Volume 6 - Low Impact Development. Table of Contents

Transcription

1 City of Tacoma Volume 6 - Low Impact Development Table of Contents Purpose of this Volume Content and Organization of this Volume Chapter 1 General Requirements Objectives Site Assessment Site Planning and Layout Retain Native Vegetation Minimize Clearing and Grading Impacts Chapter 2 Low Impact Development Best Management Practices Application Best Management Practices Site Design BMPs BMP L620 Preserving Native Vegetation BMP L621 Better Site Design Structural Low Impact Development BMPS BMP L601 Rain Gardens BMP L630 Bioretention BMP L631 Vegetated Rooftops (Green Roofs) BMP L632 Rainwater Harvesting BMP L633 Permeable Pavements BMP L634 Minimal Excavation Foundations BMP L635 Reverse Slope Sidewalks Appendix A Recommended Modifications to ASTM D2434 When Measuring Hydraulic Conductivity for Bioretention Soil Mixes Appendix B Green Stormwater Infrastructure Details i

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3 City of Tacoma VOLUME 6 Low Impact Development Purpose of this Volume This volume focuses on the concept of low impact development. Low impact development is a stormwater and land use management strategy that strives to mimic predisturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of onsite natural features, site planning, and distributed stormwater management practices that are integrated into a project design. Low impact development is sometimes considered to be a component of Green Stormwater Infrastructure which is a set of distributed stormwater best management practices that seek to mimic natural systems and deliver multiple community benefits in addition to stormwater management. Green stormwater infrastructure can be used at a wide range of landscape scales in place of more traditional stormwater control elements to support the principles of Low Impact Development. The terms should not be used interchangeably as some elements that are considered green stormwater infrastructure may not be considered low impact development. This section should be used in conjunction with Low Impact Development: Technical Guidance Manual for Puget Sound, which can be found at: stormwater/municipal/lid/resources.html Volume 3 contains BMPs that are also considered low impact development techniques. Content and Organization of this Volume Volume 6 contains two chapters: Chapter 1 provides an overview of the general requirements for low impact development. Chapter 2 provides detailed information pertaining to Best Management Practices (BMPs) for low impact development. The City of Tacoma is developing Standard Plans for Green Stormwater Infrastructure Best Management Practices. These standard plans are available in Volume 6, Appendix B of this manual and may be used in conjunction with the design criteria in each BMP when designing projects. As plans are fully developed and incorporated into the City of Tacoma Right-of-Way Design Manual, the plans will be available at under the Standards Plan tab. Purpose 6-1 Volume 6

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5 City of Tacoma Chapter 1 General Requirements 1.1 Objectives The goal of low impact development is to manage stormwater generated from new development and redevelopment on-site so there will be no negative impacts to adjacent or downstream properties and no degradation to ground or surface waters. The following are objectives for low impact design: Minimize the impacts of increased stormwater runoff from new development and redevelopment by maintaining flow frequencies and durations of the site s undisturbed hydrologic condition. Retain and/or restore native soils and vegetation to the maximum extent practicable. Retain and incorporate natural site features that promote infiltration of stormwater on the developed site. Manage stormwater as close to the source as possible. Promote groundwater recharge. Provide visible sustainable facilities. Minimize impervious surfaces. 1.2 Site Assessment Before implementing LID practices it is necessary to perform a site assessment which includes an assessment of both on-site and off-site conditions and features. See Volume 1, Chapter 4 for Stormwater Site Plan requirements, including site assessment. See Chapter 2 of the Low Impact Development: Technical Guidance Manual for Puget Sound for more information on steps for performing a site assessment. 1.3 Site Planning and Layout Sites should be configured to reduce impervious surfaces and utilize natural drainage features. Chapter 3 of the Low Impact Development: Technical Guidance Manual for Puget Sound contains information and techniques for site planning. 1.4 Retain Native Vegetation Retain native vegetation to the maximum extent practicable in order to: Reduce total impervious surface coverage Provide infiltration areas for overland flows generated in adjacent developed portions of the project Maintain the natural hydrology of the site. See BMP L620 in Chapter 2 of this volume, Tree Retention and Transplanting BMPs in Volume 3, Chapter 5, and Chapter 4 of Low Impact Development: Technical Guidance Manual for Puget Sound for techniques on retaining native vegetation and trees. Chapter Volume 6

6 City of Tacoma 1.5 Minimize Clearing and Grading Impacts Conduct a soils analysis prior to clearing and grading to identify predevelopment soil types and infiltration capabilities. Keep grading to a minimum by incorporating natural topography. Always use appropriate erosion and sediment control techniques when clearing and grading. See Volume 2 of this manual for erosion and sediment control measures. Utilize techniques from Chapter 5 of Low Impact Development: Technical Guidance Manual for Puget Sound. Volume Chapter 1

7 City of Tacoma Chapter 2 Low Impact Development Best Management Practices This Chapter presents the methods for analysis and design of on-site stormwater management Best Management Practices (BMPs). Design procedures and requirements for stormwater management BMPs meeting Minimum Requirement #7, Flow Control, are contained in Volume Application Low impact development techniques should be used at every site when feasible. Using low impact development techniques may eliminate the need to install more costly flow control or water quality devices. When utilized in accordance with this manual, most low impact development techniques provide flow credits. Flow credits only apply to flow control thresholds. Flow credits do not apply to water quality thresholds. 2.2 Best Management Practices The following Low Impact Development BMPs are discussed in this Chapter: Site Design BMPs BMP L620 Preserving Natural Vegetation BMP L621 Better Site Design Structural Low Impact Development BMPs BMP L601 Rain Gardens BMP L630 Bioretention Areas BMP L631 Vegetated Rooftops BMP L632 Rainfall Re-use BMP L633 Alternate Paving Systems BMP L634 Minimal Excavation Foundations BMP L635 Reverse Slope Sidewalks Infiltration, dispersion, soil quality and tree retention BMPs can be found in Volume 3, Volume 5 and Volume Site Design BMPs BMP L620 Preserving Native Vegetation Purpose Preserving native vegetation on-site to the maximum extent practicable will minimize the impacts of development on stormwater runoff. Chapter Volume 6

8 City of Tacoma Applications and Limitations On lots that are one acre or greater, preservation of 65 percent or more of the site in natural vegetation will allow the use of full dispersion techniques presented in BMP L614 (Volume 3, Section 3.3). Sites that can fully disperse are not required to provide runoff treatment or flow control facilities. Design Criteria Situate the preserved area to maximize the preservation of wetlands, and to buffer stream corridors and minimize clearing of existing forest. Place the preserved area in a separate tract or protect through recorded easements for individual lots. If feasible, locate the preserved area downslope from the building sites, since flow control and water quality are enhanced by flow dispersion through duff, undisturbed soils, and native vegetation. Show the preserved area on all property maps and clearly mark the area during clearing and construction on the site. Maintenance Do not remove vegetation and trees from undisturbed areas, except for approved timber harvest activities and the removal of dangerous and diseased trees BMP L621 Better Site Design Purpose Fundamental hydrological concepts and stormwater management concepts can be applied at the site design phase that are: More integrated with natural topography, Reinforce the hydrologic cycle, More aesthetically pleasing, and Often less expensive to build. Design Criteria Define Development Envelope and Protected Areas The first step in site planning is to define the development envelope. This is done by identifying protected areas, setbacks, easements and other site features, and by consulting applicable local standards and requirements. Site features to be protected may include important existing trees, steep slopes, erosive soils, riparian areas, and wetlands. By keeping the development envelope compact, environmental impacts can be minimized, construction costs can be reduced, and many of the site s most attractive landscape features can be retained. In some cases, economics or other factors may not allow avoidance of all sensitive areas. In these cases, care can be taken to mitigate the impacts of development through site work and other landscape treatments. Minimize Directly Connected Impervious Areas Impervious areas directly connected to the storm drain system are the greatest contributors to urban nonpoint source pollution. Minimize these directly connected impervious areas. This can be done by limiting overall Volume Chapter 2

9 City of Tacoma impervious land coverage or by infiltrating and/or dispersing runoff from these impervious areas. Maximize Permeability - Within the development envelope, many opportunities are available to maximize the permeability of new construction. These include minimizing impervious areas, paving with permeable materials, clustering buildings, and reducing the land coverage of buildings by smaller footprints. All of these strategies make more land available for infiltration and dispersion through natural vegetation. Build Narrower Streets - More than any other single element, street design has a powerful impact on stormwater quantity and quality. In residential development, streets and other transportation-related structures typically can comprise between 60 and 70 percent of the total impervious area, and, unlike rooftops, streets are almost always directly connected to the stormwater conveyance system. Maximize Choices for Mobility - Given the costs of automobile use, both in land area consumed and pollutants generated, maximizing choices for mobility is a basic principle for environmentally responsible site design. By designing residential developments to promote alternatives to automobile use, a primary source of stormwater pollution can be mitigated. Use Drainage as a Design Element - Unlike conveyance storm drain systems that hide water beneath the surface and work independently of surface topography, a drainage system for stormwater infiltration or dispersion can work with natural land forms and land uses to become a major design element of a site plan Structural Low Impact Development BMPS Low impact development BMPs are structural BMPs that can be used to manage stormwater onsite. Using LID techniques can reduce surface runoff. For each category, basic design criteria is included. The design criteria components in this manual must be used in order to obtain runoff credits. The guidance provided in Low Impact Development: Technical Guidance Manual for Puget Sound, found on the Puget Sound Partnership website: can also be used in design BMP L601 Rain Gardens Purpose and Definition Rain gardens are non-engineered, shallow, landscaped depressions with compost-amended soils and adapted plants. Rain gardens temporarily store stormwater runoff from adjacent areas. Stormwater passes through the amended soil profile and into the native soil beneath. Stormwater that exceeds the rain garden is designed to overflow to an adjacent drainage system. Volume 6, Appendix B of this manual contains several rain garden figures Design Criteria Phosphorus-sensitive Waterbodies Rain gardens constructed with imported compost material shall not be used within 1/4 mile of phosphorus-sensitive waterbodies if the underlying soils do not meet the soil suitability criteria for treatment (Volume 5, Section 7.3). This restriction does not apply to rain garden construction within 1/4 mile of Wapato Lake. Consult with Environmental Services to determine if a waterbody is considered phosphorus-sensitive. Chapter Volume 6

