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HomeMy WebLinkAboutDrainage Reports - 07/01/2021Project No. 635-190702-000-8427 1 Drainage Memorandum Date: February 12, 2021 To: Brian Schlichting, Poudre School District From: Kent Bruxvoort, P.E. Re: Poudre School District Transportation Facility PEC Project No. 635-190702-000-8427 1.0 Project Overview Poudre School District (PSD) provides administrative services including transportation services at its Johannsen Support Services Center (JSSC) at 2407 LaPorte Avenue, between Taft Hill Road and Overland Trail. PSD owns and operates 150 school buses and employs 200 individuals in transportation administration, fleet maintenance, dispatch, routing, and driving its buses. Current operations are scattered in existing buildings at JSSC, including outdated modular facilities. PSD and its consultants are in the design and construction process for an upgraded Transportation Service Center (proposed facility) that will centralize many of these functions. The proposed facility will be located in the southwest portion of the JSSC and would entail removal of a number of these outdated structures and construction of an approximately 29,000 square foot facility. Locating the new building at the JSSC will allow these operations to continue to be centralized and will limit the footprint of new construction to an area already heavily used. This location will also keep bus- miles at their current levels in comparison to a more remote location at other properties owned by PSD, will allow PSD to continue to use the existing bus barn for covered bus parking, and to continue to use the existing wash bay adjacent to the bus barn. Building permits for the facility have been submitted for review to the State of Colorado per 8 CCR 1507-30 and site design drawings and this memorandum is submitted for Site Plan Advisory Review (SPAR) to the City of Fort Collins. City of Fort Collins Approved Plans Approved by: Date: Dan Mogen 07/01/2021 Project No. 635-190702-000-8427 2 2.0 Existing Conditions Poudre River. Regional stormwater runoff flows through and from the property toward the north and northeast. Local drainage from the property is generally conveyed in pipes and concrete pans toward LaPorte Avenue, with three sub-basins contributing surface runoff to three outfalls: at the northwest, north central, and northeast portions of the JSSC site. Lidstone & Anderson, Inc. prepared an analysis and design for the JSSC in 1996 (Final Drainage Plan for the Poudre School District R-1 Facility Services Site, Revised November 22, 1996). The purpose of the report and resulting designs was to reduce the incidence of flood-related damages on the site and to provide drainage infrastructure that would not increase the peak discharge from the site during the 100- year storm, even after proposed JSSC site development with new buildings and additional parking. Two stormwater ponds were proposed and implemented: a South Detention Pond for the primary purpose of providing channelization of the main regional flow path from the neighborhood to the west through the south half of the site, and a North Retention Pond (called the Stephens Street Retention Pond) for the westerly portion of JSSC site runoff. The North Retention Pond was to provide storage in the northwestern portion of the site to result in a -existing conditions in that area. The North Retention Pond was constructed adjacent to the northwest entrance off LaPorte Avenue and adjacent to 1 depicts existing topographic conditions at the pond. The North Retention Pond, if overtopped, provides a potential site outfall, and the sub- Sub-basin. This sub-basin has an approximate area of 6.0 acres. Figure 2 depicts existing stormwater flow directions and site drainage sub-basins in the project area and includes the proposed building footprint for context. Existing concrete pans (Pans 1 and 2) to either side of Building G are located within the Northwest Sub-basin and convey stormwater north to the North Retention Pond. The basin divide between the Northwest and the other two sub-basins is located within and immediately to the east of the proposed building footprint in the vicinity of the existing modular buildings which are t -basin is identified herein as the North Sub- basin. An existing concrete pan (Pan 3) north of the bus barn conveys stormwater to LaPorte Avenue through a sidewalk chase as depicted in Figure 2. The North Sub-basin has an area of approximately 4.5 acres. The remainder of the site, identified herein as the East Sub-basin, drains south and east from the proposed building area and through the eastern portion of the JSSC site, discharging near the intersection of Impala Drive and LaPorte Avenue. An existing concrete pan (Pan 4), southwest of the project area, drains south and into the South Detention Pond. Existing concrete pans to the west of the bus barn (Pans 5 and 6) convey stormwater south to a storm inlet, as shown on Figure 2. The pipe that leaves that inlet conveys flow toward South Impala Drive and LaPorte Avenue. The East Sub-basin has an area of approximately 8.1 acres. Pre-project pervious area, consisting of gravel, hard-pan parking area and lawn, exists between and south of the pre-project modular Buildings H, J and L, as shown in Figure 3. Inspection of area photography for the pre-project area allows an estimation of pervious area that is relatively undisturbed Project No. 635-190702-000-8427 3 ions, and an estimation of pervious areas used by south of Pan 5. Pan 5 will be demolished and replaced 20 feet to the south as part of this project. These pre-project, generally pervious areas, total approximately 15,960 square feet. Of this pre-project total, approximately 9,980 square feet is classified as gravel and approximately 5,980 square feet as lawn area. . Project No. 635-190702-000-8427 4 3.0 Project Description and Change in Pervious Area The proposed facility is a non-residential use that will include drive-through maintenance bays in the western two-thirds of the building, with related work areas and storage areas for parts, tools