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HomeMy WebLinkAbout008 Stormwater Report Nexus Point Stormwater Design Report Page 1 of 2 NEXUS POINT MASTER SITE PLAN 3747 SOUTH 19TH AVENUE, BOZEMAN, MT STORMWATER DESIGN REPORT A. Introduction The proposed development for the Nexus Point Master Site Plan will be constructed at 3747 South 19th Avenue in Bozeman, MT. The site will be developed in phases consisting of residential and office buildings and associated infrastructure and parking facilities. The infrastructure will be installed for the entire site during Phase 1 of development. The site will contain stormwater infrastructure including curb inlets throughout the site that drain to a stormwater detention basin with underground R Tank chamber system located underneath. Said basin and R Tank system is located in the northwest portion of the site. The following references were used in the preparation of this report: a. COB Design Standards and Specifications Policy, 2004 (DSSP). Addendum #6 b. COB Modifications to Montana Public Works Standard Specifications (MPWSS) B. Stormwater Management Stormwater from the proposed parking lot areas and roads will be graded to flow into various curb inlets and area drains throughout the site. An underground pipe network consisting of 15- inch to 21-inch diameter Advanced Drainage Systems (ADS) plastic corrugated pipe will carry stormwater to the proposed detention basin with underground R Tank chamber system located underneath. Stormwater will enter the R Tank system first. When the R Tank becomes full, it will overflow into the at-grade detention basin through a 24-inch by 36-inch area drain overflow structure. Stormwater run-off was calculated based on the Rational Method and the detention basin was sized per the City of Bozeman’s design standards for a storm intensity of 10-year frequency and a 2-hour duration. Additionally, the stormwater runoff from the first half inch of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation was calculated by multiplying 0.5 inches by the total impervious area within the site (per Bozeman DSSP II.A.4). The difference between the required detention volume and the volume of the first half inch of rainfall is detained within the detention basin. The volume of the first half inch of rainfall is retained within an underground R Tank chamber system located underneath the detention basin. A summary table of the detention system is provided in Table 1. Nexus Point Stormwater Design Report Page 2 of 2 Property Detention Basin R Tank Side slopes 4:1 (H:V) N/A Water design depth (ft) 1.0 2.17 Freeboard (ft) 1.0 N/A Bottom elevation (ft) 4949.33 4953.0 Storage volume (cf) 3,438 14,577 Number of chambers N/A 2,297 Table 1 Detention basin and R Tank summary C. Gutter and curb inlet adequacy check The gutters and curb inlets located in the streets that received surface flow from the site were checked for capacity. All gutters and standard Neenah Foundry R-3067-L 24-inch by 36-inch curb inlet grates are adequate to handle the runoff from a 25-year storm event. The curb inlets located in low points within the parking lots were check for capacity using a capacity chart provided by Neenah Foundry for the typical R-3067-L inlet grate. See Appendix B for more information. D. Pipe sizing All ADS storm drain pipes were sized to accommodate the 25-year storm event per Bozeman design standards. See Appendix C for more information. Appendices A. Stormwater basin calculations B. Gutter and curb inlet adequacy check C. Pipe sizing calculations D. Stormwater Maintenance Plan E. Drainage Exhibit 11 My Commission expires __________________ EXHIBIT A STORMWATER MAINTENANCE PLAN Association’s responsibility for routine inspection and maintenance. 1. Association to maintain and fund Operation and Maintenance of stormwater detention facilities located within Nexus Point Subdivision. 2. Keep the outlet and inlets of the storm water facilities free of leaves, rocks, and other debris. 2. The storm water detention facility is to be mowed regularly. During the summer, approximately once every two weeks, the grass is to be mowed and the cuttings are to be promptly removed and disposed of. Unless visibly tainted, dispose of lawn clippings in the same manner as yard waste. Otherwise, bag and take to a sanitary landfill. 3. Remove sediment by hand with a flat bottom shovel during the summer months whenever sediment covers vegetation. Have the grass cut short in that particular location so that the bed can be made as level as possible. 5. Re-sod damaged or maintained areas immediately, or use grass plugs from the adjacent up- slope area. 6. Inspect the facilities periodically, especially after heavy rains (preferably monthly and after each storm that delivers .5 inches of rainfall). 7. Inspect flow control outlet semi-annually. Clean outlet when soil and vegetation buildup interfere with flow introduction. 9. See that litter and other debris are removed in order to reduce the tendency to channel when trash accumulates.