 ft drape on substrate Seasonal Deployment Removed in winter Removed in winter Year round deployment

Based on 1984observations ofbarrier net performance, significant changes were made to the barrier net system in September of 1984. Inspections in 1984determinedthat in eleven ofnineteen post deployment dive inspections (conducted a day or so after the net was changed)the barrier net was off the bottom at one or more pilings. 

### To alleviate this problem, a 

### 380-ft skirt of 0.75-in. stretch mesh was added along the 

### bottom of the inner pilings. It was further determined that smalljuvenile crabs could pass through the single 1.5-in.stretch mesh net. 

### In September 1984,the mesh size of the net was reduced to 0.75411. stretch (Figure2)and an outer net of 1-5-h stretch mesh was added. Themesh size of the inner net was now slightly smaller than the 0.375411. stainless steel mesh of the travelling screens. The method ofnet deploymentwas also changed. The net was deployed 15 

### ft. in fi-ontofthe pilings and allowedto floatback against the pilings, whch allowed the bottom of the net to lay on the substrate several feet in front of the pilings and form a good seal. Prior to this, the net was simply dropped directly in front of the pilings,

3which resulted in gaps under the net in areas where depressions occurred in the substrate. 

### As a result ofthe design and operational improvements,an analysis ofperformance improvement was conducted. This analysis focused on benthic species (blue crab, hogchoker and white perch) that could have gone under the net or through the larger mesh size in 1984 (Table 

### 2) Up until 1996the barrier nets were removed during the winter, from early December through late February, due to concern over ice damage. 

### In late fall of 1996, a singlebarrier net with 1.5411. stretch mesh was deployed year round at the inner net supportpilings. However, to addressthe concern of ice damage, the top ofthe net was submerged several feet below the water. The length 

### of time that it would remain submerged was based on water temperature,but usually lasted for a period of approximatelysix weeks.


### costs

The primary capital cost ofthe barrier net system was installation 

### of the 40-ft. long barrier-net support pilings. The pilings were hydraulically sunk into the sediment to a depth 

### of 10 

### ft. From 3-5 

### ft. 

### ofpiling remains above water at mean high tide. The capital cost to install the 91 pilings that make up the barrier net system was $100,000. The 40 inner net pilings were deployed in 1982 and the 5 

### 1 outer net pilings were

~ deployed in 1984.

One set ofnets 

### (2 inner and 3 outer) cost approximately $13,000 and each year one set ofreplacementnets is purchased as the life ofthe nets is about three years. 

### A local contractorperforms the barrier net changes and diving inspections. The annual cost of

### this contract is approximately 

### $75,000.

Costs ofbarrier net systems may vary considerably due to site-specific circumstances. Wisconsin Energy Corporationreports that the barrier net used at their Pine HydroelectricFacility in northeastern Wisconsin is almost never changed and associated O&M costs are minimal. 

### In addition,nets are replaced less frequently since 

### UV damage due to exposure to sunlight during the drylngprocess (to remove fouling) is eliminated (Dave Michaud, personal communication). 

### In contrast, DTE Energy reports significantlyhigher costs at their LudingtonPump Storage Plant in Michigan. 

### In this case, a 2.5-mi. long barrier net is deployed seasonallyfi-omApril through October and requires frequent maintenanceby divers. The capital cost of this net was 

### $1.5 million and annual O&M costs are $1.3 million (Robert Reider personal communication).


### Effectiveness Evaluation


### Methods

1. ImpingementMonitoring Methods

1976-1977

### Pre-Barrier Net Impingement Study

Between June 1976 and November 1977impingement monitoring was conducted to estimate impingement of fish and blue crabs. Two 0.5-hour collections, 3 hours apart, were made on each of 

### 6 consecutive days 

### in order to determine diel impingementrates. These sampleswere followed by a 2 or 3 day period during which no samplingwas done and then a new six-day seriesbegan. The time schedule for the collection of the first sample of each 6-day series was: 0000 hr., 0400 hr., 0800 hr., 1200hr., 1600

### hr. and 2000 hr.. After the sixth sampling series, the start time would go back to 0000 hr. Unit 1was sampled first, immediately followedby a Unit 2 sample. This procedure was followed for 4.8months.

