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Home My WebLink AboutROCKBRIDGE CONDO RESIDENCES PUD MINOR SUBDIVISION - 82 93B - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTNo Text Rockbridge Garden Apartments P.U.D. Page 3 and pavement (Basin 1) will generate runoff to fill this group of ponds. The release from the pond will be through a restricted outlet on the most downstream area inlet before being piped to the new 24 inch storm drain pipe along Harmony Road. The other two detention ponds are located on Basin 2 at the Northwest comer of the site and on Basin 5 which is the Clubhouse and its parking lot. The new 24 inch pipe proposed along Harmony Road will replace the existing swale and concrete drain pan and will be extended to the west to pick up pond release and runoff from the Courtney Park area. An area inlet will be constructed at the west end of this pipe to pick up surface flow. It is desirable to install a pipe instead of the considerably deep swale so that the 800 foot strip adjacent to Harmony Road can be bermed and landscaped, and also to provide a route for the Harmony corridor path. The problem of standing water in the existing borrow ditch along the north side of Harmony Road will be relieved by extending a 15 inch storm drain pipe to the ditch and constructing a new area inlet to pick up the flow. This inlet and pipe will drain directly into the 24 inch pipe running east under Wheaton Drive. The Harmony Road flows will be intercepted by the new area inlet and therefore will not flow into the Wheaton Drive gutters. This will help relieve the possible problem of ponding at the existing 4 foot catch basin at the low point in Wheaton Drive. The runoff from Harmony Road plus the runoff from the southerly portion of the landscaped area of the Rockbridge development can be handled by the existing ditch along Harmony. The added runoff from the landscaped area of 0.73 acres is QZ = 0.37 c.f s. and Qioo = 1.32 c.f.s.. There is a dry creek bed and an entry bridge at the entrance driveway into the site. This landscaping feature will have no effect on storm drainage. EROSION CONTROL The entire site will be overlot graded to provide building pads, parking lots and landscape areas. A silt fence will be installed along the back of the curb along Wheaton Drive immediately after grading begins. The underground utility lines and storm drain pipes will then be installed, followed by concrete curbs and inlets. Sediment traps shall be placed around the inlets for sediment control, and gravel base in the pavement areas will provide erosion control The building pad construction will begin and landscaped areas can be mulched and seeded. All erosion control methods shall comply with City of Fort Collins Criteria. The Rockbridge site is located in a moderate rainfall'and wind erodibility zone. t6e„ Phillip I. Robinson, P.E. & L.S. �I Rockbridge Garden Apartments P.U.D. Page 2 PROPOSED DEVELOPMENT AND DRAINAGE FACILITIES i The site will be developed with eight apartment buildings and the accompanying parking and recreation facilities. The density will be approximately 20 units per acre. The Rockbridge Apartments site is located in the southwest comer of the Golden Meadows site approximately 3000 feet from the regional detention pond. All of the drainage facilities, piping and swales are in place between Rockbridge and the pond, but these facilities are not adequate to carry the entire 100-year runoff; so we plan to have onsite detention in the parking lots and landscaped areas. The capacity of the major swale along Harmony Road is about 150 c.f s., per the Golden Meadows Fifth report, but the 24 inch pipes under Wheaton, McMurray and Innovation have capacities of approximately 30 c.f.s. before overflow occurs. Also, the crossing under the Warren Lake Outlet Ditch, which is 3-24 inch pipes, has a capacity of about 90 c.f s. before overflow begins. As previously stated, these pipes are in need of maintenance. ' The 1978 Resource Consultants report considered this site to be 40 percent pervious, which equates to a C factor of 0.60. In the 1980 drainage report for Golden Meadows Fifth, a C factor of 0.5 was estimated for this Rockbridge site, along with a detention quantity