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<br />City of Everett • 2024 Comprehensive Sewer Plan • October 2025 <br />Executive Summary • ES-8 <br />Wastewater Flow Characteristics <br />The existing wastewater flows were determined using data from flow monitoring locations in the City’s <br />wastewater collection system for the 2021/2022 wet season. The wastewater flow metrics determined <br />were existing average dry weather flow (ADWF), diurnal patterns, and rainfall dependent inflow and <br />infiltration (RDII). The wastewater flow metrics were used to develop and calibrate the H/H model used for <br />the project. Figure ES-2 shows a typical diurnal pattern for weekdays and weekends at meter <br />EV_SMH1785H04. <br />Figure ES-2 | Average Diurnal Pattern as meter EV_SMH1785H04 <br />F <br />The RDII analysis evaluated how much flow enters the wastewater conveyance system from groundwater <br />and surface water because of rainfall. A high RDII could be indicative of infrastructure that requires <br />maintenance or replacement. Chapter 5 determines severity of basin areas by ranking them based on RDII <br />rates. RDII was not computed for combined basins as the system is designed to include sources of inflow. <br />Future flow rates were determined from population growth and pipe condition deterioration. Population <br />growth in the City service area is assumed to increase by approximately 26 percent over the 20-year <br />planning horizon (see Chapter 4). Therefore, ADWF average values for each meter basin were increased by <br />26 percent in the H/H model. This assumes that future water usage is similar to current water usage. It was <br />also assumed that the usage of water throughout the day would remain consistent, therefore no <br />modifications to diurnal patterns were made. <br />The RDII degradation is assumed to be 7 percent per decade which is a value used by neighboring <br />jurisdiction King County Wastewater Treatment Division in their planning efforts. The H/H model was <br />adjusted so that RDII was increased in future modeling scenarios. See the H/H modeling technical <br />memorandum in Appendix E and Chapter 7 for more information.