MEMORANDUM

Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

Phone 703-385-6000

Fax 703-385-6007

TO: Paul Shriner and Lisa Biddle, EPA

FROM: John Sunda, Tetra Tech

DATE: April 10, 2013

SUBJECT: Analysis of Survey Intake Velocity Data

The detailed technical questionnaire (DQ

1

) requested that facilities report through-screen

velocity for each intake and the flow basis upon which the value was calculated. In the 

development of compliance costs, EPA estimated IM compliance with respect to the 0.5 feet per 

second (fps) through-screen velocity for existing intakes using reported velocity data but did not 

take into consideration the reported flow basis. This memo examines the implications of this 

approach as it relates to the maximum impingement mortality (IM) compliance alternative based 

on intake velocity as defined in the final rule.

The DQ specifically requested data for the “Design Through-screen Velocity” in fps and the 

“Source Water Flow Basis for Design Through-screen Velocity” (flow basis). The flow basis 

was reported by checking one of the following:

1. Critical Low Flow

2. Mean Flow

3. Don’t Know

Tables 1-3 below summarize the number of intakes for which velocity data was reported and the 

distribution by flow basis for generators, manufacturers and all facilities, respectively. The 

Tables also contain distribution data for the subset of intakes that reported a velocity that was 

≤0.5 fps and the subset of that group that were once-through (i.e., not closed-cycle recirculating 

cooling systems as defined in the final rule (CCRS)).

2

In most cases, the subset of intakes that 

were not closed-cycle and reported a velocity ≤0.5 fps were assumed to be IM compliant in the 

baseline solely on the basis of the reported velocity regardless of the reported source water 

 

1

316(b) Detailed Industry Questionnaire: Phase II Cooling Water Intake Structures. See DCN 3-0030 in the Phase I 

docket.

2 This is important because closed-cycle systems are IM compliant regardless of intake velocity.
basis.

3 One concern is that the through-screen velocity based on the minimum ambient water 

level will, in most cases, be a higher value than the through-screen velocity based on the mean 

flow. Note that this data is unweighted.

As can be seen from the tables, 39% of all intakes did not report velocity data. In the cost model, 

these intakes were assigned the default values shown in Table 4. These default values were 

derived based on the average reported velocity for each sub-group. All of these default values 

exceed the 0.5 fps intake velocity threshold and so all are assumed not to meet the intake velocity

compliance alternative in the baseline. However, it is likely that some portion of these intakes 

that are assumed to not to have an intake velocity ≤0.5 fps (because no velocity data was 

reported) may have design or actual through-screen velocities that would meet the ≤0.5 fps 

threshold.

As shown in Table 3, roughly 11% of intakes were once-through and reported a velocity of ≤0.5 

fps; therefore they were assumed to comply with the IM standard in the baseline through the low 

intake velocity compliance alternative. Some public comments have expressed concern that EPA 

overestimated the number of facilities/intakes that were IM compliant in the baseline. Although 

no commenters specifically cited EPA’s assumptions for baseline compliance via the IM velocity 

compliance alternative or flow basis, many argued that defining the IM velocity threshold based 

on the minimum ambient water depth would prevent many facilities from complying even 

though they would meet the ≤0.5 fps intake velocity threshold most of the time. The minimum 

ambient depth is analogous to the critical low flow and so those that reported a design velocity of 

≤0.5 fps based on the critical low flow would clearly be IM compliant in the baseline. This group 

comprises only 1% of all non-CCRS intakes. But there may be more in that category since 41%

of those reporting a design velocity ≤0.5 fps and operating as once-through fall into the “don’t 

know” flow basis category. If we assume that for those reporting a velocity of ≤0.5 fps the 

proportion of those reporting critical low flow as the basis is similar for those where the basis is 

unknown and apply this proportion to those that are unknown, the result is that an additional 1% 

is projected to be clearly compliant via the design velocity compliance alternative (increasing the 

total to an estimated 2%).

