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HomeMy WebLinkAboutHOWES SIX TWO NINE - FDP210025 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORT Final Drainage and Erosion Control Report for Howes Six Two Nine Fort Collins, Colorado October 15, 2021 Prepared for: Carlson Land Development Blake Carlson 14570 Clay St. Broomfield, CO 80023 Prepared by: 301 N. Howes St. Suite 100 Fort Collins, Colorado 80521 Phone: 970.221.4158 www.northernengineering.com Project Number: 1590-002 This Drainage Report is consciously provided as a PDF. Please consider the environment before printing this document in its entirety. When a hard copy is necessary, we recommend double-sided printing. October 15, 2021 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, CO 80521 RE: Preliminary Drainage and Erosion Control Report for Howes Six Two Nine Dear Staff: Northern Engineering is pleased to submit this Preliminary Drainage and Erosion Control Report for your review. This report accompanies the combined Preliminary & Final Plan submittal for the proposed Howes Six Two Nine Development. This report has been prepared in accordance with the Fort Collins Stormwater Criteria Manual (FCSCM) and serves to document the stormwater impacts associated with the proposed Howes Six Two Nine project. We understand that review by the City of Fort Collins is to assure general compliance with standardized criteria contained in the FCSCM. If you should have any questions as you review this report, please feel free to contact us. Sincerely, Northern Engineering Services, Inc. Austin Snow, PE Danny Weber, PE Project Engineer Project Manager Compliance Statement: I hereby attest that this report for preliminary drainage design for Howes Six Tow Nine 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. Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Table of Contents Table of Contents I. General Location and Description ........................................................................ 1 II. Drainage Basins and Sub-Basins .......................................................................... 3 III. Drainage Design Criteria ..................................................................................... 4 IV. Drainage Facility Design ..................................................................................... 7 V. Conclusions ......................................................................................................... 9 VI. References ......................................................................................................... 11 Tables and Figures Figure 1 – Vicinity Map .............................................................................................. 1 Figure 2 – Aerial Photograph ..................................................................................... 2 Figure 3 – FEMA Firmette (Map Number 08069C0979H) .......................................... 3 Appendices Appendix A – Hydrologic Computations Appendix B – Hydraulic Computations Appendix C – Water Quality/Lid Design Computations Appendix D – Erosion Control Report Appendix E – USDA Soils Report Map Pocket DR1 – Drainage Exhibit Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 1 of 12 I. General Location and Description A. Location 1. Vicinity Map Figure 1 – Vicinity Map 2. The Howes Six Two Nine project site is located within the North half of Lot 2 and the South half of Lot 3 of Block 93 of the Harrison subdivision, City of Fort Collins, County of Larimer, State of Colorado. 3. The project site (refer to Figure 1) is bordered on the North and South by parking lots for multifamily residences; to the West by an inverted-crown alleyway; to the East by the public right-of-way for South Howes. B. Description of Property 1. The Howes Six Two Nine development is comprised of ±0.43 acres. Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 2 of 12 2. The site is currently occupied by one two-story brick building planned to remain, a garage adjacent the alleyway which is planned for removal, as well as various concrete sidewalk and pavers, and an approximately 4240 square-foot asphalt parking area connected to the alley. Figure 2 – Aerial Photograph 3. The existing groundcover consists of grasses, concrete, pavers, asphalt, and the existing structures’ rooftops. The existing on-site runoff generally drains Westward into the inverted crown alleyway and Eastward to South Howes on shallow grades (e.g., <2.00%). From there, the drainage follows existing City of Fort Collins stormwater infrastructure. 4. According to the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Soil Survey website: (http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx), the site consists primarily of Fort Collins loam (Hydrologic Soil Group C). 5. The proposed development will consist of two townhome buildings (connected above-ground) containing a total of 7 single-family attached units. Other proposed improvements include: an enlarged asphalt parking lot, walkways and a courtyard, and landscaping. Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 3 of 12 6. The project site is located in the Campus North sub-district of the Downtown District. The proposed use (Multi-family, 8-units or less/building) is an approved use in the Downtown District, subject to Type 1 Administrative Review. 7. No significant offsite flows are directed into the site. Off-site flows are prevented from entering the site by existing topography. 8. The site is not required to provide on-site detention as it is located in the Old Town Basin and is increasing total impervious area by less than 5,000 square feet. Additionally, the standard water quality requirement for this site has been provided for in the City’s Udall water quality facility. This site will meet the City’s on-site LID treatment requirements via a bioswale included in the design. C. Floodplain 1. The subject property is not located in a FEMA or City regulatory floodplain according to FEMA Firm No. 08069C0979H, effective on 05/02/2012. 2. Figure 3 – Existing Floodplain(s) II. Drainage Basins and Sub-Basins A. Major Basin Description Howes Six Two Nine is located within the City of Fort Collins Old Town major drainage basin. Specifically, the project site is situated in the lower center of this Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 4 of 12 major drainage basin. This basin is located in north-central Fort Collins and has a drainage area of approximately 2,120 acres. The entire basin is urbanized and generally drains from west to east. Most of the water from the Old Town basin drains to the Poudre River, just to the east. B. Sub-Basin Description 1. The outfall for the project site is the Cache la Poudre River, to the east. 2. The subject site can be defined with three (3) historical sub-basins that encompass the entire project site. 3. The existing site runoff drains to four different places, including the alley to the west, the parking lot to the north of the site, low areas within the site itself, or east to the Howes gutter. 4. The project site does not receive notable runoff from off-site properties. III. Drainage Design Criteria A. Optional Provisions There are no optional provisions outside of the FCSCM proposed with the Howes Six Two Nine project. B. Stormwater Management Strategy The overall stormwater management strategy employed with the Howes Six Two Nine project utilizes the “Four Step Process” to minimize adverse impacts of urbanization on receiving waters. The following is a description of how the proposed development has incorporated each step. Step 1 – Employ Runoff Reduction Practices. The first consideration taken in trying to reduce the stormwater impacts of this development is the site selection itself. By choosing an already developed site with public storm sewer currently in place, the burden is significantly less than developing a vacant parcel absent of any infrastructure. The Howes Six Two Nine project aims to reduce runoff peaks, volumes and pollutant loads from frequently occurring storm events (i.e., water quality (i.e., 80th percentile) and 2-year storm events) by implementing Low Impact Development (LID) strategies. Wherever practical, runoff will be routed across landscaped areas or through an infiltration gallery. These LID practices reduce the overall amount of impervious area, while at the same time Minimizing Directly Connected Impervious Areas (MDCIA). The combined LID/MDCIA techniques will be implemented, where Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 5 of 12 practical, throughout the development, thereby slowing runoff and increasing opportunities for infiltration. Step 2 – Implement BMPs that Provide a Water Quality Capture Volume (WQCV) with Slow Release. The efforts taken in Step 1 will help to minimize excess runoff from frequently occurring storm events; however, urban development of this intensity will still have stormwater runoff leaving the site. The primary water quality treatment will occur off-site at the City’s Udall Water Quality facility. Step 3 – Stabilize Drainageways. The project will pay one-time stormwater development fees, as well as ongoing monthly stormwater utility fees, both of which help achieve Citywide drainageway stability. Additionally, LID BMPs provide downstream Step 4 – Implement Site Specific and Other Source Control BMPs. This step typically applies to industrial and commercial developments. C. Development Criteria Reference and Constraints 1. The subject property is part of the Old Town basin drainage master plan. 2. The site plan is constrained on the east by a public street, on the west by an alley, and by existing development on the north and south. D. Hydrological Criteria 1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure 3.4-1 of the FCSCM, serve as the source for all hydrologic computations associated with the development. Tabulated data contained in Table 3.4-1 has been utilized for Rational Method runoff calculations. 2. The Rational Method has been employed to compute stormwater runoff utilizing coefficients contained in Chapter 3 of the FCSCM. 3. The Rational Formula-based Federal Aviation Administration (FAA) procedure has not been utilized for detention storage calculations since detention is not required for the project. 