Tetra Tech, Inc.

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TO: Paul Shriner and Jan Matuszko, EPA

FROM: Tim Havey and Kelly 

DATE: June 23, 2008

SUBJECT: California Offshore Intake Structures with Velocity Caps 

Cooling water intake structures located in deeper offshore waters may offer impingement and/or 

entrainment (I&E) reductions when compared with a similar intake structure located at the shoreline. 

I&E reductions may be achieved 

the water body, although this is somewhat dependent on local aquati

When compared with a shoreline intake, an offshore l

also alter the I&E species profile. 

Any impingement reductions realized by the intake’s physical locatio

cap, which changes the intake water vector from vertical to horizontal, thus enabling larger fish to detect 

the current and escape. Impingement reductions observed at velocity cap facilities along the southern 

California Bight have been gen

percent. These reduction values must be qualified, however, based on the methods used to collect and 

analyze the samples as well as the species on which the reduction is calculated. Ear

the 1958 El Segundo Generating Station 

important species only, leaving aside forage and non

value. 

Velocity cap performance may vary significantly based on temporal or local factors. Significant diurnal 

fluctuations in impingement rates have been observed with nighttime performance often well below 

daytime values. At Huntington Beach Generating Station (HBGS), for example, obser

impingement rates were 12 to 37 percent higher during nighttime collection. 

In addition, there are several factors that may influence velocity cap effectiveness and may be unique to 

southern California’s facilities:

 It is worth noting that coastal w

long-term periodic shifts in ocean temperatures that can affect the 

potentially affected by the intake. Two major climatic factors, the Pacific Decadal Osc

MEMORANDUM

Paul Shriner and Jan Matuszko, EPA

and Kelly Meadows

California Offshore Intake Structures with Velocity Caps 

Cooling water intake structures located in deeper offshore waters may offer impingement and/or 

reductions when compared with a similar intake structure located at the shoreline. 

I&E reductions may be achieved through the intake’s location in less productive, less sensitive areas of 

the water body, although this is somewhat dependent on local aquatic life and physical characteristics. 

When compared with a shoreline intake, an offshore location may reduce overall I&E 

also alter the I&E species profile. 

Any impingement reductions realized by the intake’s physical location are increased 

the intake water vector from vertical to horizontal, thus enabling larger fish to detect 

Impingement reductions observed at velocity cap facilities along the southern 

California Bight have been generally been significant, with overall reductions ranging from 65 to 95 

percent. These reduction values must be qualified, however, based on the methods used to collect and 

analyze the samples as well as the species on which the reduction is calculated. Ear

the 1958 El Segundo Generating Station report, tended to focus on commercially and recreationally

important species only, leaving aside forage and non-use species that were presumed to be of little 

y vary significantly based on temporal or local factors. Significant diurnal 

fluctuations in impingement rates have been observed with nighttime performance often well below 

At Huntington Beach Generating Station (HBGS), for example, obser

impingement rates were 12 to 37 percent higher during nighttime collection. 

In addition, there are several factors that may influence velocity cap effectiveness and may be unique to 

oastal waters along the southern California Bight are subject to short and 

term periodic shifts in ocean temperatures that can affect the number and composition of species 

potentially affected by the intake. Two major climatic factors, the Pacific Decadal Osc

- 1 -

Cooling water intake structures located in deeper offshore waters may offer impingement and/or 

reductions when compared with a similar intake structure located at the shoreline. 

intake’s location in less productive, less sensitive areas of 

c life and physical characteristics. 

ocation may reduce overall I&E rates but may

n are increased by the velocity 

the intake water vector from vertical to horizontal, thus enabling larger fish to detect 

Impingement reductions observed at velocity cap facilities along the southern 

reductions ranging from 65 to 95 

percent. These reduction values must be qualified, however, based on the methods used to collect and 

analyze the samples as well as the species on which the reduction is calculated. Earlier studies, such as 

, tended to focus on commercially and recreationally

use species that were presumed to be of little 

y vary significantly based on temporal or local factors. Significant diurnal 

fluctuations in impingement rates have been observed with nighttime performance often well below 

At Huntington Beach Generating Station (HBGS), for example, observed 

In addition, there are several factors that may influence velocity cap effectiveness and may be unique to 

aters along the southern California Bight are subject to short and 

number and composition of species 

potentially affected by the intake. Two major climatic factors, the Pacific Decadal Oscillation (PDO) 
- 2 -

and the El Niño Southern Oscillation (ENSO), can significantly raise or lower water temperatures 

compared with long-term averages. During the El Niño phase of the ENSO, warmer waters from the 

south generally replace the cooler water of the California Current along the bight. During the La Niña 

phase, the pattern may shift and result in colder than normal temperatures. 

Each shift has the potential to alter the species mix in the vicinity of the intake, with El Niño cycles 

driving cold water species further from shore and into areas where they may be affected by the intakes.

