2010/09/15 Council Agenda Packet

Scope of Services 1 Continued c. Delineate sewer basins for the study area, format data, and input into hydrologic model. d. Define land slopes, format data,and input into hydrologic model. e. Define land cover types (pervious/impervious), format data, and input into hydrologic model. Land use data will be defined for three (3) time frames as follows: i. June 9th land use is land cover that existed on June 9, 2010.The Providence Hospital tower is only partially constructed. ii. Current land use is the existing land use with current construction at the Providence Hospital complete (early 2011, approximately). iii. Future land use is the land cover that will exist when all phases of Providence Hospital are built-out. f. Precipitation Evaluation/Input for Hydrologic/Hydraulic Model MGS Engineering Consultants, Inc., as a subconsultant to Otak,will provide precipitation evaluation services. Refer to Scope of Services—MGS,Attachment A, for a detailed description of specific services provided. Precipitation collected by the City of Everett or Seattle Public Utilities (SPU), based on a quality review of precipitation records,will be used to perform long- term simulations using the SWMM model to identify capacity issues in the Everett combined sewer.The precipitation time series data will be reviewed for completeness and will be quality checked to ensure there are no erroneous precipitation values, and those values corrected.A flood-frequency analysis on flow time series will be computed using the SWMM model following the long- term simulation. Modify the existing spreadsheet tools for compatibility with output time series computed by SWMM.These spreadsheets compute flood- frequency statistics using a non-parametric plotting position approach. Develop storms using synthetic design hyetographs and regional precipitation-frequency statistics where the ten-year time series used to analyze the sewer capacity may not contain storm(s) that are sufficiently large enough to design mitigation alternatives. 2. Construct Existing Conditions Hydraulic Model for Flood Prone Area (Study Area) Otak will review and update the existing InfoSWMM hydraulics model.The following steps will be used to update the hydrologic model: a. Compare the sewer system data in the existing InfoSWMM model to (1) sewer data in the City's GIS database, (2) any as-builts or design drawings provided by the City, and (3) any structures surveyed by Otak. Where discrepancies exist, priority will given to the Otak survey data, as-built data, design drawing, then the City of Everett—North Everett Combined Sewer System Evaluation 4 otak K_\project\31500\31520\Contract\Amendll_NAreaFlooding\31520 Everett Sewer Amendment 1 Scope082510.doc 6