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Home My WebLink AboutMIRAMONT OFFICE PARK PUD - PRELIMINARY & FINAL - 54-87AN - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT�;�.,...: �.�, :.. �c ,, 1 Final Dr�ainage and Erosion Conirol Study for Miramont Off�ce Park Fart Collir�s, Coiorado i�'Iarch 27,1997 prepared f'or: Miramont Associates 309 West Harmony Road Fort Collins, Colorado 80526 .: . •:__ THE SEAR-BROWN GROUP Starytidarcl� i�rti L�:�ce�le�lc�� .�� -ra-� r� ��AR-BR4WN G Rou r� FULLrSERVICE DESIGN PROFESSIONALS FORMERLY RBD, INC. 209 SOUTH MELDRUM FORT COLLINS, COLORADO 80521-260� 970-482-5922 FAX:970-482-6368 A�arch 27, i 997 A'Ir. B.tsil F�amdan City of Fort Collins Utility Services Stormwater 235 Mathews Street Fort Collins, Colorado 80522 RE: Final Drainage and Erosion Control Study for Miramont Office Park Dear Basil: We are pleased to submit to you for your review and approval, this Final Drainage and Erosion Control Study for the Miramont Office Park. All computations within this report have been performed in compliance with the City of Fort Collins Storm Drainage Design Criteria. We appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. Respectfully, RBD Inc. Engineering Consultants Prepared by: Reviewed by: P rry C t, EI Project ineer cc: 504-021(A) Correspondence H:\SITE\\proj ects�miramont. of�drainfin.rpt i��r��,,�� `r� ��i �i'rr�ir�:_�,'ri'dn��ii,� � � �i..� �i �ni �� • i rrni i '�'1'ANQARDS II\I EXC .F,LLENCE (NOT AVAII.ABLE) Jim Allen-Morley, P.E. Project Reviewer F�� ,I�,�I � il'I'��I?'I'llr�lfr'1' f��ll�l � �,�I ' � � � �I TABLE OF CONTENTS DESCRIPTION PA E GENERAL LOCATION AND DESCRIl'TION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Location 1 Description of Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 DRAINAGE BASINS ....................................................... 1 � Major Basin Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Sub-Basin Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 I � � � � � � DRAINAGE DESIGN CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Regulations......................................................... 2 Development Criteria Reference and Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Hydrological Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Hydraulic Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Variances from Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 DRAINAGE FACII.ITY DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 GeneralConcept ..................................................... 3 Specific Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 STORM WATER QUALITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 EROSION CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 � GeneralConcept ..................................................... 4 Specific Details . . . . . . . . . . . . . . . . . . . . . . . 5 � CONCLUSIONS ........................................................... 5 Compliance with Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Drainage Concept .................................................... 5 � Storm Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 � Erosion Control Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 REFERENCES........................................................... APPENDIX VICII�TITY MAP HYDROLOGIC CALCULATIONS DETENTION POND CALCULATIONS FRONT RANGE BAPTIST CHURCH SUPPLEMENT SWMModel FOR McCLELLAND'S MASTER DRAINAGE BASIN EROSION CONTROL TABLES AND FIGURES 7 2 3 9 12 17 28 FINAL DRAINAGE AND EROSION CONTROL STUDY FOR THE MIRAMONT OFFICE PARK FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A, ocati n The Miramont Office Park is located south of Harmony Road between Boardwalk Drive and the Fairway Estates Third Filing. It can also be described as being north of the Upper Meadow at Miramont First Filing P.U.D. and the proposed Miramont Self-Storage P.U.D. More specifically, this site is located in the Northeast Quarter of Section 1, Township 6 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins, Larimer County, Colorado. B. Description of Propertv The Miramont Office Park site is currently undeveloped and sparsely covered with natural ground vegetation. The site slopes from north to south at a grade of approximately 1%. The proposed purpose for the subject site is currently understood to be for a privately owned and operated office park. IIe DRAINAGE BASINS A. Major Basin Descri,ption The project site is located within the McClellands Basin, SWMModeled as Basin 202. No major drainage way exists within the Miramont Office Park site. B. �ub-Basin Descri tion As stated, the Miramont Office Park lies within Basin 202, as laid out in the McClellands Basin Regional SWMModel. Basin 202 also contains the proposed Miramont Self Storage P.U.D., Front Range Baptist Church and Bank One at Harmony Market. The completion of this project will partially develop Basin 