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HomeMy WebLinkAboutPOUDRE VALLEY PLAZA PUD - PRELIMINARY - 30-95A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTCASH �9 OF, BO LEV RD HORSETOOTH A NUE PROJECT AVENUE SITE ARBOR BROOK DRIVE > �(� � 2�G 0 � 0 � � Z W O Ld � O U � :0 N rrn V) 0 ' J W LOPEZ f= ELEMENTARY H VICINITY MAP SCALE: 1" = 1000' N VICINITY MAP 1. 2. 3. 4. Storm Drainage Design Criteria and Construction Standards, by the City of Fort Collins, Colorado, May 1984, revised January 1991. Erosion Control Reference Manual for Construction Sites, by the City of Fort Collins, Colorado, January 1991. McClellands and Mail Creek Major Drainageway Plan, by Cornell Consulting Company, December 1980. Mail Creek Hydrology Update - Existing Conditions, by Water Engineering & Technology, Inc., July 1991. 7 plans adequately provide for the conveyance and release of developed on -site flows into the existing curb and gutter on Horsetooth Road. The combination of on -site parldng lot capacities and storm drain system will provide for the 10 and 100 year developed flows to reach the northeast corner of the site for release onto I Horsetooth Road or be conveyed to the proposed on -site retention facility. The existing Regional Detention Pond downstream has been designed and built to account for flows released from the Poudre Valley Plaza site at a rate of 56.5 cfs. If groundwater is encountered at the time of construction, a Colorado Department of Health Construction Dewatering Permit will be required. 6 The volume required for the retention pond is twice the volume required for runoff generated from sub -basins 2, 3, and 4 during a 100 year storm. Calculations (included in the appendix) show that 0.176 acre-feet are generated, which means that 0.35 acre-feet are required for the retention pond. V. STORM WATER QUALITY A. General Concept The water quality of storm water runoff must be addressed on all final design utility plans. The Poudre Valley Plaza P.U.D. is anticipating construction beginning in 1996. Best Management Practices for the treatment of storm water runoff will be determined during final design for this project. However, BMP's may include grass -lined swales, minimized DCIA, and installation of silt fence and gravel inlet filters. All construction activities must comply with the State of Colorado permitting process for Storm water Discharges Associated with Construction Activity. A Colorado Department of Health NPDES Permit will be required before any construction grading can begin. VI. EROSION CONTROL A. General Concept During final design for this site, appropriate measures will be designed to control erosion from the site. These measures to limit erosion will typically include installation of silt fence, reseeding and mulching all disturbed areas which will not be paved, and using a gravel mulch on areas which are to be paved. VII. CONCLUSIONS A. Compliance with Standards All computations that have been completed within this report are in compliance with the City of Fort Collins Erosion Control Reference Manual for construction Sites and the Storm Drainage Design Criteria Manual. B. Drainage Concept iThe proposed drainage concepts presented in this report and on the construction 5 The proposed drainage and grading plan is included in the back pocket of this report. B. Specific Details The Poudre Valley Plaza P.U.D. has been broken down into 6 sub -basins, four of which are on -site sub -basins and two of which are offsite sub -basins. Runoff from sub -basin 3 will be collected in a trench drain and conveyed by a storm pipe, south into the retention pond. Sub -basin 3 contains the courtyard in front of the eastern building. Flows from sub-basiri 2 will also be collected by a trench drain and flow into the retention pond via storm piping. Sub -basin 2 includes the courtyards in front of the two buildings in the southeast corner of the site. Sub -basin 4 includes the retention pond and one building. Runoff from this sub -basin flows by grassed swales into the retention pond. Sub -basin 1 contains the remaining on site area including the parking areas and 6 buildings. Flows from sub -basin 1 will be conveyed by sheet flow, by concrete pans, and by concrete curb and gutter to the northeast corner of the site in the parking lot. The runoff will flow onto Horsetooth Road via a concrete trickle channel and sidewalk culvert. Sub -basin 0-1 is a part of Basin 168 and includes the east half of Shields Street, the south half of Horsetooth Road, and the north 160' of the west half of Century Drive. Runoff from this sub -basin will flow around the Poudre Valley Plaza site to Detention Pond #103 via Horsetooth Road. Sub -basin 0-2, which contains the south 330' of the west half of Century Drive, flows through Basin 167 and to Detention Pond #104 as it has historically done. ' Sub -basin 1 generates 45.8 cfs in the 100 year storm which will be routed to the northeast corner of the site and onto Horsetooth Road. Sub -basin 0-1 generates 10.7 cfs during a 100 year event, which flows to the Regional Detention Pond. The total allowable release rate for Basin 168, as determined by the Mail Creek Hydrology Update, is 56.5 cfs for the 100 year event. The amount of runoff generated by sub -basins 1 and 0-1 totals 56.5 cfs therefore, no detention is required on this site. The downstream Regional Detention Pond #.103 has been sized to accept flows from the Poudre Valley Plaza site at the release rates discussed previously. The flows from sub -basin 2, 3, and 4 total 5.1 cfs for the 100 year storm and will be collected by trench drain and conveyed via a storm pipe to the on -site retention pond in sub -basin 4 in the southeast corner of the site. 4 SWMM model will be revised at the time of the final submittal. I Following this modification, the Poudre Valley Plaza Site will be all of Basin 168 that is remaining to drain onto Horsetooth Road and Detention Pond #103. The runoff from the south half of Horsetooth Road and the east half of Shields Street Iwill flow off site undetained. Runoff from the site will be routed to the northeast corner of the site through the