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HomeMy WebLinkAbout3. 2011 Comprehensive Stormwater StudyMount Prospect Public Works Department 1 INTEROFFICE MEMORANDUM TO: VILLAGE MANAGER MICHAEL E. JANONIS FROM: DIRECTOR OF PUBLIC WORKS DATE: MARCH 5, 2012 SUBJ: 2011 COMPREHENSIVE STORMWATER STUDY FINAL REPORT Background This memorandum transmits a copy of the 2011 Comprehensive Stormwater Study final report as prepared by the Village's engineering consultant, Burns & McDonnell of Oak Brook, Illinois in conjunction with Village staff. This report culminates five (5) months of data collection, and analysis pertaining to the extensive flooding that beset the Village following torrential rain on July 23, 2011. The report also presents a series of recommendations intended to cost - effectively mitigate defined and recurrent flooding problems. At over 100 pages, the attached report is an expansive and compendious examination of the subject. However, the essence of the report can be distilled to the following central themes: The July 23, 2011 rainstorm was an extraordinary event. Analysis of official National Weather Service data indicates that rainfall received in the early morning hours on that day was the most intense ever recorded in the Chicago area. Technically, the storm is considered a "400 + - year" event; a storm that has a 0.25% statistical probability of occurring in any given year. Although technically feasible, constructing sewer improvements to convey a 400 -year storm is cost prohibitive. Conservative estimates place the financial cost of such a feat in excess of $200 million. Furthermore, even if the local sewer systems could collect and convey the storm, downstream conduits such as the Metropolitan Water Reclamation District of Greater Chicago ( MWRDGC) interceptor sewers, MWRDGC "Deep Tunnel" system, and local waterways do not have the capacity to accept the discharge. Consequently, substantial additional expenditures would be required to procure property and construct detention facilities to temporarily store the excess volume. Rainfall- induced flooding is not a new, or even recent, development in the Chicago area. The entire region has been plagued with flooding problems dating all the way back to initial settlements. In Mount Prospect, flooding issues predate incorporation of the Village and highlight each of the intervening decades until this day. Locally, tens of millions have been spent in efforts to mitigate the problem. Regionally, billions have been expended on projects such as the "Deep Tunnel ". Inevitably, all of these improvements have been rendered ineffective by storms that exceed their designs. The common thread uniting all of these efforts has been the unequivocal pursuit of a "build a bigger pipe" solution. That is, all of these Page 2 of 3 Transmittal of Comprehensive Stormwater Study Final Report March 5, 2012 improvements have featured the construction of larger pipes, or the installation of relief sewers and pumping stations, designed to handle the last big storm. Burns and McDonnell recommends, and staff concurs, that the most effective and cost - efficient go- forward stormwater management s trategy involves a combination of private sector and public sector improvements: ➢ Recommended private sector improvements include the disconnection of footing tile drains and the installation of overhead sewers in the 379 homes that reported sanitary backup. These improvements will eliminate the prevalent pathway for sanitary sewer water to enter basements and below -grade living spaces — the gravity drain. In many cases, these improvements will also eliminate the introduction of clear water into separate sanitary sewers. The estimated cost of this work is $9,475,000. ➢ Recommended public sector improvements include the installation of larger pipes, and /or the construction of relief sewers, at specified locations. These improvements will be constructed where engineering evaluation revealed severe and geographically concentrated flooding associated with a pronounced lack of local sewer conveyance capacity. Improvements will be designed to provide a 25 -year level of protection. The estimated cost of this work is $14,440,000. ➢ Four (4) drainage basins were identified that warrant additional engineering evaluation. The estimated cost for this work is $28,000. The total cost for all improvements recommended by the attached 2011 Comprehensive Stormwater Study is $23,943,000. Discussion Staff seeks Village Board direction on the following items: Is there support for Village participation in the cost of footing tile disconnection and overhead sewer installation on private property? a. If there is support, at what level? i. Should the Basin 14 model be applied? (50% grant, 50% simple- interest loan payable upon sale of the home, $18,000 cap per home.) ii. If the Basin 14 model is inadequate, how should the Village participate in the cost of private sector improvements? 