EPA Smart Growth
Getting Started
The National Green Values™ Calculator is a tool for quickly comparing the performance, costs, and benefits of Green Infrastructure, or Low Impact Development (LID), to conventional stormwater practices. The GVC is designed to take you step-by-step through a process of determining the average precipitation at your site, choosing a stormwater runoff volume reduction goal, defining the impervious areas of your site under a conventional development scheme, and then choosing from a range of Green Infrastructure Best Management Practices (BMPs) to find the combination that meets the necessary runoff volume reduction goal in a cost-effective way.
A few important points to keep in mind:
- The National GVC is currently focused on runoff volume reduction. It does not produce any peak flow results. Volume reduction in this context implies infiltration, evapotranspiration and reuse, and does not include detention in ponds or vaults. All runoff volume captured in BMPs is assumed to be kept on site.
- The National GVC is meant for a single site or a campus of buildings contained on a single site. If you are interested in looking at the performance and cost/benefit analysis of Green Infrastructure BMPs applied on a neighborhood or watershed scale, consider using the original GVC and/or some of the other stormwater tools provided below.
To get started, select a tab at the top to enter site information. Default values (that can always be changed by the user) are provided throughout the calculator, so you can begin on any step. However, we recommend starting on the Lot Information page and proceeding through each step. Below is a brief description of the information you can provide on each page:
- Lot Information
- Site location by zip code, which defines the average precipitation for your site. The user can override the average precipitation values if desired, e.g. if the user knows and wants to enter the amount of precipitation for the 2-year 24-hour design storm.
- Site size
- Site Hydrologic Soil Group (A-D)
- Predevelopment
- Site predevelopment conditions, meaning either the state of the site before a particular project is built, or the pre-human development condition of the site. The user provides the percent area of the site covered by different possible land uses, e.g. lawn, trees, impervious, etc.
- Runoff Reduction Goal
- Runoff volume reduction goal. The user can choose from a list of regulations requiring different quantities of precipitation and/or runoff to be infiltrated, evapotranspirated, or reused on site.
- Conventional Development
- Conventional development site conditions, including roof size, parking lot area, and street, sidewalk, and driveway area.
- Green Improvements
- Green Infrastructure BMP options. The user can select from a wide range of BMPs to determine which combination will best meet runoff reduction goals and budget considerations.
- Advanced Options
- Lifecycle. The user can choose to analyze the lifecycle cost and benefits of green and conventional stormwater systems over 5 to 100 years.
- Discount rate. The user can set the amortization rate which affects how the dollar value of maintenance and future replacement costs affects lifecycle costs.
- Curve numbers. The user can choose to use the default curve numbers provided in the calculator for various soil groups or fill in their own curve number values.
Results
The results statement at the bottom of the page continuously updates to reflect site conditions and BMPs defined by the user. The user does NOT need to click on a “Calculate” button to see the affect of different choices. The results are presented in detail on separate tabs. They include progress toward the specified runoff volume reduction goal, changes in the total runoff volume annually and for the average storm event for the predevelopment, conventional, and green scenarios, changes in the land use between the conventional and green scenarios, and detail on the lifecycle cost/benefit analysis.
For more information on the conceptual, hydrologic, and financial basis of this tool, please see the methodology link provided below.
Download Methodology
Other Resources
Original Green Values Stormwater Calculator
EPA Office of Water Managing Wet Weather with Green Infrastructure
* Required fields.
+ Must have at least one of these fields filled in.
Enter the site zip code to view the annual average precipitation and the 90% storm for your site. You can input any precipitation quantity into these values if you are interested in looking at the total runoff volume produced by each scenario under different precipitation conditions then those determined here. For example if you know and the amount of precipitation for a particular design storm and are interested in seeing total runoff volume changes under those conditions, enter that value here as either the annual or 90% storm and see the results for that same category.
Rainfall estimates may not match other reported standard values. Please consult local regulatory authorities when designing or implementing projects. Please see the Green Values methodology for details on rainfall calculation.
* Required fields.
+ Must have at least one of these fields filled in.
This land cover includes paved parking lots, roofs, driveways, and paved streets.
This category includes open space like lawns, parks, golf courses, and cemeteries. It includes landscaped areas that are not solely turf grass, e.g. ornamental landscaping and non-dense trees. Good condition is indicated by a grass or vegetated cover greater than 75%.