4 Many of the remaining 9% that EPA assumed are IM compliant in the 

baseline via the velocity compliance alternative that are not based on critical low flow should be 

able to comply as well without requiring technology or operational modifications that would 

result in increased costs. An important consideration is the fact that many intakes that reported 

velocity values along with a “Mean Flow” or “Don’t Know” basis had values low enough that 

even when adjusted for low ambient water levels they should be able to comply with the design 

flow velocity compliance alternative. For example, a mean flow design velocity of ≤0.35 would 

be able to comply with the 0.5 fps velocity compliance alternative as long as the total wetted 

screen depth was reduced by no more than 30%. Table 3 shows that about two thirds of those 

that reported a velocity ≤0.5 fps were also ≤0.35 fps (an estimated 5% of total). In addition, 11 

(about 1% of total) of the intakes reporting a velocity of exactly 0.5 fps with once-through flow 

and a flow basis of “Mean Flow” or “Don’t Know” were submerged wedgewire screens that 

would be unaffected by water level.

 

3 A few intakes in this group may also be IM compliant through the use of IM compliant technologies such as 

modified traveling screens or existing offshore velocity caps.

4 Applying the “critical low flow”/”mean flow” distribution (2% & 9%) of intakes with velocity ≤0.5 fps to the 4% 

“Don’t Know” intakes adds 1%.
All of these estimations are based on the reported design through-screen velocity; EPA allows 

intakes to also comply via the velocity based on actual intake flow (AIF) as well. Thus, for the 

remaining 3% where the assumption of compliance remains uncertain, this option is available as 

well but in some instances they may require some operational adjustments during periods when 

water levels are very low.

Finally, if we apply the reported velocity data distribution to the 39% that did not report velocity 

data, the result is an estimated additional 5% of the total intakes that may have a velocity ≤0.5 

fps and thus be potentially compliant via either the DIF or AIF velocity compliance alternatives.

5

With the exception of those that were compliant via another alternative, these intakes were 

assumed non-compliant and were assigned compliance technology costs.

Conclusion

In summary:

 Only 11% of non-CCRS intakes were assumed IM compliant in the baseline based on the 

velocity compliance alternative alone.

 An estimated 2% meet the compliance alternative based on a design velocity ≤0.5 fps at 

critical low flow.

 An estimated additional 5% meet the compliance alternative because the velocity is low 

enough that an adjustment of design water level by minus 30% would still meet design 

velocity ≤0.5 fps.

 An estimated 1% would meet the compliance alternative because reporting velocity is

≤0.5 fps and the intake screen is submerged wedgewire.

 Many of the remaining 3% could likely meet the velocity compliance alternative based 

on the AIF but some may not without added costs.

 The sum of these estimates suggest that EPA did not overestimate baseline compliance 

on the basis of reported through-screen velocity even considering the fact that, in many

instances, the reported values did not correspond to the maximum velocity at the lowest 

ambient water level. In fact, EPA may have underestimated baseline compliance via the 

design and actual intake velocity compliance alternatives since EPA assumed a velocity 

of >0.5 fps for all of the 39% of model intakes where velocity data was unreported. The 

portion of these intakes assumed non-compliant with regard to the velocity alternative

could represent nearly 5% of all intakes. While some of this 5% may not be compliant via 

the IM velocity alternative, those that are compliant should more than offset the estimated 

up to 3% assumed compliant that may require some operational adjustments or other 

measures in order to comply.

