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Home My WebLink AboutGUARDIAN SELF STORAGE - FDP210018 - DOCUMENT MARKUPS - ROUND 3 - DRAINAGE REPORTFINAL DRAINAGE REPORT FOR Guardian Storage Fort Collins Prepared For: Guardian Storage Fort Collins 1555 South 76th Street Superior, CO 80027 Kevin Cohen (303) 250-2059 July 2021 Project No. 39737.01 Prepared By: JR Engineering, LLC 2900 South College Avenue, Suite 3D Fort Collins, CO 80525 Guardian Storage Fort Collins Final Drainage Report ii TABLE OF CONTENTS TABLE OF CONTENTS ......................................................................................................................... ii APPENDIX ..................................................................................................................................... iii Engineer’s Certification Block ........................................................................................................... iv GENERAL DESCRIPTION AND LOCATION ............................................................................................ 1 LOCATION ..................................................................................................................................... 1 DESCRIPTION OF PROPERTY .......................................................................................................... 1 FLOODPLAIN SUBMITTAL REQUIREMENTS ..................................................................................... 1 DRAINAGE BASINS AND SUB-BASINS ................................................................................................. 3 MAJOR BASINS AND SUB-BASINS ................................................................................................... 3 EXISTING SUB-BASIN DESCRIPTION ................................................................................................ 3 EXISTING OFFSITE DRAINAGE BASINS ............................................................................................ 3 EXISTING OFFSITE DRAINAGE DESCRIPTIONS ................................................................................. 3 PROPOSED SUB-BASINS DESCRIPTIONS ......................................................................................... 4 OFFSITE SUB-BASINS DESCRIPTIONS .............................................................................................. 6 DRAINAGE DESIGN CRITERIA ............................................................................................................. 8 REGULATIONS ............................................................................................................................... 8 FOUR STEP PROCESS TO PROTECT RECEIVING WATERS ................................................................. 8 DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS ............................................................. 8 HYDROLOGIC CRITERIA .................................................................................................................. 9 HYDRAULIC CRITERIA ..................................................................................................................... 9 FLOODPLAIN REGULATIONS COMPLIANCE ................................................................................... 10 MODIFICATIONS OF CRITERIA ...................................................................................................... 11 BEAT THE PEAK ANALYSIS ............................................................................................................ 11 DRAINAGE FACILITY DESIGN ............................................................................................................ 12 GENERAL CONCEPT ..................................................................................................................... 12 WATER QUALITY/ DETENTION FACILITIES .................................................................................... 13 LOW IMPACT DEVELOPMENT ...................................................................................................... 14 CONCLUSIONS ................................................................................................................................. 15 COMPLIANCE WITH STANDARDS ................................................................................................. 15 DRAINAGE CONCEPT ................................................................................................................... 