GSI-CALC USER S MANUAL

Transcription

1 GSI-CALC USER S MANUAL Version 1.0 Kitsap County Public Works Department Prepared by Herrera

2 Note: Some pages in this document have been purposefully skipped or blank pages inserted so that this document will copy correctly when duplexed.

3 GSI-CALC USER S MANUAL GREEN STORMWATER INFRASTRUCTURE SIZING TOOL FOR WESTERN WASHINGTON LOWLANDS Prepared for Kitsap County Public Works Department 614 Division Street, MS-26 Port Orchard, Washington Prepared by Herrera Environmental Consultants, Inc Sixth Avenue, Suite 1100 Seattle, Washington Telephone: 206/ July 13, 2011 Final

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5 CONTENTS Acknowledgements... iii Funding... iii Project Partners... iii Introduction... 1 GSI-Calc Applicability... 1 Project Location... 2 Stormwater Management Standards... 2 Native Soil Infiltration Rates... 5 Drainage Area Landcover... 5 GSI BMPs... 6 GSI-Calc Development... 6 GSI Design Requirements... 9 Bioretention Facilities... 9 Permeable Pavement Trees Installing GSI-Calc System Requirements Downloading and Installing GSI-Calc Opening GSI-Calc Uninstalling GSI-Calc Using GSI-Calc Entering Site Information Entering Calculator Inputs Selecting Tree Credits Sizing Permeable Pavement Sizing Bioretention Saving Inputs Generating Project Reports Glossary References Disclaimers i jr gsi-calc user's manual

6 TABLES Table 1. BMP design configurations included in GSI-Calc Table 2. Precipitation ranges for each Western Washington Lowland Region included in GSI-Calc FIGURES Figure 1. GSI-Calc coverage and mean annual precipitation in western Washington Figure 2. Bioretention cell schematic Figure 3. Linear bioretention cell schematic Figure 4. Bioretention cell with underdrain schematic Figure 5. Bioretention planter schematic Figure 6. Low slope permeable pavement schematic Figure 7. Higher slope permeable pavement schematic Figure 8. Menu of GSI-Calc options by calculator Figure 9. Screen shot of GSI-Calc main screen Figure 10. Screen shot of forest duration flow control standard screen Figure 11. Screen shot of tree credit screen Figure 12. Screen shot of permeable pavement screen Figure 13. Screen shot of bioretention screen Figure 14. Example GSI-Calc project report jr gsi-calc user's manual ii

7 ACKNOWLEDGEMENTS Funding GSI-Calc was developed through a Washington State Department of Ecology Grant of Regional or Statewide Significance and administered by Kitsap County. Project Partners Kitsap County Thurston County Pierce County City of Seattle City of Bellevue City of Everett City of Issaquah City of Sammamish Home Builders Association of Kitsap County Herrera Environmental Consultants MGS Engineering Consultants, Inc. Washington State University Puyallup Research and Extension Center Final GSI-Calc User s Manual iii

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9 INTRODUCTION The Western Washington NPDES (National Pollutant Discharge Elimination System) Phase I and Phase II Municipal Stormwater Permits include several provisions related to low impact development (LID). GSI-Calc was developed to assist permittees in implementing these requirements by providing a regional sizing tool for LID stormwater Best Management Practice (BMP) design in western Washington lowlands (Figure 1). GSI-Calc allows sizing of LID BMPs, or green stormwater infrastructure (GSI), as a function of contributing impervious area, prevalent soil types in the region, representative site infiltration rates, and mean annual precipitation. This program is intended to assist developers and regulatory agency reviewers in sizing and designing LID BMPs without need for continuous simulation modeling, thereby reducing the barriers to the implementation of LID. The scope of this initial version of GSI-Calc was developed in conjunction with project partners and additional interested NPDES Phase I and Phase II permittees. The BMP types, configurations, and other sizing tool variables were prioritized to maximize the benefits of GSI-Calc given finite grant funding. Future versions of the GSI-Calc tool could be developed to expand the options and applicability of the tool to meet ongoing regional needs in stormwater management and LID implementation. To increase the utility of GSI-Calc, supplemental sizing equations were integrated from a recent effort conducted for Pierce County, increasing the types of BMPs included for the precipitation zones covering the Pierce County area. This User s Manual provides an explanation of the applicability of GSI-Calc, the design requirements for the GSI facilities included (e.g., required material specifications, side slopes, ponding depths, maximum contributing drainage areas) and step-by-step instructions on how to use the program. GSI-Calc Applicability GSI-Calc is applicable to projects that meet the following requirements: Site is located in the lowland areas of Western Washington Project is subject to one of the stormwater management standards included in GSI-Calc (i.e., Ecology predeveloped forest duration standard, Ecology water quality treatment standard, or Kitsap County predeveloped recharge standard) Native soil design infiltration rates greater than inches per hour for sites predominantly underlain by till, and greater than 0.5 inches per hour for sites predominantly underlain by outwash Drainage areas contributing runoff to BMPs are predominantly impervious Final GSI-Calc User s Manual 1

