Traveling screens as sampling gear for vertical distribution studies

Coastal and Estuarine Research Federation

Traveling Screens as Sampling Gear for Vertical Distribution StudiesAuthor(s): Roger A. Rulifson and B. J. CopelandSource: Estuaries, Vol. 5, No. 2 (Jun., 1982), pp. 82-94Published by: Coastal and Estuarine Research FederationStable URL: http://www.jstor.org/stable/1352105 .

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Estuaries Vol. 5, No. 2, p. 82-94 June 1982

Traveling Screens as Sampling Gear for

Vertical Distribution Studies

ROGER A. RULIFSON AND B. J. COPELAND

Department of Zoology North Carolina State University Raleigh, North Carolina 27650

ABSTRACT: A sampling technique has been developed for increasing the information gathered during routine monitoring of impingement at water intake structures. Samples from impingement catches on traveling screens were taken from the sluiceway of the Brunswick Steam Electric Plant near Southport, North Carolina during the screen wash process so as to divide the catch into vertical catch components. Each component represented 1.2 m of the water column. Results showed differences in day and night vertical distributions of impinged organisms according to the spatial preferences of each species. Impingement during daytime was lower than during night. Impingement of surface- oriented species during daytime occurred at mid-depth, midwater species near the bottom, and bottom species were seldom impinged. During the night surface-oriented species were impinged at the surface, midwater species at mid-depth, and bottom species near the bottom. Residue (animals and debris) which remained within the screen wash system from collection of the previous sample, and those organisms which became impinged on the screens during retrieval of the sample, were used to calculate the rate of "continuous impingement" on the traveling screens. These rates were compared to the impingement catch in order to determine biases in the vertical catch components. Comparison of these rates indicated that impingement estimates determined by routine monitoring methods may under- estimate true impingement rates for certain species. We have concluded that this sampling technique for monitoring impingement at intake structures will increase knowledge of the local biologic system while minimizing the cost of obtaining the information. The technique will also aid in pinpointing specific impingement problems which may be corrected through modifications to the design of the intake structure.

Introduction Power plants designed with once-through

cooling systems require large volumes of water in order to operate efficiently. Pe- riodic monitoring of entrainment and impingement at water intake sites is used to identify and enumerate those organisms removed from the environment by plant operation. The impact of the power plant on the aquatic environment is estimated by comparing these data to data gathered through extensive monitoring efforts within the surrounding areas.

The sampling methods, as well as the extent of monitoring efforts, should be chosen to best reflect the rhythmic patterns of activity displayed by local populations of organisms residing near the plant site. The initial course of action may involve extensive monitoring efforts, combined with var-

ious sampling techniques, to determine the most efficient procedure for gathering the necessary biologic information in order to reduce the extent and cost of the sampling program. At present there is no standard operations procedure for monitoring impingement at water intake structures. Cri- teria for monitoring procedures are developed for each power plant based on the extent of the impingement problem and the unique geographic and biological characteristics of the site area. Monitoring procedures range from minimum efforts designed for catch extrapolation to daily counts of all impinged organisms.

This study was proposed to develop a sampling technique for increasing the information gathered during routine monitoring of impingement at water intake structures. We believe that this technique will: (1) in-

? 1982 Estuarine Research Federation 82 0160-8347/82/020082-13$01.50/0



Traveling Screens as Sampling Gear 83

Fig. 1. Cape Fear estuary showing the location of the Brunswick Steam Electric Plant (BSEP).

crease knowledge of the local biologic system while minimizing the cost of obtaining the information; and (2) pinpoint specific impingement problems which perhaps can be corrected through modifications to the design of the intake structure.

Methods SITE DESCRIPTrION

The Brunswick Steam Electric Plant (BSEP) is a nuclear-fueled, electric generating plant located on the Cape Fear River estuary in the southeastern portion of North Carolina approximately 11 km north of the confluence of the Cape Fear River and At- lantic Ocean (Fig. 1). The plant, owned and operated by Carolina Power and Light Com- pany (CP&L), consists of two units, each with a net generating capacity of 821 megawatts (MWe).

