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HomeMy WebLinkAbout1C Markley Lake - Council Work Session slidesPete Young Andy Brotzler Brian Welch Markley Lake Flooding July 15, 2019 Workshop Purpose The purpose of this meeting is to review the current water level at Markley Lake and discuss short-term and long- term options within historical context. Physical Characteristics •Closed basin –no natural outlet •water leaves via evaporation and infiltration •19 acres + 3.5 acres compensatory storage (2013) •Watershed of ~350 acres, roughly 60% developed land Map 2018 Aerial Photo Historical Lake Levels Compiled data from aerial photos: •1937 (Dust Bowl) –lake is reduced to small marsh •1957 –lake is partially refilled •1964 (drought) –multiple small ponds •1970-1990 –lake fills to current level •1997-2013 –lake level varies from year to year, but seems to be rising Map 1937 Aerial Photo (Dust Bowl) Map 2018 Aerial Photo 1957 Aerial Photo Map 1964 Aerial Photo Map 1970 Aerial Photo Map 1980 Aerial Photo Map 1990 Aerial Photo Map 1997 Aerial Photo Map 2000 Aerial Photo Map 2007 Aerial Photo Map 2013 Aerial Photo (April, before Excavation) Map 2015 Aerial Photo (Comp. Storage) Map 2018 Aerial Photo Historical Data Compiled data from: •DNR: sporadic lake level measurements since 1994 •City documents: lake levels related to 1998-99 pumping permit application and reporting •Recent level data from city loggers •Nat’l Weather Service (Chanhassen) precipitation data Historical Records 1998-’99 Flood Lake level range: 11 ft 1998-1999 Event Note the wet lead-in to winter City, WMO, Twpgetpumping permit from DNRCity, WMO, Twprenewpumping permit from DNRLake drops: ~0.03”/day w/o pump ~0.15”/day w/pump Historical Records 1998-’99 Flood 2018-’19 Flood Markley LakeFlood Study (2009) 2019 Event Very wet summer, but no flood Lake drops at ~0.03”/day, same as Prior Lake Very wet fall, deep snowpack, very wet spring Initial Interpretations •Markley Lake level has varied by 11 ft over the last 20 years •Conditions leading to high water: •Wet fall right before freeze-up •Wet winter and/or spring •Continued rain into summer •or: Single very wet month (2014) •Pumping lowers lake 3-5 times faster (0.15”/day) than normal (0.03”/day) •Theoretically, 2019 should have been worse than 1998 What’s Next? Near-term: •Work with affected city residents •Continue to maintain upstream water retention infrastructure •Work with Twp , WMO, DNR on short-term response(s) Long-term: •Work with Twp , WMO, DNR on long- term policy/mitigation •WMO ideally is the lead agency since watershed spans multiple jurisdictions Questions/Discussion