of 1.2 acre ' feet and a release of 5.0 c.f s.. The calculated composite C factor for the proposed Rockbridge site is 0.643. The historic ' 2-year runoff and release rate is calculated to be 2.3 c.f s., and the detention volume required for the difference from this 2-year historic runoff and the 100-year developed runoff is 1.24 acre feet. ' We proposed to provide 1.28 acre feet of onsite detention in three ponding areas, and the total combined release rate from the three ponds will be 1.34 c.f. s.. There are two areas of the site, one being the bermed landscaped area along Harmony Road and the other being the beamed, ' fairly steep, landscaped area adjacent to Wheaton Drive which cannot be routed through the detention ponds. The combined 100-year runoff of these two areas is 4.67 c.f s., which becomes ' a total 100-year site release rate of 6.01 c.f s. when added to the 1.34 c.f s. restricted release from the three onsite ponds. We request a variance from the Storm Drainage Criteria to release the undetained runoff from Basins 3 and 4. This request is reasonable since detention volume has ' already been provided for these basins in the Golden Meadows Regional Pond (as per Resource Consultants report), and also because this additional release will have a minimal impact on the downstream conveyance system. There are three onsite detention ponds. The large ponding system is a group of ponds located in the paved parking areas. This group of ponds has the same high water elevation, and they are connected by a 15 inch piping system as well as surface overflow, so they act as one pond. Any overflow from the entire ponding system will be through the entrance driveway into Wheaton Drive. The building garage floors are set a minimum one foot above the high water line, 1 and the detached garage floors are set one-half foot above the high water line. All building roofs FINAL DRAINAGE REPORT FOR ROCKBRIDGE GARDEN APARTMENTS P.U.D. ' PROJECT LOCATION AND EXISTING CONDITIONS ' Rockbridge Garden Apartments P.U.D. is located in the Southwest 1/4 of Section 31, Township 7 North, Range 68 West, of the Sixth P.M., City of Fort Collins, County of Larimer, ' State of Colorado. The site is in the Golden Meadows area and is located in the Fox Meadows drainage basin. The site has a net area of 8.2 acres and a gross area of 10.0 acres, including one- half of Harmony Road and Wheaton Drive. The adjacent streets, Harmony Road and Wheaton Drive, are My constructed, and all offsite drainage facilities are in place. The site drains easterly to Wheaton Drive and also south to a major drain swale which runs west to east along the north side of Harmony Road. The runoff from Wheaton Drive goes into this swale via catch basins and storm drain pipes. This major swale carries runoff east to the west side of the U.P. Railroad, and then it turns north along the railroad and then dumps into the Golden Meadows regional pond. This regional detention pond was designed in 1978 by Resource Consultants to provide detention for the total Golden Meadows developments, including the Rockbridge Apartments site. This 1978 drainage study and detention pond design was prepared for the 230-acre Collindale South development (now called Golden Meadows), and the pond was sized to detain runoff from this entire 230-acre development. The Rockbridge Garden Apartments site is included in the southern drainage area of this 230-acre site. A map of the Golden Meadows area showing the location of this major swale, the location of the regional detention pond, and flow patterns of the Golden Meadows area is included with this report. Also shown on this map at crossings and at concentration points are developed flow quantities, some of which were calculated by previously prepared storm drainage. reports. We have included calculations of the capacity of the pipes and Swale. Also included is a copy of the 1979 plan of the piped Swale crossing of the Warren Lake Outlet Ditch. Onsite inspection of this crossing found the pipes in need of maintenance and cleaning. Included in the Appendix are copies of a portion of the 1978 Resource Consultants drainage study of the Golden Meadows area along with a map of the regional detention pond. The runoff from