HomeMy WebLinkAbout15-1 - Drainage Report update 2020-03-031
Stormwater Management Update
The original Preliminary Grading and Drainage Plan provided hydrologic and hydraulic calculations
for sizing storm water conveyance, detention storage facilities and other storm drainage structures for
the entire Bozeman Gateway PUD. A modification to the Preliminary Grading and Drainage Plan
for Bozeman Gateway Development, was done in May 2018 and showed changes to the boundaries of
the minor basins as individual site plans were more defined. The drainage plan consists of two
systems, the major drainage system and the minor drainage system. The major drainage system shall
convey 100-year events without inundating building structures. The minor system is designed to
provide public convenience, to accommodate moderate, relatively frequent flows and to convey
runoff from the site.
Phase 5 lies within scope of the previously approved drainage submittal is composed of one 5.22-acre
basin called Minor Basin 1 shown in Figure 1 “Minor Drainage Basin Phase 5 Prelim Plat”. Minor
Basin 1 is the entirety of Phase 5 to the north of and including Technology Blvd West. The remainder
of Phase 5 south of Technology Blvd West is open space and will not require any detention as the
runoff coefficient will remain unchanged. The detention ponds located on Phase 5 are existing and
used to detain runoff from Fowler Avenue.
Runoff from Minor Basin 1 basin is treated and detained with a “StormTech” pre-fabricated
underground storage and infiltration facility. The City’s design standards require Low Impact
Development (LID) practices that infiltrate, evapotranspire or capture runoff, to the extent feasible,
for a specified design runoff event. This requirement is addressed using existing and proposed
StormTech underground detention and storage facilities. These systems combine infiltration and
runoff capture. The StormTech system is a widely accepted LID practice found to remove 80 percent
of total suspended solids from municipal storm runoff. It meets LEED criteria for storm water
treatment (SS6.2 water quality credit).
The facilities for this project provide over 6 inches of capture depth for infiltration and runoff volume
reduction, plus treatment of additional runoff by deposition as water slowly travels through the long,
linear rows of chambers and gravel base. The system solves the problem of runoff “short-circuiting”
from inlet to outlet that is inherent in surface detention ponds. Runoff from the first ½” of rainfall
will percolate through and fill a gravel filter before finding its way to a perforated low-flow outlet
pipe that discharges into the detention outlet structure. Additional runoff from larger storms can
bypass this discharge route, but only after storage depth reaches 2 feet (this correlates to about
64 percent of the total available storage volume). Although the detention calculations assume zero
infiltration, the existing alluvial (gravel/pit run) soils directly under the StormTech systems provide
excellent infiltration, which further improves runoff treatment. We believe this type of treatment not
only meets, but exceeds, the City’s current design standards.
The underground storage is designed using a conservative post development coefficient of 0.8 for the
entire basin. As the post development coefficient will be less than or equal to 0.8 for the lots in Phase
5, the previous calculations in the Preliminary Grading and Drainage Plan meets the required storm
design criteria for Phase 5. Like the previous phases 1-3, the final grading and drainage plans will be
developed once lots in Phase 5 reach the site plan stage.