HomeMy WebLinkAboutWATERGLEN SELF-STORAGE - REPLAT TRACT ''W'' - 71-93D - CORRESPONDENCE - DRAINAGE REPORTcurrent basins and rerouting the water to the existing ponds will not affect the water quality
volumes required as mentioned in the approved drainage report.
Existing Ponds D, E, and F were all modeled by Lidstone & Anderson, Inc. and reported in the
Regional Drainage Evaluation for the Waterglen P.U.D. dated November 30, 1998. These ponds
are hydraulically connected in the following manner. Pond D's outlet releases into Pond E and
Pond E's outlet releases into Pond F. Pond F discharges into Cooper Slough. The pond outlets
act as orifices and the water surface of each pond produces a tail water effect on the upstream
pond's outlet. Therefore, the water surface of Pond F controls the release of Pond E and the
water surface of Pond E controls the release of Pond D.
The proposed development will redirect the runoff from the revised basins to Pond F instead of
the current flow path to Pond D. This will raise the water surface of Pond F faster than originally
designed. However, this will in turn restrict the releases from Pond E until Pond E builds
enough head to push flows through its outlet structure. The water surface elevation in Pond E
required to do this will still be the same water surface elevation as in the approved drainage
report for Waterglen P.U.D. As the water surface in Pond E rises, the flows leaving Pond D will
be restricted until the water surface elevation in Pond D reaches the same water surface elevation
as that approved in the above mentioned drainage report for Waterglen P.U.D. The control in
this situation is the sizing of the pond outlets acting as orifices and the required head differential
between ponds to pass the previously designed flows.
JR Engineering has reviewed and concluded that the revised development will not negatively
impact the site storm sewer system in any way and that the existing detention ponds are
adequately sized for the proposed development. All computations within this addendum have
been completed in compliance with the most recent rainfall data and the City of Fort Collins
Storm Drainage Design Criteria.
We appreciate your time and consideration in reviewing this submittal. If you have any
questions or need additional information please call (970) 491-9888.
Prepared by:
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Amber Kauffman, . .
Design Engineer II
Reviewed by:
Robert Almirall, P.E.
Director of Operations
X:\3910000.a11\3914510\Word\Letters\drainage update.doc
RECEIVED
NOV 2 P. /,","IV
CURRENT PLANNING
November 22, 2002
Mr. Basil Harridan
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, CO 80521
fV
J•R ENGINEERING
A Subsidiary of Westrian
RE: Waterglen P.U.D. — Drainage Addendum for Tract W, "Waterglen Self Storage"
Dear Basil:
We are pleased to submit for your review and approval this addendum showing the minor
changes being made to the approved Tract W, Waterglen P.U.D. dated 11/6/98. Tract W is still
considered a commercial development as previously approved in the Final Drainage and Erosion
Control Report Waterglen, P.U.D. dated August 11, 1997 and the Addendum to Final. Drainage
and Erosion Control Report Waterglen, P.U.D. dated May 18, 1999. The only existing basins
affected by the revised development are Basins S-1, T-3 & 0-3R. Attached is sheet 29A/31A of
84 showing the revisions to Tract W.
Existing basin S-1 has reduced in size by 0.27 acres from 7.74 acres to 7.47 acres of commercial
area. This contributing area on Tract W currently drains across the existing residential lots to the
south. With this proposed development the contributing area is redirected to basin 0-3R. This
will thereby reduce the amount of water going to design point 22 and reduce the resulting
carryover to design point 25 and existing Pond D.
Existing basin T-3 has also reduced in size. The current size of basin T-3 is 2.99 acres. The
revised basin is 2.55 acres. The difference of contributing area, being 0.44 acres, will be directed
towards basin 0-3R. This will reduce the amount of runoff to design point 21 and existing Pond
D.
Basin 0-3R was reported in the May 1999 Addendum as having a contributing area of 6.53 acres
and a runoff coefficient (C) of 0.35. With this addendum, the revised area of basin 0-3R is 7.24
acres and a revised C of 0.41. The time of concentration shall remain the same at 23.8 minutes
for the 10-year and 100-year events as shown in the approved drainage report mentioned above.
This means that the peak flow from Basin 0-3R is increased from 16.9 cfs to 17.5 cfs in the 100-
year event. Basin 0-3R ultimately releases into existing Pond F.
Existing Pond F contains all the required water quality capture volume for the runoff originating
in the contributing areas to existing Ponds D, E and F. Therefore, changing the sizes of the
2620 East Prospect Road, Suite 190, Fort Collins, CO 80525
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