This category includes open space like lawns, parks, golf courses, and cemeteries. It includes landscaped areas that are not solely turf grass. Fair condition is indicated by a grass or vegetated cover between 50-75%.
This category is not included in the TR-55 tables, but is a frequently found predevelopment condition of urban sites. It includes a mix of sparse grass (less than 50%), weeds, gravel, and compacted soils.
A landscaping definition commonly found in Western desert areas.
This category applies to pervious areas only and implies areas with no vegetation.
This category is meant to describe agricultural fields being converted to land for development. Good condition implies that a variety of conditions encourage better than average infiltration and decrease runoff.
This category is for any grassland or range in continuous use for foraging and grazing. Fair condition means that the area has 50-75% ground cover and is not heavily grazed.
This category implies continuous grassland or open space that is protected from grazing. If defining predevelopment conditions as prior to human inhabitant, this category would be a good descriptor for plains or natural grass areas.
This category implies an area of trees protected from grazing with adequate brush and forest litter coverage of the soil. This category can also be used to describe predevelopment conditions prior to human habitation.
This category can be used to define undeveloped land in arid and semiarid regions. It covers open space with vegetation like saltbrush, mesquite, and cactus.
This category can be used to define an undeveloped portion of the site, or site conditions prior to human habitation. It covers wetland areas, as well as swamp and marsh lands.
This category can describe any natural or man-made water features found on the site. Water features have a very high curve number, similar to impervious surfaces.
Local stormwater regulations may define the "predevelopment" condition as the existing condition of the site prior to the development project in question, OR as the original condition of the site prior to any human development (e.g. forest or meadow). Either definition can be satisfied here by defining the different land cover types represented on your site for either predevelopment condition. Check with local authorities to determine which definition you should use to define the predevelopment scenario in this calculator.
Most of the land cover categories below are chosen from the USDA Soil Conservation Service Technical Release 55 (TR-55): Urban Hydrology for Small Watersheds Table 2.2a-d. We tried to provide a short list of land cover categories that can be used in combination to describe a variety of sites around the country, and fit both interpretations of predevelopment. The land cover category is associated with a curve number (either a default value or user-specified) and the area assigned to each land cover type creates a composite curve number for the site under predevelopment conditions.
* Required fields.
+ Must have at least one of these fields filled in.
Select a runoff reduction goal from the following drop-down list. The selected value defines the amount of precipitation (0.5 – 1.5 inches) over the impervious area of the site that must be infiltrated, evapotranspirated, and reused on site with Green Infrastructure BMPs.
The example list on the right demonstrates how these goals are included in stormwater ordinances across the country. Click on any of the examples to open a brief summary of their language and a source where they can be found.
- National Standards
- The Energy Independence Security Act (ESIA). Section 438. Stormwater Runoff Requirements for Federal Development Projects. Option 1
Option 1: Design, construct, and maintain stormwater management practices that infiltration, evapotranspiration, and/or reuse the precipitation from the 95th percentile rainfall event. Applies to any development or redevelopment project involving a Federal facility with a footprint that exceeds 5,000 square feet.Flow Regulations Presentation, U.S. EPA, 06/07
- LEED ND Green Technology and Construction Credit 9: Stormwater Management
1 - 5 certification credits available depending on how much precipitation from the impervious area is infiltrated, evapotranspired, or reused in arid, semi-arid, and humid watersheds. For semi arid (between 20”-40” rain/year) the minimum amount is 0.225 inch and the max is 1.125 inches. The maximum of all categories is 1.5 inches
- The Energy Independence Security Act (ESIA). Section 438. Stormwater Runoff Requirements for Federal Development Projects. Option 1
- State and Local Standards
- California: LA County Public Works
LA County offers two stormwater options for development and redevelopment in unincorporated areas. Option 1 requires that small residential projects include at least two GI BMPs. Examples include pourous pave on 50% of driveway surface,a rain garden, rain barrels, and a green roof on 50% of roof. Option 2 requires large residential and non-residential projects to infiltrate or reuse the excess volume of runoff resulting from 0.75 inches of precipitation on the post-development site compared to the pre-development site.
- California: Los Angeles Regional Water Quality Control Board for Ventura County
Control pollutants, pollutant loads, and runoff volume emanating from impervious surfaces through percolation, infiltration, storage, or evapo-transpiration, by reducing the percentage of Effective Impervious Area to less than 5 percent of total project area
- Connecticut: Town of Tolland
Bioretention, Filtration, Infiltration, Level Spreaders, and permeable pavement BMPs are required to provide enough volume storage capacity for the 1” rainfall event.