Water for the once-through cooling system is withdrawn from the estuary through a dredged canal 5.5 km long which extends from the plant site through Snows Marsh to the Cape Fear River ship channel. At mean sea level (MSL), the canal has a width of 94.5 m at the surface, 51.2 m at the bottom, and an average depth of 5.5 m. Water is discharged into the near-shore ocean off Oak Island through a canal 8.8 km long. The cooling system was designed to withdraw 68 m3 per s of water from the estuary for condenser cooling (Carolina Power and Light Company 1980).

WATER INTAKE STRUCTURE

The circulating water intake structure comprises eight separate intake bays (four bays for each unit) each approximately 4.3-m wide (Fig. 2). The bottom of each bay is -6.1 m MSL and the operating deck is +6.1 m MSL. The two sets of intake bays are separated by a 3.65-m wide pipe cham-

FRONT VIEW SIDE VIEW

Sample collection area Intake bay Cowling for from sluiceway IC7 Traveling screens

- J I -

11 I J I I I [ I I I I -

Pipe 2D 2C 2B 2A chamber 1D _ 1,__ 1B 1A

-uM-

-LM- --B-

Trash

-MSL-

Fig. 2. Front and side view representations of the BSEP water intake structure showing relative positions of the intake bays and locations of the vertical components of the impingement catch from intake bay 1C. (S = surface; UM = upper midwater; M = midwater; LM = lower midwater; B = bottom).



84 R. A. Rulifson and B. J. Copeland

ber, making the overall dimension for the intake structure 53.3-m wide and 13.4-m high (Carolina Power and Light Company 1979).

Coarse bar racks ("trash racks") have been placed in front of the intake bays to prevent large pieces of trash and debris from entering the bays. The accumulated debris is removed manually. The trash racks are constructed in two sections with vertical slit openings 5.1-m wide. Growth of filter-feed- ing marine organisms must be periodically removed by divers to minimize the obstruc- tion of water flow through the racks.

Vertical traveling screens, located in each intake bay behind the trash racks, prevent aquatic organisms and debris from entering the plant and clogging the condenser tubes (Fig. 2). These screens are constructed as a continuous belt of 43 0.6-m high panels of 9.5-mm wire mesh which are designed to rotate automatically when there is an increase in pressure drop across the screen or when manually initiated by the screen operator. The screens travel upward to the operating deck where they are washed with water sprayed from jets located behind the screens. Water, debris and organisms fall into a sluiceway and flow into a spiral chute which empties into a floating basket located in the intake canal. The basket is lifted out of the water by a hoist and can be emptied either into a collection basket for exami- nation or into the holding tank aboard the nekton return boat (MS SLUICE) for trans- port back to the Cape Fear River.

VERTICAL DISTRIBUTION OF IMPINGED ORGANISMS

An impingement study was conducted October 7-8, 1978 to determine the vertical distribution of impinged organisms over a 27-h period. Samples were taken from the traveling screen of intake bay 1 C every two hours by inserting a specially constructed dip net (5-mm round mesh) into the sluiceway just prior to starting the screen wash process. On command, the screen operator initiated a slow-speed rotation of the traveling screen (2.54 cm per s). As each screen panel was washed, organisms and debris fell into the sluiceway which were then captured by dip net. Lag time between wash and cap- ture was approximately seven seconds.

Due to the change in tide height within the intake bay, either 9, 10 or 11 of the 43 available screen panels were used to determine the vertical distribution of impinged organisms (Fig. 2). The sampling time for these screen panels comprised three parts: (1) positioning of the panels prior to the start of the vertical distribution sample, (2) actual vertical distribution sample, and (3) retrieval of the sample. Since all panels had to travel vertically from the bottom of the intake bay through the surface water at the same rate, the transit time (positioning + retrieval) for all panels was the same (Fig. 3).