10 The GSI BMP design configurations included in GSI-Calc are appropriate for sitespecific stormwater management needs A more detailed description of the applicability and limitations of GSI-Calc is provided below. Project Location Mean annual precipitation in western Washington ranges from 24 inches west of central Puget Sound to more than 270 inches in the Olympic Mountain range (see Figure 1). Based on the precipitation data used to develop GSI-Calc (see the GSI-Calc Development section), the region is divided into four precipitation zones: the western central Puget Sound (Puget West), the eastern central Puget Sound (Puget East), the Vancouver/Castle Rock corridor, and the western coastal region. GSI-Calc covers the lowland areas of these western Washington regions (i.e., up to approximately 1,500 feet in elevation). The hydrology at higher elevations is affected by snow accumulation and snowmelt processes that are not accounted for in the continuous simulation models currently used for stormwater facility design in western Washington. The areas where altitudes exceed 1,500 feet (and where GSI-Calc is not applicable) are shaded in light gray in Figure 1. Additional areas are excluded from GSI-Calc because the mean annual precipitation is too high to reasonably extrapolate the sizing equations built into the program. These areas are hatched in purple in Figure 1. Stormwater Management Standards BMPs included in GSI-Calc were sized to meet up to three stormwater management performance standards: the Ecology forest duration standard, the Ecology water quality treatment standard, and/or the Kitsap County forest recharge standard. Bioretention and permeable pavement facilities were sized to meet Ecology s minimum requirement for flow control assuming a pre-developed forest landcover (referred to in this document as the forest duration standard). This standard requires matching flow durations from half of the 2-year to the 50-year recurrence interval flows to a pre-developed forest condition (on till or outwash soil). Because a similar sizing tool was developed for Kitsap County under a separate effort (Herrera 2010), an additional standard was used to size bioretention and permeable pavement in the Kitsap County area. The County requires maintaining the recharge (i.e., average annual volume of water that infiltrates to groundwater on a site) at or above pre-development levels. This standard (assuming a predeveloped forest cover on till or outwash soil) was included for the precipitation regions covering the county (i.e., Puget West precipitation zones from 32 to 80 inches). Bioretention facilities were also sized to achieve the Ecology water quality treatment requirement (i.e., facilities were sized to infiltrate 91 percent of all runoff volume for the period modeled). Bioretention facilities meet Ecology s basic and enhanced water quality treatment requirements when at least 91 percent of the total runoff volume is infiltrated 2 Final GSI-Calc User s Manual

11 Figure 1. GSI-Calc coverage and mean annual precipitation in western Washington. Final GSI-Calc User s Manual 3

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13 through soil meeting Ecology s treatment soil requirements (such as 18 inches of the City of Seattle bioretention soil mix). For bioretention facilities intended for treatment, Ecology also requires that the water quality volume (i.e., the 91st percentile, 24-hour runoff volume) infiltrate through the entire column of bioretention soil within 48 hours (Ecology 2005). Allowing the soil to periodically dry out is necessary to restore the hydraulic capacity of the system for subsequent storms, maintain infiltration rates, maintain adequate soil oxygen levels for healthy soil biota and vegetation, and provide proper soil conditions for treatment. This 48-hour drawdown period is exceeded for larger contributing areas when site soils are till and soil infiltration rates are or 0.25 inches per hour. GSI-Calc reports a warning message when the 48-hour drawdown period is exceeded. Native Soil Infiltration Rates The native soil design infiltration rates included in GSI-Calc vary by BMP, design configuration and soil type, as shown in Table 1. Up to three rates were evaluated for sites predominantly underlain by till soil (i.e., 0.125, 0.25, and 0.5 inches per hour 1 ). Up to three somewhat higher infiltration rates were evaluated for sites predominantly underlain by outwash soils (i.e., 0.5, 1, and 3 inches per hour 2 ). It is unnecessary to include design infiltration rates higher than 3 inches per hour for bioretention because the design infiltration rate for the imported bioretention soil is 3 inches per hour, and therefore will be limiting regardless of the underlying soil permeability. Similarly, the aggregate thickness necessary to meet flow control requirements for low slope permeable pavement with design infiltration rates exceeding 3 inches per hour becomes very small, often less that can be feasibly constructed. For sites with higher or intermediate design infiltration rates, the user must rounded down to the nearest rate included in GSI-Calc (e.g., for a site with a design rate of 0.75 inches per hour, 0.5 inches per hour must be selected). This will result in conservative facility sizing. Drainage Area Landcover GSI-Calc was developed to size BMPs for a 100 percent impervious contributing drainage area. Future versions may include the option to size BMPs for runoff from a mix of impervious and pervious drainage areas. In the meantime, if a drainage area includes both impervious and pervious surface areas, a BMP may be sized using GSI-Calc for the total contributing area (including pervious areas). In this case, the facility size will be conservatively large, because there is less runoff from pervious areas than impervious areas. 1 BMP design configurations included in the Piece County calculator were also sized for 1 and 2 inches per hour for sites predominantly underlain by till. 2 BMP design configurations included in the Piece County calculator were only sized for 4 inches per hour for sites predominantly underlain by outwash. Final GSI-Calc User s Manual 5

14 GSI BMPs GSI-Calc can be used to size the following GSI BMPs: Bioretention cell: 3H (horizontal):1v (vertical) side slopes with any bottom geometry Linear bioretention: 3H:1V side slopes with 2 foot bottom width Bioretention cell with underdrain: 3H:1V side slopes Bioretention planter with underdrain: vertical side slopes Low slope permeable pavement: up to 2 percent subgrade slope (no run-on from other areas) Higher slope permeable pavement: >2 to 5 percent subgrade slope with subsurface measures to create ponding in the aggregate storage layer (no run-on from other areas) Additional BMP configurations are included for the precipitation zones covering the Pierce County area, including: Bioretention cell: 4H (horizontal):1v (vertical) side slopes with any bottom geometry Low slope permeable pavement: up to 2 percent subgrade slope (may receive run-on from other areas) Higher slope permeable pavement: >2 to 5 percent subgrade slope with subsurface measures to create ponding in the aggregate storage layer (may receive no run-on from other areas) In addition to permeable pavement and bioretention BMPs, GSI-Calc includes impervious surface reduction credits for newly planted trees and retained trees. Table 1 presents these BMPs by design configuration (e.g., ponding depth), type of sizing performed (i.e., performance standards used for sizing), and infiltration rate evaluated. To use GSI-Calc, BMPs must meet the requirements provided in the GSI Design Requirements section below. It is important to note that the bioretention cells bottom area for is optimized for a square bottom geometry. Sizing for bioretention BMPs with other bottom geometries will be conservative. GSI-Calc Development To develop GSI-Calc, continuous simulation hydrologic modeling was conducted to evaluate a suite of bioretention and permeable pavement facilities relative to selected stormwater standards for the range of soil and climate conditions prevalent in the western Washington lowlands. Based on modeling results, simple mathematical relationships were developed that relate BMP performance to contributing impervious area, mean annual precipitation, soil type, and infiltration rate. The BMP sizing equations were folded into a computer program (GSI-Calc) that automates calculations and provides standardized output for design review submittals. 6 Final GSI-Calc User s Manual