4. Two separate design storms have been utilized to address distinct drainage scenarios. The first event analyzed is the “Minor,” or “Initial” Storm, which has a 2-year recurrence interval. The second event considered is the “Major Storm,” which has a 100-year recurrence interval. Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 6 of 12 E. Hydraulic Criteria 1. The drainage facilities proposed for the Howes Six Two Nine project are designed in accordance with criteria outlined in the FCSCM and/or the Mile High Flood District’s (MHFD) Urban Storm Drainage Criteria Manual. 2. As stated in Section I.C.1, above, the subject property is not located in any FEMA or City floodplains. F. Floodplain Regulations Compliance 1. As previously mentioned, this project is not subject to any floodplain regulations. G. Modifications of Criteria No formal modifications are requested at this time. Quantity detention is required in the Old Town Basin if the site increases impervious area by greater than 5000 square feet. This site does not exceed/trigger this requirement. H. Conformance with Water Quality Treatment Criteria City Code requires that 100% of runoff from a project site receive some sort of water quality treatment. As per staff guidance, his project will utilize the Udall water quality facility to treat the runoff emanating from the site. Additionally, the site will feature LID treatment for at least the required treatment area. Due to the physical constraints associated with a project of this nature/size and the prohibition of providing water quality facilities within the public right-of-way, there are some small, narrow areas around the perimeter of the project that cannot be captured. The uncaptured areas tend to be narrow strips of landscaped area between the property line and buildings or curbs. While these small areas will not receive formal water quality treatment, most areas will still see some treatment as runoff is directed across through the landscaped areas or LID system before outletting to the storm sewer off-site. I. Conformance with Low Impact Development (LID) The project site will conform with the requirement to treat a minimum of 75% of the project site using a LID technique. Please see Appendix C for LID design information, table, and exhibit(s). As shown in the LID table provided in the appendix, 86.4% of the proposed site impervious area will receive LID treatment, which exceeds the minimum required. Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 7 of 12 J. Sizing of LID Facilities Bioswale 1. The bioswale was sized by first determining the required treatment area for the system (75% of new/modified impervious area). New and modified impervious area equals approximately 11,591 square feet, so the required treatment area equals approximately 8,693 square feet. 2. After preliminary placement was identified, the contributing area was delineated in order to determine if the treatment area requirement in 1, above, was being met. The treatment area, as designed, equals 8,836 square feet, or 76.2% of the new & modified impervious area. 3. Once it was determined that the treatment area requirement was met, the bioswale was sized using the method described in UDFCD/MHFD Treatment BMP Fact Sheet T-03, referenced in Appendix C (LID Implementation Manual) of the FCSCM. A modification was applied to this sizing method: the requirement for storage of 120% of the WQCV as per the FCSCM. The sizing worksheet is included in the Appendices of this report. 4. A volume calculation utilizing the contributing area flow rate into the bioswale and the calculated release rate through the filter area was completed as per Fact Sheet T-03. IV. Drainage Facility Design A. General Concept 1. The main objective of the Howes Six Two Nine drainage design is to maintain existing drainage patterns, while not adversely impacting adjacent properties. 2. No notable off-site runoff passes directly through the project site. 3. A list of tables and figures used within this report can be found in the Table of Contents at the front of the document. The tables and figures are located within the sections to which the content best applies. 4. Drainage for the project site has been analyzed using thirteen (13) drainage sub- basins, designated as sub-basins A through G and H-C. The drainage patterns anticipated for the basins are further described below. Sub-Basin A Sub-basin A encompasses approximately 4.5% the total site area. This sub-basin is comprised primarily of asphalt parking/drive and landscaped area. The sub- basin will drain to the alley located along the west property line and be captured Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 8 of 12 by an area drain which will then convey runoff from the basin through the storm drain system off-site. Historically, approximately 360 square feet of impervious area drains to the alley. The proposed development will increase the impervious area draining to the alley by 262 square feet to approximately 622 square feet. Since the increase is less than 350 square feet, as per City staff, drainage analysis of the