A portion of Lots 8 and 9, Block 1, Fountainhead Subdivision PRELIMINARY DRAINAGE REPORT Crescent View Academy 10855 E. Bethany Drive Aurora, Colorado Prepared for: Crescent View Academy II, LLC 10958 E. Bethany Drive Aurora, Colorado 80014 303-745-2245 Contact: Anwar Zuraigt Prepared by: Vermilion Peak Engineering LLC 1745 Shea Center Drive, 4th Floor Highlands Ranch, CO 80129 720-402-6070 Contact: Brian Krombein, PE, PLS June 12, 2020 Job Number 19037 06/23/2020 1 TABLE OF CONTENTS Page 1.0 INTRODUCTION............................................................................................................ 4 Location Proposed Development 2.0 HISTORIC DRAINAGE ................................................................................................. 4 Overall Basin Description Drainage Patterns Through Property Outfalls Downstream from Property 3.0 DESIGN CRITERIA ....................................................................................................... 5 List of References Hydrologic Criteria Hydraulic Criteria 4.0 DRAINAGE PLAN .......................................................................................................... 6 General Concept Specific Details 5.0 CONCLUSIONS .............................................................................................................. 8 Compliance with Standards Summary of Concept 6.0 LIST OF REFERENCES ................................................................................................ 9 APPENDIX Hydrologic Computations Hydraulic Computations Copies of graphs, tables and nomographs used 2 ENGINEER’S CERTIFICATION This report for the preliminary drainage design of a portion of Lots 8 and 9, Block 1, Fountainhead Subdivision was prepared by me (or under my direct supervision) in accordance with the provisions of City of Aurora Storm Drainage Design and Technical Criteria and was designed to comply with the provisions thereof. I understand that the City of Aurora does not and will not assume liability for drainage facilities designed by others. __________________________________________________ Brian Krombein, PE, PLS Colorado Registered Professional Engineer No. 34294 For and on behalf of Vermilion Peak Engineering LLC 3 1.0 INTRODUCTION Location The project site contains 1.00 acres and is located in the NW 1/4, Section 34, Township 4 South, Range 67 West of the 6th Principal Meridian in the City of Aurora, Arapahoe County, Colorado (see Vicinity Map). The property is platted as A portion of Lots 8 and 9, Block 1, Fountainhead Subdivision). It is bounded on the south by E. Bethany Drive and on the north, east and west by existing office buildings. Proposed Development The development at 10855 E. Bethany Drive contains an existing office building with associated parking areas and a small amount of landscaped areas. The disturbed area covers approximately 2,500 square feet. Proposed improvements include interior remodeling of the building, a small building expansion to contain only a new elevator and fire riser room, the addition of a one-way, paved drive aisle and sidewalk along the west side of the building and a new sidewalk connection to E. Bethany Drive. According to FEMA flood insurance rate map 08005C0186K, dated December 17, 2010, this property lies within Zone X, which is defined as, "areas determined to be outside the 0.2% annual chance floodplain". Soils on the site consist of Blakeland loamy sand, 1 to 20 percent slopes and Bresser- Truckton sandy loams, 3 to 5 percent slopes. They belong to Hydrologic Soil Group A. The site impervious area will be increased by a total of 2,230 square feet which will increase the total site imperviousness from 78% to 83%. 2.0 HISTORIC DRAINAGE Overall Basin Description The project site is located in the Cherry Creek drainage basin, with Cherry Creek being located approximately 1/4 mile southwest of the project site. This basin contains a mix of residential and commercial properties. 4 Drainage Patterns Through Property In the existing condition site runoff drains either directly to Bethany Drive or through the adjacent office development to the west from where it drains to Bethany Drive. The proposed site improvements will not alter these existing drainage patterns. Runoff from 100% of the additional impervious areas will drain directly to Bethany Drive and not through neighboring properties. Concentrated drainage will not be allowed to flow over sidewalks. No previous drainage reports were found that affect this property. There are no existing wetland or irrigation facilities that affect the project site. There are no offsite flows which affect the project site. Outfalls Downstream from Property Once site runoff reaches Bethany Drive it drains westerly to public storm sewer inlets which collect runoff and convey it westerly then southerly in public storm sewer beneath Bethany Drive. Flows ultimately outfall to Cherry Creek a short distance southeast of the intersection of Havana Street and Dartmouth Avenue. 