HomeMy WebLinkAboutWestside Flats Stormwater Design Report 10-28-2020
INTRODUCTION
The proposed Westside Flats project is a residential multi-family development within an existing
subdivision. The project will include the extensions of May Fly Street and Dragon Fly Street east
into the project to tie into a proposed center drive aisle running north-south, which functions as a
drive access and parking area for the site. Nine buildings are proposed for construction along the
center drive aisle to create 132 – 2 bedroom apartments in a series of 12-plex and 18-plex buildings.
The center drive aisle has proposed perpendicular parking along both sides and has been designed
with a 1.5% cross slope (to the east) instead of a standard crowned design. The center drive aisle
also generally slopes from south to north at the two east-west connection aisles. The stormwater
will be collected in catch curb on the east side of the center drive aisle and perpendicular parking.
The stormwater will flow to a series of inlet curb chases. The curb chases will run to storm swales
that convey water to a set of retention ponds on the east side of the project.
Additional parking areas are also provided along the west side of the property. Generally, these
parking areas are graded to a curb chase on the north side of the parking area, where it is conveyed
to a stormwater retention pond. Stormwater from the parking area at the north end of the project,
in Phase 3, is graded to the north, where it will be conveyed through curb cuts to grass lined storm
swales. The stormwater is then conveyed to Retention Pond #4 and Retention Pond #11, which are
separated from the existing detention pond on the south side of West Babcock Street.
The existing detention pond on the north end of the site is not functioning properly and retains
water permanently. The existing detention pond will not be changed with construction of this
project. A portion of the site to the north will remain landscaped and not contribute any additional
runoff to the existing stormwater infrastructure.
STORM SEWER FACILITIES DESIGN
Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation. For each inlet,
the contributing area, weighted C factor, and time of concentration were calculated. These values
were input into Manning’s Equation to check capacity and flow characteristics for curbs chases,
storm swales, and curb gutters. All curbs are designed to maintain 0.15’ freeboard per C.O.B.
Design Manual Section IV.C.5. For the purposes of this report, each curb chase and swale was
named for the associated drainage areas. Curb chase and storm swale sizing calculations are
included in Appendix C.
RETENTION POND DESIGN
All ponds have been sized according to City of Bozeman Design Standards. The proposed retention
ponds are sized to capture the entire volume of the 10-year, 2-hour storm event. The site lies in an
area of known high groundwater. Appendix D contains groundwater monitoring data from Valley
West Subdivision and Norton Ranch Phase 4 Subdivision. Groundwater Monitoring Well #10 from
valley west lies ~00 feet from the Westide Flats project site, and had a minimum groundwater
depth of approximately 0.4-0.5 feet below ground surface. Groundwater Monitoring Well #2 from
Norton Ranch Phase 4 is approximately 700 feet from the project site and had minimum
groundwater depths of approximately 3 feet. This information was used to make the assumption
that the Seasonal High Ground Water Level (SHGWL) is approximately 1.5 to 2.0 feet below
existing grade on the site. Proposed retention ponds are designed to be less than 1.5’ below existing
grade.
Calculations used for sizing each pond can be found in Appendix c. Design pond capacities were
calculated using volume surfaces in AutoCAD Civil 3D.
Proposed Retention Pond 1
Retention Pond 1 is located along the western edge of the property in Phase 1 of the project. The
pond receives runoff from the proposed parking lot north of May Fly Street. The runoff is conveyed
via sheet flow and curb and gutter to an outlet chase in the northwest corner of the parking lot. The
required pond volume is 394 cubic feet, and the proposed pond volume is 403 cubic feet. The
proposed pond is 1.5’ deep with 4:1 side slopes. The bottom of Retention Pond 1 is less than 1.5’
below existing grade to avoid the Seasonal High Groundwater Level (SHGWL).
Proposed Retention Pond 2
Retention Pond 2 is located along the western edge of the property in Phase 3 of the project. The
pond receives runoff from the proposed parking lot in the northwest corner of Phase 2 of the project
and proposed landscaped areas in Phase 3. The runoff is conveyed via sheet flow and curb and
gutter to an outlet chase in the northwest corner of the parking lot. The required pond volume is
673 cubic feet, and the proposed pond volume is 843 cubic feet. The proposed pond is 1.5’ deep
with 4:1 side slopes. The bottom of Retention Pond 2 is less than 1.5’ below existing grade to
avoid the Seasonal High Groundwater Level (SHGWL).
Proposed Retention Pond 3
Retention Pond 3 is located along the western edge of the property in Phase 2 of the project. The
pond receives runoff from the proposed Building #6 and the landscaped area behind it. The runoff
will be conveyed via gutters, downspouts and landscaping swales to the pond. The required pond
volume is 339 cubic feet, and the proposed pond volume is 463 cubic feet. The proposed pond is
1.5’ deep with 4:1 side slopes. The bottom of Retention Pond 3 is less than 1.5’ below existing
grade to avoid the Seasonal High Groundwater Level (SHGWL).
Proposed Retention Pond 4
Retention Pond 4 is located on the northwestern part of the project in Phase 3 of the project. The
pond receives runoff from the proposed Building #9, the large parking lot in front of Building 9
and associated landscaping areas. The runoff will be conveyed via gutters, downspouts and curb
chases to the pond. The required pond volume is 2,162 cubic feet, and the proposed pond volume
is 2,487 cubic feet. The proposed pond is 1.5’ deep with 4:1 side slopes. The bottom of Retention
Pond 4 is less than 1.5’ below existing grade to avoid the Seasonal High Groundwater Level
(SHGWL).