and tires. The eastern one-third of the building will include a two-story section for offices, conference space, drivers lounge, training, and information technology rooms. The facility will have a footprint of 21,328 square feet. landscaped area is proposed with the project on the east side and southeast corner of the facility, as shown in Figure 4. This landscaping will comprise approximately 4,125 square feet. The net loss of pervious area (net increase in impervious area) is estimated at 11,835 square feet east and south of the new building. With the project PSD proposes to remove approximately 11,205 square feet of existing asphalt west of the proposed facility, as shown on Figure 5. This removed asphalt will offset most of the pervious area lost in constructing the proposed facility, resulting in a net loss of approximately 630 square feet of pervious ground from the JSSC site with the project. and will not trigger Land Use Code requirements for water quality or Low Impact Development (LID) treatment of stormwater runoff. Project No. 635-190702-000-8427 5 4.0 Proposed Drainage System and Potential Drainage Impacts Figure 4 depicts the site drainage basins after the project is completed. Site grading will be provided to raise the finished floor at least 1.5 feet above the Base Flood Elevation. The Transportation Facility roof will have a peak along an east-west center line, with roof flow captured by gutters and draining to the surface. The net result of building construction and site grading will be minor changes in the three sub- basin boundaries. Roof drainage will flow to existing pans and pipes which will collect and convey the water as currently occurs. Runoff from the north half of the building will flow to Pans 1 and 2 and then to the existing North Retention Pond. Runoff from the south half of the building will flow to Pan 5 and then to the existing stormwater inlet located southeast of the project facility. Project changes affecting runoff from the site result from slight changes in sub-basin boundaries and in perviousness. We estimated area-weight runoff coefficients as applicable in the Rational Method for the three JSSC site sub-basins, under pre- and post-project conditions. Table 1 presents the changes in sub- basin drainage area and in area-weighted runoff coefficient. The majority of the JSSC site is underlain by the Paoli fine sandy loam soil, with a Hydrologic Soil Group A classification as determined from the Natural Resources Conservation Service (NRCS) Web Soil Survey. Table 1: Summary of Changes in Runoff Conditions Due to the Project Northwest Sub-basin North Sub-basin East Sub-basin Sub-basin Area, acres, Pre-Project 5.971 4.546 8.048 Sub-basin Area, acres, Post-Project 5.934 4.505 8.126 Change, % -0.62% -0.90% 0.97% Runoff Coefficient, Pre-Project 0.8361 0.9118 0.8309 Runoff Coefficient, Post-Project 0.8301 0.9162 0.8335 Change in Runoff Coefficient, % -0.72% +0.48% +0.31% As can be seen in Table 1, changes in drainage area and in area-weighted runoff coefficient as a result of the project are less than 1.0 percent. Thus, construction of the proposed facility will cause a very small increase in runoff from the site due to the approximately 630 square foot increase in impervious area. Project No. 635-190702-000-8427 6 5.0 Floodplain and Floodway Collins floodplain and floodway extend through the JSSC property. No FEMA floodplain exists at this location. FEMA mapping in Panel No. 08069CO978G ind property. Updated hydraulic modeling and remapping of the West Vine Basin is currently underway by ICON Eng The edge of the southeast corner of the proposed Transportation Facility will clip the mapped 100-year floodplain through the property, as shown on Figure 6, but will be located outside the floodway. The project grading plan has been submitted to the City of Fort Collins as part of the Floodplain Use Permit package. The proposed facility is a non-residential and non-critical facility used for bus maintenance, driver training, driver lounge, and offices for Poudre R- proposed structure will include a continuous concrete footing foundation with a concrete slab providing the base for the first floor. No crawl space or basement will be constructed with the building. Table 2 presents a summary of floodplain data for the project. All elevation data are based on the NAVD 1988 datum. Table 2: Summary of Floodplain Data for the Transportation Facility City Base Flood Elevation (BFE) at the upstream end of the structure 5077.33 feet FEMA BFE at the upstream end of the structure Not applicable Regulatory flood protection elevation 5077.33 feet Lowest floor elevation 5079.50 feet HVAC elevation * 5079.50 feet * The mechanical room is located on the second floor, but ductwork and fans extend to the ceiling of the first floor. The proposed lowest floor elevation is 5079.50 feet (NAVD88), providing 2.17 feet or 26 inches of -inches requirement for new construction in City basin floodplains other than the Poudre River floodplain. All HVAC equipment is more than 2.17 feet above the BFE. The project is in compliance with Chapter 10 Flood Prevention and Protection of the City of . Project No. 635-190702-000-8427 7 6.0 Conclusions at the site. With the project, approximately 630 square feet of current pervious area will be replaced with the building footprint and parking. This change is less than 1,000 square feet and no LID or other water quality-related measures are proposed. Evaluation of this change on runoff conditions was performed by looking at the pre-project and post-project area-weighted runoff coefficients for the three sub-basins which drain from the JSSC site toward LaPorte Avenue. We conclude that the post-project changes in area-weighted runoff coefficient is less than 1.0 percent in each of the sub-basins, as documented above. Design of the proposed Transportation Facility floodway and includes a finished floor elevation to be 2.17 feet above the base flood elevation established in current hydraulic modeling of the West Vine Basin as provided to us by the City. This freeboard meets City code requirements.