10 City of Tacoma Groundwater Separation A minimum of one foot of separation is required between the lowest elevation of the rain garden soil or any underlying gravel infiltration layer and the seasonal high groundwater elevation or other impermeable layer Infeasibility Criteria A rain garden is not required if any of the following infeasibility criteria are met. If a project proponent wishes to use a rain garden facility even when not required to based upon the infeasibility criteria, the criteria in this section apply. Environmental Services will review the proposed design and may allow the installation on a case by case basis. See Volume 3 Appendix B to determine if a soils report is required. These criteria are in addition to setback criteria described in Volume 3, Section These criteria are in addition to any competing needs criteria (Vol 1, Sec ). Setback distances are measured from the bottom edge of the rain garden soil mix. The following infeasibility criteria require evaluation of site specific conditions and a written recommendation from a Washington state licensed Professional Engineer or Professional Geologist. Where professional geotechnical evaluation recommends infiltration not be used do to reasonable concerns about erosion, slope failure, or down gradient flooding. Within an area whose groundwater drains into an erosion hazard or landslide hazard area. Where infiltrating water would threaten existing below grade basements. Where infiltrating water would threaten shoreline structures such as bulkheads. Where the only area available for siting would threaten the safety or reliability of preexisting underground utilities, preexisting underground storage tanks, preexisting structures, or preexisting road or parking lot surfaces. The following criteria can be sited as reasons for a finding of infeasibility without a written recommendation from a licensed professional engineer or licensed professional geologist, though some require site specific evaluation by an appropriate licensed professional. Where the only area available for siting does not allow for a safe overflow pathway to the City storm system or a private storm system. Where there is lack of usable space onsite for rain garden facilities at redevelopment sites. For work within the public right of way, where there is insufficient space. Where they are not compatible with surrounding drainage system (e.g., project drains to an existing stormwater system whose elevation precludes proper connection to a rain garden). Where land for the rain garden is within an area designated as an erosion hazard or landslide hazard. Where the site cannot be reasonably designed to locate rain gardens on slopes less than 8%. For properties with known soil or groundwater contamination: Within 100 feet of an area known to have deep soil contamination; Volume Chapter 2

11 City of Tacoma Where groundwater modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the groundwater; Wherever surface soils can be found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltration area; Any area where these facilities are prohibited by an approved cleanup plan under the state Model Toxics Control Actor or Federal Superfund Law, or an environmental covenant under Chapter RCW. Within 100 feet of a closed or active landfill. Within 100 feet of a drinking water well, or a spring used for drinking water supply. Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and grey water reuse systems. For setbacks from a large on-site sewage disposal system, see WAC Chapter B. Within 10 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is 1100 gallons or less. (As used in this criteria, an underground storage tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which 10% or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface.) Within 100 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is greater than 1100 gallons. Where the field testing indicates potential rain gardens sites have a measured native soil saturated hydraulic conductivity less than 0.30 inches per hour. If the measured native soil infiltration rate is less than 0.30 in/hour, this option should not be used to meet the requirements of MR#5. Soils testing shall comply with Volume 6, Section Small Scale PIT Test Other Site Suitability Factors The following factors should be considered when siting the rain garden facility. Utility conflicts: See Volume 3, Chapter 10 for utility separation requirements. Transportation safety: The design configuration and selected plant types should provide adequate sight distances, clear zones, and appropriate setbacks for roadway applications in accordance with City of Tacoma Design Standards. Impacts of surrounding activities: Human activity influences the location of the facility in the development. For example, locate rain gardens away from traveled areas on individual lots to prevent soil compaction and damage to vegetation or provide elevated or bermed pathways in areas where foot traffic is inevitable. Provide barriers, such as wheel stops, to restrict vehicle access in roadside applications. Visual buffering: Rain gardens can be used to buffer structures from roads, enhance privacy among residences, and for an aesthetic site feature Flow Entrance/Presettling Use one of the following types of flow entrances (other alternatives may be considered on a case-by-case basis): Dispersed across a landscaped area. Dispersed through an open swale with plants and decorative rock. For slopes greater than 2%, add rock check dams every 5 to 10 feet to slow water flow. Chapter Volume 6

12 City of Tacoma Pipe flow entrance. Place a rock pad where stormwater enters the rain garden from a swale or pipe. It is recommended to use washed round rock that is a minimum of 2 inches in diameter. Rock pad should be 4 thick and 2 feet wide and extend 2 feet. Avoid the use of angular rock or quarry spalls as sediment removal is difficult. Environmental Services may require flow dispersion for rain gardens depending upon type of inlet design and size of the facility. Do not place plants directly in the entrance flowpath as they can restrict or concentrate flows. Install flow diversion and erosion control measures to protect the rain garden from sedimentation until the upstream area is stabilized. Based upon project site characteristics, Environmental Services may require a presettling facility per Volume Cell Ponding Area The ponding depth shall be 6 inches, minimum and 18 maximum. See section for rain garden sizing and geometry. The minimum freeboard measured from the maximum ponding water surface elevation to the top of the facility shall be 2 for drainage areas less than 1,000 square feet and 6 for drainage areas 1,000 square feet or greater. If berming is used to achieve the minimum top elevation, maximum slope on berm shall be 2H:1V, and minimum top width of design berm shall be 1 foot. Berm shall be a material which is water tight. Imported soil may be necessary to ensure berm does not fail. Berm shall be tightly packed during construction Overflow Provide an overflow pathway lined with a 4 thick washed rock pad. Washed rock shall be a minimum of 2 inches in diameter. Extend overflow 4 feet past rain garden edge. Overflow shall not be directed to structures, neighboring properties, or over sidewalks. Overflow shall not cause damage to downstream properties or receiving waters Rain Garden Soil Mix Minimum depth of rain garden mix shall be 12 inches. The compost component shall: Meet the definition of composted material in WAC and comply with testing parameters and other standards in WAC Be Fine Compost meeting the following size gradation (by dry weight) when tested in accordance with TMECC test method B, Sample Sieving for Aggregate Size Classification. Sieve Size Minimum Percent Passing /8 90 Volume Chapter 2

13 City of Tacoma Compost specifications are also contained in Volume 5, Appendix E. Obtain a rain garden soil mix by one of the following methods. Method 1: Excavate and replace existing soil with a rain garden soil mix. Use this method if existing soils are poor quality. If clay content is greater than 5%. In gravel soils as plant growth will be inhibited. A rain garden soil mix typically contains about 60% sand content and 40% compost by volume. Method 2: Excavate and amend existing soil Use 1/3 compost to every 2/3 of existing soil. Use this method when existing soils are moderately good to good quality soils. Do not use this method if clay content is greater than 5%. Use Method 1. Method 3: Amend Soil in Place Use this method if the existing soils are good quality. Do not use this method if the clay content is greater than 5%. Use Method 1. Use this option only if the infiltration rate is 1 inch per hour or greater. Amend soil by excavating to the ponding depth plus 3 inches. Spread 3 inches of compost and till to depth of 4 to 5 inches to fully incorporate compost component Planting ¼ 75 Submit a planting plan showing the type (species), location and size of each plant. Plants must be tolerant of summer drought, ponding fluctuation, and saturated soil conditions. Rain gardens have three planting zones. Zone 1 is bottom area of the rain garden, which is frequently wet during the rainy season. Zone 2 includes the side slopes, which occasionally become wet during rain events. Zone 3 includes the area around the perimeter of the rain garden, starting above the top surface of ponding elevation, which has drier soil. Additional information on planting zones and appropriate plants for each zone can be obtained through the Rain Garden Handbook for Western Washington, available at raingarden. In general, the following guidelines should be used when considering which plants to use: At least 50% (by quantity) should be evergreen plants. Leaf fall can reduce the function of the facility. Chapter Volume 6

14 City of Tacoma Do not leave large areas of the soil unplanted/uncovered. Exposed soil can cause erosion and reduce the function of the facility. Plant types can be overlapped (tree canopy can overlap shrubs and groundcover) to reduce the area of exposed soil. Provide a variety of plant types with various rooting structures. Plant variety encourages good soil health. Typical plant types used in rain garden construction include: Emergents: Rushes, grasses, and sedges have shallow, fibrous roots that remain close to the soil surface. Planting solely with shallow rooted plants can cause thick root mats to form decreasing the infiltrative ability of the soil. Woody shrubs and trees: Woody shrubs and trees have a mixture of shallow fibrous roots and deeper structural roots that can penetrate deeper into the soil and increase soil porosity over time. Groundcover: Groundcover tends to form dense masses of vegetation low to the ground surface. Low, dense vegetation can be effective at consolidating soil in areas otherwise prone to erosion such as the side slopes of the facility (Zone 2). Herbaceous perennials: Herbaceous perennials do not have woody plant parts and the above ground growth typically dies back in the winter. Roots must not damage underground infrastructure. Consider adjacent plant communities and avoid potential invasive species. Consider aesthetics, rain gardens should blend into surrounding landscapes. Irrigation may be required until plants are fully established and in the summer months Mulch Layer Rain garden facilities should be designed with a mulch layer. Properly selected mulch material reduces weed establishment, regulates soil temperatures and moisture, and adds organic matter to the soil. Mulch should be free of weed seeds, soil, roots, and other material that is not trunk or branch wood and bark. Mulch shall not include grass clippings, mineral aggregate, pure bark, or beauty bark. Mulch should be coarse mulch. Mulch should be: Wood chip mulch composed of shredded or chipped hardwood or softwood, depth 2-3 inches. Additional rain garden depth will be needed to ensure appropriate ponding and freeboard. A dense groundcover can be used as an alternative to mulch although mulch should be used until the dense groundcover is established Sizing and Geometry The top of the ponded surface area below the overflow shall be at least 5% of the total hard surface area draining to it. If lawn/landscaped area will also be draining to the bioretention facility, it is recommended that the top of the ponded surface area below the overflow be increased by 2% of the lawn/landscaped area. Volume Chapter 2

15 City of Tacoma The maximum side slopes allowed shall be 2:1. The following table provides the minimum top of ponded surface area, minimum bottom pond area, and minimum top of berm area based upon side slope and contributing area. The applicant shall determine the geometry of their rain garden based upon contributing square footage, minimum sizing criteria and preferred side slope. The applicant shall submit the proposed ponding area, bottom ponding area and top of berm area to Environmental Services for review. Side Slope Table 6-1: Rain Garden Sizing and Geometry For sites proposing an underdrain, an engineered design will be required. Refer to Volume 6, BMP L630 (Section ) for sizing guidance Flow Credit Contributing Area (square feet) No flow credits are allowed for rain gardens. Minimum Top of Ponding Area (square feet) General Construction Criteria Minimum Bottom of Ponding Area (square feet) 2:1 800 or less : or less : or less Minimum Top of Berm Area (square feet) Do not install media or excavate rain garden during soil saturation periods. Excavation and soil placement should be done from equipment operating adjacent to the facility no heavy equipment should be operated in the facility. If equipment must be operated within the facility for excavation, use lightweight, low ground pressure equipment and scarify the base to reduce compaction upon completion. Do not use equipment on top of rain garden soil mix. Do not use fully excavated rain garden for erosion and sedimentation control during construction. Clogged soil and silt shall be removed during excavation to finished bottom grade prior to installing rain garden soil mix. Scarify sides and bottom to roughen where equipment may have compacted soil. Ensure the rain garden is protected from erosion and sedimentation until all contributory areas are fully stabilized. If sedimentation occurs within the rain garden, excavate the area a minimum of 12 inches below final grade to remove sediment and replace media, mulch, and plants as necessary Maintenance Criteria Per Minimum Requirement #9, an operation and maintenance plan shall be prepared for all stormwater management facilities. See Volume 1, Appendix C, Maintenance Checklist #22 for specific maintenance requirements for rain gardens. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility. Any standing water removed during the maintenance operation must be disposed of in a City approved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment Chapter Volume 6