Proposed Retention Pond 5
Retention Pond 5 is located along the eastern side of the property in Phase 3 of the project. The
pond receives runoff from the proposed drive aisle, proposed Building #7 and Building #8 and the
landscaped area behind it. The runoff will be conveyed via curb and gutter, a curb chase and a
storm swale to the retention pond. The required pond volume is 2,153 cubic feet, and the proposed
pond volume is 2,259 cubic feet. The proposed pond is 1.5’ deep with 4:1 side slopes. The bottom
of Retention Pond 5 is less than 1.5’ below existing grade to avoid the Seasonal High Groundwater
Level (SHGWL).
Proposed Retention Pond 6
Retention Pond 6 is located along the eastern side of the property in Phase 2 of the project. The
pond receives runoff from the proposed drive aisle, proposed Building #4 and the landscaped area
behind it, as well as a portion of Building #5. The runoff will be conveyed via curb and gutter, a
curb chase and a storm swale to the retention pond. The required pond volume is 1658 cubic feet,
and the proposed pond volume is 1,939 cubic feet. The proposed pond is 1.5’ deep with 4:1 side
slopes. The bottom of Retention Pond 6 is less than 1.5’ below existing grade to avoid the Seasonal
High Groundwater Level (SHGWL).
Proposed Retention Pond 7
Retention Pond 7 is located along the eastern side of the property in Phase 1 of the project. The
pond receives runoff from the proposed drive aisle, proposed Building #2 and the landscaped area
behind it, as well as a portion of Building #3. The runoff will be conveyed via curb and gutter, a
curb chase and a storm swale to the retention pond. The required pond volume is 1,686 cubic feet,
and the proposed pond volume is 2,079 cubic feet. The proposed pond is 1.5’ deep with 4:1 side
slopes. The bottom of Retention Pond 7 is less than 1.5’ below existing grade to avoid the Seasonal
High Groundwater Level (SHGWL).
Proposed Retention Pond 8
Retention Pond 8 is located along the western side of the property in Phase 1 of the project. The
pond receives runoff from the proposed drive access from May Fly Street, portion of proposed
Building #3 and the landscaped area behind it. The runoff will be conveyed via gutter, landscaping
swales to the retention pond. The required pond volume is 607 cubic feet, and the proposed pond
volume is 639 cubic feet. The proposed pond is 1.5’ deep with 4:1 side slopes. The bottom of
Retention Pond 8 is less than 1.5’ below existing grade to avoid the Seasonal High Groundwater
Level (SHGWL).
Proposed Retention Pond 9
Retention Pond 9 is located along the western side of the property in Phase 1 of the project. The
pond receives runoff from the proposed main drive aisle, the southernmost parking lot, proposed
Building #1 and the landscaped area behind it. The runoff will be conveyed via gutter, landscaping
swales to the retention pond. The required pond volume is 2,024 cubic feet, and the proposed pond
volume is 2,111 cubic feet. The proposed pond is 1.5’ deep with 4:1 side slopes. The bottom of
Retention Pond 9 is less than 1.5’ below existing grade to avoid the Seasonal High Groundwater
Level (SHGWL).
Proposed Retention Pond 10
Retention Pond 10 is located along the eastern side of the property in Phase 2 of the project. The
pond receives runoff from the proposed main drive aisle, the southernmost parking lot, proposed
Building #5 and the landscaped area behind it. The runoff will be conveyed via gutter, landscaping
swales to the retention pond. The required pond volume is 1,391 cubic feet, and the proposed pond
volume is 1,532 cubic feet. The proposed pond is 1.5’ deep with 4:1 side slopes. The bottom of
Retention Pond 10 is less than 1.5’ below existing grade to avoid the Seasonal High Groundwater
Level (SHGWL).
Proposed Retention Pond 11
Retention Pond 11 is located along the northern side of the main parking Aisle in Phase 3 of the
project. The pond receives runoff from the main parking aisle. The runoff will be conveyed via
gutter to the retention pond. The required pond volume is 418 cubic feet, and the proposed pond
volume is 593 cubic feet. The proposed pond is 1.5’ deep with 4:1 side slopes. The bottom of
Retention Pond 11 is less than 1.5’ below existing grade to avoid the Seasonal High Groundwater
Level (SHGWL).
Retention Pond Summary
Pond Drainage Area Pond Depth
(ft)
Required Storage
Volume (cf)
Provided Storage
Volume (cf)
1 1A & 1B 1.5 394 403
2 2A & 2B 1.5 673 843
3 3A 1.5 339 463
4 4A & 4B 1.5 2,162 2,487
5 5A & 5B 1.5 2,153 2,259
6 6A & 6B 1.5 1,658 1,939
7 7A, 7B, & 7C 1.5 1,686 2,079
8 8A & 8B 1.5 607 639
9 9A & 9C 1.5 2,024 2,111
10 10A & 10B 1.5 1,391 1,532
11 11A 1.5 418 593
G:\C&H\18\180820\Design Reports\Stormwater\Westside Flats Master Stormwater Plan\180820_ Westside Flats Stormwater Design Report
R1.Docx