Residue (animals and debris) which remained within the wash system from collection of the previous sample was removed from the sluiceway by dip net during retrieval of the vertical distribution sample. As the panels containing the sample were washed, the sample was collected at two-panel intervals by simultaneously removing one dip net from the sluiceway while inserting another. This technique divided the impingement catch into vertical catch components with each component representing 1.2 m of the water column.

The screen panels following those containing the vertical distribution sample were impinging animals and debris as they trav- eled upward through the water column. The catch from these panels was collected by the same method described for the vertical distribution sample until the screen wash system was turned off by the operator. This catch was then used to calculate the rate of "continuous impingement" on the traveling screens.

Organisms and Spartina debris from each subsample were preserved in 10% formalin and then taken back to the laboratory for analysis. Each organism was identified and weighed to the nearest 0.05 g; standard length (SL) and total length (TL) were mea- sured to the nearest mm. Spartina debris was squeezed and blotted to remove excess water and then weighed to the nearest 0.05 g. Coelenterates and trash were discarded and not used in the analysis. Blue crabs (Callinectes sapidus) were very agile and were observed to continually drop from the traveling screen and become reimpinged as the panels rotated upward. Therefore, blue




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11 12 13 14 15 16 17 18 19 20 21 22 PANEL NUMBER

Fig. 3. Time required for screen panels to travel from the bottom of the intake bay upward through the water column and surface water, showing the time used for placement of panels prior to starting a vertical distribution sample and time for retrieval of the sample. In this example, panel 12 is the surface panel and panel 20 is (always) the bottom panel.

crabs greater than 90-mm carapace width (CW) were not used in the analysis. The spatial preference of each species, oriented toward the bottom, midwater, surface or other (pilings, reefs, floating debris), was determined by descriptions of species distributions and ecologies summarized by U.S. Fish and Wildlife Service (1978).

HYDROGRAPHY NEAR THE WATER INTAKE STRUCTURE

In order to interpret the vertical distribution of impinged organisms, it was necessary to determine the flow patterns and physical characteristics of the water being withdrawn from the intake canal. In conjunction with the vertical distribution study, salinity (%c) and temperature (C) were taken every hour at 0.5-m intervals to the bottom directly in front of intake bay 1C trash rack.

During March 4-5, 1975, a velocity profile study was conducted by members of the Environmental Assessment Unit of CP&L to determine the patterns of water flow through the intake bays. With all four Unit

2 circulating pumps in operation, five evenly-spaced General Oceanics flowmeters (model 2030) attached to a metal frame were positioned in front of the traveling screen within intake bay 2A. Measurements were taken at 0.3, 1.5, 3.0, 4.6 m and bottom once per hour over a complete tidal cycle (25 h). The flowmeters were left at each depth for five minutes. At the end of the study the flowmeters were again lowered into position and then immediately raised to determine the number of counts accumulated in raising and lowering the frame. Concurrent velocity measurements at similar depths were taken directly in front of the Unit 2A trash rack with an Endeco flow- meter (type 110). Tide height data was taken every 0.5 h by a tide gauge (Fischer and Porter Co., model 35-1542C-DA12A) mounted on pilings in the Snows Marsh area.

SCREEN SELECTIVITY

To predict the effects of impingement on populations of organisms, it is important to




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temperature salinity temperature sal inity

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1630 1830 2030 TIME

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Fig. 4. Surface and bottom salinities and temperatures of the water column directly in front of trash rack 1C during October 7-8, 1978 showing the extent of vertical mixing of the water column.

determine when the young grow from entrainable to impingeable sizes. This transition is a function of the overall dimensions of the organism in relation to the mesh size of the traveling screen. The effect of screen selectivity on local populations of brown shrimp (P. aztecus) was estimated by measuring the total body length (TBL, tip of rostrum to tip of uropods) and the body depth (distance from the ventral to dorsal surface on the carapace, not including the walking legs) on specimens collected from BSEP entrainment and impingement, and by trawl (1.9-cm bar mesh) from nearby Dutchman Creek. The size range at which the entrainment-impingement transition oc- curs for brown shrimp was then determined by comparing these measurements to the dimensions of the traveling screen mesh.