2. Several property owners affected by the July 23, 2011 flood have subsequently installed overhead sewers and removed footing tile connections. Should these property owners be retroactively eligible to access Village financial aid? 3. Is there support for the construction of recommended public sector improvements? Page 3 of 3 Transmittal of Comprehensive Stormwater Study Final Report March 5, 2012 4. It is the opinion of staff that sufficient capacity does not presently exist in the water /sewer enterprise fund to facilitate recommended repairs. As such, serious consideration of this report and its findings inevitably will broach the issue of financing options. To this end, staff suggests that the following alternatives might be worthy of deliberation: Increase water /sewer rates The current water /sewer rate structure provides little capacity to address long -term capital improvements. Historically, our water /sewer rates have demonstrated the capability to fund only operational costs and satisfy Northwest Suburban Municipal Joint Action Water Agency (NWSMJAWA) commitments. However, recent rate increases by the City of Chicago, our water supplier, has severely curtailed the potential for funding improvements with user fees. Extend or Increase Sewer Maintenance Flat Fee The existing flat fee applied to water /sewer bills to finance the sewer improvements is set to expire in 2016. This fee typically generates approximately $800,000 per year. The sunset on this fee could be extended and /or the fee could be increased to fund recommended improvements. Utilize available capacity on the quarter -cent sales tax In the past, a number of flood relief and sewer improvement projects have been funded utilizing this mechanism. However, much of the revenue generated from this tax is currently being utilized to service existing debt. The Finance Department forecasts that capacity will start to become available from this revenue source in 2013. Typically, it generates $1.2 million per year. At the present time, approximately half of the revenue is applied to retire debt incurred to fund sewer improvements implemented following the 1990 flood study. The other half is utilized to fund flood control capital projects. We will continue to retire debt until 2019 when all loans will be repaid. Create a special service area The entire Village could be defined as a special service area. This classification would enable access to the property tax levy for the purpose of funding stormwater improvements. This solution has the potential to be an acceptable long -term solution if it can be structured to replace Special Service Area 5. Special Service Area 5 is due to expire in 2017. It was originally implemented to pay for the costs of bringing Lake Michigan water to the Village. Recommendation With your concurrence, staff asks that this matter be presented for the Village Board's consideration at the April 10, 2012 Committee of the Whole meeting. ) Sean P. Dorsey EXECUTIVE SUMMARY The Village of Mount Prospect (Village) contracted Burns & McDonnell Engineering Company, Inc. (Burns & McDonnell) to determine the reasons for the significant impacts to the Village that occurred with the July 23, 2011 storm event and identify potential improvements to the Village's stormwater management system that would help to diminish the impact that a future storm of this magnitude would generate. The Village provided storm flood survey information to Burns & McDonnell which was gathered from the residents through an extensive public outreach program. This outreach program included having Village employees and Burns & McDonnell representatives visiting residents' homes or contacting them via telephone to gather information regarding the magnitude and type of flooding experienced by each affected resident. Further, the intent of the study was to quantify and analyze the various drainage basins within the Village based on the findings of the flood surveys. Using the information gathered, areas that experienced the highest density of flooding were selected to be studied in greater detail using hydraulic modeling software. Basins that were not modeled were analyzed based on the types of flooding reported in each area. This study is intended to serve as a tool to assist the Village in evaluating and prioritizing capital improvements and in maintaining its storm and combined sewer system, as well as its separate sanitary sewer system. It is important to emphasize that the July 23r