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Home My WebLink AboutMELDRUM OFFICE BUILDING - PDP - PDP130027 - REPORTS - DRAINAGE REPORTADDRESS: 200 S. College Ave. Suite 10 Fort Collins, CO 80524 PHONE: 970.221.4158 FAX: 970.221.4159 WEBSITE: www.northernengineering.com D:\Projects\838-007\Drainage\Reports\Drainage Report\838-007_Drainage-Letter_2013-10-16.docx Page 1 of 2 Drainage Letter Report Date: October 16, 2013 Project: Meldrum Office Building Project No. 838-007 Fort Collins, Colorado Attn: Mr. Wes Lamarque City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 Dear Wes: This letter serves to address the stormwater impacts of the above referenced project. The site currently consists of rooftop, concrete walks, asphalt parking area, and limited vegetation. The east half of the site contains a vacant 2-story office building, while the west half contains the parking lot. Roof drainage from the existing building is currently directed towards the rear parking lot. The parking lot is extremely flat with poor drainage, but does appear to eventually drain towards the alley to the west before heading north to Mountain Avenue. Gutter flows from both Mountain and Meldrum converge at two existing inlets at the southwest corner of the intersection. Historic drainage patterns will generally be preserved with the proposed redevelopment in terms of maintaining the existing outfall location at Mountain and Meldrum. The total existing impervious area (as verified by field survey) within the limits of construction is approximately 19,814 square-feet. Additionally, this parcel has historically paid monthly stormwater utility fees commensurate with a fully impervious value (0.95). Even though the site is being completely redeveloped, a variance is hereby requested to utilize the historic impervious area for which this parcel has paid into the stormwater utility system. The proposed redevelopment will completely remove all existing structures and on-site paving. The new project will consist of a larger building, as well as streetscape improvements along Meldrum and a reconstructed parking lot west to the alley. The streetscape improvements provide significant parkway enhancements, including the replacement of existing diagonal parking spaces with a large bioretention rain garden. The total impervious area within the limits of construction of the proposed redevelopment is roughly 16,051 square-feet. The total impervious area proposed with this project will be decreased by approximately 3,782 square-feet (19,814 – 16,051). A detailed breakdown of the existing and proposed impervious areas is attached to this letter. (Note: these areas will be confirmed again during the Final Plan phase.) Since the proposed redevelopment actually results in a lower impervious area than the historic condition, on-site detention is not required. However, water quality requirements still apply, as do Low-Impact Development (LID) regulations pursuant to Fort Collins Ordinance No. 152, 2012. According to said ordinance, no less than fifty percent (50%) of any newly added impervious area must be treated using one or a combination of LID techniques; and no less than twenty five percent (25%) of any newly added pavement areas must be treated using a permeable pavement technology. While this proposal has no newly added impervious area or pavement, an attempt has been made to meet the spirit of Ordinance 152 to maximum extent feasible. D:\Projects\838-007\Drainage\Reports\Drainage Report\838-007_Drainage-Letter_2013-10-16.docx Page 2 of 2 The site is divided into two primary developed drainage basins. The rooftop of the new office building, which will drain east to Meldrum Street, and the rear parking lot which drains north to Mountain Avenue. The existing rooftop currently discharges into the rear parking lot where it has a poor drainage path. This stormwater then collects additional surface pollutants before reaching the inlet at the southwest corner of Mountain and Meldrum. The strategy with this proposal is to ameliorate the current condition by directing the roof water towards Meldrum Street. The vast majority of the building rooftop will be routed through the new bioretention rain garden to be constructed in Meldrum. The water will be directed through roof leaders and sidewalk chases into the rain garden. Pass through runoff will head north to the historic drainage inlet. However, the bioretention rain garden is only expected to see pass through in major storm events. Routine (first flush, 80th