Results HYDROGRAPHY

During the October, 1978 study, the range of water temperatures (20.18-23.30 ?C) and salinities (29.32-32.55%o) that occurred over the 27-h period indicated the Cape Fear River was in a condition of low flow which

allowed the warmer, more saline ocean water to pervade the estuarine system. Data showed a consistent, uniform increase in salinity with depth. Temperature fluctua- tions were slight and inconsistent with depth. A general trend of decreased salinities and temperatures throughout the study was indicative of increased freshwater input to the system.

The extent of mixing in the water column directly in front of the intake structure fluc- tuated throughout the study (Fig. 4). Ver- tical mixing of the water column was assumed to be greatest when differences between surface and bottom temperatures and salinities were smallest. Mixing was assumed to be less intense when differences in the values were greatest.

The velocity profile study (CP&L) during March, 1975 showed that water was withdrawn by the plant from all levels of the water column throughout a complete tidal cycle (Fig. 5). Water velocities in front of trash rack 2A were generally fastest near the surface and decreased with depth, ranging from 0.58 m per s at the surface to 0.06 m per s at the bottom. Fastest velocities

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Fig. 5. Water velocities which occurred throughout a complete tidal cycle (24 h) directly in front of trash rack 2A during March 4-5, 1975 (CP&L data).

were recorded at low tides and decreased with increased tide height (Fig. 6). A similar trend occurred within the intake bay behind the trash rack as water flow was fastest at low tides and decreased with increased tide height. Water flow was consistently fastest on the left (north) side within the bay (Fig. 7) with occasional "hot spots" appearing at random near mid-depth. Most water velocities observed within the bay were 0.3 to 0.6 m per s.

The Unit 2A pumping rate was estimated as a function of the observed average water velocity behind the trash rack and the width and water depth of the intake bay. Water was withdrawn from the canal at a rate of 10.63 m3 per s (+0.4387 SE) with flow rates ranging from 0.6-11.2 m3 per s, a 10% fluc- tuation from the mean. The total withdrawal rate by Unit 2 was estimated by multiplying the 2A rate by the total number of pumps in operation (four). The mean total with-

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I I I I I I I I I I I I I ' I ,I I I' 1 1430 1530 1730 1930 2130 2330 0130 0330 0530 0730 0930 1130 1330 1530

Fig. 6. The relationship of tide height in the intake canal to the water velocities in front and behind trash rack 2A and to the estimated pumping rate for Unit 2 during March 4-5, 1975 (CP&L data).

drawal rate was 42.53 m3 s (+1.755 SE). Most of these values were within one standard deviation from the mean (Fig. 6). The calculated mean withdrawal rate was within the capacity of the cooling system during 1975 when the study was completed.

SCREEN SELECTIVITY

The theoretical minimum impingeable size for an organism should be equal to the smallest dimension of the screen mesh, in this case equal to a body depth of 9.5 mm, assuming the screen mesh to be clean and free of debris. The theoretical maximum entrainable size should be no greater than a body depth of 13.5 mm, the diagonal dimension of the screen mesh. On the basis of the length-depth ratio, the smallest brown shrimp (P. aztecus) that theoretically could be impinged by orienting head-first with the body aligned to the square measure of the screen mesh should be 70 mm total body

length (TBL). The largest brown shrimp which theoretically could be entrained head- first with its body aligned to the diagonal measure of the screen mesh would be 93 mm TBL (Fig. 8). The measurement of body depth did not include legs, fins, "shoulder width," or other body features. These features, in combination with approach to the intake screen (other than a head-first position) and partial clogging of the screens, will significantly reduce the size range of the entrainment-impingement transition. The actual impingeable size (at BSEP) for shrimp appears to be about 40 mm TL (Fig. 9) or approximately 49 mm TBL.