4Prior to sampling,the screens were 

### runfor 20 to 

### 30 minutes to clean them of debris and impinged organisms. After the initial cleaning, impinged organismswere collectedby placing a dip net in the screenwashdischarge sluiceway for 0.5 hour. The dip net was made of 1.27-cm stretch-meshnylon attachedto a steel frame made to 

### fit precisely in the dischargetrough.

All collected organisms were counted and weighed by species. In addition, up to 

### 50 individuals of each fish species was measured for total length.


### 1984-85Post 

### Barrier NetDeployment Impingement Study

Between March 1, 1984 and September

### 6, 1985

### two methods were used to monitor fish impingement. The first method involved conducting a complete count of all organisms impingedthroughout the entire samplingperiod. The complete count method was an attempt to more accurately determine impingement numbers, specificallyfor blue crabs. Although fish data was also collected, there was concern that, due to screenpredation by crabs or deteriorationin hot weather, fish counts could be skewed. Thereforethe fish data collectedusing this method was onlyused during the period when crabs were less active and the temperatureswere below 14OC. (from November through April), The other method involved sampling one day a week, twice a day, for 

### 0.5 hour to monitor impingementrates in a way that could compare current impingementlevels to the pre-barrier net levels ofthe 1976/1977 study.

The complete count was generally conductedby collecting daily samplesthroughoutthe study period. Somevariances in the samplingregime occurred from March 1to April 13,1984and September 17to May 24,1985 when weekends were included in Monday counts. Also between June 16 and July 24,1984 sampling was done twice a day to help evaluatepotential losses due to consumption of impinged organisms by blue crabs. These samples were collected as in the pre-bank net study,by deploying a net in the screenwash sluiceways of Units 

### 1 and 2. The nets remained deployedhowever, to ensure that, in the event that plant personnel turned the screens for operationalpurposes, any fish or crabs washed from the screens would be collected. The frame size of these samplingnets was 17in. wide and 27 in. high, and the mesh
was 0.25-in. square.
For impingementrate estimates, 0.5-hour daytime and nighttime sampleswere collected once per week. Collectionswere scheduled so that there would be complete diel coverage each month. The methods used to collect these samples were the same as those used in the complete counts, except that the screenswere rotated for 30 minutesprior to taking the sample in order to clean them of accumulated debris and organisms. This material was still collected and included in the complete count numbers (as were the samples themselves).


### 1989-1999 Barrier Net Performance Monitoring

From June 1989through 1999 qualitative samplingwas conducted to monitor barrier net performance. Twenty-fourhour sampleswere collected from the screenwash sluicewaytrough. These sampleswere collected weekly from Units 1 and 

### 2 until 1991when samplingwas reduced to once every other week. Samplingwas fktherreduced in 1992to Unit 1 only. The screenswould be run to clean them of debris and fish and the final screen would be marked. The next day a 24-hr. impingement samplewould be collected from the screens.

52. 

### Long-term Relative Abundance Monitoring Methods

Two sources of fish and crab data were used to evaluatebarrier net effectivenessin the context of inter-year relative abundance changes. Maryland Department ofNatura1 Resources 

### (MD DNR)juvenile index data for the Choptank and Nanticoke Rivers, the closestpermanentjuvenile index locations to Chalk Point (Figure l), and the Chalk Point Patuxent River benthic trawl catch. The MD DNR index is based on 

### 2 seine hauls with a 100-ft. bagless seine with 0.25-in. bar mesh. Threerounds of seine collections are made monthly fiom July to September. The index value used in the analysis is the geometric mean over 

### 8 stations. The 

### MD DNR data and a more detailed description ofmethods are available at 

### http://www.dnr.state.md.us/fisheries/juvindex/index.html.

The Patuxent River trawl catch data is the mean number of fishper minute of trawling over 23 stations for fish and 24 stations for crabs. The trawl was a 16-ft. otter trawl with 1.25411. stretch-mesh in the body and 0.5-in. mesh in the bag. Sampling frequency varied from weekly to monthly from 1982to the present 

### (Loos and Perry, 2001).


### 3. 

### Analytical Methods

The yearly estimates ofimpingement 

### fiom 30-minute samples were calculated using the formula: 

### Impingement Estimate 

### = mean # fish per 

### 0.5 hour 

### * 

### 2 units 

### * 48 half-hourdday 

### * 365 dayslyr. 

### (1) These estimates are for the hypothetical case of full operation 

### ofboth units year round. 

### In the 19844985 period, 0.5-hour sampleswere weighted by the duration of day and night periods. This was not necessary in the 1976/1977period because the number of samples in eachperiod was proportional to the duration.