16 City of Tacoma may be necessary. Solids must be disposed of in accordance with state and local waste regulations. Facilities shall be designed and constructed such that the facility can be safely and easily inspected by one person and safely and easily maintained. This may require the construction of additional inspection ports or access manholes BMP L630 Bioretention Purpose and Definition Bioretention areas are shallow landscaped depressions, with a designed soil mix and plants adapted to the local climate and soil moisture conditions that receive stormwater from a contributing area. Bioretention areas can provide effective removal of many stormwater pollutants, and provide reductions in stormwater runoff quantity and surface runoff flow rates. Bioretention Standard Plans can be found at govme.org under Standard Plans. These Standard Plans can be used for projects in the public ROW and on private property. In this manual, bioretention areas are categorized as follows: Bioretention cells: Shallow depressions with a designed planting soil mix and a variety of plant material, including trees, shrubs, grasses, and/or other herbaceous plants. Bioretention cells may or may not have an underdrain and are not designed as a conveyance system. Bioretention swales: Incorporate the same design features as bioretention cells; however, bioretention swales are designed as part of a system that can convey stormwater when maximum ponding depth is exceeded. Bioretention swales have relatively gentle side slopes and ponding depths that are typically 6 to 12 inches. Bioretention planters and planter boxes: Designed soil mix and a variety of plant material including trees, shrubs, grasses, and/or other herbaceous plants within a vertical walled container usually constructed from formed concrete, but could include other materials. Planter boxes are completely impervious and include a bottom (must include an underdrain). Planters have an open bottom and allow infiltration to the subgrade. These designs are often used in ultra-urban settings. NOTE: Ecology has approved use of certain patented treatment systems that use specific, high rate media for treatment. Such systems are not considered LID BMPs and are not options for meeting the requirements of Minimum Requirement #5. The Ecology approval is meant to be used for Minimum Requirement #6, where appropriate Phosphorus-sensitive Waterbodies Bioretention areas constructed with imported compost material shall not be used within 1/4 mile of phosphorus-sensitive waterbodies if the underlying soils do not meet the soil suitability criteria for treatment (Volume 5, Section 7.3). Facilities with an underdrain that discharge to a phosphorus-sensitive waterbody or to a storm system that discharges to a phosphorus-sensitive waterbody shall not be used within ¼ mile of phosphorus-sensitive waterbodies. This restriction does not apply to bioretention facility construction within 1/4 mile of Wapato Lake. Consult with Environmental Services to determine if a waterbody is considered phosphorus-sensitive Setback Criteria Setback requirements are generally required by the Tacoma Municipal Code, Uniform Building Code, the Tacoma-Pierce County Health Department, or other state regulation. Where a conflict between setbacks occurs, the City shall require compliance with the most stringent of the Volume Chapter 2

17 City of Tacoma setback requirements from the various codes/regulations. The following are the minimum setbacks required per this manual. Additional setbacks may be required by other local, state, or federal agencies. See the individual BMPs for BMP specific setback criteria. A least 10 feet from any building structure and at least 5 feet from any other structure or property line unless approved in writing by Environmental Services. All facilities shall be a minimum of 50 feet from the top of any steep (greater than 15%) slope. A geotechnical analysis must be prepared addressing impacts to facilities proposed within 50 feet of a steep (greater than 15%) slope. More stringent setbacks may be required based upon other portions of Tacoma Municipal Code. At least 10 feet from septic tanks and septic drainfields. Shall not be located upstream of septic systems unless topography or a hydrologic analysis clearly indicates that subsurface flows will not impact the drainfield. Environmental Services may require additional setback or analysis for infiltration facilities proposed to be sited within the influence of known contaminated sites or abandoned landfills Site Suitability Groundwater Separation A minimum of 3 feet of vertical separation is required between the lowest elevation of the bioretention soil or any underlying gravel infiltration layer and the seasonal high groundwater elevation or other impermeable layer if the area draining to the bioretention facility has 5,000 square feet or more of pollution-generating impervious surface, or 10,000 square feet or more of impervious surface, or ¾ acre or more of pervious surface; and cannot be reasonably broken into smaller amounts. A minimum of 1 foot of vertical separation is required between the lowest elevation of the bioretention soil or any underlying gravel infiltration layer and the seasonal high groundwater elevation or other impermeable layer if the area draining to the bioretention facility has less than 5,000 square feet of pollution-generating impervious surface, or less than 10,000 square feet of impervious surface, or less than ¾ acre of pervious surface Infeasibility Criteria A bioretention facility is not required if any of the following infeasibility criteria are met. If a project proponent wishes to use a bioretention facility even when not required to based upon the infeasibility criteria; Environmental Services will review the proposed design and may allow the installation on a case by case basis. See Volume 3, Appendix B to determine if a soils report is required. These criteria are in addition to setback criteria described in Section These criteria are in addition to any competing needs criteria (Vol 1, Sec ). Setback distances are measures from the bottom edge of the bioretention soil mix. A site characterization study must be completed in order to determine if the following infeasibility criteria apply. The following infeasibility criteria require evaluation of site specific conditions and a written recommendation from an appropriate licensed professional (e.g., Professional Engineer, Professional Geologist, Professional Hydrogeologist) Where professional geotechnical evaluation recommends infiltration not be used due to reasonable concerns about erosion, slope failure, or down gradient flooding. Chapter Volume 6

18 City of Tacoma Within an area whose groundwater drains into an erosion hazard or landslide hazard area. Where infiltrating water would threaten existing below grade basements. Where infiltrating water would threaten shoreline structures such as bulkheads. Where the only area available for siting would threaten the safety or reliability of preexisting underground utilities, preexisting underground storage tanks, preexisting structures, or preexisting road or parking lot surfaces. The following criteria can be sited as reasons for a finding of infeasibility without a written recommendation from a professional engineer though some require site specific evaluation by an appropriate licensed professional. Where the only area available for siting does not allow for a safe overflow pathway to the City storm system or a private storm system. Where there is lack of usable space onsite for bioretention facilities at redevelopment sites. For work within the public right of way, where there is insufficient space. Where they are not compatible with surrounding drainage system (e.g., project drains to an existing stormwater system whose elevation precludes proper connection to the bioretention facility). Where land for the bioretention facility is within an area designated as an erosion hazard or landslide hazard. Where the site cannot be reasonably designed to locate bioretention facilities on slopes less than 8%. For properties with known soil or groundwater contamination: Within 100 feet of an area known to have deep soil contamination; Where groundwater modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the groundwater; Wherever surface soils can be found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltration area; Any area where these facilities are prohibited by an approved cleanup plan under the state Model Toxics Control Act or Federal Superfund Law, or an environmental covenant under Chapter RCW. Within 100 feet of a closed or active landfill. Within 100 feet of a drinking water well, or a spring used for drinking water supply. Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and grey water reuse systems. For setbacks from a large on-site sewage disposal system, see WAC Chapter B. Within 10 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is 1100 gallons or less. (As used in this criteria, an underground storage tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which 10% or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface.) Within 100 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is greater than 1100 gallons. Volume Chapter 2

19 City of Tacoma Where the field testing indicates potential bioretention sites have a measured native soil saturated hydraulic conductivity less than 0.30 inches per hour. If the measured native soil infiltration rate is less than 0.30 in/hour, this option should not be used to meet the requirements of MR#5. In slow draining soils, a bioretention facility with an underdain may be used to treat pollution-generating surfaces to meet Minimum Requirement #6. If the underdrain is elevated within a gravel course, it may provide some flow reduction to help achieve compliance with Minimum Requirement # Other Site Suitability Factors The following factors should be considered when siting the bioretention facility. Utility conflicts: See Volume 3, Chapter 10 for utility separation requirements. Transportation safety: The design configuration and selected plant types should provide adequate sight distances, clear zones, and appropriate setbacks for roadway applications in accordance with local jurisdiction requirements. The following website contains information for landscaping within the City of Tacoma right of way: cms.cityoftacoma.org/enviro/urbanforestry/landscape%20standards%20row.pdf. The following website contains information for clearance standards within the City of Tacoma: Impacts of surrounding activities: Human activity influences the location of the facility in the development. For example, locate bioretention areas away from traveled areas on individual lots to prevent soil compaction and damage to vegetation or provide elevated or bermed pathways in areas where foot traffic is inevitable. Provide barriers, such as wheel stops, to restrict vehicle access in roadside applications and parking areas. Visual buffering: Bioretention facilities can be used to buffer structures from roads, enhance privacy among residences, and for an aesthetic site feature Site Characterization One of the first steps in siting and designing infiltration facilities is to conduct a characterization study. Information gathered during initial geotechnical investigations can be used for the site characterization. Information gathered during site characterization is used to determine if the infeasibility criteria (Section ) apply to the site. See Volume 1, Chapter 4 for site assessment requirements Subsurface Characterization See Volume 3, Appendix B for soils report requirements. Conduct pit/hole explorations during the wet season (December 1 through March 31st). Subsurface explorations (test holes or test pits) to a depth below the base of the bioretention facility of at least 3 feet. Subsurface characterization shall be conducted by a qualified engineer with geotechnical and hydrogeologic experience, or an equivalent professional acceptable to the City, under the seal of a registered Professional Engineer. The design engineer may utilize a team of certified or registered professionals in soil science, hydrogeology, geology, and other related fields. The professional can exercise discretion concerning the need for an extent of infiltration rate (saturated hydraulic conductivity) testing. The professional can consider a reduction in the extent of infiltration testing if, in their judgment, information exists confirming that the site is unconsolidated outwash material with high infiltration rates, and there is adequate separation from groundwater. Chapter Volume 6

