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Home My WebLink About8A - P.L./S.L. Watershed Report CITY COUNCIL AGENDA REPORT 16200 Eagle Creek Avenue S.E. Prior Lake, MN 55372-1714 MEETING DATE: March 15,2004 AGENDA #: SA PREPARED BY: Frank Boyles, City Manager AGENDA ITEM: PRIOR LAKE I SPRING LAKE WATERSHED REPORT - "OVERVIEW OF PRIOR LAKE VOLUME MANAGEMENT" (Shannon Lotthammer, Administrator) ) PRESENTATION: At the March 1, 2004 City Council Work Session, the City Council viewed a presentation prepared by Shannon Lotthammer, the Prior Lake I Spring Lake Watershed District Administrator, regarding the management of surface water on Upper and Lower Prior Lake. The presentation summarizes an outlet channel and lake volume study, identifies potential issues, the implications of those issues, as well as addresses opportunities or partnership between the Watershed and the City. In an effort to further communicate the report to the public, Shannon will be present to give a similar presentation for the public. The Council, of course, will have an opportunity to ask additional questions as a follow-up to their earlier work session. No Council action necessary or expected for this agenda item. www.cityofpriorlake.com 1:\COUNCIL\AGNRPTS\2004\WATE~~~.447.4230 / Fax 952.447.4245 Overview of Prior Lake Volume Management City of Prior Lake Planning Commission February 23, 2004 Purpose . Summarize the Outlet Channel and Lake Volume Management Study . Highlight Watershed District's efforts to address volume management/lake level issues . Discuss implication for future development within the watershed . Discuss opportunities for further partnership with City of Prior Lake 1 Background . Prior Lake is landlocked with natural overflow at about 916 feet · Historically had extreme lake level fluctuations . Flooding concerns . No wake ordinance at 904 feet . Outlet and channel first operated in 1983 . Frequency of operation has increased Outlet Channel and Lake Volume Management Study . Completed in May 2003 . Two main areas of focus: . Managing Prior Lake levels in the face of increasing development · Operating the Prior Lake Outlet Channel after 20 years of use, given increasing development . Process involved many stakeholders, including cities, Scott County, state agencies, townships 2 Study Com.ponents . Assessment of current and future conditions . Modeled future land use conditions and associated runoff and lake levels . Analysis of management options . Selection of strategies . Conceptual plan for volume management and addressing outlet system needs Future Conditions Model . Lake and outlet channel . Based on city and county Comprehensive Plans . Examined effects of outlet improvements, volume management and future development . Peak water levels of Prior Lake . Duration of lake levels above 904.0 . Used to evaluate volume management options 3 Example model result (1999 compared to year 2020) IfDl I I I I I I I I I I .1- Peak Lake Level - 1/ increases 1.6 feet under future (40 years) I / -.: Future increased . I -. '" : " discharge to the 1 Lake is 3 000 ac- ft , 1"" 'I .- . . 'IJ' -.Ii""" ':"11 - Duration over 904 = - 30 days existing conditions - 79 days future (40 years) conditions I I I I I I I I I I I IIII IiIiI g c o i- iii I 4! :- i ~1DIl IIIDl .. -------...........--- 0.. - - Existing . . . -Future Analysis of Managetnent Options . Evaluated strategies to address future conditions . Five strategies were selected: . Outlet Structure Modifications . Rilles for New Development . Land Management Program for Upstream Storage . Low home floodproofing, buy-out or emergency preparedness . Outlet Channel Improvements 4 Managem.ent Plan . Addition of up to 3,000 acre-feet of storage in the watershed . District rules . Land management (filter strips, wetlands) . Outlet system improvements . Oudet structure, channel . Low home floodproof111g, buy-out or emergency preparedness Im.plem.entation Status . New rules adopted in 2002, revised in 2003 . Pursuing wetland restoration and enhancement projects . Two scheduled for construction in 2004 . Preliminary design for Outlet Channel Improvements . Outlet improvements will be designed in 2004 . Constructed when channel improvements are completed 5 So how does this all apply to City of Prior Lake development? · Acquiring additional storage can be difficult and expensIve . Another option for managing volume is to create less runoff from new development . Study also evaluated effects of development density on future lake levels Developtnent Scenarios . "High Density Residential" . Extension of municipal services to the watershed southwest of Spring Lake . Development density of approx. 4 units per acre. . "Medium Density Residential" . More moderate density, approximately V2 acre lots . Scenarios compared to 1999 lake levels 6 Watershed District, City of Prior Lake & Annexation Areas Legend o PLSLINO c:J Qty of Prior Lake Annexation Ai'll.. by Yall/' D- O- -- lIB- D- O,... 0,... D- =:~ Modeling Results Even with outlet & channel improvements and more storage, development density will impact water levels, duration over 904.0 and flooding Scenario Peak Lake Lake Level Homes With Low Level (feet) Above 904.0 Floors Below (days) Peak Water Level 1999, Existing Land Use 904.9 30 20 (Current Conditions) 2020, High Density 905.2-905.4* 33-36* 23-24* Residential (4 units/acre) 2020, Medium Density 905.0-905.2* 29-33* 22-23* Residential (approx. Y2 acre lots) *Range depends on amount of upstream storage that is accomplished through wetland restoration, additional ponding (beyond current rule and ordinance requirements), and best management practices. 7 Options for Addressing Future Development Impacts . Decreased density . Below the 3.6 units/acre net density allowed by the R-L/MD designation . Identify and protect specific areas for open space and water storage . Encourage low-impact development techniques 8 Benefits . Improved water quality . Lower runoff volumes . Less potential for flooding . Fewer recreational impacts . Protection of groundwater recharge areas . Potential for more open space and wildlife habitat Partnership Opportunities . City-Watershed District partnership to increase storage and protect inflltration areas . Promote low-impact development . Homeowner and business-owner education to decrease runoff from current developments . Continued coordination on development review and long-term planning 9 Questions /Discussion Thank you! 10