More realistic estimatesbased on actualplant operation and census estimates of impingement were also made for the 1984/1985period. The census data were considered

### to provide more reliable estimates because they did not require making the assumptionthat impingementon a fieshly cleaned screen is representative of normal operation. However, the data were used selectively, due to concerns about bias that could result fiom predation of impinged fishby crabs and from deterioration in hot weather. For purposes 

### of computing annual impingement estimates, the census data were used during the period when crabs were less active and the temperatureswere below 14 C. (fiom November through March). Crab
census data were used year round. The composite of 30 minute impingement estimates and census estimateswere made by summingmonthly estimates of impingement over the period that each method was used.

For the purpose 

### of comparing fish impingementrelative to barrier net deployment, estimates for individual fish specieswere computed only for those species that made up more than 1% of impingement levels in the 19764977 and 1984/1985 study periods. The blue crab and six fish species met this criterion.

We consideredthe change in number impingedbefore and after the net system deployment was the most logical comparisonto evaluate the effectiveness ofthe net systembecause it allows a direct estimate of percent reduction. However, for that to be a fair comparison,the number of fish and crabs in the river would need to be comparable for the before and after periods. That was not the case. Comparing the 

### MD 

### DNRjuvenile index values for major species with available data showed substantial differences. Therefore
we elected to make adjustments in the estimatesbased on changes in population abundances.

6Adjusted estimates ofpercent reduction were done in two steps. First, an expectedimpingement in the 1984/1985period was computedbased on fish abundance using the following equation: 

### Expected Impingementin 1984/1985 

### = Impingement in 1976/1977 

### * (2) 

### (Juvenile Index in 1984/Juvenile Index in 1976).
Second, the percent reductionwas computed using expected and actual impingementin 1984/1985using this equation: 

### Percent Reduction 

### = (Expected Impingement

## - Actual impingement)/ 

### (3) Expected Impingement *I 

### 00. Population-abundance adjustmentswere made using the 

### MD DNRjuvenile index data where it could be shown that there was a reasonably close relationshipbetween impingement and the index. This wasjudged using R-square values of approximately0.4 or higher for the Impingement

- Juvenile Index regression for log transformed data. The 

### two most abundant fish species in 1976/1977met this criterion.
The 

### MD DNRjuvenile index data were not useful for makmg adjustmentsfor some species eitherbecause index data were not available or because there was little or no relationshipbetween the index and impingement. Therefore, for those species, trawl data were evaluated for inter-yearrelative aburidance adjustments. Blue crab weekly census data for impingement were correlated with weekly trawl catches, while annual impingement for 

### two fish specieswhere correlatedwith mean trawl catches fi-omMay 

### to September.


### A limitation 

### ofthe trawl data 

### is that it was not available for the 1976/1977 studyperiod. Therefore data 

### fi-om 1984and 1985were used to computepercent reduction, since many improvementswere made prior to the 1985crab season. 

### As noted in the description 

### ofbarrier net deploymentthere were many improvements in design and operation ofthe net that would be expected to reduce impingement between 1984 and 1985, especiallyforbenthic oriented species like crabs, hogchoker and white perch. The equations used to computepercent reduction were similarto those used for thejuvenile index except that the ratio ofmean trawl catch in 1985over mean trawl catch in 1984was used to computethe expected impingement in 1985. The census data collectedbetween April and August in 1984and 1985were used for the impingement numbers.

This would not take into accountperiods when the barrier net was not deployed. For crabs, however, there
was almost no impingement during this period (winter). For the other species, there could be some overestimatebut this could be reduced since the net is now deployed year-round.

Table 2 provides a 

### summarycomparison ofthe 1976/1977and 1984/1985impingement studies. 

### Blue crabs
were the most commonly impinged organismmaking up 45.1% ofthe total impingementin 197611977 and 78% in 1984/1985 

### (38.1% using the census data in 1984/1985). Atlantic menhaden were the most commonly impinged fish species making up 56.9% oftotal fish impingement in 197611977 and 36.5% in 198411985 (54.1% using the census data in 1984/1985). These two species made up 76.4% ofthe total impingementprior to and 84.2% (71-6%) of total impingement after the doublebarrier net system was deployed.

