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HomeMy WebLinkAboutVariance Request 6-12-19 7325 Janes Avenue, Woodridge, I llinois 60517 | 630.724.9200 V3 | Visio, Vertere, Virtute … The Vision to Transform with Excellence May 17, 2019 Alex Jordan Engineer City of Carmel Engineering Department 1 Civic Square, First Floor Carmel, Indiana 46032 RE: Estates at Towne Meadow | Board of Public Works Waiver Request Dear Mr. Jordan V3 Companies, Ltd. (V3), on behalf of the developer PLP UHS Properties LLC is requesting waivers from the following section of the City of Carmel Stormwater Technical Standards Manual: 1. Section 302.08.(2) requires that for residential developments, the maximum planned depth of storm water stored shall not exceed four (4) feet. A variance is requested to allow us to store greater than four feet of water in the “Southwest Basin BMP.” The basin is an expansion of an existing detention facility that has transformed into a wetland. Coordination with the Indiana Department of Environmental Management (IDEM) made it clear that the proposed expansion would only be allowed if we leave the grading in the bottom of the wetland untouched. The extra depth within the southern portion of the existing wetland will create a small area that exceeds the maximum depth matching a conditions that even exists today. This conflict between IDEM requirements and the need to detain water during peak storms creates a hardship and a variance is requested. Since we cannot vary the amount of water entering the basin, nor change the wetland per IDEM requirement we are asking for a variance. 2. Section 501.01. requires that there should be no less than 2.5 feet of cover along any part of the pipe from final pavement elevation or final ground surface elevation to the top of the pipe. A variance is requested in regards to storm sewers that run between storm structures 604, 604A, 617, 618, and 601. For 617, 618, and 601, such a requirement creates a hardship by restricting our ability to adequately capture off site runoff from the northern property while also maintaining minimum pipe capacity. These structures are part of a continuous drainage chain which begins with street inlets and discharges into a basin called "Northwest Basin BMP." Due to the relative elevations of the road verses the bottom elevation of the Basin it is impossible to maintain cover as well as minimum discharge capacity. We have designed such that we maintain discharge velocity, maximizing cover, yet maintain a swale that is low enough to capture offsite runoff. Inlets Page 2 Alex Jordan 5/17/2019 7325 Janes Avenue, Woodridge, Illinois 60517 | 630.724.9200 V3 | Visio, Vertere, Virtute … The Vision to Transform with Excellence of the subject drainage structures must hold their current elevation to allow for offsite drainage encroaching our site from the property to the north. For structures 604 and 604A, the pipe is also controlled by a downstream outlet condition that does not allow us to install a pipe at a lower elevation. Despite running at minimum slopes, the cover is still less than 2.5’. With all inlets listed being located in rear yards and away from vehicular traffic, we request a reduction in minimum cover to no less than 1.5’. 3. Section 302.05 states “Runoff from all upstream tributary areas (off-site land areas) may be bypassed around the detention/retention facility without attenuation. Such runoff may also be routed through the detention/retention facility, provided that a separate, secondary outlet system is incorporated for the safe passage of such flows, i.e., not through the primary outlet of a detention facility.” Due to the existence of a regulated drain with existing detention running through the center of the site currently, capturing the offsite runoff and rerouting it around the site would create a severe hardship due to the inability to provide slope for such reroute and the negative impact on usability of land that would need to be set aside for the rerouted regulated drain. Additionally, the depth of the existing dual pipeline running generally perpendicular to the regulated drain easement restricts crossing locations. Therefore we request an allowance to be able to provide a combined multistage outlet for both offsite and onsite water to minimize pipeline crossings while still providing adequate detention for the proposed improvements. 4. Section 302.11 requires the design to provide emergency overflow facilities designed to handle 1.25 x the peak inflow from the entire contributing watershed. The proposed design provides 1.25 x the peak inflow after accounting for upstream detention instead of an undetained watershed. For this project the upstream contributing watershed is large and contains a significant amount of existing and proposed detention that will drastically reduce peak inflows flows making an undetained analysis excessive. Due to significant upstream detention and the excess storage provided in the onsite detention system we ask for a waiver from the standard in 302.11 to allow us to consider the detained inflow for upstream / offsite watersheds when sizing emergency overflows. 5. Section 303.07 requires the design to consider a fully clogged condition when providing emergency overflow routes. Two large parallel pipes connect the North and Southwest basin across Towne Meadow Drive. Due to the unlikely event that both pipes would be clogged, we requests that we Page 3 Alex Jordan 5/17/2019 7325 Janes Avenue, Woodridge, Illinois 60517 | 630.724.9200 V3 | Visio, Vertere, Virtute … The Vision to Transform with Excellence be allowed to not consider these pipes clogged in the emergency overflow analysis. Additional freeboard exists within the North basin and the proposed homes in the area are already elevated due to the localized flood routing analysis. Reducing the road to provide a lower overflow path would cause cover issues with infrastructure in the area. For these reasons we request a waiver that would eliminate the need to analyze this crossing using a fully clogged condition. It is requested that the above variances be granted by the Board of Public Works. V3 has, and will continue to coordinate closely with City Staff in determining appropriate and agreeable solutions to infrastructure in the City of Carmel. Should you have any questions or concerns, please do not hesitate to contact me. Sincerely, Jim Rinehart Project Manager V3 Companies