20 City of Tacoma On single, smaller commercial properties where one bioretention facility is appropriate, conduct a Small-Scale PIT test per Volume 3, Section On larger commercial and industrial sites, conduct a Small-Scale PIT test every 5000 square feet. On multi-lot residential developments, multiple bioretention facilities or a shared facility serving multiple lots may be provided. A Small-Scale PIT test at the location of every bioretention site is required. For linear bioretention facilities, a Small-Scale PIT test is required every 200 lineal foot, minimum. If the site subsurface characteristics across the site indicate consistent soil characteristics and depths to seasonal high groundwater conditions or hydraulic restriction layer, the number of test locations may be reduced to a frequency recommended by the geotechnical professional. If a bioretention area will serve a drainage area exceeding 1 acre, a groundwater mounding analysis may be required Correction Factors The hydraulic saturated conductivity obtained from the PIT test or Soil Grain Size Analysis Method is an initial rate. This initial rate shall be reduced through correction factors that are appropriate for the design situation to produce a design infiltration rate. Use the correction factor from Table 6-2 below or alternative values can be proposed based upon the professional judgment of the licensed engineer or site professional. Justification for alternate values must be provided to Environmental Services. Table 6-2: Measured Hydraulic Saturated Conductivity Rate Reduction Factors Issue Site Variability and Number of Locations Tested CF v = 0.33 to 1.0 Partial Correction Factor The number of locations tested must be capable of producing a picture of the subsurface conditions that fully represents the conditions throughout the facility site. The partial correction factor used for site variability depends on the level of uncertainty that adverse subsurface conditions may occur. If the range of uncertainty is low - for example, conditions are known to be uniform through previous exploration and site geological factors - one pilot infiltration test (or grain size analysis location) may be adequate to justify a partial correction factor at the high end of the range. If the level of uncertainty is high, a partial correction factor near the low end of the range may be appropriate. This might be the case where the site conditions are highly variable due to conditions such as a deposit of ancient landslide debris, or buried stream channels. In these cases, even with many explorations and several pilot infiltration tests (or several grain size test locations), the level of uncertainty may still be high. A partial correction factor near the low end of the range could be assigned where conditions have a more typical variability, but few explorations and only one pilot infiltration test (or one grain size analysis location) is conducted. That is, the number of explorations and tests conducted do not match the degree of site variability anticipated. Volume Chapter 2

21 City of Tacoma Design Criteria Flow Entrance/Presettling Flow velocity entering the facility shall be less than 1 ft/sec for the 100-year, 24-hour storm event. If 1 ft/sec cannot be obtained, a designed flow dispersion or energy dissipation device may be required depending on the type of inlet and the size of facility. Use the following types of flow entrances (other alternatives may be considered on a case-by-case basis): Dispersed, low velocity flow across a landscape area: Landscape areas and vegetated buffer strips slow incoming flows and provide an initial settling of particulates and are the preferred method of delivering flows to the bioretention cell. A minimum 1-inch grade change between the edge of the contributing area and the flow entrance is required. Dispersed or sheet flow across pavement or gravel and past wheel stops for parking areas. Curb cuts for roadside, driveway or parking lot areas: Curb cuts should include a rock pad, concrete or other erosion protection material in the channel entrance to dissipate energy. Minimum curb cut width: 12 inches; maximum curb width: 18 inches. Avoid the use of angular rock or quarry spalls and instead use round (river) rock if needed. Removing sediment from angular rock is difficult. Flow entrance should drop 2 to 3 inches from curb line and provide an area for settling and periodic removal of sediment and coarse material before flow dissipates to the remainder of the cell. Curb cuts used for bioretention areas in high use parking lots or roadways require increased level of maintenance due to high coarse particulates and trash accumulation in the flow entrance and associated bypass of flows. The following are methods recommended for areas where heavy trash and coarse particulates are anticipated: Curb cut width: 18 inches. At a minimum, the flow entrance should drop 2 to 3 inches from gutter line into the bioretention area and provide an area for settling and periodic removal of debris. Anticipate relatively more frequent inspection and maintenance for areas with large impervious areas, high traffic loads and larger debris loads. Catch basins or forebays may be necessary at the flow entrance to adequately capture debris and sediment load from large contributing areas and high use areas. Piped flow entrance in this setting can easily clog and catch basins with regular maintenance are necessary to capture coarse and fine debris and sediment. Chapter Volume 6

22 City of Tacoma Pipe flow entrance: Piped entrances should include rock or other erosion protection material in the channel entrance to dissipate energy and disperse flow. Catch basin: In some locations where road sanding or higher than usual sediment inputs are anticipated, catch basins can be used to settle sediment and release water to the bioretention area through a grate for filtering coarse material. Trench drains: can be used to cross sidewalks or driveways where a deeper pipe conveyance creates elevation problems. Trench drains tend to clog and may require additional maintenance. Do not place plants directly in the entrance flowpath as they can restrict or concentrate flows. Install flow diversion and erosion control measures to protect the bioretention area from sedimentation until the upstream area is stabilized. If the catchment area exceeds 2,000 square feet, a presettling facility may be required Bottom Area and Side Slopes Bioretention areas are highly adaptable and can be configured to fit the site by adjusting bottom area and side slope configuration. The following are required maximum and minimum dimensions: Maximum planted side slope if total cell depth is greater than 3 feet: 3H:1V. Planted side slopes steeper than 3:1 may be approved if overall facility depth is less than 3 feet. If steeper side slopes are necessary rockeries, concrete walls or soil wraps may be effective design options. Additional safety features may be required where steep side slopes are adjacent to sidewalks, walkways, bike lanes, and parking areas. Minimum bottom width for bioretention swales: 1 foot minimum, 2 feet recommended. Carefully consider flow depths and velocities, flow velocity control (check dams) and appropriate vegetation, rock, or mulch to prevent erosion and channelization at bottom widths less than 2 feet. Bioretention areas shall have a minimum shoulder of 24 inches between the road edge and beginning of the bioretention side slope where flush curbs are used. Compact shoulder to minimum 90% standard proctor density. Bottom area shall be level with no cross slope Cell Ponding Area The surface pool drawdown time shall be less than 24 hours. Maximum ponding depth shall be 12 inches. The minimum freeboard measured from the overflow elevation to the top of the facility shall be 2 for drainage areas less than 1,000 square feet and 6 for drainage areas 1,000 square feet or greater. If berming is used to achieve the minimum top elevation, maximum slope on berm shall be 3H:1V, and minimum top width of design berm shall be 1 foot. Soil for berming shall be imported bioretention soil or amended native soil compacted to a minimum of 90% dry density Overflow Unless designed for full infiltration of the entire runoff volume, bioretention systems must include an overflow. Volume Chapter 2

23 City of Tacoma A drain pipe installed at the designed maximum ponding elevation and connected to a downstream BMP or an approved discharge point can be used as the overflow. Overflows shall be designed to convey the 100-year recurrence interval flow or maximum flow that can reach the facility if a flow splitter is utilized. Overflow channels shall be rock-lined. An emergency overflow pathway shall be provided for all facilities to ensure that all potential overflows are directed into the downstream conveyance system or the public right of way. See Volume 3, Chapter 11 for additional guidance on rock protection Bioretention Soil Media Minimum depth of bioretention soil mix shall be 18 inches Default Bioretention Soil Media Projects that use the following bioretention soil mix do not have to test the media for the saturated hydraulic conductivity. The bioretention soil media (BSM) shall meet the following standards: Have a 60-65% mineral aggregate component by volume as specified below and a 35-40% compost component by volume as specified below. Have organic matter content of 5-8% by weight. Have a cation exchange capacity (CEC) 5 milliequivalents/100 g dry soil. BSM meeting the above specifications do not have to be tested for CEC value; it is assumed the CEC value is met. The mineral aggregate component shall meet the following standards: Have a maximum of 5% fines with an ideal range between 2-4% fines using ASTM D422. Be well graded. According to ASTM D , well graded sand should have the following gradation coefficients: Coefficient of Uniformity (Cu=D60/D10) equal to or greater than 4, and Coefficient of Curve equal to or greater than 1 and less than or equal to 3. Meet the gradation presented in Table 6-3. Table 6-3: Bioretention Soil Mix Aggregate Component Gradation Sieve Size Aggregate Component (60% by Volume) Percent Passing 3/8 100 # # # # # Chapter Volume 6

24 City of Tacoma Compost used as a component of the default bioretention soil mix (BSM) in bioretention facilities shall meet the specifications below: Meets the definition of composted material in WAC and complies with testing parameters and other standards in WAC Produced at a composting facility that is permitted by the jurisdictional health authority. Permitted compost facilities in Washington are included on a list available at: The compost product must originate from a feedstock that contains a minimum of 65% by volume recycled plant waste comprised of yard debris, crop residues, and bulking agents. The remainder of the feedstock may contain a maximum of 35% by volume post-consumer food waste. Biosolids are not allowed. Terms are defined in WAC Stable (low oxygen use and CO2 generation) and mature (capable of supporting plant growth) by tests shown below. This is critical to plant success in a bioretention soil mix. Moisture content range: no visible free water or dust produced when handling the material. Tested in accordance with U.S. Composting Council Test Method for the Examination of Compost and Composting (TMECC), as established in the Composting Council s Seal of Testing Assurance (STA) program. Most Washington compost facilities now use these tests. Be fine compost meeting the following size gradation (by dry weight) when tested in accordance with TMECC test method B, Sample Sieving for Aggregate Size Classification. Sieve Size Minimum Percent Passing /8 90 ¼ 75 ph between 6.0 and 8.5 (TMECC A). Physical contaminants (as defined in WAC ) content less than 1% by weight (TMECC A) total, not to exceed 0.25 percent film plastic by dry weight. Minimum organic matter content of 40% (TMECC A Loss on Ignition ) Soluble salt content less than 4.0 ds/m (mmhos/cm) (TMECC A Electrical Conductivity, 1.5 Slurry Method, Mass Basis ) Maturity indicators from a cucumber bioassay (TMEC A Seedling Emergence and Relative Growth ) must be greater than 80% for both emergence and vigor Stability of 7 mg CO2-C/g OM/day or below (TMECC B Carbon Dioxide Evolution Rate ) Carbon to nitrogen ratio (TMECC 05.02A Carbon to Nitrogen Ratio which uses Organic Carbon and 04.02D Total Nitrogen by Oxidation ) of less than 25:1. The C:N ratio may be up to 35:1 for planting composed entirely of Puget Sound Lowland native species and up to 40:1 for coarse compost to be used as a surface mulch (not in a soil mix). Compost specifications are also found in Volume 5, Appendix E. Volume Chapter 2

