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Home My WebLink AboutHOMESTEAD AT CLARENDON HILLS - FDP200032 - - DRAINAGE REPORT (4) December 18, 2020 DRAINAGE LETTER AND LID REPORT HOMESTEAD AT CLARENDON HILLS Fort Collins, Colorado Prepared for: Kenneth R. Mitchell Mosaic Real Estate LLC 1021 Nightingale Drive Fort Collins, Colorado. 80525 Prepared by: 301 North Mason Street, Suite 100 Fort Collins, Colorado 80524 Phone: 970.221.4158 www.northernengineering.com Project Number: 1013-008 Drainage Letter and LID Report – Homestead at Clarendon Hills Date: December 18, 2020 Project: Homestead at Clarendon Hills Project No. 1013-008 Preliminary Development Plan (PDP) Fort Collins, Colorado Attn: City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 To whom it may concern: This letter serves to address the stormwater impacts of the proposed project known as “Homestead at Clarendon Hills”. The site is located in southwest Fort Collins bounded to the north by Front Range Community College, to the east by existing Clarendon Hills Subdivision, to the south by Clarendon Hills Drive and Langdale Drive, and to the west by South Shields Street. The project is part of the Drainage Report for Clarendon Hills by Stuart and Associates written in 1986. The proposed development consists of 8 low-density single-family lots and a detention area. The project site also contains an existing irrigation ditch along the north property line. The project was included in the drainage report for Clarendon Hills Subdivision. Therefore, this letter intends to show conformance with the approved drainage design, including general drainage patterns and imperviousness. This letter also documents how the project meets new Low Impact Development requirements implemented City of Fort Collins since the approval of the original project. Original Drainage Patterns The original Clarendon Hill drainage design divided the subdivision into 3 basins. The current project site is split by two of these basins - Drainage Basin A draining to the north, and Drainage Basin B draining to the south. Drainage Basin B flows into Clarendon Hills Drive and Langdale Drive and travels via gutter and pan flow south along Clarendon Hills Drive. The flow is then collected by inlets at Clarendon Hills Drive and Hinsdale Drive and conveyed to a series of existing detention ponds. Drainage Basin A generally flows to the northeast to an existing irrigation ditch on the property. A small undisturbed area north of the irrigation ditch will flow into the Front Range Community College drainage system. From there, it flows through existing drainage ways east to Mail Creek, and eventually to Fossil Creek. An Existing Drainage Exhibit and excerpts from the original report are provided in the appendices for reference. Page | 3 Proposed Drainage Patterns The Homestead at Clarendon Hills project was divided into seven proposed drainage basins that essentially follow the established drainage patterns, as well as the drainage patterns approved with the Drainage Report for Clarendon Hills. Basin A encompasses the rear of the lots and open space between the rear lot lines and the existing irrigation ditch. The only proposed improvements with Basin A are landscaping in the rear of the lots. Flows from the basin will drain north into the irrigation ditch. Basin A is consistent with Basin A from the Drainage Report for Clarendon Hills. Basin B will roughly match the existing area draining south to the existing Clarendon Hills ponds. Proposed improvements will include single-family homes, the front and side of the lots, driveways, walks, and detention area. Basin B is further subdivided into Basins B1, B2, B3, and B4. Basins B1 and B3 consists of the open space at the west and east ends that will discharge into the Clarendon Hills and Langdon Drives curb and gutter. Basins B2 consists of Lots 1 through 8 building envelopes, driveways and detention pond. The project was graded to allow the building envelopes to drain to south, and small swales and curb and gutter in the shared driveway will convey drainage towards the detention pond. Basin B4 consists of the landscaping along the front of the pond that is incapable of draining into the detention pond. The release rate from the detention pond is reduced to accommodate the undetained releases from Basins B1, B3, and B4. An exhibit calculating the release rate is included in the appendices. The percent impervious for the lots was developed by assuming a typical building footprint of 3,250 square feet. The building footprint was placed along the rear building envelope to maximize the length of the driveway. This resulted in 49% imperviousness on a 7,982 sq. ft. lot. The area of the driveway was adjusted on the other lots to maintain a 49% imperviousness on the other lots. Along with the shared driveway, this resulted in a percent imperviousness of 51% in Basin B3. An exhibit is included in the appendices. Additionally, Basin OS1 and OS2 are on the east and the north sides of the site to quantify flows to the Front Range Community College campus. Basin OS1 consists of the side and rear of Lot 8 and adjacent open space. Basin OS2 is a small sliver of open space on the north side of the irrigation ditch that drains directly to the Front Range Community College campus. Development of Existing Runoff Coefficient The drainage for Clarendon Hills Subdivision was originally analyzed in the Drainage Report: Clarendon Hills: Fort Collins, Colorado prepared by Stuart and Associates in 1986 as part of the approval process for Clarendon Hills 1st Filing. Homestead for Clarendon Hills is in the northwest corner of the original drainage master plan for Clarendon Hills 1st Filing. In the approved original drainage report for Clarendon Hills (1st Filing), the project site was planned as office and multi-family with a developed runoff coefficient of C=0.50. According to the approved drainage report for Clarendon Hills 4th Filing, prepared by Land Development Services and dated April 9, 1991, a church site of 8.27 acres in the middle of the Clarendon Hills 4th Filing was assumed to be an offsite undeveloped drainage basin with a developed Page | 4 runoff coefficient of C=0.25. According to the approved drainage report for Clarendon Hills 6th Filing, prepared by