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Home My WebLink AboutSite Certifications - 07/16/1996ASSOCIATES X STORMWATER March 19,1996 ,) p `VRECEIVED \� (/ MAR 2 0 1996 Mr. Basil Harridan City of Fort Collins Stormwater tility 235 Mathews Fort Collins, Colorado 80524 Re: North Lemay Plaza PUD Drainage Certification Dear Basil, This letter is in reference to the site grading and drainage certification for the above -mentioned PUD. It summarizes the results of our field examination and computations performed to certify that the as -constructed elements succeed in operating according to the approved design. Ayres Associates surveyed the site in May and August 1995. The as -constructed elevations are shown on the accompanying marked grading plan. We note that a check mark next to an elevation indicates that the as -constructed elevation equals the design elevation. The site was visited on March 18, 1996, to verify the removal of a tree adjacent to the area inlet at the southeast corner of the site. The tree has been removed. We note the following items: • Surveyed spot elevations in the parking lots, drainage swales, drainage divides, and area inlets located in the Phase I area were within 0.10 foot of design elevation shown on the Grading and Paving Plan by Parsons & Associates, Inc. (approved November 18, 1994) and subsequent revisions provided by Parsons on March 23, 1995. The building first floor elevation, 56.0, agrees exactly with the design elevation and is 1 foot above the pond overflow elevation (the entrance off of Lemay Avenue) and 1 foot above the computed detention pond 100-year water surface elevation of 55.0. In the Phase II area, which is not yet developed, most spot elevations are within 0.2 foot of the approved interim condition grading plan shown on the marked grading plan. The actual 55-foot contour, which corresponds to the peak 100-year detention pond water surface elevation, encompasses a slightly larger area than the design 55-foot contour; therefore, the actual.storage volume at that elevation should exceed the design volume. • The "special' area inlet located at the southeast corner of the site varies from the approved design in several ways. First, the inlet was rotated approximately 90° due to a conflict with an existing telephone vault. The actual size is 48 inches by 102 inches (the design shows an inlet of 48 by 103.5 inches). The size difference results from use of a different grate than that specified. The as -constructed inlet and grate provide an open area of 17.8 square feet (computations shown on the attached sheets), exceeding the required 14.3 square feet computed in the drainage report. The outlet pipe draining the area inlet is a 42-inch diameter RCP; the design called for a 36-inch pipe. Owen Ayres & Associates, Inc. Engineers/Scientists/Surveyors HAMD-3A.LTR 3665 JFK Parkway, Building 2, Suite 300, P.O. Box 270460, Fort Collins, CO 80527 34-0368.00 (970) 223-5556, Denver Metro (303) 572=1806, FAX (970) 223-5578 Printed on recycled paper Mr. Basil Harridan Page Two March 19, 1996 • The area inlet at the southwest corner of the PUD is of the design dimensions and is located at the design elevation. The 18-inch RCP pipe draining this inlet lies at a slope of 0.21 percent (design grade is 0.25 percent). The slope reduction is a result of changes at the southeast area inlet described above. The required capacity of the 18-inch RCP is difficult to determine from the approved drainage report. The report states that the pipe is intended to convey nuisance flows and is designed for the 10-year discharge of 11 cfs; however, the computations in the report show a pipe capacity of only 5.3 cfs. The slope reduction reduces the pipe capacity to 4.8 cfs (as shown in the attached computations). • The largest difference between design grades and actual grades occurs on top of the berm at the southwest corner of Phase II. The design elevation in that location is 56 while the actual grade is 55.5. This should not be significant for two reasons: (1) there are design spot elevations of 55.5 on top of the berm elsewhere, and (2) the existing grades of the pond just south of the area indicate a top -of -bank elevation of 55.0; water will overtop at this location prior to overtopping the berm along the south edge of Phase II. The as -constructed site grading and drainage facilities fulfill the intent of the design grading plan. This letter does not certify that erosion control features are complete. Please call me if you have questions regarding our findings. Sincerely, Owen Ay s & Associates, Inc. 4 Scott Qu en, P.E. Civil Engineer SQ:sp Enclosure HAMD-3A.LTR 34-0368.00 CC O I , NORT! G C C F Storm Sewer Line A - N. Lemay Plaza PUD Worksheet for Circular Channel Project Description Project File untitled Worksheet North Lemay Plaza PUD Flow Element Circular Channel Method Manning's Formula Solve For Discharge Input Data Mannings Coefficient 0.013 Channel Slope 0.002100 ft/ft Depth 1.50 ft Diameter 1.50 ft Results Discharge 4.81 ft3/s Flow Area 1.77 ftz Wetted Perimeter 4.71 ft Top Width 0.00 ft Critical Depth 0.84 ft Percent Full 100.00 % Critical Slope 0.005701 ft/ft Velocity 2.72 ft/s Velocity Head 0.12 ft Specific Energy FULL ft Froude Number FULL Maximum Discharge 5.18 ft3/s Full Flow Capacity 4.81 ft3/s _ Full Flow Slope 0.002100 ft/ft gLl a r a rweod e7a� . 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