alley’s conveyance is not required. Sub-Basin B Sub-basin B encompasses approximately 40.6% the total site area. This sub-basin is comprised primarily of roof area, concrete flatwork, an asphalt parking lot, and landscaped area. Flows for the sub-basin will drain to a curb-cut in the southeast corner of the parking lot that flows into Sub-Basin F and the LID system. Sub-Basin C Sub-basin C encompasses approximately 21.4% the total site area. This sub-basin is comprised primarily of roof area, concrete flatwork and landscaped areas. The sub-basin will sheet flow to a trench drain, which will then convey flows to an area inlet leading to the off-site storm sewer in Howes. Sub-Basin D Sub-basin D encompasses approximately 2.3% the total site area. This sub-basin is comprised primarily of landscaped area and roof area. Flows from the sub- basin will flow to an area inlet which will convey runoff from the basin into the off-site storm sewer. Sub-Basin E Sub-basin E encompasses approximately 16.5% the total site area. This sub-basin is comprised primarily of roof area. Flows from the sub-basin will flow via gutters into Sub-Basin F and the LID system. Sub-Basin F Sub-basin F encompasses approximately 8.2% the total site area. This sub-basin is comprised primarily of landscaped area. Flows from the sub-basin will enter the LID system (bioswale) which has been designed to capture the major flows from the contributing area and features an underdrain conveying flows to the off-site storm sewer in Howes. An area drain is also featured to collect flows greater than the major event. Since this Sub-Basin receives flows from multiple other Sub- Basins, it has been designed to overflow into the Sub-Basin D outlet, an area inlet that leads directly to the off-site storm sewer, in case of a long-return period event or clogging in the LID system. Sub-Basin G Howes Six Two Nine Preliminary Drainage Report October 15, 2021 Carlson Development Page 9 of 12 Sub-basin G encompasses approximately 6.2% the total site area. This sub-basin is comprised primarily of roof area, concrete flatwork and landscaped areas. Flows from this sub-basin cannot be captured due to existing topography at the property boundary and will flow off-site to the parking lot to the north or the gutter in Howes, following historical drainage patterns. A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the end of this report. B. Specific Details 1. Since detention is not required with this site, the existing impervious area has not been considered in determining allowable release from the property. 2. An allowable release rate was not determined for this project due to an increase in impervious area less than 5,000 square feet from historical. 3. The FAA method was not used to size the on-site detention volume for quantity detention since detention is not required. 4. Storm infrastructure was sized to the major event flows from contributing areas as per the FCSCM. V. Conclusions A. Compliance with Standards 1. The design elements comply without variation and meet all LID requirements. 2. The drainage design proposed with the Howes Six Two Nine project complies with the City of Fort Collins Master Drainage Plan for the Old Town Basin. 3. There are no FEMA regulatory floodplains associated with the development. 4. The drainage plan and stormwater management measures proposed with the project are compliant with all applicable State and Federal regulations governing stormwater discharge. B. Drainage Concept 1. The drainage design proposed with this project will effectively limit potential damage associated with its stormwater runoff. The project will not detain for the pervious area converted to impervious areas to release at the 2-year existing rate during the 100-year storm as per staff instruction, however, LID will be implemented to treat new impervious area. X X X X X X X X X X XLID BIOSWALE EXTENTS 250 CUBIC FEET CAPACITY 7.2" MAXIMUM WATER DEPTHLID BIOSWALE BOTTOM FILTER AREA 160 SQUARE FEET IMPERMEABLE LINER SheetHOWES SIX TWO NINEThese drawings areinstruments of serviceprovided by NorthernEngineering Services, Inc.and are not to be used forany type of constructionunless signed and sealed bya Professional Engineer inthe employ of NorthernEngineering Services, Inc.NOT FOR CONSTRUCTIONREVIEW SETENGINEERNGIEHTRONRNFORT COLLINS: 301 North Howes Street, Suite 100, 80521GREELEY: 820 8th Street, 80631970.221.4158northernengineering.comof 15 ( IN FEET ) 0 1 INCH = 10 FEET 10 10 LEGEND BIOSWALE TREATMENT AREA UNTREATED AREA NOTES: 1.AREA NOT TREATED BY LID CONSISTS OF COURTYARD/WALKWAY, EXISTING ROOFTOP AREA, LANDSCAPING AND LAWNS. 2.BIOSWALE LID TREATMENT AREA CONSISTS OF ROOF AREA OF NEW BUILDING(S), THE PORTION OF THE ASPHALT PARKING AREA NOT DRAINING TO THE ALLEY, AND LANDSCAPED AREAS THAT ARE TRIBUTARY. LID1 LID EXHIBIT NA Howes Six Two Nine LID Summary Project Summary Total New/Modified Impervious Area 10,847 sf Target Treatment Percentage 75% Minimum Area to be Treated by LID Measures 8,135 sf LID - Bioswale New/Modified Impervious Area Treated by LID 8,480 sf Percent of New/Modified Impervious Area Treated 78.2%