
                                  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) 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)). 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 basis. 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%). 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.

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. 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.


                                    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 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
