1

Reductions in Impingement Mortality 

Resulting from Enhancements to Ristroph 

Traveling Screens at an Estuarine Cooling 

Water Intake Structure

Kenneth A. Strait, John H. Balletto,

L. Raymond Tuttle, Shawn L. Shotzberger

A Symposium on Cooling Water Intake Technologies to 

Protect Aquatic Organisms

May 6-7, 2003
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Salem 

Generating 

Station • Delaware Estuary 

Steam Electric Plant

• Approx. 30 miles SW 

of Philadelphia

• Each unit rated at 

1,162 Mwe.

• Commercial 

Operation – Unit 1: 1977 – Unit 2: 1981

• Delaware Estuary 

Steam Electric Plant

• Approx. 30 miles SW 

of Philadelphia

• Each unit rated at 

1,162 Mwe.

• Commercial 

Operation – Unit 1: 1977 – Unit 2: 1981
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Cooling Water 

Intake Structure 

(CWIS) • 12 Intake Bays • Monthly Average Flow 

of 3,024 MGD

• Approach Velocity – 1.0 ft/s at low tide – 0.87 ft/s at high tide • 12 Intake Bays • Monthly Average Flow 

of 3,024 MGD

• Approach Velocity – 1.0 ft/s at low tide – 0.87 ft/s at high tide
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CWIS 

Traveling 

Screens • 12 screens (one per intake bay) • Continuously rotating to remove 

detritus and marine life

• Modified in 1996 to improve 

efficacy: – Enhanced bucket profile – Lighter construction – Finer Smooth-Tex TM Mesh (0.25” x 

0.5” vs. 0.375” x 0.375” with old 

screens)

– Modified spray wash configuration

• 12 screens (one per intake bay) • Continuously rotating to remove 

detritus and marine life

• Modified in 1996 to improve 

efficacy: – Enhanced bucket profile – Lighter construction – Finer Smooth-Tex TM Mesh (0.25” x 

0.5” vs. 0.375” x 0.375” with old 

screens)

– Modified spray wash configuration
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FLOW

Original Screens

Bucket Profile & Screen Mesh

Modified Screens

Bucket Profile & Screen Mesh
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Salem CWIS Fish Return System

Top Right: Fish spray and flap seals

Below: Fish and debris return troughs

Bottom Right: Fish return trough terminus
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Salem CWIS Fish Collection & Holding Facilities

Impingement Abundance 

Sampling in North Fish 

Counting Pool

Impingement Abundance 

Sampling in North Fish 

Temporary Latent 

Impingement Mortality 

(LIM) Holding Tank

Temporary Latent 

Impingement Mortality 
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1995 Impingement Mortality Direct 

Comparison Study - Methods

• Unit 2 modified with improved Ristroph screens, Unit 

1 retained original screens

• Discharge split to north (U1) and south (U2) pools in 

4 to 6 minute samples for comparison

• LIM Samples collected on 19 dates between June 20 

and August 24, 1995

• Sampled entire tidal cycle

• Weakfish, bay anchovy and spot targeted for study

• Fish held in six 100 gallon tanks

• Survival fraction observed after 12, 24, and 48 hours
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Original Screens Modified Screens

MONTH

Number of 

fish 

examined

Impingement 

Mortality Rate

Number of 

fish 

examined

Impingement 

Mortality Rate

June 111 33% 366 17%

July 367 31% 473 18%

August 553 51% 623 25%

TOTAL 1031 38% 1462 20%

Summary of Results from the 1995 Direct 

Comparison Study - Weakfish

Original Screens versus Modified Screens
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1997–2000 Impingement Mortality Study -

Methods

• Modified Ristroph screen improvements completed 

for both units

• Discharge combined and directed in the direction of 

the tide

• Sampled entire tidal cycle

• Study targeted weakfish, bay anchovy, spot, alewife, 

blueback herring, American shad, striped bass, white 

perch and Atlantic croaker

• Fish held in six 100 gallon tanks

• Survival fraction observed after 12, 24, and 48 hours
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RANK SPECIES ANNUAL 

MORTALITY

* (%)

MINIMUM 

(%)

MAXIMUM 

(%)

TOTAL 

NUMBER 

SAMPLED

1 Striped Bass 4.66 2.10 6.87 1,505

2 White Perch 6.29 0.95 33.63 25,757

3 Spot 6.67 -- -- 132

4 Atlantic Croaker 22.64 3.86 44.86 35,186

5 American Shad 23.95 -- -- 40

6 Blueback Herring 27.39 14.11 43.38 4,150

7 Alewife 39.15 17.41 43.01 551

8 Weakfish 47.77 10.28 65.25 26,400

9 Bay Anchovy 58.02 27.48 83.97 10,235

* Calculated from abundance-weighted monthly mortality estimates.

Mortality Rate Ranking (Lowest to Highest) for RIS 

Species Based on 1997 through 2000 Data
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BLUEBACK HERRING AND ALEWIFE

Month

Percent Mortality

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

1978 - 1982 (old screens)

1995 - 2000 (new screens)

Relative abundance
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ATLANTIC CROAKER

Month

Percent Mortality

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

1978 - 1982 (old screens)

1995 - 2000 (new screens)

Relative abundance
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BAY ANCHOVY

Month

Percent Mortality

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCY NOV DEC

1978 - 1982 (old screens)

1995 - 2000 (new screens)

Relative abundance
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WHITE PERCH

Month

Percent mortality

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

1978 - 1982 (old screens)

1995 - 2000 (new screens)

Relative abundance
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Fish Collection Pool and “End-of-Pipe” Evaluatio

Methods

• Fish collection pool and “End-of-Pipe” models 

constructed off-site

• Tests conducted with alewife and weakfish

• Testing performed in both models as well as in 

the Salem fish collection pools

• Marked control fish included in each replicate

• Survival fraction enumerated after 12, 24 and 

48 hours
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“End-of-Pipe” Model 

6-foot drop configuration
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Treatment Immediate 48 Hours

Existing 

Configuration

0.9965 

(0.0035)

0.9964 

(0.0059)

1.3-ft Freefall 1.0 (N/A) 1.0140 

(0.0098)

6-ft Freefall 1.0 (N/A) 1.0034 

(0.0034)

Estimates of survival (standard error) from pooled 

replicates by treatment with alewife for the endof-pipe experiment

Note: Values > 1 indicate higher control mortality
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Fish Collection Pool Model
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Estimates of survival (standard error) from 

pooled replicates by treatment for the fish 

collection pool experiment (Model) 

Treatment Immediate 48 Hours

3 cfs / 25 cm of 

cushion water

1.0 (N/A) 1.16434 

(0.0058)

3 cfs / 50 cm of 

cushion water

1.0 (N/A) 1.0315 

(0.0379)

Note: Values > 1 indicate higher control mortality
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Treatment Immediate 48 Hours

3 cfs / 25 cm of 

cushion water

1.0034 

(0.0058)

1.0034 

(0.0058)

3 cfs / 50 cm of 

cushion water

1.0067 

(0.0047)

1.0067 

(0.0047)

13 cfs / 25 cm of 

cushion water

0.9966 

(0.0034)

0.9966 

(0.0034)

Estimates of survival (standard error) from 

pooled replicates by treatment for the fish 

collection pool experiment (Station)

Note: Values higher > 1 indicate higher control mortality
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Summary

• Properly designed traveling water screen/fish 

return systems can effectively reduce 

impingement mortality rates

• Impingement mortality is variable & can be 

affected by fish distribution, condition factor, 

temperature and salinity

• Properly designed fish collection, counting 

and return systems do not contribute to 

reported impingement mortality rates