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HomeMy WebLinkAboutSALUD FAMILY HEALTH CENTER - ODP - ODP150003 - REPORTS - DRAINAGE REPORT1 Overall Development Plan (ODP) Drainage Report Date: March 8, 2016 Project: Salud Fort Collins, Colorado Attn: Mr. Wes Lamarque Fort Collins Utilities 700 Wood Street Fort Collins, Colorado 80524 Mr. Lamarque: This letter report accompanies the submittal for the Salud Development Plan (ODP). Specifically, this letter report serves to document the overall drainage impacts associated with the proposed development. The proposed development site is located just northeast of the intersection of Laporte Avenue and Taft Hill Road in Fort Collins, Colorado (Please see Figure 1, below). The project involves site improvements including parking, roadway, and utilities. The large existing building located on the project site is planned to remain and to be renovated. The overall Salud property is broken into 6 lots which are labeled on the Overall Drainage Plan (Attached). In general, there will be 2 detention ponds which will serve all 6 lots. The west Vine Floodplain encroaches a portion of the property as shown in Figure 2, below. A notable feature of the West Vine floodplain is the spill entering the floodplain in vicinity of the current project site from the Larimer Canal No. 2 irrigation ditch. Current floodplain mapping changes are under way by the City of Fort Collins, and we are working with City Stormwater Utility Staff to incorporate the changes regarding this spill from the Larimer No. 2. A future submittal to the City involving modeling of the spill from the ditch and grading the project site to allow for this spill to safely pass along the western perimeter of the site (just west of the large existing building that is to remain and to be renovated). The West Vine flood zone is a City-designated floodplain; there is no FEMA designated flood zone in vicinity of the project site. The proposed development site is in the City of Fort Collins West Vine Master Basin. Detention requirements for this basin are to detain the difference between the 100-year developed inflow rate and the historic 2-year release rate. However, due to site constraints, the proposed drainage concept for the site is to place an interim pumped detention pond in the northwest corner of Lot 6. The Overall Drainage Plan (Attached) shows the proposed location of this pond 2 and illustrates the concept to provide detention in this pond for Lots 4 through 6. We anticipate, based on conceptual grading completed to date, that drainage from Lot 5 will be piped from this lot into the pumped detention pond in the northwest corner of Lot 6. The pumped detention pond is to be located so that in the future a gravity tie-in to a future City of Fort Collins Regional Pond, which is anticipated to be constructed just northwest of the project site. A rough anticipated construction date for this pond is the year 2025. In the interim period, the onsite detention pond will be pumped. The pump will be sized to discharge at a rate that adequately evacuates all storm runoff from the pond to meet State of Colorado revised Statute 37-92-602(8). This states that 97% of all the stormwater runoff from a 5-year or less event be released within 72 hours of the storm event, and 99% of all of the stormwater runoff from greater than a 5-year event be released within 120 hours of the storm event. Calculations will be provided at Preliminary and Final stages of the design process to show proper pump sizing meeting these criteria. Additionally, the pumped detention pond will be required to obtain a variance from the City, as pumped detention is not allowed per the Fort Collins Stormwater Criteria Manual. FIGURE 1 Vicinity Map 3 FIGURE 2 City Floodplain Map (Floodplain shown is a City Designated Flood Zone-No FEMA Flood Zones encroach project site) Developed runoff from the majority of southern portion of the project site, which encompasses Lots 2 and 3, and a portion of Lot 1 will be detained and released into the Larimer Canal No. 2 irrigation ditch at a historic 2-year rate. We have discussed this concept with the Larimer Canal No. 2 Irrigation Company, and they are initially agreeable to this concept. We will provide further documentation to the irrigation company when available, and will need ultimate approval and signatures prior to construction. A portion of Lot 1, which is comprised of the Laporte Avenue frontage, has historically drained into Laporte Avenue. This area is composed of an existing parking lot as well as landscaped areas. We will maintain historic runoff rates from this area and ensure no increase in runoff going into Laporte Avenue. In doing so, we will not detain this historic flow going into Laporte Avenue. All water quality treatment requirements and LID requirements will be satisfied with the proposed development. Means of providing for these requirements will be worked through at the Preliminary and Final stages of the design process. It is acknowledged that all existing imperviousness that is to be removed and redeveloped will be required to be treated for water quality. 4 In summary, this Overall Drainage Report letter adequately addresses any potential stormwater changes associated with the proposed development. In general, there are no significant changes proposed at a major drainage level. The ODP complies with the governing City of Fort Collins Master Drainage Plans, and the previously approved drainage plans specific for the subject property. Please feel free to contact me with any questions you may have. Sincerely NORTHERN ENGINEERING Aaron Cvar, PhD, PE Senior Project Engineer