IMPINGEMENT

The traveling screen of Unit 1C impinged 836 organisms, representing 35 taxa, over the 27-h period during October 7-8, 1978. Total impingement for BSEP was estimated by multiplying this catch by the number of

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1 .0

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/ \ pumps in operation (six), resulting in an es- /J, }} \^timated catch of 5,016 organisms weighing f / [ 31.9 kg. Total weight of Spartina debris im-

pinged was estimated as 22.2 kg for a total catch weight of 54.1 kg. Two days after com-

\ pletion of our study, CP&L fishery biolo- gists examined the impingement catch of October 9-10 over a 24-h period by subsam- pling portions of the catch and expanding the data by methods described in Carolina Power and Light Company 1979. Total impingement was estimated as 5,579 organisms representing 41 taxa, results similar to our study. Weight of the organisms and

\ trash was 80.6 kg. ol u ( ] ) J Impingement of Spartina debris over the

27-h period was tidally influenced while organism impingement was influenced to a

3\ ' 0 4. f greater extent by ambient light conditions

0 4.0 ?

(Fig. 10). Most animals were impinged during night (91.5%) with the largest catches

n) asinte bolateA occurring on ebbing tides just after sunset as Geological Sur- and several hours before sunrise. into the figure. Vertical distribution of impinged organ-

PENAEUS AZTECUS

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max. entrainable size

Mesh size of traveling screen (mm)

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I I I I I I I I I I I I I I I I" 20 40 60 80 100 120 140

TOTAL BODY LENGTH (mm)

Fig. 8. Theoretical entrainment-impingement transition for brown shrimp (Penaeus aztecus) based on the relationship of body size to the dimensions of the screen mesh.

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Fig. 9. Minimum and maximum lengths (TL in mm) of brown shrimp (Penaeus aztecus) impinged at BSEP from January 1974 through December 1976. Values given are the smallest and largest for the date indicated.

isms reflected responses to ambient light levels characteristic of the spatial preference of each species. Impingement was in- frequent during the day. Surface-oriented species were impinged toward the bottom (Fig. 11). Bottom-oriented species were seldom impinged during daytime, and animals associated with pilings and floating debris were sparsely impinged throughout the water column. Impingement was greatly increased during nighttime. Most surface- oriented animals were impinged near the surface while midwater-associated and "other'-associated organisms were impinged throughout the water column (Fig. 11). Bottom-oriented species were impinged in greatest numbers toward the bottom.

Rates of impingement were calculated for each two-hour sample by dividing the number of organisms caught by the sample time. The average rate of impingement during the day (0.28 organisms per min) was significantly lower (a = .05) than the rate during the night (1.13 organisms per min) using Duncan's multiple range test. Impingement rates were also calculated for the residue

retained within the screen wash system, vertical distribution samples, and the catch representing "continuous impingement." There were no significant differences (a = 0.5) among the three areas for daytime samples. The three impingement rates calculated for nighttime samples were significantly different (a = .05) from each other. The highest mean rate of impingement was calculated for the continuous impingement catch (1.22 organisms per min). The residue impingement rate was 0.77 organisms per min, and the lowest impingement rate was calculated for vertical distribution samples (0.30 organisms per min).

Discussion HYDROGRAPHY

The hydrography of the system must be considered when interpreting the vertical distribution of impinged organisms on traveling screens. Both the horizontal and vertical components of water flow within the intake canal are not uniform (Carolina Pow- er and Light Company, unpublished data), but probably do not directly affect the ver-

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Fig. 10. Number of organisms and weight of Spar- tina debris impinged on traveling screen IC over a 27-h period during October 7-8, 1978.

tical distribution of organisms at the plant. The intensity of stratification and the extent of vertical mixing of the water column may be more important in determining the distribution of organisms near the intake structure.

