HomeMy WebLinkAboutPINECONE PUD THE TOWER SHOPPES SONIC DRIVE IN PRELIMINARY - 60 91N - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTII
ADDENDUM TO THE
FINAL DRAINAGE AND EROSION CONTROL STUDY
TOWER SHOPPES P.U.D.
Prepared for:
The W.W. Reynolds Companies
1600 Specht Point Drive, Suite C
Fort Collins, CO 80525
Ph. 970.482.4800
Fax 970.686.5011
Prepared by:
Interwest Consulting Group
1218 W. Ash, Suite C
Windsor, Colorado 80550
_ Ph. 970.674.3300
T Fax 970.674.3303
June 29, 2005
INTERWEST . CONSULTING GROUP
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6. REFERENCES
1. "Addendum to Final Drainage and Erosion Control Study for the Tower Shoppes,
P.U.D., Lot 3 (a.k.a. Pad `B') Temporary Drainage Utilities, RBD, Inc., August 1995.
2. City of Fort Collins, "Storm Drainage Design Criteria and Construction Standards"
(SDDCCS), May 1984.
3. "Final Drainage Report for the New Fort Collins High School", Meline & Irelan, Inc.,
August 1994.
4. Letter Report RE: New Fort Collins High School Detention Pond Overtopping
Analysis. Lidstone and Anderson, Inc, March 1995
5. Urban Drainage and Flood Control District, "Urban Storm Drainage Criteria
Manual", Volumes 1 and 2, dated June 2001, and Volume 3, dated September 1992.
Page 7
Addendum to the Final Drainage and Erosion Control Study June 29, 2005
Tower Shoppes P.U.D.
INTERWEST M CONSULTING GROUP
June 29, 2005
Mr. Basil Hamdan
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, CO 80521
RE: Addendum to the Final Drainage and Erosion Control Study
Tower Shoppes P.U.D.
Dear Basil:
We are pleased to submit for your review this Addendum to the Final Drainage and Erosion Control
Study for the Tower Shoppes P.U.D. This report describes and details the proposed water quality
facility that is required under the current City of Fort Collins Storm Drainage Criteria for the
development of Lots 1, 2, and 4 to occur. All computations within this report have been completed
in compliance with the City of Fort Collins Storm Drainage Design Criteria and the "Urban Storm
Drainage Criteria Manual" (UDFCD) Volume 3.
We greatly appreciate your time and consideration in reviewing this submittal. Please call -if -you --
any questions.
Sincerely,
J er Zun
rado Profes
Engineer No. 34
TABLE OF CONTENTS
TABLE OF CONTENTS
PAGE
ii
1. INTRODUCTION..................................................................................................................1
2. PROJECT BACKGROUND.................................................................................................2
3. DETENTION REQUIREMENTS........................................................................................2
3.1 Site Description................................................................... ............................................2
3.2 Regional Pond Description.............................................................................................3
3.3 On -site Detention Calculations.......................................................................................4
WATER QUALITY REQUIREMENTS..............................................................................4
4.
4.1
4.2
Site Description...............................................................................................................4
Extended Detention Basin Design..................................................................................
5
4.3
Outlet Structure Design...................................................................................................
5
5. CONCLUSIONS.....................................................................................................................6
6. REFERENCES.......................................................................................................................7
Appendix A: Detention Calculations
Appendix B: Extended Detention Basin Design
Appendix C: Excerpts from other Reports
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1. INTRODUCTION
This report is an addendum to the Final Drainage and Erosion Control Study for the Tower
Shoppes P.U.D. (RBD, Inc.,1995) and provides details of the proposed extended detention
basin (EDB) sedimentation facility for Lots 1, 2 and 4 of the Tower Shoppes P.U.D. The
Tower Shoppes P.U.D. is located in the northeast comer of the intersection of Horsetooth
Road and Timberline Road in the City of Fort Collins, Colorado. Lots 1, 2 and 4 of the
Tower Shoppes P.U.D., hereafter referred to as "site", are currently undeveloped land. The
New Fort Collins High School lies on the north and east sides of the site and a McDonalds
Restaurant occupies Lot 3. Construction plans for a Sonic Drive -In Restaurant on Lot 4 are
currently undergoing development review and are being submitted in conjunction with this
Drainage Report Addendum.
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Addendum to the Final Drainage and Erosion Control Study Page 1
June 29, 2005
Tower Shoppes P.U.D.
