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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 it 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 it 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. . assac u r'n9r S4 Norvrood Ln c .... vEt�,a 42 ti 7 Q. ilE Be? pringwoodDr c o `aoWdlovCree watertord Lq 'aftkin '" ➢ ITE 111 Collindale Dr o 4 ` - �� Quail Hollow Dr pike rofi Preston Tit C2N } � w "dyc E HorsetooUt Rd � P o R'+tE/aA4 pd 'Wod fid �W aI� � Bighorn Rd Kadtiax 43e 9S ' Arobc Fox Dr Bison Rd _ Corrindalc U corj Conrcc a Sunburst Dr %@ C Sunston n Paddingto Cantiou Dr c c Figure 1— Vicinity Map !l� Addendum to the Final Drainage and Erosion Control Study Page 1 June 29, 2005 Tower Shoppes P.U.D. ;T�I9 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. i 0 I 9' 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. Id 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. r r I I I 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. I 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