Harmony Road drains into an existing borrow ditch which is located about 8 feet north of the pavement. This borrow ditch slopes to the east and is very flat. This ditch has no positive flow outlet and water must pond about 1 foot deep before overflow occurs to the north along Wheaton Drive. There is offsite drainage onto the site from the existing Courtney Park development lying to the west. One point of offsite drainage is at the southwest comer of the site where a 15-inch pipe from an existing detention pond drains into the existing major Swale running west to east across the site. The other offsite runoff comes onto the west side of the site from 0.67 acres of the rear yards of the Courtney Park apartments. STEWARr&AsSOCIATES Consulting Engineers and Surveyors ' May 30, 1995 Mr. Basil Hamdan Stormwater Utility City of Fort Collins P.O. Box 580 Fort Collins, CO 80522 Dear Basil: Following is the revised storm drainage and erosion control report for the Rockbridge Garden Apartment P.U.D. project. This project has been on and off for the last several months, but now the owners are prepared to go ahead. I have made the final revisions to this report and to the plans. The design and calculations included in this report and on the plans comply with the City of Fort Collins Storm Drainage Design Criteria. A variance is being requested for the allowable release rate from the site. In 1980, Stewart & Associates prepared the drainage report for Golden Meadows Fifth Filing, and in that report a release rate of 5.0 c.f.s. was estimated for the Rockbridge site. The total Rockbridge site runoff will be 6.01 c.fs., which includes the restricted release from the detention ponds plus the unrestricted runoff from landscaped areas around the perimeter of the site. This variance request is for the additional 1.01 c.f.s. release above the 1980 estimate. Sincerely, Phillip I. Robinson, P.E. & L.S. Vice President rsc enclosures James H. Stewart and Associates, Inc. 103 S. Meldrum Street P.O. Box 429 Ft. Collins, CO 80522 303/482-9331 Fax 303/482-9382 Fina - .Report .4AL STORM DRAINAGE AND EROSION CONTROL REPORT FOR ROCKBRIDGE GARDEN APARTMENTS P.U.D. PREPARED FOR JIM LOFTUS PREPARED BY STEWART & ASSOCIATES, INC. 103 SOUTH MELDRUM STREET FORT COLLINS, COLORADO 80521 970/482-9331 DATE MAY 309 1995 STEWART&JkSSOCIATES 103 S. MEL9 DRUM , FORT COLLINS, CO 80521 Consulting Engineers and Surveyors By: PI Date: i2 1 5 Client: Fa n�- x Al ; C-Ad I5 Sheet No. of Project: Rd c L< b r- i C14 P. CO n d o 1 n ifs S V! S (d i✓ Subject: n7/a StTF %ZFLrASE R,-TF �i STEWART&O SSOCIATES 10 S- ME 3D1RUM, FAX FORT COLLINS, CO 80521 Consulting Engineers and Surveyors By: F ) T<1 Date: Z ? 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MELDRUM, FORT COLLINS, CO 80521 PH. 482-9331 FAX 482-9382 By: P1 R Date: i/ / > Client: and i r hot J Sheet No. ot� Project: i7o cJ-T Can d rJM / i11510 Al Subject: t3 eV i S e c Gas in # $asl✓1 1 4,17 Ac j-iar� sur_�G�F . Z3 1.0(4/+..c. 3 ✓�� ► 1. / B L� �F TE i701V ✓_. o,�vn!i � - �. ZS .9 � Ft_ .. 5 NJ8 cv. F7. FF_LIT-A3S&- 1F/-)7E Use same rare as or;5 %��,/ basis j = d, 71. c DJ tlolvMe �e�vJ-e� F� " % Mass D.1 a9 raM__ � L 6A !� V iRe(cofe Tie Jlre W 5-2 S..Z3 27.2 3Z.C3 30 f-.i5 z1.7 3996e /4-zz 37G4-6 0 -3..5 � /-.3 .4-343z - .leyt .¢Z03G SU .... 3-.0 ( 15.7 47070. 2370 4-4-70CJ C0 2, 13.1/ 4CF-cr5-J 2k1-4- 1410.1 _.70__... .2. 3 ! z, 0 s0 r 2. l 33 l 0 ¢7 Z03 60 .. 2,0 to, 7 5/46 3 3771 4-V V .. qo . 1, 8S y. 7. izo 1,1-5 7. 5"01 Hl?3. 1S-0 1•05' s.s 5-9308 3Z 5-0774' 4--- M/Ax1MVM REQV)REP PETE010d/ VUU/MF_ = TO70. GF 11 1. 17 /}-L 1 F D December 19, 1995 Page 2 There will be only two detention ponding areas for the revised layout. The original small pond located on the parking lot at the entrance will be eliminated. The original small pond located in the green space at the northwest comer of the site will be retained. The shape of this pond will be revised, but the volume and release will be the same as originally calculated, since the area draining to the pond is virtually the same. The large ponding area located both on parking lots and green space is being revised from the original plan in that the original ponding was mostly on the parking lots with a small amount on a