15 BMP Retained Tree Newly Planted Tree Bioretention Cell (3H:1V Side Slopes) Bioretention Cell (Linear) (3H:1V Side Slopes) Bioretention Planter with Underdrain Bioretention Cell with Underdrain (3H:1V SS) Permeable Pavement Surface (with no run-on) Design Configuration Evergreen & Deciduous Evergreen & Deciduous 3-inch Ponding Depth 6-inch Ponding Depth 12-inch Ponding Depth 3-inch Ponding Depth 6-inch Ponding Depth 12-inch Ponding Depth 6- and 12-inch Ponding Depth 6- and 12-inch Ponding Depth 2% Subgrade Slope >2-5% Subgrade Slope with Subsurface Ponding Table 1. Forest Duration 1 Impervious Reduction Factor Impervious Reduction Factor Area Sizing Area Sizing Area Sizing Area Sizing Area Sizing Area Sizing BMP design configurations included in GSI-Calc. Sizing Type by Performance Standard Forest Recharge 2 Treatment 3 Impervious Reduction Factor Impervious Reduction Factor Area Sizing Area Sizing Area Sizing Area Sizing Area Sizing Area Sizing Design Infiltration Rate Included (inches per hour) Sites Predominantly Underlain by Till Soils Sites Predominantly Underlain by Outwash Soils N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A Area Sizing Area Sizing Area Sizing N/A N/A Area Sizing N/A N/A Area Sizing Aggregate Sizing Average Ponding Depth Aggregate Sizing Average Ponding Depth X X X 5 X X X X X 4 X 5 X X X X 4 X X X 5 X X X X X 4 4 X 5 X X X X N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A 4 X X X X X X 4 X X X X X X Final GSI-Calc User s Manual 7

16 BMP Bioretention Cell (4H:1V Side Slopes) 6 Design Configuration 2-inch Ponding Depth Table 1 (continued). Forest Duration 1 Area Sizing BMP design configurations included in GSI-Calc. Sizing Type by Performance Standard Forest Recharge 2 Treatment 3 4 Area Sizing Design Infiltration Rate Included (inches per hour) Sites Predominantly Underlain by Till Soils Sites Predominantly Underlain by Outwash Soils X 7 X X 6-inch Ponding Depth Area Sizing 4 Area Sizing 4 X 7 X X 12-inch Ponding Depth Area Sizing 4 Area Sizing 4 X 7 X X Permeable Pavement Surface (with run-on) 6 2% Subgrade Slope >2-5% Subgrade Slope with Subsurface Ponding Aggregate Sizing Average Ponding Depth 4 4 X 7 X 7 X ,8 4 4 X 7 X 7 X ,8 X included in GSI-Calc not included in GSI-Calc N/A not applicable 1 BMPs sized for the Ecology forest duration standard (i.e., match flow durations from half of the 2-year to the 50-year recurrence interval flow to a predeveloped forest condition). 2 BMPs sized for the Kitsap County forest recharge standard (i.e., maintaining the average annual volume of water that infiltrates at a predeveloped forest level). This standard is only included for the precipitation regions covering Kitsap County (i.e., Puget West precipitation zones from 32 to 80 inches). 3 BMPs sized for the Ecology water quality treatment standard (i.e., infiltrate 91 percent of the total runoff volume). 4 These BMP design configurations are not included in this version of GSI-Calc due to budget limitations, but may be included in future versions. 5 For these ponding depth and infiltration rate combinations the surface pool drawdown time is 48 hours, exceeding Ecology s recommended maximum of 24 hours. 6 These BMP design configurations are included for precipitation zones covering the Pierce County area only (i.e., Puget West 40 to 56 on till, Puget East 32 to 56 on till and Puget East 40 to 56 on outwash). 7 Design infiltration rates of 1 and 2 inches per hour are also included for this BMP design configuration when the site is predominantly underlain by till. 8 A design infiltration rate of 4 inches per hour is included when the site is predominantly underlain by outwash. 8 Final GSI-Calc User s Manual

17 Modeling was performed using MGSFlood Version 4. The range of rainfall depths and patterns in western Washington lowlands were represented by an extended precipitation and evaporation time series developed by MGS Engineering Consultants, Inc. (MGS 2002), and supplemented for this project (MGS 2010). Detailed modeling methods and quality assurance measures are described in Draft Low Impact Development Best Management Practice Simplified Sizing Tool for Western Washington Lowlands GIS-Calc (Herrera 2011a) and LID Best Management Practice Simplified Sizing Tool for Pierce County (Herrera 2011b). In addition to bioretention and permeable pavement facilities, GSI-Calc includes flow control credits for newly planted and retained trees. These credits were developed based on a literature review of infiltration, evapotranspiration, and interception of evergreen and deciduous trees (Herrera 2008), and discussions with the City of Seattle and Ecology. If approved for use by the user s local jurisdiction, the impervious surface reduction credits in GSI-Calc may be applied to the total site impervious area, thereby reducing the total impervious area that must be mitigated with engineered flow control facilities. GSI Design Requirements To use GSI-Calc, BMPs must meet the design requirements outlined in this section. Additional LID BMP siting and design requirements not presented in this manual will also apply, such as infiltration rate testing requirements and correction factors, setbacks, and vertical separation between the bottom of the facility and the underlying water table. LID design requirements for the state are presented in the Ecology Stormwater Management Manual for Western Washington (Ecology 2005). Supplemental design resources for GSI (e.g., recommended construction specifications) are available in the City of Seattle Stormwater Flow Control and Water Quality Treatment Technical Requirements Manual (Seattle 2009). Additional state-of-the-practice design guidance for LID BMPs will be forthcoming in 2011 in an update to the Low Impact Development Technical Guidance Manual for Puget Sound (WSU 2005) being prepared by the Washington State University Cooperative Extension for the Puget Sound Partnership. Designers must be familiar with local requirements and regional best practices to ensure LID BMPs are designed, constructed, and operated per applicable requirements. Bioretention Facilities Bioretention facilities and rain gardens are shallow depressions with a specified soil mix and plants adapted to the local climate and soil moisture conditions. The healthy soil structure and vegetation promote infiltration, water storage, and slow release of stormwater flows to more closely mimic natural conditions. When used to achieve the forest duration or forest recharge standard, bioretention facilities must not have an underdrain to intercept infiltrated runoff or an impermeable liner that impedes infiltration to underlying soil. Four facility design variations were evaluated and included in the GSI-Calc tool: bioretention cells, linear bioretention, bioretention cells with underdrains, and bioretention planters. Each is discussed in detail below. Final GSI-Calc User s Manual 9