15 Guardian Storage Fort Collins Final Drainage Report iii APPENDIX Appendix A – Figures Appendix B – Hydrologic Calculations Appendix C – Hydraulic Calculations Appendix D – Water Quality Calculations Appendix E – Offsite SWMM Results Appendix F – “Beat the Peak” UD SWMM Analysis Appendix G – Maps Appendix H– Variance Forms Guardian Storage Fort Collins Final Drainage Report iv Engineer’s Certification Block I hereby attest that this report for the final drainage design for the Guardian Storage Fort Collins was prepared by me or under my direct supervision, in accordance with the provisions of the Fort Collins Stormwater Criteria Manual. I understand that the City of Fort Collins does not and will not assume liability for drainage facilities designed by others. Rebecca A. Brush, PE Registered Professional Engineer State of Colorado No. 53260 Guardian Storage Fort Collins Final Drainage Report 1 GENERAL DESCRIPTION AND LOCATION LOCATION The site is located in the North West Quarter of the South West Quarter of Section 1, Township 6 North, Range 69 West of the 6th Principal Meridian in the City of Fort Collins, Larimer County, Colorado. More specifically, the site is a 1.857 acre property that is currently undeveloped. The Fossil Creek Design Center PUD lies to the north of the site. Directly to the east of this site are lots one through four of the Phase 1 Fossil Creek Meadows subdivision. Across Fossil Creek Pkwy to the south lies Lot 2 of Discount Tire at Fossil Creek. The site is located adjacent to S. Collage Ave, and Fossil Creek Pkwy. DESCRIPTION OF PROPERTY The Guardian Self Storage site is 1.857 acres at the northeast corner to S. College Ave and Fossil Creek Pkwy. The existing site currently has dirt stockpiled on-site. The existing site generally slopes from northwest to southeast with slopes ranging between 0.70% and 25%. The property is generally covered with sparse native grasses. The site currently drains from the northwest corner to the southeast corner to an existing 29”x45” HERCP which outfalls to Fossil Creek, which is located approximately 190 feet south of the site. The site is tributary to the Fossil Creek drainage basin. The Guardian Storage site will be developed in concurrence to Snead Drive directly adjacent to the site along its eastern border as well as open space Tract A with areas of 0.5187 acres and 0.292 acres respectively. FLOODPLAIN SUBMITTAL REQUIREMENTS The site is on FEMA FIRM panel 08069C1000F. The flood map for this location has a status of "not printed". This means that the entire area of the panel is in a single flood zone, so FEMA chose to economize and not create a printable image for this location. The site lies within FEMA Zone X, areas which are determined to be outside the 0.2% annual chance floodplain. The site is not located within a FEMA regulated floodplain, but the eastern portion of the site is located within the City of Fort Collins Fossil Creek floodplain and is characterized by the city as “High Flood Risk – Floodway” as shown in Appendix A, FC Maps – Guardian Self Storage. The City of Fort Collins is in the process of reanalyzing and updating the Fossil Creek floodplain mapping and the BFEs (Base Flood Elevations) for Fossil Creek Guardian Storage Fort Collins Final Drainage Report 2 near the site. The 2019 Fossil Creek LOMR floodplain model and report have been approved and have now moved into the floodplain adoption process. This development is in compliance with Chapter 10 of the City Code which contains the floodplain regulations. The floodplain use permit will be submitted when at time of building permit application. The FEMA Elevation Certificate will be completed and approved before the CO is issued. The proposed non- residential building is intended to be constructed with a slab-on-grade foundation. The revised BFE at out outfall location was estimated to be 4945.00 (NAVD88) and the lowest floor elevation is set higher than the regulatory flood protection elevation (RFPE) at 4946.60 (NAVD88). Any item that can float such as the bike rack will be anchored to the ground. The HVAC for the building is set above the RFPE. The completion of the development meets the no-rise criteria from Chapter 10 of the City Code. See structure floodplain information table below for more details. Structure Floodplain Information Elevation (NAVD88) City BFE upstream of structure 4945.0 Interpolated BFE at site 4944.8 Regulatory flood protection elevation 4946.3 Lowest floor elevation 4946.6 HVAC elevation 4946.6 Guardian Storage Fort Collins Final Drainage Report 3 DRAINAGE BASINS AND SUB-BASINS MAJOR BASINS AND SUB-BASINS The proposed Guardian Self Storage site is located in the Fossil Creek basin. The Fossil Creek drainage basin extends along the south end of Fort Collins, from the foothills across Interstate 25 past CR 5. It encompasses 32 square miles in the City of Fort Collins and Larimer County. Historically, the basin consisted of agricultural land, but the basin has experienced significant