### 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 , 201 3 SUBJECT

: Analysis of Survey Intake Velocity Data The detailed technical questionnaire (D Q 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 implication s 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 specificall y requested data for the “Design Through

- screen Velocity” in fps and the “Source Water Fl ow Basis for Design Through

- screen Velocity” (flow basis). The flow basis was reported b y checking one of the following: 1. Critical Low Flow 2. Mean Flow 3. Don’t Know Table s 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

- c ycle recirculating cooling s ys tems as defined in the final rule (CCRS) )

. 2 In most cases, th e subset of intakes that were not closed

- cycle and reported a velocity ≤0.5 fps were assumed to be IM compliant in the baseline solel y 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 T his is im

portant 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. Inthe 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 like ly 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 , roughl y 1 1 % 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 c ompliance via the IM velocity compliance alternative or flow basis , m any argued th at 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 th at 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 criti cal low flow as the basis is similar for those where the basis is unknown and a

ppl y this proportion to those that are unknown , the result is that an additional 1 % is projected to be clearl y compliant via the design velocity compliance alternative ( increas ing 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 “M ean F low ” 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 compl y with the 0.5 fps velocity compliance alternative as long as the total wetted screen depth w as 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 exactl y 0.5 fps with once - through flow and a flow basis of “M ean F low ” or “D on’t K now ” were submerged wedgewire screens that would be unaffected b y water level

.

3 A few intakes in this group may also be IM compliant through the use of IM compliant technologies such as mo dified traveling screens or existing offshore velocity caps

. 4 Applying the “critical low flow”/”mean flow” distribution ( 2% & 9%) o f 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

intake s 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 appl y 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 w ere 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 with out 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 nearl y 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 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* 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 Ty

pe 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