Effects of El Niño/La Niña events may be magnified or moderated depending on the concurring phase 

of the PDO, which may take 20-30 years to complete a full cycle. Temperatures may fluctuate by 2.5° F 

or more during the event peaks. Comparisons between historical and current information do show 

differences in species abundance, although a direct correlation is difficult.

 Benefits of offshore intakes with respect to entrainment have not been studied in as much detail 

as impingement, although recent sampling efforts by several facilities offer a substantial data set from 

which entrainment reductions may be calculated. 

 Several of California’s southern coastal facilities with offshore intakes are located in areas with 

rocky substrates that support giant kelp forests. These kelp forests support larger nursery and spawning 

areas offshore than are generally found off the Atlantic coast.

 Some entrainment analyses have noted that entrainment rate comparisons between offshore and 

shoreline locations may not accurately reflect actual impact reductions when evaluating all taxa instead 

of target or relevant species. Changes in currents due to ENSO or other climatic events may result in the 

presence of some species that are not native to the area and are effectively “dead” despite their potential 

for entrainment. 

Conclusions presented by several facilities that have completed their most recent sampling analyses are 

typically discussed in terms of population level impacts as opposed to percentage reductions. Because 

the Phase II rule has been suspended, the facilities have reverted to the previous BPJ-based permitting 

approach. Based on EPA’s 1977 guidance document, they have sought to quantify whether AEI was 

occurring as a result of the intake structure’s operation and whether that impact was “significant.” As 

such, performance is broadly based on whether these impacts are significant or not instead of the 

“percent reduction” approach used in Phase II. Nevertheless, data are available from which to calculate 

percentage reductions from different baseline or relative values.

Huntington Beach Generating Station (HBGS)—Huntington Beach, CA

Intake Description:

 Single intake located 1,500 feet from shore in Pacific Ocean

 Velocity cap approximately 18 feet below MLLW and 5 feet above intake riser

 Large marine life exclusion 

 Intake velocity: 2.8 fps (at cap)

 Design flow: 517 mgd
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Velocity cap impingement studies conducted at HBGS have been among the more comprehensive and 

rigorous of all the southern California facilities. This is due, in part, to its selection by Southern 

California Edison (SCE) as a model facility to use in determining velocity cap effectiveness at its other 

facilities in the region, including the El Segundo, Ormond Beach and Redondo Beach generating 

stations. Other facilities with velocity caps (Scattergood and SONGS) have also used data collected at 

HBGS as part of their own effectiveness studies. 

The initial study at HBGS was conducted by the University of Washington from 1978 through 1979 

and consisted of seven trial periods with 123 hourly impingement estimates as well as source water 

abundance estimates using hydroacoustic sampling techniques. Velocity cap performance was

calculated by comparing impingement the relative impingement rates of a capped versus uncapped 

intake. This was done by reversing the intake and discharge locations, both of which are located 

offshore in the same general area. Intake tunnels were purged of any resident fish prior to testing by 

injecting chlorine into the tunnel.

Results from the comparative tests showed the velocity cap was effective in reducing impingement by 

as much as 99% during they day but as low as 53% at night. Overall effectiveness averaged 82% for all 

sampling events regardless of time. As part of its NPDES permit requirements, HBGS has continued 

impingement monitoring during all heat treatments and representative operating periods.

Entrainment analyses were not conducted at HBGS in the late 1970s. Rather, data collected at two other 

SCE facilities (Ormond Beach and SONGS) were used to extrapolate HBGS entrainment rates based 

on local conditions. Entrainment performance was not calculated because source water references were 

not developed on which any reduction could be based.

HBGS conducted additional I&E sampling in 2003 and 2004 as part of its relicensing agreement with 

the state. These samples included source water abundance monitoring for both I&E at several reference 

monitoring stations located near the intake and along the shoreline. Because these data were considered 

representative of current conditions, HBGS did not collect additional data in order to comply with 

Phase II CDS requirements. 

Various models were used to estimate entrainment impacts relative to the source water. Depending on 

the target species, AEL, FH, and ETM methods were used to estimate the percent mortality (Pm), 

which, in turn, provided the basis for APF estimates.1 HBGS proposed to use this method to determine 

the calculation baseline and any existing design credits under Phase II. 

HBGS concluded that I&E impacts were not significant, although raw data supporting this 

determination were not available for review. Presumably, data collected in 2003 and 2004 would be 

able to show entrainment rates relative to the source water body abundance. HBGS also conducted an 

entrainment survival study (through condenser), but results are not yet available. 

 

1 AEL = Adult Equivalent Loss, FH = Fecundity Hindcasting, ETM = Empirical Transport Model, APF = Area of 

Production Forgone. These are methods of normalizing I&E data from eggs and larvae to something meaningful. 