development in the recent past. EXISTING SUB-BASIN DESCRIPTION There are offsite areas that flow across the proposed development site. The existing drainage basins within the site flow to the southeast of the site. The existing sub-basins can be found on the Offsite SWMM Map located in Appendix G EXISTING OFFSITE DRAINAGE BASINS Currently the site collects offsite runoff from 3 locations: the northwest corner (Q100 = 33 cfs), the northeast corner (Q100 = 66 cfs) and the southeast corner (Q100 = 28.0cfs). Offsite flows were evaluated utilizing EPA SWMM Version 5.1. A map summarizing the SWMM model and flows can be found in Appendix G. The results from the SWMM modeling as well as all flows entering the site for both the 100-year storm event and 2-year storm event can be found within Appendix E. These offsite flows will be collected in inlets and piped through the site to the outfall location, an existing 29”x45” HERCP in the southeast corner of the site that drains directly to Fossil Creek. In the current site condition, when a 100-year storm event occurs, runoff builds up in the drainage channel located at the southeast corner of the site. Approximately, 32.45 cfs will pass through the existing 29” X 45” HERCP with approximately 78 cfs overtopping the ditch and traveling east along the north side of Fossil Creek Parkway. EXISTING OFFSITE DRAINAGE DESCRIPTIONS The offsite flows will be routed through the site. Specifically, 24.6 of the 33 cfs coming onsite from the northwest will be piped around the west and south side of the site bypassing the on- site sand filter. The remaining 8.4 cfs of off-site runoff will enter an inlet at the northwest of the site and combine with on-site flows to be treated by sand filter which will provide LID Guardian Storage Fort Collins Final Drainage Report 4 treatment and water quality. Treating the 8.4 cfs of offsite runoff will account for the combined 4.2 cfs generated in Basins E1, E2, and G2 which bypass the LID sand filter. Offsite runoff that enters the site from the northeast will combine with runoff generated on proposed Snead Drive and bypass the on-site sand filter. Offsite flows that enter the site from the southeast will flow to a sump in Snead Drive where they will be captured in an inlet and piped to Fossil Creek. PROPOSED SUB-BASINS DESCRIPTIONS The proposed Sub-Basins are discussed below, and are shown in the Drainage Plan in Appendix G. Basin A1: Basin A1 (Q2=1.4 cfs Q100= 4.8 cfs) consists of 0.68 acres of the Guardian Storage building roof that drains to the north and gets collected in roof drains. The roof drains route the total flow into the storm drain system then to design point 10.1. Basin B1: Basin B1 (Q2=0.1 cfs Q100= 0.3cfs) consists of 0.03 acres of drive area located at the northeast corner of the building; the runoff from this basin drains to an inlet located at design point 10. Basin B2: Basin B2 (Q2=0.1 cfs Q100= 0.5 cfs) consists of 0.05 acres of drive area bordering the norther facing portion of the building; the runoff from this basin drains toward an inlet located at design point 9.0. Basin B3: Basin B3 (Q2=0.1 cfs Q100= 0.4 cfs) consists of 0.04 acres of drive area bordering the norther facing portion of the building; the runoff from this basin drains toward an inlet located at design point 8.0. Guardian Storage Fort Collins Final Drainage Report 5 Basin B4: Basin B4 (Q2=0.1 cfs Q100= 0.2 cfs) consists of 0.02 acres of covered drive area bordering the norther facing portion of the building; the runoff from this basin drains toward an area inlet located at design point 7.0. Basin B5: Basin B5 (Q2=0.2 cfs Q100= 0.6 cfs) consists of 0.10 acres of drive area bordering the norther facing portion of the building; the runoff from this basin drains toward an inlet located at design point 1.0. Basin C1: Basin C1 (Q2=0.2 cfs Q100= 0.8 cfs) consists of 0.13 acres of existing drive area located at the northern portion of the site. The runoff from this basin drains to the east along the proposed curb and gutter to be captured in a curb inlet located at design point 5.0. Basin D1: Basin D1 (Q2=1.2 cfs Q100= 4.1 cfs) consists of 0.30 acres of pervious area along the sites eastern boundary. The runoff from this basin is collected in area inlets and piped to design point 13. Basin E1: Basin E1 (Q2=0.6 cfs Q100= 2.1 cfs) consists of 0.37 acres of Snead Drive and is characterized by both pervious and impervious area. Runoff generated within this basin flows south along the curb and gutter until collected in a curb inlet located at design point 15. Basin E2: Basin E2 (Q2=0.5 cfs Q100= 1.8 cfs) consists of 0.26 acres of Snead Drive and is characterized by both pervious and impervious area. Runoff generated within this basin