grassed area, while the revised layout has most of the detention volume in a grassed area with a smaller amount in the parking lots. The original pond had a volume of 1.13 acre feet with a release of 0.79 cfs. We propose to keep the 0.79 cfs release rate and add volume for the 0.05 acre feet of detention which was lost when the small pond at the entrance was eliminated. The volume of the proposed pond is a total of 1.25 acre feet. Most of this volume is in the grassed area and the maximum depth in the parking lots will be 1.0 foot instead of the original 1.5 feet. The maximum ponding depth in the grassed area will be 5.2 feet at the outlet pipe. A variance from the maximum ponding depth of 4.0 feet is hereby requested for the 5.2 foot depth. The outlet pipe from this pond will tie into the back of the existing catch basin located on the west side of Wheaton Drive. This is the same location where the outlet pipe from the original small pond was to be tied into the existing storm drainage system. The original total 2 year release from the site was 2.60 cfs and the revised 2 year release will be 2.45 cfs. ... The original total 100 year release from the site was 6.01 cfs and the revised 100 year release will be 5.96 cfs. The revised plan has very little effect on the original drainage and detention quantities. The largest change in the drainage is that most of the detention will now be on a grassed pond rather than in the parking lots, and therefore, the ponding depth in the parking lots can be lowered from 1.5 feet to 1.0 foot. I have included in this report a copy of the original approved drainage report along with calculations of the revised detention volumes and release rates. Also included are copies of both the original and revised grading, drainage and erosion control plans. Sincerely, Phillip I. Robinson, P.E. & L.S. Vice President rsc enclosures 1 L 1 II it II II I I I 1 STEWART$,LSSOCIATES Consulting Engineers and Surveyors December 19, 1995 Mr. Basil Hamdan Stormwater Utility City of Fort Collins P.O. Box 580 Fort Collins, CO 80522 Dear Basil: Re: Rockbridge Condominium Residences, a revision of Rockbridge Apartments The layout of the Rockbridge project, which was approved July 21, 1995, has been revised. The buildings have been changed from apartments to condominiums and the locations of buildings E, F, G and H have been shifted. The hardsurface area of the buildings remain the same. The hardsurface of the parking lot and driveways is being increased from 2.55 acres to 2.63 acres. The most change on the project is the relocation of the clubhouse from the front of the property to the center of the site. The original hardsurface area of the pool and clubhouse combined was 0.29 acres, and the hardsurface area of the revised pool and clubhouse area is 0.21 acres. The total revised layout ends up with no net change of hardsurface area. Since the hardsurface and therefore the runoff coefficient of the site is unchanged by the revisions, a total detention volume and release rate which are equal to or less than the originally calculated quantities should be acceptable. The original site had three separate detention ponding areas which were as follows: 1. The larger ponding volume of 49367 cubic feet with a release of 0.79 cfs was located mostly on the parking areas and with a smaller portion located on a green space. This pond had a maximum depth in the parking lots of 1.5 feet. ' 2. A small pond was located in a green space at the northwest comer of the site. This pond had a volume of 4284 cubic feet and a release rate of 0.36 cfs into Wheaton Drive through a sidewalk culvert. ' 3. A small pond was located on the parking area near the entrance to the site. This pond had a volume of 2207 cubic feet and a release rate of 0.19 cfs. The maximum depth of this pond was 1.5 feet. James H. Stewart and Associates, Inc. 103 S. Meldrum Street P.O. Box 429 Ft. Collins, CO 80522 303/482-9331 Fax .gO3/482-9382 REVISED FINAL STORM DRAINAGE AND EROSION CONTROL REPORT FOR ROCKBRIDGE CONDOMINIUM RESIDENCES P.U.D. PREPARED FOR THE NICHOLS PARTNERSHIP, INC. RANDY G. NICHOLS PREPARED BY STEWART & ASSOCIATES, INC. 103 SOUTH MELDRUM STREET FORT COLLINS, COLORADO 80521 970/482-9331 DATE DECEMBER 2091995