18 Bioretention Cell Bioretention cells are earthen depressions with 3H:1V side slopes (Figure 2). In order to use GSI-Calc, the following bioretention cell design requirements must be met: Bioretention bottom area 3,4 shall be sized using GSI-Calc, and must be a minimum of 4 square feet. The drainage area contributing runoff to an individual bioretention facility shall be no larger than 5,000 square feet 5,6. Bottom area shall be flat (0 percent slope). Side slopes within the ponded area shall be no steeper than 3H:1V 7. Imported bioretention soil mix per City of Seattle specifications shall be used. This specification is posted on the Seattle Public Utilities (SPU) green stormwater infrastructure website at This soil mix meets Ecology infiltration treatment soil requirements, has a design infiltration rate of 3.0 inches per hour, and has 40 percent porosity. Because imported bioretention soil is used, the design infiltration rate of the underlying native soil does not require a correction factor (i.e., the design, or longterm infiltration rate selected may be the same as the initial infiltration rate). For design infiltration rates that are higher than or intermediate to those included in GSI-Calc, the designer must rounded down to the nearest rate available (e.g., for a site with a design rate of 0.75 inches per hour, the designer would use 0.5 inches per hour). Such rounding will result in conservative facility sizing. Bioretention soil depth shall be a minimum of 12 inches for flow control, and a minimum of 18 inches for water quality treatment. No underdrain or impermeable layer shall be used. Minimum ponding depth shall be as specified (i.e., 3, 6, or 12 inches). For deeper or intermediate ponding depths, the designer must round down to the nearest depth 3 Top area (total facility footprint) will be larger than the bottom area, and can be calculated as a function of the bottom area, the side slopes, and the total facility depth (e.g., ponding and freeboard depth. 4 Bottom area is optimized for a square bottom geometry. Sizing for bioretention with other bottom geometries will be conservative. 5 Contributing drainage area limitations are applicable to individual bioretention facilities sized using the tool. For larger contributing areas, the designer may split the area into multiple drainages (less than 5,000 square feet) routed to separate facilities or size a single facility with an approved continuous hydrologic model. 6 Bioretention cells with 4H:1V side slopes are also available for precipitation zones covered by Pierce County (Puget West 40 to 56 on till, Puget East 32 to 56 on till, and Puget East 40 to 56 on outwash). 7 Sizing for bioretention with flatter side slopes will be conservative. 10 Final GSI-Calc User s Manual

19 included in GSI-Calc (e.g., for a ponding depth of 8 inches, the designer would use 6 inches of ponding). Such rounding will result in conservative sizing. Additional design requirements presented in the Stormwater Management Manual for Western Washington (Ecology 2005) may also apply. Figure 2. Bioretention cell schematic. In addition to the requirements listed above, Ecology recommends a maximum surface pool drawdown time of 24 hours (Ecology 2005). Because some jurisdictions are more flexible with drawdown limitations, two of the ponding depth and infiltration rate combinations evaluated (6-inch ponding with inches per hour and 12-inch ponding with 0.25 inches per hour) result in higher surface pool drawdown periods (i.e., 48 hours). GSI-Calc will report a warning message when the recommended 24-hour surface drawdown period is exceeded. Linear Bioretention Linear bioretention is a linear earthen depression with 3H:1V side slopes and a bottom width of 2 feet (Figure 3). For linear bioretention, all requirements presented for bioretention cells apply, except as noted below: Bottom width shall be 2 feet. Minimum ponding depth shall be as specified (i.e., 3, 6, or 12 inches). For deeper or intermediate ponding depths, the designer must round down to the nearest depth included in GSI-Calc. Final GSI-Calc User s Manual 11

20 Figure 3. Linear bioretention cell schematic. Bioretention with Underdrain This configuration consists of a bioretention cell (earthen depression with 3H:1V side slopes) with an underdrain (Figure 4). For this BMP, all requirements presented for bioretention cells apply, except as noted below: Underdrain must be used (City of Seattle specifications for underdrain and aggregate blanket are recommended). Impermeable layer below facility may be used. Minimum ponding depth shall be as specified (i.e., 6 or 12 inches). For deeper or intermediate ponding depths, the designer must round down to the nearest depth included in GSI-Calc. Bioretention soil depth shall be a minimum of 18 inches (BMP is applicable for water quality treatment only). 12 Final GSI-Calc User s Manual