flows south along the curb and gutter until collected in a curb inlet located at design point 14. Guardian Storage Fort Collins Final Drainage Report 6 Basin F1: Basin F1 (Q2=0.3 cfs Q100= 1.0 cfs) consists of 0.10 acres of impervious parking lot area. Runoff generated within this basin sheet flows to the south then along the curb and gutter to the east until collected in a curb inlet located at design point 11. Basin F2: Basin F2 (Q2=0.7 cfs Q100= 2.4 cfs) consists of 0.26 acres of impervious parking lot area. Runoff generated within this basin sheet flows to the southeast then along the curb and gutter to the east to a curb cut at design point 12. Basin G1: Basin G1 (Q2=0.1 cfs Q100= 0.4 cfs) consists of 0.28 acres and is characterized by pervious landscape area as well as a pervious LID sand filter. Runoff generated within this basin sheet flows to design point 16 where it is treated by the sand filter area. Basin G2: Basin G2 (Q2=0.1 cfs Q100= 0.3 cfs) consists of 0.15 acres and is characterized by pervious landscape area as well as an impervious sidewalk area. Runoff generated within this basin sheet flows to design point 18 where it is treated by the sand filter area. OFFSITE SUB-BASINS DESCRIPTIONS Basin OSA (Q2=1.2 cfs Q100=4.1 cfs) is composed of 0.68 acres and is characterized by portions of South College Ave and Fossil Creek Parkway. Runoff generated in this basin is conveyed via sheet flow and curb and gutter to design point OS2. Basin OSB (Q2=0.4 cfs Q100=1.5 cfs) is composed of 0.22 acres and is characterized by portions Fossil Creek Parkway. Runoff generated in this basin is conveyed via sheet flow and curb and gutter to design point OS3. Guardian Storage Fort Collins Final Drainage Report 7 Other offsite flows and upstream basins were evaluated utilizing SWMM in order to accurately represent the off-site flows that enter the site. The SWMM results as well as an offsite basin map can be found in Appendices E and G respectively. Guardian Storage Fort Collins Final Drainage Report 8 DRAINAGE DESIGN CRITERIA REGULATIONS This report was prepared to meet or exceed the City of Fort Collins stormwater criteria. The City of Fort Collins Stormwater Criteria Manual (dated 2018)(FCSCM) and the Urban Drainage Flood Control District’s (UDFCD) Drainage Criteria Manual (USDCM) Volumes 1, 2 and 3 were referenced as guidelines for this design. FOUR STEP PROCESS TO PROTECT RECEIVING WATERS City of Fort Collins requires the Four Step Process to protect receiving waters. The four step process incorporates reducing runoff volume, treating water quality capture volume, stabilizing streams, and implementing long-term source controls. Volume reduction is an important part of the Four Step Process and is fundamental to effective stormwater management. Per City criteria, a minimum of 75 percent of new impervious surface area must be treated by a Low-Impact Development (LID) best management practice (BMP). The site incorporates a sand filter designed to capture and treat 120% of the sites water quality capture volume. The sand filter media needs to be properly cleaned and media maintained to allow for long term protection of the receiving waters. The proposed LID BMPs will have the effect of slowing runoff through the site lot and increasing infiltration and rainfall interception by encouraging infiltration and careful selection of vegetative cover. The improvements will decrease the runoff coefficient from the site and are expected to have no adverse impact on the timing, quantity, or quality of stormwater runoff. The majority of new impervious area is treated by the proposed sand filter in the southeast corner of the site as illustrated in the LID/Surface Map is provided in Appendix G. Runoff generated on the proposed Snead Drive will not be treated by sand filter. In order to compensate for portions of Snead drive not being treated via LID, untreated offsite runoff that historically entered the site from the north west will be routed to the sand filter and treated. See the LID Map & Table 3 for a summary of the areas to be treated. