HomeMy WebLinkAboutHEWLETT-PACKARD TEMPORARY FACILITIES - I-L SITE PLAN REVIEW - 54-88B - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFINAL DRAINAGE REPORT
FOR THE HEWLETT PACKARD TEMPORARY BUILDING
(Preston -Kelley Subdivision)
Fort Collins, Colorado
Prepared For:
Neenan Company
P.O. Box 2127
Fort Collins, Colorado
and
Hewlett Packard, Fort Collins
3404 East Harmony Road
Fort Collins, Colorado
Prepared By:
RBD, Inc. Engineering Consultants
2900 South College Avenue
Fort Collins, Colorado
Job # 033-012
Engineering Consultants
2900 South College Avenue
Fort Collins, Colorado 80525
3031226-4955
September 7, 1988
Mr. Glen D. Schlueter
City of Fort Collins
Stormwater Utility
P.O. Box 580
Fort Collins, Colorado 80522
RE: FINAL DRAINAGE REPORT FOR THE HEWLETT PACKARD TEMPORARY
BUILDING - PRESTON-KELLEY SUBDIVISION
Dear Glen:
Hewlett Packard of Fort Collins is located on the northeast cor-
ner of the intersection of County Road Number 9 and Harmony Road
(Colorado State Highway 68). More specifically this project is
located in the southwest quarter of Section 33, Township 7 North,
Range 68 West of the 6th Principal Meridian. This report is for
a temporary building to be located just north and east of the
current Hewlett Packard entrance off County Road Number 9. The
location of this temporary building is shown on the vicinity map
included with the drainage plan (fold -out exhibit A) included
with this report. This temporary building is to be replaced by
permanent building number 6. Building number 6 is currently
being designed and it is anticipated that a drainage report will
be submitted to the City by the end of September.
Following is Exhibit B which shows the overall master plan for
the HP site as it is presently envisioned. Noted on this plan
are the existing buildings (1,2, & 3), the proposed building 6,
and the location of the temporary building.
The temporary building is to be located in a grassed area ad-
jacent to existing parking. The Drainage Plan (fold -out exhibit
"A") shows the drainage basin boundaries before and after the
construction of the temporary building. In both the "before" and
"after" conditions, runoff is directed to an existing inlet
designated as K-7 on previous plans. Stormwater entering inlet
K-7 is carried by pipes (ranging in size from 18-inch diameter to
48-inch diameter) to the main (DAM) detention pond in the
southeast corner of the HP site. Foldout exhibit "C" highlights
Other Offices: Vail, Colorado 303/476-6340 • Colorado Springs, Colorado 3031574-3504
in yellow the storm sewer routing.
If a detention pond were located adjacent to the temporary
building, its volume would need to 0.125 ac.-ft. in order that
stormwater runoff from the building does not exceed the 2-year
historic runoff rate. This volume was calculated using the ra-
tional mass diagram method as shown in Exhibit "D". Attached as
fold -out exhibit "E" is a portion of the construction drawing for
the DAM POND which shows the pond layout and Pond Area -Capacity
Curve. From field verifications obtained to date, it appears
that the DAM pond was indeed constructed according to this plan.
The pond -area -capacity curve indicates that at the DAM POND
Spillway crest elevation of 4892.0, a total of 865,000 cubic feet
of water will be retained. Low water elevation is at 4887 which
relates to 290,000 cubic feet of water storage. Therefore, the
detention capacity available to stormwater runoff is (865,000 -
290,000) 575,000 cubic feet, or 13.2 acre-feet.
An overall master site drainage plan prepared by Anderson & Hast-
ings prior to the construction of the DAM POND identified that a
storage volume of 574,000 cubic feet be provided between eleva-
tions 93 and 78. Of this storage volume, 388,000 cubic -feet was
identified as that needed to detain site runoff if no other on -
site detention ponds were constructed. The present DAM POND con-
struction has the overall volume identified for it by the Ander-
son & Hastings master plan. Consequently, it can be assumed that
of the total 575,000 cubic feet provided, 388,000 cubic feet (8.9
acre-feet) is provided for detention of onsite stormwater runoff.
In July, 1977, Anderson & Hastings prepared a quantity take -off
of anticipated total site development area. Developed area was
defined to be building area and pavement area. The total
developed area at site buildout was calculated at that time to be
60.53 acres. Recently, RBD has calculated the present site
developed area to be 32.97 acres or 54.5% of the previously cal-
culated total. The recent site developed area was calculated
using site aerial photogrammetry and field survey information.
Roughly speaking then, it can be assumed that 54.5% of the DAM
POND's 8.9 acre-feet stormwater storage volume is utilized by the
existing site development. This means that 4.0 acre-feet of
stormwater storage is still available for use by new site
development such as the temporary building. Obviously, the 4.0
acre-feet of available storage is more than enough to accommodate
the 0.124 acre-feet storage identified earlier for the temporary
building.
