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

I

1 /+ Bailev, D. J. Loos, A. Weannoutli, P. Lungley, and E

.* ? 

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 Statistical Consultant 

Huntingtown MD 

Barrier nets at the Chalk Point Generating Station on the Patuxent River Estuary are deployed to reduce 

environmental and operational impacts of fish and blue crab impingement. A temporary prototype net was 

deployed in 1981;a single permanent 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 of the station intake canal which has a maximum 

depth of 15 feet. The barrier nets cost $100,000 to install and $75,000 to $88,000 for annual operation and 

maintenance. After deployment of the second net, impingement liabilitywas evaluated based on size and 

number of organisms impinged as specified in the Maryland Code of Regulation. There was no estimation 

of overall percent reduction in impingement. In the current evaluation, percent reduction was estimated by 

comparing numbers impinged in a 12 month period in 1984 and 1985 after the second barrier net was 

deployed with baseline numbers in an 18-monthperiod in 1976 and 1977before deployment. There were 

78% and 18% reductions in fish and crab impingement totals, respectively. However these estimates were 

confounded by changes in river populations. To alleviatethe confounding, the estimated reductions were 

adjusted where this could be supported by finding of good correlations (R-square > 0.4) between 

impingement and relative abundance for seven species which represented more 1% of impingement totals 

both evaluation periods. The relationshp was judged 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 of reduction for the former ranged fiom 82 to 98%. 

Introduction 

The Chalk Point Generating Station is currently owned and operated by 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 experiencing frequent outages during a period of peak energy demand due to 

condenser blockages by juvenile blue crabs. Subsequently the barrier net was used to satisfy requirements 

for Best Technology Available for impingement under 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 impingement impacts at Chalk Point (Bailey et. al, 2000). This paper will 

discuss the current state of the barrier net system development in terms of its design and operation. The 

paper will also provide information on 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 calculating percent reductions for total 

fish and crabs, as well as the major fish species, by comparing impingement levels before and after the 

1 
barrier net system was installed. The second method uses relative abundance data from long-term seining 

and trawling studies to adjust performance estimates for the most commonly impinged species based on 

their inter-year relative abundance. 

Design, Operation and Maintenance 

The Chalk Point Generating Station is 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 Station use 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 first barrier net of the current net system (a 

prototype was temporarily installed on hand 

placed gill net poles from August -October of 

1981)was deployed at the entrance to the 

intake canal in Swanson’s Creek in April of 

1982. It is currently made up of two (2) 300 ft. 

long by 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 of the 

first net was deployed in September of 1984 to 

relieve pressure on the inner net. This net 

currently consists of three (3) panels, 300 ft. 

long by 27 ft. deep, supported by fifty-one (5 1) 

10 

-

0 10 20 Miles 

Inset Scale 

0.5 

P 

0 0.5 1 Mile 

Figure 1. Map showing location of Chalk Point 

Generating Station and River Population Monitoring 

Stations 

10-in.pilings. The woven netting mesh width is 1.5411. stretch. Both sets of pilings now have a 4-ft. skirt 

attached along the bottom in the deepest part of the chmel to help ensure a good seal during times when 

there is a lot of pressure on the nets. The skirt is made up of 0.75411. stretch mesh. There is 0.5-in. wire 

mesh fencing located on supports between the shore and the first pilings. The top of each net panel is hung 

from hooks attached to the support pilings 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 of the Patuxent where several biofouling organisms are found. The dominant 

species during peak fouling season are a colonial hydroid (G~ruia~franciscana) and a bryozoan (Victorella 

pavida). In addition, debris (primarily leaves in the fall) and jellyfish (in the summer) can accumulate on 

the outer net. To control fouling growth and remove debris and jellyfish, 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. The net is 

changed in a manner such that 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 during this 

process. The new net is deployed about 15 ft. in 

fi-ontof the pilings and relies on the current to 

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 of the inner net. 

After the change, divers inspect and adjust the 

bottom of the nets to ensure there is a good seal 

with the river substrate. 

Each fall around mid-November the barrier nets 

are removed for ten days to two weeks. Ths is 

done to prevent impingement of menhaden in 

the fall. It is believed that smalljuvenile or late 

larval stage menhaden 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 meshnetting) 

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 1984 observations of barrier net performance, significant changes were made to the barrier net 

system in September of 1984. Inspections in 1984 determined that in eleven of nineteen 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 

(Figure 2) and an outer net of 1-5-hstretch mesh was added. The mesh size of the inner net was now 

slightly smaller than the 0.375411. stainless steel mesh of the travelling screens. The method of net 

deployment was also changed. The net was deployed 15 ft. in fi-ontof the pilings and allowed to float back 

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, 

3 
which resulted in gaps under the net in areas where depressions occurred in the substrate. As a result of the 

design and operational improvements, an analysis of performance 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 single barrier net with 1.5411. stretch mesh was 

deployed year round at the inner net support pilings. However, to address the concern of ice damage, the 

top of the 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 approximately six weeks. 

costs 

The primary capital cost of the 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. of piling 

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 51outer net pilings were ~

deployed in 1984. 