- Connecticut: Town of Deep River
All developments shall be designed to the extent practicable with the goal of no-net run-off from the site. That is, the volume of run-off from the site after development shall not, to the extent practicable, exceed the volume of site run-off prior to the proposed development. Infiltration of stormwater shall be accomodated to the extent possible through limitation of land disturbance and grade changes, retention of existing natural drainage area and wetlands, and creation of vegetated islands, vegetated medians, and vegetated perimeter buffer strips.
- Illinois: City of Chicago
The City of Chicago Stormwater Ordinance requires that sites disturbing more than 7500 sf retain 0.5 inches of precipitation over the impervious area on the site or reduce the site impervious area by 15%.
- Massachusetts Department of Environmental Protection
Stormwater Management Standard 3 requires that the annual recharge from the post-development site shall approximate the annual recharge from pre-development conditions based on soil type. Loss of annual recharge to groundwater shall be eliminated or minimized through the use of infiltration measures including environmentally sensitive site design, low impact development techniques, stormwater best management practices, and good operation and maintenance.
- Minnesota: Ramsey-Washington Metro Watershed District
For sites that disturb 1 acre or more, stormwater runoff volume reduction shall be achieved onsite in the amount equivalent to the runoff generated from a one inch rainfall over the impervious surfaces of the development.
- New Hampshire: Town of Thornton
An option for meeting the Storm Treatment goal of providing 80% removal of TSS and 40% removal of phosphorous is to provide a mechanism for infiltrating 3600 cf of runoff (one acre-inch) per acre of impervious surface. Drainage designs must also demonstrate that the average annual recharge rate found on site are maintained in order to preserve existing groundwater table elevations.
- New Jersey Department of Environmental Protection
For new major development, the design engineer shall, either: (a) Demonstrate through hydrologic and hydraulic analysis that the site and its stormwater management measures maintain 100 percent of the average annual pre-construction groundwater recharge volume for the site; or (b) Demonstrate through hydrologic and hydraulic analysis that the increase of stormwater runoff volume from pre-construction to post-construction for the 2-year storm is infiltrated.
- North Carolina Department of Environment and Natural Resources
For residential development disturbing less than 1 acre, stormwater runoff from all built upon areas of the site from the first 1.5 inches of rain shall be controlled using a combination of rain barrels/cisterns, permeable pavements, and rain gardens.
- Ohio Environmental Protection Agency: Big Darby Creek Watershed
The Big Darby Creek Watershed (near Columbus) construction general permit requires that post-development groundwater recharge be equal to or exceed the pre-construction groundwater recharge.
- Oregon: Portland Bureau of Environmental Services
Stormwater must be infiltrated onsite to the maximum extent feasible, before discharging any flows offsite. Vegetated and subsurface infiltration facilities should be used to completely infiltrate the 10-year storm (3.4 inches of rainfall over 24 hours). If this cannot be accomplished due to site constraints, an offsite discharge point must be identified.
- Oregon: City of Gresham
Green Development Practices, including rain gardens, stormwater planters, porous pavements, and tree planting must be used to manage 100% of the stormwater runoff from private property. The Master Plans also require that all new streets incorporate Green Street rain gardens and stormwater planters to manage stormwater prior to discharge off-site.
- Pennslyvania Department of Environmental Protection
For any size of regulated activity, do not increase the post-development total runoff volume for all storms equal to or less than the 2-year/24-hour event.For projects less than 1 acre at least the first 1 inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow — i.e. it shall not be released into the surface Waters of this Commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration.
- West Virginia Department of Environmental Protection
Site design standards for all new and redevelopment that require, in combination or alone, management measures that infiltrate, evapotranspirate and reuse of, at a minimum, the first one inch of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation. This first one inch of rainfall must be 100% managed with no discharge to surface waters. For projects that cannot meet 100% of the infiltration/evapotranspiration/reuse requirement on-site, two alternatives are available: off-site mitigation and payment in lieu.
- California: LA County Public Works
* Required fields.
+ Must have at least one of these fields filled in.
* Required fields.
+ Must have at least one of these fields filled in.
