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MWMC FACILITIES PLAN 2004). The peak day average flow (PDAF) was predicted using daily precipitation and flow data for January through.May, the months that groundwater affects I/I rates in the system. Using a probability distribution, the peak week and peak instantaneous flow rates were predicted. Table 4.1.3-1 presents the peaking factors derived from the DEQ predicted flows. TABLE 4.1.3-1 DEQ Methodology Flow Peaking Factors MWMC Facilities Plan, Eugene-Springfield Dry Weather Wet Weather Wet Weather Peak Maximum Month Maximum Month Maximum Week Peak Day Instantaneous 2.0 1.4 1.7 2.6 6.1 4.1.4 Infiltration and Inflow Many communities typically experience higher flow rates in their wastewater collection systems during rain events. These flow responses to rainfall (and sometimes snowmelt) will usually vary according to storm volume and intensity, as well as the amount and duration of antecedent rainfall (rainfall in the days preceding the particular rain event). The flow response to rainfall/snowmelt can generally be referred to as RDII (rainfall dependent infiltration and inflow). It corresponds with that portion of a wastewater collection system hydrograph that is above the normal base flow. The ratio of RDII (volume or peak flow rate) to precipitation is generally referred to as the RDII "return rate." Infiltration and inflow are two distinct contributors to a wet weather hydrograph, but there is no clear demarcation between them. Inflow enters sewers through direct stormwater connections such as roof leaders, illegal drain connections, and leaky manhole covers in depressed or sump areas. Infiltration is produced by rainfall and/or snowmelt that has been sustained long enough to soak into the soil and produce temporary saturated soil conditions. Infiltration enters sewers directly through cracks and faulty deteriorated joints, and indirectly through basement sump pumps. RDII can be reduced through rehabilitation of the wastewater collection system. Rehabilitation can include repair or replacement of facilities, relining of facilities, and disconnection of inflow sources. Previous Infiltration and Inflow Studies During the mid 1970s, a Sewer System Evaluation Study (SSES) was performed for the Eugene and Springfield sewer system study areas to assess the amount of RDII entering the collection systems. The study estimated that in 1978 the peak flow to the planned regional WPCF would be 264 mgd. The SSES also predicted quantities of RDII that could be cost- effectively removed. These removal estimates formed the basis for the 175-mgd design flow rate of the WPCF. Subsequent studies for Eugene (City of Eugene Public Works/Engineering Urban Sanitary Sewer Master Plan, 1992) and Springfield (City of Springfield Sanitary Sewer Master Plan, July 1980), documented RDII as significant contributors to systemwide wastewater peak 4-4 MWMC_4.0_REV8. DOC