R2018-13 Professional Services Agreement AECOM for Ellen and Division Inflow and Infiltration Reduction

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�� Exhibit A-2 Page 2 City of Decatur,IL Ellen and Division I&I Reduction Program PCSWMM is a graphical user interface for the USEPA Stormwater Management Model (SWMM). SWMM is a dynamic rainfall-runoff simulation model that is widely used to assess mechanisms of flooding in urban areas. The baseline hydraulic model will be developed by leveraging digital sanitary sewer system data maintained by the City of Decatur. This will include pipe lengths, manholes and outfalls where applicable. Any data gaps will be supplemented by site visits performed by AECOM in partnership with City of Decatur staff. As a part of the private property inspections performed by AECOM (see Task 4), AECOM will also measure basement heights (i.e. floor to ceiling) to assist in predicting basement backups. This data will be used with LIDAR data and/or existing City manhole GPS data to determine the approximate elevation of basements. However, no topographic surveying is included in this proposal. Subcatchments, which are used to route surface water runoff to manholes, will be delineated using the Thiessen (Voronoi) polygon generation approach. Flow lengths and slopes for each subcatchment will be derived from Light Detection and Ranging (LIDAR) data. Land use data will be used to inform the percentage of impervious area in each subcatchment and to make assumptions on the infiltration capacity and depression storage available. Calibration of the model will then be undertaken for both dry weather and wet weather flows based on flow meter data. AECOM has already specified locations for the installation of eight flow meters (installed by others) within the proposed study area. Average Dry Weather Flow (ADWF) will be derived for each meter location by identifying metered days that were not influenced by previous rainfall events and have a regular diurnal pattern. Patterns will be created for both week days and weekends for use in the model. ADWF values will be applied to the contributing reaches of the collection upstream of the relevant flow meter. These flows will be multiplied by a typical diurnal pattern to account for the daily variations in flow. The calibration will meet EPA standards (WAPUG calibration and validation criteria as defined in Table 5.2 of WEF Manual of Practice No. FD-17 — Prevention and Control of Sewer System Overflows). Wet weather flows are characterized by a measured increase in flow within the collection system following the onset of rainfall, AECOM will endeavour to calibrate the model to three(3) rainfall events at each of the seven flow metering locations. This calibration process is dependent on the receipt of good quality metered data and the identification of rainfall events of sufficient magnitude to produce a response in the collection system. Model parameters will be altered in an effort to calibrate the model include percentage impervious area, infiltration rates, flow path, slope and roughness. Once the model has been calibrated, it will be compared with an independent wet weather flow event. Model parameters may be further adjusted to ensure the model performs adequately during both the calibration and verification events. If rainfall data is available for wet weather events known to have caused SSOs at Manhole No. 10-13, up to two of these events will also be run through the model. This will ensure that the model replicates locations and mechanisms of flooding accurately. Also, it should be noted that the model may predict SSOs at manholes that are not currently known to overflow. A range of storm events will then be applied to the calibrated and verified hydraulic model to determine the critical storm duration. Typically, the 1-hour, 2-hour, 3-hour, 6-hour, 12-hour, 18-hour and 24-hour durations will be run for the 2-year, 5-year, 10-year, and 25-year events. In total, twenty- eight(28) model runs will be undertaken. The critical storm will be the event that creates the greatest stress on the sewer system. Stress is categorized by the highest wet weather peak flow rate or largest predicted flood volume.