The Green Infrastructure BMPs included below can provide runoff reduction benefits through infiltration, evapotranspiration, and reuse of captured stormwater for irrigation and other non-potable uses. Green infrastructure BMPs provide additional environmental benefits including carbon sequestration, reduced energy use, and groundwater recharge in addition to reduced construction and maintenance costs and extended design life. Experiment with applying different combinations of BMPs the see how you can progress towards meeting the specified runoff reduction goal, reduce total runoff volume from the site (annually and for the average storm), reduce site imperviousness, and affect lifecycle costs and benefits.
Click on each BMP title below to see a description of the BMP, as well as some common assumptions on their design and construction.
Green Roof
Green Roof, Peggy Notebaert Nature Center
Green roofs are vegetated roof covers, with growing media and plants taking the place of bare membrane, gravel ballast, shingles or tiles. Green roofs help to both absorb and evapo-transpirate stormwater runoff, as well as delay and filter runoff flows that exceed the roofs runoff storage capacity. Green roofs have a variety of design options, including extensive and intensive and modular or non-modular.
Green roofs also reduce building energy use and provide the resulting reduction in carbon emissions. This reduction is due to the green roofs’ natural thermal insulation properties— structures are cooler in summer and warmer in winter.
The National GVC uses a default depth of 6 inches of growing media.1 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Design Guidelines for Green Roofs” Canada Mortgage and Housing Corporation. http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/design_guidelines_for_green_roofs.pdf
Planter Boxes (disconnect downspout)
Planter Box Illustration, The Low Impact Development Center
Planter boxes are elevated structures that intercept, store and filter stormwater runoff from disconnected downspouts. Planter boxes reduce stormwater runoff volumes through ponding, infiltration, and evapotranspiration. Runoff is retained and stored in the soil substrate and intercepted by plants which evapo-transpire moisture. Planter boxes are most commonly used in urban areas adjacent to buildings and along sidewalks.
The National GVC uses a default depth of 12 inches of soil media and 12 inches of gravel aggregate.1 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Green Development Practices for Stormwater Management”, City of Gresham, Oregon http://www.greshamoregon.gov/city/city-departments/environmental-services/watershed-management/template.aspx?id=3288
Rain Gardens (disconnect downspout)
Rain Garden in Chicago, CNT
Rain gardens are bowl-shaped gardens filled with native plants. Runoff from a disconnected roof downspout collects in the bowl and infiltrates into the soil with the help of the long-rooted native plants. Rain gardens are a simple and affordable technique that communities can use to reduce damage from flooding and drainage overflow. The native plants in rain gardens provide valuable wildlife habitat and reduce irrigation needs and maintenance costs in comparison to traditional forms of landscaping. The National GVC uses a default depth of 18 inches of amended soil substrate and 12 inches of gravel aggregate.1 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Green Development Practices for Stormwater Management”, City of Gresham, Oregon http://www.greshamoregon.gov/city/city-departments/environmental-services/watershed-management/template.aspx?id=3288
Cisterns/Rain barrels (disconnect downspout)
Cistern in Lake County, IL
Rain Barrel
Rain barrels and cisterns are containers that collect stormwater runoff from a roof through a disconnected downspout. Rain barrels and cisterns help to reduce stormwater runoff and protect water quality by capturing runoff on-site and keeping it out of the sewers. They also contribute to water conservation. Landscape irrigation can account for 38% of a building’s water use.1 Residences with a rain barrel or larger facilities with a cistern can eliminate the use of potable water for irrigation.
- “Existing Buildings: Operations and Maintenance Reference Guide”, U.S. Green Building Council, August 2008
Native Vegetation
Residential landscaping with native plants, CNT
Native vegetation refers to plant species that were growing naturally in an area before humans introduced plants from distant places. They evolved to survive in the soil, moisture, and weather conditions of a particular location and are vigorous and hardy, with an increased ability to survive winter cold and summer heat. They are resistant to most pests and diseases. Once established, they require no irrigation or fertilization, thereby increasing water conservation through reduced irrigation and protecting water quality by eliminating the lawn pollutants that otherwise are carried into local water bodies via stormwater runoff. Native plants provide food and shelter for birds, butterflies and other desirable wildlife, allowing biodiversity to thrive even in highly urbanized areas. Many help to enrich the soil and their root systems help rainfall percolate into the soil, reducing erosion and runoff. The diversity of native plants includes interesting flowers and foliage. Native shrubs and trees provide a variety of heights, shapes and textures in the landscape and contribute to defining a unique sense of place for any landscape.1
- Wild Ones, www.for-wild.org
Vegetated Filter Strips
Vegetated Filter Strip, California Stormwater Quality Association
Vegetated filter strips are low-angle vegetated slopes designed to treat sheet flow runoff from adjacent impervious areas. Filter strips function by slowing runoff velocities, filtering out sediment and other pollutants, and providing some infiltration into underlying soils. Filter strips were originally used as an agricultural treatment practice, but have recently been used in more urban and suburban locations. They differ slightly from buffer strips, which are natural vegetated areas alongside streams and lakes; buffer strips are left undisturbed for habitat protection and visual screening, while filter strips are altered areas designed primarily for stormwater management. Like many other LID techniques, vegetated filter strips can add aesthetic value to development. They cost significantly less than conventional stormwater infrastructure and also provide a convenient and effective area for snow storage and treatment.