percentile) storms should infiltrate through the rain garden media before being intercepted by the underdrain. The new underdrain will connect into the gravity storm sewer in Meldrum Street. Additional details of the rain garden can be found on the Landscape Plans, as well as the (construction) Utility Plans to be provided during Final Design. In addition to receiving piped roof runoff from the new office building, the bioretention rain garden will also be designed to intercept gutter flows from Meldrum Street. This will provide significant water quality enhancement to public stormwater previously reaching the Poudre River without any treatment at all. The reduced impervious area and increased water quality treatment offered with the new rain garden results in a noticeable public benefit, not only from an ecological standpoint, but from an aesthetic perspective and pedestrian experience as well. The other drainage improvement comes by way of the reconstructed rear parking lot. Whereas this area is currently very flat with poor drainage, the new parking lot will be rebuilt with improved slope. Additionally, a new trench drain and discharge pipe will be installed to provide a better outfall path. The trench drain will discharge through an existing retaining wall to a parking lot north of the Meldrum Office Building redevelopment. Said parking lot (401 W. Mountain Ave.) is under common ownership with the 111 Meldrum property, and a letter of intent can be provided prior to hearing, if necessary. The need for a formal drainage easement will be explored during the Final Plan process, as will the need for a continued trench drain or similar measures north to Mountain Avenue. In addition to the aforementioned LID rain garden, stormwater quality mitigation will be further addressed by both temporary and permanent Best Management Practices (BMPs). During construction, the Contractor will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control. A comprehensive Erosion Control Plan and Report will be provided during the Final Plan process. 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, not to mention the LID components. The project site is located in the Old Town Basin, and meets all associated drainage master plan stipulations. There are no floodplains impacting the subject property. Additional design details will be provided during the Final Plan process. Standard Operating Procedures (SOPs) for the LID facilities will be incorporated into the Development Agreement. The drainage outfall location at the corner of Mountain and Meldrum will be preserved, and modifications only serve to improve the existing condition. Therefore, it is my professional opinion that the new building and site improvements proposed with the Meldrum Office Building redevelopment meet or exceed all applicable stormwater criteria. Please do not hesitate to contact me if you have questions or require additional information. Sincerely, Nicholas W. Haws, PE Vice President APPENDIX A PRELIMINARY RAIN GARDEN DESIGN CONCEPTS MELDRUM OFFICE BUILDING Preliminary Rain Garden Concepts The rain garden is envisioned to closely mimic the concepts approved on Sheet D10 of The Grove at Fort Collins. The rain garden will be pass-through in nature with prevailing longitudinal grade allowing overflow to continue to the north. Depressed areas will be provided to allow a true water quality capture volume above the ground surface. Additional grading detail on the Final Plans and diligent coordination with the landscape contractor are expected to help ensure the depressed portions of the rain garden are achieved. Preliminary bio-retention sizing was performed utilizing UDFCD’s UD-BMP (v3.02) Rain Garden (RG) worksheet. A maximum ponding depth of 12” was assumed, and a conservative side slope of 4:1 was utilized. Actual side slopes may vary with final design. The following three scenarios were analyzed. 