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2. PROJECT BACKGROUND
The Final Drainage and Erosion Control Study for the Tower Shoppes P.U.D. was approved
on June 7, 1995. Since the time that this drainage study was completed, the City of Fort
Collins (CFC) has modified their stormwater design criteria to include stormwater quality
enhancement requirements and the use of a larger design storm based on a 1998 precipitation
study. Because more than three years has elapsed from the time of plan approvals, the
undeveloped Lots 1, 2 and 4 of the Tower Shoppes P.U.D. are now required to meet the
updated design criteria.
On March 31, 2005 representatives from the CFC Stormwater Utility, CFC Attorney's office,
WW Reynolds (property owner), Sonic (prospective purchaser), and Interwest Consulting
Group met to discuss what would now be required for Lots 1, 2 and 4 to be developed. It
was agreed that detention should be provided for the undeveloped portions of Lot 2 and Lot 4
and that water quality enhancements should be provided for the undeveloped portions of Lot
2, Lot 4 and the west half (parking area) of Lot 1. The required detention volume is based on
the difference between the old design storm (2.89" over 2 hrs) and the current design storm
(3.67" over 2 hrs) for fully developed conditions. The water quality enhancement method
that was recommended by the CFC is an extended detention basin with a 40-hr drain time.
3. DETENTION REQUIREMENTS
3.1 Site Description
The portion of the site for which additional detention is required includes the undeveloped
portions of Lot 2 (Basin A, 0.78 acres) and Lot 4 (Basin C, 0.76 acres) for a total of 1.54
acres of commercial property. Historically the site drains to the northeast to the New Fort
Collins High School stormwater conveyance system via two temporary swales that were
constructed with the McDonalds Restaurant on Lot 3 per the Addendum to the Final
Drainage and Erosion Control Report for the Tower Shoppes, P.U.D, Lot 3 (RBD, 1995).
Detention for the site in fully developed conditions was accounted for in the regional
detention pond located along the eastern side of the New Fort Collins High School property
(Meline & Irelan,1993). This regional pond is known as Detention Pond 213 in the SWM
model for the High School site and in the SWMM model for the entire Foothills Basin. The
proposed percent imperviousness for the Tower Shoppes P.U.D. (71-79%) is equal to or less
m to the Final Drainage and Erosion Control Study Page 2
Addendu
June 29, 2005
Tower Shoppes P.U.D.
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than the assumed percent imperviousness used to design the regional pond (85-89%). The
proposed percent imperviousness for the new Sonic Drive -In on Lot 4 is 80% and the
remaining undeveloped lots are expected to have a percent imperviousness that meets or is
less than the designed values.
Figure 2 — Existing temporary swale from west side of the site
3.2 Regional Pond Description
�1 The regional detention pond on the eastern side of the New Fort Collins High School site,
Pond 213, was originally designed with 7.8 ac-ft of storage and 0.6-ft of freeboard during a
100-year storm under the old rainfall conditions. A variance for less than I -foot of freeboard
was requested and granted with the New Fort Collins High School site. In 1995 an
overtopping analysis of this pond was completed by Lidstone & Anderson and a revised
storage -discharge rating curve was incorporated into the Foothills Basin S WMM model. The
overtopping analysis assumed that the outlet was completely plugged and indicated that Pond
213 begins to overtop when it reaches a storage volume of 11.77 ac-ft.
The most current SWMM model for the Foothills basin that includes the updated rainfall
data shows that Pond 213 reaches a peak discharge of 44.2 cfs at a storage volume of 11.40
ac-ft during a 100-year event. Thus, the regional detention pond has 0.37 ac-ft of extra
storage capacity before the pond begins to overtop during a 100-year stone event (11.77 —
11.40 ac-ft).
Addendum to the Final Drainage and Erosion Control Study Page 3
June 29, 2005
Tower Shoppes P.U.D.
3.3 On -site Detention Calculations .
The area for which additional detention is being required is less than 5-acres and therefore
the Rational Volumetric FAA Method was used to calculate the required detention volume
(directed by Basil Harridan, CFC Stormwater Utility). The total tributary area is 1.54-acres
(Lots 2 & 4) and a minor storm "C" coefficient of 0.74 (equal to that used in the Tower
�I Shoppes Drainage Report) and a major "C" coefficient of 0.93 was used for the 100-year
storm. The allowable release rate was set to 12.0 cfs which is equal to the developed
undetained peak flow rate from this area under the old rainfall standards. The input storm is
the 100-year storm using the new rainfall standards and the resulting detention volume
requirement is 0.03 ac-ft.
The excess storage capacity of the regional detention Pond 213 is 0.37 ac-ft and the required
storage volume for the difference in rainfall standards for Lots 2 & 4 is 0.03 ac-ft. In the
worse case scenario where the peak detention requirements for Pond 213 and Lots 2 & 4
occurred simultaneously then the impact to the regional pond is still negligible. The peak
discharge rate would remain the same at 44.2 cfs with no change in the water surface
elevation. Thus detention for Lots 2 & 4 will be provided in the regional pond and on -site
detention for Lots 2 & 4 will not be required. This conclusion was discussed and agreed
upon with Basil Hamdan during a phone conversation in early April, 2005.
