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HomeMy WebLinkAboutClay Township Addition - Drainage Summary 10.26.22115N 17th AVE, BEECH GROVE, IN 46107 // 317.780.1555 // CROSSROADENGINEERS.COM CrossRoad Engineers, PC 115 N 17th Avenue Beech Grove, IN 46107 Phone No.: (317) 780-1555 Fax No.: (317) 780-6525 Email: info@crossroadengineers.com Stormwater & Water Quality Calculations Drainage Summary Clay Township Government Center Building Addition 10701 N College Ave Carmel, Indiana 46032 Drainage Submittal: Oct 26, 2022 Prepared By: 03/10/23 Oct 26, 2022 // Clay Township Government Center Building Addition Calcs Section 1: Stormwater Calculations Summary Introduction This submittal of stormwater drainage calculations and construction plans are for the review and approval of the site design for the proposed Clay Township Government Center Building Addition. The site is located 10701 N College Avenue in Carmel, IN. Refer to Exhibit 1 for the Location Map and Vicinity Map. Pre-Development Conditions The existing project site consists of the existing government building and fire station on a ±2.01 acre lot located along the east side of College Avenue between 107th and 108th Streets. Existing storm infrastructure routes runoff from the site to the 108th Street 24-inch trunkline that presumably outlets into the dry detention basin to the north of the site. The ultimate receiving water is Haverstick Creek. See Exhibit 2 for the pre-developed watershed maps. The subject property is located within Flood Zone X (areas to be determined to be outside the 500-year floodplain) per Community Panel Number 18057C0209G of the Flood Insurance Rate Map of Hamilton County, Indiana and the City of Carmel (map dated November 19, 2014). There are no wetland areas present according to the National Wetlands Inventory Map. Post-Development Conditions This project includes the construction of a ±2,700 sq. ft. addition that includes a single bay garage. Infrastructure including asphalt drives and parking areas, storm structures, sanitary sewer laterals, and water quality measures will be constructed with the development. No new storm sewer is proposed, only new storm structures utilizing existing pipe. All stormwater shall be collected via an existing storm sewer network and routed to the existing storm sewer system which outlets into the 24- inch trunk line in 108th Street. See Exhibit 3 for the post-developed watershed maps. Per discussions with the City of Carmel, the site will provide water quality volume as necessary to treat the increase in runoff from the existing conditions of Government Center site. Since only a portion of the site will be disturbed, the net increase will be determined based on the disturbed area since undisturbed areas of the site will not be able to be routed to the proposed water quality basin. To meet these requirements, a raingarden will be designed and constructed as shown in the enclosed calculations and plans. The City is not requiring detention on this project. 03/10/23 Oct 26, 2022 // Clay Township Government Center Building Addition Calcs Stormwater System Design As discussed above, no detention will be required but water quality for increase in impervious area will be provided. The proposed storm sewer network was designed to accommodate a 10-year storm event and does not overburden the existing storm sewer network. The Rational Method was used to perform the storm sewer pipe network calculations. 03/10/23 Development Consultants Transportation &NORTHLOCATION MAP VICINITY MAP EXHIBIT 1: LOCATION AND VICINITY MAPS CITY OF CARMEL PROJECT LOCATION 108th Street 107th StreetCollege AvenuePROJECT LOCATION 03/10/23 03/10/23 Oct 26, 2022 // Clay Township Government Center Building Addition Calcs Section 2: Runoff Summary All runoff and water quality calculations were completed per the City of Carmel Stormwater Technical Standards (STSM) Manual for development sites less than or equal to 5 acres in size, with a contributing drainage area less than or equal to 50 acres and no depressional storage. The Rational Method was utilized to calculate the runoff for direct discharge areas. The Modified Rational Method was utilized to calculate the peak