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HomeMy WebLinkAboutDrainage ReportPreliminary Drainage Report Meridian Pointe Lofts Phase 1 Prepared for: Prepared by: SNELLING ENGINEERING, LLC 1 12'fl Illinois Street, Suite 14 Carmel, IN 46032 Ph: 1317) 663 -3206 Fax: (317) 663- 08 www.snellingeng.cam Date: January 13, 2010 DRAINAGE SUMMARY General The existing 6.25 acre site is located between Old Meridian Street, Pennsylvania Street and Carmel Drive south of the Meijer detention basin within the city limits of Carmel. The site is presently occupied by a combination of lawn and woods with several small old structures. The site drains to the east with runoff exiting the site at a culvert under construction for Old Meridian Street. The culvert discharges into the Fertig Drain. The site is not located within a FEMA flood hazard area. Anderson Birkla Associates is proposing to construct a mixed -use residential and future office development on the site. Phase 1 of the project will be the residential portion only. The residential building will total approximately 235,000 SF and will have 276 surface parking spaces, including on- street parking. Pre Development Conditions Due to downstream restrictions, the 100 -year allowable discharge for projects within the Fertig Drain watershed is 0.25 cfs /acre. The City of Carmel also requires that the fully developed right of -way be detained in the post developed conditions. Pre development conditions were studied for two separate drainage basins: a northern area and a southern area. For the northern area, the pre developed right -of -way area flowing to the site is 0.8 acres. The northern Phase 1 area (not include areas proposed for direct discharge along Old Meridian Street) is 4.0 acres. Therefore, the 100 -year allowable discharge for the northern area is 1.2 cfs [(0.8 +4.0) x 0.25]. For the southern area, the pre developed right -of -way area flowing to the site is also 0.8 acres. The sorthern Phase 1 area (not include areas proposed for direct discharge along Old Meridian Street) is 0.7 acres. Therefore, the 100 -year allowable discharge for the northern area is 0.4 cfs [(0.8 +0.7) x 0.25]. Post Developed Conditions In the northern area, which is comprised mostly of the main apartment building site, a 55,126 CF Stormtech SC -740 underground detention system will be installed under the parking lot to control the peak release rate. Modeling of the underground detention system included volume of upstream pipes, bioretention areas, and parking lot storage up to 6- inches. The total volume of storage available is 69,000 CF. Due to the footprint of the underground system with respect to the outlet location, a 0.45 acre offsite area (two separate basins), which are parts of the rear of the adjacent Hamilton Crossing retail center to the west and a small are within the Meijer detention parcel to the north, must be routed through the detention system. However, the underground system was initially sized for only the onsite and right -of -way area. Calculations were performed to determine the outlet pipe size required to accommodate the complete watershed. For the initial onsite calculation, a 4.4 -inch orifice would be needed to control the peak discharge to 1.17 cfs, with parking lot storage occurring only within the proposed bioretention BMP areas. With the offsite runoff included using the minimum a 6 -inch orifice the underground detention system will fill with a 100 -year flood elevation of 855.26. Emergency overflow for the system will occur through the top of an open grate manhole near the northeast property corner along Old Meridian Street. This manhole will also serve as the outlet structure for the detention system. A 48 -inch diameter pipe at 0.22% upstream of this manhole will carry 1.25 times the peak 100 -year inflow rate of 46 cfs. The top of casting of this manhole will be set at the 100 -year flood elevation of the system. As required by the City of Carmel two water quality BMP's will be installed installed in series for the project. The BMP's will consist of six (6) bioretention areas and three (3) Stormtech Isolator Rows. To our knowledge, both BMP's have been pre- approved in the City of Carmel. In the southern area, which is comprised mostly of the main apartment building site, a 15,054 CF Stormtech SC -740 underground detention system will be installed under the future parking lot and connector driveway to control the peak release rate. Modeling of the underground detention system included volume of upstream pipes, bioretention areas and parking lot storage up to 6- inches. The total volume of storage available is 22,830 CF. A 0.25 acre offsite area from the adjacent Hamilton Crossing retail center to the west will also be routed through the detention system. Similar to the northern area, the underground system was initially sized for only the onsite and right -of -way area. Calculations were performed to determine the outlet pipe size required to accommodate the complete watershed. For the initial onsite calculation, a 2.4 -inch orifice would be needed to control the peak discharge to 0.4 cfs. With the offsite runoff included using the minimum a 6 -inch orifice the underground detention system will fill with a 100 -year flood elevation of 853.03. Emergency overflow for the system will occur through a 30 -inch diameter pipe at 0.24% that will carry 1.25 times the peak 100 -year inflow rate of 16 cfs. The outlet structure of the detention system will have an emergency overflow weir within the manhole that will allow the 100 -year flow into the 30 -inch pipe. As required by the City of Carmel two water quality BMP's will be installed installed in series for the project. The BMP's will consist of a bioretention areas and two (2) Stormtech Isolator Rows. To our knowledge, both BMP's have been pre- approved in the City of Carmel. A Drainage Area Map and calculations are included in the Appendix. Individual storm sewers will be designed at a later date as part of the development of the final construction plans. Phase 1 Drainage Summary: NORTH AREA: TOTAL DRAINAGE AREA (INCL. R/W) 100 YR. ALLOWABLE RELEASE RATE STORAGE VOLUME REQUIRED FOR ONSITE ONLY 4.8 ACRES 0.25 CFS /AC. (FERTIG DRAIN RESTRICTION) 1.2 CFS 64,982 CF (INCLUDING PIPES AND PARKING LOT VOLUME) 100 YR. PEAK RELEASE RATE FOR ONSITE ONLY 100 YR. PEAK RELEASE RATE W /OFFSITE 100 -YEAR ELEVATION W/ OFFSITE SOUTH AREA: TOTAL DRAINAGE AREA (INCL. R/W) 100 YR. ALLOWABLE RELEASE RATE STORAGE VOLUME REQUIRED FOR ONSITE ONLY 100 YR. PEAK RELEASE RATE FOR ONSITE ONLY 100 YR. PEAK RELEASE RATE W /OFFSITE 100 -YEAR ELEVATION W/ OFFSITE Conclusion 1.17 CFS (W/ 4.4 INCH ORIFICE) 2.13 CFS (W/ 6 INCH ORIFICE) 855.26 (64,070 CF STORAGE USED) 1.5 ACRES 0.25 CFS /AC. (FERTIG DRAIN RESTRICTION) 0.4 CFS 20,085 CF (INCLUDING PIPES AND PARKING LOT VOLUME) 0.36 CFS (W/ 2.4 INCH ORIFICE) 1.86 CFS (W/ 6 INCH ORIFICE) 853.03 (15,874 CF STORAGE USED) Final construction plans for the project will be generally designed in accordance with current City of Carmel and Hamilton County standards. The project is not anticipated to cause any adverse effects to upstream of downstream properties.