The National GVC uses a default depth of 12 inches of amended soil substrate.1 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Low Impact Development for Big Box Retailers”, The Low Impact Development Center Inc. November 2005 http://www.lowimpactdevelopment.org/lid%20articles/bigbox_final_doc.pdf
Amended Soil
Courtesy of Stormwater Magazine
Amended soil refers to the practice of amending the existing soil at a site by adding compost and rototilling it in. Other options include importing a topsoil mix with 8% to 13% soil organic matter and placing it on the surface of existing soil that is too rocky, compacted, or poorly drained to amend effectively. A developer can also stockpile the site’s existing soil and reapply it when the disturbance work is completed. In cases where native soil exists, the forest duff layer and topsoil must be stockpiled and reapplied separately, and no soil amendment is required. For disturbed soil, the site soil is reapplied, and then the soil must be amended with compost.
The National GVC uses a default depth of 8 inches of amended soil.1 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Setting Soil Standards With the Future in Mind”, Stormwater Magazine, Jan-Feb 2008
Roadside Swales (elimination of curb and gutter)
Street Edge Alternatives, Seattle Public Utilities
Bioswales are gently sloped ditches vegetated with native plants that reduce runoff volumes through infiltration and evapo-transpiration. This category of bioswale is located next to roads and used in lieu of curbs and gutters to infiltrate and convey runoff from the road surface. Bioswales can absorb low flows or carry runoff from heavy rains to storm sewer inlets or directly to surface waters. The majority of annual precipitation comes from frequent, small rain events that produce quantities of runoff that can be completely infiltrated by a bioswale. However, in the event of a heavy storm, they also provide an alternative to conventional curb, gutter, and sewer design that slows the flow of runoff from large rain events into the sewer system. Bioswales improve water quality by infiltrating the first flush of storm water runoff and filtering the large storm flows they convey.
The National GVC uses a default depth of 24 inches of amended soil and aggregate.1 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Pennsylvania Model Ordinance for Crum Creek”, Pennsylvania Department of Environmental Protection http://www.depweb.state.pa.us/southeastro/lib/southeastro/storm_model_ordinance_crumcreekrev.pdf
Swales in Parking Lot
Parking Lot Bioswale, CNT
Bioswales are gently sloped ditches vegetated with native plants that reduce runoff volumes through infiltration and evapo-transpiration. This category of bioswale refers is located in a parking lot to absorb runoff from the lot. Bioswales can absorb low flows or carry runoff from heavy rains to storm sewer inlets or directly to surface waters. The majority of annual precipitation comes from frequent, small rain events that produce quantities of runoff that can be completely infiltrated by a bioswale. However, in the event of a heavy storm, they also provide an alternative to conventional curb, gutter, and sewer design that slows the flow of runoff from large rain events into the sewer system. Bioswales improve water quality by infiltrating the first flush of storm water runoff and filtering the large storm flows they convey.
The National GVC uses a default depth of 24 inches of amended soil and aggregate.1 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Pennsylvania Model Ordinance for Crum Creek”, Pennsylvania Department of Environmental Protection http://www.depweb.state.pa.us/southeastro/lib/southeastro/storm_model_ordinance_crumcreekrev.pdf
Trees
Trees provide multiple environmental and economic benefits in urban, suburban, and rural settings. Trees help to intercept rainfall in their leafy canopies, allowing for the water to evaporate without ever becoming runoff. In addition, trees provide open space around their bases where runoff can be infiltrated. Trees can significantly reduce energy use and the resulting carbon emissions through shading buildings and reducing the urban heat island effect. Trees also contribute to reducing air pollution and carbon sequestration. Trees can contribute significantly to raising property value and appreciate in value over time.1 Trees have also been shown to reduce crime by increasing open space even in heavily urbanized areas.2
Tree Box Filters
Tree Box Filter, Reno Nevada
Tree box filters are areas designed to provide trees with a healthy amount of space and soil in which to grow. This soil can be designed as “structural soil” and have specific void space and pollutant removal properties. The soil around street trees can be compacted during the construction of paved surfaces and minimized as underground utilities encroach on root space. If tree roots are surrounded by compacted soils or are deprived of air and water by impervious streets and sidewalks, their growth will be stunted, their health will decline, and their expected life span will be cut short. By providing adequate soil volume and a good soil mixture, the runoff infiltration benefits of street trees can be maximized.
