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HomeMy WebLinkAboutCITY OF FORT COLLINS UTILITIES CUSTOMER SERVICES BUILDING - PDP - PDP140005 - REPORTS - DRAINAGE REPORTDecember 24, 2014 PRELIMINARY DRAINAGE AND EROSION CONTROL REPORT FOR CITY OF FORT COLLINS UTILITIES ADMINISTRATION BUILDING Fort Collins, Colorado Prepared for: City of Fort Collins Prepared by: 200 South College Avenue, Suite 10 Fort Collins, Colorado 80524 Phone: 970.221.4158 Fax: 970.221.4159 www.northernengineering.com Project Number: 940-002  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 absolutely necessary, we recommend double-sided printing. December 24, 2014 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 RE: Preliminary Drainage and Erosion Control Report for CITY OF FORT COLLINS UTILITIES ADMINISTRATION BUILDING Dear Staff: Northern Engineering is pleased to submit this Preliminary Drainage and Erosion Control Report for your review. This report accompanies the Project Development Plan submittal for the proposed City of Fort Collins Utilities Administration Building. This report has been prepared in accordance to Fort Collins Stormwater Criteria Manual (FCSCM), and serves to document the stormwater impacts associated with the proposed project. We understand that review by the City 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. Aaron Cvar, PE Project Engineer Utilities Administration Building Preliminary Drainage Report TABLE OF CONTENTS I. GENERAL LOCATION AND DESCRIPTION ................................................................... 1 A. Location ............................................................................................................................................. 1 B. Description of Property ..................................................................................................................... 2 C. Floodplain.......................................................................................................................................... 3 II. DRAINAGE BASINS AND SUB-BASINS ....................................................................... 5 A. Major Basin Description .................................................................................................................... 5 B. Sub-Basin Description ....................................................................................................................... 5 III. DRAINAGE DESIGN CRITERIA ................................................................................... 5 A. Regulations........................................................................................................................................ 5 B. Four Step Process .............................................................................................................................. 5 C. Development Criteria Reference and Constraints ............................................................................ 6 D. Hydrological Criteria ......................................................................................................................... 6 E. Hydraulic Criteria .............................................................................................................................. 6 F. Modifications of Criteria ................................................................................................................... 7 IV. DRAINAGE FACILITY DESIGN .................................................................................... 7 A. General Concept ............................................................................................................................... 7 B. Specific Details .................................................................................................................................. 7 V. CONCLUSIONS ........................................................................................................ 8 A. Compliance with Standards .............................................................................................................. 8 B. Drainage Concept .............................................................................................................................. 8 EROSION CONTROL REPORT ............................................................................................ 15 APPENDICES: APPENDIX A – Hydrologic Computations APPENDIX B - Water Quality Design Computations APPENDIX C – Erosion Control Report Utilities Administration Building Preliminary Drainage Report LIST OF FIGURES: Figure 1 – Aerial Photograph ................................................................................................ 2 Figure 2– Proposed Site Plan ................................................................................................ 3 Figure 3 – Existing Floodplains ............................................................................................. 4 MAP POCKET: Proposed Drainage Exhibit Utilities Administration Building Preliminary Drainage Report 1 I. GENERAL LOCATION AND DESCRIPTION A. Location 1. Vicinity Map 2. The project site is located in Section 11, Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado . 3. The project site is located just northeast of the intersection of Laporte Avenue and Howes Street. 4. The project site lies within the Old Town Basin. Due to the site’s proximity to the Howes Street outfall, there are no detention requirements for the site. However, the site still must provide water quality treatment. Several water quality treatment methods are proposed for the site, and are described in further detail below. 5. As this is an infill site, the area surrounding the site is fully developed. 6. No offsite flows enter the site from the south, west, or east. A small area to the north of the site sheet flows onto the site. Utilities Administration Building Preliminary Drainage Report 2 B. Description of Property 1. The development area is roughly 1.25 net acres. Figure 1 – Aerial Photograph 2. The subject property is currently composed of existing buildings, and landscaped areas. Existing ground slopes are mild to moderate (i.e., 1 - 6±%) through the interior of the property. General topography slopes from northwest to southeast. 3. 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 of Nunn Clay Loam, which falls into Hydrologic Soil Group C. 4. The proposed project site plan is composed of the development of a City Utilities Administration Building. Associated site work, water, and sewer lines will be constructed with the development. Onsite water quality treatment is proposed and will consist of several features which are discussed in Section IV, below. Utilities Administration Building Preliminary Drainage Report 3 Figure 2– Proposed Site Plan 5. There are no known irrigation laterals crossing the site. 6. The proposed land use is a City customer service building. C. Floodplain 1. The project site is encroached by a City designated 100-year floodplain. The City of Fort Collins Stormwater Utility has identified a 100-year floodplain and floodway in Laporte Avenue, as well as a shallow flooding area through the project site. The proposed building footprint has been located outside of the flood fringe and floodway identified along Laporte Avenue. Utilities Administration Building Preliminary Drainage Report 4 Figure 3 –Area Floodplain Mapping 2. Due to the site’s proximity to the floodway identified in Laporte Ave., and because modifications to Laporte Ave. are proposed, no-rise modeling is required for the proposed development. No-rise modeling for the proposed modifications to Laporte Ave. will be provided at Final. Utilities Administration Building Preliminary Drainage Report 5 II. DRAINAGE BASINS AND SUB-BASINS A. Major Basin Description 1. The project site lies within the Old Town Basin. Detention requirements are to detain the difference between the 100-year developed inflow rate and the historic 2-year release rate. However, the site is adjacent to the Howes Street Outfall, and all runoff will be directed into this outfall. Therefore, detention is not required for this particular site. Water quality treatment is required and will be provided, as discussed below. B. Sub-Basin Description 1. The subject property