Res-2021-076 FCZD - Laserfiche WebLink

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Attachment 1 to Amendment2 - $cope of V\lork Reecer Creek Flood plain Mapping and Levee Certification Support Services Design and Gertification Services Scope of Work Aspect Gonsulting LLC Prepared January th, 2021 Background Aspect Consulting, LLC (Aspect) has developed this Amendment 2 Scope of Work to provide additional consulting services to the City of Ellensburg for the Reecer Creek Ftoodplain Mapping and Levee Certification Support Seryices project (project). Pursuing certification of the Reecer Creek levee system (with a reduced freeboard of 2 ft in some areas)will require additional analyses and technical reports not developed during prior work: a. Hydraulic assessment report to support the certification and reduced freeboard request. An engineering analyses to show adequate protection with reduced freeboard must submitted to support a levee freeboard reduction request. The analysis must evaluate uncertainty in base flood elevation and include, but not necessarily be limited to an assessment of statistical confidence limits of the 10O-year discharge; changes in stage-discharge relationships; and the sources, potential, and magnitude of debris, sediment, and ice accumulation. lt must be also shown that the levee will remain structurally stable during the base flood when additional loading considerations are imposed. Aspect assumes that the County's modeling consultant (WSE) would use their model for the overtopping analysis and to provide scour velocities for the high 1ow condition. Aspect's engineers would complete the stability assessment and produce the hydraulic report. b. Embankment protection. Documentation is required to show that no significant erosion of levee embankments will occur during the base flood (also for the "worst case scenario" in areas where we want the reduced levee freeboard). This includes assessing erosion risk due to water velocities, debris, ice, etc., and needs to account for the duration of the erosive conditions as well as the embankment materials and susceptibility of different areas of the levee to erosion (for instance at bends, or areas of deepest/fastest flowing water). Aspect engineers will complete this assessment, focusing on several key risk areas. Scour was considered during the prior design work but will be reassessed given the higher flows that need to be reviewed. c. Embankment and foundation stability. An engineering evaluation of seepage issues is required to document that the levee embankment and foundation is designed/built to be stable. Aspect engineers will complete this assessment using information and data from prior geotechnical reports, which is not expected to be a significant effort. Reecer Creek Floodplain Mapping and Levee Certification Support Services Project i. I page