The above logic in identifying storage needs and availabilities
is conservative. It does not take into account travel time in
the onsite storm sewers. More importantly it does not consider
the effect of other, existing and proposed onsite storage. Both
of these factors increase the actual amount of storage available
onsite. However, because of the small volume of storage required
by the temporary building, these other effects did not need to be
studied at this time but instead will be addressed with the
design of building 6.
The DAM POND is provided with a spillway (crest elevation 4892.0)
to handle flood flows in excess of the 100-year frequency event.
A second outlet from the DAM POND is provided by an 18-inch pipe.
Exhibit "H" provides the depth -discharge relationship for this
existing outfall structure. The 24-inch outlet pipe referred to
in Exhibit "H" is usually gated shut. Discharges from the DAM
POND goes into the Fossil Creek Ditch and flow south. The tem-
porary building will not change the quality or quantity of
stormwater runoff which is now be accepted by the Fossil Creek
Ditch.
Storm sewer design calculations made by Anderson and Hastings in
1976 and 1977 for many of the onsite sewers indicate they were
sized for the 10-year frequency storm which matches current City
requirements. The calculation method used (rational formula)
also matches current City criteria. Exhibits "F" and "G" provide
the hydrology calculations for a 10-year and 100-year frequency
storm for drainage basin A - both as it exists now and as it will
exist when the temporary building is constructed. These tables
show that stormwater flow to existing inlet K-7 will increase
from 4.4 to 4.8 cfs with the 10-year frequency storm. Similarly,
stormwater flow will increase from 9.0 to 9.6 with the 100-year
frequency storm. The hydrology calculations are supported by
calculation sheets 1 of 3 and 2 of 3.
Calculation sheet 3 of 3 provides capacity check calculations for
the existing 18-inch RCP storm sewer leaving existing inlet K-7.
With a 10-year frequency storm, this sewer flows slightly less
than full - a condition which will not be significantly affected
by the addition of the temporary building. For the 100-year
frequency storm, this storm sewer presently surcharges and causes
a portion of the runoff to travel across the parking lot -
primarily to the south. This situation will continue with the
addition of the temporary building.
This report was prepared to satisfy current City of Fort Collins
criteria in design storm and drainage calculation methodology.
Sincerely,
7
1111,
RBD, Inc.GO���'��t`�''
- •
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Joseph C.
10: : cc
GolA - P.E. .' �
cc: Greg
Fisher;�••B�•�``��``
033-012
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LIST OF EXHIBITS
EXHIBIT
LETTER TITLE
A Drainage Plan
W
HP Master Site Plan
C Existing Site Utilities
D Temp. Bldg. Detention Volume
E Dam Pond Construction Drawing
F 10-Year Storm Frequency
G 100-Year Storm Frequency
H Dam Pond Outlet -Capacity Curve
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DATK 8-29-88
PROJECT: HP TEMPORARY BUILDING
')U? NiNlAE|l: V33-012
DESIGN ENGIMEERi
BASIN DESIGNATION = TOTAL SITE
DE[ERMIHE EXISTING Q FOR 2 YEAR STORM
=====================================================================
HISTORIC RUNOFF COEFFICIENT .2
BASIN SLOPE: 1 PERCENT
BASIN LENGTH 60 FEE'',,'
AREA: .7 ACRES
HIST TC = 1.87*(1.1-( 1 * .2
)>*SQRROOT 60 >)/ 1
13.03646 MINUTES
HISTORIC RAINFALL INTENSITY: 2.277083 INCHEWHOUR
HISTORIC OUTFALL: .3187916 CUBIC FEET PER SECOND
DETERMINATION OF DETENTION POND VOLUME FOR DEVELOPED CONDITIONS
==================================================================
DEVELOPED S[()RM: 100
DEVELOPED RUNOFF COEFFICIENT
CCFAC = 1
=
m|RATION
MIN
5
10
15
3(,,�
45
60
90
120
180
360
*
.875
GROSS VOL.
CU. FT.
1915.2
2948.4
3754.8
5191.2
6217
6577.2
7106.4
7257.6
7862.4
87b9.599
YEAR STORM
1
1.25 *
OUTFL{�� VOL.
CU. FT.
95.63748
191.275
286.9125
573.8249
860.7373
1147.65
1721.475
2295.3
3442.949
6985.899
DETENTION VOL.
CU. FT
1819'563
2757.125
3467.888
4617.375
5376.263
5429.55
5384.926
4962.3
4419.451
1883.701
REQUIRED DETEH|IOH VOLUME IS 5429.55 CU. FT. OR
.1246453 ACRE FEET
EXHIBIT ^D^
TEMPORARY BUILDING DETENTION POND
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