HomeMy WebLinkAbout19 - Design Report - Northwest Crossing - Storm Drainage L� Morrison
® Maierle
engineers surveyors planners scientists memo
TO: Matt Ekstrom
FROM: James Nickelson
DATE: August 13, 2019
JOB NO.: 5659.005
RE: Northwest Crossing Master Plan — Drainage Concept
CC:
❑Urgent ❑For Review ❑Please Comment ❑Please Reply ®For Your Use
Northwest Crossing consists of approximately 160 acres. The property is master planned to be
developed at various densities that will generate varying amounts of stormwater runoff. The
topography of the property naturally drains to the north and to the northwest. Post development
runoff will need to be controlled in order to comply with the City of Bozeman Design Standards.
Water quality provisions will also need to be incorporated into the site to comply with the Design
Standards.
As the master site plan does not fully define quantities or locations of impervious surfaces it is not
possible to complete a full stormwater analysis of the property. A conceptual volumetric approach
is utilized at this point to get a broad understanding of the drainage needs for the property.
It is anticipated that a combination of stormwater facilities will be utilized for the project.
Conveyance facilities will likely consist of drainage swales, natural drainage ways, and storm
sewers. Treatment and stormwater basins will likely consists of open ponds, underground storage
systems and be a combination of retention and detention facilities.
In order to arrive at an approximate land area that will be needed for stormwater basins two
approaches have been utilized. The first methodology uses the City of Bozeman Design Standard
water quality requirement to address the first 0.5 inches of rainfall and the second utilizes a
retention volume approach.
Utilizing the 0.5 inch water quality guideline results in a storage volume of 4.4 acre-feet and
considering a maximum depth of 1.5 feet and a factor that considers side slopes an approximate
area of 5.9 acres would be needed for stormwater storage.
Page 1
i 11 Morrison
Northwest Crossing Master Plan—Drainage Concept ® Maierle
Using a retention volume approach and an estimated average runoff coefficient of 0.5 (based on
Dense Residential classification from the City of Bozeman Design Standards) for the entire 160
acres results in a storage volume of 5.4 acre-feet which may require 7.2 acres of land for ponds.
Both of these methodologies provide an approximate area of required property within the 160
acre to address stormwater storage requirements. Depending on the type, location, and number
of storage facilities the land area could be lower than the areas estimated by these methods.
It is anticipated that stormwater storage facilities will be concentrated to the north and to the
northwest portion of the property based on topography and other considerations. In addition, a
number of smaller facilities will likely be located within the various development parcels in order
to reduce the magnitude of stormwater conveyance facilities and address concerns regarding
phasing of the development.
In summary it is anticipated that the 160 acre development will require approximately 6 to 7 acres
dedicated to stormwater storage. This value may change as the development is further defined
and that time the drainage plan will be updated.
Page 2