7

### In 1976/1977, spot were the next most abundant fishmaking up 27.3 percent of total fish, followed by hogchoker 

### (8.1 %), white perch (1.8%),bay anchovy (1.4%), gizzard shad and Atlantic silversides (1.3%). Fish species making up more than 1% of fish impingement in 1984/1985included hogchoker (21.9%), Atlantic silverside (14.9%),weakfish (5.5%), spot (3.7%), skilletfish (3.3), whiteperch (2.9),bay anchovy (2.3%) and winter flounder (1.7%). Comparisons ofblue crab and fish species that made up more than 

### 1% of fish impingementboth before and after the barrier net system deployment are shown in Figure 3. Based on a simple comparison of impingement samples collectedbefore (1976/1977) and after (1984/1985)the barrier net systemwas deployed there was an overall 78% reduction for total fish. There were reductions 

### of 86% for menhaden and 97% for spot, which accounted 

### for 84.2% 

### of 

### fish impingementprior to the barrier net system deployment. Hogchoker, white perch and bay anchovyhad reductions of 40%, 64% and 62% respectively. These three species made up 1 1.3% of fish impingement prior to the net system deployment. Atlantic silversides,however, showed a 162% increase in impingement afterthe net system'deploymentand blue crabs, the most commonly impinged organism had a reduction of only 18%.

All Fish Menhaden spot Hogchoker White Perch By Anchovy e1anti.c Silverside Blue Crab
0.5 1.0 a.5 20 2.5 Impingement(millions)

Raw Percent Reduction

Figure 3. Impingement before (1976/1977) and after (1984/1985) barrier net deployment reduction and unadiusted (raw) percent reduction after deployment.

8U 

## s

a: c (c c .- E C C

s O b d -IC

S 0

9Table 3. Results of regression analysis of impingement vs. measures of river population abundance. Species Years Dependent IndependentVariable Parameter P-Value Variable Value Atlantic Menhaden 4 985-1 999 Imp. Sample' Juvenile. Index' Bay Anchovy

" Atlantic Silverside '

" White Perch
" spot Atlantic Menhaden 1985-1999 Imp. Sample' Trawl Catch'

la 'I Bay Anchovy ' ' Atlantic Silverside White Perch ' spot Hogchoker Blue Crab 1984/1985 Weekly Imp. WeeklyPrawlCatch Census Year 1984=1; 1985=2 (x 1000) TrawlCatch*Year 1.382 0.0001 0.68 -0.765 0.01 06 0.36 0.055 0.8879 0 0.184 0.1122 0.1 8 0.829 0.0037 0.49 0.660 0.6721 0.02 -0.622 -0.61 6 0.0554 0.7995 0.27 0 0.340 0.0088 0.42 0.318 0.0064 0.45 0.402 0.0026 0.52 1.8684 <0.0001 0.79 15.554 0.0246 -0.917 <0.0001

spot R-Square=O .49 Loglo Transformed
Table 3 shows the results of correlation analysis conducted to determine if impingementlevels 

### ofthe commonly impinged species could be correlated with either the 

### MD DNRjuvenile index
or Patuxent River trawl catch. Results ofthis analysis 

### show that for Atlantic menhaden 

### and spot there is a good correlationbetween the MD DNR juvenile index survey and numbers of fish impinged. No correlationwas found between impingement and the juvenile index for white perch bay anchovies and Atlantic silversides and the survey does not include data for blue crabs and hogchoker. Applying the barrier net system adjustment for species with some degree 

### of correlation, the barrier net systemreduction for Atlantic menhaden was decreased to 83%, spot was increased to 98% and white perch to 95% ( Table 4).
Another method to evaluatethe barrier net systemperformance in the context of inter-year species abundancevariability of commonlyimpinged species was to correlateresults with the 1984 and 1985 benthic trawl survey. As indicated earlier in this paper, a singlebarrier net AtlanticMenhaden

0.2 0.6 1.0 1.4 0.0 0.4 0.8 1.2 Log Juv. Index Log Juv. Index

Figure4. Plot of Impingement vs. Maryland Juvenile Index for Atlantic menhaden and spot. Trace indicates predicted value from regression analysis.

