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Home My WebLink AboutLIBERTY COMMON HIGH SCHOOL EXPANSION - SPAR - SPA110003 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTMr. Wes Lamarque November 1, 2011 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. Silt fence will require periodic replacement. Maintenance is the responsibility of the contractor. All disturbed areas must be seeded and mulched within 30 days of project start. 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. I trust that I have been able to address your concerns. If you have any questions or comments please contact me at (970) 674-3300 Ext. 106. Sincerely, Pp00 AQG,S p T� Robert Almirall, P.E. p� °v O 33441 Colorado Professional Engineer 33441 ! -o r Cr Interwest Consulting Group 90,E °�� NAL� Attachments Mr. Wes Lamarque November 1, 2011 Page 3 of 4 remaining concentrated areas are conveyed under the sidewalk via proposed channels and/or sidewalk chases to the flow line of Custer Drive. Basin 2 is comprised mainly of Pond 217, the northern half of the school building and the proposed north phase I and II building expansions, the southwest half of the proposed parking lot and the field area north of the channel. Pond 217 discharges to the east under Minnesota Drive via an existing storm culvert and into Extended Detention (Water Quality) Pond 218 prior to being released into the Foothills Regional Channel. The majority of Basin 3 drains to the north to Limon Drive via proposed sidewalk chases and curb cuts and runoff is conveyed in the existing storm drainage system to pond 218 which is just east of this site. This basin consists of the northeast half of the proposed parking lot and the existing sidewalk. Filing Six similarly utilized an "Ultimate Development" condition for the basins that make up the School site. All the area contained within the site was assumed to be Commercial or Multi - Family with an imperviousness of 70% used for the middle basin (SWMM Sub basin 583) and 90% for both the north and south basins (SWMM Sub basins 584 and 58) for an average site imperviousness of 0.79. The actual imperviousness of the site is 0.51, with a composite C Factor of 0.62. Therefore, it is requested that the site be deemed in substantial conformance with the previously approved Final Drainage Reports. EROSION CONTROL Erosion and sedimentation will be controlled on -site by use of silt fences, culvert inlet protection, a gravel construction entrance, 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 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. Wes Lamarque November 1, 2011 Page 2 of 4 EXISTING CONDITIONS The site currently has an existing building and parking lot on the southern portion. A drainage channel/ pond exists through the middle of the site as do the inlet and outlet culverts. The northern portion is vacant. The site drains in three distinct directions 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 The proposed development is phased and consists of an approximately 6,200 square foot school addition in phase 1 with approximately 15,500 square feet possible in a fixture phase for a total of 21,700 square feet. The site will also include a play field, future parking areas and a reconfigured drainage/detention area. The existing parking lot south of the channel will remain in its current configuration Portions of the existing pond are to be re -graded with this development. During phase one, the northwest portion of the pond will be re -graded. Because the proposed phase two building expansion causes a pinch point in the detention pond, during phase two the pond will be split by a retaining wall into two and connected via a 30=inch reinforced concrete pipe. The southeast comer of the pond will also be re -graded in order to contain the 100-year WSEL. A portion of the existing drainage easement will be vacated and a proposed Drainage Easement is shown for this area to contain the 100-year WSEL. Attached are the previously approved stage -storage calculations and the calculations with the proposed improvements. The storage is shown to be equal to or greater than the previously approved design. Therefore, we request that the pond volume be deemed in substantial conformance with the approved design. The existing pedestrian bridge is to be relocated during phase two. 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. BASIN DESCRIPTIONS The site is divided into three basins, the southern (Basin 1), the middle (Basin 2) and the northern (Basin 3). Basin 1 is comprised of the southern half of the school building, the proposed south phase II building expansion and the entire existing parking area. 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. Portions of this basin sheet flow over the existing Custer Drive sidewalk in accordance with the LCUASS. The INTERWEST = CONSULTING G R O U P November 1, 2011 Mr. Wes Lamarque City of Fort Collins Storm Water Utility 700 Wood Street Fort Collins, CO 80522 Re: Liberty Common Charter School — Storm Drainage Report Dear Wes, Please accept the following letter report on behalf of the Liberty Common 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 and the Drainage Letter for Pioneer Charter School dated August 20, 2003, as prepared by Interwest Consulting Group. INTRODUCTION This site is legally known as Tract E of Rigden Farm Filing One. It contains 4.72 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. The design for this pond was included as part of the Filing One Drainage Report and was constructed with that Filing. This pond was amended and is only for detention. The original stage -storage table is attached for reference. 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 Rigden 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. Excerpts from the Filing Six report are attached for reference. 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. The Pioneer Charter School Drainage letter interpolated from the original stage -storage curve to determine a new 100-year water surface elevation for the pond during the ultimate development. 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. The pond was modified as part of that project. 1218 WEST ASH, SUITE C WINDSOR, COLORADO 80550 TEL. 970.674.3300 • FAX. 970.674.3303