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HomeMy WebLinkAboutWaiver Request to Direct Discharge Telephone 317.844.6777 www.cripe.biz 9339 Priority Way West Drive, Suite 100, Indianapolis, Indiana 46240 August 9, 2022 Technical Advisory Committee City of Carmel City Hall One Civic Square Carmel, IN 46032 317-571-2400 RE: Flora PUD on Spring Mill Road – Waiver Request Dear Committee members, On behalf of our client, Onyx + East, for the Flora PUD Improvements on Spring Mill Road, we are requesting a waiver per Section 302.02 of the Stormwater Technical Standards Manual as described below: Section 302.03 General Release Rates Cripe is requesting a waiver to direct discharge off-site via overland flow a combined runoff amount of 2.03 cfs during the 10-year storm event and 4.43 cfs in the 100-year storm event over the allowable release rates of 1.7 cfs and 5.1 cfs. These flows occur on the steep western slopes along the west property line that are designated “tree preservation” areas and occur as sheet flow over grass in areas designated as 2S and 3S – see the Post-Developed Drainage Exhibit attached. These slopes designated as “tree preservation” areas occur along the western border of property line. They are also considered unstable slopes and there is approximately 20 feet of vertical relief from the top of the bank to the floodplain along Williams Creek Regulated Drain. Excavating these slopes to drain toward the pond or site for capture and detention will jeopardize root systems of the trees to be preserved. Disturbing root systems may cause trees to die and could lead to further destabilization of slopes. The upper north-west slope, area 2S, will only discharge 0.33 cfs and 0.81 cfs in the 10- and 100-year storm events as sheet flow. The lower north-west slope, area 3S, will discharge 1.74 cfs and 4.27 cfs in the 10 - and 100-year storm events. However, there are two depressional pockets for storage where a good portion of this runoff will be detained naturally as it already does in the pre - developed condition. Even though the direct discharges areas cannot be conveyed to the wet detention pond, the pond has been sized for the entire developed area (inclusive of these areas) and off-site right-of-way along Spring Mill Road. The actual drainage area to the wet detention pond results in a 100-year WSE of 818.85 and the emergency spillway has been set to elevation 819.00. To meet adjusted allowable release rates after deducting direct discharge area 4S, the required storage volume is 5.21 acre-ft. With the emergency spillway higher than the 100-year WSE, the storage volume provided is 5.41 acre-ft. So, the wet detention pond will detain more than the 100 -year storm in an extreme event. Additionally, the direct discharges will not be concentrated flows and will be ultimately drain to the Williams Creek Regulated Drain. Lastly, area 3S, that presently drains to the depressional pockets is substantially less than the area that is draining there today. Please take these considerations in account and grant the requested waiver as all efforts to increase on-site detention have been made to avoid “tree preservation” areas. Sincerely, Stev Pierre, P.E. Director of Engineering Cripe 9339 Priority Way West Drive Suite 100 Indianapolis, IN 46240 317-844-6777 Attachment EMIIIIIIIIIIIIIIIIII I I I I I I I I I I WILLIAMS CREEKPOST-DEVELOPED CONDITIONS - WEIGHTED CN CLACULATIONS (On-Site Detained)DetainedArea 1STotal Area(Acres)HeF Area(acres)CN=74MmB2 Area(acres)CN=74CrA Area(acres)CN=80UcfA Area(acres)CN=80PerviousArea (acres)Impervious Area(acres) CN=98Paved / CN=100PondCN x AreasWeighted CN14.270.0030.6660.9290.0001.5982.668385.3289021.810.0310.3570.2030.0000.5911.218164.3499132.910.0610.8220.0670.0000.9501.958262.5459043.580.0730.8720.2230.0001.1692.409323.85691Pond0.890.0000.0000.0000.0000.0000.89089.000100TOTAL13.450.1682.7171.4220.0004.3089.1431225.07991Off-SiteDrainageArea 1RAreaPerviousImerviousWeighted(acres)UcfA (acres)CN(acres)CNCNGrass1.161.16800.009880Walk0.150800.159898Paved0.760800.769898Totals:2.071.16800.919888POST-DEVELOPED CONDITIONS - WEIGHTED CN CALCULATIONS - Site Direct DischargeAreaTotal Area(Acres)HeF Area(acres)CN=74MmB2 Area(acres)CN=74CrA Area(acres)CN=80UcfA Area(acres)CN=80PerviousArea (acres)Impervious Area(acres) CN=98Paved / CN=100PondCN x AreasWeighted CN2S0.160.1580.0000.0000.0000.1580.00011.692743S0.840.4740.3610.0000.0000.8350.00061.790744S0.480.1270.3550.0000.0000.4820.00035.66874TOTAL1.480.7590.7160.0000.0001.4750.000109.15074On-Site /Off-SiteArea (acres)(cfs)(cfs)Site & ROW17.01.705.10Dir. Disc. 4S0.48--Rev. Allowed-0.883.06Post-Developed15.520.883.10Q10AllowedQ100Allowed(cfs)(cfs)Q10ProposedQ100Proposed-0.82-2.000.842.45POST-DEVELOPED CONDITIONS - WEIGHTED CN CLACULATIONS (Off-Site ROW Detained)ALLOWABLE RELEASE RATE LESS DIRECT DISCHARGETITLE:PROJECT:OWNER:Date:SCALE:PROJECT NO:SHEET No.(317) 844-6777INDIANAPOLIS, INDIANA 462409339 PRIORITY WAY WEST DRIVE, SUITE 100www.cripe.bizREAL ESTATE SERVICESSURVEY +3D LASER SCANNINGENERGY + FACILITIESEQUIPMENT PLANNINGCIVIL ENGINEERINGARCHITECTUREARCHITECTURE + INTERIORSO:\2021\210114\20000\Calcs\Engr\Detention\_In Progress\Post Developed Conditions Exhibit.dwg, August 8, 2022 10:14 AM, JOE BYRNE, © Paul I. Cripe, Inc.1 of 108-09-22210114-200001"-150'317-559-0169INDIANAPOLIS, IN 462021828 CENTRAL AVENUEONYX + EASTFLORA PUD ON SPRING MILLPOST-DEVELOPEDCONDITIONSDRAINAGE EXHIBIT