3.0 DRAINAGE DESIGN CRITERIA List of References The hydrologic analysis and hydraulic design for this site is based upon criteria established in the City of Aurora Storm Drainage Design and Technical Criteria Manual and the Urban Storm Drainage Design Manual prepared by Mile High Flood District (MHFD). Hydrologic Criteria The Rational Method was used to calculate peak runoff rates for the sub-basins during the 2-year and 100-year storm events. Basin imperviousness was obtained from Table 1 (City of Aurora), runoff coefficients were obtained from Table 1 (City of Aurora), rainfall intensities were obtained from Equation 5.5 (City of Aurora) using the 1-hour point rainfall data from Figures RA-1 and RA-6 (MHFD). Runoff velocities were obtained from Figure 1 (City of Aurora). Times of concentration were determined using form SF-2, and runoff flow 5 rates were determined using form SF-3. Runoff tables and calculations can be found in the appendices of this report. None of the existing developments in the surrounding office park appear to contain onsite detention or water quality treatment. Although we could not find any previous drainage studies for this property it appears that detention and water quality were not required for this entire area due to its proximity to nearby Cherry Creek and Cherry Creek Reservoir. Hydraulic Criteria Inlet sizing calculations are included in the appendix of this report. Hydraulic grade line calculations will be provided in the final drainage report. 4.0 DRAINAGE PLAN General Concept Site grading has been designed to direct runoff away from the building where it will sheetflow across the parking lots to curb and gutter which conveys flows to Bethany Drive. A new 5’ Type R inlet will be installed in the new drive aisle through the southwest corner of the property in order to collect a majority of onsite runoff. Collected flows will be routed through a mechanical water quality device then released into public storm sewer. Specific Details The project site has been analyzed as a single drainage basin for determination of historic and proposed runoff rates: Basin H-1 (1.0 acres) contains the entire property in the existing condition. It contains building, pavement and landscaped areas. Runoff from Basin H-1 drains either directly to Bethany Drive or to the parking lot of the adjacent office building to the west, from which point it drains to Bethany Drive. The historic 2-year and 100-year flowrates at Design Point H1 are 2.2 cfs and 6.7 cfs. Upon reaching Bethany Drive runoff drains approximately 330 feet west to a public storm inlet in the north gutter from Bethany Drive. Public storm sewer conveys collected flows to a point of discharge into Cherry Creek approximately 1/4 mile to the southwest. 6 Basin A (0.70 acres) contains the northwest two-thirds of the property. It contains building, pavement and landscaped areas. Runoff from Basin A drains to an on-grade, 5’ Type R inlet at Design Point 1. The proposed 2-year and 100-year flowrates at Design Point 1 will be 1.8 cfs and 5.5 cfs. Basin B (0.30 acres) contains the southeast portion of the property. It contains pavement and landscaped areas. Runoff from Basin B drains directly to Bethany Drive as occurs in the existing condition. The proposed 2-year and 100-year flowrates at Design Point 2 will be 0.5 cfs and 1.6 cfs. Runoff from the building expansion and a portion of the new concrete walk will drain across landscaped areas prior to sheetflowing into Bethany Drive. The remainder of the new impervious area will drain to the proposed 5’ Type R inlet at Design Point 1. Total site 2-year runoff rates will increase from 2.2 cfs to 2.3 cfs and 100-year rates will increase from 6.7 cfs to 7.1 fs. We hereby request a variance from the typical requirement to provide additional detention volume due to an increase in impervious area. We offer the following support for this request: • Approximately 2,500 square feet of additional impervious area is proposed. • The proposed improvements are located in the lowest-elevation corner of the property with no area available to add detention facilities onsite. • It does not appear that any detention or water quality facilities currently exist to serve this entire commercial subdivision. • The proposed total site imperviousness is 83%. While we were unable to find an existing drainage study for this subdivision, it is reasonable to assume that the drainage infrastructure for this subdivision (downstream roadway and storm sewer) was designed with an assumed imperviousness