White Perch

0.0 0.5 1.0 1.5 2.0 2.5 Log Trawl Catch
Figure 5. Plot of Impingement vs. benthic trawl for white perch and hogchoker. Trace indicates predicted value from regression analysis.

10 1

### of 1.5-in. stretch mesh was deployed in the summer of 1984. The use of a single net, combined with poor deployment methods, frequentlyresulted 

### in the net having gaps along the bottom which allowed benthic species (in particular)to pass under the net. It is these speciesthat are collectedin greatest abundancein benthic trawl surveys. 

### In this analysis, the more accurate complete census impingement data was used for correlationwith the benthic trawl data. 

### A good correlation Number of 

# ' 'I I Blue Crabs It. Impinged �a 

# - 1984Qbserved 1985 Observed 60 1985 Predicted

40

20

0 was found for blue crabs (Figure 

### 6)and Feb Mar Apr May Jun Jul Aug Sep Oct Nov DF?c a weak correlationwas found for 

# I 198411985 hogchoker and white perch when complete census data was compared with data collected in the benthic trawl study (Figure 5)

. The results of estimates ofbarrier net performance based on adjustments for inter-year relative abundance are shown in Table 4, For all three species the relative abundancewas higher 

### in 1985than 1984. Figure 7 shows the results of barrier net systemperformance for the blue crab based on a comparison of 1984 and 1985weekly impingement census data, versus expected 1985weekly impingement adjusting for relative abundanceusing the weeklybenthic trawl data. This comparison indicates the barrier net systemresulted in an estimated 

### 82% reduction 

### in blue crab impingement. Similar analysis (i.e. based on annual impingement and trawl data rather than weekly data) for Figure 6. Plot of weekly impingementvs. mean weekly trawl catch for blue crab. Trace indicates predicted value from regression analysis.

80
9 70 4. 8 c 3, Number of Crabs 

## i% 1984 gf50 -?-. 1985 f m 240 R Square = 0.79 0 f t30 L C 0, $20

u) f

- '- 

# e- 10 

# ____-----­ m, 0 40 Man Number Der Trawl Cotledion Figure 7. Observed weekly impingement in 1984 and observed and predicted weekly impingement in 1985 with predicted impingement based on river population abundance hogchoker and white perch indicate an 

### 83% and greater than 95% reduction respectively for these two species (Table 4).

Atlantic silverside impingement numbers could not be correlatedwith the 

### MDDNRjuvenile index and they
were not collected in benthic trawl samples. However, it is importantto note that in 1984/1985the barrier net system was deployed on a seasonalbasis; being removed in December and re-deployed in late February
or early March due to concern over ice damage. Analysis of impingement samples on a weekly basis determinedthat 95% of the Atlantic silverside impingement occurred in the winter during the period ofnet system removal. The Atlantic silverside 1984 and 1985 complete census data were comparedto evaluate

11the benefit of the doublebarrier net system during the periods when the net was deployed in those years. There were 

### 58% fewer of this species impingedin 1985 after the doublebarrier net system was deployed.

Table 4. Unadjusted and adjusted estimates of percent reduction in impingement following barrier net deployment.

SDecies Impingement Reduction Based Impingement Reduction Adjusted for on'Rab Data Inter-\/ear Relative Abundance Blue Crab 18% 82% Atlantic Menhaden 86% 85% spot 97% 98% (") Hogchoker 40% 83% White Perch 64% 95% (") * R square less for impingement vs. abundance regression less than 0.5 (0.45 for spot and 0.42 for white perch). Bay anchovyimpingement samples were also not correlatedwith either MD 

### DNRjuvenile index or trawling samples. A comparison of the 1984 and 1985 census data showed a 55% reduction in observed impingement. 

### Discussion

Based on impingementreductions reported, the barrier net was determinedto satisfj

### 3 16(b)requirements under Maryland State regulations 

### (Loos 1987). The Maryland impingementreduction regulations are based on American Fishery Societyreplacementvalue costs rather than a numeric reduction in impingement. 

### In addition to providing a fish and shellfishprotection benefit, Chalk Point Generating Stationpersonnel have reported that the technologyhas been beneficial in terms ofpreventing outages due to condenserblockage by aquatic organisms or debristhat by-passes the travelling screens 

### (Loos 1986).

The primary cost component ofthe operation and maintenanceofthis technology is the cost for barrier net changes to controlbiofouling and debris. 