21 Figure 4. Bioretention cell with underdrain schematic. Bioretention Planter A bioretention planter has vertical side slopes and an underdrain (Figure 5). For bioretention planters, all requirements presented for bioretention cells apply, except as noted below: Underdrain must be used (City of Seattle specifications for underdrain and aggregate blanket are recommended). Impermeable layer below facility may be used. Side slopes within the ponded area may be vertical. Top area (total facility footprint) will be the same as the bottom area if vertical walls are used. Minimum ponding depth shall be as specified (i.e., 6 or 12 inches). For deeper or intermediate ponding depths, the designer must round down to the nearest depth included in GSI-Calc. Bioretention soil depth shall be a minimum of 18 inches (BMP is applicable for water quality treatment only). Final GSI-Calc User s Manual 13

22 Figure 5. Bioretention planter schematic. Permeable Pavement Permeable pavement allows rainfall to percolate into an underlying aggregate storage reservoir, where stormwater is stored and infiltrated to underlying soil. A permeable pavement system consists of a pervious wearing course (e.g., porous asphalt concrete, porous cement concrete, paver blocks, or open-celled paving grids) and an aggregate sub-base course installed over native soil. The design requirements for permeable pavement vary depending upon subgrade slope. Installations on a sloped subgrade have an increased potential for lateral flow through the aggregate storage reservoir along the top of the lower permeability subsurface soil. For subgrade slopes less than or equal to 2 percent, it is assumed that this lateral flow is negligible and that the full depth of the storage reservoir aggregate provides storage. For subgrade slopes exceeding 2 percent, lateral flow cannot be neglected and subsurface features (e.g., check dams) are required to ensure ponding in the aggregate storage reservoir. The permeable pavement BMPs included in GSI-Calc for most western Washington lowland areas are designed to manage only the water that falls upon it (i.e., they are not intended to take significant stormwater run-on from other areas). The exception to this is the precipitation zones covered by Pierce County (Puget West 40 to 56 on till, Puget East 32 to 56 on till, and Puget East 40 to 56 on outwash) where permeable pavement may receive run-on from additional impervious areas up to 200 percent of the pavement facility area. Two permeable pavement design configurations are included in GSI-Calc: low slope permeable pavement surfaces (up to 2 percent subgrade slope; Figure 6) and higher slope permeable pavement surfaces (>2 to 5 percent subgrade slope; Figure 7). As explained above, where subgrade slopes exceed 2 percent, the sub-base must be designed to create subsurface ponding within the aggregate course to detain subsurface flow and increase infiltration. 14 Final GSI-Calc User s Manual

23 Ponding may be accommodated using design features such as terracing berms (check dams) or intermittent infiltration trenches. Figure 6. Low slope permeable pavement schematic. Figure 7. Higher slope permeable pavement schematic. For the aggregate sizing results in GSI-Calc to be used, the following design requirements must be met for low slope permeable pavement surfaces (up to 2 percent subgrade slope): Final GSI-Calc User s Manual 15

24 The minimum aggregate depth below the wearing course required for stormwater management performance shall be determined using GSI-Calc. The aggregate depth shall be the greater of the depth reported by GSI-Calc or the minimum aggregate required for the design loading (or other design considerations). The total depth of aggregate shall be fully below the surrounding ground surface elevation on all sides of the facility. The infiltration rate that the user enters into GSI-Calc shall be the design, or longterm, rate, and must be calculated using correction factors (safety factors) per the Stormwater Management Manual for Western Washington (Ecology 2005). Based on conversations with Ecology staff, a minimum correction factor of 2 is recommended. For design infiltration rates that are higher than or intermediate to those included in GSI-Calc, the designer must rounded down to the nearest rate available (e.g., for a site with a design rate of 0.75 inches per hour, the designer would use 0.5 inches per hour). Such rounding will result in conservative facility sizing. Aggregate shall have a minimum void volume of 20 percent. No underdrain or impermeable layer shall be used. The pavement surface shall not receive stormwater run-on from other areas. Additional design requirements presented in the Stormwater Management Manual for Western Washington (Ecology 2005) may also apply. For higher slope permeable pavement surfaces (up to 5 percent subgrade slope), all requirements presented for low slope permeable pavement surfaces apply, except as noted below: The average subsurface ponding depth 8 within the aggregate storage reservoir required for stormwater management performance shall be determined using GSI-Calc. Subsurface ponding must be created using design features such as terracing berms (e.g., check dams). The aggregate depth shall be the greater of the depth required for subsurface ponding or the minimum aggregate required for the design loading or other design considerations). Trees Trees provide flow control via interception, evapotranspiration, and increased infiltration. Additional environmental benefits include improved air quality, carbon sequestration, reduced heat island effect, pollutant removal, and habitat preservation or formation. When implemented in accordance with the criteria outlined below, retained and newly planted trees can provide credit toward meeting stormwater flow control requirements. The degree 8 The depth specified is the average depth of aggregate in which ponding occurs (assuming a void volume of 20 percent). 16 Final GSI-Calc User s Manual

25 of flow control provided by a tree depends on the tree type (i.e., evergreen or deciduous), canopy area, and whether or not the tree canopy overhangs impervious surfaces. For the impervious surface reduction credits in GSI-Calc to apply, the following requirements must be met for both retained and newly planted trees: Trees shall be retained, maintained, and protected on the site after construction and for the life of the development, or until any approved redevelopment occurs in the future. Trees that are removed or that die shall be replaced with like species during the next planting season (typically in autumn). Trees shall be pruned according to industry standards. Tree credits are not applicable to trees in areas used for flow dispersion or other flow control credit. Trees must be on the development site and within 20 feet of new or replaced ground level impervious surfaces (e.g., driveway, patio, or parking lot). Distance from impervious surfaces is measured from the edge of the surface to the center of the tree at ground level. Trees planted in planter boxes are not eligible for flow control credit. The total tree credit for retained and newly planted trees shall not exceed 25 percent of the total impervious surface area requiring mitigation. The following additional requirements apply to retained trees: Trees must be viable for long-term retention on the site (i.e., in good health and compatible with proposed construction and site plans). Retained trees shall be a minimum of 6 inches in diameter at breast height (DBH). DBH is defined as the outside bark diameter at 4.5 feet above the ground on the uphill side of a tree. For existing trees smaller than this, the newly planted tree credit may be applied (see below). The retained tree canopy area shall be measured at the time of permit application as the area within the tree drip line. A drip line is the line encircling the base of a tree, which is delineated by a vertical line extending from the outer limit of a tree's branch tips down to the ground. If trees are clustered, overlapping canopies are not double counted. The existing tree roots, trunk, and canopy shall be fenced and protected during construction activities to avoid damage to the tree. The following additional requirements apply to newly planted trees: New deciduous trees shall be at least 1.5 inches in diameter measured 6 inches above the ground. New evergreen trees shall be at least 4 feet tall. Approved tree species are listed in the Seattle Green Factor Tree List, available via link from the SPU GSI web site Trees must be Medium/Small or larger. Final GSI-Calc User s Manual 17