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS The master drainage report for the Fossil Creek Drainage Basin was prepared by ICON Engineering, Inc. April 9th, 2002. Guardian Storage falls within element 30 in the SWMM file associated with the master drainage report. The development of the Guardian Storage site must not have a negative impact Guardian Storage Fort Collins Final Drainage Report 9 on downstream discharges, volumes and water surface elevations. The elements directly downstream from the Guardian Storage site are elements 47 and 29 from the master SWMM file. HYDROLOGIC CRITERIA The rational method was performed to calculate the peak runoff rates for each basin. Weighted percent imperviousness and weighted runoff coefficients were calculated for each basin using the Fort Collins Stormwater Criteria Manual Table 3.2-2 and Table 4.1-2. The time of concentration was calculated using FCSCM Equation 5-3 and the intensity was interpolated from Table 3.4-1 in the FCSCM. The City of Fort Collins area has 2-year, 1-hour rainfall depth of 0.82 inches and a 100-year, 1-hour rainfall depth of 2.86 inches. These depths do account for the 1997 adjusted rainfall depths. HYDRAULIC CRITERIA The ultimate Guardian Self Storage storm drainage system will be designed to convey the minor and major storm events through the property with the proposed inlets, storm sewer pipes, and swales for the flows being calculated by this report. Per the requirements provided by the City of Fort Collins Storm Drainage Design Criteria and Construction Manual, all inlets and storm pipes will be designed to convey the 100-year storm flows. During the major storm event, the developed condition 100-year storm, the accumulated water depth in the sand filter will be held to a maximum level of one-foot below all building finished floor elevations. There are a series of custom inlets proposed for the Guardian Storage site. These inlets have been designed to both receive larger pipes than standard Type R inlets while also still capturing the 100 year surface flow. Custom structures 2 and 4 are on-grade inlets designed to reduce the amount of flow that ultimately makes it to the sump inlets custom structure 4 and the 10’ Type R inlet located at DP14. Custom structure 4 is responsible for taking 29.9 cfs in the 100-yr event with the 10’ type R on the east side of the sump responsible for 7.4 cfs. The eastern Type 4 inlet is only taking 7.4cfs due to a 15” RCP pipe leaving the structure being the largest that can cross Snead Drive due to the bury depth of the existing sanitary sewer. If the tailwater in the creek during the 100-year storm is low, than the inlets will be able to intercept the full 100-year event. If the tailwater in Fossil Creek is at the 10-year water level than the sump inlets will fill up past the top back of the curb and approximately 71 cfs of flow will travel along the 100-year emergency overflow path out of the site to the southeast. Guardian Storage Fort Collins Final Drainage Report 10 INLET DESIGNATION INTERCEPTED FLOW (CFS) FLOW BYPASSED (CFS) BYPASS INLET DESIGNATION CUSTOM STRUCTURE 2 (DP5)8.7 6.3 CUSTOM STRUCTURE 3 CUSTOM STRUCTURE 3 (DP15)29.9 0 N/A CUSTOM STRUCTURE 4 (DPOS3)15.2 12.8 CUSTOM STRUCTURE 3 Type R Inlet (DP14)7.4 8.7 CUSTOM STRUCTURE 2 The East half of Fossil Creek Parkway has adequate capacity to handle the 28 cfs of runoff that it receives between the offsite flow from College Avenue and design points 18, OS2 and OS3. Two different sections showing the capacity in Fossil Creek Parkway can be found located in appendix B. It is important to note that the site’s designed hydraulic system is controlled by the tailwater in Fossil Creek at the outfall of the storm system. When the tailwater in Fossil creek was set at the crown of the discharge pipe, the Guardian drainage system has enough capacity to convey the full 100-year flowrate (or 141.1-cfs) as seen in the “100 YR Profile-Tailwater @ Crown” profile in Appendix C. When the tailwater is set to the 10-Year WSEL in Fossil Creek, the drainage system will pass approximately 70 cfs through the outfall pipe while the remaining 71 cfs leaves the site via surface flow to the East. In this scenario, the 71 cfs leaving the site is less than the ditch overflow leaving the site in the existing condition. The 70 cfs passing through the outfall pipes was determined by iterating the flow in the system until the hgl at the sump in proposed Snead Drive was at the overflow elevation in the curb and gutter as shown in the “100 Yr Profile-HGL Set to Overflow @ Sump” StormCAD Profile in Appendix C. Results for the 100 Year scenario, the 100 Year with the tailwater set at the Crown, and the 100 Year with the HGL Set to Overflow at the Sump inlets can all be found in Appendix C. FLOODPLAIN REGULATIONS COMPLIANCE As previously stated, the eastern portion of the Guardian Self storage site is within the City of Fort Collins Fossil Creek flood plain. The City of Fort Collins is in the process of reanalyzing and updating the Fossil Creek floodplain and the BFEs (Base Flood Elevations) for Fossil Creek near the site. The revised BFE at out outfall location was estimated to be 4945.00 (NAVD88) and as such the lowest building finished floor elevation is set at least 18” above that at 4946.60 (NAVD88). The portion of the site that is within the floodplain is located within the ineffective