111 Meldrum Property Only This scenario was analyzed to compute the private development responsibility for the 111 Meldrum property only. The lot area is 9,735 sq.ft., and a conservative value of 100% imperviousness was utilized. Interim Condition The scenario accounts for the 111 Meldrum property and adjacent streetscape, as well as existing upstream tributary areas. An existing high point in Meldrum at Oak Street, as well as an existing inlet at the northwest corner of the intersection, limits the area contributing to public gutter flow to approximately 13,070 sq.ft. Additionally, roughly 34,723 sq.ft. of area from the existing surface parking lot south of the site will drain to the new rain garden. Ultimate Condition This scenario assumes the existing surface parking lot to the south redevelops in similar fashion to the mixed-use parking structure presented at the 07.15.13 Conceptual Review. It is assumed that such a redevelopment would capture all stormwater on-site rather than sheet flowing across the asphalt parking lot towards the rain garden, as it does in the interim condition. Additionally, the mixed-use parking structure development would likely introduce its own curb extensions and rain gardens into the existing diagonal spaces along Meldrum Street. This is assumed to reduce the incoming gutter flow tributary area to 1,458 sq.ft. The following table summarizes the required water quality capture volumes (WQCV) and associated surface areas for each scenario, as well as the approximate WQCV and flat surface area provided. Table 1 – Rain Garden Scenario Summary Scenario Tributary Area (sq.ft.) Effective Imperviousness (percent) WQCV (cu.ft.) Surface Area (sq.ft.) 111 Meldrum 9,735 100 325 216 Interim 61,208 94 1,785 1,190 Ultimate 14,873 90 398 265 Provided (approx.) - - 846 450 The proposed rain garden is expected to provide over double the LID quantity required by the 111 Meldrum property itself. The proposed rain garden is also envisioned to provide over twice the WQCV for the entire tributary area in the ultimate condition (assuming a mixed-use parking structure develops south of the Meldrum Office Building). However, the rain garden would be slightly undersized in the interim condition (or in the long-term if the adjacent surface parking lot does not redevelop). MELDRUM OFFICE BUILDING Preliminary Rain Garden Concepts The undersized bio-retention area is not expected to be problematic if proper design and maintenance procedures are followed. Chris Carlson with the City of Loveland Stormwater Department has conducted research on a very similar situation proving the effectiveness of such urban LID retrofits. One key design enhancement intended to improve the performance of the bio- retention area and assist in maintenance operations is the upstream inlet forebay. The inlet forebay will utilize a standard City of Fort Collins curb (“combo”) inlet box and grate/frame, with some key modifications. A solid 6” PVC outlet pipe will drain the bottom of the inlet box into the adjacent rain garden underdrain. A steel orifice plate with a ½“ diameter hole will restrict flows into the outlet pipe. The bottom ±8” of the box will be filled with No. 57 drain rock, upon which ±6” of pea gravel will be placed. Another 6” to 8” of filter sand will sit atop the pea gravel. The top of the sand layer will be approximately 2’ below the gutter flowline. This 2’ sump depth is intended to collect large trash, debris, and sediment. This will trap these pollutants in the concrete inlet box allowing for easy maintenance and proper disposal, while also preventing clogging and other damage to the rain garden itself. Portions of the filter sand are expected to be periodically removed and replaced with routine cleaning of the forebay inlet box. While the 2’ sump will trap the heavy sediments and debris, it also has the potential to hold standing water. The concern of standing water will be mitigated with two outflow points. The first outflow point is the aforementioned 6” solid PVC outlet pipe with a ½“ diameter orifice. Water trapped in the sump will slowly percolate through the sand, pea gravel, and drain rock before flowing downstream through the underdrain and into the public storm sewer. However, this is not intended to be the preferred flowpath. This is purely meant to drain the sump volume only. It is desired to direct as much flow as possible through the rain garden. An 18” wide by 6” tall cutout will be provided in the back of the inlet box to allow flow to continue north into the rain garden. The bottom of the cutout will be roughly 12” above the top of sand. This elevation will be flush with the bottom of the rain garden cobble channel, which will be approximately 12” below the incoming gutter flowline. A Low Profile Snout will be mounted in front of the rectangular cutout. This will further limit large debris and floatables from entering the rain garden. It also will minimize the introduction of hydrocarbons into the rain garden. The small, simple forebay inlet box is expected to provide many enhancements to the overall performance of the rain garden, particularly when it comes to long-term operations and