historically drains overland from northwest to southeast. Runoff from the majority of the site has historically been collected in the existing Laporte Avenue storm system running along the southern boundary of the site. 2. A more detailed description of the project drainage patterns is provided below. III. DRAINAGE DESIGN CRITERIA A. Regulations There are no optional provisions outside of the FCSCM proposed with the proposed project. B. Four Step Process The overall stormwater management strategy employed with the proposed 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 Several techniques have been utilized with the proposed development to facilitate the reduction of runoff peaks, volumes, and pollutant loads as the site is developed from the current use by implementing multiple Low Impact Development (LID) strategies including: Conserving existing amenities in the site including the existing vegetated areas. Providing vegetated open areas throughout the site to reduce the overall impervious area and to minimize directly connected impervious areas (MDCIA). Routing flows, to the extent feasible, through vegetated swales to increase time of concentration, promote infiltration and provide initial water quality. Step 2 – Implement BMPs That Provide a Water Quality Capture Volume (WQCV) with Slow Release The efforts taken in Step 1 will facilitate the reduction of runoff; however, urban development of this intensity will still generate stormwater runoff that will require additional BMPs and water quality. The majority of stormwater runoff from the site will ultimately be intercepted and treated using extended detention methods prior to exiting the site. Step 3 – Stabilize Drainageways There are no major drainageways within the subject property. While this step may not seem applicable to proposed development, the project indirectly helps achieve stabilized drainageways nonetheless. By providing water quality treatment, where none previously existed, sediment with erosion potential is removed from downstream drainageway Utilities Administration Building Preliminary Drainage Report 6 systems. Furthermore, this project will pay one-time stormwater development fees, as well as ongoing monthly stormwater utility fees, both of which help achieve City-wide drainageway stability. Step 4 – Implement Site Specific and Other Source Control BMPs. The proposed project will improve upon site specific source controls compared to historic conditions: The proposed development will provide LID and water quality treatment; thus, eliminating sources of potential pollution previously left exposed to weathering and runoff processes. C. Development Criteria Reference and Constraints The subject property is surrounded by currently developed properties. Thus, several constraints have been identified during the course of this analysis that will impact the proposed drainage system including: Existing elevations along the property lines will generally be maintained. As previously mentioned, overall drainage patterns of the existing site will be maintained. Elevations of existing downstream facilities that the subject property will release to will be maintained. D. Hydrological Criteria 1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure RA-16 of the FCSCM, serve as the source for all hydrologic computations associated with the proposed development. Tabulated data contained in Table RA-7 has been utilized for Rational Method runoff calculations. 2. The Rational Method has been employed to compute stormwater runoff utilizing coefficients contained in Tables RO-11 and RO-12 of the FCSCM. 3. Three separate design storms have been utilized to address distinct drainage scenarios. A fourth design storm has also been computed for comparison purposes. The first design storm considered is the 80th percentile rain event, which has been employed to design the project’s water quality features. The second event analyzed is the “Minor,” or “Initial” Storm, which has a 2-year recurrence interval. The third event considered is the “Major Storm,” which has a 100-year recurrence interval. The fourth storm computed, for comparison purposes only, is the 10-year event. 4. No other assumptions or calculation methods have been used with this development that are not referenced by current City of Fort Collins criteria. E. Hydraulic Criteria 1. As previously noted, the subject property maintains historic drainage patterns. 2. All drainage facilities proposed with the project are designed in accordance with criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual. 3. As stated above, the subject property is located in a City designated floodplain. The proposed project does not propose to modify any natural drainageways. Utilities Administration Building Preliminary Drainage Report 7 F. Modifications of Criteria 1. The proposed development is not requesting any modifications to criteria at this time. IV. DRAINAGE FACILITY DESIGN A. General Concept 1. The main objectives of the project drainage design are to maintain existing drainage patterns, and to ensure no adverse impacts to any adjacent properties. 2. Onsite water quality treatment will be provided, as well as other LID features, which are discussed further below. Water quality capture volume for the purpose of water quality treatment conforming to porous landscape detention (PLD) criteria will be provided in a holding cell in the northern portion of the site. 3. Drainage patterns anticipated for drainage basins shown in the Drainage Exhibit are described below. Drainage basins have been defined for preliminary design purposes an are subject to change at Final design; however, general drainage patterns and concepts are not expected to be significantly altered. Basin 1 Basins 1 will generally drain via overland flow and roof drains into the proposed water quality facilities along the north side of the site. A drainage swale will direct discharge from the proposed roof drain system into a porous landscape detention (PLD) basin, which will provide water quality treatment. Please see further discussion of water quality and LID features in Section IV.B, below. Basin 2 Basin 2 will generally drain via sheet flow into a series of proposed Rain Gardens as shown on the Drainage Exhibit. Please see further discussion of water quality and LID features in Section IV.B, below. Basin OS1 and OS2 Basins OS-1 and OS2 consists portions of the site which physically cannot be directed into the proposed water quality facilities. Runoff from these basins will be directed via sheet flow into adjacent Right of Way, where it will be conveyed via curb and gutter into existing storm line systems. We do not anticipate any issues arising from the free release of these basins into adjacent Right of Way, as there is no significant change from historic drainage conditions. 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. A porous landscape detention (PLD) holding cell is proposed in the north portion of the site. This pond will provide standard 12-hour porous landscape detention (PLD) treatment. 2. A series of Rain Gardens will be provided along the Laporte Ave. frontage of the site. It is our understanding that the Landscape Architect will design these Rain Gardens, and will also design an underdrain system which will daylight into the existing Howes Street Outfall reinforced concrete box (RCB) running along the east side of the site. Utilities Administration Building Preliminary Drainage Report 8 We have currently designed the majority of grading within Basin 2 to direct developed runoff into the Rain Gardens. 3. The following table summarizes LID features and overall percentage of the basin being treated by the proposed LID features. Table 1 – LID Summary Basin Basin Area (sq.ft.) Portion of Basin Captured (sq.ft.) Treatment Type 1 37815 37815 Porous Landscape Detention 2 7777 5274 Rain Gardens OS1 499 0 None OS2 8503 0 None Total 54594 43089 Total % Treated: 78.93% 4. Final design details, construction documentation, and Standard Operating Procedures (SOP) Manual shall be provided to the City of Fort Collins for review prior to Final Development Plan approval. A final copy of the approved SOP manual shall be provided to City and must be maintained on-site by the entity responsible for the facility maintenance. Annual reports must also be prepared and submitted to the City discussing the results of the maintenance program (i.e. inspection dates, inspection frequency, volume loss due to sedimentation, corrective actions taken, etc.). 