flow area of the Fossil Creek floodplain model and a no-rise memo will be completed to show that the proposed development is within the ineffective flow area and will therefore result in no-rise of the BFE. Guardian Storage Fort Collins Final Drainage Report 11 MODIFICATIONS OF CRITERIA Chapter 6, Section 1.1 of the City of Fort Collins Stormwater Criteria Manual states that stormwater detention is required on site. Due to the site's proximity to Fossil Creek, a "beat the peak" alternative in conjunction with a sand filter sized to treat 100% of the proposed impervious area is the proposed alternative to the detention requirement. A dewatering system will be placed around the proposed sand filter in order to keep the groundwater level a minimum of 12 inches below the invert of the outfall underdrain system. 12 inches of separation between the groundwater and outfall pipe of the sand filter is a variation from the 24 inch standard depth. BEAT THE PEAK ANALYSIS Per Chapter 6, Section 2.5 of the City of Fort Collins Stormwater Criteria, if a site is nearby to a drainage or water way and the proposed site hydrograph is shown to “beat the peak” stormwater detention may be eliminated from the site. The City of Fort Collins provided the master hydrology model for Fossil Creek. The Guardian Storage site is located within Basin 30 in the SWMM model. Basin 30 was modified per the proposed Guardian Storage site condition within the model. A summary of the results can be seen in Table 1 below. Table 1 Element 30 (channel) Element 47 (direct flow) Element 29 (channel) From the City Peak (cfs)451.9 cfs 3488.9 cfs 3430.8 cfs Stage (ft)0.5'-7.5' Time (hr/min)36 min 2hr 47 min 3hr 5 min Revised UD SWMM Peak (cfs)456 cfs 3489 cfs 3431 cfs Stage (ft)0.5'-7.5' Time (hr/min)36 min 2hr 47 min 3hr 5 min Element 30 contains the proposed site and element 47 and 29 are the next two elements downstream. The full UDSWMM output is located in Appendix F. The was essentially no increase in flow rate to Fossil Creek in downstream elements, basin volumes or basin water surface elevations as a result of the development. Guardian Storage Fort Collins Final Drainage Report 12 DRAINAGE FACILITY DESIGN GENERAL CONCEPT The proposed improvements to the Guardian Storage site will result in developed condition runoff being conveyed around the proposed building and to the east via storm sewer and surface flow. Low-impact development best management practices are proposed to improve the quality of runoff and aid in reducing peak flows. Specifically, a sand filter is proposed on the southeast sides of the site to improve water quality. A dewatering system is to be placed around the LID sand filter in order to provide adequate separation between the ground water and release pipe from the sand filter. Runoff from the site is captured in inlets located in the parking lot, the drive aisle to the north of the building. Runoff is conveyed in storm sewers, along curb and gutter, and via surface flow to the proposed sand filter at the south end of the site. Secondly, surface overflow paths have been provided, such that the 100-year storm flows remain at least one foot below the finished floor elevation in a fully clogged inlet condition. Runoff generated from proposed Snead Drive will not be routed to the sand filter but instead be bypassed directly to Fossil Creek. To ensure that water quality standards are maintained, a large portion of existing impervious area from the northwest of the site’s runoff will be conveyed to the sand filter and treated in place of Snead. The proposed site configuration and storm drainage pipe system cannot currently convey all 100 year offsite flows within the storm drain system without bolting the lids of some of the proposed storm manholes. The 100-year offsite flows being conveyed through the property’s storm system are limited by both a 29”x45” HERCP outfall pipe to Fossil Creek as well as a tail-water condition of 4943.19 which is the interpolated 10 year water surface elevation in Fossil Creek from the Cities HEC-RAS model. In order to convey developed 100-yr flows from the site and ensure that the drainage configuration in the area is better than the existing condition, an additional 29”x45” HERCP outfall pipe to Fossil Creek is being proposed in the design. A StormCAD analysis of the system was utilized to analyze the 100-year storm event and the results can be found in Appendix C. The remaining flows that can’t be passed through the system due to tailwater effects are conveyed via surface flow to the sump in Snead Drive where they overtop the ROW and travel down the emergency overflow path to the southeast corner of the site. The proposed site drainage configuration conveys 100-yr