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2020 Eugene Wastewater Master Plan Chapter 4: Design Criteria 32 • Rainfall-dependent I/I (RDI/I) occurs during and shortly after a rainfall event and includes both infiltration and inflow. The stormwater inflow results from surface runoff sources and the rainfall-dependent infiltration (RDI) results from saturated soil conditions. Various factors influence the I/I flow rate. GWI peaks during the high groundwater period, usually between January and March. The RDI peak occurs when the soil is highly saturated and has limited capacity to store additional water. New Systems New system design should not allow stormwater inflow. Ongoing system management and inspection will prevent connection of catch basins, area drains, and roof drains to the wastewater systems. An allowance for infiltration should still be included because pipe and joint materials will develop some defects during the long service life of the system. New pipe materials are expected to have a service life of 100 years or more. Over that long service life, considerable damage should be expected. Therefore, the recommended peak I/I rate for new wastewater system design is: 2000 gal/gross acre/day or 2500 gal/net acre/day This is an increase from the previous master plan but is consistent with the DEQ recommendation and the criteria used in the MWMC Facility Plan. Existing Systems For basins and study areas that include an existing wastewater system, an allowance for both infiltration and inflow must be included for the existing system. Peak I/I flow rates can vary widely, depending on the decade of construction, material type and groundwater conditions. Because of the complexity of variables causing I/I, rather than calculating a value based on these factors, the best way to estimate I/I is to use hydraulic model results for the study area. As the wastewater hydraulic model is developed, more and more basin-specific I/I rate data is becoming available. This data is based on field measured flow rates. If the study area does not have model results available, estimate the capacity of an existing wastewater line by reviewing the age of the system, the type of pipe materials, and whether any rehabilitation has been completed. If this review indicates less than 50 percent of the system is rehabilitated or PVC pipe, the peak infiltration rate of 4000 gal/gross acre/day should be used. SUMMARY OF PROPOSED DESIGN CRITERIA FOR NEW SYSTEMS This section provides a summary of the design criteria developed in the previous sections. The basic components of the design flow are shown in the following equation: Design Flow = (ADWF x PFF) + (I/I) where ADWF = Average Dry Weather Flow, PFF = Peak Flow Factor, and I/I = Infiltration and Inflow. Average Dry-Weather Flow The ADWF is the total of the base wastewater flows from all types of land use designations within the design basin, shown in Table 4-3. Refined flow information for specific developments (especially