FINAL REPORT 

BIOLOGICAL EVALUATION 

OF A MODIFIED TRAVELING SCREEN 

MYSTIC STATION - UNIT NO. 7 

Prepared for 

BOSTON EDISON COMPANY 

AUGUST 1981 

By 

STONE & WEBSTER ENGINEERING CORPORATION 
SUMMARY 

On the basis of concerns raised by the U.S. Environmental Protection Agency 

(EPA), Boston Edison Company has conducted a study to determine the 

effectiveness of a modified, traveling screen in reducing mortality of fish 

at Mystic Station - Unit No. 7. Modifications included the attachment of 

fish lifting buckets to each debris tray on one Unit 7 screen and the 

installation of a low-pressure spraywash to gently wash fish from the screen 

into a fish trough. A new transmission on the screen motor allowed the 

screen to be rotated at test speeds of approximately 3.3, 7.5, 10, and 

15 ft/min. 

Impingement survival studies were conducted on four separate occassions 

between October 1980 and June 1981; these studies encompassed the fall, 

winter, and spring seasons and the period of peak fish impingement. The 

studies were concentrated on obtaining survival data for four species of 

concern to EPA: winter flounder, rainbow smelt, alewife, and blueback 

herring. 

Sampling consisted of collecting and identifying all fish removed from the 

modified screen and categorizing them as live, dead, or stunned. Live and 

stunned fish were placed in holding pools and monitored for 96 hours. 

Overall survival was then calculated by combining initial and latent 

mortality values. 

Results indicate that reasonable survival can be achieved among the key test 

species at Mystic Station by modifying traveling screen design and 

operation. Survival of winter flounder was high under all operating 

conditions and exceeded 96 percent during the peak abundance period. 

Rainbow smelt, alewife, and blueback herring exhibited lower survival rates, 

ranging from 23 to 67 percent depending on screen rotation speed and 

species, with the exception of adult herring which showed low survival under 

all conditions. 

On the basis of these results, it would appear that a modified traveling 

screen offers an effective means of alleviating fish mortality at Mystic 

Station - Unit 7. Further, since the majority of fish impingement occurs 

over a period of 3 to 4 months, continuous screen operation only during 

these months should afford a high level of protection on a yearly basis. 

Finally, on the basis of cost-benefit considerations, it is recommended that 

the modified screens be operated at intermediate travel speeds on the order 

of 8 ft/sec. 
FINAL REPORT 

BIOLOGICAL EVALUATION OF A MODIFIED TRAVELING SCREEN 

MYSTIC STATION - UNIT NO. 7 

BOSTON EDISON COMPANY 

INTRODUCTION 

One of the two traveling water screens at Mystic Station - Unit No. 7 has 

been modified to permit a test of the effectiveness of the screen, equipped 

with lift buckets and low-pressure sprays, in reducing fish mortality. On 

the basis of discussions with the Environmental Protection Agency, four 

species were selected as being of primary concern at Mystic: winter 

flounder (Pseudopleuronectes americanus), rainbow smelt (Osmerus mordax), 

alewife (Alosa pseudoharengus), and blueback herring (Alosa aestivalis). 

The latter two species are difficult to distinguish from each other; 

therefore, in this report, they are grouped as Alosa spp. 

Previous studies at Mystic Station (Stone & Webster, 1978) had shown that 

flounder are capable of withstanding impingement and removal from the 

existing, unmodified screens with relatively high survival. It was probable 

that the modified screen would enhance survival of this species further. 

Smelt and Alosa spp. had not been evaluated at this site, but available data 

indicated that adequate survival might be obtained under certain conditions 

of continuous screen rotation. Therefore, in order to determine the 

survival of the four species of concern at Mystic, an experimental screen 

system was installed in the fall of 1980. 

1 
One Unit No. 7 traveling water screen was modified in an effort to enhance 

fish survival. The existing Unit No. 7 screenwell is shown on Figure I. 

Modifications included: 

1. the attachment of fish lifting buckets to each of the existing 

debris trays; 

2. installation of a low pressure spray header with nozzles to gently 

wash fish off of the screen after passing over the head shaft; 

3. installation of a fish trough to collect fish removed from the 

screen; 

4. incorporation of a fish collection area from which the fish can be 

gently removed and transferred to holding tanks for latent 

survival studies; 

5. movement of the existing high pressure debris spray header from 

the ascending to the descending run of the screen; and 

6. installation of a new debris trough (located directly under the 

fish trough) to carry debris around the screen to the existing 

Unit No. 7 debris trough. 