One set of nets (2 inner and 3 outer) cost approximately $13,000 and each year one set of replacement nets 

is purchased as the life of the nets is about three years. A local contractor performs the barrier net changes 

and diving inspections. The annual cost of this contract is approximately $75,000. 

Costs of barrier net systems may vary considerably due to site-specific circumstances. Wisconsin Energy 

Corporation reports that the barrier net used at their Pine Hydroelectric Facility 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 drylng process (to remove fouling) is eliminated 

(Dave Michaud, personal communication). In contrast, DTE Energy reports significantly higher costs at 

their Ludington Pump Storage Plant in Michigan. In this case, a 2.5-mi. long barrier net is deployed 

seasonally fi-om April through October and requires frequent maintenance by 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. Impingement Monitoring Methods 

1976-1977 Pre-Barrier Net Impingement Study 

Between June 1976 and November 1977 impingement 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 impingement rates. These samples were followed by a 2 or 3 day period during 

which no sampling was done and then a new six-day series began. The time schedule for the collection of 

the first sample of each 6-day series was: 0000 hr., 0400 hr., 0800 hr., 1200 hr., 1600 hr. and 2000 hr.. 

After the sixth sampling series, the start time would go back to 0000 hr. Unit 1 was sampled first, 

immediately followed by a Unit 2 sample. This procedure was followed for 4.8 months. 

4 
Prior to sampling, the screens were runfor 20 to 30 minutes to clean them of debris and impinged 

organisms. After the initial cleaning, impinged organisms were collected by placing a dip net in the 

screenwash discharge sluiceway for 0.5 hour. The dip net was made of 1.27-cm stretch-mesh nylon 

attached to a steel frame made to fit precisely in the discharge trough. 

All collected organisms were counted and weighed by species. In addition, up to 50 individuals of each fish 

species was measured for total length. 

1984-85 Post Barrier Net Deployment 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 impinged throughout the entire 

sampling period. 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 

screen predation by crabs or deterioration in hot weather, fish counts could be skewed. Therefore the fish 

data collected using this method was only used during the period when crabs were less active and the 

temperatures were below 14 OC. (from November through April), The other method involved sampling one 

day a week, twice a day, for 0.5 hour to monitor impingement rates in a way that could compare current 

impingement levels to the pre-barrier net levels of the 1976/1977 study. 

The complete count was generally conducted by collecting daily samples throughout the study period. 

Some variances in the sampling regime occurred from March 1 to April 13,1984 and September 17 to 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 evaluate potential 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 deployed however, to ensure that, in the event 

that plant personnel turned the screens for operational purposes, any fish or crabs washed from the screens 

would be collected. The frame size of these sampling nets was 17 in. wide and 27 in. high, and the mesh 

was 0.25-in. square. 

For impingement rate estimates, 0.5-hour daytime and nighttime samples were collected once per week. 

Collections were 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 screens were 

rotated for 30 minutes prior 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 1989 through 1999 qualitative sampling was conducted to monitor barrier net performance. 

Twenty-four hour samples were collected from the screenwash sluiceway trough. These samples were 

collected weekly from Units 1 and 2 until 1991 when sampling was reduced to once every other week. 

Sampling was fkther reduced in 1992 to Unit 1 only. The screens would be run to clean them of debris 

and fish and the final screen would be marked. The next day a 24-hr. impingement sample would be 

collected from the screens. 

5 
2. Long-term Relative Abundance Monitoring Methods 

Two sources of fish and crab data were used to evaluate barrier net effectiveness in the context of inter-year 

relative abundance changes. Maryland Department ofNatura1Resources (MD DNR) juvenile index data 

for the Choptank and Nanticoke Rivers, the closest permanent juvenile 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. Three rounds 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 of methods are available at 

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

The Patuxent River trawl catch data is the mean number of fish per 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 1982 to the present (Loos 

and Perry, 2001). 

3. Analytical Methods 

The yearly estimates of impingement 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 of both units year round. In the 19844985 

period, 0.5-hour samples were weighted by the duration of day and night periods. This was not necessary in 

the 1976/1977period because the number of samples in each period was proportional to the duration. 

