HomeMy WebLinkAboutStormwater Maintenance PlanBozeman Gateway
Storm Water Maintenance Plan
April, 2020
The Bozeman Gateway development has a site-wide storm drainage system that is being
constructed in phases as needed to treat, store and convey storm and snowmelt runoff through
the site to receiving waters, in accordance with City of Bozeman requirements at the time of
each phase of development and in general accordance with the Bozeman Gateway PUD,
Preliminary Grading and Drainage Plan.
Typically, runoff collects in street gutters and in grass swales, enters the piped drainage system
(storm drain) through curb inlets, drop inlets, manholes and culverts, and discharges primarily
through underground detention and treatment basins to one of two discharge points: a culvert
under Fowler Avenue just south of Huffine Lane; and a 36” storm drain under Huffine Lane and
the Gallatin Valley Mall.
The Bozeman Gateway storm drain system is owned and operated by Mitchell Development
Group, LLC and includes all storm drain and surface water treatment and conveyance facilities
within the private property, including interior streets. It does not include the following systems
around the site perimeter, which are operated and maintained by others:
· Farmers Canal Pipeline: A pipeline on Bozeman Gateway property that crosses Fowler
Avenue just south of Technology Boulevard West, runs south along the east side of
Fowler Avenue, then east along the north side of West Garfield Street.
· West Side Storm Drain: A pipeline on Bozeman Gateway property that crosses Fowler
Avenue just north of Technology Boulevard West, then runs north along the east side of
Fowler Avenue and across Huffine Lane.
· Garfield Culverts: Culverts under West Garfield Street for Upper Cattail Creek (a.k.a.
Harmon Stream), and for an un-named drainage approximately 850 feet east of Harmon
Stream.
Mitchell Development Group, LLC shall maintain their components of the storm drainage system
in accordance with this maintenance plan and with the applicable City of Bozeman regulations.
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.
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
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.
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