 

5 Assuming a similar distribution in each group the estimated 5% of facilities should be analogous and in addition to 

the 11% that EPA assumed compliant based on a reported velocity of ≤0.5 fps. 
Table 1

Generator Model Intake Velocity Data Summary

Survey Velocity 

Data Basis

All Model Intakes Velocity ≤0.5 fps Velocity ≤0.5 fps and Once-through (OT) Velocity ≤0.35 fps and Oncethrough

Intake 

Count

Percent 

of Total

Percent 

of 

Those 

With 

Data

Intake 

Count

Percent 

of Total

Percent 

of 

Those 

With 

Data

Percent 

of 

Those 

With 

≤0.5 fps

Intake 

Count

Percent 

of Total

Percent 

of 

Those 

With 

Data

Percent 

of Those 

With 

≤0.5 fps 

& OT

Intake 

Count

Percent of 

Those With 

≤0.5 fps & OT

No Data* 132 33%

Critical Low 

Flow

100 25% 36% 6 1% 2% 10% 1 0% 0% 3% 0 0%

Mean Flow 146 36% 53% 37 9% 14% 59% 21 5% 8% 64% 9 27%

Don't Know 28 7% 10% 20 5% 7% 32% 11 3% 4% 33% 5 15%

Total With Data 274 67% 100% 63 16% 23% 100% 33 8% 12% 100% 14 42%

Total 406 100%

OT – Once-through cooling

fps – feet per second

* Intake or through-screen velocity was not reported.

Table 2

Manufacturer Model Intake Velocity Data Summary

Velocity Basis All Model Intakes Velocity ≤0.5 fps Velocity ≤0.5 fps and Once-through Velocity ≤0.35 fps and 

Once-through

Intake 

Count

Percent 

of Total

Percent 

of 

Those 

With 

Data

Intake 

Count

Percent 

of Total

Percent 

of 

Those 

With 

Data

Percent 

of 

Those 

With 

≤0.5 fps

Intake 

Count

Percent 

of Total

Percent 

of 

Those 

With 

Data

Percent 

of 

Those 

With 

≤0.5 fps 

& OT

Intake 

Count

Percent of 

Those With 

≤0.5 fps & 

OT

No Data* 150 47%

Critical Low Flow 34 11% 20% 7 2% 4% 12% 4 1% 2% 9% 2 5%

Mean Flow 71 22% 43% 28 9% 17% 48% 19 6% 11% 44% 18 42%

Don't Know 62 20% 37% 23 7% 14% 40% 20 6% 12% 47% 14 33%

Total With Data 167 53% 100% 58 18% 35% 100% 43 14% 26% 100% 34 79%

Total 317 100%

OT – Once-through cooling

fps – feet per second

*Intake or through-screen velocity was not reported
Table 3

All Facility Model Intake Velocity Data Summary

Velocity Basis All Model Intakes Velocity ≤0.5 fps Velocity ≤0.5 fps and Once-through Velocity ≤0.35 fps and 

Once-through

Intake 

Count

Percent of 

Total

Percent 

of Those 

With 

Data

Intake 

Count

Percent 

of Total

Percent 

of Those 

With 

Data

Percent 

of Those 

With 

≤0.5 fps

Intake

Count

Percent 

of Total

Percent 

of Those 

With 

Data

Percent 

of Those 

With 

≤0.5 fps 

& OT

Intake 

Count

Percent of 

Those With 

≤0.5 fps & OT

No Data* 282 39%

Critical Low Flow 134 19% 30% 13 2% 3% 11% 5 1% 1% 7% 2 3%

Mean Flow 217 30% 49% 65 9% 15% 54% 40 6% 9% 53% 27 36%

Don't Know 90 12% 20% 43 6% 10% 36% 31 4% 7% 41% 19 25%

Total With Data 441 61% 100% 121 17% 27% 100% 76 11% 17% 100% 48 63%

Total 723 100%

OT – Once-through cooling

fps – feet per second

* Intake or through-screen velocity was not reported

Table 4

Default Intake Through-screen Velocity Values Used in Cost Model

Facility Type

Default Screen Velocity (fps)

DIF ≥ 50 MGD DIF < 50 MGD

Generators 1.5 0.6

Manufacturers 1.2 0.8

DIF – design intake flow

MGD – million gallons per day