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2020 Eugene Wastewater Master Plan Chapter 4: Design Criteria 31 adjusted as necessary. More accurate estimates can be made when specific site plans are developed, and then actual flow rates can be determined when the development is in operation. This process would allow preliminary design flow rates to be verified and provides more valid information to monitor system capacities. PEAK FLOW FACTOR The preceding sections have established estimates of average base wastewater flows for proposed developments. Average base wastewater flow is defined as the average daily wastewater contribution from residential, commercial, and industrial users. To determine the required pipe sizes for the wastewater collection system, estimates of the peak hourly flow rate are required. Residential, commercial, and industrial flows typically follow a regular pattern, the maximum peak occurring in the morning and a lesser peak generally occurring in the evening. The peaks correspond to high water usage in homes, commercial institutions, and industries. The 1992 USSMP includes an extensive analysis for the development of the peak flow factor used in Eugene and there is no basis to modify this methodology. The base peak flow factor should not be more than 3.5 nor less than 1.5. It is based on an exponential curve which can be calculated by the formula: Peak Flow Factor = 25 - 20.20 (ADWF)0.0165 where ADWF = average dry-weather flow expressed in 1000s of gallons/day. Design Depth of Flow Wastewater systems are often designed to flow at a d/D ratio (depth of peak flow/pipe diameter) of 0.5 to 0.7 during peak flow conditions. This serves two purposes: • Maintains ventilation throughout the pipeline. • Provides some reserve capacity for future flow increases which may occur from land use or zoning changes, high-volume commercial or industrial businesses, or concentration of high-volume users in certain areas. To simplify the design process, the peak flow factor has been calculated to provide a variable safety factor. When the proposed design criteria indicate a pipe is flowing full, the actual d/D ratio (depth of peak flow/pipe diameter) is estimated to vary from 0.65 for 8-inch pipes to 0.85 for 60-inch pipes. This allows the designer to accurately select pipe sizes based on their capacity when flowing full. INFILTRATION/INFLOW General Infiltration/inflow (I/I), combined with peak base wastewater flow from residential, commercial, and industrial users makes up the total collection system design flow. In the Eugene collection system, I/I constitutes a majority of the total peak flow during the wet weather periods of the year. Total infiltration/inflow consists of two components: • Groundwater infiltration (GWI) occurs when a non-watertight wastewater pipe or structure is submerged or partially submerged beneath the groundwater table.