The extent to which the water column is stratified may restrict the diel vertical migration of some organisms within the canal, an effect which may be exhibited in impingement catches taken by the method described in this study. Well-defined halo- clines and thermoclines developed within

the canal during the summer of 1978 (Ru- lifson 1980). However, the slight differences between the surface and bottom salinities and temperatures which occurred during this study should not be expected to limit the diel vertical migration of organisms.

The intensity of vertical mixing of the water column may affect the tendency of organisms to become impinged on the screens at a depth different from where they were originally swimming. Salinity and temperature data taken at two stations during the summer of 1978 (Rulifson 1980) indicated that the tendency for deep vertical mixing was weak away from the plant but strong near the intake structure, affected by tidal amplitude, tidal seiche (end of the tidal basin) and rate of water withdrawal from the canal by the plant. Salinity and temperature data taken in front of the intake structure during this study indicated deep vertical mixing of the water column near the power plant through the first ebb tide and a portion of the following flood tide (Fig. 4). Presum- ably, deep vertical mixing near the plant ceased as the tide height increased, causing the water from the canal to remain stratified as it was withdrawn by the plant. Some vertical mixing occurred on the second ebb tide and for the remainder of the study, but the surface and bottom waters still retained their salinity-temperature identities. The amplitude of the second high tide in this study was smaller than the first and could be a partial explanation for the lack of deep vertical mixing near the plant for the remainder of the study.

Once organisms passed through the trash racks and began approaching the traveling screens, several factors may have affected the level at which they became impinged. Water flow data collected by CP&L during March, 1975 showed turbulent but fairly uniform water velocities with depth (Fig. 7), indicating that the trash racks may serve as baffles to distribute the flow more evenly throughout the bay. However, flow data collected within intake bay 1C during March, 1979 (Schneider 1980) indicated that water flow was consistently faster near the surface and became slower with depth. This flow pattern is similar to those recorded for the 1975 study directly in front of trash rack 2A (Fig. 5). Schneider's data also showed

I




NIGHT Spatial Preference:

RESIDUE

SURFACE

UPPER MIDWATER

MIDWATER

LOWER MIDWATER

BOTTOM

OVERFLOW

"CONTINUOUS

IMPINGEMENT"

Bottom - oriented

Surface -oriented

Midwater - oriented

Other-pilings, etc.

' I

I-r-- I 1 I I I 0 10 20 0 25 50 75

NUMBER OF ORGANISMS IMPINGED

i I 100 125

Fig. 11. Day and night vertical distributions of impinged organisms on traveling screen 1C over a 27-h period during October 7-8, 1978.

a deflection in the water flow within the intake bay caused by the juncture of the trash rack sections at mid-depth. Thus, water entering the bay at mid-depth was deflected toward the surface and bottom and was shown to affect the vertical distribution of entrainable organisms as they entered the plant. During the present study, reduced impingement at mid-depth occurred from 0130-0508 hours on the last high tide (Fig. 10) which could have been caused by water deflection within the intake bay. Buildup of large pieces of trash and growth of organisms on the trash rack may have contributed to flow deflection within the bay.

RETENTION OF THE CATCH BY THE SCREENS

The greatest source of variability con- cerned the capability of the screens to retain the catch until washing of the screens was accomplished. During each two-hour sampling period some organisms may have been able to escape impingement on the traveling screens for indefinite periods before complete exhaustion caused final impingement. This has been demonstrated for some fish species (Rulifson 1975), Other organisms may have been able to resist impingement by assuming a position in front of impinged trash or animals. This condition was prob-

ably the contributing factor for the large portion of the catch designated as "overflow" (Fig. 11) which was caught at the bottom by clean panels that rotated into position as the vertical distribution sample was retrieved. This event may also have taken place for panels following these which were also presumably measuring the rate of "continuous impingement." Once the traveling screens began to travel upward and out of the water, certain species may have been able to dislodge themselves from the screens and fall back into the surface water where they became reimpinged at a site different from the original impingement site.

