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Home My WebLink AboutRIGDEN FARM, PIONEER CHARTER SCHOOL - SITE PLAN ADVISORY REVIEW - 56-98U - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTI Mr. Basil Hamdan June 20, 2003 Page 4 of 4 All temporary and permanent erosion and sediment control practices must be maintained and repaired as needed to assure continued performance of their intended function. Straw bale dikes or silt fences will required periodic replacement. Sediment traps (behind straw bale barriers) shall be cleaned when accumulated sediments equal approximately one-half of trap storage capacity. Maintenance is the responsibility of the contractor. A vegetative cover shall be established within one and one-half years on disturbed areas and soil stockpiles not otherwise permanently stabilized. Vegetation shall not be considered established until a ground cover is achieved which is demonstrated to be mature enough to control soil erosion to the satisfaction of the City Inspector and to survive severe weather condition. If you have any questions or comments please do not hesitate to contact me at my office. Sincerely, XQ Co S�. Robert Almirall, P.E. :a: Colorado Professional Engineer 33441 9 33441 o FSS�G Attachment AN Mr. Basil Hamdan June 20, 2003 Page 3 of 4 Minnesota Drive and into Extended Detention (Water Quality) Pond 218 prior to being released into the Foothills Regional Channel. The southern most basin is comprised of the southern half of the school building and the entire parking area south of the channel. This basin drains to the south toward Custer Drive where it is intercepted and conveyed via curb, gutter and storm drainage pipes to the Water Quality ponds located north of the Foothills Regional Channel. Since the parking lot is proposed to be a highly pervious gravel system, virtually all runoff generated from the travel aisle and parking lot will infiltrate prior to reaching the public street. Therefore, a storm inlet or by-pass culvert is not necessary to avoid runoff from crossing the public sidewalk. Filing Six utilized an "Ultimate Development" condition for the basins that make up the School site. All the area contained within the site were assumed to be Commercial or Multi -Family with an imperviousness of 70% used for the middle basin and 90% used for both the north and south basins for an average site imperviousness of 0.79. The actual imperviousness of the site, including the future parking and building site to the north of the channel, is 0.61. Therefore, it is requested that the site be deemed in substantial conformance with the previously approved Final Drainage Report. EROSION CONTROL Erosion and sedimentation will be controlled on -site by use of silt fences, culvert inlet protection, gravel constructions entrances and seeding and mulch. The measures are designed to limit the overall sediment yield increase due to construction as required by the City of Fort Collins. During overlot and final grading the soil will be roughened and furrowed perpendicular to the prevailing winds. During the performance of the work required by these specifications or any operations appurtenant thereto, whether on right-of-way provided by the City or elsewhere, the contractor shall furnish all labor, equipment, materials, and means required. The Contractor shall carry out proper efficient measures wherever and as necessary to reduce dust nuisance, and to prevent dust nuisance that has originated from his operations from damaging crops, orchards, cultivated fields, and dwellings, or causing naissance to persons. The Contractor will be held liable for any damage resulting from dust originating from his operations under these specifications on right- of-way or elsewhere. It is unlawful to track or cause to be tracked mud or other debris onto city streets or rights -of - way unless so approved by the Director of Engineering in writing. Wherever construction vehicles access routes or intersect paved public roads, previsions must be made to minimize the transport of sediment by runoff or vehicles tracking onto the paved surface. Stabilized construction entrances are required per the detail shown on the Erosion Control Plan, with base material consisting of 6" coarse aggregate. The contractor will be responsible for clearing mud tracked onto city streets on a daily basis. Mr. Basil Hamdan June 20, 2003 Page 2 of 4 and these general basin lines have been maintained with the proposed development and in conformance with the Ultimate Development basin delineations as shown in the Rigden Farm Filing Six Final Drainage Report. PROPOSED DEVELOPMENT Portions of the existing pond are to be re -graded with this development. The northwest corner of the pond has been re -graded in order to contain the 100-year WSEL within the recorded Drainage Easement. The southwest corner has been re -graded to incorporate a small amphitheatre into the embankment. A proposed Drainage Easement is shown for this area to contain the 100-year WSEL. The southeast corner of the pond has been steepened to a 3:1 slope, in accordance with the previously approved variance made part of the Filing One Final Drainage Report, in order to accommodate the proposed improvements adjacent to the pond Attached are the previously approved stage -storage calculations and the calculations with the proposed improvements. At each critical stage the storage is shown to be equal to or greater than the previously approved design. Therefore, we request that the pond be deemed in substantial conformance with the approved design. A pedestrian bridge is proposed to cross the drainage channel/pond. The bottom chord of the bridge is to be set at a minimum elevation of 4924.0 in order to free span the proposed 100-year WSEL. While the abutments will reduce the overall width of the channel the pond functions in a backwater condition; therefore, the constraint will not impact the function of the pond. The site development proposes the use of Gravel Pave (or approved equal) for the parking lot area in the southeast portion of the site. This system incorporates a large layer of pit -run gravel (typically around 12-inches deep) and an interlocking structural honeycomb system just below the surface and the combs are filled with smaller size gravel. This system provides the benefit of a highly pervious surface with the structural stability significantly better than a typical gravel section. Because the structural member on the surface does not allow the rutting and compaction often seen with standard gravel sections, this system remains significantly more porous over it's life span. For the purpose of impervious calculations, this area has been assigned a C Factor of 0.50 which is consistent with standard gravel sections. I: RVI log)]*%%RI"CII&I The site is divided into three basins. The northern basin drains to the north toward Limon Drive and runoff is conveyed in the existing storm drainage system to pond 218 which is just east of this site. This basin is currently proposed to remain undisturbed; however, this area has the potential to be developed in the future with additional parking and another building. Therefore, the calculations provided have assumed this ultimate development. The middle basin is comprised mainly of the existing drainage channel. The northern half of the proposed school building will also drain into this basin. Pond 217 discharges to the east under June 20, 2003 Mr. Basil Hamdan City of Fort Collins Storm Water Utility 700 Wood Street Fort Collins, CO 80522 Re: Pioneer Charter School — Storm Drainage Report CO-03-03-1 Dear Basil, I N T E R W E S T CONSULTING G R O U P Please accept the following letter report on behalf of the Pioneer Charter School to demonstrate the site's compliance with the previously approved Final Drainage Reports for Rigden Farm Filing One and Filing Six, as prepared by JR Engineering. INTRODUCTION This site is legally known as Tract E of Rigden Farm Filing One. It contains 4.27 acres and is bounded on all four sides by existing city roadways, namely being Custer Drive, Kansas Drive, Limon Drive and Minnesota Drive. The site is bisected by a previously constructed storm drainage channel/detention pond which is referred to as Pond 217. This pond was designed as part of the Filing One Drainage Report and was constructed with that Filing. There are three ponds in series which drain from west to east (Ponds 216, 217 and 218) which were modeled using EXTRAN and SWMM. All detention and water quality requirements for this site have been met by the overall Ridgen Farm development. Rigden Farm Filing Six updated the basins in the vicinity of this pond based upon anticipated ultimate development; however, the original pond sizing (stage -storage) was not altered. Filing Six did update the SWMM model using the updated basin information; however, the EXTRAN model was not re -run with that Filing. Pursuant to our previous conversations, the Filing Six updated SWMM model shows that the peak storage of Pond 217 during a 100-year storm event is 2.2 acre-feet. By interpolating from the original stage -storage curve, it was determined that the new 100-year water surface elevation for the pond during ultimate development is 4924.0. We have shown a 30-foot wide emergency spillway into the parking lot at elevation 4924.3 and a minimum top of pond elevation of 4925.0 in order to comply with minimum freeboard criteria. EXISTING CONDITIONS The site is currently vacant and contains a high stand of grasses. The channel exists through the middle of the site as do the inlet and outlet culverts. The site drains in three distinct directions 1235 HONEYSUCKLE CT. • WINDSOR, CO 80550 r E L. 970. 222. 9133 • FAX 970. 686. 9576