5. Proper maintenance of the drainage facilities designed with the proposed development is a critical component of their ongoing performance and effectiveness. V. CONCLUSIONS A. Compliance with Standards 1. The drainage design proposed with the proposed project complies with the City of Fort Collins’ Stormwater Criteria Manual. 2. The drainage design proposed with this project complies with requirements for the Old Town Basin. 3. The drainage plan and stormwater management measures proposed with the proposed development 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 any potential damage associated with its stormwater runoff by providing detention and water quality mitigation features. 2. The drainage concept for the proposed development is consistent with requirements for the Old Town Basin. Utilities Administration Building Preliminary Drainage Report 9 References 1. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by Ordinance No. 174, 2011, and referenced in Section 26-500 (c) of the City of Fort Collins Municipal Code. 2. Larimer County Urban Area Street Standards, Adopted January 2, 2001, Repealed and Reenacted, Effective October 1, 2002, Repealed and Reenacted, Effective April 1, 2007. 3. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation Service, United States Department of Agriculture. 4. Old Town Master Drainage Plan, Baseline Hydraulics, Volume II, Anderson Consulting, July 15, 2003. 5. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control District, Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008. APPENDIX A HYDROLOGIC COMPUTATIONS CHARACTER OF SURFACE: Runoff Coefficient Percentage Impervious Project: 940-002 Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: ATC Asphalt ……....……………...……….....…...……………….…………………………………. 0.95 100% Date: Concrete …….......……………….….……….………………..….……………………………… 0.95 90% Gravel ……….…………………….….…………………………..………………………………. 0.50 40% Roofs …….…….………………..……………….…………………………………………….. 0.95 90% Pavers…………………………...………………..…………………………………………….. 0.40 22% Lawns and Landscaping Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0% Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0% 2-year Cf = 1.00 100-year Cf = 1.25 Basin ID Basin Area (s.f.) Basin Area (ac) Area of Asphalt (ac) Area of Concrete (ac) Area of Roofs (ac) Area of Gravel (ac) Area of Lawn, Rain Garden, or Landscaping (ac) 2-year Composite Runoff Coefficient 10-year Composite Runoff Coefficient 100-year Composite Runoff Coefficient Composite % Imperv. 1 37815 0.87 0.000 0.087 0.346 0.000 0.435 0.60 0.60 0.75 45% 2 7777 0.18 0.000 0.089 0.004 0.000 0.086 0.61 0.61 0.77 47% OS1 499 0.01 0.000 0.005 0.000 0.000 0.006 0.56 0.56 0.70 40% OS2 8503 0.20 0.000 0.163 0.033 0.000 0.000 0.95 0.95 1.00 90% DEVELOPED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I. 10-year Cf = 1.00 December 1, 2014 Overland Flow, Time of Concentration: Project: 940-002 Calculations By: Date: Gutter/Swale Flow, Time of Concentration: Tt = L / 60V Tc = Ti + Tt (Equation RO-2) Velocity (Gutter Flow), V = 20·S½ Velocity (Swale Flow), V = 15·S½ NOTE: C-value for overland flows over grassy surfaces; C = 0.25 Is Length >500' ? C*Cf (2-yr Cf=1.00) C*Cf (10-yr Cf=1.00) C*Cf (100-yr Cf=1.25) Length, L (ft) Slope, S (%) Ti 2-yr (min) Ti 10-yr (min) Ti 100-yr (min) Length, L (ft) Slope, S (%) Velocity, V (ft/s) Tt (min) Length, L (ft) Slope, S (%) Velocity, V (ft/s) Tt (min) 2-yr Tc Rational Method Equation: Project: 940-002 Calculations By: Date: From Section 3.2.1 of the CFCSDDC Rainfall Intensity: 1 1 0.87 11 8 6 0.60 0.60 0.75 2.13 4.21 9.63 1.1 2.2 6.3 2 2 0.18 6 5 5 0.61 0.61 0.77 2.76 4.87 9.95 0.3 0.5 1.4 OS1 OS1 0.01 5 5 5 0.56 0.56 0.70 2.85 4.87 9.95 0.0 0.0 0.1 OS2 OS2 0.20 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 0.5 0.9 1.9 DEVELOPED RUNOFF COMPUTATIONS C100 Design Point Flow, Q100 (cfs) Flow, Q2 (cfs) 10-yr Tc (min) 2-yr Tc (min) C2 Flow, Q10 (cfs) Intensity, i100 (in/hr) Basin(s) ATC December 1, 2014 Intensity, i10 (in/hr) Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1 C10 Area, A (acres) Intensity, i2 (in/hr) 100-yr Tc (min) Q  C f  C i  A APPENDIX B WATER WAWAWATER QUALITY DESIGN COMPUTATIONS WATER QUALITY DESIGN CALCULATIONS Water Quality Capture Volume (12-Hr. PLD) Project: 940-002 By: ATC Date: 12/1/14 REQUIRED STORAGE & OUTLET WORKS: BASIN AREA (ac) = 0.870 <-- INPUT from impervious calcs BASIN IMPERVIOUSNESS PERCENT = 45.00 <-- INPUT from impervious calcs BASIN IMPERVIOUSNESS RATIO = 0.4500 <-- CALCULATED WQCV (watershed inches) = 0.160 <-- CALCULATED from Figure EDB-2 WQCV (ac-ft) = 0.012 <-- CALCULATED from UDFCD DCM V.3 Section 6.5 WQ Depth (ft) = ** <-- INPUT from stage-storage table **To be completed at final design Calculating the WQCV and Volume Reduction Chapter 3 3-6 Urban Drainage and Flood Control District August 2011 Urban Storm Drainage Criteria Manual Volume 3 Once the WQCV in watershed inches is found from Figure 3-2 or using Equation 3-1 and/or 3-2, the required BMP storage volume in acre-feet can be calculated as follows: 𝑉 = � WQCV 12 � 𝐴 Equation 3-3 Where: V = required storage volume (acre-ft) A = tributary catchment area upstream (acres) WQCV = Water Quality Capture Volume (watershed inches) Figure 3-2. Water Quality Capture Volume (WQCV) Based on BMP Drain Time Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-1 Urban Storm Drainage Criteria Manual Volume 3 Terminology The term bioretention refers to the treatment process although it is also frequently used to describe a BMP that provides biological uptake and retention of the pollutants found in stormwater runoff. This BMP is frequently referred to as a porous landscape detention (PLD) area or rain garden. Photograph B-1. This recently constructed rain garden provides bioretention of pollutants, as well as an attractive amenity for a residential building. Treatment should improve as vegetation matures. Description A BMP that utilizes bioretention is an engineered, depressed landscape area designed to capture and filter or infiltrate the water quality capture volume (WQCV). BMPs that utilize bioretention are frequently referred to as rain gardens or porous landscape detention areas (PLDs). The term PLD is common in the Denver metropolitan area as this manual first published the BMP by this name in 1999. In an effort to be consistent with terms most prevalent in the stormwater industry, this document generally refers to the treatment process as bioretention and to the BMP as a rain garden. The design of a rain garden may provide detention for events exceeding that of the WQCV. There are generally two ways to achieve this. The design can provide the flood control volume above the WQCV water surface elevation, with flows bypassing the filter usually by overtopping into an inlet designed to restrict the peak flow for a larger event (or events). Alternatively, the design can provide and slowly release the flood control volume in an area downstream of one or more rain gardens. This infiltrating BMP requires consultation with a geotechnical engineer when proposed near a structure. A geotechnical engineer can assist with evaluating the suitability of soils, identifying potential impacts, and establishing minimum distances between the BMP and structures. Bioretention (Rain Garden) Functions LID/Volume Red. Yes WQCV Capture Yes WQCV+Flood Control Yes Fact Sheet Includes EURV Guidance No Typical Effectiveness for Targeted Pollutants3 Sediment/Solids Very Good1 Nutrients Moderate Total Metals Good Bacteria Moderate T-3 Bioretention B-2 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Site Selection Bioretention can be provided in a variety of areas within new developments, or as a retrofit within an existing site. This BMP allows the WQCV to be treated within areas designated for landscape (see design step 7 for appropriate vegetation). In this way, it is an excellent alternative to extended detention basins for small sites. A typical rain garden serves a tributary area of one impervious acre or less, although they can be designed for larger