These modifications are shown on Figure 2. In addition to these 

modifications, the screen was equipped with a two-speed motor and a 

four-speed transmission. The transmission and motor permitted approximate 

2 
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'to 

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77'- 6.. 

• 

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PL AN 

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DIRECTION 

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FIGURE 1 

CIRCULATING WATER INTAKE 

MYSTIC STATION - UNIT 7 

BOSTON EDISON COMPANY 

STONE & WEBSTER ENGINEERING CORPORATION 
SHEET PILING -Or

AQAM 

I

DEBRIS TROUGH 

(BELOW FISH TROUGH) 

G SCREENS 

i

MMILMWMWIPM..11,■• 

DEBRIS TROUGH 

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ti 

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0 

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(TYP) 

APPROX HOUSING LOW PRESSURE FISH 

AREA SPRAY WASH TROUGH 

FLOW 

FIGURE 2 

MODIFIED TRAVELING WATER SCREEN 

MYSTIC STATION - UNIT 7 

BOSTON EDISON COMPANY 

STONE & WEBSTER ENGINEERING CORPORATION 
screen travel speeds of 2.5, 3.3, 5, 7.5, 10, 15, and 30 ft/min to be 

selected. Finally, water supply and return systems were installed in the 

Unit Nos. 1 and 2 screenhouse to service the fish holding facility located 

in this area. 

METHODS 

The biological evaluation of the modified screen began on October 6, 1980. 

Two types of sampling were conducted: 1) total impingement collections, and 

2) impingement survival testing. Total impingement sampling consisted of 

counting and identifying all fish collected on both Unit 7 traveling water 

screens throughout the study period. To accomplish this, the contents of 

all screen washings for both screens were picked and sorted for fish several 

times per week. All fish were identified to species and recorded. The 

primary purpose of this sampling effort was to monitor impingement patterns 

in order to select periods of fish abundance for impingement survival 

sampling and thus ensure meaningful sample sizes. Total impingement 

sampling was suspended on April 27, 1981 due to a unit outage and subsequent 

shutdown of the circulating water pumps. An abundance of jellyfish, present 

after the late May restart of the unit and the circulating water pumps, 

prevented all further total impingement sampling. 

Impingement survival sampling, which was the primary purpose of the study, 

was intended to determine survival rates of fish collected by the modified 

screen. Prior to the start of each test, the traveling screen was rotated 

for a period of time at the appropriate test speed and all previously 

impinged debris and fish were removed from the screen and a specially3 
constructed collection area (Figure 2). Each test began after several 

screen rotations. As fish were washed from the buckets and screens into the 

collection area, they were categorized as live, dead, or stunned and were 

transported in a water-filled bucket to holding pools. The pools were 

supplied with water on a flow-through basis from a submersible pump located 

off the southeast corner of the intake building. 

Testing was primarily concerned with determining the screen speed which 

would maximize the survival of a species. Since screen speed is directly 

related to impingement duration and, therefore, stress, it is reasonable to 

assume that higher speeds would result in lower mortality. However, it is 

important to clearly define the nature of this speed/mortality relationship, 

since higher speeds result in increased maintenance and reduced reliability 

of the screen. While faster rotation may produce higher survival, at some 

point the incremental increase in survival may be negligible. Therefore, 

the intent of this study was to determine the optimal screen speed which 

would yield adequate fish survival while not jeopardizing screen 

reliability. 

The method used to determine the optimal screen speed for each species 

involved a sequential sampling strategy in which two or three speeds were 

evaluated during each sampling period. Testing in each period lasting no 

longer than one month. During all test periods, fish were collected at 

screen speeds of 3.3 and 15 ft/min, which represented the longest and 

shortest impingement durations tested. During fall testing, an intermediate 

speed of 7.5 ft/min was tested. During winter testing, an intermediate 

4 
speed of 10 ft/min was tested. In the spring, an intermediate speed was not 

evaluated. 

As stated previously, all fish collected from the modified screen were 

identified, categorized as live, dead, or stunned, and recorded. These 

numbers were used to calculate initial survival values. All live and 

stunned fish were held in holding pools for up to 96 hr to determine latent 

survival. Moribund fish were removed from tanks at time intervals of 1, 24, 

48, 72, and 96 hours from time of sampling. All test fish were measured and 

weighed at time of death or at 96 hours if alive. The overall survival 

following impingement and removal from the modified screen system was 

calculated by combining initial and latent mortality values. 

In addition to recording mortality over time, all fish collected were 

weighed and measured and their condition recorded to determine whether 

differences in size or condition affect survival. Examination of length 

data from past studies indicated that both young-of-the-year and adults of 

the genus Alosa are abundant at Mystic. Therefore, collection of length 

data for Alosa spp. also allowed for differentiation of survival by 

lifestage. 

