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HomeMy WebLinkAbout2.A. Surface Water Management Plan Review A f ROSE �'lOUN MM ; EXECUTIVE SUMMARY CITY COUNCIL City Council Work Session: April 11, 2012 AGENDA ITEM: Surface Water Management Plan Review 4GENDA S CTIION: PREPARED BY: Andrew J. Brotzler, PE, /AGENDA NO. Director of Public Works /City Enginee A�. ,4- ATTACHMENTS: Star Tribune Article APPROVED BY: Dal RECOMMENDED ACTION: Discussion ISSUE In 2011, a review of the City's storm water utility and trunk area charges was initiated with the Utility Commission based on discussions about the justification the rates that are within the City Council adopted fee list. As the rates are directly related to the City's storm water management policies as well as the corresponding capital improvement plan, the initial review and discussion with the Utility Commission led to a more in -depth discussion about the rationale and basis used in developing Rosemount's current policies, its Surface Water Management Plan, and its associated capital improvement plan. Subsequent discussions with the Utility Commission focused on reviewing the justification for the current policies and plan, as well as evaluating and discussing other potential alternatives that could be considered for use within the City. Since the policies and strategies adopted by the City directly impact the developments cost, future tax base, level of flood protection, and the design and cost the public drainage system, a determination to continue with the current plan or modify the direction of the Surface Water Management Plan needs to be considered now, before any update to future Utility or trunk area charges can be completed. Additionally, given the significance of the Surface Water Management Plan related to the development of property and with the current uptick in development, the Utility Commission requested a joint discussion with the Council on the topic to further review and consider the direction for surface water management in the City: The Utility Commissioner's will be in :attendance at the meeting to participate with the Council in a discussion on the direction of the Surface Water Management Plan. G: \ENGPROJ \ENG 0089 - Comprehensive Stormwater Management Plan \20120411 CWS Surface Water Management Plan.docx BACKGROUND 1998 Stormwater Plan Prior to 2002, the City's surface water management activities were based on a plan adopted in February 1998. The framework of the plan was based on the following: } • The City was a member of the developing Vermillion River Watershed Management Organization (VRWMO) and with the exception of two small areas, all water from the City was anticipated to be directed to the Vermillion River. • The plan anticipated multiple discharge locations from Rosemount through Lakeville, Nininger Township and Empire Township that ultimately would discharge to the Vermillion River. Following the adoption of this Surface Water Management Plan in 1998, two water resource management actions occurred that were viewed to have a significant impact on the ability of the City to implement the policies and strategies outlined within this 1998 Surface Water Management Plan. These actions included: • The Minnesota Department of Natural Resources and State Legislature evaluating and designating 45 miles of the Vermillion River as a trout stream. This resulted in new limitations and criteria for increasing the rate or adding any new volume of runoff to the Vermillion River. This also called into question the City's ability to construct and convey stormwater runoff to the Vermillion River as outlined in it's1998 plan. • In 2000, the Metropolitan Council Environmental Services (MCES) began evaluating the expansion of its wastewater treatment plant in Empire Township which is adjacent to the Vermillion River and had an existing discharge to the Vermillion River. As part of MCES plans to expand the facility, an increase in discharge of only 15 cfs to the Vermillion River was proposed. With the pending designation of the Vermillion River as a designated trout stream, this proposed 'increase in flow was not approved and MCES was required to develop a new outlet from the Empire facility north through Empire and Rosemount, discharging to the Mississippi River at an additional cost to MCES of over $63 Million. Given the City was anticipating constructing a system to add a new flow rate of over 300 cfs to the Vermillion in the future, getting approvals to construct this alternative began looking increasingly unlikely. Based on the above actions, the City re- evaluated its long term strategy for directing its storm water outlets to the Vermillion as outlined in its 1998 plan, and ultimately determined the construction of outfalls to the Vermillion River was no longer a viable plan. Development of Current Adopted Plan In the early 2000's, the City began evaluating other options for the ultimate management of surface water and developed an updated plan that has been in the process of being implemented since 2003. The plans strategy relies on infiltration of a 100 -year, 24 hour rainfall event combined with an overflow. This overflow system was incorporated into the plan to eliminate the need for other lateral conveyance systems, allow for more cost effective regional systems to be constructed, and accommodate runoff from upstream areas that: • Developed before infiltration was required (approximately 1/3 of the City). • Have infiltration systems that fail due to frozen ground conditions or deterioration of'infiltration capacity. • Cannot meet the infiltration standard due to non - infiltrating soils on the site. • Cannot hold the runoff from events exceeding the runoff volume from a 100 year 24 hour event without inundating homes with flood waters. 