flows from the site and improves the drainage in the area by allowing more of the 100-year generated flows to pass via pipe underneath Fossil Creek and reduces the amount of 100 year runoff that would leave the site at the southeast corner. In Guardian Storage Fort Collins Final Drainage Report 13 the 100 year peak flow condition, a 100-year water surface elevation of 4944.50 will be present in the sand filter. Adjacent property will be protected via an engineered swale that travels south than east and conveys that flows to the properties southeast corner. WATER QUALITY/ DETENTION FACILITIES The Guardian Self Storage sand filter water–quality treatment facility is located at the southern portion of the site near Snead Drive and Fossil Creek Pkwy. The Urban Drainage UD-BMP spreadsheet version 3.07 was utilized to design and size the proposed sand filter. The sand filter design details can be found in Appendix D. Due to the several engineering challenges of the site including the complexity of the stormwater system & the need to convey offsite stormwater from the north to Fossil Creek to the south it has been deemed infeasible to convey runoff from Snead Drive to the proposed sand filter. Since runoff from Snead Drive cannot be treated for water quality, runoff from the existing Fossil Creek Design Center in the northwest corner is collected in a proposed inlet conveyed to the sand filter which will provide water quality for the offsite area in exchange for Snead Drive. A summary of the water quality & L.I.D. requirements are provided in Table 3. Sub-basins A, B. F, & G and some offsite area to the northwest all ultimately end up contributing runoff to the sand filter and account for 75,359 square feet. The composite percent impervious of all of the sub-basins is equal to 80 percent. The calculated watershed capture volume is equal to 0.26 watershed inches which is equivalent to 1,649 cubic feet. Fort Collins requires 120 percent of the calculated water quality capture volume so 1,979 cubic feet have been provided in the proposed sand filter at a depth of 1.57 feet (see Table 2 below). At a depth of 1.57 feet, the WSEL in the sand filter would be approximately 4941.97 which is more than 18 inches lower than the finished floor elevation of the building of 4946.60. Guardian Storage Fort Collins Final Drainage Report 14 Table 2 Stage Stage Elevation Stage Surface Area Stage Volume Cumulative Volume Cumulative Volume (square feet)(cubic feet)(cubic feet)(acre feet) 0.00 4940.40 0 0 0 0.00 1.00 4941.00 1,224 245 245 0.01 2.00 4942.00 2,456 1,805 2,050 0.05 3.00 4943.00 3,813 3,110 5,160 0.12 4.00 4944.00 5,801 4,772 9,932 0.23 The surface of the sand filter is at an elevation of 4941.00. The 2-year HGL within Fossil Creek was interpolated from the Cities HEC-RAS model at the site outfall to be 4940.34 which is below the surface of the sand filter. The 5-year HGL within Fossil Creek was interpolated from the Cities HEC-RAS model at the site outfall to be 4942.41 which would be above the surface of the sand filter. Soil bores taken back in January of 2021 indicate an existing groundwater level of approximately 4939.45. A dewatering system will be installed surrounding the sand filter in order to get the groundwater elevation to lower than a depth of 4937.91 which would provide at least 12 inches of separation from the groundwater to the invert of the sand filter underdrain discharge pipe. LOW IMPACT DEVELOPMENT A minimum of 75 percent of new impervious surface area is treated by a Low-Impact Development (LID) best management practice (BMP) in accordance with City criteria. The BMPs proposed for the Guardian Storage site are a sand filter for water quality treatment. Of the total 55,841 sf of new impervious area, 51,707 sf are treated by LIDs (93%) with an additional 6,794 sf of existing impervious area being treated for a total of 58,501 sf (105%). Table 3 shows the water quality and LID requirements below. An illustrative LID/ Surface Map is provided in Appendix G and the sizing of the LID sand filter is provided in Appendix D. Guardian Storage Fort Collins Final Drainage Report 15 Table 3 WATER QUALITY & LID REQUIREMENT AREA DESCRIPTION AREA (SF) % OF TOTAL REQUIRED WATER QUALITY TREATMENT AREA (ON-SITE NEW IMPERVIOUS AREA) 55,841 100% REQUIRED LID TREATMENT AREA (75% OF WATER QUALITY AREA)41,881 75% PROVIDED LID TREATMENT AREA (NEW IMPERVIOUS AREAS TREATED WITH LID) 51,707 93% NEW IMPERVIOUS AREA NOT TREATED BY LID 4,134 7% PROVIDED STANDARD WATER QUALITY TREATMENT AREA (AREA TREATED WITH EXTENDED DETENTION) 0 0% PROVIDED WATER QUALITY TREATMENT AREA FOR OFFSITE AREAS 6,794 TOTAL IMPERVIOUS AREA RECEIVING WATER QUALITY TREATMENT 58,501 105% CONCLUSIONS COMPLIANCE WITH STANDARDS