detention volume. EXISTING CONDITIONS: ATOT = 2.008 Ac.CAVG = 0.645 APERVIOUS = 0.636 Ac.C = 0.20 AIMPERVIOUS = 1.373 Ac.C = 0.85 TC = 5.0 min. (assumed) • I10 = 6.12 in/hr Q10 = (0.645)( 6.12)(2.008) = 7.93 cfs See Exhibit 2 for existing watershed map. PROPOSED CONDITIONS: ATOT = 2.008 Ac.CAVG = 0.678 APERVIOUS = 1.476 Ac.C = 0.20 AIMPERVIOUS = 0.533 Ac.C = 0.85 TC = 5.0 min. (assumed) • I10 = 6.12 in/hr Q10 = (0.678)( 6.12)(2.008) = 8.33 cfs 03/10/23 200-6 TABLE 201-2: Rainfall Intensities for Various Return Periods and Storm Durations Rainfall Intensity (Inches/Hour) Return Period (Years) Duration 2 5 10 25 50 100 5 Min. 4.63 5.43 6.12 7.17 8.09 9.12 10 Min. 3.95 4.63 5.22 6.12 6.90 7.78 15 Min. 3.44 4.03 4.55 5.33 6.01 6.77 20 Min. 3.04 3.56 4.02 4.71 5.31 5.99 30 Min. 2.46 2.88 3.25 3.81 4.29 4.84 40 Min. 2.05 2.41 2.71 3.18 3.59 4.05 50 Min. 1.76 2.06 2.33 2.73 3.07 3.47 1 Hr. 1.54 1.80 2.03 2.38 2.68 3.03 1.5 Hrs. 1.07 1.23 1.42 1.63 1.91 2.24 2 Hrs. 0.83 0.95 1.11 1.37 1.60 1.87 3 Hrs. 0.59 0.72 0.84 1.04 1.22 1.42 4 Hrs. 0.47 0.58 0.68 0.84 0.99 1.15 5 Hrs. 0.40 0.49 0.58 0.71 0.83 0.97 6 Hrs. 0.35 0.43 0.50 0.62 0.72 0.85 7 Hrs. 0.31 0.38 0.44 0.55 0.64 0.75 8 Hrs. 0.28 0.34 0.40 0.49 0.57 0.67 9 Hrs. 0.25 0.31 0.36 0.45 0.52 0.61 10 Hrs. 0.23 0.28 0.33 0.41 0.48 0.56 12 Hrs. 0.20 0.24 0.29 0.35 0.41 0.48 14 Hrs. 0.17 0.22 0.25 0.31 0.36 0.42 16 Hrs. 0.16 0.19 0.23 0.28 0.32 0.38 18 Hrs. 0.14 0.17 0.20 0.25 0.29 0.34 20 Hrs. 0.13 0.16 0.19 0.23 0.27 0.31 24 Hrs. 0.11 0.14 0.16 0.20 0.23 0.27 Source: Purdue,A.M., et. al., "Statistical Characteristics of Short Time Incremental Rainfall", Aug., 1992. (Values in this table are based on IDF equation and coefficients provided for Indianapolis, IN.) TABLE 201-3: Rainfall Depths for Various Return Periods 03/10/23 TABLE 201-1: Runoff Coefficients © for Use in the Rational Method TYPE OF SURFACE RUNOFF COEFFICIENT © Non-Urban Areas Bare earth 0.55 Steep grassed areas (slope 2:1) 0.60 Turf meadows 0.25 Forested areas 0.20 Cultivated fields 0.30 Urban Areas All watertight roof surfaces 0.90 Pavement 0.85 Gravel 0.85 Impervious soils (heavy) 0.55 Impervious soils (with turf) 0.45 Slightly pervious soil 0.25 Slightly pervious soil (with turf) 0.20 Moderately pervious soil 0.15 Moderately pervious soil (with turf)0.10 Business, Commercial & Industrial 0.85 Apartments & Townhouses 0.70 Schools & Churches 0.55 Single Family Lots < 10,000 SF 0.45 Lots < 12,000 SF 0.45 Lots < 17,000 SF 0.40 Lots > ½ acre 0.35 Park, Cemetery or Unimproved Area 0.30 200-5 03/10/23 EXHIBIT 2: PRE-DEVELOPED WATERSHED MAP Development Consultants Transportation &NORTH03/10/23 EXHIBIT 3: POST-DEVELOPED WATERSHED MAP Development Consultants Transportation &NORTH03/10/23 Oct 26, 2022 // Clay Township Government Center Building Addition Calcs Section 3: Water Quality Calculations Water Quality requirements for the project will be met utilizing a bioretention basin in the form of a rain garden. The proposed water quality measure provides sufficient removal of sediment and floatables prior to runoff entering the existing on-site storm system. In accordance with the City of Carmel Stormwater Manual, the treatment volume has been determined for 1 inch of rainfall in 24-hours utilizing the detention BMP sizing formula in Section 701.04 of the STSM. As mentioned in Section 1, the site will provide water quality volume as necessary to treat the increase in runoff from the disturbed area of Government Center site. The calculations below determine the net increase in WQV required based on the net increase in impervious area. See Exhibits 4 & 5 for the Water Quality details. Formulas utilized: WQ VOLUME REQUIRED DISTURBED AREA (ac) IMP % Rv WQv (ac-ft) WQv (cft) PRE 0.578 62% 0.605 0.029 1,270 POST 0.578 79% 0.764 0.037 1,605 Net increase 335 Per the above calculations, the proposed 454 cft rain garden meets the treatment volume requirements set forth by the City of Carmel. 03/10/23 EXHIBIT 4: WATER QUALITY WATERSHED MAP Development Consultants Transportation &NORTH03/10/23 EXHIBIT 5: RAIN GARDEN DETAILS Development Consultants Transportation & A B C B A RAIN GARDEN PLANTING ZONE DETAIL RAIN GARDEN SECTION DETAIL TYPICAL UNDERDRAIN DETAIL 03/10/23