2 • Based'on this plan the critical event for the City to provide flood protection is a 100 -year, 10 -day' event. This event has the same probability of occurring as a 100 year, 24 hour event (rainfall of about 6 inches in 24 hours) but' creates approximately two times the 24 hour runoff volume over 10 days. • It should be noted that the infiltration component of this updated plan reflects one of the more rigorous and burdensome development standards in the metro area and is achievable due to the granular soils present throughout much of the City. When fully implemented, it also allows for no discharge from Rosemount or the 100 year, 24 hour event compared to a previous planned discharge of 328 cfs. The key components of the City's current Surface Water Management Plan are as follows: • New development is required to infiltrate the runoff from a 100 : year,24 -hour event without discharge. • • The post - development allowable peak overflow discharge rate is .05 cfs/acre'. • On average, for a 100,acre development, 7 acres (7 %) will need to be utilized to provide surface 'water management and infiltration. , • The maximum overflow rate for the system wiltrange from 65 cfs near the Bloomfield Pond increasing to 870 cfs at the outfall into the Mississippi River • This compares to a typical peak flow rate of 25,000 to 30,000' cfs during a typical year when the - City would not likely experience an overflow, to the 100 year flow rate of the Mississippi river that exceeds 150,000 cfs. • The estimated cost for the trunk overflow system to the Mississippi River is $28.5 million, and the -- cost for easements and right- of- way`acquisition is $1.5 million. Immediate Need for Plan Implementation in Selected Areas Through discussions with the Utility Commission, alternatives to the current policies for stormwater management along with changes to or elimination of the overflow system to the Mississippi River were identified and evaluated. As part of this evaluation, it was further highlighted that there is an urgency to complete selected components of the current plan, specifically noted were the following three pond areas which do not yet have outlets constructed. Should the critical 100 -year, 10 -day event occur today, the structures noted below would be inundated. • Shannon Pond (Shannon Parkway and CR 46) 50 to 57 homes /properties • Wachter Lake (West side of TH 3 south of Canada Avenue) 94 to 102 homes /properties • Bloomfield Pond (North side of CR 42 east of Auburn Avenue) 130 to 135 homes /properties Discussion Regarding Alternatives to Current Plan • Alternatives to the current Surface Water Management Plan that have been evaluated include the following • Redirect outlet for Shannon Pond to Empire Township and Vermillion River Watershed Joint Powers Organization (VRWJPO). • Allow no future outlet or storm water overflow from the UMore Park Property (2,91 1 acres) or Flint Hills property north of Bonaire Path (approximately 1,870 acres). o Not certain this is viable option in all areas. The 'development of mining on the UMore Park property may accommodate storm water flow from the west but east properties already overflow to the east (approximately 1,000 acres). 3 o Storm water utility rates and trunk area charges are based in part on the estimated costs to construct storm water management facilities. Another factor for the calculation of trunk area charges is the remaining undeveloped area to include in the calculation of the trunk area rates. Including Or excluding these two large property owners and the associated land areas that have a significant impact on the trunk area rate. • Develop overflow system from Akron Avenue east approximately 3,300 feet to daylight and discharge to surface flow in agriculture area, but provide no further downstream overflow inlet. Acquire property, remove threatened structures and build regional ponds to accommodate this overflow. o For critical event, an estimated 1,080 acres will be inundated including some developed properties. o At $60,000 per acre for easement and $2 per cubic yard for excavation, the estimated cost associated with this option is $82 million. o Additional financial considerations may include the" cost to acquire impacted structures and ' future lost development opportunities and associated tax revenue. Challenges Associated with Implementation of Current or Modified Plan • Schedule — immediate need for plan implementation , • Permits /approvals of plan modifications • Changes to development requirements /cost to developers both in fees and impact to developable acreage. • Changing weather patterns — attached is an article from the Star Tribune about the potential effects of changing weather patterns. SUMMARY At this time, the first itemato address are the goals, policies and infrastructure plans in the current adopted plan and determine if the City will continue forward with the current plan or modify the direction of the plan for surface water management. Should Council determine to continue with the goals, policies and infrastructure plans in the current adopted plan, the following actions should be considered: • ' Continue implementation of improvement projects of the plan. „ • Proceed with the preparation and submittal of a permit for an overflow to the Mississippi River. • Evaluate storm water utility rates arid area charges based on estimated costs and land areas to be assessed (Include or not include UMore Park and /or Flint Hills properties). - Should Council determine to pursue theidentified alternatives to the current adopted plan, the following actions should be considered: • Contact Empire Township and Vermillion River Joint.Powers Organization to