HomeMy WebLinkAboutLARIMER COUNTY COMMUNITY CORRECTIONS FACILITY ADDITION, PHASE II - SITE PLAN ADVISORY REVIEW - 19-03A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTNo Text
Based on the grading shown in the attached drainage and erosion control plan the revised 100-
year pond elevation is 4907.47 ft and the storage provided totals 0.79 ac-ft. This can be
compared to the original 100-year pond elevation of 4907.65 feet. This reduction in 100-year
water surface resulted in the net reduction in discharge out of the orifice plate. The discharge
out of pond 102 (as shown on the attached sheet) is controlled by a 4 5/16" orifice plate. The
computations for the discharge out of this orifice plate are included on attached spreadsheet and
are expected to be 0.65 cfs, lower than the required 0.72 cfs.
Should you have any questions regarding this analysis or explanation, feel free to contact me at
970-221-2400.
Sincerely,
Nolte Associates, Inc.
/'-� .1
c� Paul Sclafani, P.E.
Sr. Engineer
Attachments
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Existing Conditions
from revious report)
Proposed Conditions
the buildin additionAreaBasin
a�aDischarge
(cfs)
rincludin
ac
Discharge (cfs)
10-Year
100-
Year
10-Year
100-
Year
100
1.66
3.84
10.76
100
1.57
4.50
13.10
101
0.27
1.26
2.70
101
102
0.71
0.29
2.40
0.38
6.14.
1.00
102
103
0.35
0.29
0.36
0.38
0.95
1.00
200
0.08
0.35
0.84
200
0.08
0.35
0.84
300
301
302
0.05
0.08
0.09
0.21
0.30
0.15
0.51
0.77
0.39
300
301
302
0.05
0.08
0.09
0.21
0.30
0.15
0.51
0.77
0.39
400
0.11
0.14
0.36
400
0.11
0.14
0.36
OS-1
0.32
1.21
3.21
OS-1
0.32 _E
1.21
3.21
As can be seen in Table 1, the only changes made to the previous drainage analysis were made
in basins 100, 101, and 102 (previous designation) or basins 100 and 101 (revised designation).
It should be noted that even though the drainage area for basin 100 was reduced; the runoff
coefficient was increased, and the time of concentration was reduced to account for the
increased impervious area. This resulted in an overall increase in discharge. The total increase
in runoff from the building addition was 4.83 cfs for the 100-year and 1.06 cfs for the 10-year.
Some of this increase in discharge went north to the Detention Basin 101 and the rest drained
into Detention Basin 100. Although the discharge reaching the detention basins is increasing,
the purpose of the detention has not changed, that is to release the historic 2-year discharge.
(0.72 cfs) from basin 102. See the attached Drainage and Erosion Control Plan for the location
of the drainage and detention basins.
In a similar procedure that was used in the previous report, the FAA methodology was used for
sizing the overall detention requirements for the site. Because the detention basins are at
relatively the same elevation and the culverts connecting them act more like equalization pipes
then control outlets, the detention for the entire site can be treated as a single detention facility.
The volume required to accommodate the increase in runoff was computed to be 0.49 ac-ft. To
accommodate this volume the northwest detention facility needs to be regraded to increase its
volume. The detention computations are included with this letter, and the extents of detention
pond 101's regrading are shown on the attached drainage and erosion control sheet. None of the
connector/outlet. pipes need to modified for the new building facility and the resulting discharge
from Basin 102 is expected to meet the required historic 2-Year discharge rate of 0.72 cfs.
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NOLTE ASSOCIATES. INC.
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NCLTE
BE YON D E N G IN E E R ING
November 22, 2005
Mr. Roger Kenney
Kenney & Associates, Architects, LLC
209 East 4' Street
Loveland, CO 80537
Re: Larimer County Corrections Center Addition
Dear Mr. Kenney:
This letter is in regard to the 8,000 square foot addition to the existing corrections facility
located at the southwest comer of Midpoint Drive and Specht Point Road in Fort Collins,
Colorado. The entire site is approximately 2.55 acres, however, the addition to the existing
facility only impacts approximately 0.29 acres. This letter addresses the effects of this addition
on the drainage system that was built at the same time as the original building and the changes
that need to be made in order to accommodate the. building addition. The necessary calculations
associated with this analysis are included with this report. The original drainage report from the
design of the original building is also included with this letter for reference.
Originally the site was subdivided into four onsite basins draining directly to an onsite detention
facility and five basins that discharge directly to the street. The detention facility was originally
designed to release the historic 2-year discharge. The computation of the historic discharge can
be found in the original drainage report dated July 31, 2003. This release rate was 0.72 cfs.
In order to ascertain the impacts of the proposed addition to the corrections facility, the drainage
basins discussed above were modified slightly to account for the increased impervious areas and
somewhat shorter time of concentration. It was assumed for this analysis that all of the roof
drains for the building addition are directed toward the detention facility. With this assumption
in mind, Basin 100's area was reduced and Basin 102's area, which includes that area draining
into the detention facility, was increased. Further, the previous Basin 101 and 102 were
combined into one drainage area. Because of this combination, the drainage areas were
renumbered. Table 1 shows the existing and proposed basins.
NOLTE ASSOCIATES, INC.
1901 SHARP POINT DRIVE, SUITE A
FORT COLLINS, CO 80525.4429
970.221.2400 TEL 970.221.2415 FAX
WWW.NOLTE.COM
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