maintenance. The rain garden itself will be designed with as large a WQCV as possible within the space available. The sizing more than meets the requirements of the Meldrum Office Building project, and should function acceptably in both the interim and ultimate condition, treating an area much larger than the 111 Meldrum lot itself. In fact, the area attributed to the Meldrum Office Building lot is somewhat nominal, and may not be of great significance if it were to ever go away. Sheet 1 of 2 Designer: Company: Date: Project: Location: 1. Basin Storage Volume A) Effective Imperviousness of Tributary Area, Ia Ia = 100.0 % (100% if all paved and roofed areas upstream of rain garden) B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 1.000 C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.40 watershed inches (WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i) D) Contributing Watershed Area (including rain garden area) Area = 9,735 sq ft E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 325 cu ft Vol = (WQCV / 12) * Area F) For Watersheds Outside of the Denver Region, Depth of d6 = in Average Runoff Producing Storm G) For Watersheds Outside of the Denver Region, VWQCV OTHER = cu ft Water Quality Capture Volume (WQCV) Design Volume H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER = cu ft (Only if a different WQCV Design Volume is desired) 2. Basin Geometry A) WQCV Depth (12-inch maximum) DWQCV =12in B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft (Use "0" if rain garden has vertical walls) C) Mimimum Flat Surface Area AMin = 216 sq ft D) Actual Flat Surface Area AActual = sq ft E) Area at Design Depth (Top Surface Area) ATop = sq ft F) Rain Garden Total Volume VT= cu ft (VT= ((ATop + AActual) / 2) * Depth) 3. Growing Media 4. Underdrain System A) Are underdrains provided? B) Underdrain system orifice diameter for 12 hour drain time i) Distance From Lowest Elevation of the Storage y = 2.0 ft Volume to the Center of the Orifice ii) Volume to Drain in 12 Hours Vol12 = 325 cu ft iii) Orifice Diameter, 3/8" Minimum DO = 0.42 in Meldrum Office Building - Fort Collins, CO Rain Garden at 111 Meldrum - private lot obligation only Design Procedure Form: Rain Garden (RG) Nick Haws Northern Engineering October 16, 2013 Choose One Choose One 18" Rain Garden Growing Media Other (Explain): YES NO UD-BMP_v3.02_on-site-area-only.xlsm, RG 10/16/2013, 10:32 AM Sheet 2 of 2 Designer: Company: Date: Project: Location: 5. Impermeable Geomembrane Liner and Geotextile Separator Fabric A) Is an impermeable liner provided due to proximity of structures or groundwater contamination? 6. Inlet / Outlet Control A) Inlet Control 7. Vegetation 8. Irrigation A) Will the rain garden be irrigated? Notes: Meldrum Office Building (Lot 1) private parcel area only = 9,735 SF Design Procedure Form: Rain Garden (RG) Nick Haws Northern Engineering October 16, 2013 Meldrum Office Building - Fort Collins, CO Rain Garden at 111 Meldrum - private lot obligation only Choose One Choose One Choose One Sheet Flow- No Energy Dissipation Required Concentrated Flow- Energy Dissipation Provided Plantings Seed (Plan for frequent weed control) Sand Grown or Other High Infiltration Sod Choose One YES NO YES NO UD-BMP_v3.02_on-site-area-only.xlsm, RG 10/16/2013, 10:32 AM Sheet 1 of 2 Designer: Company: Date: Project: Location: 1. Basin Storage Volume A) Effective Imperviousness of Tributary Area, Ia Ia = 94.0 % (100% if all paved and roofed areas upstream of rain garden) B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.940 C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.35 watershed inches (WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i) D) Contributing Watershed Area (including rain garden area) Area = 61,208 sq ft E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 1,785 cu ft Vol = (WQCV / 12) * Area F) For Watersheds Outside of the Denver Region, Depth of d6 = in Average Runoff Producing Storm G) For Watersheds Outside of the Denver Region, VWQCV OTHER = cu ft Water Quality Capture Volume (WQCV) Design Volume H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER = cu ft (Only if a different WQCV Design Volume is desired) 2. Basin Geometry A) WQCV Depth (12-inch maximum) DWQCV =12in B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft (Use "0" if rain garden has vertical walls) Meldrum Office Building - Fort Collins, CO Rain Garden at 111 Meldrum - Interim Condition Design Procedure Form: Rain Garden (RG) Nick Haws Northern Engineering October 16, 2013 C) Mimimum Flat Surface