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HomeMy WebLinkAboutDrainage Reports - 01/16/1986MASTER DRAINAGE REPORT FOR ARBOR PLAZA AND FINAL DRAINAGE REPORT FOR ARBOR PLAZA PHASE I PREPARED BY: DREXEL, BARRELL & CO. 1700 38TH STREET BOULDER, COLORADO 80301 PREPARED FOR: KROH BROS. DEVELOPMENT CORP. 5690 OTC BLVD., SUITE 200 ENGLEWOOD, CO 80111 NOVEMBER 19, 1985 REVISED DECEMBER 5, 1985 REVISED JANUARY 7, 1986 REVISED JANUARY 16, 1986 E-3357 (0271R) DREXEL, BARRELL & CO. — ENGINEERS 1700 38TH STREET Of BOULDER, COLORADO 80301 (303)442-4338 January 7, 1986 SURVEYORS OFFICES: BOULDER COLORADO SPRINGS This "Piaster Drainage Report for Arb r Plaza and Final Drainage Report for Arbor Plaza Phase I", complies with the C�tA of Fort Collins' "Storm Drainage Design Criteria and Construction Standards', ay, 1984 edi,.,iorvF#,,,,_ E. y#1 �r4nof MASTER DRAINAGE REPORT FOR ARBOR PLAZA AND FINAL DRAINAGE REPORT FOR ARBOR PLAZA PHASE I PURPOSE: This report serves as both a Master Drainage Report for Arbor Plaza and a Final Drainage Report for Arbor Plaza Phase I. SITE DESCRIPTION: Arbor Plaza is a 20.2 acre parcel of land located in the northeast quarter of Section 2, Township 6 North, Range 69 West of the 6th P.M. The property is bordered by the Poudre Valley REA property on the south, College Avenue (U.S. Highway 287) on the east, Harmony Road on the north and the Burlington Northern Railroad property on the west. The proposed land use is commercial and will include the construction of one 102,000 s.f. archor store, 51,913 s.f. of retail space in a shopping center type layout, seven separate building pads, related water, sanitary sewer and storm sewer, approximately 900 L.F. of South Mason Street and approximately 1000 L.F. of 8'x6' box culvert. Phase I is approximately 15.1 acres of the site and includes the construction of South Mason Street, the 8'x6' box culvert, and all buildings, parking lots and related utilities located to the west and south of South Mason Street. HISTORIC DRAINAGE CHARACTERISTICS: As shown on the vicinity map, the Mail Creek traverses the northerly portion of the property from the west to the east. Drainage runoff from the northern 11.4 acres of the site sheetflows to the Mail Creek at slopes of 2-5%. The drainage runoff from the southern 8.8 acres sheetflows at a 3% slope to the south and east. The 20.2 acre parcel is primarily grassland, except for the immediate area of the Mail Creek which is heavily wooded. Presently, there are four wood sheds, a brick and wood frame house, and a garage on the parcel. These buildings will be removed during construction of Phase I. No offsite drainage enters the site. PROPOSED DRAINAGE CHARACTERISTICS: To allow for a more efficient land use, the Mail Creek flow will be conveyed through the site in an 8' wide by 6' high concrete box culvert. The box will carry 600 cfs with a one foot freeboard. Proposed grading of the site will direct all runoff to Mail Creek. The Master Drainage Plan for McClelland Mail Creek indicates that no detention facilities would be required for properties abutting the Creek. Since the Mail Creek does traverse this site, typically, no detention would be required for that portion of the site which historically drained to Mail Creek. However, the existing downstream Mail Creek facilities are inadequate to convey any amount of increased runoff. Therefore, as a temporary measure, onsite detention will be provided to store the 100 year developed runoff of the entire 20.2 acre site and the total site release to Mail Creek will not be more than the 100 year historic runoff of the northern 11.4 acres of the site which historically drains to Mail Creek (Sub -basin A-1). Once the downstream facilities have been improved, the total release rate may be increased to allow for full release of the developed northern 11.4 acres (Sub -basin A-1). Detention will continue to be provided for the southern 8.8 acres (Sub -basin A-2) as it historically drained to the south and not to Mail Creek. Any reevaluation of the site release characteristics will not be undertaken without prior approval of the City of Fort Collins. With the temporary release rate restrictions in mind, the Master Drainage Plan presented in this report utilitizes rooftop detention, parking lot ponding and a landscaped pond to store the developed runoff. Each rooftop will have control structures which will limit the runoff release rate to 1 cfs. Where the storm sewer serving the parking lot detention ponds drain directly into the Mail Creek box culvert, orifice type plates will be placed in the inlets to restrict the pond's outflow to the appropriate historic runoff release rate. All other storm sewer will drain into the proposed landscaped detention pond at the eastern side of the site. A weir structure will be placed in this detention pond to limit the release to Mail Creek. The total amount of developed runoff released to Mail Creek will be restricted to the historic 100 year runoff and 10 year runoff rates for the northern 11.4 acres of the site. The Final Drainage Plan for Arbor