parking lot and released at the rate determined for Basin 168. There will also be a retention area in the southeast corner of the site that will receive runoff from the courtyards. C. Hydrological Criteria The Rational Method for determining surface runoff was used for the project site. The 10-year and 100-year storm event criteria, obtained by the City of Fort Collins, were used in calculating runoff values. These calculations and criteria are included in the Appendix. ID. Hydraulic Criteria IAll hydraulic calculations within this report have been prepared in accordance with the City of Fort Collins Storm Drainage Criteria and are included in the Appendix. E. Variances from Criteria No variances from City of Fort Collins Storm Drainage Criteria are being sought for this project. I IV. DRAINAGE FACILITY DESIGN A. General Concept The majority of the on -site runoff produced by the proposed Poudre Valley Plaza P.U.D. will flow to the northeast corner of the site and will be released at the predetermined rate of 56.5 cfs onto Horsetooth Road. The released storm water will flow on Horsetooth to existing Detention Pond #103 of the Mail Creek ' Hydrology Update. A portion of the site, including the courtyard areas and one building roof, will flow ' to an on -site retention pond located in the southeast corner of the site. The retention pond will be sized to contain twice the 100 year runoff. Following a storm event, the retention pond will be pumped to the northeast corner of the site and released onto Horsetooth Road. 3 determined using a C-value of 0.45 representing a low density residential development. The UDSWM2 model shows runoff from this basin draining onto Horsetooth Road and being conveyed by gutter flow to Detention Pond 103. Since the completion of the Mail Creek Hydrology Update for Existing Conditions, Basin 168 was modified by Northern Engineering Services, Inc. in April, 1994. This modification included decreasing the size of Basin 168 from an area of 13.0 acres to an area of 10.5 acres. The area from the centerline of Century Drive to the east (2.5 acres) was added to Basin 165 which flows southeast and into Detention Pond #104. The allowable release rate from Basin 168 was not changed with this modification. An exhibit showing this modification to Basin 168 is shown in the Appendix. DRAINAGE DESIGN CRITERIA A. Regulations The City of Fort Collins Storm Drainage Design Criteria is being used for the subject site. B. Development Criteria Reference, Constraints, and Comparison The criteria established by McClellands and Mail Creek Major Drainageway Plan, prepared by Cornell Consulting Company, December 1980 and the Mail Creek Hydrology Update for the Existing Conditions, prepared by Water Engineering & Technology, Inc. in July 1991 and modified by Northern Engineering Services, Inc in April, 1994 will be used for this site. With this development, we are proposing that Basin 168 be further modified which will reduce the area from 10.5 acres to approximately 7.7 acres. A triangular portion of Basin 168 (area 'A' on the exhibit in the Appendix) which lies south of the proposed site at the corner of Shields Street and the future western extension of Arbor Avenue was designated in the Hydrology Update to drain north through the proposed Poudre Valley Plaza site to Horsetooth Road. The only way this would be accomplished is to establish a low point in the extension of Arbor Avenue to allow area 'A' to surface drain in a pan across the street. Since the pan across Arbor Avenue is not desirable, the other option is to grade the area to drain into area 'B' and then onto Century Drive following its natural grade. This will required revising the SWMM model to incorporate the area 'A' into Basin 167. This area, which is approximately 2.8 acres, would be modeled to outfall at the equivalent flow of a single family residential development from area 'B' into Basin 167. This imposes a slight increase in the detention requirements for this combined area. The stormwater utility will require written proof that the current property owner of this combined area agrees to this increased detention requirement. The 2 PRELIMINARY DRAINAGE STUDY FOR POUDRE VALLEY PLAZA FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The proposed development is located in the Southeast quadrant of the intersection of Horsetooth Road and Shields Street in southwest Fort Collins. The project is bounded to the north by Horsetooth Road, to the west by Shields Street, to the east by residential development, and to the south by undeveloped land. More precisely the proposed development is located in the Northwest Quarter of Section 35, Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins, Larimer County, Colorado. The project site is shown on the Vicinity Map in the Appendix. B. Description of Property The Poudre Valley Plaza commercial site contains approximately 7.7 acres and is generally undeveloped at the present. The existing site is presently open ground covered with native grasses. The site topography generally slopes toward the northeast corner at approximately 1.2%. IL DRAINAGE BASINS AND SUB -BASINS A. Major Basin Description The project site is located in the Mail Creek Basin and is described in the report entitled McClellands and Mail Creek Major Drainageway Plan, prepared by Cornell Consulting Company in December, 1980. B. Sub -Basin Description This site is included as part of Basin 168 in the Mail Creek Hydrology Update for the Existing Conditions, as prepared by Water Engineering & Technology, Inc. in July 1991. A portion of the exhibit from that hydrology update is included in the Appendix. IIn the study by Water Engineering & Technology, Inc., flows from Basin 168 were 1 PRELEMINARY DRAINAGE STUDY FOR POUDRE VALLEY PLAZA FORT COLLINS, COLORADO June 19, 1995 Prepared for: Wheeler Commercial Property Services, LLC 1125 W. Drake Road, P.O. Box 272030 Fort Collins, Colorado 80527 Prepared by: RBD, Inc. Engineering Consultants 209 South Meldrum Street Fort Collins, Colorado 80521 (303) 482-5922 RBD Job No. 154-008 :LIMINARY DRAINAGE STUDY OR POUDRE VALLEY PLAZA ?ORT COLLINS, COLORADO