4. WATER QUALITY REQUIREMENTS
--4-.1--Site Description
The portion of the site for which water quality enhancements is required includes Lot 2, Lot 4
and the west half of Lot 1 for a total of 4.33-acres of commercial property. The west half of
Lot 1 contains a future parking lot and the east half of Lot 1 is the location of a future
building. Only the west half of Lot 1 is being included in the water quality area because
parking areas will generate more pollutants which should be removed.
Currently the site drains to the northeast to the New Fort Collins High School stormwater
conveyance system via two temporary swales that were constructed with the McDonalds
Restaurant on Lot 1. As mentioned in the previous section, the percent imperviousness for
Addendum to the Final Drainage and Erosion Control Study Page 4
June 29, 2005
Tower Shoppes P.U.D.
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this area has been assumed to be 71 % to 89% in previous studies. For this study, the higher
and more conservative percent impervious of 89% was used.
Stormwater quality was briefly addressed in the original Drainage Report for the Tower
Shoppes P.U.D. and stated that runoff from the site would "enter a permanent stormwater
detention pond serving as a means to settle out suspended particles". However facilities that
meet the current criteria were not constructed and thus this study and water quality
enhancement design was prompted.
4.2 Extended Detention Basin Design
The recommended water quality enhancement measure is the use of an extended detention
basin (EDB) designed using guidelines set forth in the Urban Storm Drainage Criteria
Manual, Volume 3 (2004). The proposed EDB is a sedimentation facility that will capture
stormwater runoff from frequently occurring rainfall events and extend the emptying time to
allow for pollutant removal through gravity settling. During the time in-between stormwater
events the majority of the basin will remain dry. A permanent micro -pool for biological.
uptake is not included in the design because the total water quality capture volume is small
and a significant amount of soluble pollutants are not expected. Additionally, since
stormwater runoff is passing through grassy swales before entering the EDB, a
presedimentation forebay was not included in this design. The design brim -full water quality
capture volume required to allow a 40-hour drain time is 0.17 ac-ft.
4.3
Outlet Structure Design
The proposed outlet structure is a drop box structure with a flow control plate -designed -to
release frequently occurring storm events over a 40-hour period. The top of the drop box
structure is equipped with a grate and the outlet pipe is designed to pass the 10-year storm.
The proposed outlet pipe is a 24" RCP which will tie into the existing manhole (MH-3A) of
the stormwater conveyance system constructed with the New High School as shown on the
approved construction plans for the Tower Shoppes P.U.D. This system was designed to
convey the 10-year developed flows from the Tower Shoppes site (Meline & Ireland,1994)
equal to 20.6 cfs. Peak flows from the 100-year storm are intended to pass over the 100-year
emergency overflow spillway that will be constructed on the east side of the EDB and pass
on to the existing 100-year overflow channel on the south side of the New High School.
Addendum to the Final Drainage and Erosion Control Study Page 5
June 29, 2005
Tower Shoppes P.U.D.
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IFigure 3 - Existing overflow channel south of the High School
After the site is fully developed, a 15' wide concrete sidewalk culvert will be located at the
spillway location just south of the existing concrete pad on the New Fort Collins High
School site as shown on the approved construction plans for the Tower Shoppes P.U.D. The
15' concrete sidewalk culvert then discharges to the 100-year overflow channel between the
High School building and the future commercial building on Lot 1. This overflow channel
will be expanded to the south when development of Lot 1 occurs. Details of the design for
the outlet structure and supporting excerpts from the previous drainage study report are
Iincluded in the Appendix.
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5. CONCLUSIONS
The proposed EDB located on the north side of Lot 1 of the Tower Shoppes PUD and
included in the proposed construction plans for Sonic — Horsetooth meets the current
drainage design criteria for the City or Fort Collins. Water quality enhancement will be
provided for the undeveloped portions of Lot 2, Lot 4 and the west half (parking area) of Lot
1 within the EDB. Stormwater detention for the difference between the old and new rainfall
standards for Lots 2 and Lot 4 can be accommodated within the existing regional detention
pond (Pond 213) constructed with the New Fort Collins High School. The existing
temporary drainage swales that were constructed with the McDonalds site will be reshaped
so that they drain into the proposed EDB.
Addendum to the Final Drainage and Erosion Control Study Page Tower Shoppes P.U.D. June 29, 2005