More realistic estimates based on actual plant operation and census estimates of impingement were also 

made for the 1984/1985 period. The census data were considered to provide more reliable estimates 

because they did not require making the assumption that impingement on 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 fish by 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 temperatures were below 14 C. (fiom November through March). Crab 

census data were used year round. The composite of 30 minute impingement estimates and census 

estimates were made by summing monthly estimates of impingement over the period that each method was 

used. 

For the purpose of comparing fish impingement relative to barrier net deployment, estimates for individual 

fish species were computed only for those species that made up more than 1% of impingement levels in the 

19764977 and 1984/1985study periods. The blue crab and six fish species met this criterion. 

We considered the change in number impinged before and after the net system deployment was the most 

logical comparison to evaluate the effectiveness of the net system because 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 

DNR juvenile index values for major species with available data showed substantial differences. Therefore 

we elected to make adjustments in the estimates based on changes in population abundances. 

6 
Adjusted estimates of percent reduction were done in two steps. First, an expected impingement in the 

1984/1985period was computed based 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 reduction was computed using expected and actual impingement in 1984/1985 using 

this equation: 

Percent Reduction = (Expected Impingement - Actual impingement) / (3)

Expected Impingement *I00. 

Population-abundance adjustments were made using the MD DNR juvenile index data where it could be 

shown that there was a reasonably close relationship between impingement and the index. This was judged 

using R-square values of approximately 0.4 or higher for the Impingement -Juvenile Index regression for 

log transformed data. The two most abundant fish species in 1976/1977 met this criterion. 

The MD DNR juvenile index data were not useful for makmg adjustments for some species either because 

index data were not available or because there was little or no relationship between the index and 

impingement. Therefore, for those species, trawl data were evaluated for inter-year relative aburidance 

adjustments. Blue crab weekly census data for impingement were correlated with weekly trawl catches, 

while annual impingement for two fish species where correlated with mean trawl catches fi-omMay to 

September. 

A limitation of the trawl data is that it was not available for the 1976/1977study period. Therefore data 

fi-om 1984 and 1985 were used to compute percent reduction, since many improvements were made prior to 

the 1985 crab season. As noted in the description of barrier net deployment there were many improvements 

in design and operation of the net that would be expected to reduce impingement between 1984 and 1985, 

especially for benthic oriented species like crabs, hogchoker and white perch. The equations used to 

compute percent reduction were similar to those used for the juvenile index except that the ratio of mean 

trawl catch in 1985 over mean trawl catch in 1984was used to compute the expected impingement in 1985. 

The census data collected between April and August in 1984 and 1985 were used for the impingement 

numbers. 

This would not take into account periods 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 

overestimate but this could be reduced since the net is now deployed year-round. 

Table 2 provides a summary comparison of the 1976/1977 and 1984/1985impingement studies. Blue crabs 

were the most commonly impinged organism making up 45.1% of the total impingement in 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% of total 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% of the total 

impingement prior to and 84.2% (71-6%) of total impingement after the double barrier net system was 

deployed. 

7 
In 1976/1977, spot were the next most abundant fish making 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 1984/1985 included hogchoker (21.9%), 

Atlantic silverside (14.9%), weakfish (5.5%), spot (3.7%), skilletfish (3.3), white perch (2.9), bay anchovy 

(2.3%) and winter flounder (1.7%). 

Comparisons of blue 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 collected before (1976/1977) and after (1984/1985) the barrier net system was 

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 impingement prior to the barrier net system 

deployment. Hogchoker, white perch and bay anchovy had reductions of 40%, 64% and 62% respectively. 

These three species made up 11.3% of fish impingement prior to the net system deployment. Atlantic 

silversides,however, showed a 162% increase in impingement after the 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 PercentReduction 

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

and unadiusted (raw) percent reduction after deployment. 

8 
U 

s 

a: 

c 

(c

.-cE

C 

C 

s 

O b d -IC 

S 

0 

9 
Table 3. Results of regression analysis of impingement vs. measures of river population abundance. 

Species Years Dependent Independent Variable Parameter P-Value 

Variable Value 

Atlantic Menhaden 4 985-1999 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 0.18 

0.829 0.0037 0.49 

0.660 0.6721 0.02 

-0.622 -0.616 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 

impingement levels of the commonly 

impinged species could be correlated 

with either the MD DNR juvenile index 

or Patuxent River trawl catch. Results 

of this analysis show that for Atlantic 

menhaden and spot there is a good 

correlation between 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 system reduction for Atlantic 

menhaden was decreased to 83%, spot 

was increased to 98% and white perch to 

95% ( Table 4). 