### This cost 

### runs approximately$75,000 to $88,000 per year at Chalk Point and can be substantially higher or lower depending 

### on site-specific circumstances.

It is important to consider the barrier net systemperformancein the context ofthe current Federal EPA rulemaking. EPA proposed regulations for existing generating stations on April 9,2002 (USEPA 

### 2002). The regulations propose performance standardsthat require reducing impingementby 80-95% for all generating stations that use more than 50 mgd and use greater than 

### 25% of their water for cooling and do not employwet closed cycle cooling. Facilities on oceans, tidal estuaries, great lakes and facilities on rivers that use more than 

### 5% of the mean annual flow are also required to reduce entrainmentby 60-90%.

EPA identified the barrier net as a technology for meeting the impingementperformance standard in both the Phase 

### I1 proposal and the Notice ofData Availability (NODA) issued March 19,2003 (USEPA 2003). The results ofthe Chalk Point barrier net systemprovide support that the technology can satisfy the performance standard. However, application of the technology shouldbe evaluated on a site-specificbasis. Successofthe barrier net system at Chalk Point required modificationsbased on site-specific conditions and impinged species. Some additional enhancementsare being consideredto further improve the barrier net system performance.


### EPA's proposed impingementperformance standard is based on requiring a reduction in impingement mortalityrather than a reduction of impingementnumbers. The Academy ofNatural Sciences of Philadelphia, which conducted the 19764977 impingement studies at Chalk Point, used impingement

12survival studies conducted at the Calvert CliffsNuclear Generating Station on the mainstem Chesapeake Bay to make observations of impingement survival for Chalk Point. 

### In addition, some effort was made to estimate impingement survival during studies conductedby Pepco at Chalk point forblue crabs, which are not only the primary organism impinged,but also accounted for the major economicvalue of impingement losses under Maryland’s 3 16(b)regulations. There is 

### 5% immediate impingementmortality forblue crabs, however, it is estimated that overallmortality is approximately 15%

### (ANSP 1983)due to thermal exposure as a result of returning impinged crabs to the heated dischargecanal. Mortality was considered a function of dischargetemperature and biocide use. Similarly,hogchoker also have very high impingement immediate survival rate, estimated to be in the range of 99%, although mortality 

### in the dischargecanal was not estimated.
The results of the Chalk Point barrier net systempoint out several issues related to impingementmonitoring relative to EPA’s proposed rule and 

### the NODA. The first is the importance ofbeing ableto relate pre and post banier-net deploymentimpingement differencesto the inter-year variabilityof the most commonly impinged species. Failureto account for inter-year variability

### ofthe major species impinged could significantlyunder or overestimate the technology’sperformance in the context ofthe proposed performance standard. 

### This is especiallytrue 

### in estuarine environments, such as the tidal Patuxent River where Chalk Point Station is located, due to salinity changes that affect species composition, in additionto normal fluctuations in aquatic fish and shellfish populations. 

### In the case of Chalk Point, there would have been a significantunderestimate ofperformance for the most commonlyimpinged species (blue crab), if the performance evaluationhad been limited to a comparison of impingementnumbersbefore and after the net system deployment. 

### In addition, effectivenessof the system’sperformance for hogchoker and white perch would have been underestimated and below the proposed performance standard.

A secondpoint is that with this technology is simplynot practical to evaluatebarrier net effectiveness in terms ofthe performance standard for each species. Skilletfishfor example where not collected 

### in the 1976/1977study,but in the post barrier net system deployment samplingthey ranked sixth in terms of the most frequentlyimpinged fish. Similarly, gizzard shad ranked sixth in terms ofthe most frequently impinged fish with an estimated 31,000 impinged in 1976/1977. However, so few were impinged in 1984/1985,the reduction would be estimatedto be well over 99% (probably an overestimate). 

### In both instances,precise quantitativeconclusionswould be unwarranted due to increased uncertaintyin years of low relative abundance.

A third point is the importance ofproper selection of the methods used to quantifyimpingement levels. As noted in Table 

### 2, the estimation samples significantly overestimatedimpingement compared to the census method samples. This phenomenon is discussed in more detailby Bailey et a1 (1998). 