The hydrologic and water quality calculations were performed using the required methods as outlined in the City of Fort Collins Stormwater Criteria Manual. The proposed drainage improvements meet or exceed the City’s requirements except for the detention requirement in Chapter 6, Section 1 and the groundwater separation requirement from the sand filter outlet pipe. A variance request has been submitted in conjunction with this report for the detention requirement and for the sand filter groundwater separation requirement. DRAINAGE CONCEPT The proposed concept for the development of the Guardian Self Storage site involves surface flows and piping of developed conditions flows to a proposed onsite sand filter. LID site enhancements will treat Guardian Storage Fort Collins Final Drainage Report 16 the site runoff at the source before outfall to Fossil Creek. All the offsite flows will be conveyed through the site and directly released at the quality and rate that has historically passed through the site. The existing conditions drainage patterns are maintained in the proposed and future conditions. The proposed improvements will have no adverse impacts on the flow rate, character, or quality of runoff leaving the site. The proposed improvements will have no adverse impacts on adjacent properties. Based on the premise of “beat the peak” theory, which suggests that the time of concentration for the subject property to release its peak 100-year flow into Fossil Creek occurs at an earlier time from the beginning of the 100-year event than that of the 100-year peak flow rate of the entire upstream basin of Fossil Creek, this development is required to only provide City required LID and water quality. This theory is mainly based on the length of travel of the 100-year peak storm flows to reach the location of Fossil Creek adjacent to the site and the time it takes it to do so. Thus, the base flood elevation (BFE) reported in the City’s most recent study of the floodplain is 4945.00 at the site’s downstream release, and is calculated based on the entire upstream basin time of concentration, which occurs at a point in time after the development’s peak flow rate has already been conveyed downstream. This occurs because this development is within a reasonably close proximity to the Fossil Creek’s Floodway. For that reason, the Snead Drive 100 year storm pipe profile is based on the 10-year water surface elevation in Fossil Creek. Due to the limited capacity of the horizontal elliptical reinforced concrete pipe (29”x45” HERCP) that exists underneath Fossil Creek Parkway and additional 29”x45” HERCP is being proposed underneath Fossil Creek Parkway. Based on the dual outfall pipes along with street surface flow being below the right-of-way in proposed Snead Drive, the proposed infrastructure is capable of conveying the on-site and off-site 100-year flows safely. The weir for the ponding that occurs during the peak 100-year flow occurs at the roadway ROW sump, and requires an engineered swale/channel that would run south than east around all existing residential lots. Subdivision:0 Project Name:Guardian Storage Fort Collins Location:Fort Collins Project No.:39737.01 Calculated By:AJ Checked By:AJH Date:12/9/20 DESIGN POINT DESIGN POINT DESIGN POINT Notes Q100 (cfs):141.1 Flows are the greater of proposed vs. future Conduit Box Culvert D c , Pipe Diameter (in):N/A W , Box Width (ft):7 H , Box Height (ft):3 Y t , Tailwater Depth (ft):4.00 If unknown, use Y t /D c (or H )=0.4 Y t /Dc or Y t /H 1.60 Q/D2.5 or Q/(WH3/2)5.10 Supercritical?No Y n , Normal Depth (ft) [Supercritical]:1.00 D a ,H a (in) [Supercritical]:N/A D a =(D c +Y n )/2 Riprap d 50 (in) [Supercritical]:N/A Riprap d 50 (in) [Subcritical]:1.34 Required Riprap Size:L Fig. 9-38 or Fig. 9-36 d 50 (in):9 Expansion Factor, 1/(2 tan):6.00 Read from Fig. 9-35 or 9-36 0.08 Erosive Soils?Yes Area of Flow,A t (ft2):28.22 A t =Q/V Length of Protection, L p (ft):0.3 L=(1/(2 tan ))(At/Yt - D) Min Length (ft)21.0 Min L=3D or 3H Max Length (ft)25.0 Max L=10D or 10H Min Bottom Width,T (ft):2.6 T=2*(Lp*tan)+W Design Length (ft)21.0 Design Width (ft)2.6 Riprap Depth (in)18 Depth=2(d50) Type II Bedding Depth (in)*6 *Not used if Soil Riprap Cutoff Wall Yes Cutoff Wall Depth (ft)24.0 Depth of Riprap and Base Cutoff Wall Width (ft)#N/A Note: No Type II Base to be used if Soil Riprap is specified within the plans * For use when the flow in the culvert is supercritical (and less than full). STORM DRAIN SYSTEM PIPE OUTFALL RIPRAP SIZING CALCULATIONS X:\3970000.all\3973701\Excel\Drainage\3973701 Drainage_Calcs_Template_v2.07.xlsm Page 1 of 2 9/13/2021 X:\3970000.all\3973701\Excel\Drainage\3973701 Drainage_Calcs_Template_v2.07.xlsm Page 2 of 2 9/13/2021