To examine this source of variability more closely, the impingement rates of two taxa were calculated for the residue, vertical distribution samples, and continuous impingement. Two anchovy species, Anchoa mitch- elli and A. hepsetus, comprised 25% of the catch over the 27-h period. They are school- ing species (U.S. Fish and Wildlife Service 1978) and have low impingement survival rates (14%) at BSEP after being removed from the traveling screens (Copeland et al. 1974). Pink shrimp, Penaeus duorarum, comprised 11% of the 27-h catch. It is a nocturnal species which remains in burrows during daytime and emerges at night to feed (Fuss 1964; Wickham 1967). The impinge-

DAY




ment survival rate is high (88%) after being removed from the traveling screens (Cope- land et al. 1974).

The average rates of impingement for pink shrimp (nighttime samples only) calculated for residue (0.12 shrimp per min) and for vertical distribution samples (0.06 shrimp per min) were not significantly different (a = .05, Duncan's multiple range test). The impingement rate calculated for continuous impingement (0.26 shrimp per min) was not significantly different (a = .05) from the residue rate. However, the impingement rate for vertical distribution samples was significantly lower (a = .05) than the rate calculated for continuous impingement catches. Because the residue remained exposed above the water surface for approximately two hours during time of sampling, and since shrimp have a high impingement survival rate, it seems reasonable to assume that a portion of the shrimp present in the residue flipped from the screen and became reimpinged on those panels which were sampling for vertical distribution. Therefore, these two predicted rates of impingement became similar. The difference between these two rates and the rate predicted by continuous impingement can be attributed to the large portion of shrimp caught in the subsample designated as "overflow" in (Fig. 11). These shrimp probably avoided impingement by positioning themselves in areas of reduced water flow, and then became impinged on panels representing continuous impingement during retrieval of the vertical distribution sample.

Impingement rates calculated for anchovies were different from those for pink shrimp. Anchovies were impinged during daytime as well as nighttime samples. The average impingement rate during daytime (0.09 anchovies per min), however, was significantly lower (a = .05) than the nighttime impingement rate (0.27 anchovies per min). The daytime impingement rates calculated for residue (0.139), vertical distribution samples (0.007), and continuous impingement catches (0.117) were not significantly different (a = .05). Since anchovies have a low impingement survival rate their ability to remove themselves from their original impingement sites was poor. Therefore, no exchange of the anchovy catch should have

occurred among the residue, vertical distribution, and continuous impingement areas. All three areas should then have similar rates of impingement, as was the case for the daytime catches shown above. How- ever, nighttime anchovy impingement rates for residue and continuous impingement were significantly higher (a = .05) than the nighttime rate calculated for vertical distribution samples. The highest rate was calculated for continuous impingement catches (0.43 anchovies per min), followed by the residue (0.33) and the vertical distribution samples (0.06). It is interesting to note that blue crabs exhibited greatest activity at night when they were impinged in great numbers, and few were impinged during the day. Many night samples contained anchovy parts such as heads, tails and stripped bones. Therefore, it can be assumed that vertical distribution samples collected at night were being cropped by impinged populations of blue crabs.

EVALUATION OF IMPINGEMENT DATA

The several problems encountered in interpreting these impingement data did not obscure the vertical trends of impingement. These trends remained consistent throughout the summer and fall of 1978 (Rulifson 1980). Retention of the catch by the screens could be improved by simple design modifications to the traveling screens. The impingement problem at BSEP appears to be occurring mostly at night with organisms being impinged fairly uniformly from the upper midwater to bottom portions of the water column. Since the water source for the cooling system is a shallow intake canal, design modifications such as skimmer walls or louvers would probably have little effect on the rate of impingement unless substan- tial reworking of the intake canal design was performed.

