2010/09/15 Council Agenda Packet

• • Scope of Services Continued Task 10.0— Phase I Evaluation of Existing Conditions and Early Actions Phase 1 of the work under this amendment (North End Combined System Evaluation) is an expedited analysis of the sewer system in the study area generally north of the 17th Street Interceptor. The objective of this evaluation is to•quickly determine if any modification should be included in the Sewer L North Area project before it goes out to bid. In addition, there is some urgency in defining the sewer improvements or modifications that should be done before the Providence Hospital"Tower"project is completed and occupied beginning in Spring 2011. Furthermore, the PEMC CUP facility is currently being commissioned and new flows will be introduced into the system immediately. This phase must be completed as soon as possible in coordination with the City's timeline and other direction by the City.The boundaries of the study area will incorporate virtually all areas that reported sewer backups or flooding related to the June 9, 2010 event and other prior flooding events. The City currently uses MHWSoft's InfoSWMM for modeling the combined sanitary sewer system. Physical components of the sewer system (pipes,manholes,weirs,pumps, etc.)were originally entered into DHI MOUSE by CH2M Hill using available data from the City (GIS, as-built drawings, survey, etc.). Details for the development of the model can be found in the CH2M Hill report titled Collection System Hydraulic Model Documentation Report prepared in 2004. The original model created by CH2M Hill described in the Collection System Hydraulic Model Documentation Report focused on the conveyance capacity of the City's interceptor mains and trunk lines. Flow input into the model, consisting of dry weather and wet weather,was done at"loading manholes" located near the centroid of the drainage basin.Therefore the smaller collection sewers in the upper reaches of the system could not be analyzed for conveyance. Otak will perform hydrologic and hydraulic modeling primarily using InfoSWMM to identify areas for improvements within the study area.This will include the following tasks: I . Construct Hydrologic Model Otak will use the hydrologic module in InfoSWMM to build a numerical hydrologic model of the study area, to simulate stormwater runoff, and to use an input into the hydraulic model of the combined sewer system in the study area.The following steps will be used to build the hydrologic model: a. Delineate detailed stormwater drainage basins (catchments) for combined sewer system north of the 17th Street Interceptor. b. Delineate less detailed stormwater drainage basins for areas south of the 17th Street Interceptor and contributing to the 17th Street Interceptor. City of Everett—North Everett Combined Sewer System Evaluation 3 otak K:\project\31500\31520\Contract\Amendll_NAreaFlooding\31520 Everett Sewer Amendment 1 Scope082510.doc 5