Land Development Services and dated January 8, 1992, the original developer of Clarendon Hills swapped the northwest corner of Clarendon Hills (including Homestead for Clarendon Hills) for the church site in Clarendon Hills 4th Filing. The old church site was then developed as Clarendon Hills 6th Filing with runoff coefficients from 0.28 to 0.70. Because of the subsequent drainage studies and land swaps, we assumed an existing runoff coefficient of C=0.25 for Homestead at Clarendon Hills. A runoff coefficient of 0.25 is consistent with the following design considerations: • The Fort Collins Stormwater Criteria Manual for lawns with clayey soil and an average slope of 2% to 7% requires a runoff coefficient of C = 0.25. The existing slopes throughout the site is between 2% to 7% with clayey soil. • The drainage report for Clarendon Hills 4th Filing assumed a runoff coefficient C=0.25 for the church site, and the church site was subsequently part of a land swap for the northwest corner of the Clarendon Hills Subdivision. Detention Per the Clarendon Hills 1st Filing Drainage Report, “The ponds were designed using a mass diagram that required 10.46 acre-feet to detain the 100-year developed storm and release at a 2-year historic rate…”. Assuming a historic runoff coefficient of 0.25, detention from Basin B was provided as long as the overall flow at Design Point B does not exceed 1.9 cfs. Because of the increased impervious area within Basin B, detention is required within Basin B2 to reduce the post-developed flow rates at Design Point B to equal or less than 1.9 cfs. Since Basins B1, B3, and B4 will be released into the surrounding curb and gutter undetained, the detention pond in Basin B2 is designed to release the 100-year event at 0.3 cfs. A release rate of 1.2 cfs is required to maintain a 100-year flow rate of 1.9 cfs at Design Point B. (See the included flowrate calculations included in the appendices.) Detention Spillway The detention pond is designed to overflow at the west driveway entrance on Clarendon Hills Drive. In the even the outlet orifice is blocked, stormwater will first overflow through a weir into the west rain garden. Then, it will overflow through a curb cut at the northwest end of the rain garden. Finally, stormwater will overflow through the curb and gutter at a high point located at the back of sidewalk at the west entrance. Hydraulic calculations are included in the appendices. Inlets By limiting the 100-year flowrate at Design Point B, the flowrate at the existing inlets will match or decrease from the projected flowrates in the previously approved Clarendon Hills drainage reports. This results in matching or decreasing runoff to the existing inlets that were designed and approved with the earlier project, so no additional inlet calculations have been provided as a part of this report. Water Quality/LID Conformance According to Section 6.0 of the City of Fort Collins Stormwater Manual, “50% site treated with ‘standard’ water quality plus 50% site treated with LID (including pavers) equals 100% of site-treated (requirements met), or 25% site treated with ‘standard’ water quality plus 75% treated with LID equals 100% of site treated Page | 5 (requirements met)”, (page 14). This project proposes to treat 90.5% of the new impervious areas through two rain gardens as LID treatment. However, due to grading constraints along the east side of the project and requirements to provide a maintenance access for the ditch company, a gravel pedestrian path (9.5% of new impervious area) will not be treated by either the rain gardens or the detention pond. An exhibit showing the area of LID treatment is included in the appendices. Although exact impervious area is unavailable at this preliminary stage, the building envelopes will drain towards the rain gardens and the detention pond. The high point along each lot line is set at the rear of the building envelopes to maximize as much of the lot draining towards the south. Space limitations on the detention pond and rain gardens, natural buffer offsets from the ditch, and fire accessibility requirements restrict the drainage towards the front. Even with the grading restrictions, the proposed flowrate into the Mail Creek Basin is 2.2 cfs less than the existing flowrate. Erosion and Sediment Control During construction, the Contractor will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control. Since more than 10,000 sf will be disturbed as a part of this project, a comprehensive Stormwater Management Plan will be prepared for this project at final design. Post construction water quality and erosion control will be achieved by a fully established and stabilized site. All areas disturbed during construction will receive permanent hardscape, landscape, or building structure. Floodplains There are no regulatory floodplains associated with the project. Conclusions The proposed grading concept matches the original drainage patterns. The site matches the designed imperviousness draining to the existing Clarendon Hills curb and gutter. Onsite detention will be provided for the increased imperviousness of the area draining to towards the existing Clarendon Hills ponds, and onsite detention will be required to maintain historic flow rates within Clarendon Hills. Stormwater quality has been provided and meets the city requirements for Low Impact Development treatment. Therefore, it is my professional opinion that Homestead at Clarendon Hills satisfies all applicable stormwater criteria and will effectively limit potential damage associated with its stormwater runoff. Please do not hesitate to contact me if you have questions or require additional information. Sincerely, Frederick S. Wegert, PE Project Engineer UD UDUDUDUDT T/ / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / // / / / / / / /RAIN GARDENL A N G D A L E D R I V ECLARENDON HILLS DRIVEPROPOSED POND100-YEAR WSE = 5089.411' CURB CUT2' WIDE CONCRETE CHASEPROPOSED DRIVEWAYVERTICAL CURBAND GUTTEROUTLET PIPEOUTLET STRUCTURE2' WIDE METALSIDEWALK CULVERTHEADWALLPROPOSEDTRENCH DRAINPROVIDE 10' WIDEWEIR AT ELEV. 5089.7RAIN GARDENFIGURE 1Spillway InundationFORT COLLINS, COHOMESTEAD AT CLARENDONENGINEERNGIEHTRONRN12.18.2020P:\1013-008\DWG\EXHIBITS\1013-008_OVERFLOW INUNDATION.DWGAREA OF INUNDATIONFROM SPILLWAY