Another method to evaluate the barrier 

net system performance in the context of 

inter-year species abundance variability 

of commonly impinged species was to 

correlate results with the 1984 and 1985 

benthic trawl survey. As indicated 

earlier in this paper, a single barrier net 

Atlantic Menhaden 

0.2 0.6 1.0 1.4 0.0 0.4 0.8 1.2 

Log Juv. Index Log Juv. Index 

Figure 4. 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 species that are collected in 

greatest abundance in benthic trawl 

surveys. In this analysis, the more 

accurate complete census impingement 

data was used for correlation with the 

benthic trawl data. A good correlation 

Number of

''II 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 correlation was 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 of barrier net performance 

based on adjustments for inter-year 

relative abundance are shown in Table 4, 

For all three species the relative 

abundance was higher in 1985 than 

1984. Figure 7 shows the results of 

barrier net system performance for the 

blue crab based on a comparison of 1984 

and 1985 weekly impingement census 

data, versus expected 1985 weekly 

impingement adjusting for relative 

abundance using the weekly benthic 

trawl data. This comparison indicates 

the barrier net system resulted 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 impingement vs. 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 f -?-. 1985 m 

240 

0 R Square = 0.79 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 correlated with the MD DNRjuvenile index and they 

were not collected in benthic trawl samples. However, it is important to note that in 1984/1985the barrier 

net system was deployed on a seasonal basis; 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 

determined that 95% of the Atlantic silverside impingement occurred in the winter during the period of net 

system removal. The Atlantic silverside 1984 and 1985 complete census data were compared to evaluate 

11 
the benefit of the double barrier net system during the periods when the net was deployed in those years. 

There were 58% fewer of this species impinged in 1985 after the double barrier net system was deployed. 

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

SDecies Impingement Reduction Based Impingement ReductionAdjusted 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 anchovy impingement samples were also not correlated with either MD DNR juvenile index or 

trawling samples. A comparison of the 1984 and 1985 census data showed a 55% reduction in observed 

impingement. 

Discussion 

Based on impingement reductions reported, the barrier net was determined to satisfj 3 16(b) requirements 

under Maryland State regulations (Loos 1987). The Maryland impingement reduction regulations are 

based on American Fishery Society replacement value costs rather than a numeric reduction in 

impingement. In addition to providing a fish and shellfish protection benefit, Chalk Point Generating 

Station personnel have reported that the technology has been beneficial in terms of preventing outages due 

to condenser blockage by aquatic organisms or debris that by-passes the travelling screens (Loos 1986). 

The primary cost component of the operation and maintenance of this technology is the cost for barrier net 

changes to control biofouling 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 system performance in the context of the current Federal EPA 

rulemaking. EPA proposed regulations for existing generating stations on April 9,2002 (USEPA 2002). 

The regulations propose performance standards that require reducing impingement by 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 employ wet 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 entrainment by 60-90%. 

EPA identified the barrier net as a technology for meeting the impingement performance standard in both 

the Phase I1 proposal and the Notice of Data Availability (NODA) issued March 19,2003 (USEPA 2003). 

The results of the Chalk Point barrier net system provide support that the technology can satisfy the 

performance standard. However, application of the technology should be evaluated on a site-specificbasis. 

Success of the barrier net system at Chalk Point required modifications based on site-specific conditions and 

impinged species. Some additional enhancements are being considered to further improve the barrier net 

system performance. 

EPA's proposed impingement performance standard is based on requiring a reduction in impingement 

mortality rather than a reduction of impingement numbers. The Academy of Natural Sciences of 

Philadelphia, which conducted the 19764977 impingement studies at Chalk Point, used impingement 

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survival studies conducted at the Calvert Cliffs Nuclear 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 conducted by Pepco at Chalk point for blue crabs, which are 

not only the primary organism impinged, but also accounted for the major economic value of impingement 

losses under Maryland’s 316(b)regulations. There is 5% immediate impingement mortality for blue crabs, 

however, it is estimated that overall mortality is approximately 15% (ANSP 1983) due to thermal exposure 

as a result of returning impinged crabs to the heated discharge canal. Mortality was considered a function 

of discharge temperature 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 discharge canal was 

not estimated. 