In the fall season, samples were collected on 9 days in October and 12 days 

in November. Winter impingement survival testing was conducted on 6 days in 

January. Also in January, a continuous 38-hour sample was collected in 

order to obtain maximum numbers of smelt and flounder. These species were 

found to be impinged in greater numbers at night. Spring sampling was 

conducted from April 13 to April 15, and again on April 23 and June 1 and 2. 

5 
Sampling was not conducted at high and low tides to account for possible 

tidal effects on mortality, as had been planned. The low and variable 

numbers of fish which occur at Mystic Station, combined with the short 

sampling durations required in the original sampling plan, would not have 

yielded sufficient numbers of test organisms to permit comparisons of tidal 

effects. In retrospect, qualitative observations of proportions of fish 

initially live, dead, or stunned at high and low tides indicate that tidal 

differences in impingement survival are probably small compared to 

differences resulting from varying screen travel speed. 

Data for comparison of screen speeds were binomial. That is, each organism 

collected at a particular screen speed was classified as live or dead. 

These data were collected in a period when other environmental variables, 

such as water temperature and salinity, did not vary to a great extent. 

Data of this type can be appropriately analyzed by contingency table or 

Chi-square analysis. These analyses test the hypothesis that the mortality 

is the same for each pair of screen speeds being compared. For details on 

the computational procedure and an example of a Chi-square analysis, see 

Steel and Torrie (1960, p. 346-351). 

RESULTS 

Results contained in this report represent sampling efforts through spring 

1981. Table 1 lists numbers of fish impinged by week for all species 

collected from October 7, 1980 through April 27, 1981. In this time period, 

a total of 103,384 fish were impinged on both traveling water screens of 

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TOTALS BY DATE 
Unit No. 7. Smelt were the most abundant species collected (45 percent), 

followed by Alosa spp. (alewives and blueback herring, 20 percent) and 

winter flounder (19 percent). It is important to note that 75 percent of 

the smelt impinged in this 29-week period were collected in 5 weeks. 

Likewise, nearly one-half of the Alosa were collected during the week of 

November 11 to November 17, and nearly 60 percent of the winter flounder 

were collected in January. 

Results of fall and winter impingement survival testing are presented in 

Tables 2 and 3. Initial mortality is the percent of the total fish that 

were dead when collected from the modified screen. Latent mortality is the 

percent of the live organisms that were placed in holding pools and then 

died within 96 hours. Results are presented by screen speed for each 

species. Since space was available in the holding facility, many species 

which were collected in low abundance and were not specifically part of the 

study were also tested. Some of these less abundant species were not 

collected at all speeds, and it was not practical to extend sampling to 

attempt to do so. Thus, results for these species are qualitative only. 

Results for Alosa in fall testing were partitioned into two length groups 

when a plot of number tested versus length revealed a distinct bimodal 

pattern separated at 10.5 cm (see Fig. 3). Data from Scott and Crossman 

(1953, pg. 103) indicate the fish in the first mode are in their first year 

of life. In Table 2, these first year Alosa are listed as "Alosa (alewife 

and blueback) small." Small Alosa were captured in abundance only at screen 

speeds of 15 and 7.5 ft/min, despite numerous samples collected at 

3.3 ft/min. These small Alosa survived impingement well, especially at the 

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15 ft/min screen speed. Results of tests at 15 ft/min show nearly 

50 percent survival (initial and latent combined) 96 hours after 

impingement. In general, impingement survival increased with screen travel 

speed. The large Alosa showed high initial survival, but latent survival 

was low at all screen speeds. 

Results of fall testing with winter flounder are also separated by size 

since a plot of number tested versus length revealed a bimodal pattern 

separated at 11.5 cm (see Fig. 4). Data from Scott and Crossman (1953, 

pg. 281) indicate the smaller fish are in their first year of life. In 

Table 2, these first year winter flounder are listed as "winter flounder 

small." From Table 2, we see that over 50 percent of both large and small 

flounder survived impingement even at the 3.3 ft/min screen travel speed. 

Results for fall testing with smelt are only marginally quantitative. Smelt 

were not greatly abundant in the fall period and were impinged less 

frequently during the day (when impingement survival testing was conducted) 

than during night hours. Results indicate about 3 to 4 percent of the smelt 

impinged at this season survive. 

Results for other species are qualitative since test organisms were few in 

number. However, in general, it appears mummichogs and cunner have good 

survival potential in the fall, while butterfish, mackerel, pollock, and 

white perch appear to have a poorer survival potential. 