### The use of the estimationmethod would be appropriateifthe screens at a facilitywere rotated on a continuousbasis but this method will lead to an overestimateof impingement at facilities such as Chalk Point, where screens tend to be rotated only once per day.
Severalideas arebeing consideredto further improve the performance ofthe barrier net. The first idea is to deploy the net system year round with the top of the net submerged during cold periods to prevent ice damage. Such deployment would probably result in a significantreduction in impingement of Atlantic silverside. The second idea is to use 0.75411. stretch mesh for both the inner 

### and outer net to provide additionalprotection for small crabs and fish. A third idea would be to furtherreduce the mesh size to provide protection to larger entrainableorganisms. A barrier net cannot achieveprotection for all life stages,but protection for larger entrainable organisms combined with other technologies or measures, could

13be a viable compliance strategyunder EPA’s proposed Phase 

### I1 rules. A final idea being considered for the Chalk Point Stationbarrier net system is to devise an escapemechanism for trapped menhaden that would eliminatethe need for temporarynet removal in the fall. Finally, it is important to note that site-specific circumstancesmay preclude use ofbarrier nets. 

### An attempt
was made to deploy a barrier net at the Morgantown Station on the lower Potomac River where there is a curtainwall in front of the facility’s intake. Delivery of fuel by barge precluded deployment of abarrier net outside the curtain wall, while the currentsinsidethe curtain wall prevented successful deployment of a barrier net there. At other locations,proximity of the intake to navigation channels or deployment in areas with rapid currents orhigh debris loads couldprevent their use. 

### Summary and 

### Conclusions

In summary, the barrier net, depending on site-specific circumstances,can be a cost effectivetechnologyto meet EPA’s proposed impingementperformance standards or as a component of 

### an overall compliance strategyto satis@ requirementsfor facilities subjectto both impingement and entrainmentperformance standards. When using a barrier net it is important to establish the calculationbaseline prior to net deployment and collect inter-year variability data on the most commonlyimpinged species 

### in order to evaluateperformancein the context of sourcewaterbodypopulation fluctuations. It is also importantto recognize that the barrier net system is likely to require modifications on a site-specificbasis to provide

. protection for the species of concern. 

### References
ANSP, 1983. Impingement. In: Chalk Point Station 316 demonstration. Technicalreports. Vol. 

### 111.pp 123-196. Prepared

### by Academy ofNatural Sciences of Philadelphia, Benedict Estuarine Research Laboratory, Benedict, MD. Bailey D. E., J. J. 

### Loos, and E. 

### S. Perry, 1998. Studies of CoolingWater Intake Structure Effects at Potomac River Generating Stations. EPRI Clean Water Act Section 

### 3 16(b)Technical Workshop. Coolfont Conference Center, Berkely Springs,

### WV. Sept. 1998 Bailey

### D. E., J. 

### J. 

### Loos, E. 

### S. Perry, R. J. Wood. 

### 2000. 

### A retrospective evaluation of 316(b) mitigation optionsusing a decision analysis framework. Environmental Science and Policy, 3 (Supplement 1): 

### Sl-S36
Loos, 

### J. 1986. Evaluation of Benefits to Pepco of Improvementsin the BarrierNet and Intake Screens at Chalk Point Stationbetween 1984 and 1985. EnvironmentalAffairs Group, Potomac ElectricPower Company, Washington DC. June 1986.


### Loos, 

### J.J. 1987. Chalk Point Station intake liability values computed after deploymentof a secondbarrier net at the mouth of the intake canal and after discontinuationof auxiliarytemperingpump usage. Environmental Affairs Group, Potomac Electric Power Company, Washington DC.


### Loos, J. J. and 

### E. 

### S. Perry, 2001. Chalk Point Station. Analysis of fish distribution relative to the Unit 

### I and 2 thermal discharge 1991

- 2001. Environmental Compliance Support,Mirant Mid-Atlantic, LLC, Upper Marlboro 

### MD. March 2001.

14USEPA, 2002. National Pollutant DischargeElimination System

- Proposed Regulationsto Establish Requirements for Cooling Water Intake Structures at Phase 

### I1 ExistingFacilities; Proposed Rule Federal Register 

### 67(68): 17122-14225.
USEPA, 

### 2003. National PollutantDischargeElimination System

- Proposed Regulations to Establish Requirements

### for CoolingWater Intake Structures at Phase 

### I1 Existing Facilities;Notice 

### ofData Availability; Proposed Rule, Federal Register 68(53): 13522-1

### 3587.

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