storm was an extreme rain event in terms of rainfall intensity and quantity. According to the hourly rainfall data obtained from the rain gauge at Chicago O'Hare International Airport, a total of 6.78 inches of rain fell during the peak 3 hours of the storm. Over a 24 hour period, a total of 8.27 inches of rain fell over the area. The storm can be classified as approximately a 400 -year storm (or a storm that has a probability of occurring once every 400 years) based on the 3 -hour peak intensity and volume of rainfall. Current industry standard is to design a typical storm sewer system to handle a 10 -year storm event using Bulletin 70 Rainfall Data. Much of the Mount Prospect storm and wastewater collection system was constructed prior to when these standards were implemented, and as such, may have a lower conveyance capacity. This circumstance is typical for older communities including the City of Chicago and most "inner ring" suburbs. Therefore, flooding of the local conveyance system during a storm with a magnitude like that of the July 23 2011 storm is unavoidable. A low end conservative estimate of the capital cost to mitigate the flooding from an event similar to the July 23r storm using public sector improvements only is in excess of $200,000,000. This cost would include the construction of relief storm sewers, combined relief sewers, underground detention, sanitary relief stations and other related improvements. Purchasing property to create additional detention would increase these costs further. Additional operation and maintenance costs would also be incurred by the Village. Even with all these improvements, protection against sewer backups from a storm event similar to the July 23 2011 event would not be as reliable as private property improvements such as installing overhead plumbing or backup prevention systems. The Village performs a very thorough maintenance and flood relief effort on an ongoing basis. Inspections of the stormwater system outfalls at detention basins and creeks are made on a twice a year Village of Mount Prospect ES - 1 February 2012 Stormwater Management Study Cost for mitigating flooding from seepage and private sector surface flooding in these basins was not determined as part of this study. It is recommended that each property owner contact a company that specializes in mitigating basement flooding caused by seepage. Based on the results of the flooding questionnaires, additional assessment and hydraulic modeling was performed for several areas where the most severe and geographically concentrated amount of flooding occurred. Based on this additional assessment improvements to upgrade the capacity of the existing combined and separate sewer systems were developed. Budgetary costs for implementing these public sector improvements are $14,440,000. Several separate sewer areas were also identified for additional assessment. These areas have a relatively low occurrence of reported flooding; however several localized areas of concentrated flooding were reported. A budgetary cost for completing the recommended evaluations is $28,000. Cost Summary The total budgetary cost to implement the recommendations presented in this study is $23,943,000. A breakdown of these costs is presented in the following table. Summary of Recommended Stormwater Management Improvements Village of Mount Prospect ES-3 February 2012 Stormwater Management Study Budgetary Area Type of Improvement Improvement Cost Haden Heights Public Sector Storm Sewer Improvements for 25- $1,500,000 Subdivision year Level of Service Hatlen Heights Public Sector Sanitary Sewer Improvements for $105,000 Subdivision 25 -year Level of Service Isabella Street Area Public Sector Combined Sewer Improvements for $12,500,000 25 -year Level of Service CS31 Combined Sewer Lonnquist Boulevard Public Sector Improvements for 25 -year Level of $200,000 Area Service Lormquist Boulevard Public Sector Adding Inlets and Televising $135,000 Area Village -wide Private Sector Disconnection of 379 Foundation $9,475,000 Drains with Overhead Plumbing Further Investigation Basin SS21, Basin SS41, Basin and Additional SS45 and Illinois American Basin $28,000 Analysis Total $23,943,000 Village of Mount Prospect ES-3 February 2012 Stormwater Management Study MAYOR Irvana K. Wilks TRUSTEES Paul Wm. Hoefert Arlene A. Juracek A. John Kom John J. Matuszak Steven S. Polit Michael A. Zadel Village of Mount Prospect 50 South Emerson Street, Mount Prospect, Illinois 60056 TO VIEW THE: VILLAGE MANAGER Michael E. Janonis VILLAGE CLERK M. Lisa Angell Phone: 847/392 -6000 Fax: 847/392 -6022 TDD: 847/392 -6064 www.moun tprospect.org 2011 COMPREHENSIVE STORMWATER STUDY IN ITS ENTIRETY GO TO: mountprospect.org/floodstudy