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HomeMy WebLinkAboutDrainage Reports - 08/24/1990-r�F,�fla. y� B/zy/90 STORM DRAINAGE REPORT FOR ONE PROSPECT P.U.D. INCLUDING HEC-2 ANALYSIS BY NORTHERN ENGINEERING SERVICES. INC. l PREPARED FOR W.W. REYNOLDS COMPANIES AS R991811 AUGUST 16, 1990 PREPARED BY STEWART & ASSOCIATES. INC. STORM DRAINAGE REPORT'.. ONE PROSPECT P.U.D. SITE LOCATION One Prospect P.U.D. is a replat of Lots 1, 2, 13, 14, 15 and 16 of Prospect Park East P.U.D. It is located in the Northwest I of Section 20, Township 7 North, Range 68 West of the Sixth P.M., City of Fort Collins, County of Larimer, State of Colorado, and is bounded by East Prospect Road on the north, Midpoint Drive on the south, Specht Point Road on the west, and Prospect Parkway on the east. The site is located in the Spring Creek drainage basin. According to the FEMA map, which indicates conditions before the Spring Creek channel was constructed, Lots 2 and 3 of One Prospect P.U.D. are within the 100 year floodway. The construction of the Spring Creek channel has moved the floodway into the Spring Creek channel and off of Lots 2 and 3, but until such time that the FEMA map is officially revised, we are required to design as though the floodway was across Lot 2 and 3. The attached sketch at page 4 of this report shows the One Prospect P.U.D. site and also shows the surrounding streets and tracts. The Spring Creek floodway, according to the FEMA map, is shown on the sketch. EXISTING SITE CONDITIONS The existing site is covered with sparse vegetation, and the surrounding streets are all paved and have curb and gutter. There is an existing drain Swale running west to east through the site. Concrete drain pans are existing across Specht Point Drive on the west and across Prospect Parkway on the east, which channel runoff water into and out of the existing swale. The entire One Prospect P.U.D. site drains to this swale. There is no storm water detention provided on developed sites or proposed on new sites in the Prospect Park East P.U.D. development. This is because the development is adjacent to the Cache La Poudre River, and drainage swales have been provided to carry runoff to the river. OFFSITE DRAINAGE ONTO THE SITE Tract "A" of Centerpoint Park, which is presently undeveloped, but which will be developed in the future under HB zoning, drains through the swale across the One Prospect P.U.D. site. The area of Tract "A", plus the surrounding streets which will contribute runoff, is 13.4 acres. 3.7 acres of this total will.drain directly into Spring Creek, leaving a total of 9.7 acres to drain through the One Prospect P.U.D. swale. According to Engineering Professionals' March 1988, Spring Creek drainage study, the 100.year existing condition storm will cause a 75 c.f.s. overflow of Timberline Road. This overflow will occur at Midpoint Drive, which is the low point at Timerline Road. The included map at page 4 of this report shows the existing elevations along Timberline Road and along the railroad. According to Master. Plan design, the 75.c.f.s. overflow of Timerline Road will be eliminated by future improvements. One Prospect P.U.D. Storm Drainage Report August 13, 1990 Page 2 The 75 c.f.s. overflow will drain east a -long Midpoint Drive and that, combined with the 100 year run—off from the Midpoint Drive right—of-way, will cause an overflow spill into Specht Point Road of 33 c.f.s. This 33 c.f.s. will then be channeled throught the One Prospect P.U.D. site. Calculation sheet 7 of this report shows how.we arrived at this quantity of 33 c.f.s. According to the Northern Engineering Services, Inc. analysis of the Spring Creek flood flows, the overflow of Timberline Road will occur more than 2 hours after the beginning of the storm, therefore this . overflow quantity will not be combined'with the offsite flows from Tract "A". DEVELOPMENT OF SITE This site will be developed as buildings and paved parking lots as shown on the plan at sheet 12 of this report. The site has three drainage sub —basins. The west basin plds the offsite flow must.be carried between the west buildings in a drainage channel. The volume of flow between the west buildings is calculated on sheet 9 of this report, and also shown is a cross—section between the buildings. . The central basin runoff consists of the west buildings roof drainage, the central parking lot and the east buildings roof drainage. The central basin plus the west basin and the offsite flow must pass through the drainage channel between the east buildings. The volume of flow between the east buildings is calculated on sheet 10 of this report, and also shown is a cross—section between the buildings. The third basin is the north parking areas and east parking areas. This runoff will be added to the offsite from the west along with the runoff from the west and central basin to produce the total flow across Prospect Parkway. This calculation is shown on sheet 8 of this report. Included with this report is a HEC-2 analysis of the flows through the site. This analysis was prepared by Mike Jones of Northern Engineering Services, Inc. Mr. Jones' report indicates that the flows will pass through the parking lots and swales between the buildings with a minimim of 1 foot from the water surface to floor elevation. Also, it shows that the flow depth across Specht Point Road and Prospect Parkway will be a maximum of 1.5 feet. 2 One Prospect P.U.D. Storm Drainage Report August 13, 1990 Page 3 . The drainage downstream from One Prospect P.U.D. will be carried through a previously constructed drain pan, and the maximum flows will overflow into parking lots adjoining this pan. The floor elevations of existing downstream buildings are constructed above the maximum 100 year high water level. The previous drainage calculations prepared for the downstream sites used criteria which resulted in smaller 100 year flows than we are now using. These larger flows will cause added depth to the flow across streets and through the drain pan and parking lots, but will not endanger any buildings. Lots 2 and 3 of One Prospect P.M. are within the Spring Creek overflow floodway per the FEMA map. Lot 1 is not in a floodway or flood plain. The FEMA maps do not reflect the revision made to the. Spring Creek channel, which physically eliminated the floodway from the One Prospect P.U.D. The FEMA map must be revised to show the floodway elimination from Lots 2 and 3 before any buildings can be constructed on Lots 2 and 3. We propose to construct the'two buildings on Lot 1 and pursue the revision of the FEMA map so that buildings can be constructed on Lots 2 and 3 in the near future. We hereby request that a variance be granted from the Storm -Drainage Criteria maximum depth of cross flow, in Specht Point.Road and in Prospect Parkway. The calculated 100 year cross flow in Specht Point Road .is 1.5 feet and in Prospect Parkway is 0.6 feet. We believe the variance is warranted due to the facts that the streets, including curb, pavement, and cross -pans are already existing, also the cross flow velocities are very low (3 f.p.s. across Prospect Parkway and 0.3 f.p.s. across Specht Point Road), also both streets.are only one block long and will have access for emergency vehicles from each end. Phillip . 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