The National GVC uses a default depth of 4 feet of structural soil in a tree box filter.3 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- Benefits of Trees, Tree Care Information, Trees are Good http://www.treesaregood.com/treecare/tree_benefits.aspx
- Kuo, Frances and William Sullivan ”Environment and Crime in the Inner City: Does Vegetation Reduce Crime?” Environment and Behavior, Vol. 33 No. 3, May 2001 343-367 https://webs.aces.uiuc.edu/herl/docs/environment&crime.pdf
- “Tree Box Filter Fact Sheet”, University of New Hampshire Stormwater Center, http://www.unh.edu/erg/cstev/fact_sheets/tree_filter_fact_sheet_08.pdf
Reduced Street Width
Many urban and suburban streets are oversized for their typical uses in order to meet code requirements for emergency service vehicles and provide a free flow of traffic. Common street designs with parking on both sides that meet standard codes can require a width of at least 34 feet. Several states have created guidelines for accommodating emergency vehicles while minimizing street widths, including solutions like alternative street parking configurations, vehicle pullout space, connected street networks, prohibiting parking near intersections, and smaller block lengths.1 Reducing street widths results in significantly less impervious area when applied throughout a city or region, provides space for other green infrastructure to be included in the streetscape, and provides additional benefits in the form of walkable neighborhoods, decreased traffic congestion, and increased pedestrian safety.
The National GVC uses a default 15% reduction in street width. The user can specify any percent reduction of street width, as determined by project constraints or local regulation.
- “Green Streets Opportunities: Managing Wet Weather with Green Infrastructure Municipal Handbook”, The Low Impact Development Center, December 2008
Permeable Pavement on Parking, Driveways, and Sidewalks
Permeable Paver Parking Lot, Colorado Association of Stormwater and Floodplain Managers
Permeable pavement systems are hardscape materials that allow liquids to infiltrate through void spaces in the material, or through open spaces between components of the material, while maintaining the functionality of an impervious surface. The permeable pavement materials used in the National GVC include: pavers, porous asphalt, porous concrete, and gravel. Concrete pavers are solid, but are installed with gaps between them that allow water to pass through to the base. Porous asphalt and concrete are similar to their impervious counterparts but are designed to have reduced fines and typically have a special binder added that allow them to maintain their strength while increasing their void spaces. All of the permeable pavement systems have an aggregate base in common which provides structural support, runoff storage, and pollutant removal through filtering and adsorption.1 Gravel surfaces with an aggregate base can maintain increased infiltration and provide a cost-effective drainage solution in appropriate circumstances.
The National GVC uses a default depth of 8 inches of aggregate.2 The user can specify any depth of subsurface storage but will incur additional cost if the soil media and/or aggregate exceed the default values.
- “Green Streets Opportunities: Managing Wet Weather with Green Infrastructure Municipal Handbook”, The Low Impact Development Center, December 2008
- “Pervious Concrete Pavement Design Requirements” pg 4, City of Noblesville Department of Engineering, http://www.cityofnoblesville.org/egov/docs/1206053715_753143.pdf
* Required fields.
+ Must have at least one of these fields filled in.
The values provided here are used in the financial and total runoff volume calculations. You can either use these defaults or input new values. Many of the curve number values are from the USDA Soil Conservation Service Technical Release 55 (TR-55): Urban Hydrology for Small Watersheds Table 2.2a-d. See the link below for more information on that resource. The curve numbers for the Green Infrastructure BMPs are taken from the original version of the GVC and from industry literature. Some of the BMPs are not included on the curve number list because they provide solely a volume capture benefit, e.g. rain barrels and cisterns, or they are directly related to another land cover that is included on the table, e.g. the Reduce Street Width BMP is reducing the amount of street cover that has a defined curve number of 98.
