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ATTACHMENT 4 To 05-06-04 Draft MWMC Minutes wet weather design capacity at 70 mgds and the average wet weather flow at 41.8 mgds (Table 3-3, page 455). DEQ, in its evaluation in 2002, said the average wet weather flow design is 75 mgds and the current average wet weather flows were 56.4 mgds. The draft facilities plan notes the average wet weather flow to be 52.5 mgds'(page 4-!3) and the design capacity to be 75 mgds [The 2005 maximum month wet weather flow is estimated at 85.7 mgd, a condition exceeding the current plant capacity] (footnote to Table 5.1.1-1, page 5-3). The draft facilities plan projects that the average wet weather flows in 2025 will be 68.2 mgds - below the 75 mgd design capacity./The 2025 maximum month flow is estimated at 1 ~ 0.8, significantly exceeding the current available capacity] The Master Plan projected that we had 18 years of remaining average wet weather flow capacity, or capacity until the year 2015. If the design capacity of 75 mgds had been used rather than the 70 mgds, the remaining years of capacity Would be further extended. The critical capacity issue involves peak wet weather flows. We have a serious FI problem. In January, 2001, CH2MHill and MWMC staff released the Wet Weather Flow Management Plan, which described the problem: The treatment plant was designed in the 1970s to provide adequate capacity through 2005. From a base flow and loading standpoint, the treatment plant performs well within its capacity (49 million gallons per day [mgd]) in dry- weather months. However, winter rainfall creates flows to the treatment plant that exceed the plant's peak capacity (175 mgd) on average several times per year and exceed full (secondary) treatment capacity (104 mgd) more frequently... Peak flow estimates for conditions associated with the 5-year storm event are used to size and plan future system improvements at the. treatment plant 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 (FI) rates in many areas of the collection system. FI occurs from extraneous water getting into the system from illegal roof drain connections, sewer pipe cracks, and other sources. FI is often associated with older pipes in the system which have deteriorated. Sanitary pipes in older areas are also more likely robe 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 FI. 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 FI are in the existing system and limited FI 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 FI from future pipes. Estimates made at the time of design of the treatment plant, relative to the amount of rainfall-derived infiltration and inflow (RDII) that could be cost-effectively Attachment 4 Page 11 of 17 4-14