25 City of Tacoma Custom Bioretention Soil Media Projects which prefer to create a custom Bioretention Soil Mix rather than using the default requirements above must demonstrate compliance with the following criteria using the specified test method: CEC 5 meq/100 grams of dry soil; USEPA 9081 ph between 5.5 and percent organic matter content before and after the saturated hydraulic conductivity test; ASTM D2974(Standard Test Method for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils) 2-5 percent fines passing the 200 sieve; TMECC A Measured (Initial) saturated hydraulic conductivity of less than 12 inches per hour; ASTM D 2434 (Standard Test Method for Permeability of Granular Soils (Constant Head)) at 85% compaction per ASTM D 1557 (Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort). Also, use Appendix V-B, Recommended Procedures for ASTM D 2434 When Measuring Hydraulic Conductivity for Bioretention Soil Mixes. Design (long-term) saturated hydraulic conductivity of more than 1 inch per hour. Note: Design saturated hydraulic conductivity is determined by applying the appropriate infiltration correction factors as shown in Table 6-5. If compost is used in creating the custom mix, it must meet all of the specifications listed above for the Default Bioretention Mix, except for the gradation specification. An alternate gradation may be proposed. The applicant is required to submit the alternate gradation specification which shall include similar particle sizes Filter Fabric Do not use filter fabric between the subgrade and the Bioretention Soil Media Underdrain Only install underdrains in bioretention areas if: Located near sensitive infrastructure where flooding is likely The facility if used for treating stormwater discharge from gas stations or other pollutant hotspots. Where a liner is used. Where infiltration is not permitted due to site suitability criteria. A bioretention facility with elevated underdrain may be used to satisfy Minimum #5 List #2 provided the following criteria are met (though an underdrain is not required): The estimated initial infiltration rate of the underlying native soils is between 0.3 and 0.6 inches. The invert of the underdrain shall be elevated 6 above the bottom of the aggregate bedding layer. The distance between the bottom of the bioretention soil mix and the crown of the underdrain pipe shall be greater than 6 but less than 12. The aggregate bedding layer must run the full length and full width of the bottom of the bioretention facility. Chapter Volume 6

26 City of Tacoma A low permeability liner must not be present. The underdrain pipe shall meet the following criteria: Underdrain pipe diameter will depend on hydraulic capacity required, 4-inch minimum. A minimum 6-inch pipe is required for installations to be maintained by the City of Tacoma. Underdrains shall be slotted, thick-walled plastic pipe. The slot opening should be smaller than the smallest aggregate gradation for the gravel filter bed (see underdrain filter bed below) to prevent migration of material into the drain. This configuration allows for pressurized water cleaning and root cutting if necessary. Perforated PVC or flexible slotted HDPE pipe cannot be cleaned with pressurized water or root cutting equipment, are less durable and are not recommended. Wrapping the underdrain pipe in filter fabric increases chances of clogging and is not recommended. Slotted subsurface drain PVC per ASTM D1785 SCH 40. Slots shall be cut perpendicular to the long axis of the pipe and be 0.04 to inches by 1 inch long and be spaced 0.25 inches apart (spaced longitudinally). Slots should be arranged in four rows spaced on 45-degree centers and cover ½ of the circumference of the pipe. Underdrains shall be sloped a minimum of 0.5 percent unless otherwise specified by an engineer. Provide a 6-inch rigid non-perforated observation pipe or other maintenance access every 250 to 300 feet to provide a clean-out port, as well as an observation well to monitor dewatering rates. Ensure access is placed so that it can be found with full vegetation growth. Place underdrain on a bed of mineral aggregate with a minimum thickness of 6 inches and cover with 6 of mineral aggregate to provide a 1-foot minimum depth around the top and sides of the slotted pipe. The following table provides the gradation for the mineral aggregate. All filter media shall be double washed to ensure removal of fines. Sieve Size Table 6-4: Underdrain sieve sizes Percent Passing ¾ inch 100 US No US No US No US No Underdrains connected directly to a storm drainage structure shall be non-slotted for at least 2 feet from the structure interface Check Dams and Weirs Check dams are necessary for reducing flow velocity and potential erosion, as well as increasing detention time and infiltration capability on sloped sites. Typical materials include concrete, wood, rock, compacted dense soil covered with vegetation, and vegetated hedge rows. Design depends on flow control goals, local regulations for structures within road right-of-ways and aesthetics. Optimum spacing is determined by flow control benefit (modeling) in relation to cost Volume Chapter 2

27 City of Tacoma consideration. See the Low Impact Development Technical Guidance Manual for Puget Sound for figures of typical check dams UIC Discharge Bioretention facilities that infiltrate may be considered Underground Injection Control facilities. Follow UIC regulations as applicable (Chapter WAC) Hydraulic Restriction Layer Adjacent roads, foundations or other infrastructure may require that infiltration pathways are restricted to prevent excessive hydrologic loading. Two types of restricting layers can be incorporated into bioretention designs: Clay (bentonite) liners are low permeability liners. Where clay liners are used underdrain systems are necessary. See Volume 5, Chapter 4 for guidelines. Geomembrane liners completely block infiltration to subgrade soils and are used for groundwater protection when bioretention facilities are installed to filter storm flows from pollutant hotspots or on sidewalls of bioretention areas to restrict lateral flows to roadbeds or other sensitive infrastructure. Where geomembrane liners are used to line the entire facility underdrain systems are necessary. The liner should have a minimum thickness of 30 mils and be ultraviolet (UV) resistant Planting Submit a planting plan showing the type (species), location and size of each plant. Plants must be tolerant of summer drought, ponding fluctuation, and saturated soil conditions. Locate slotted pipe at least 5 feet from tree roots and other utilities. Bioretention cells have three planting zones. Zone 1 is bottom area of the rain garden, which is frequently wet during the rainy season. Zone 2 includes the side slopes, which occasionally become wet during rain events. Zone 3 includes the area around the perimeter of the bioretention cell, starting above the top surface of ponding elevation, which has drier soil. Additional information on planting zones and appropriate plants for each zone can be obtained through Low Impact Development: Technical Guidance Manual for Puget Sound. In general, the following guidelines should be used when considering which plants to use: At least 50% (by quantity) should be evergreen plants. Leaf fall can reduce the function of the facility. Do not leave large areas of the soil unplanted/uncovered. Exposed soil can cause erosion and reduce the function of the facility. Plant types can be overlapped (tree canopy can overlap shrubs and groundcover) to reduce the area of exposed soil. Provide a variety of plant types with various rooting structures. Plant variety encourages good soil health. Typical plant types used in bioretention construction include: Emergents: Rushes, grasses, and sedges have shallow, fibrous roots that remain close to the soil surface. Planting solely with shallow rooted plants can cause thick root mats to form decreasing the infiltrative ability of the soil. Woody shrubs and trees: Woody shrubs and trees have a mixture of shallow fibrous roots and deeper structural roots that can penetrate deeper into the soil and increase soil porosity over time. Chapter Volume 6

28 City of Tacoma Groundcover: Groundcover tends to form dense masses of vegetation low to the ground surface. Low, dense vegetation can be effective at consolidating soil in areas otherwise prone to erosion such as the side slopes of the facility (Zone 2). Herbaceous perennials: Herbaceous perennials do not have woody plant parts and the above ground growth typically dies back in the winter. Roots must not damage underground infrastructure. Consider adjacent plant communities and avoid potential invasive species. Consider aesthetics, bioretention facilities should blend into surrounding landscapes. Irrigation may be required until plants are fully established and in the summer months Mulch Layer Bioretention facilities should be designed with a mulch layer. Properly selected mulch material reduces weed establishment, regulates soil temperatures and moisture, and adds organic matter to soil. Mulch should be free of weed seeds, soil, roots, and other material that is not trunk or branch wood and bark. Mulch shall not include grass clippings, mineral aggregate or pure bark. Mulch shall be: Coarse compost in the bottom of the facilities, depth 3 inches. Compost shall: Meet the definition for composted material per WAC and comply with standards in WAC Be coarse compost meeting the following size gradations (by dry weight) when tested in accordance with the U.S. Composting Council Test Methods for the Examination of Compost and Composting (TMECC) Test Method B. Sieve Size Minimum Percent Passing /4 70 ¼ 40 Have no visible water or dust during handling. Wood chip mulch composed of shredded or chipped hardwood or softwood on side slopes, above ponding elevation, depth 4 inches. Do not use wood chips that may have been contaminated by preservatives, such as construction debris. A dense groundcover can be used as an alternative to mulch although mulch shall be required until the dense groundcover is established Modeling and Sizing For facilities sized to meet Minimum Requirement #5, Onsite Stormwater Management using the list approach: The top of the ponded surface area below the overflow shall be at least 5% of the total impervious surface area draining to it. If lawn/landscaped area will also be draining to the Volume Chapter 2

29 City of Tacoma bioretention facility, it is recommended that the top of the ponded surface area below the overflow be increased by 2% of the lawn/landscaped area. For facilities sized to meet Minimum Requirement #6, Water Quality: Use WWHM and model the facility using the bioretention element. Size the facility to treat or infiltrate the water quality design volume which is 91% of the total runoff volume as predicted by WWHM. Alternatively, size the facility to treat 100% of the water quality design volume. The surface pool drawdown time shall be 24 hours or less. For facilities sized to meet Minimum Requirement #7, Flow Control: Use WWHM and model the facility using the bioretention element. Size the facility to meet the flow control requirements. The surface pool drawdown time shall be 24 hours or less. Variable Computational Time Step Inflows to Facility Precipitation and Evaporation applied to Facility Bioretention Soil Infiltration Rates Safety Factor for Saturated Hydraulic Conductivity (K SAT ) Bioretention Soil Depths Native Soil Infiltration Rate Infiltration Across Wetted Surface Area Table 6-5: WWHM Modeling Assumptions for Bioretention Assumptions 15 minutes Surface flow and interflow from drainage area routed to facility Yes Default Bioretention Soil Media: WWHM assumes a default infiltration rate of 12 inches per hour. Choose the soil layer SMMWW and the appropriate factor of safety below. Custom Bioretention Soil Media Use ASTM D2434 Standard Test Method for Permeability of Granular Soils (Constant Head) with a compaction rate of 85% using ASTM D1557 Test Method for Laboratory Compaction Characteristics of Soil Using Modified Effort. See Appendix A for recommended modifications for ASTM D2434. Enter the derived value in the View/Edit Soil Types pull down menu. Model using the appropriate factor of safety below. Enter a K SAT Safety Factor of 4 if the contributing area to the facility equals or exceeds any of the following: 5,000 square feet of pollution-generating impervious surface; 10,000 square feet of impervious surface; ¾ acres of lawn and landscape. If the contributing area is less than all of the above or if the design includes a pretreatment device for solids removal, enter a K SAT Safety Factor of 2. Minimum of 18 inches Measured infiltration rate. See Volume 3 for more information on infiltration rate determination. Only if sides slopes are 3:1 or flatter Chapter Volume 6