26 Mature tree height, size, and rooting depth must be considered to ensure that the tree location is appropriate given adjacent and above- and below-ground infrastructure. To help ensure tree survival and canopy coverage, the minimum tree spacing for newly planted trees shall accommodate the mature tree size. In no circumstance shall flow control credit be given for new tree density exceeding 10 feet on center spacing (i.e., trees spaced closer than 10 feet on center). Provisions shall be made for supplemental irrigation during the first three growing seasons after installation to help ensure tree survival. 18 Final GSI-Calc User s Manual

27 INSTALLING GSI-CALC System Requirements GSI-Calc can be installed on any of the following Windows operating systems: Windows XP Service Pack 3 Windows XP x64 Edition Service Pack 2 Windows Server 2003 R2 Service Pack 2 Windows Vista Service Pack 2 Windows Server 2008 Service Pack 2 or R2 Windows 7 Your computer needs to have an Intel or AMD x86 processor supporting the SSE2 instruction set (which includes most processors commonly in use). Free disk space required is 1 gigabyte. GSI-Calc s RAM requirement is 1,024 megabytes, though at least 2,048 megabytes are recommended for best performance. It is also recommended that your computer have a screen size of at least 900 (width) by 750 (height) pixels to properly view GSI-Calc screens. GSI-Calc interfaces with Microsoft Excel for generating project reports. To generate project reports with Excel, your computer needs to have Excel 2003, 2007, or 2010 installed. GSI-Calc also interfaces with Google Earth to automate precipitation data inputs. To use this feature, the latest version of Google Earth should be installed on your computer (and the computer should be restarted after Google Earth installation). Google Earth can be downloaded ( If Google Earth is not installed on your computer, you can manually enter the precipitation data for your site. Downloading and Installing GSI-Calc The instructions for downloading and installing GSI-Calc are provided below. 1) Download GSI_Calc_Installer.exe to a folder you create called GSI-Calc Version 1 in your Documents or My Documents folder or another folder on a local drive having read/write access. [Note: If for any reason you have to reinstall GSI-Calc, download GSI_Calc_Installer.exe to a newly created folder. Do not attempt to reinstall GSI-Calc into the folder you used on previous installations.] 2) When GSI_Calc_Installer.exe has finished downloading to your newly created folder, double click to run it (you will need administrator privileges to successfully run the Final GSI-Calc User s Manual 19

28 installer executable). You should see a DOS window popup, similar to the one pictured below. It describes all of the files that are being decompressed. 3) When the installer has finished decompressing files, it will start to install the required libraries for GSI-Calc. First, it will install the Visual C++ runtime libraries if you do not already have these on your machine. The image below shows what this looks like (the dialog may vary slightly from operating system to operating system). You can select Install without a need to highlight either of the Visual C++ runtimes shown. 4) Next, the installer for the Matlab Compiler Runtime (MCR) will be started automatically. GSI-Calc is written in Matlab and the MCR is a requirement to run GSI-Calc. When the MCR installer starts, it will ask for your installation language. 20 Final GSI-Calc User s Manual

29 Select a language and then accept all the default options as the installer proceeds. The first MCR installer screenshot is shown below. 5) If you have already installed a previous version of GSI-Calc or any other software that depends on Matlab Compiler Runtime 7.15, you may see the screen pictured below during your installation. If so, select the Modify radio button and click on the Next button, and the installer will complete successfully. 6) When the MCR installer is complete, you should see a screen similar to the one pictured below: Final GSI-Calc User s Manual 21

30 7) Once you see that the MCR installer is complete, the DOS window will be closed for you, and you are ready to run GSI-Calc. Your folder should look like the image below. At this point, optionally you can delete or move GSI_Calc_Installer.exe, _install.bat, and MCRInstaller.exe. Never move the folders shown in the image below, and do not rename any files in those folders, or GSI-Calc may not work. 22 Final GSI-Calc User s Manual

31 8) If you ever need to reinstall GSI-Calc, create a different folder name in Documents, My Documents, or elsewhere with read/write access on your local drive. Download GSI-Calc-Installer.exe to that folder, and double-click to run from that new folder. Do not attempt to reinstall GSI-Calc into a folder used for previous installations, or it will not function properly. Opening GSI-Calc To run GSI-Calc, double click GSI_Calc.exe in the folder that you created in Documents or My Documents. Note there is a delay in starting GSI-Calc.exe after you double click it (between 45 to 90 seconds on most systems), and there is not a splash screen while you wait. Please wait for it to load the necessary libraries and open instead of double clicking GSI-Calc.exe again. Uninstalling GSI-Calc 1) Navigate to your machine s Control Panel and then to Add/Remove Programs (on Windows XP) or to Programs and Features (on Windows 7). Find Matlab Compiler Runtime in the list of programs and features installed on your machine. The image below shows what this looks like on Windows 7. From this screen, highlight Matlab Compiler Runtime 7.15 and select Uninstall. You do not want to do this if you have other programs that depend on Matlab Compiler Runtime 7.15 (e.g., Aquarius). Final GSI-Calc User s Manual 23