202. Detention for Basin 202 is provided by on-site ponds - one for each of the four sites. The Miramont Office Park will provide an on-site detention pond in the southeast corner of the site. 1 This site will be receiving flows from the future detention pond on the Front Range Baptist Church, to the west. Currently, the church free releases stormwater onto the property that is planned for the office park. The Bank One at Harmony Market P.U.D. site to the north of the office park will release its detained runoff to the Boardwalk as well. However, when the Bank One drainage report was submitted by Meline & Irelan, Inc. it planned for a swale (Section G-G) starting at the southwest corner of their property. This swale was designed by Meline & Irelan since the Front Range Baptist Church's historic runoff was flowing onto their property as well. When the Front Range Baptist Church access drive plan and profile was approved earlier this year, this swale was connected to a culvert under the access drive. The culvert has been constructed, and now discharges into a swale with a similar cross-section, which releases into the office park site. Since the Miramont Ofiice Park is in the flow way of the historic drainage path for Front Range Baptist Church, it was necessary to deflect flow around the office park, so that it would still reach Boardwalk. A swale has been proposed along the west property line of the church to cany the church's historic runoff to the south, and then release it into the Miramont Office Park access drive. Since this access drive is integral not only to the office pazk and the church, but also the Miramont Self-Storage Units, it imposes grading cross-restrictions on aIl the sites. In order to drain the church's runoff into Boardwalk Drive without intercepting it between the swale and the drive, it was necessary to plan for this driveway to free release into Boardwalk Drive. The Front Range Baptist Church access drive was planned to release directly to Boardwalk Drive and was approved by the Stormwater Utility. A portion of that access drive will now be diverted into the Bank One at Harmony Market P.U.D. and the Miramont Office Park. Based on this, we are proposing that the portion of the drive common to the office park and the storage units be routed into Boardwalk Drive directly since the existing grade is too flat to store the runoff from this drive and at the same time, allow for the church to inevitably discharge from their site. III. DRAINAGE DESIGN CRITERIA A. Regulations The City of Fort Collins Storm Drainage Design Criteria is being used for this subject site. B. Development Criteria Reference and Constraints � As stated earlier, flows from the site are detained on-site in a pond. Basin 202, as � 2 � shown on the Overall Drainage Plan for the McClellands Basin, had only one pond within it, but the individual property owners have elected to prorate the detention requirements so that each of the pieces have their own detention ponds. Basin 202 was originally modeled with an impervious percentage of 50.00%. C. Hydrolog�ica! Criteria The Rational Method is being used to determine run-off peak flows from the study site. Rainfall criteria for this study was obtained from the City of Fort Collins and is incIuded in the Appendix. The 10- and 100-year rainfall criteria were utilized to determine the developed flows. The release rate from the pond was obtained by proportioning the acceptable release rate from Basin 202 on a per-acre basis, and then applying that figure to the Miramont Office Park site area. D. Aydraulic Criteria All calculations within this study have been prepared in accordance with the City of Fort Collins Drainage Criteria. E. Variances from Criteria A variance for erosion control is being sought. The effectiveness during construction is 62.7 instead of the rainfall performance standard of 77.0. We feel that the measures provided will e$'ectively prevent sediment transport to adjacent areas and any further measures are unnecessary. IV. DRAINAGE FACILITY DE I N A. �eneral Concgpt — Stormwater for the office park site will sheet flow southward across the parking lots and be directed by curb & gutter to sidewalk culverts. The sidewalk culverts discharge the flow into the detention pond. Discharge from the detention pond will � be regulated through an orifice plate structure attached to a 15-inch PVC drain pipe. � The drain pipe will terminate at an earthen swale that will carry the flow into a metal covered sidewalk culvert. The sidewalk culvert discharges the on-site flow into the flowline of Boardwalk Drive, as indicated in the Master Plan. � B. Specific Details � The required storage for the detention pond has been calculated to be 0.73 acre-feet � � � . . r � � � V. � using the FAA method. The average release rate from the site has been calculated to be 1.60 cfs. In order to achieve this release rate, the acceptable release from Basin 202, 11.1 