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HomeMy WebLinkAbout08 Stormwater Maintenance Plan_NWX Building I 1 of 3 BUILDING B at NORTHWEST CROSSING Storm Water Maintenance Plan November 2021 Building B at Northwest Crossing has a sub-surface storm drainage system that is being constructed to treat, store and convey storm and snowmelt runoff through the site to receiving waters, in accordance with City of Bozeman requirements. Typically, runoff collects in street gutters, enters the piped drainage system (storm drain) through curb inlets, drop inlets, manholes and culverts, and is stored primarily through underground detention and treatment basins. Building B storm drain system is owned and operated by NWX, LLC, and includes all storm drain and surface water treatment and conveyance facilities within the private property, including interior parking facilities. It does not include the road systems around the site perimeter, which include Twin Lakes Avenue and West Oak Street. These designations will be operated and maintained by City of Bozeman. NWX, LLC, shall maintain their components of the storm drainage system in accordance with this maintenance plan and with the applicable City of Bozeman regulations. NWX, LLC will maintain a written account of all maintenance and repair activities, such as a log book, for future reference. Every Month, May through October 1. Grass Swales (Lawns): Mow to maintain maximum grass height of 6 inches. Do not allow mulch to discharge or accumulate within 6 feet of the flowline. 2. Storm Drain Pipe and Culverts: Trim and control vegetation near open ends of pipes to prevent any significant restriction of flow. 3. Surface Detention/Treatment Facilities: Clean the area to keep free of leaves, grass clippings, excess vegetation and debris (paper, cardboard, plastic bags, etc.). If the basin contains water at the time of inspection, return later to clean the area. If ponded water persists over a dry period of a few days, remove cover from the outlet control structure and unclog the orifice (remove debris) to ensure unimpeded flow to the outlet pipe. If sediment sump is full to the orifice level, remove all sediment from the sump. Every Three Months, and Immediately after Major Storm or Snowmelt Events 1. Grass Swales (Lawns and Native Grasses): Walk along each swale to inspect. Clean as needed to keep free of silt, debris, excess vegetation, or any other material that impedes flow. Note areas of ponding and areas with dense weeds or sparse grass cover, and repair within 1 year (see “Repair Procedures” below). 2. Storm Water Manholes and Inlets: Inspect each inlet grate and grated manhole cover. Clean as needed to keep free of leaves, debris, excess vegetation, or any other material (paper, cardboard, plastic bags, etc.) that impedes flow. In winter, ice or snow may remain over inlet grate. If water ponds excessively and creates a problem during snowmelt events, chip ice to provide a drainage channel into the manhole or inlet. I I I 2 of 3 3. Surface Detention/Treatment Facilities: From November through April, continue inspection and cleaning procedures as stated under the “monthly inspection” category above, every three months and immediately after major storm or snowmelt events. 4. Underground Detention/Treatment Facilities: Open inspection port(s) in the isolator row, and view with a flashlight. If the geotextile fabric (at the bottom of the chamber) is obscured by sediment, measure depth to sediment level, and subtract from baseline depth (full depth to geotextile fabric). If the result (sediment level) is greater than 3 inches, schedule to have sediment removed, as stated under the “annual inspection” category below. Annually 1. Storm Drain Pipe (pipe terminating in manhole or inlet): Pull inlet grates and manhole covers as needed, and inspect each end of each pipe. If observed sediment level in the sump is above the lowest (discharge) pipe flowline, clean all sediment from the pipes and the manhole or inlet sump. 2. Storm Drain Pipe or Culvert (pipe terminating in open ditch or swale): If sediment has accumulated to 3 inches or more above the pipe flowline, remove sediment from the outlet pipe and ditch to provide free drainage and re-seed or sod the area of disturbance. If soil has eroded and un-vegetated rills are visible, re-seed or sod the area. If there are recurring problem areas, repair as stated in “Repair Procedures” below. 3. Underground Detention/Treatment Facilities: Remove manhole covers at each end of the isolator row. If the observed sediment level is above the lowest pipe flowline, clean all sediment from the pipes and the manhole or inlet sump. Also, look down the isolator row to see if sediment level is at or above the lower row of sidewall holes (about 3 inches above the geotextile bottom). Follow OSHA regulations for confined space entry, or use pole-mounted mirrors or cameras. If this observation or previous inspections through the inspection ports noted sediment levels greater than 3 inches, employ a pipe cleaning service to remove all sediment from the isolator row in accordance with the detention system manufacturer’s recommendations. Every 5 years 1. Surface Detention/Treatment Facilities: Sediment, windblown dust and thatch will build up over time and reduce the storage capacity. Storage capacity is the basin’s airspace volume between two elevations measured in the outlet control structure: the top of the overflow riser, and the flowline elevation of the discharge pipe. Employ a land surveyor or engineer to determine the airspace volume. If it is less than the minimum volume indicated on the approved construction drawings (record drawings), remove excess material and replace landscape materials to originally constructed conditions. Repair Procedures 1. Grass Swales with Dense Weeds or Sparse Grass Cover: Re-seed or sod the area, but first determine and address the source of the problem. Expand irrigation coverage, add 3 of 3 soil amendments, fertilize, etc., as needed to improve growth media and grass health, and to control weeds. 2. Grass Swales, Areas of Ponding: Cut sod and re-grade the area for consistent downgradient slope along the swale. Then re-seed or sod the area of disturbance. 3. Pipe Outlet Ditch, Excessive Sediment Accumulation: Cut sod, remove sediment, and re-grade the area to a consistent downgradient slope along the outlet ditch or swale. Extend the re-grading as far as needed to provide positive drainage. Then re-seed or re-sod the area of disturbance. 4. Pipe Outlet Ditch, Erosion: It is recommended to hire a Professional Engineer to address this issue. Another option is to cut sod and re-grade the area, install a permanent, non-degradable turf reinforcement mat (TRM) per the manufacturer’s recommendations, and re-seed the area of disturbance through the TRM per the manufacturer’s recommendations.