tributary areas. Multiple installations can be used within larger sites. Rain gardens should not be used when a baseflow is anticipated. They are typically small and installed in locations such as:  Parking lot islands  Street medians  Landscape areas between the road and a detached walk  Planter boxes that collect roof drains Bioretention requires a stable watershed. Retrofit applications are typically successful for this reason. When the watershed includes phased construction, sparsely vegetated areas, or steep slopes in sandy soils, consider another BMP or provide pretreatment before runoff from these areas reaches the rain garden. The surface of the rain garden should be flat. For this reason, rain gardens can be more difficult to incorporate into steeply sloping terrain; however, terraced applications of these facilities have been successful in other parts of the country. When bioretention (and other BMPs used for infiltration) are located adjacent to buildings or pavement areas, protective measures should be implemented to avoid adverse impacts to these structures. Oversaturated subgrade soil underlying a structure can cause the structure to settle or result in moisture-related problems. Wetting of expansive soils or bedrock can cause swelling, resulting in structural movements. A geotechnical engineer should evaluate the potential impact of the BMP on adjacent structures based on an evaluation of the subgrade soil, groundwater, and bedrock conditions at the site. Additional minimum requirements include:  In locations where subgrade soils do not allow infiltration, the growing medium should be underlain by an underdrain system.  Where infiltration can adversely impact adjacent structures, the filter layer should be underlain by an underdrain system designed to divert water away from the structure.  In locations where potentially expansive soils or bedrock exist, placement of a rain garden adjacent to structures and pavement should only be considered if the BMP includes an underdrain designed to divert water away from the structure and is lined with an essentially impermeable geomembrane liner designed to restrict seepage. Benefits  Bioretention uses multiple treatment processes to remove pollutants, including sedimentation, filtering, adsorption, evapotranspiration, and biological uptake of constituents.  Volumetric stormwater treatment is provided within portions of a site that are already reserved for landscaping.  There is a potential reduction of irrigation requirements by taking advantage of site runoff. Limitations Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-3 Urban Storm Drainage Criteria Manual Volume 3 Designing for Maintenance Recommended maintenance practices for all BMPs are in Chapter 6 of this manual. During design, the following should be considered to ensure ease of maintenance over the long-term:  Do not put a filter sock on the underdrain. This is not necessary and can cause the BMP to clog.  The best surface cover for a rain garden is full vegetation. Do not use rock mulch within the rain garden because sediment build-up on rock mulch tends to inhibit infiltration and require frequent cleaning or removal and replacement. Wood mulch handles sediment build-up better than rock mulch; however, wood mulch floats and may clog the overflow depending on the configuration of the outlet, settle unevenly, or be transported downstream. Some municipalities may not allow wood mulch for this reason.  Consider all potential maintenance requirements such as mowing (if applicable) and replacement of the growing medium. Consider the method and equipment for each task required. For example, in a large rain garden where the use of hand tools is not feasible, does the shape and configuration of the rain garden allow for removal of the growing medium using a backhoe?  Provide pre-treatment when it will reduce the extent and frequency of maintenance necessary to maintain function over the life of the BMP. For example, if the site is larger than 2 impervious acres, prone to debris or the use of sand for ice control, consider a small forebay.  Make the rain garden as shallow as possible. Increasing the depth unnecessarily can create erosive side slopes and complicate maintenance. Shallow rain gardens are also more attractive.  Design and adjust the irrigation system (temporary or permanent) to provide appropriate water for the establishment and maintenance of selected vegetation. Design Procedure and Criteria The following steps outline the design procedure and criteria, with Figure B-1 providing a corresponding cross-section. 1. Basin Storage Volume: Provide a storage volume based on a 12-hour drain time.  Find the required WQCV (watershed inches of runoff). Using the imperviousness of the tributary area (or effective imperviousness where LID elements are used upstream), use Figure 3-2 located in Chapter 3 of this manual to determine the WQCV based on a 12-hour drain time.  Calculate the design volume as follows: 𝑉𝑉 = � WQCV 12 � 𝐴𝐴 Equation B-1 Where: V= design volume (ft3) Is Pretreatment Needed Designing the inflow gutter to the rain garden at a minimal slope of 0.5% can facilitate sediment and debris deposition prior to flows entering the BMP. Be aware, this will reduce maintenance of the BMP, but may require more frequent sweeping of the gutter to ensure that the sediment does not impede flow into the rain garden. T-3 Bioretention B-4 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Benefits of Shredded Paper in Rain Garden Growing Media  Shredded paper, similar to other woody materials, captures nutrients from the compost and slowly releases them as the paper decomposes. Compost alone will leach more nutrients than desired.  As the paper decomposes, nutrients stored in the material are available to the vegetation.  Paper temporarily slows the infiltration rate of the media and retains moisture, providing additional time for a young root system to benefit from moisture in the growing media. A = area of watershed tributary to the rain garden (ft2) 2. Basin Geometry: A maximum WQCV ponding depth of 12 inches is recommended to maintain vegetation properly. Provide an inlet or other means of overflow at this elevation. Depending on the type of vegetation planted, a greater depth may be utilized to detain larger (more infrequent) events. The bottom surface of the rain garden, also referred to here as the filter area, should be flat. Sediment will reside on the filter area of the rain garden; therefore, if the filter area is too small, it may clog prematurely. Increasing the filter area will reduce clogging and decrease the frequency of maintenance. Equation B-2 provides a minimum filter area allowing for some of the volume to be stored beyond the area of the filter (i.e., above the sideslopes of the rain garden). Note that the total surcharge volume provided by the design must also equal or exceed the design volume. Use vertical walls or slope the sides of the basin to achieve the required volume. Use the rain garden growing medium described in design step 3 only on the filter area because this material is more erosive than typical site soils. Sideslopes should be no steeper than 4:1 (horizontal:vertical). 