R2021-200 Resolution Authorizing Amendment No. 2 for Professional Engineering Services Agreement with Northwater Consulting for Lake Decatur Watershed Management Plan City Project 2019-38

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Lake Decatur Watershed Management Plan—Phase III Scope of Services&Budget Task 3.8: Lake Sedimentation and Bathymetry Program Considering the recent $92M investment in dredging, lake bathymetry and sediment accumulation monitoring is important to track the loss of lake storage capacity both spatially and temporally. It also serves as a means to estimate sediment yields and track progress towards reducing loading. It will also be a valuable resource for the City and community to have access to an updated and accurate map of the lake. The lake bathymetry and sediment accumulation monitoring strategy were developed during Phase II and the monitoring will commence in Phase III. The plan is to perform baseline lake-wide mapping in Phase III and establish between 6 and 11 formal cross-sections for monitoring every 2-3 years in subsequent phases. Lake-wide Bathymetric Baseline Mapping Northwater will perform the survey planning,execution,data post-processing,and production map(s)for internal and external use. It is estimated that the field survey will take between 5 and 8 weeks,depending on lake conditions. Survey will be performed using a motorized boat equipped with a XYLEM Sontek M9 Acoustic Doppler Profiler and GPS receiver. The M9 can profile bathymetry up to 40 m depth with 0.001 m depth resolution. HYPACK hydrological surveying software will be used to plan the survey to optimize results in key areas of interest and cross sections for follow-up monitoring. HYPACK will also be utilized to post-process the data and develop bathymetry profiles. When future monitoring is performed, the software can be utilized to calculate changes in lake storage (sediment accumulation). Lake level transducers will be installed during the entire duration of the survey to record accurate lake levels,this is important to properly correct the survey data for changing lake levels throughout the duration the survey period. High resolution and accurate surveying is necessary to monitor sediment accumulation. Upon completion of the survey and the data processing,Northwater will produce a professional lake bathymetry map, and establish the baseline conditions to enable efficient continued monitoring of lake bathymetry and sediment accumulation. Task 3.9: Policy& Ordinance Support Development, modifications and alterations along the shoreline of Lake Decatur and the contributing waterways impacts water quality, infrastructure, and the efficacy to cost-effectively manage and maintain the lake as a drinking water supply and recreational resource. This task continues from Phase II to support the City as needed related to policy and ordinance initiatives relevant to enhancing and protecting the lake and water resources. Specific tasks and activities are note defined but may include: • Policy and ordinance reviews. • Recommendations and proposed language for ordinances,policies etc. • Strategies and recommendations to integrate the Bluffs watershed plan into policies and ordinances. Task 3.10: General Consulting Due to the diverse range of tasks and activities in Phase III, a general consulting task is included to efficiently support the City as needed in case out of scope engineering or environmental consulting is requested. nor-filvvro'e'r