Results of winter testing are presented in Table 3. During this period, 

only 16 Alosa were captured due to their low abundance at this time. Data 

8 
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indicate Alosa survivorship was no better than that for large Alosa 

collected during fall testing. 

Flounder tested during winter showed very high survival at all screen travel 

speeds, as shown in Table 3. 

Results for smelt are separated by size since a plot of number tested versus 

length revealed a bimodal pattern separated at 14 cm (see Fig. 5). Data 

from Scott and Crossman (1953, pg. 138) and Liem and Scott (1966, pg. 121) 

indicate that smaller smelt comprising the 9 cm to 14 cm mode were too large 

to be first year fish. Therefore, it seems more probable that the two modes 

represented, respectively, age II fish and age III fish. Initial survival 

for large and small smelt was high for all speeds tested. Latent survival 

was significantly lower at the 3.3 ft/min speed than at the 10 or 15 ft/min 

screen speed for both the large and small smelt. The 15.0 ft/min screen 

speed yielded the greatest survival for both the large and small smelt, 

however, it was not significantly different than the 10 ft/min speed. 

Interestingly, the smaller size smelt had greater survival than the larger 

test specimens at a given speed. 

Results for other species tested in the winter are more qualitative. 

Sculpin survival was very high as was that of Atlantic tomcod and threespined stickleback. Pollock of all sizes appear to have better survival in 

the winter season at a screen speed of 15 ft/min than in fall. The few 

white perch collected all died. 

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very low abundance of fish at times when samples could be taken. Tests 

conducted in the April 13 to April 15 period were terminated prior to the 

normal 96 hour holding period due to operational problems with the fish 

holding facility water supply. These shorter term (24 hour) holding results 

indicated good survival of flounder and poor survival of smelt (Table 4). 

Later impingement survival testing resulted in a limited collection of test 

specimens, especially in June. Fish impingement on the Unit 7 screens was 

extremely low at this time. Latent survival results show good survival of 

sticklebacks, flounder, pipefish, and eels and poor survival of smelt 

(Table 5). 

SUMMARY OF TEST RESULTS 

Previous studies of winter flounder survival after impingement on the 

unmodified Unit No. 7 traveling water screens showed 85 percent survival 

with continuously rotated screens and 55 percent survival with 2-hour screen 

wash intervals. The data from the present study show an improved survival 

of winter flounder impinged on the modified screen, and a virtual 

elimination of mortality when the screens are continuously rotated at all 

speeds during the period of peak abundance in the winter. 

Survival of Alosaand smelt was lower than that of winter flounder. These 

species are difficult to maintain under laboratory conditions and overall 

survival would probably exceed observed values if these fish were to be 

returned directly to the river. In addition, differences in survival due to 

screen speed for Alosaand smelt are not as dramatic as those for winter 

10 
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flounder. Regardless, small Alosaand smelt of all sizes displayed moderate 

survival at screen speeds of 15.0 ft/min in fall and winter. 

Less abundant species such as tomcod, stickleback, sculpin, mackerel, 

mummichogs, and cunner appear to have good survival potential at any screen 

speed. 

An interesting and very positive general conclusion is that species show 

high survival when they are abundant. 

DISCUSSION 

It would appear from the results of this study that a modified, traveling 

screen system offers an effective means of alleviating fish mortality at 

Mystic Station - Unit 7. Should such a system be incorporated as a 

permanent solution to impingement losses, the engineering implications of 

continuous operation during periods of fish impingement should be 

considered: 

1) Continuous screen operation reduces the life of screen components, 

2) Increasing speed of screen rotation under a continuous operating 

mode increases the rate of wear of screen components. 

Since these factors have a major impact on maintenance requirements and 

operating costs and can jeopardize unit reliability, every attempt should be 

made to minimize the duration and speed of continuous screen operation. 

11 
Based on the seasonal patterns of impingement noted in this study (refer to 

Table 1), as well as those observed in past impingement monitoring studies 

at Mystic Station, it appears that a major portion of the fish impinged 

could be protected by operating modified screens at Unit 7 on a continuous 

basis for only several months each year. While impingement sampling was not 

conducted from April 27 to June 1, it was evident from observations at Unit 

6 during this period that numbers of fish impinged remained at the low level 

observed for the prior ten weeks. Historically, the number of fish impinged 

at Mystic Station from June through September is very low, while jelly fish 

(Aurelia aurita) are in abundance. Therefore, it is believed that the 

impingement numbers presented in Table 1 represent the majority of the fish 

impinged at Unit 7 on a yearly basis. Using these numbers, it is clear that 

two peaks in impingement of the four species of concern occurred. The 

periods of high abundance were short in duration, while the remainder of the 

year numbers were low. As shown in Table 6, at least 80 percent of each 

species occurred during the two seven week periods of October 21 to December 

8 and December 30 to February 16. Therefore, if modified screens had been 

run continuously during fourteen weeks of the year, protection would have 

been afforded to a large percentage of the fish collected during the entire 

year. 

