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ATTACHMENT 4 To 05,06-04 Draft MWMC Minutes' to growth also seems right as it is slightly higher than the population increase over the study period (36. 7%). The second major issue I have is the 29% allocation of peak flow to growth. The 2001 Wet Weather Study, prepared by CH2MHill, provided the following information: Peak flow estimates for conditions associated with the 5-year storm event are used to size and plan future system improvements at the treatment plan! and in the collection system. Through system modeling, the 5-year peak was estimated at 264 mgd. Peak flows are attributed to high infiltration and inflow (VI) rates in many areas of the collection system. III occurs from extraneous water getting into the system from illegal roof drain connections,' sewer pipe cracks, and other sources. VI is often associated with older pipes in the system which have deteriorated. Sanitary pipes in older areas are also more likely to be subject to . improper storm drainage (inflow) connections when construction inspection practices were more lenient and / or such connections were allowed, creating a combined flow system. Newer pipe systems reflect improvements in construction techniques, materials, and inspection and typically exhibit far less VI. In Eugene, 11 percent of the pipes are at least 50 years old. In Springfield, the percentage of pipes at least 50 years old is 15 percent. Because the primary sources of I/I are in the existing system and limited III is anticipated from system expansion, growth in the system does not contribute significantly to projected system deficiencies. ' The 5-year peak is estimated at 298 mfd. Of this peak, only 4 percent or 12 mgd, is estimated to be the result of VI from future pipes. In a response to a question from Chris Clemow on the CAC, I believe that CH2MHill increased the 4% to 4.7%, which I have no objection to. [The current SDC methodology attributes 5.2% of the peal( flow I/I to growth: 14.5mgd / 277mgd. This 5.2% is in line with the previous 4.7'% and is slightly higher primarily because the updated collection system modeling is now estimating a slightly reduced peak flow of 27'7 mgd. Future total peak flows are estimated to be less due to I/I reduction practices in the existing system, therefore, the percentage due to growth increases] The Point is that we are not responsible for 29% of peak flow management costs [29% is growth's share of the peak flow capacity needed through 2025 30 mgd divided by ~02 mgd] Peak flow is made up of the average flow (currently less than 30 mgds) [This is incorrect. Current maximum month dry weather flows] and the rest is III. We are already allocated 100% of the average flow costs under the methodology and pay that separately. Let's assume that average flow in 2005 will be 49 mgds. Our portion of peak flow costs would be 4.7% of 249 mgds or 11.7 mgds. [bio, incorrect. The portion of the peak flow is 30 mgd divided by ~02 mgd. The 4.7' % value is a percentage of l/I only. If growth's allocation is going to be based on I/I only and not peak flow, then we have to be consistent and allocate to existing users based on I/! only as well- whereas the average flow would have to be subtracted out of the remaining peak flow which is 7'2 mgd (~02 mgd less 30 mgd)] I am willing to accept the 14 mgds of I/I that CH2MHill suggested Attachment 4 Page 4 of 17 4-7