30 City of Tacoma Variable Assumptions Overflow Underdrain Underdrain Aggregate General Construction Criteria: Model as riser outlet structure. If underdrain is proposed, click underdrain is used box. Use Gravel as the Material Layer for the Underdrain Aggregate Layer. Only the void volume of the aggregate below the underdrain invert and above the bottom of the bioretention facility can be used for flow control benefits. Do not install media or excavate bioretention facility during soil saturation periods. Excavation and soil placement should be done from equipment operating adjacent to the facility or by conveyor no heavy equipment should be operated in the facility. If equipment must be operated within the facility, use lightweight, low ground pressure equipment and scarify the base to reduce compaction upon completion. Clogged soil and silt shall be removed during excavation to finished bottom grade prior to installing bioretention cell profile. Scarify sides and bottom a minimum of 3 inches to roughen before BSM placement. Ensure the bioretention facility is protected from erosion and sedimentation until all contributory areas are fully stabilized. If sedimentation occurs within the bioretention facility, excavate the area as necessary in minimum 6-inch lifts to remove sediment. Subgrade infiltration rates shall be field tested per Vol 6, Sec and compared to design rates. Failure to meet or exceed the design infiltration rate will require revised engineering design to verify achievement of treatment and flow control benefits that were estimated in the design. Prior to placement of the BSM, the subgrade shall be inspected by the responsible engineer to verify condition. Do not mix or place soil if the BSM or subgrade is saturated. Place the BSM in horizontal layers not to exceed 12 inches per lift. Compact the BSM to a relative compaction of 85% of modified maximum dry density. Compaction can be achieved by boot packing and applying 0.2 inches of water per 1 inch of BSM depth. Water for settling shall be sprayed or sprinkled. Do not use bioretention facilities as temporary sediment control facilities. Construction of the bioretention area should not begin until contributing areas are stabilized. Inlets should be blocked until BSM has been placed and bioretention facility has been planted. All inlet protection shall be in place before inlets are unblocked Verification Testing If using the default bioretention soil media, laboratory analysis for saturated hydraulic conductivity of the bioretention soil media is not required. The applicant shall provide verification of the mineral aggregate gradation, compliance with the compost specifications, and the mix ratio before soil placement. If using a custom bioretention soil media, verification of compliance with the minimum design criteria cited above for such custom mixes must be provided. This will require laboratory testing of the material that will be used in the installation. Testing shall be Volume Chapter 2

31 City of Tacoma performed by a Seal of Testing Assurance, AASHTO, ASTM or other standards organization accredited laboratory with current and maintained certification. Samples for testing must be supplied from the BSM that will be placed in the bioretention areas. If testing infiltration rates is necessary for post-construction verification, use the Pilot Infiltration Test (PIT) method or a double ring infiltrometer test. If using the PIT method, do not excavate Bioretention Soil Mix (conduct test at level of finished Bioretention Soil Mix elevation), use a maximum of 6 inch ponding depth and conduct test before plants are installed. Environmental Services will determine the need for post construction verification on a case-by-case basis Operation and Maintenance Per Minimum Requirement #9, an operation and maintenance plan shall be prepared for all stormwater management facilities. See Volume 1, Appendix C for specific maintenance requirements for bioretention facilities. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility. Any standing water removed during maintenance operation must be disposed of in a City approved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment may be necessary. Solids must be disposed of in accordance with state and local waste regulations. Facilities shall be designed and constructed such that the facility can be safely and easily inspected by one person and safely and easily maintained. This may require construction of additional inspection ports or access manholes to allow inspection access to be opened by one person BMP L631 Vegetated Rooftops (Green Roofs) Purpose and Definition A vegetated rooftop, also known as a green roof, is a rooftop that is partially or completely covered with vegetation and a growing medium planted over a waterproofing membrane. The green roof will also contain a root repelling membrane and drainage system Applications and Limitations Vegetated rooftops offer a practical method of managing runoff in densely developed urban neighborhoods and can be engineered to achieve specific stormwater runoff control objectives Design Guidelines See the Low Impact Development Technical Guidance for Puget Sound for design criteria. Figure 6-8 provides a general schematic of a Green Roof Section Flow Credits for Vegetated Roofs Modeling in WWHM is required to obtain flow credits. Model the facility using the Green Roof element Maintenance Criteria Per Minimum Requirement #9, an operation and maintenance manual shall be prepared for all stormwater management facilities. See Volume 1, Appendix C, Maintenance Checklist #26 for specific maintenance requirements for vegetated roofs. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility. Any standing water removed during the maintenance operation must be disposed of in a Cityapproved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment Chapter Volume 6

32 City of Tacoma may be necessary. Solids must be disposed of in accordance with state and local waste regulations BMP L632 Rainwater Harvesting Purpose and Definition Rainwater harvesting systems are designed to collect stormwater and use the collected water Applications and Limitations Approval of the water re-use system requires approval of the appropriate state and local agencies as required for any water right Design Guidelines Figure GSI (see Volume 6, Appendix B) shows a conceptual plan for a vegetated roof section. If a rainwater re-use system holds more than 6 inches depth of water, the system should not be easily accessible except for maintenance purposes. Design and maintain the system to minimize clogging by leaves and other debris. Restrict use to locations with no more than 4 homes/acre when captured water is solely used for outdoor use Flow Credits for Rainwater Harvesting The drainage area to the rainfall re-use system does not need to be entered into the runoff model when: 100% of the annual average runoff volume (using WWHM) is re-used, or Interior uses have a monthly water balance that demonstrates adequate capacity for each month and re-use of all stored water annually Maintenance Criteria Per Minimum Requirement #9, an operation and maintenance manual shall be prepared for all stormwater management facilities. See Volume 1, Appendix C, Maintenance Checklist #24 for specific maintenance requirements for cisterns. Maintenance shall be a basic consideration in design and cost-determination of the stormwater management facility. Any standing water removed during the maintenance operation must be disposed of in a Cityapproved manner. See the dewatering requirements in Volume 4 of this manual. Pretreatment may be necessary. Solids must be disposed of in accordance with state and local waste regulations BMP L633 Permeable Pavements Purpose and Definition Permeable paving surfaces are an important integrated management practice within the LID approach and can be designed to accommodate pedestrian, bicycle and auto traffic while allowing infiltration, treatment and storage of stormwater. The general categories of permeable paving systems include: Porous hot or warm-mix asphalt pavement is a flexible pavement similar to standard asphalt that uses a bituminous binder to adhere aggregate together. However, the fine Volume Chapter 2

33 City of Tacoma material (sand and finer) is reduced or eliminated and, as a result, voids form between the aggregate in the pavement surface and allow water to infiltrate. Pervious Portland cement concrete is a rigid pavement similar to conventional concrete that uses a cementitious material to bind aggregate together. However, the fine aggregate (sand) component is reduced or eliminated in the gradation and, as a result, voids form between the aggregate in the pavement surface and allow water to infiltrate. Permeable interlocking concrete pavements (PICP) and aggregate pavers. PICPs are solid, precast, manufactured modular units. The solid pavers are (impervious) highstrength Portland cement concrete manufactured with specialized production equipment. Pavements constructed with these units create joints that are filled with permeable aggregates and installed on an open-graded aggregate bedding course. Aggregate pavers (sometime called pervious pavers) are a different class of pavers from PICP. These include modular precast paving units made with similar sized aggregates bound together with Portland cement concrete with high-strength epoxy or other adhesives. Like PICP, the joints or openings in the units are filled with open-graded aggregate and placed on an open-graded aggregate bedding course. Aggregate pavers are intended for pedestrian use only. Grid systems include those made of concrete or plastic. Concrete units are precast in a manufacturing facility, packaged and shipped to the site for installation. Plastic grids typically are delivered to the site in rolls or sections. The openings in both grid types are filled with topsoil and grass or permeable aggregate. Plastic grid sections connect together and are pinned into a dense-graded base, or are eventually held in place by the grass root structure. Both systems can be installed on an open-graded aggregate base as well as a dense-graded aggregate base. The City of Tacoma has developed Standard Plans for permeable pavement sections that can be used for both public and private projects within the City of Tacoma. These plans are available at govme.org under the Standard Plans link Permeable Pavements - Applications and Limitations Permeable paving systems can generally be used where traditional paving surfaces are used. See Volume 6, Appendix B for permeable pavement figures and standard plans. Limit run-on to permeable pavement surfaces to the maximum extent practicable. Run-on shall only be allowed from fully stabilized areas. Unless the pavement, base course, and subgrade have been designed to accept runoff from adjacent impervious surfaces, slope impervious runoff away from the permeable pavement to the maximum extent practicable. Sheet flow from up-gradient impervious areas is not recommended, but permissible if the permeable pavement area is greater than the impervious pavement area Permeable Pavements Setbacks Setback requirements are generally required by the Tacoma Municipal Code, Uniform Building Code, the Tacoma-Pierce County Health Department, or other state regulation. Where a conflict between setbacks occurs, the City shall require compliance with the most stringent of the setback requirements from the various codes/regulations. The following are the minimum setbacks required per this manual. Additional setbacks may be required by other local, state, or federal agencies. See the individual BMPs for BMP specific setback criteria. All facilities shall be a minimum of 50 feet from the top of any steep (greater than 15%) slope. A geotechnical analysis must be prepared addressing impacts to facilities Chapter Volume 6

34 City of Tacoma proposed within 50 feet of a steep (greater than 15%) slope. More stringent setbacks may be required based upon other portions of Tacoma Municipal Code. At least 10 feet from septic tank and septic drainfields. Shall not be located upstream of septic systems unless topography or a hydrologic analysis clearly indicates that subsurface flows will not impact the drainfield. Environmental Services may require additional setback or analysis for infiltration facilities proposed to be sited within the influence of known contaminated sites or abandoned landfills Permeable Pavements - Groundwater Separation A minimum of 1 foot of vertical separation is required between the bottom of the lowest gravel base course and the seasonal high groundwater elevation or other impermeable layer to ensure that a saturated condition is not created Permeable Pavements - Infeasibility Criteria Permeable pavements are not required if any of the following infeasibility criteria are met. If a project proponent wishes to use permeable pavement even when not required to based upon the infeasibility criteria; Environmental Services will review the proposed design and may allow the installation on a case by case basis. See Volume 3, Appendix B to determine if a soils report is required. These criteria also apply to impervious pavements that would employ stormwater collection from the surface of the impervious pavement with redistribution below the pavement. These criteria are in addition to setback criteria described in Section These criteria are in addition to any competing needs criteria (Vol 1, Sec ). Setback distances are measures from the edge of the pavement section. A site characterization study must be completed in order to determine if the following infeasibility criteria apply. The following infeasibility criteria require evaluation of site specific conditions and a written recommendation from an appropriate licensed professional (e.g., Professional Engineer, Professional Geologist, Professional Hydrogeologist) Where professional geotechnical evaluation recommends infiltration not be used due to reasonable concerns about erosion, slope failure, or down gradient flooding. Within an area whose groundwater drains into an erosion hazard or landslide hazard area. Where infiltrating and ponded water below new permeable pavement would compromise existing adjacent impervious pavements. Where infiltrating and ponded water below new permeable pavement would threaten existing below grade basements. Where infiltrating water would threaten shoreline structures such as bulkheads. Downslope of steep, erosion prone areas that are likely to erode sediment. Where fill soils are used that can become unstable when saturated. Excessively steep slopes where water within the aggregate base layer or at the subgrade surface cannot be controlled by detention structures and may cause erosion and Volume Chapter 2