32 2) Optional Step: Delete your GSI-Calc Version 1 folder, but first move out any project files that you would like to keep out of the Project Files directory. 24 Final GSI-Calc User s Manual

33 USING GSI-CALC This section provides step-by-step instructions on how to use GSI-Calc, including: Entering Site Information Entering Calculator Inputs Selecting Tree Credits Sizing Permeable Pavement Sizing Bioretention Saving Inputs Generating Project Reports Every screen of GSI-Calc includes a button with a? symbol on the top right corner. The user may click this button for guidance on using the program. Figure 8 shows the menu of GSI-Calc options by calculator. Final GSI-Calc User s Manual 25

34 Figure 8. Menu of GSI-Calc options by calculator. 26 Final GSI-Calc User s Manual

35 Entering Site Information This section provides step-by-step guidance for entering site information on the main screen (Figure 9) of GSI-Calc. Information entered on this screen includes precipitation data, soil type, site area requiring mitigation, the applicable stormwater management standard, and calculator selection Figure 9. Screen shot of GSI-Calc main screen. Step 1 Enter Precipitation Information Enter precipitation information including the precipitation zone and the depth of mean annual precipitation (in inches) for your site. Table 2 shows the ranges of mean annual precipitation depth by region. There are two options for this step: Option 1: Use Google Earth to automatically enter the precipitation information for your site location. To do this, select the Google Earth button, zoom into your site location, return to GSI-Calc and select the Accept Location button. Note that the Google Earth map displays the western Final GSI-Calc User s Manual 27

36 Washington precipitation region bounds for reference. In addition, higher elevation areas for which the program is not applicable are shaded in light gray and cannot be selected. Table 2. Precipitation ranges for each Western Washington Lowland Region included in GSI-Calc. Western Washington Lowland Region Mean Annual Precipitation Range in GSI-Calc (inches) Western Central Puget Sound (Puget West) 18 to 100 Eastern Central Puget Sound (Puget East) 22 to 85 Vancouver/Castle Rock 40 to 85 Western Coastal 70 to 120 Option 2: Manually enter the precipitation zone and the depth of mean annual precipitation for your site. Refer to Figure 1 in this manual or the Map button on the upper right corner of the screen. Step 2 Enter Soil Type Enter the predominant soil type for your site. The options include outwash (Type A/B) and till (Type C). This will specify the predeveloped soil condition associated with the forest duration and forest recharge standards. Refer to the Ecology Stormwater Management Manual for Western Washington (Ecology 2005)for methods to determine the soil type and infiltration rate at your site. Step 3 Enter Project Area Requiring Mitigation (Optional) Enter the new plus replaced impervious area and pollution-generating impervious surface area requiring mitigation (refer to the Glossary section for definitions). These fields are optional, but can help you to track if you are meeting the stormwater management standard for the total area requiring mitigation. Step 4 Select Applicable Standard Select the applicable set of standards for your site. The options include Ecology, Pierce County, and Kitsap County. See Figure 8 for the menu of performance standard and BMP design configuration options. The Ecology set of standards is applicable for all western Washington lowlands and will be applicable to most sites. The Kitsap County set of standards is only applicable for the precipitation zones covering Kitsap County (i.e., Puget West precipitation zones from 32 to 80 inches). This calculator includes an additional stormwater performance standard (i.e., the forest recharge standard). The Pierce County set of standards is only applicable for the precipitation zones covering the Pierce County area (i.e., Puget West precipitation zones from 40 to 56 on till, and Puget East precipitation zones from 32 to 56 on till and 40 to 56 on outwash). This calculator includes 28 Final GSI-Calc User s Manual

37 alternative BMP design configurations (i.e., bioretention cells with 4H:1V side slopes and permeable pavement sized to receive run-on from other impervious areas). Step 5 Select Calculator Select the calculator that you would like to use by clicking one of the yellow-shaded buttons: The forest duration standard (matching flow durations from 50 percent of the 2-year to the 50-year recurrence interval flow to a predeveloped forest condition) The forest recharge standard is applicable to Kitsap County (maintaining the average annual volume of water that infiltrates at a predeveloped forest level) The water quality standard (infiltrating 91 percent of the average annual runoff volume) Once you select a calculator, a new window will open. Entering Calculator Inputs This section provides step-by-step guidance for entering inputs on the main calculator screen (Figure 10) Figure 10. Screen shot of forest duration flow control standard screen. Final GSI-Calc User s Manual 29

38 Step 1 Enter Project Description (Optional) Enter a short project description in this box. This description will be included in the Project Report. Step 2 Review Site Information Site information entered on the main screen is displayed in this box. Select the Revise button to update information. Step 3 Select GSI Facilities Select the type of GSI facility that you would like to size. You can size multiple facilities and this screen will provide a tally of the total area mitigated. The yellow-shaded buttons will become green-shaded when GSI facility information has been entered. Step 4 Review Total Area Mitigated After selecting and sizing GSI facilities (see Selecting Tree Credits, Sizing Permeable Pavement and Sizing Bioretention sections below), return to this screen to review the area mitigated by GSI facility type and the total area mitigated for the site. Iteratively design site GSI facilities until the desired area is mitigated. Step 5 Generate Project Report (Optional) When GSI facility sizing is complete, you may select Generate Project Report under the dropdown File menu in the upper left-hand corner of the GSI-Calc window. Selecting Tree Credits This section provides step-by-step guidance for selecting impervious surface reduction credits for newly planted and retained trees (see Figure 11). Step 1 Select Newly Planted Trees Enter the number of new evergreen and deciduous trees that will be planted as part of your project (new trees must meet detailed requirements, see Step 4). Step 2 Select Existing Trees Enter the number and/or canopy area of existing evergreen and deciduous trees that will be retained as part of your project (retained trees must meet detailed requirements, see Step 4). Step 3 Review Tree Credits Refer to the maximum area that may be mitigated using tree credits. This is calculated as 25 percent of the area requiring flow control mitigation. The total area mitigated by trees is also listed. 30 Final GSI-Calc User s Manual

