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A P P E N O= X
1. HISTORIC RUNOFF .......................................... 1
2. DEVELOPED RUNOFF ......................................... 4
3. ORIFICE PLATE CALCULATIONS ............................... 9
4. MASS DIAGRAMS ............................................ 11
5. ORIFICE DISCHARGE BALANCE SHEET .......................... 15
6. INLET CALCULATIONS ....................................... 16
7. STORM SEWER PIPE CALCULATIONS ............................ 17
8. EROSION CONTROL CALCULATIONS ............................. 20
East Drake TesfWce P.U.D.
Drainage Repol_
page 4
V. EROSION CONTROL
It is anticipated that the proposed improvements for the East
Drake Terrace P.U.D. will be completed by the Spring of 1994. Lot
grading is to begin after approval, with construction of utilities,
drainage structures and appurtenances, and parking areas to follow.
iTemporary erosion control measures consist of gravel filters
to be placed around the three inlets as shown on the attached
Drainage Plan. The filters should be monitored by the contractor
during construction and maintained as necessary.
Permanent vegetative erosion control will be used in
conjunction with landscaping. Permanent revegetation of areas
disturbed by construction will be the responsibility of the
contractor. Seed will be applied by drilling and protected by
crimped straw mulch. Straw shall be applied at a rate of two tons
�. per acre (minimum).
e
1
17
i.
East Drake Te�ce P.U.D.
Drainage Repor:
page 3
IV PROPOSED IMPROVEMENTS (Cont.)
When developed, the runoff from the site will be directed to
inlets located near the center of each parking area. The 100 yr.
storm will be detained to the historic 2 yr. storm runoff rate on
site within the parking areas. Each inlet will have an orifice
plate installed over the discharge pipe to limit the discharge
rate. Storm water will then be piped from these inlets into the
existing 48" C.M.P. storm sewer pipe located on the site.
A portion of the site, noted as Area D-4 on the plans, will
have developed runoff surface flowing directly off site. Due to
the smallness of the site and grading contraints to provide
detention within the parking areas, it will not be possible to
divert runoff from Area D-4 into the detention basins. The net
result will be a decrease in the surface flow off site from 3.21
cfs (Historic) to 1.04 cfs (Developed) in the 100 year storm. The
orifices used in the inlets were minimized to 2 1/2 inch diameter
in order to limit developed 100 year discharge to the historic 2
year rate. The capacity of the basins is fully utilized with every
effort being made to reduce the 100 year discharge. Therefore, due
to the reduction in off -site flow, the nominal amount of this flow
and the fact that the three basins together release less developed
100 year flow than the 2 year historic rate, we request a variance
from the city standards on the 100 year release rate.
Also, due to the grading contraints to provide detention in
the parking areas and the fact that the existing grade is higher
than the surrounding gutter lines, it will not be possible to
intercept any of the street flow to detain on -site. Therefore, we
request a variance from the city standard to detain the run off
from the crown of the streets adjacent to site.
Without a complete analysis of the storm sewer system
contributing to the 48" C.M.P. pipe passing through this property,
it is impossible to determine whether the pipe is flowing under
pressure at this point. We don't believe that it is under pressure
and feel that it won't present a problem in the future. We have,
however, provided overflow structures to divert the flow in case
it does overflow.
East Drake Te*ce P.U.D.
Drainage Repor:
page 2
7
III. HISTORIC DRAINAGE
All of the Historical Runoff from the site flows to the east
through the apartment complex and northeast to Marquette Street.
It then makes its way into a detention basin originally designed
i for the overall Cottonwood P.U.D. The basin was designed to detain
the 50 yr. return period storm. No off -site drainage flows across
this site.
The Rational Method was used to predict the peak runoff. The
weighted runoff coefficient for pasture land (C = 0.26) was used
to obtain the 2 and 100 year peak storm water flows at the
concentration point of the site. Table A shows the results of peak
historical runoff calculations.
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IV. PROPOSED IMPROVEMENTS
�• The development of the site would include 7 general office
buildings, approximately 41,000 square feet of asphalt -paved
concrete -curbed parking with islands, reconstructing the 24 ft.
wide access drive, and landscaping. Existing water and sanitary
sewer structures and piping will be utilized to the extent possible
with new water and sanitary sewer constructed as needed to service
the buildings (see plans for locations of proposed service
connections). Any existing sanitary sewer manholes not utilized
in the new development will be removed. Existing water and
sanitary sewer piping not used will be abandoned in place except
where such piping is under a proposed building in which case the
piping shall be removed.
Nhern Engineering Seri: )eS, Inc.
FINAL DRAINAGE REPORT
for
EAST E)FiAi, KE TERRACE P _ U
Fort Collins, Colorado
December, 1993
I. GENERAL
This report summarizes the results of a storm drainage study
conducted for the proposed development of the East Drake Terrace
P.U.D. to be located in the Southeast quarter of Section 24,
Township 7 North, Range 69 West of the 6th Prime Meridian, in the
City of Fort Collins, County of Larimer, Colorado. Methods
outlined in the City of Fort Collins, Colorado Storm Drainage
Design Criteria and Construction Standards Manual were used in the
calculations for the storm water runoff portion of this study. The
Rational Method was used to estimate peak storm water runoff from
the site. Summary calculations and other supporting material are
contained in an appendix to this report.
II. SITE DESCRIPTION
The proposed East Drake Terrace P.U.D. is a 7 building general
office complex, being 1.84 acres in size. It is located on Tract
B of the Cottonwood P.U.D. The site is bounded on the north by
Marquette Drive, on the east by a portion of Cottonwood
Subdivision, on the south by Drake Road, and on the west by Stover
Street. The existing site is an open field covered by medium size
native grass on a one foot depth of silty clay with an existing 24
ft. wide asphalt drive connecting Stover Street with the apartment
complex to the east. Existing grade is approximately 1.6% from
west to east.
There are both water and sanitary sewer services currently on
the site. A 48" C.M.P. storm sewer pipe runs through the property
near the north and east property lines. Existing 3-phase power is
located on the east side of the property. This property is located
in the Spring Creek Storm Drainage Basin.
420 S. Howes • Suite 202 • Fort Collins, Colorado 80521 • (303) 221-4158 • Fax (303) 221-4159
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VICINITY MAP
Northern Engineering Services, Inc.
December 3, 1993
Mr. Glen Schleuter
Stormwater Utility
City of Fort Collins
235 Matthews
P.O. Box 580
Fort Collins, Colorado 80522-0580
Re: East Drake Terrace P.U.D.
Final Drainage Report
Project No. 9337.00
Dear Glen:
Northern Engineering is pleased to submit this report of a
stormwater investigation for the East Drake Terrace P.U.D. It
represents a study of existing and proposed runoff characteristics
of the project site.
we would like to thank you for your valuable input during the
research and review phases of this study. If you have any
questions or comments as you review this report, please feel free
to contact me at your earliest convenience.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
M -p4
Gregory P. Henson
Project Engineer
Reviewed/by:
s/ L��j �u✓
Michael F. J nes, P.
Project Manager
420 S. Howes • Suite 202 • Fort Collins, Colorado 80521 • (303) 221-4158 • Fax (303) 221-4159
I
E
FINAL DRAINAGE REPORT
for
E A S T D RA K E T E R RA C E
Fort Collins, Colorado
December, 1993
R . U . E:) .
prepared by
NORTHERN ENGINEERING SERVICES, INC.
420 S. Howes, Suite 202
Ft. Collins, Colorado 80521
Project No. 9337.00