The impingement problem at BSEP is not in the design of the intake structure itself but rather concerns the number of organisms within the canal as a source of impingement. A reduction in the source of organisms would reduce the number of organisms impinged per unit volume of water filtered, thus reducing overall impact to the estuarine system.

During fall 1978, CP&L engineers de-




signed and constructed a screen diversion device at the entrance to the intake canal. Although there have been several problems in keeping the device operational, it has reduced the number of organisms impinged at the intake structure (W. T. Hogarth, per- sonal communication).

Conclusions 1) Traveling screens at water intake struc-

tures can be used as sampling gear to determine the vertical distribution of impinged organisms.

2) The sampling technique described by our study reduces the proportion of the impingement catch needed to estimate impingement impact while increasing the amount of information gathered about the biologic system.

3) Biases in estimating impingement impact can be calculated and corrected for each species.

4) This sampling technique would be most effective if used in conjunction with present monitoring efforts.

ACKNOWLEDGMENTS

We extend our sincere appreciation to the following people without whose help this study could not have been completed: Dr. R. Monroe, J. Hackman, data analysis; M. Shepherd, J. Geaghan, computer; Dr. R. Hodson, Dr. C. Knowles, R. Laney, J. Schneider, presentation; Dr. W. Hogarth, J. Donahey, W. Yontz, K. McPherson, CP&L data; H. Parrish, A. Bence, W. Fallon, D. Grande, D. Sonnenberg, B. Wester, data collection; B. Blackmon, R. Inman, M. Ward, screen operators. The work was supported through a contract "Effect of Power Plant Construction and Operation on

the Lower Cape Fear River and Ocean off Oak Island," B. J. Copeland, Project Leader.

LITERATURE CITED

CAROLINA POWER AND LIGHT COMPANY. 1979. Im- pingement studies at the Brunswick Steam Electric Plant, Southport, North Carolina, 1974-1978. July 1979.

CAROLINA POWER AND LIGHT COMPANY. 1980. Brunswick Steam Electric Plant Cape Fear Studies, Interpretive Report. January 1980.

COPELAND, B. J., W. S. BIRKHEAD, AND R. G. HOD- SON. 1974. Intensive sampling of the Brunswick Power Plant intake area and discharge canal. Janu- ary-September 1974. Report to Carolina Power and Light, 1 November 1974, 100 p.

Fuss, C. M., JR. 1964. Observations on the burrowing behavior of the pink shrimp, Penaeus duorarum Burkenroad. Bull. Mar. Sci. 14:62-73.

RULIFSON, R. A. 1975. The effect of temperature and current velocity on the swimming performance of juvenile striped mullet (Mugil cephalus Linnaeus), spot (Leiostomus xanthurus Lacepede) and pinfish (Lagodon rhomboides (Linnaeus)). M.S. Thesis, North Carolina State Univer., Raleigh. 45 p.

RULIFSON, R. A. 1980. Assessing the vulnerability of penaeid shrimp to impingement on the traveling screens of the Brunswick Steam Electric Plant near Southport, North Carolina. Ph.D. Dissertation, North Carolina State Univer., Raleigh. 176 p.

SCHNEIDER, J. W. 1980. Vertical distribution of estuarine meroplankton in the vicinity of a power plant cooling water intake, Southport, North Carolina. M.S. Thesis, North Carolina State Univer., Raleigh. 97 p.

U.S. FISH AND WILDLIFE SERVICE. 1978. Develop- ment of fishes of the Mid-Atlantic Bight. An atlas of egg, larval and juvenile stages. Vol. I-VI. Bio- logical Services Program, FWS/OBS-78/12, January 1978.

WICKHAM, D. A. 1967. Observations on the activity patterns in juveniles of the pink shrimp, Penaeus duorarum. Bull. Mar. Sci. 17:769-786.



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Traveling screens as sampling gear for vertical distribution studies

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