𝐴𝐴 ≥ (2/3) V 1 foot Equation B-2 Where: V= design volume (ft3) A = minimum filter area (flat surface area) (ft2) The one-foot dimension in this equation represents the maximum recommended WQCV depth in the rain garden. The actual design depth may differ; however, it is still appropriate to use a value of one foot when calculating the minimum filter area. 3. Growing Medium: For partial and no infiltration sections, provide a minimum of 18 inches of growing medium to enable establishment of the roots of the vegetation (see Figure B-1). Previous versions of this manual recommended a mix of 85% sand and 15% peat (by volume). Peat is a material that typically requires import to Colorado and mining peat has detrimental impacts to the environment (Mazerolle 2002). UDFCD partnered with the University of Colorado to perform a study to find a sustainable material to replace peat. The study was successful in finding a replacement that performed well for filtering ability, clogging characteristics, as well as seed germination. This mixture consists of 85% coarse sand and a 15% compost/shredded paper mixture (by volume). The study used thin (approximately 1/4 inch) strips of loosely packed shredded paper mixed with an equal volume of compost. Based on conversations with local suppliers, compost Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-5 Urban Storm Drainage Criteria Manual Volume 3 containing shredded paper is not an uncommon request, although not typically provided in the proportions recommended in this BMP Fact Sheet. Compost suppliers have access to shredded paper through document destruction companies and can provide a mixture of Class 1 compost and shredded paper. The supplier should provide the rain garden compost mixture premixed with coarse sand. On- site mixing is not recommended. Rain Garden Compost Mixture (by volume)  50% Class 1 STA registered compost (approximate bulk density 1000 lbs/CY)  50% loosely packed shredded paper (approximate bulk density 50 to 100 lbs/CY) When using diamond cut shredded paper or tightly packed paper, use the bulk densities provided to mix by weight. The supplier should premix the rain garden compost mixture (above) with coarse sand, in the following proportions, prior to delivery to the site: Rain Garden Growing Medium  15% rain garden compost mixture described above (by volume)  85% coarse sand (either Class C Filter Material per Table B-2 or sand meeting ASTM C-33) (by volume) Table B-1 provides detailed information on Class 1 compost. Be aware, regular testing is not required to allow a compost supplier to refer to a product as a specific STA class. However, regular testing is required and performed through the United States Compost Council (USCC) Seal of Testing Assurance (STA) Program to be a STA registered compost. To ensure Class 1 characteristics, look for a Class 1 STA registered compost. Other Rain Garden Growing Medium Amendments The growing medium described above is designed for filtration ability, clogging characteristics, and vegetative health. It is important to preserve the function provided by the rain garden growing medium when considering additional materials for incorporation into the growing medium or into the standard section shown in Figure B-1. When desired, amendments may be included to improve water quality or to benefit vegetative health as long as they do not add nutrients, pollutants, or modify the infiltration rate. For example, a number of products, including steel wool, capture and retain dissolved phosphorus (Erickson 2009). When phosphorus is a target pollutant, proprietary materials with similar characteristics may be considered. Do not include amendments such as top soil, sandy loam, and additional compost. Full Infiltration Sections A full infiltration section retains the WQCV onsite. For this section, it is not necessary to use the prescribed rain garden growing medium. Amend the soils to provide adequate nutrients to establish vegetation. Typically, 3 to 5 cubic yards of soil amendment (compost) per 1,000 square feet, tilled 6 inches into the soil, is required for vegetation to thrive. Additionally, inexpensive soil tests can be conducted to determine required soil amendments. (Some local governments may also require proof of soil amendment in landscaped areas for water conservation reasons.) T-3 Bioretention B-6 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Table B-1. Class 1 Compost Characteristic Criteria Minimum Stability Indicator (Respirometry) Stable to Very Stable Maturity Indicator Expressed as Ammonia N / Nitrate N Ratio < 4 Maturity Indicator Expressed as Carbon to Nitrogen Ratio < 12 Maturity Indicator Expressed as Percentage of Germination/Vigor 80+ / 80+ pH – Acceptable Range 6.0 – 8.4 Soluble Salts – Acceptable Range (1:5 by weight) 0 – 5 mmhos/cm Testing and Test Report Submittal Requirement Seal of Testing Assurance (STA)/Test Methods for the Examination of Composting and Compost (TMECC) Chemical Contaminants Equal or better than US EPA Class A Standard, 40 CFR 503.13, Tables 1 & 3 levels Pathogens Meet or exceed US EPA Class A standard, 40 CFR 503.32(a) levels Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-7 Urban Storm Drainage Criteria Manual Volume 3 4. Underdrain System: Underdrains are often necessary and should be provided if infiltration tests show percolation drawdown rates slower than 2 times the rate needed to drain the WQCV over 12 hours, or where required to divert water away from structures as determined by a professional engineer. Percolation tests should be performed or supervised by a licensed professional engineer and conducted at a minimum depth equal to the bottom of the bioretention facility. Additionally, underdrains are required where impermeable membranes are used. Similar to the terminology used for permeable pavement sections, there are three basic sections for bioretention facilities:  No-Infiltration Section: This section includes an underdrain and an impermeable liner that does not allow for any infiltration of stormwater into the subgrade soils. It is appropriate to use a no- infiltration system when either of the following is true: o Land use or activities could contaminate groundwater when stormwater is allowed to infiltrate, or o The BMP is located over potentially expansive soils or bedrock and is adjacent (within 10 feet) to structures.  Partial Infiltration Section: This section does not include an impermeable liner and, therefore; allows for some infiltration. Stormwater that does not infiltrate will be collected and removed by an underdrain system.  Full Infiltration Section: This section is designed to infiltrate all of the water stored into the subgrade below. Overflows are managed via perimeter drainage to a downstream conveyance element. UDFCD recommends a minimum infiltration rate of 2 times the rate needed to drain the WQCV over 12 hours. When using an underdrain system, provide a control orifice sized to drain the design volume in 12 hours or more (see Equation B-3). Use a minimum orifice size of 3/8 inch to avoid clogging. This will provide detention and slow release of the WQCV, providing water quality benefits and reducing impacts to downstream channels. Space underdrain pipes a maximum of 20 feet on center. Provide cleanouts to enable maintenance of the underdrain. Cleanouts can also be used to conduct an inspection (by camera) of the underdrain system to Important Design Considerations The potential for impacts to adjacent buildings can be significantly reduced by locating the bioretention area at least 10 feet away from the building, beyond the limits of backfill placed against the building foundation walls, and by providing positive surface drainage away from the building. The BMP should not restrict surface water from flowing away from the buildings. This can occur if the top of T-3 Bioretention B-8 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 ensure that the pipe was not crushed or disconnected during construction. Calculate the diameter of the orifice for a 12-hour drain time using Equation B-3 (Use a minimum orifice size of 3/8 inch to avoid clogging.): 𝐷𝐷12 hour drain time = � 𝑉𝑉 1414 𝑦𝑦0.41 Equation B-3 Where: D = orifice diameter (in) y = distance from the lowest elevation of the storage volume (i.e., surface of the filter) to the center of the orifice (ft) V = volume (WQCV or the portion of the WQCV in the rain garden) to drain in 12 hours (ft3) In previous versions of this manual, UDFCD recommended that the underdrain be placed in an aggregate layer and that a geotextile (separator fabric) be placed between this aggregate and the growing medium. This version of the manual replaces that section with materials that, when used together, eliminate the need for a separator fabric. The underdrain system should be placed within an 6-inch-thick section of CDOT Class C filter material meeting the gradation in Table B-2. Use slotted pipe that meets the slot dimensions provided in Table B-3. Table B-2. Gradation Specifications for CDOT Class C Filter Material (Source: CDOT Table 703-7) Sieve Size Mass Percent Passing Square Mesh Sieves 19.0 mm (3/4”) 100 4.75 mm (No. 4) 60 – 100 300 µm (No. 50) 10 – 30 150 µm (No. 100) 0 – 10 75 µm (No. 200) 0 - 3 Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-9 Urban Storm Drainage Criteria Manual Volume 3 Table B-3. Dimensions for