evaluate proposed overflow discharges to adjacent communities and ultimately the Vermillion River: Approval from these agencies will be necessary to this action. • Initiate and complete an update to the City Surface Water Management Plan to address new surface water management goals, strategies and policies. This will be necessary to develop new standards for the development of property, identify and update trunk storm system improvements 4 - fi R t to include with development, and identify downstream land areas that easements will be required from for the development of additional storage areas. r � 5 March 23.2012 Wrong as rain, planners try again BILL McAULIFFE, Star Tribune Intense rainfalls are getting bigger and more frequent,, causing local governments, engineers and landowners to rethink whether sewer systems and other drainage features are up to their tasks. The storm water filtration pond near Cedar Lake in Minneapolis, for instance, needed to be cleaned out six years after it was built, instead of the 25 years designers expected. "I'm just a guy at the end of the rainfall who has to deal with it," said Brian Wilson, a storm `water policy analyst with the Minnesota Pollution Control Agency. "It really got my attention because these big storms seem to just be rolling through more and more frequently." Researchers point directly at a warming climate as the cause. Warmer air holds more water vapor, and a longer storm season means intense storms can occur more frequently in what once were regarded as the off - seasons, said JayLawrimore, a climate scientist with the center. As more northerly latitudes have warmed,' the storm- producing clashes between warm and cold air masses have moved northward as well, a key reason why the Upper Midwest has seen a steep increase in extreme precipitation, Lawrimore said The Upper Midwest saw a 31 percent increase in "intense" rainfalls -- the statistical 1 percent events -- from 1958 to 2007, over previous decades, according to the National Climactic Data Center. That was the second - highest increase among eight U.S. regions, including Alaska and Hawaii. New England and the Northeast saw a 67 percent increase. Overall, intense doses of precipitation have become more frequent and more intense in recent decades than at any other time in the historical record and account for a larger percentage of total precipitation, according to a study by the U.S. Climate Change Science Program. "The atmosphere is laden with more water vapor than it has been historically," University of Minnesota Extension climatologist Mark Seeley said. "I think we've transitioned into conditions where convection [severe storms] is the more dominant form of precipitation. We've seen flash floods in recent years in October, for crying out loud, when we should be seeing mild polar fronts across the state." Any explanation for the increase, other than climate change, would be "a stretch," Seeley said. �. How much water in big storms? Across Minnesota there were seven 1 percent storms in the 1940s and 17 in the 2000s, though some of that apparent increase might be attributable to an increase in observers across the state, according to DNR climatologist Greg Spoden. In the Twin Cities, where a 1 percent storm brings 6 inches of rain in 24 hours, only two are on record: a 7.28 - incher on Aug. 30, 1977, and a9.15- incher on July 23, 1987. But what amount of rainfall occurs 1 percent of the time these days? Is it still 6 inches, as it has been since a federal determination in 1961? Or is it 7 inches or 8 inches? It matters, Wilson said, because it's a figure that determines how public infrastructure -- dams, sewer systems, -- gets designed. Wilson has spearheaded an effort to determine more exactly what a 1 percent storm might be these days: With $210,000 from the Environmental Trust Fund and the Minnesota Department of Transportation, Minnesota has joined 10 other Midwestern states in a $3 million study expected to result in new federal figures next year. Those aren't the only public agencies reassessing rainfall risks. The Minnehaha Creek Watershed District has also launched an effort with local communities to develop new standards not only for } A � floodwater management and flood mitigation, but for how new developments might be designed and located. That might include everything from the size of new sewerpipes to restrictions on impervious pavement. The issues also extend to water quality and habitat, Wilson noted, since heavy rain often carries street pollutants directly into streams grid raises their temperature dramatically. Latham Stack, a consultant on the Minnehaha:Creek project, `saidcommunities ought to look for a variety of strategies, not just an expensive upgrade of a storm sewer system. "If a 50- or 100 -year storm happens every 10 years, that has some pretty major impacts on the ability of communities to absorb damages," Stack said. Drought, floods can coexist Meanwhile, recent weather in Minnesota is hinting that a drought that has persisted for about seven months across much of the state -- .longer in the Arrowhead -- might be loosening its grip: But those kinds of variations don't mean the longer -term trends of increases in intense rainfall might also reverse quickly, Seeley said; excessive rain has been overpowering drought trends in recent decades. E' At the same time, the classic climate warming scenario dictates that both excess rain and drought } are likely simultaneously, as heat evaporates moisture from the land, then dumps it elsewhere in spotty, unpredictable, summer -storm patterns. "It's all going to be about winners and losers," Seeley said. Bill McAuliffe • 612 -673 -7646 }` i r t x C ¢ _ } q a k 1 — t e{> t � a ` Y � 1 a � } . t 1