35 City of Tacoma structural failure, or where surface runoff velocities may preclude adequate infiltration at the pavement surface. Where permeable pavements cannot provide sufficient strength to support the anticipated loads. Where underlying soils are unsuitable for supporting traffic loads when saturated. Where installation of permeable pavement would threaten the safety or reliability of preexisting underground utilities, preexisting underground storage tanks, preexisting structures, or preexisting road or parking lot surfaces. The following criteria can be sited as reasons for a finding of infeasibility without a written recommendation from a professional engineer though some require site specific evaluation by an appropriate licensed professional. Within an area designated as an erosion hazard or landslide hazard. For properties with known soil or groundwater contamination: Within 100 feet of an area known to have deep soil contamination; Where groundwater modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the groundwater; Wherever surface soils can be found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltration area; Any area where these facilities are prohibited by an approved cleanup plan under the state Model Toxics Control Act or Federal Superfund Law, or an environmental covenant under Chapter RCW. Within 100 feet of a closed or active landfill. Within 100 feet of a drinking water well, or a spring used for drinking water supply. Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and grey water reuse systems. For setbacks from a large on-site sewage disposal system, see WAC Chapter B. Within 10 feet of an underground storage tank and connecting underground pipes, regardless of tank size. As used in this criteria, an underground storage tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which 10% or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface. At multi-level parking garages, and over culverts and bridges. Where the site cannot reasonably be designed to have a porous asphalt surface at less than 5% slope, or a pervious concrete surface at less than 10% slope, or a permeable interlocking concrete pavement surface at less than 12% slope. Grid systems upper slope limit can range from 6-12%; check with manufacturer. Where the native soils below a pollution-generating permeable pavement do not meet the soil suitability criteria for providing treatment. These include: Cation exchange capacity (CEC) of the treatment soil must be 5 milliequivalents CEC/100g dry soil (USEPA Method 9081). Consider empirical testing of soil sorption capacity, if practicable. Ensure that soil CEC is sufficient for expected pollutant loadings, particularly heavy metals. CEC values of >5 meq/100g are expected in loamy sands, according to Rawls, et. al Chapter Volume 6

36 City of Tacoma Depth of soil used for infiltration treatment must be a minimum of 12 inches Organic content shall be a minimum of 1%. Organic content shall be measured on a dry weight basis using ASTM D2974. A measured (initial) saturated hydraulic conductivity greater than 9 inches/hour. If the applicant wishes to use permeable pavement but the criteria above for treatment are not met, they can elect to use a 6 layer of media meeting the soil suitability criteria or a 6 sand layer meeting the sand filter specification (Vol. 5 Ch. 8) to provide treatment. Where appropriate field testing indicates soils have a measured native soil saturated hydraulic conductivity less than 0.30 inches per hour. An underdrain may be used in this scenario if desired and properly designed. Where the road receives more than very low traffic volumes and more than very low truck traffic. Very low traffic volumes are those with a projected average daily traffic volume of 400 vehicles or less (AASHTO, 2001)(U.S. Department of Transportation, 2013). Very low truck traffic is defined as those areas with less than 2% of ADT as through truck traffic. Very low truck traffic may have up to weekly use by utility trucks (ex. garbage), daily bus use, and multiple daily use by pick-up trucks, mail/parcel delivery trucks, and maintenance vehicles. See RCW , RCW , and RCW This infeasibility criteria does not extend to sidewalks and other non-traffic bearing surfaces. Where replacing existing impervious surfaces unless the existing surface is a nonpollution generating surface over an outwash soil with a saturated hydraulic conductivity of 4 inches/hour or greater. At sites defined as high-use sites. In areas with industrial activity as identified in 40 CFR (b)(14). Where the risk of concentrated pollutant spills in more likely such as gas stations, truck stops, and industrial chemical storage sites. Where routine, heavy applications of sand occur in frequent snow zones to maintain traction during weeks of snow and ice accumulation. Most lowland western Washington areas do not fit this criterion Permeable Pavement Site Characterization One of the first steps in siting and designing infiltration facilities is to conduct a characterization study. Information gathered during initial geotechnical investigations can be used for the site characterization. Information gathered during site characterization is used to determine if the infeasibility criteria (Section ) apply to the site. See Volume 1, Chapter 4 for site assessment requirements Permeable Pavement Subsurface Characterization See Volume 3, Appendix 1 for soils report requirements. Conduct pit/hole explorations during the wet season (December 1 through March 31st). Perform tests in the soil profile at the estimated bottom elevation of base materials for the permeable pavement. If no base materials, perform the testing at the estimated bottom elevation of the pavement. Subsurface explorations (test holes or test pits) to a depth below the base of the permeable pavement section of at least 1 foot. Volume Chapter 2

37 City of Tacoma Subsurface characterization shall be conducted by a qualified engineer with geotechnical and hydrogeologic experience, or an equivalent professional acceptable to the City, under the seal of a registered Professional Engineer. The design engineer may utilize a team of certified or registered professionals in soil science, hydrogeology, geology, and other related fields. The professional can exercise discretion concerning the need for an extent of infiltration rate (saturated hydraulic conductivity) testing. The professional can consider a reduction in the extent of infiltration testing if, in their judgment, information exists confirming that the site is unconsolidated outwash material with high infiltration rates, and there is adequate separation from groundwater. Conduct a Small-Scale PIT Test per Volume 3, Section for every 5,000 square feet of permeable pavement but not less than 1 test per site. On multi-lot residential developments, perform a Small-Scale PIT test at every proposed lot, at least every 200 feet of roadway and within each length of road with significant differences in subsurface characteristics. If the site subsurface characteristics across the site indicate consistent soil characteristics and depths to seasonal high groundwater conditions or hydraulic restriction layer, the number of test locations may be reduced to a frequency recommended by the geotechnical professional. If a permeable pavement will serve a drainage area exceeding 1 acre, a groundwater mounding analysis may be required Permeable Pavements - Correction Factors The hydraulic saturated conductivity obtained from the PIT test is an initial rate. This initial rate shall be reduced through correction factors that are appropriate for the design situation to produce a design infiltration rate. Use the correction factor from Table 6-6 below or alternative values can be proposed based upon the professional judgment of the licensed engineer or site professional. Justification for alternate values must be provided to Environmental Services. Table 6-6: Measured Hydraulic Saturated Conductivity Rate Reduction Factors Issue Site Variability and Number of Locations Tested CF v = 0.33 to 1.0 Quality of Pavement Aggregate Base Material CF m = 0.9 to 1 Total Correction Factor (CF t ) = CF v * CF m Partial Correction Factor The number of locations tested must be capable of producing a picture of the subsurface conditions that fully represents the conditions throughout the facility site. The partial correction factor used for site variability depends on the level of uncertainty that adverse subsurface conditions may occur. If the range of uncertainty is low - for example, conditions are known to be uniform through previous exploration and site geological factors - one pilot infiltration test (or grain size analysis location) may be adequate to justify a partial correction factor at the high end of the range. If the level of uncertainty is high, a partial correction factor near the low end of the range may be appropriate. This might be the case where the site conditions are highly variable due to conditions such as a deposit of ancient landslide debris, or buried stream channels. In these cases, even with many explorations and several pilot infiltration tests (or several grain size test locations), the level of uncertainty may still be high. Chapter Volume 6

38 City of Tacoma A partial correction factor near the low end of the range could be assigned where conditions have a more typical variability, but few explorations and only one pilot infiltration test (or one grain size analysis location) is conducted. That is, the number of explorations and tests conducted do not match the degree of site variability anticipated. A correction factor of 1 for the quality of pavement aggregate base material may be appropriate if the aggregate base is clean washed material with 1% or less fines passing the 200 sieve Design Criteria At a minimum, comply with the design criteria in the sections below. The City of Tacoma Design Manual, Chapter 4 contains additional requirements for permeable pavement sections that are placed in the ROW. The Low Impact Development Technical Guidance Manual for Puget Sound is also a good guide for general guidance when designing permeable pavements Subgrade Subgrade shall be a maximum 3% slope. If roadway surface exceeds 3%, subsurface detention structures per may be required. Compact the subgrade to the minimum necessary for structural stability and at a minimum be "firm and unyielding". See the City of Tacoma Right of Way Design Manual for additional subgrade preparation for applications within the City ROW. The exposed subgrade shall maintain preconstruction infiltration rates. The subgrade shall be protected during construction including minimizing traffic to the subgrade. To prevent compaction when installing the aggregate base, the following steps are recommended: Dump aggregate onto subgrade from the edge of the installation. Dump subsequent aggregate base from on top of the aggregate base Separation or Bottom Filter Layer A layer of sand or crushed stone (0.5 inches or smaller) is recommended to promote infiltration across the surface, stabilize the base layer, protect underlying soils from compaction, and serve as a transition between the base course and the underlying geotextile material or subgrade Geotextile and GeoGrids (Optional) Geotextiles between the subgrade and aggregate base are not required or necessary for many soil types. Geotextile is recommended on the side slopes of the open graded base perimeter next to the soil subgrade if concrete curbs or impermeable liners are not provided that extend the full depth of the base Permeable Ballast Base Course Refer to the City of Tacoma Standard Plan PD-01 for the minimum permeable ballast thickness necessary for structural integrity for permeable pavement roadways, accessways, and trails. The applicant shall submit WWHM calculations showing the proposed ballast thickness is sufficient as a reservoir layer to manage stormwater runoff based on the design criteria applicable to the project. For private onsite permeable pavement surfaces, the minimum permeable ballast thickness shall be 6 inches for structural integrity and to function as a minimum reservoir layer. The applicant shall submit WWHM calculations showing the required ballast thickness is sufficient as a reservoir layer based on the design criteria applicable to the project. Projects required to comply Volume Chapter 2