cfs was apportioned on a per acre basis over the entire basin, giving a 0.51 cfs per acre release. The value was then multiplied by the acreage of the site, 0.88 acres, to give 2.23 cfs. Instead of using the traditional freeboard requirement of 1.0', the release rate for the pond was dropped to allow for a conservative pond volume. This rate will be controlled by an orifice plate structure attached to the upstream end of the 15" PVC drain pipe. The detention pond has been designed to realize a 100-year water surface elevation of 5099.5. No overflow spillway has been designed, since the stormwaer can flow out of the access drive Boardwalk in an emergency. STORM WATER QUALITY A. General Concept The water quality of storm water runoff must be addressed on all final design utility � plans. Construction is anticipated to begin in Summer of 1996. We have sought to find various Best Management Practices for the treatment of storm water runof� Though there is little that may be done as most of the drainage facility is already � provided, we have sought to improve Storm water quality with the methods called out in the Erosion Control and Drainage Plan. The on-site detention pond also provides some time for impurities to settle out of the stormwater runoff. � `VI> EROSION CONTROL � � � � � � � A. General Concept The Miramont Of�ice Park lies within the Moderate Rainfall and Wind Erodibility Zone per the City of Fort Collins zone maps. The potential exists for silt movement during construction of the parking/driveways and building. Thus the new improvements will be subjected to both wind and rainfall erosion. Per the City of Fort Collins Erosion Control Reference Manual for Construction Sites and related calculations in the appendix, the erosion control performance standard (PS) for the subject site during construction is 79.6 and after construction is 93.7 From the calculations in the appendix, the effectiveness(EFF) of the proposed erosion control plan is 72.9, during construction and 98.07 after construction. It is anticipated that the construction duration of this project will be less than 8 weeks due to the limited amount of work to be completed, therefore the erosion control plan as specifically detailed below, should meet the City of Fort Collins requirements. A copy � � � ;� r � � r . � of the erosion control calculations is included in the Appendix. An erosion control escrow cost estimate of $2427 is also included in the Erosion Control Section of the Appendix. Specific Details Immediately upon completion of over lot grading the silt fencing shown on the drainage and erosion control plan should be installed. In order to protect the curb chases from receiving excessive silt deposits, the curb chases will have gravel filters installed around them. The above items can be removed once asphalt and landscaping in their respective areas have been completed. � VII. �ONCLUSIONS A. : C. � Compliance with Standards All computations within this report have been completed in compliance with the City of Fort Collins Storm Drainage Design Criteria. Drainage Conce.pt The proposed drainage concepts presented in this report and shown on the Drainage and Erosion Control Plan are in compliance with the City of Fort Collins drainage criteria. This project was designed to be in conformance with the intent of the McClelland's Basin Master Drainage Plan. Storm water Qualitx Because storm water quality has become a requirement, the site has addressed this storm water aspect. We have designed the erosion control to reduce the sediment which runs into the storm water system. The on-site detention pond also provides a settling period during which impurities may be filtered out of the stormwater runoff before leaving the site. Erosion Control Conce�t The proposed erosion control plan adequately provides for the control of wind and rainfall erosion from the study site. A variance has been requested to allow an � � effectiveness during construction of 37.7 instead of the calculated performance standard of 83.8. � The proposed erosion control concepts presented in this report and shown on the Erosion Control Plan are in compliance with the City of Fort Collins erosion control criteria. � � � � � � � � � � � � � � � G � REFERENCES 1. City of Fort Collins, Storm DrainaQe DesiQn Criteria and Construction Standards, May 1984, Revised 1992. � 2. City of Fort Collins, P_r�sion Control Reference Manual for construction Sites, January 1992. 3. Overall Drainage Study for Oak/Cottonwood Farm, McClellands Basin, Fort Collins, Colorado, by RBD, Inc., May 4, 1992, revised October 10, 1994. 4. McClellands Basin 100-year Master Plan Update (County Road 9 to Fairway Estates) for Proposed Modifications to the Oakridge Village Regional Detention Pond, Fort Collins, Colorado, by RBD, Inc., April 1, 1996 7 � � � � � � � � � � � � �I !1 � �� VICII�TITY MAP L��.' � � � �� ■ A A � �' A VICINITY MAP APPROXIMATE SCALE: 1 "=2000'