The results of the Chalk Point barrier net system point out several issues related to impingement monitoring 

relative to EPA’s proposed rule and the NODA. The first is the importance of being able to relate pre and 

post banier-net deployment impingement differences to the inter-year variability of the most commonly 

impinged species. Failure to account for inter-year variability of the major species impinged could 

significantly under or overestimate the technology’s performance in the context of the proposed 

performance standard. This is especially true in estuarine environments, such as the tidal Patuxent River 

where Chalk Point Station is located, due to salinity changes that affect species composition, in addition to 

normal fluctuations in aquatic fish and shellfish populations. In the case of Chalk Point, there would have 

been a significant underestimate of performance for the most commonly impinged species (blue crab), if the 

performance evaluation had been limited to a comparison of impingement numbers before and after the net 

system deployment. In addition, effectivenessof the system’s performance for hogchoker and white perch 

would have been underestimated and below the proposed performance standard. 

A second point is that with this technology is simply not practical to evaluate barrier net effectiveness in 

terms of the performance standard for each species. Skilletfishfor example where not collected in the 

1976/1977 study, but in the post barrier net system deployment sampling they ranked sixth in terms of the 

most frequently impinged fish. Similarly, gizzard shad ranked sixth in terms of the 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 estimated to be well over 99% (probably an overestimate). In both 

instances, precise quantitativeconclusions would be unwarranted due to increased uncertainty in years of 

low relative abundance. 

A third point is the importance of proper selection of the methods used to quantifyimpingement levels. As 

noted in Table 2, the estimation samples significantly overestimated impingement compared to the census 

method samples. This phenomenon is discussed in more detail by Bailey et a1 (1998). The use of the 

estimation method would be appropriate if the screens at a facilitywere rotated on a continuousbasis but 

this method will lead to an overestimate of impingement at facilities such as Chalk Point, where screens 

tend to be rotated only once per day. 

Several ideas are being considered to further improve the performance of the 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 significant reduction in impingement of Atlantic 

silverside. The second idea is to use 0.75411. stretch mesh for both the inner and outer net to provide 

additional protection for small crabs and fish. A third idea would be to further reduce the mesh size to 

provide protection to larger entrainable organisms. A barrier net cannot achieve protection for all life 

stages, but protection for larger entrainable organisms combined with other technologies or measures, could 

13 
be a viable compliance strategy under EPA’s proposed Phase I1 rules. A final idea being considered for the 

Chalk Point Station barrier net system is to devise an escape mechanism for trapped menhaden that would 

eliminate the need for temporary net removal in the fall. 

Finally, it is important to note that site-specific circumstancesmay preclude use of barrier nets. An attempt 

was made to deploy a barrier net at the Morgantown Station on the lower Potomac River where there is a 

curtain wall in front of the facility’s intake. Delivery of fuel by barge precluded deployment of a barrier net 

outside the curtain wall, while the currents inside the 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 or high debris loads could prevent their use. 

Summary and Conclusions 

In summary, the barrier net, depending on site-specific circumstances, can be a cost effective technology to 

meet EPA’s proposed impingement performance standards or as a component of an overall compliance 

strategy to satis@ requirements for facilities subject to both impingement and entrainment performance 

standards. When using a barrier net it is important to establish the calculation baseline prior to net 

deployment and collect inter-year variability data on the most commonly impinged species in order to 

evaluate performance in the context of source waterbody population fluctuations. It is also important to 

recognize that the barrier net system is likely to require modifications on a site-specific basis to provide . 

protection for the species of concern. 

References 

ANSP, 1983. Impingement. In: Chalk Point Station 316 demonstration. Technical reports. Vol. 111.pp 

123-196. Prepared by Academy of Natural Sciences of Philadelphia, Benedict Estuarine Research 

Laboratory, Benedict, MD. 

Bailey D. E., J. J. Loos, and E. S. Perry, 1998. Studies of Cooling Water 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 

options using a decision analysis framework. Environmental Science and Policy, 3 (Supplement 1): Sl-S36 

Loos, J. 1986. Evaluation of Benefits to Pepco of Improvements in the Barrier Net and Intake Screens at 

Chalk Point Station between 1984 and 1985. Environmental Affairs Group, Potomac Electric Power 

Company, Washington DC. June 1986. 

Loos, J.J. 1987. Chalk Point Station intake liability values computed after deployment of a second barrier 

net at the mouth of the intake canal and after discontinuation of auxiliary tempering pump 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. 

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USEPA, 2002. National Pollutant Discharge Elimination System - Proposed Regulations to 

Establish Requirements for Cooling Water Intake Structures at Phase I1 Existing Facilities; Proposed Rule 

Federal Register 67(68): 17122-14225. 

USEPA, 2003. National Pollutant Discharge Elimination System -Proposed Regulations to Establish 

Requirements for Cooling Water Intake Structures at Phase I1 Existing Facilities; Notice of Data 

Availability; Proposed Rule, Federal Register 68(53): 13522-13587. 

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