39 City of Tacoma with Minimum Requirements #6, 7, or 8 shall submit WWHM calculations. Commercial projects where runon from adjacent surfaces may reach the permeable pavement surface shall submit WWHM calculations. Single/family duplex projects are not required to submit calculations unless the permeable pavement surface is being used to meet Minimum Requirement #6, 7, or 8. For pervious concrete sidewalks the minimum ballast thickness shall be 4 inches. Where runon is minimized to the pervious sidewalk sections, calculations are not necessary to determine if the reservoir layer is sufficient for stormwater management. If runon to the permeable sidewalk is allowed, additional modeling may be required by Environmental Services. Permeable ballast base course shall meet the requirements of WSDOT Standard Specifications Section (2) - Permeable Ballast except as modified by this section. The permeable ballast base course shall be seated or compacted until no visible movement of aggregate is observed. Immediately following spreading and final shaping each layer of surfacing shall be lightly compacted in one lift to a firm and unyielding condition. Permeable ballast base course shall not be manufactured from recycled concrete. The materials shall be uniform in quality and substantially free from wood, roots, bark, and other extraneous material and shall meet the following quality test requirements: Los Angeles Wear, 500 Rev 30% maximum, WSDOT Test Method T 96 Degradation Factor: 30 minimum, WSDOT Test Method T 113 Minimum Void Ration Content: 30% as determined by AASHTO T19 or ASTM C29, rodding procedure Table 6-7: Permeable Ballast Grading Requirement Sieve Size Percent Passing 1 2-1/2 inch inch /2 inch inch /2 inch 0-5 No No % Fracture 95 1.All percentages are by weight The fracture requirement shall be at least two fractured face and will apply the combined aggregate retained on the No. 4 sieve in accordance with FOP for AASHTO T335. Permeable ballast base course shall meet the requirements for grading and quality when placed in hauling vehicles for delivery to the site, after placement in temporary location, when in stockpiles on site, during installation, and after installation and in place after compacted to project specifications Subsurface Detention Structures For permeable pavements on slopes 3%, provide subsurface detention structures or terrace subgrade to increase infiltration, improve flow attenuation, and reduce structural issues with Chapter Volume 6

40 City of Tacoma subgrade erosion on slopes. See the Low Impact Technical Guidance Manual for Puget Sound for examples and design recommendations Wearing Layer The wearing course includes the actual driving surface and the asphalt treated permeable base layer. For permeable pavement roadways and accessways refer to City of Tacoma Standard Plan PD-01 for minimum wearing layer thickness. For private onsite permeable pavement surfaces, the minimum wearing layer thickness shall be 4 for porous asphalt and 6 for pervious concrete though actual thickness shall be based on projected traffic use. The maximum slope for porous asphalt and permeable concrete shall be 10%. A minimum initial infiltration rate of 20 inches per hour is required. High infiltration rates are recommended. For porous asphalt, products shall have adequate void space, commonly 16-25%. For pervious concrete, products shall have adequate void space, commonly, 15-35%. For permeable interlocking concrete pavement and aggregate pavers, pavement joints should be filled with No. 8, 89, or 9 stone. For grid/lattice systems filled with gravel, sand, or a soil of finer particles with or without grass, the fill material shall be at least a minimum of 2 inches of sand, gravel, or soil Permeable Pavement - Underdrain If an underdrain is placed at or near the bottom of the aggregate base in a permeable pavement design, the permeable pavement is not considered a low impact development technique and cannot be used to satisfy Minimum Requirement #5. Elevated underdrains that are placed within the aggregate base course to protect the pavement wearing course from saturation can be used to satisfy Minimum Requirement #5 though an underdrain is not required Drainage Conveyance Ensure there is an appropriate conveyance path to safely move water away from the road prism Acceptance Test For driveways or small permeable pavement sections (1000 square feet or less), test by pouring a bucket of water on the permeable surface. Additional testing prior to acceptance will be needed if there is runoff from the surface. For permeable pavement sections greater than 1000 square feet, test the initial infiltration rate using ASTM C1701. Test to determine if the minimum infiltration rate of 10 inches per hour is met. The number of test locations is specified in ASTM C Permeable Pavements Maintenance Per Minimum Requirement #9, an operation and maintenance plan shall be prepared for all stormwater facilities. See Volume 1, Appendix C for specific maintenance requirements. Maintenance shall be a basic consideration in design and costdetermination of the stormwater management facility. Any standing water or sediment removed during maintenance operation must be disposed of in a City approved manner. See the dewatering requirements in Volume 4 of Volume Chapter 2

41 City of Tacoma this manual. Pretreatment may be necessary. Solids must be disposed of in accordance with state and local waste regulations. Facilities shall be designed and constructed such that the facility can be safely and easily inspected by one person and safely and easily maintained. Follow manufacturer s suggestions for maintenance. Inspect project upon completion to correct accumulation of fine material. Conduct periodic visual inspections to determine if surfaces are clogged. Sweep non-planted surfaces with a high-efficiency sweeper twice per year, once in autumn and once in early spring. Sweeping frequency can be reduced if infiltration rate testing indicates that a rate of 10 inches/hour or greater is being maintained. The design engineer should consider the turning radius of the sweeper when designing permeable pavement surfaces to ensure the facility can be properly maintained Maintenance records shall be retained and provided to the City upon request Permeable Pavements Sizing Criteria and Flow Credits Modeling in WWHM is required to obtain flow credits and required for all projects except single/ family duplex and sidewalks that are only required to comply with Minimum Requirements #1-#5. Model the facility using the Permeable Pavement element BMP L634 Minimal Excavation Foundations Purpose and Definition Minimal excavation foundation systems are those techniques that minimize disturbance to the natural soil profile within the footprint of the structure. This preserves most of the hydrologic properties of the native soil. Pin foundations are an example of a minimal excavation foundation Applications Limitations Heavy equipment cannot be used within or immediately surrounding the building. Terracing of the foundation area may be accomplished by tracked, blading equipment not exceeding 650 lbs/ft Design Criteria See Chapter 6 of Low Impact Development: Technical Guidance Manual for Puget Sound for design information Flow Credits for Minimal Excavation Foundation Systems Modeling in WWHM is required to obtain flow credits. Where roof runoff is dispersed on the up gradient side of a structure in accordance with the design criteria in Downspout Dispersion, model the tributary roof area as pasture on the native soil. Where step forming is used on a slope, the square footage of roof that can be modeled as pasture must be reduced to account for lost soils. In step forming, the building area is terraced in cuts of limited depth. This results in a series of level plateaus on which to erect the form boards. The following equation can be used to reduce the roof area that can be modeled as pasture. Chapter Volume 6

42 City of Tacoma dc 0.5 A A dp 1 = A 2 Where: A 1 = roof area draining to up gradient side of structure dc = depth of cuts into the soil profile dp = permeable depth of soil (the A horizon plus an additional few inches of the B horizon where roots permeate into ample pore space of soil). A 2 = roof area that can be modeled as pasture on the native soil If roof runoff is dispersed down gradient of the structure in accordance with the design criteria and guidelines in BMP L603: Downspout Dispersion, and there is at least 50 feet of vegetated flowpath through native material or lawn/landscape area that meets the guidelines in Volume 3, Chapter/Section 4.1 BMP L613 Post-Construction Soil Quality and Depth on page 39, model the tributary roof areas using the lateral flow element to send impervious area runoff onto the lawn/landscaped area that will be used for dispersion BMP L635 Reverse Slope Sidewalks Definition and Purpose Reverse slope sidewalks are sloped to drain away from the road and onto adjacent vegetated areas Design Criteria for Reverse Slope Sidewalks There must be greater than 10 feet of vegetated surface downslope that is not directly connected into the storm drainage system. Vegetated area receiving flow from sidewalk must be native soil or meet the guidelines in Volume 3 Chapter/Section 4.1 BMP L613 Post-Construction Soil Quality and Depth on page Flow Credits for Reverse Slope Sidewalks Model the sidewalk area as landscaped area over the underlying soil type. Modeling in WWHM is required to obtain flow credits. Model the facility using the Lateral Flow Element to send impervious areas onto lawn areas for dispersion. Volume Chapter 2

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45 City of Tacoma Appendix A Recommended Modifications to ASTM D2434 When Measuring Hydraulic Conductivity for Bioretention Soil Mixes Developed by the City of Seattle in cooperation with local soils laboratories. Proctor method ASTM D1557 Method C (6-inch mold) shall be used to determine maximum dry density values for compaction of bioretention soil sample. Sample preparation for the Proctor test shall be amended in the following ways: 1. Maximum grain size within the sample shall be no more than ½ inches in size. 2. Snip larger organic particles (if present) into ½ inch long pieces. 3. When adding water to the sample during the Proctor test, allow the sample to pre-soak for at least 48 hours to allow the organics to fully saturate before compacting the sample. This pre- soak ensures the organics have been fully saturated at the time of the test. ASTM D2434 shall be used and amended in the following ways: 1. Apparatus: a. 6-inch mold size shall be used for the test. b. If using porous stone disks for the testing, the permeability of the stone disk shall be measured before and after the soil tests to ensure clogging or decreased permeability has not occurred during testing. c. Use the confined testing method, with 5- to 10-pound force spring d. Use de-aired water. 2. Sample: a. Maximum grain size within the sample shall not be more than ½ inch in size. b. Snip larger organic particles (if present) into ½-inch long pieces. c. Pre-soak the sample for at least 48 hours prior to loading it into the mold. During the pre-soak, the moisture content shall be higher than optimum moisture but less than full saturation (i.e., there shall be no free water). This pre-soak ensures the organics have been fully saturated at the time of the test. 3. Preparation of Sample: a. Place soil in cylinder via a scoop. b. Place soil in 1-inch lifts and compact using a 2-inch-diameter round tamper. Preweigh how much soil is necessary to fill 1-inch lift at 85% of maximum dry density, then tamp to 1-inch thickness. Once mold is full, verify that density is at 85% of maximum dry density (+ or 0.5%). Apply vacuum (20 inches Hg) for 15 minutes before inundation. c. Inundate sample slowly under a vacuum of 20 inches Hg over a period of 60 to 75 minutes. d. Slowly remove vacuum (> 15 seconds). e. Sample shall be soaked in the mold for 24 to 72 hours before starting test. Appendix A 6-43 Volume 6

46 City of Tacoma 4. Procedure: a. The permeability test shall be conducted over a range of hydraulic gradients between 0.1 and 2. b. Steady state flow rates shall be documented for four consecutive measurements before increasing the head. c. The permeability test shall be completed within one day (one-day test duration). Volume Appendix A

47 City of Tacoma Appendix B Green Stormwater Infrastructure Details Appendix B 6-45 Volume 6

48 Volume 6 City of Tacoma 6-46 Appendix B