Area AMin = 1190 sq ft D) Actual Flat Surface Area AActual = sq ft E) Area at Design Depth (Top Surface Area) ATop = sq ft F) Rain Garden Total Volume VT=cu ft (VT= ((ATop + AActual) / 2) * Depth) 3. Growing Media 4. Underdrain System A) Are underdrains provided? B) Underdrain system orifice diameter for 12 hour drain time i) Distance From Lowest Elevation of the Storage y = 2.0 ft Volume to the Center of the Orifice ii) Volume to Drain in 12 Hours Vol12 = 1,785 cu ft iii) Orifice Diameter, 3/8" Minimum DO = 0.97 in Choose One Choose One 18" Rain Garden Growing Media Other (Explain): YES NO UD-BMP_v3.02_interim.xlsm, RG 10/16/2013, 10:46 AM Sheet 2 of 2 Designer: Company: Date: Project: Location: 5. Impermeable Geomembrane Liner and Geotextile Separator Fabric A) Is an impermeable liner provided due to proximity of structures or groundwater contamination? 6. Inlet / Outlet Control A) Inlet Control 7. Vegetation 8. Irrigation A) Will the rain garden be irrigated? Notes: Interim condition. 13,415 SF from 111 Meldrum, plus 34,723 SF from existing surface parking lot to the south, plus 13,070 SF from existing west gutter section of Meldrum south to Oak. Design Procedure Form: Rain Garden (RG) Nick Haws Northern Engineering October 16, 2013 Meldrum Office Building - Fort Collins, CO Rain Garden at 111 Meldrum - Interim Condition Choose One Choose One Choose One Sheet Flow- No Energy Dissipation Required Concentrated Flow- Energy Dissipation Provided Plantings Seed (Plan for frequent weed control) Sand Grown or Other High Infiltration Sod Choose One YES NO YES NO UD-BMP_v3.02_interim.xlsm, RG 10/16/2013, 10:46 AM Sheet 1 of 2 Designer: Company: Date: Project: Location: 1. Basin Storage Volume A) Effective Imperviousness of Tributary Area, Ia Ia = 90.0 % (100% if all paved and roofed areas upstream of rain garden) B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.900 C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.32 watershed inches (WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i) D) Contributing Watershed Area (including rain garden area) Area = 14,873 sq ft E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 398 cu ft Vol = (WQCV / 12) * Area F) For Watersheds Outside of the Denver Region, Depth of d6 = in Average Runoff Producing Storm G) For Watersheds Outside of the Denver Region, VWQCV OTHER = cu ft Water Quality Capture Volume (WQCV) Design Volume H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER = cu ft (Only if a different WQCV Design Volume is desired) 2. Basin Geometry A) WQCV Depth (12-inch maximum) DWQCV =12in B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft (Use "0" if rain garden has vertical walls) C) Mimimum Flat Surface Area AMin = 265 sq ft D) Actual Flat Surface Area AActual = sq ft E) Area at Design Depth (Top Surface Area) ATop = sq ft F) Rain Garden Total Volume VT= cu ft (VT= ((ATop + AActual) / 2) * Depth) 3. Growing Media 4. Underdrain System A) Are underdrains provided? B) Underdrain system orifice diameter for 12 hour drain time i) Distance From Lowest Elevation of the Storage y = 2.0 ft Volume to the Center of the Orifice ii) Volume to Drain in 12 Hours Vol12 = 398 cu ft iii) Orifice Diameter, 3/8" Minimum DO = 0.46 in Design Procedure Form: Rain Garden (RG) Nick Haws Northern Engineering October 16, 2013 Meldrum Office Building - Fort Collins, CO Rain Garden at 111 Meldrum - Ultimate Condition (if parking garage develops) Choose One Choose One 18" Rain Garden Growing Media Other (Explain): YES NO UD-BMP_v3.02_ultimate.xlsm, RG 10/16/2013, 10:47 AM Sheet 2 of 2 Designer: Company: Date: Project: Location: 5. Impermeable Geomembrane Liner and Geotextile Separator Fabric A) Is an impermeable liner provided due to proximity of structures or groundwater contamination? 6. Inlet / Outlet Control A) Inlet Control 7. Vegetation 8. Irrigation A) Will the rain garden be irrigated? Notes: Ultimate condition. 13,415 SF from 111 Meldrum, plus 1,458 SF from west portion of Meldrum street section. Design Procedure Form: Rain Garden (RG) Nick Haws Northern Engineering October 16, 2013 Meldrum Office Building - Fort Collins, CO Rain Garden at 111 Meldrum - Ultimate Condition (if parking garage develops) Choose One Choose One Choose One Sheet Flow- No Energy Dissipation Required Concentrated Flow- Energy Dissipation Provided Plantings Seed (Plan for frequent weed control) Sand Grown or Other High Infiltration Sod Choose One YES NO YES NO UD-BMP_v3.02_ultimate.xlsm, RG 10/16/2013, 10:47 AM EXHIBIT 1 EXISTING & PROPOSED IMPERVIOUS AREAS