Unfortunately, the numbers have a tendency of saying just about anything when you’re dealing with large populations.
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Scattergood Generating Station (SGS)—Los Angeles, CA

Intake Description:

 Single intake located 1,600 feet from shore in Santa Monica Bay

 Velocity cap approximately 17 feet below MLLW 

 Large marine life exclusion (installed February 2008)

 Intake velocity: 1.5 fps (at cap)

 Design flow: 495 mgd

Site-specific evaluations of the SGS velocity cap’s impingement performance were first conducted in 

the early 1970s when a storm damaged the original velocity cap. The cap was removed and, at the 

request of California Department of Fish and Game, left off so as to allow a comparison of 

impingement rates between the capped and uncapped intake. Using a biomass metric normalized to the 

intake flows reported during the monitoring periods, SGS estimated the velocity cap’s impingement 

reduction effectiveness at 83% compared with the uncapped intake. As part of its NPDES permit 

requirements, SGS has continued impingement monitoring during all heat treatments and representative 

operating periods.

The most recent velocity cap effectiveness study at SGS was conducted in 2006 as part of Phase II CDS 

compliance. This study again compared the performance of a capped versus uncapped intake by

reversing the operating flows; i.e., the intake becomes the discharge, and vice versa. Unlike the previous 

analysis, the 2006 study included source water reference monitoring using hydroacoustic sampling 

techniques to estimate aquatic biomass abundance and distribution. Over a 12-week period, 

impingement data were collected under normal operating conditions (with cap) and reverse flow 

(without cap), alternating every two weeks. Source water characterizations were augmented with 

impingement sampling at reference monitoring stations along the shoreline and in the vicinity of the 

intake.

Using these data as well as weekly impingement monitoring information, normalized based on intake 

flow, the SGS velocity cap effectiveness was calculated at 95% using a biomass metric and more than 

97% based on abundance. 

Entrainment analyses at SGS were first conducted in 1978 but only focused on commercially and 

recreationally important species. As part of its Phase II CDS compliance requirement, SGS conducted 

additional entrainment monitoring in 2006. Samples were collected from the intake structure as well as 

several reference stations along the shoreline and in the vicinity of the intake structure. In contrast to the 

1978 efforts, all taxa were identified as accurately as possible.

Various models were used to estimate entrainment impacts relative to the source water. Depending on 

the target species, AEL, FH, and ETM methods were used to estimate the percent mortality (Pm), 

which, in turn, provided the basis for APF estimates. SGS proposed to use this method to determine the 

calculation baseline and any existing design credits under Phase II. An aggregate “percent reduction” 

value is not explicitly presented in the final report, although raw data are available from both the intake 

and reference stations that would enable such a determination.
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SGS bases its discussion of entrainment impacts on guidelines set forth in EPA’s 1977 guidance 

document, which categorizes AEI as significant or insignificant relative to the known source 

populations. SGS concludes that the current intake’s impacts are insignificant.

El Segundo Generating Station (ESGS)—El Segundo, CA

Intake Description:

 Two intakes located 2,600 feet from shore in Santa Monica Bay (only one operational)

 Velocity cap approximately 15 feet below MLLW 

 Intake velocity: 2.4 fps (at cap)

 Design flow: 399 mgd

The original velocity cap effectiveness study at ESGS was conducted in 1958 and consisted of a full 

year of impingement monitoring before and after the velocity cap was installed. As with other earlier 

studies, the 1958 analysis primarily focused on commercially and recreationally important species and 

excluded forage and non-use species. A comparison between the before and after periods showed an 

impingement reduction of 95% using a biomass metric. Original data are not available and cannot be 

reviewed to determine relative species abundance. Source water sampling was not conducted.

Impingement sampling was conducted in 2006-2007 as part of Phase II CDS compliance, but ESGS did 

not perform a site-specific analysis of its velocity cap effectiveness. Instead, it relies on extrapolations 

from the previous HBGS study and the more recent SGS study. The relative close proximity between 

ESGS and SGS would appear to support similar conclusions for velocity cap effectiveness.

Entrainment analyses were not conducted at ESGS in the late 1970s. Rather, data collected at Ormond 

Beach were used to extrapolate ESGS entrainment rates based on local conditions. These data are not 

considered reliable for ESGS because of the distance separating the two facilities (60 miles) and the 

sample collection and analysis methods used that the time. Entrainment performance was not calculated 

because source water references were not developed on which a reduction could be based.

ESGS did conduct additional entrainment monitoring as part of Phase II CDS compliance. Samples 

were collected in the intake forebay as well as at several reference monitoring stations along the 

shoreline and in the vicinity of the intake. Total entrainment values were estimated based on actual and 

design flows.

Various models were used to estimate entrainment impacts relative to the source water. Depending on 

the target species, AEL, FH, and ETM methods were used to estimate the percent mortality (Pm), 

which, in turn, provided the basis for APF estimates. ESGS proposed to use this method to determine 

the calculation baseline and any existing design credits under Phase II. 

ESGS bases its discussion of entrainment impacts on guidelines set forth in EPA’s 1977 guidance 

document, which categorizes AEI as significant or insignificant relative to the known source 

populations. ESGS concludes that the current intake’s impacts are insignificant.
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Others

SONGS: Awaiting final characterization report from SCE

Redondo: Most data and conclusions are extrapolated from ESGS and HBGS; did not conduct a 

site specific velocity cap analysis

Ormond: Awaiting final characterization report