HomeMy WebLinkAboutDrainage Reports - 01/29/1991STORM DRAINAGE REPORT
FOR
BUILDING ADDITIONS TO DUNN ELEMENTARY SCHOOL
PREPARED FOR
POUDRE SCHOOL DISTRICT R-1
PREPARED BY
STEWART & ASSOCIATES. INC.
January 14, 1992
3
STORM DRAINAGE
FOR
BUILDING ADDITIONS TO DUNN
SITE LOCATION
REPORT.
ELEMENTARY SCHOOL
Dunn Elementary School is located at the Southwest corner of West
Mulberry Street and South Washington Avenue. The net area of the -site is
4.95 acres. The site is located in the Old Town Drainage Basin.
EXISTING CONDITIONS _ddZc a��"'• fj rC
The existing site consists of 1.50 acres building and asphalt, 1.80
acres gravel playground and 1.65 acres lawn. There is a 15—inch City storm
drain running from west to east across the south side of the site. Located
at the west end of this 15—inch storm drain is an area inlet. This inlet
picks up runoff from the west playground area of the school. This west
playground consists of 1.741 acres gravel surface and 0.561 acres asphalt
surface. All roof drainage from the existing building is carried by an
internal piping system to the northeast corner of the building, from which
point it is piped underground to the City's storm drainage system in West
Mulberry Street. The lawn area lying east of the building drains on the
surface to the west gutter of South Washington Avenue, and then is carried
north by the gutter to an inlet into the West Mulberry storm drain. I
understand the existing 15—inch pipe running along the south side of the
site is undersized for the area it drains.
PROPOSED IMPROVEMENTS
Two building additions are to be constructed. The total amount of new
hard surface will be 6480 square feet. The additional runoff generated by
the new building additions is 483 cubic feet during the 2—year storm and
1584 cubic feet for the 100—year storm. This runoff will end up in the West
Mulberry Street storm drain system.
We plan to provide onsite detention to compensate for the additional
- runoff. The detention pond will be constructed in the west playground area
and will cut down the rate of flow into the existing 15—inch pipe along the
-isouth side of the site. We plan to restrict the release rate from the west
�j ,.pri $ aygroun area to th 2—year runoff of 1.8 c.f.s. The required detention
pond volume is calculated to be 7759 cubic feet, and the volume of the
proposed pond is 7765 cubic feet.
The planned detention pond capacity is quite a bit larger than the
additional volume of water caused by the present new building additions,
but there are possible future building additions. When, and if, these future
additions are constructed, detention will already be in place to compensate
for additional runoff.
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Storm Drainage Report/Dunn Elementary School
Page 2
The proposed release rate of 1.8 c.f.s. is considerably less than the
4.5 c.f.s. capacity of the existing grated inlet into the 15—inch pipe along
the south side of the site. Since the 15—inch pipe is undersized, the
restricted flow into it will be a benefit to the Myrtle Street storm drain
system.
We plan to leave the existing"area inlet with grated inlet in place
as it exists for any possible overflows, although with the new detention
pond, there will be very little runoff which reaches this inlet.
The attached plan shows the detention pond and its outflow pipe tied
into the existing area inlet. Also shown are the building additions. The
attached computation sheets are calculations of runoff, detention, release
rate, inlet capacity and pipe sizing.
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Phillip . Robinson, P.E. & L.S.
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COMPUTATIONS
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