While the periods of peak fish abundance at Unit 7 vary in time from year to 

year, impingement data gathered over a number of years show that definite 

peaks occur each year. Therefore, should modified screens be utilized at 

Unit 7, it is recommended that they only be run continuously during periods 

of peak abundance. Such an operational mode would provide a high degree of 

fish protection in a cost-effective manner while limiting the potential for 

12 
TABLE 6 

Percent of Total No. 

Collected During 

Species Period of Peak Abundance the Two Peak Periods 

Rainbow smelt December 30 - February 16 90 

Alosa spp. October 21 - December 8 93 

Winter flounder December 30 - February 16 80 

TABLE 7 

Mean Percent Survival During Period 

of Peak Abundance by Screen Speed 

Species Size Low Medium High 

Rainbow smelt Large 11.0 31.3 40.0 

Small 22.5 58.3 66.7 

Alosa spp. Large 0.8 0 0.5 

Small 6.7 23.4 48.1 

Winter flounder All 96.8 100 98.6 
problems associated with operation, maintenance, and general reliability. 

Since the peak periods of fish abundance at Mystic Station are 

distinguishable from periods of lower abundance, the decision to operate 

continuously could be made on the basis of periodic observations during 

intermittent operation. 

A final consideration in employing modified screens at Unit 7 would be the 

speed at which to rotate the screen during times of continuous operation. 

As shown on Table 7, higher survival was obtained at the highest speed than 

at the lower speeds. It would appear that the lowest speed would achieve a 

high level of survival with flounder only. On the other hand, the 

intermediate and high speeds both resulted in good survival for most species 

tested. The final decision to run at a specific speed must take into 

consideration engineering reliability and cost considerations, as well as 

survival. 

The engineering implications of operating at higher speeds involve greater 

wear of the rotating parts of the screens: the rollers, bushings, and pins 

of the chains; the foot shaft and headshaft bearings and bushings; and the 

tooth-inserts on the sprockets. Wear of these components increases 

exponentially with screen speed. Wear of the rotating components results in 

slack in the chains and the interference of the baskets with the bottom boot 

plate where the baskets rotate around the foot shaft and sprockets. This 

can result in breaking away of the basket, jamming of the traveling screens, 

breakdown of the tooth-inserts, and breaking of the chains (Mussalli et al. 

1977). 

13 
Wear of the screens can lead, therefore, to increased maintenance costs and 

jeopardize unit reliability. The estimated incremental maintenance cost for 

high speed versus medium speed operation of two modified screens at Mystic 

Station - Unit 7 would be $5,000.00 annually based on 14 weeks of continuous 

operation. This cost includes materials and labor to replace the footshaft 

and headshaft bushings and bearings and the chains approximately every five 

years. Potentially more important than routine maintenance costs would be 

the major costs associated with a unit outage resulting from equipment 

failure due to high screen speed operation. 

The specific drawbacks of operating a modified screening system at Mystic 

Station - Unit 7 at a high travel speed rather than medium speed can be 

summarized as follows: 

1. Wear of screen components would be four times greater at the 

higher speed, 

2. Annual maintenance costs would be higher, 

3. The reliability of Unit 7 would be placed in greater jeopardy, and 

additional costs could be incurred due to unit outage as a result 

of equipment failure, 

4. Flounder, the most valuable commercial species, are afforded equal 

protection at all operating speeds. 

14 
For these reasons, it is recommended that modified screens be operated at 

intermediate travel speeds should they be required at Mystic Station - 

Unit 7. Considering the relatively low commercial value of Alosaand smelt, 

the costs and potentially high reliability risks of high-speed operation 

would not be warranted. 

During periods when flounder are the only abundant species even intermittent 

(Stone & Webster 1978) screen operation would afford an adequate level of 

protection. 

LITERATURE CITED 

Steel and Torrie. 1960. Principles and Procedures of Statistics. 

McGraw-Hill, New York. 481 pages. 

Stone & Webster Engineering Corporation . 1978. Fish Impingement 

Monitoring, Mystic Station - Unit No. 7. July 1978. 54 pages. 

Mussalli, Y. G., Taft, E. P. III, and Hofmann, P. 1977. Engineering 

Implications of New Fish Screening Concepts. Proceeding of the Fourth 

National Workshop on Entrainment and Impingement, Chicago, Illinois, 

December 5-7, 1977, pp. 367-376. 

15