Plaza Phase I complies with the Master Drainage Plan described above. Phase I is divided into two sub -basins. The northern 1.7 acre sub -basin utilizes rooftop ponding and a parking lot detention pond. The detention pond will discharge into the proposed box culvert. Rooftop ponding will also be utilized in the southern sub -basin of Phase I. Drainage runoff not collected on the rooftops will be directed overland and in pipes to a temporary landscaped pond located on the east side of South Mason Drive. A weir structure will be used to control the landscaped pond's release to the Mail Creek. The total Phase I runoff release will be restricted to the historic 100 year runoff and 10 year runoff rates for the northern 6.3 acres of Phase I which historically drained into the Mail Creek. When Phase II is developed, the available capacity of the downstream Mail Creek will be re-evaluated. If improvements have been made such that Arbor Plaza's runoff release rate can be increased, the detention volumes will be decreased. If downstream improvements have not been made, the detention volume for the entire 20.2 acre site calculated in this Master Report will be provided. In either case, the temporary landscaped detention pond utilized in Phase I will be abandoned and graded over. A combination of rooftop ponding, parking lot ponding, and a possible landscaped pond will be utilized to provide the necessary detention ponding for the 20.2 acre site. Control structures on rooftops, orifice type plates in inlets, and weir structures in ponds will be used to control the total runoff release rate to Mail Creek. This release rate and specific detention details will be determined in the Final Drainage Report for Arbor Plaza Phase II. BOX CULVERT: As previously stated, the Mail Creek flow will be conveyed through the site in an 8 foot wide by 6 foot high concrete box culvert. The foundations for the buildings in the vicinity of the culvert will be on caissons which bear on soil below the box culvert. This will allow the box to be worked on or replaced without affecting the building or requiring special excavation procedures. ON -SITE STORM WATER COLLECTION SYSTEM The on -site storm water collection system has been sized within the parameters of the calculated runoff and future basin release rates studied in the master drainage portion of this report. Pipe sizes and capacities for each run of storm main are indicated on the Master Utility Plan for the development and were calculated using the Mannings Equation and the various pipe slopes. Please note that, except for rooftop detention areas and the permanent detention pond in the northwesterly portion of the site, no attempt has been made to size the detention ponds, other than for Phase I. At the time of Phase II development it is the intent to review the drainage concept in detail and provide a workable drainage plan using this Master Study and City of Fort Collins criteria as the basis for future study. CRITERIA: The City of Fort Collins "Storm Drainage Criteria and Construction Standards" as well as the "Urban Drainage and Flood Control Storm Drainage Criteria Manual" were followed in preparation of this report. The rational method was used to determine the historic and developed flows for the 2 year, 10 year and 100 year storms. U.S. Department of Transportation's "Hydraulic Design of Improved Inlets for Culvert", August, 1972, was utilized in designing the inlet for the proposed box culvert. The hydraulic grade line for the box culvert was calculated using the Direct Step Method, Mannings Equation, and the friction slope, using flow data supplied by the City of Fort Collins for the 100 year storm occurance. CONCLUSIONS: This report serves as a Master Drainage Report for Arbor Plaza and a Final Drainage Report for Arbor Plaza Phase I. Since the existing downstream facilities of Mail Creek are inadequate to convey any increased runoff, this master plan utilizes temporary detention facilities to limit the developed release rate to the amount that historically entered the creek during a 100 year and 10 year year storm. As the downstream facilities are improved, the allowable release rate from the site will be increased and the temporary detention facilities will be abandoned. Since Phase I is scheduled to be constructed prior to any downstream improvements, detention will be provided for the developed runoff from a 100 year storm. The Phase I release rate will be limited to the historic 100 year, and 10 year release rate for that portion of Phase I which historically drained to Mail Creek. At the time Phase II is developed, a Final Drainage Plan for Phase II will be prepared and will re-evaluate the allowable release rate of the entire project with respect to the capacity of the downstream Mail Creek facilities. Detention volumes and release rates may be modified at that time. A vicinity map, calculations and Drexel, Barrell & Co. Drawing No. 3D-701, Sheets 3 and 14 are included for your review. Respectfully submitted, Debra J n Northrop, P.E. 1 Reviewe `�y: George N. bl�pers P.E. #18056