Slotted Pipe Pipe Diameter Slot Length1 Maximum Slot Width Slot Centers1 Open Area1 (per foot) 4” 1-1/16” 0.032” 0.413” 1.90 in2 6” 1-3/8” 0.032” 0.516” 1.98 in2 1 Some variation in these values is acceptable and is expected from various pipe manufacturers. Be aware that both increased slot length and decreased slot centers will be beneficial to hydraulics but detrimental to the structure of the pipe. 5. Impermeable Geomembrane Liner and Geotextile Separator Fabric: For no-infiltration sections, install a 30 mil (minimum) PVC geomembrane liner, per Table B-5, on the bottom and sides of the basin, extending up at least to the top of the underdrain layer. Provide at least 9 inches (12 inches if possible) of cover over the membrane where it is attached to the wall to protect the membrane from UV deterioration. The geomembrane should be field-seamed using a dual track welder, which allows for non-destructive testing of almost all field seams. A small amount of single track and/or adhesive seaming should be allowed in limited areas to seam around pipe perforations, to patch seams removed for destructive seam testing, and for limited repairs. The liner should be installed with slack to prevent tearing due to backfill, compaction, and settling. Place CDOT Class B geotextile separator fabric above the geomembrane to protect it from being punctured during the placement of the filter material above the liner. If the subgrade contains angular rocks or other material that could puncture the geomembrane, smooth-roll the surface to create a suitable surface. If smooth-rolling the surface does not provide a suitable surface, also place the separator fabric between the geomembrane and the underlying subgrade. This should only be done when necessary because fabric placed under the geomembrane can increase seepage losses through pinholes or other geomembrane defects. Connect the geomembrane to perimeter concrete walls around the basin perimeter, creating a watertight seal between the geomembrane and the walls using a continuous batten bar and anchor connection (see Figure B-3). Where the need for the impermeable membrane is not as critical, the membrane can be attached with a nitrile-based vinyl adhesive. Use watertight PVC boots for underdrain pipe penetrations through the liner (see Figure B-2). T-3 Bioretention B-10 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Table B-4. Physical Requirements for Separator Fabric1 Table B-5. Physical Requirements for Geomembrane Property Thickness 0.76 mm (30 mil) Test Method Thickness, % Tolerance ±5 ASTM D 1593 Tensile Strength, kN/m (lbs/in) width 12.25 (70) ASTM D 882, Method B Modulus at 100% Elongation, kN/m (lbs/in) 5.25 (30) ASTM D 882, Method B Ultimate Elongation, % 350 ASTM D 882, Method A Tear Resistance, N (lbs) 38 (8.5) ASTM D 1004 Low Temperature Impact, °C (°F) -29 (-20) ASTM D 1790 Volatile loss, % max. 0.7 ASTM D 1203, Method A Pinholes, No. Per 8 m2 (No. per 10 sq. yds.) max. 1 N/A Bonded Seam Strength, % of tensile strength 80 N/A Property Class B Elongation Test Method < 50%2 Elongation > 50%2 Grab Strength, N (lbs) 800 (180) 510 (115) ASTM D 4632 Puncture Resistance, N (lbs) 310 (70) 180 (40) ASTM D 4833 Trapezoidal Tear Strength, N (lbs) 310 (70) 180 (40) ASTM D 4533 Apparent Opening Size, mm (US Sieve Size) AOS < 0.3mm (US Sieve Size No. 50) ASTM D 4751 Permittivity, sec-1 0.02 default value, must also be greater than that of soil ASTM D 4491 Permeability, cm/sec k fabric > k soil for all classes ASTM D 4491 Ultraviolet Degradation at 500 hours 50% strength retained for all classes ASTM D 4355 1 Strength values are in the weaker principle direction 2 As measured in accordance with ASTM D 4632 Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-11 Urban Storm Drainage Criteria Manual Volume 3 Designing for Flood Protection Provide the WQCV in rain gardens that direct excess flow into to a landscaped area providing the flood control volume. Design the flood control outlet to meter the major event (100-year event) and slowly release the difference in volume between the EURV and the WQCV. (This assumes that the runoff treated by the rain gardens is routed directly into the outlet or infiltrates.) Providing treatment in this manner will reduce inundation in the landscaped area to a few times per year, resulting in an area better suited for multipurpose uses. 6. Inlet/Outlet Control: In order to provide the proper drain time, the bioretention area can be designed without an underdrain (provided it meets the requirements in step 4) or the outlet can be controlled by an orifice plate. Equation B-3 is a simplified equation for sizing an orifice plate for a 12-hour drain time. 7. How flow enters and exits the BMP is a function of the overall drainage concept for the site. Inlets at each rain garden may or may not be needed. Curb cuts can be designed to both allow stormwater into the rain garden as well as to provide release of stormwater in excess of the WQCV. Roadside rain gardens located on a steep site might pool and overflow into downstream cells with a single curb cut, level spreader, or outlet structure located at the most downstream cell. When selecting the type and location of the outlet structure, ensure that the runoff will not short-circuit the rain garden. This is a frequent problem when using a curb inlet located outside the rain garden for overflow. For rain gardens with concentrated points of inflow, provide for energy dissipation. When rock is used, provide separator fabric between the rock and growing medium to minimize subsidence. 8. Vegetation: UDFCD recommends that the filter area be vegetated with drought tolerant species that thrive in sandy soils. Table B-6 provides a suggested seed mix for sites that will not need to be irrigated after the grass has been established. All seed must be well mixed and broadcast, followed by hand raking to cover seed and then mulched. Hydromulching can be effective for large areas. Do not place seed when standing water or snow is present or if the ground is frozen. Weed control is critical in the first two to three years, especially when starting with seed. Do not use conventional sod. Conventional sod is grown in clay soil that will seal the filter area, greatly reducing overall function of the BMP. Several successful local installations have started with seed. Photograph B-2. The curb cut shown allows flows to enter this rain garden while excess flows bypass the facility. Note: trees are not recommended inside a rain garden T-3 Bioretention B-12 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 When using an impermeable liner, select plants with diffuse (or fibrous) root systems, not taproots. Taproots can damage the liner and/or underdrain pipe. Avoid trees and large shrubs that may interfere with restorative maintenance. Trees and shrubs can be planted outside of the area of growing medium. Use a cutoff wall to ensure that roots do not grow into the underdrain or place trees and shrubs a conservative distance from the underdrain. 9. Irrigation: Provide spray irrigation at or above the WQCV elevation or place temporary irrigation on top of the rain garden surface. Do not place sprinkler heads on the flat surface. Remove temporary irrigation when vegetation is established. If left in place this will become buried over time and will be damaged during maintenance operations. Irrigation schedules should be adjusted during the growing season to provide the minimum water necessary to maintain plant health and to maintain the available pore space for infiltration. Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-13 Urban Storm Drainage Criteria Manual Volume 3 Table B-6. Native Seed Mix for Rain Gardens 2 1 Wildflower seed (optional) for a more diverse and natural look. 2 PLS = Pure Live Seed. Common Name Scientific Name Variety PLS2 lbs per Acre Ounces per Acre Sand bluestem Andropogon hallii Garden 3.5 Sideoats grama Bouteloua curtipendula Butte 3 Prairie sandreed Calamovilfa longifolia Goshen 3 Indian ricegrass Oryzopsis hymenoides Paloma 3 Switchgrass Panicum virgatum Blackwell 4 Western wheatgrass Pascopyrum smithii Ariba 3 Little bluestem Schizachyrium scoparium Patura 3 Alkali sacaton Sporobolus airoides 3 Sand dropseed Sporobolus cryptandrus 3 Pasture sage1 Artemisia frigida 2 Blue aster1 Aster laevis 4 Blanket flower1 Gaillardia aristata 8 Prairie coneflower1 Ratibida columnifera 4 Purple prairieclover1 Dalea (Petalostemum) purpurea 4 Sub-Totals: 27.5 22 Total lbs per acre: 28.9 T-3 Bioretention B-14 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Reflective Design A reflective design borrows the characteristics, shapes, colors, materials, sizes and textures of the built surroundings. The result is a design that fits seamlessly and unobtrusively in its environment. Aesthetic Design In addition to providing effective stormwater quality treatment, rain gardens can be attractively incorporated into a site within one or several landscape areas. Aesthetically designed rain gardens will typically either reflect the character of their surroundings or become distinct features within their surroundings. Guidelines for each approach are provided below. Reflecting the Surrounding  Determine design characteristics of the surrounding. This becomes the context for the drainage improvement. Use these characteristics in the structure.  Create a shape or shapes that "fix" the forms surrounding the improvement. Make the improvement part of the existing surrounding.  The use of material is essential in making any new improvement an integral part of the whole. Select materials that are as similar as possible to the surrounding architectural/engineering materials. Select materials from the same source if possible. Apply materials in the same quantity, manner, and method as original material.  Size is an important feature in seamlessly blending the addition into its context. If possible, the overall size of the improvement should look very similar to the overall sizes of other similar objects in the improvement area.  The use of the word texture in terms of the structure applies predominantly to the selection of plant material. The materials used should as closely as possible, blend with the size and texture of other plant material used in the surrounding. The plants may or may not be the same, but should create a similar feel, either individually or as a mass. Creating a Distinct Feature Designing the rain garden as a distinct feature is limited only by budget, functionality, and client preference. There is far more latitude in designing a rain garden that serves as a distinct feature. If this is the intent, the main consideration beyond functionality is that the improvement create an attractive addition to its surroundings. The use of form, materials, color, and so forth focuses on the improvement itself and does not necessarily reflect the surroundings, depending on the choice of the client or designer. Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-15 Urban Storm Drainage Criteria Manual Volume 3 Figure B-1 – Typical Rain Garden Plan and Sections T-3 Bioretention B-16 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-17 Urban Storm Drainage Criteria Manual Volume 3 T-3 Bioretention B-18 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-19 Urban Storm Drainage Criteria Manual Volume 3 Figure B-2. Geomembrane Liner/Underdrain Penetration Detail Figure B-3. Geomembrane Liner/Concrete Connection Detail T-3 Bioretention B-20 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Photograph B-3. Inadequate construction staking may have contributed to flows bypassing this rain garden. Photograph B-4. Runoff passed the upradient rain garden, shown in Photo B-3, and flooded this downstream rain garden. Construction Considerations Proper construction of rain gardens involves careful attention to material specifications, final grades, and construction details. For a successful project, implement the following practices:  Protect area from excessive sediment loading during construction. This is the most common cause of clogging of rain gardens. The portion of the site draining to the rain garden must be stabilized before allowing flow into the rain garden. This includes completion of paving operations.  Avoid over compaction of the area to preserve infiltration rates (for partial and full infiltration sections).  Provide construction observation to ensure compliance with design specifications. Improper installation, particularly related to facility dimensions and elevations and underdrain elevations, is a common problem with rain gardens.  When using an impermeable liner, ensure enough slack in the liner to allow for backfill, compaction, and settling without tearing the liner.  Provide necessary quality assurance and quality control (QA/QC) when constructing an impermeable geomembrane liner system, including but not limited to fabrication testing, destructive and non-destructive testing of field seams, observation of geomembrane material for tears or other defects, and air lace testing for leaks in all field seams and penetrations. QA/QC should be overseen by a professional engineer. Consider requiring field reports or other documentation from the engineer.  Provide adequate construction staking to ensure that the site properly drains into the facility, particularly with respect to surface drainage away from adjacent buildings. Photo B-3 and Photo B-4 illustrate a construction error for an otherwise correctly designed series of rain gardens. Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-21 Urban Storm Drainage Criteria Manual Volume 3 Photograph B-5. Rain garden is staked out at the low point of the parking area prior to excavation. Construction Example Photograph B-6. Curb and gutter is installed. Flush curbs with wheel stops or a slotted curb could have been used in lieu of the solid raised curb with concentrated inflow. T-3 Bioretention B-22 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Photograph B-7. The aggregate layer is covered with a geotextile and growing media. This photo shows installation of the geotextile to separate the growing media from the aggregate layer below. Cleanouts for the underdrain system are also shown. Note: The current design section does not require this geotextile. Photograph B-8. Shrubs and trees are placed outside of the ponding area and away from geotextiles. Photograph B-9. This photo was taken during the first growing season of this rain garden. Better weed control in the first two to three years will help the desired vegetation to become established. Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-23 Urban Storm Drainage Criteria Manual Volume 3 Design Example The UD-BMP workbook, designed as a tool for both designer and reviewing agency is available at www.udfcd.org. This section provides a completed design form from this workbook as an example. T-3 Bioretention B-24 Urban Drainage and Flood Control District November 2010 Urban Storm Drainage Criteria Manual Volume 3 Sheet 1 of 2 Designer: Company: Date: Project: Location: 1. Basin Storage Volume A) Effective Imperviousness of Tributary Area, Ia Ia = 95.0 % (100% if all paved and roofed areas upstream of rain garden) B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.950 C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.36 watershed inches (WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i) D) Contributing Watershed Area (including rain garden area) Area = 32,000 sq ft E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 954 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 = 12 in B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 0.00 ft / ft (Use "0" if rain garden has vertical walls) C) Mimimum Flat Surface Area AMin = 636 sq ft D) Actual Flat Surface Area AActual = 955 sq ft E) Area at Design Depth (Top Surface Area) ATop = 955 sq ft F) Rain Garden Total Volume VT= 955 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 = 2.7 ft Volume to the Center of the Orifice ii) Volume to Drain in 12 Hours Vol12 = 954 cu ft iii) Orifice Diameter, 3/8" Minimum DO = 0.67 in Design Procedure Form: Rain Garden (RG) J. Tann BMP, Inc. November 24, 2010 Shops at 56th NW corner of 56th Ave. and 27th St. Choose One Choose One 18" Rain Garden Growing Media Other (Explain): YES NO Bioretention T-3 November 2010 Urban Drainage and Flood Control District B-25 Urban Storm Drainage Criteria Manual Volume 3 References Erickson, Andy. 2009. Field Applications of Enhanced Sand Filtration. University of Minnesota Stormwater Management Practice Assessment Project Update. http://wrc.umn.edu. Guo, James C.Y., PhD, Anu Ramaswami, PhD, and Shauna M. Kocman, PhD Candidate. 