39 Step 4 Review Tree Requirements Select the Requirements button for a list of detailed requirements that must be met to get credit for newly planted and retained trees. Step 5 Select OK When tree credit selection is complete, select OK to return to the main calculator screen Figure 11. Screen shot of tree credit screen. Sizing Permeable Pavement This section provides step-by-step guidance for entering permeable pavement information including design infiltration rate, run-on area, permeable pavement area, and notes (see Figure 12). Step 1 Select Design Infiltration Rate Enter the design infiltration rate for the native soil underlying the permeable pavement BMP. The long-term design rate must be determined using the methods outlined in the Ecology manual (Ecology 2005), and include the application of a correction factor. The infiltration rates included in GSI-Calc vary by standard, BMP design configuration and soil type (see Table 1). To be conservative, design infiltration rates for the native soils must be rounded Final GSI-Calc User s Manual 31

40 down to the nearest rate included in GSI-Calc (e.g., for a site with a design rate of 0.75 inches per hour, 0.5 inches per hour must be selected) Figure 12. Screen shot of permeable pavement screen. Step 2 Enter Run-on Area This option is currently only available for Pierce County set of standards (see Entering Site Information section). If applicable, enter the additional impervious area contributing run-on to the permeable pavement facility, up to a maximum of 200 percent of the permeable pavement area. Step 3 Enter Permeable Pavement Area Enter the permeable pavement area. Step 4 Enter Notes (Optional) Enter identifying characteristics (e.g., location, type of permeable pavement) for each permeable pavement area. Notes will be included in the Project Report. 32 Final GSI-Calc User s Manual

41 Step 5 Review Aggregate Depth Refer to the screen to see what aggregate depth is required. For low slope permeable pavement BMPs, this will be the minimum aggregate depth below the wearing course required for stormwater management performance. The design aggregate depth shall be the greater of the depth reported by GSI-Calc or the minimum aggregate required for the design loading or other design considerations. For higher slope permeable pavement BMPs, this will be the average subsurface ponding depth within the aggregate storage reservoir required for stormwater management performance. Subsurface ponding must be created using design features such as terracing berms (e.g., check dams). The aggregate depth shall be the greater of the depth required for subsurface ponding or the minimum aggregate required for the design loading or other design considerations. The minimum aggregate depth reported by GSI-Calc is 0.3 feet. Step 6 Review Area Mitigated Refer to the screen to see how much area is mitigated. For configurations with no run-on, this will be equal to the permeable pavement area. Step 7 Review Design Requirements Select the Requirements button for a list of detailed design requirements that must be met to use the aggregate depths reported by GSI-Calc. Step 8 Select OK When permeable pavement sizing is complete, select OK to return to the main calculator screen. Sizing Bioretention This section provides step-by-step guidance for entering bioretention information including facility shape, ponding depth, design infiltration rate, bottom area, and notes (see Figure 13). Step 1 Select Facility Shape This option is currently only available for bioretention cells with 3H:1V side slopes sized for the forest duration standard. Select your preferred facility shape based on site constraints. A linear bioretention cell must have a bottom width of 2 feet. The non-linear bioretention cell may be used for any other bottom geometry. It is important to note that the non-linear sizing assumes a square bottom area and will result in conservative sizing for other bottom geometries. Step 2 Select Ponding Depth Select the ponding depth. Ponding depths included in GSI-Calc vary by standard, BMP design configuration and soil type (see Table 1). For deeper or intermediate ponding depths, Final GSI-Calc User s Manual 33

42 rounded down to the nearest depth included in GSI-Calc (e.g., for a ponding depth of 8 inches, select 6 inches of ponding). This will result in conservative sizing Figure 13. Screen shot of bioretention screen. Step 3 Select Design Infiltration Rate Select the design infiltration rate for the native soil underlying the permeable pavement BMP. The long-term design rate must be determined using the methods outlined in the Ecology manual (Ecology 2005), and include the application of a correction factor. The infiltration rates included in GSI-Calc vary by standard, BMP design configuration, and soil type (see Table 1). To be conservative, design infiltration rates for the native soils must be rounded down to the nearest rate included in GSI-Calc (e.g., for a site with a design rate of 0.75 inches per hour, 0.5 inches per hour must be selected). Step 4 Size Bioretention Bottom Area Iteratively adjust the facility size (bioretention bottom area) until the desired contributing area is mitigated. The minimum bioretention bottom area is 4 square feet. An individual bioretention cell can mitigate a maximum of 5,000 square feet of contributing impervious area. When GSI-Calc reports a mitigated area of >5000 square feet, the program 34 Final GSI-Calc User s Manual

43 will give you a warning message suggesting that you reduce your bioretention bottom area to optimize your facility size. The program will also give you a warning message if either the recommended surface pool drawdown period of 24 hours or the maximum facility drawdown period of 48 hours is exceeded. Step 5 Enter Notes (Optional) Enter identifying characteristics (e.g., location, type of bioretention facility) for each bioretention cell. Notes will be included in the Project Report. Step 6 Review Design Requirements Select the Requirements button for a list of detailed design requirements that must be met to use the bioretention areas reported by GSI-Calc. Step 7 Select OK When bioretention sizing is complete, select OK to return to the main calculator screen. Saving Inputs To save your work, select Save from the dropdown File menu in the upper left-hand corner of the GSI-Calc window. This will allow you to return to your file at a later time for reference or revisions. Generating Project Reports To generate a report, select Generate Project Report from the dropdown file menu in the upper left-hand corner of the GSI-Calc window. All program inputs and outputs will be exported into an Excel template (Figure 14). Final GSI-Calc User s Manual 35

44 Figure 14. Example GSI-Calc project report. 36 Final GSI-Calc User s Manual