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Nexus Point Stormwater Design Report
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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
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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
G:\MADISON ENGINEERING\PROJECTS\2018\18-127 Nexus Point\Storm\STORMWATER MAINTENANCE PLAN.doc
STORMWATER MAINTENANCE PLAN
Nexus Point
Owner’s responsibility for routine inspection and maintenance
1. Keep the inlets of the facilities free of leaves, rocks, and other debris.
2. The storm water basin 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. Re-sod damaged or maintained areas immediately, or use grass plugs from the
adjacent up-slope area.
4. See that litter and other debris are removed from inlets, swales, and vegetated and
paved areas.
5. Maintenance of the underground systems are as follows:
• Minimum required maintenance includes a quarterly inspection during the
first year of operation and a yearly inspection thereafter. Utilize inspection
ports for inspections.
• The inspection ports can be used to pump water into the system and re-
suspend accumulated sediment so that is may be pumped out. Flush and
pump as inspections deem necessary.
• Utilize the adjacent upstream storm sewer manhole or cleanout for removal
(vacuuming) of debris from the end of the storm drain at the connection
point with the underground chamber.
6. Owner to maintain and fund Operation and Maintenance of stormwater facilities.
_______________________________
Steve Moore, Owner’s Representative