2010. Sustainable Design of Urban Porous Landscape Detention Basin. University of Colorado Denver Mazzerolle, Marc J. 2002. Detrimental Effects of Peat Mining on Amphibian Abundance and Species Richness in Bogs. Elsevier Science Limited. 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 NO SPRINKLER HEADS ON THE FLAT SURFACE A) Will the rain garden be irrigated? Notes: November 24, 2010 Shops at 56th NW corner of 56th Ave. and 27th St. Design Procedure Form: Rain Garden (RG) J. Tann BMP, Inc. 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 APPENDIX C EROSION CONTROL REPORT Utilities Administration Building Preliminary Erosion Control Report EROSION CONTROL REPORT A comprehensive Erosion and Sediment Control Plan (along with associated details) will be included with the final construction drawings. It should be noted, however, that any such Erosion and Sediment Control Plan serves only as a general guide to the Contractor. Staging and/or phasing of the BMPs depicted, and additional or different BMPs from those included may be necessary during construction, or as required by the authorities having jurisdiction. It shall be the responsibility of the Contractor to ensure erosion control measures are properly maintained and followed. The Erosion and Sediment Control Plan is intended to be a living document, constantly adapting to site conditions and needs. The Contractor shall update the location of BMPs as they are installed, removed or modified in conjunction with construction activities. It is imperative to appropriately reflect the current site conditions at all times. The Erosion and Sediment Control Plan shall address both temporary measures to be implemented during construction, as well as permanent erosion control protection. Best Management Practices from the Volume 3, Chapter 7 – Construction BMPs will be utilized. Measures may include, but are not limited to, silt fencing along the disturbed perimeter, gutter protection in the adjacent roadways and inlet protection at existing and proposed storm inlets. Vehicle tracking control pads, spill containment and clean-up procedures, designated concrete washout areas, dumpsters, and job site restrooms shall also be provided by the Contractor. Grading and Erosion Control Notes can be found on the Utility Plans. The Final Plans will contain a full-size Erosion Control sheet as well as a separate sheet dedicated to Erosion Control Details. In addition to this report and the referenced plan sheets, the Contractor shall be aware of, and adhere to, the applicable requirements outlined in the Development Agreement for the development. Also, the Site Contractor for this project will be required to secure a Stormwater Construction General Permit from the Colorado Department of Public Health and Environment (CDPHE), Water Quality Control Division – Stormwater Program, prior to any earth disturbance activities. Prior to securing said permit, the Site Contractor shall develop a comprehensive StormWater Management Plan (SWMP) pursuant to CDPHE requirements and guidelines. The SWMP will further describe and document the ongoing activities, inspections, and maintenance of construction BMPs. MAP POCKET DRAINAGE EXHIBITS ST ST ST ST FO FO FO FO FO FO FO FO FO FO FO FO FO FO FO FO ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST SS SS SS SS SS SS SS SS SS SS ST ST ST SS SS SS SS SS SS SS SS SS SS ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST ST FO FO FO FO FO FO FO FO FO FO FO FO FO FO FO 2" W 2" W 2" W 2" W 2" W 2" W 2" W 2" W W RD RD RD EXISTING RIGHT-OF-WAY EXISTING RIGHT-OF-WAY 1 2 OS1 OS2 PROPOSED FLOWLINE OF DRAINAGE SWALE AT MINIMUM 2% GRADE PROPOSED CONCRETE FOREBAY FUTURE PEDESTRIAN WALKWAY PROPOSED 6" STORM SEWER CONNECT PROPOSED STORM SEWER INTO EXISTING BOX CULVERT PROPOSED WATER QUALITY OUTLET STRUCTURE PROPOSED 12" STORM SEWER PROPOSED 6" STORM SEWER PROPOSED EMERGENCY ACCESS ALIGNMENT 1 2 OS2 OS1 EXISTING RIGHT-OF-WAY EXISTING 100-YR CITY HIGH RISK FLOOD FRINGE EXISTING 100-YR CITY FLOODWAY EXISTING RIGHT-OF-WAY EXISTING STORM SEWER MANHOLE (TYP.) PROPOSED RAIN GARDEN (RE: LANDSCAPE ARCHITECT) PROPOSED RAIN GARDEN (RE: LANDSCAPE ARCHITECT) PROPOSED SIDEWALK PROPOSED UTILITIES ADMINISTRATION BUILDING FF = 4985.50 RELOCATED BUTTERFLY BLDG FF = 4984.0 4984.1 100-YR FLOOD ELEVATION (FT.COLLINS NGVD29) PROPOSED RAIN GARDEN (RE: LANDSCAPE ARCHITECT) LAPORTE AVENUE (100' ROW) HOWES STREET (100' ROW) EXISTING 215 N. MASON BUILDING EXISTING 214 N. HOWES BUILDING PROPOSED SIDEWALK PROPOSED SIDEWALK APPROXIMATE LOCATION OF 16' WIDE x5' TALL CONCRETE BOX CULVERT PROPOSED CONCRETE TERRACE (RE: LANDSCAPE ARCHITECT) PROPOSED SIDEWALK PROPOSED SIDEWALK PROPOSED SIDEWALK PROPOSED WATER QUALITY CONTROL POND Monday, April 14, 2014 10:17 AM D:\Projects\940-002\Dwg\Drng\940-002_DRNG.dwg Wegert Provencio C600 DRAINAGE PLAN CITY OF FORT COLLINS 222 Laporte Avenue Fort Collins, CO 80524 UTILITIES ADMINISTRATION BUILDING PROJECT DEVELOPMENT PLAN 1050 17th STREET, SUITE A200 DENVER, CO 80265 303 295 1717 t 303 292 0845 f DRAWN BY: CHECKED BY: A B C D E F G H I J A B C D E F G H I J 1 2 3 4 5 6 7 8 9 10 11 12 13 14 14 13 12 11 10 9 8 7 6 5 4 3 2 1 15 15 K K NOTES: A B2 1.45 ac NOT FOR CONSTRUCTION LEGEND: ( IN FEET ) 10 0 10 20 30 1 INCH = 10 FEET UD ST CALL 2 BUSINESS DAYS IN ADVANCE BEFORE YOU DIG, GRADE, OR EXCAVATE FOR THE MARKING OF UNDERGROUND MEMBER UTILITIES. CALL UTILITY NOTIFICATION CENTER OF COLORADO R PROJECT DATUM: NGVD29 Unadjusted (Old City of Fort Collins Datum) BENCHMARK #1: City of Fort Collins Benchmark 1-13 Southwest corner of College Avenue and Maple Street, on a concrete traffic signal base. Elevation= 4976.58 NOTE: If NAVD 88 Datum is required for any purpose, the following equation should be used: NAVD88 = NGVD29 Unadjusted + 3.17' PROJECT BENCHMARKS: the perimeter wall for the BMP impedes flow away from the building. Always adhere to the slope recommendations provided in the geotechnical report. In the absence of a geotechnical report, the following general recommendations should be followed for the first 10 feet from a building foundation. 1) Where feasible, provide a slope of 10% for a distance of 10 feet away from a building foundation. 2) In locations where non-expansive soil or bedrock conditions exist, the slope for the surface within 10 feet of the building should be at least 5% away from the building for unpaved (landscaped) surfaces. 3) In locations where potentially expansive soil or bedrock conditions exist, the design slope should be at least 10% away from the building for unpaved (landscaped) surfaces. 4) For paved surfaces, a slope of at least 2% away from the building is adequate. Where accessibility requirements or other design constraints do not apply, use an increased minimum design slope for paved areas (2.5% where non- expansive soil or bedrock conditions exist).  Additional design and construction steps are required for placement of any ponding or infiltration area near or upgradient from a building foundation and/or when expansive (low to high swell) soils exist. This is discussed in the design procedure section.  In developing or otherwise erosive watersheds, high sediment loads can clog the facility. Other Considerations Life-cycle Costs4 Moderate 1 Not recommended for watersheds with high sediment yields (unless pretreatment is provided). 3 Based primarily on data from the International Stormwater BMP Database (www.bmpdatabase.org). 4 Based primarily on BMP-REALCOST available at www.udfcd.org. Analysis based on a single installation (not based on the maximum recommended watershed tributary to each BMP). (min) 10-yr Tc (min) 100-yr Tc (min) 11No0.25 0.60 0.75 55 2.00% 9.4 5.5 3.9 180 0.50% 1.41 2.1 0 0.00% N/A N/A 11 8 6 22No0.55 0.61 0.77 35 2.00% 4.8 4.3 2.9 75 0.75% 1.73 0.7 0 0.00% N/A N/A 6 5 5 OS1 OS1 No 0.25 0.56 0.70 10 2.00% 4.0 2.5 1.9 0 0.00% N/A N/A 0 0.00% N/A N/A 5 5 5 OS2 OS2 No 0.95 0.95 1.00 25 1.00% 1.4 1.4 0.9 130 1.70% 2.61 0.8 0 0.00% N/A N/A 5 5 5 DEVELOPED TIME OF CONCENTRATION COMPUTATIONS Gutter Flow Swale Flow Design Point Basin Overland Flow ATC December 1, 2014 Time of Concentration (Equation RO-4)  3 1 1 . 87 1 . 1 * S Ti C Cf L  