site design measures - flowstobay...synthetic fertilizers, your drainage design may route the runoff...

Chapter

44 Site Design Measures In this Chapter:

How si t e des ign measures can reduce s tormwater treatment measure s ize Tree preservat ion and plant ing Reducing the s ize o f impervious areas Sel f - treat ing areas On-si te water s torage Technical guidance for green roo fs , pervious paving, unit pavers & turf b lock

Site design measures for water quality protection are used to reduce the project’s impact on water quality and beneficial uses. Site design measures are not treatment measures. Including site design measures in a project does not meet the C.3 requirements for stormwater treatment, but it can help reduce the size of treatment measures (see Section 4.1). Site design measures can be grouped into two categories:

Site design measures that preserve sensitive areas and high quality open space, and

Site design measures that reduce impervious surfaces in a project. This chapter focuses on site design measures that reduce impervious surfaces, which can reduce the amount of stormwater runoff that will require treatment. This translates into smaller treatment measures than would have been required without the site design measures. Site design measures are also important in minimizing the size of any required hydromodification management measures for the site. Site design measures to reduce stormwater runoff can be incorporated in your project in the following ways: SSiittee ddeessiiggnn mmeeaassuurreess

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Preserve existing trees and vegetation.

Use self-treating areas.

Reduce the size of impervious features in the project.

Use cisterns or rain barrels to store rainwater onsite.

Use alternative surfaces that allow infiltration into the soil.

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Where landscaped areas are designed to have a stormwater drainage function, it is important that they be installed and maintained without the use of fertilizers and pesticides. Consult Bay Friendly Landscaping (see Resources) and the Planting Guidelines in Appendix B. Landscaped areas with stormwater drainage functions also need to be carefully integrated with other landscaping features on the site early in project design. This may require coordinating separate designs prepared by different professionals.

Remember that any site design measures (including self-treating areas) used to reduce the size of stormwater treatment measures must not be removed from the project without a corresponding resizing of the stormwater treatment measures. For this reason, your municipality may require you to include site design measures in the maintenance agreement or maintenance plan for stormwater treatment measures, or otherwise record them with the deed. Depending on the municipality, site design measures may be subject to periodic operation and maintenance inspections. Check with the municipal staff regarding the local requirements.

4.1 Tree Preservation and Planting Trees perform a variety of functions that reduce runoff volumes and improve water quality. Leaf canopies intercept and hold rainwater on the leaf surface, preventing it from reaching the ground and becoming runoff. Root systems create voids in the soil that facilitate infiltration. Trees also absorb and transpire large quantities of groundwater, making the soil less saturated, which allows more stormwater to infiltrate. Through the absorption process, trees remove pollutants from stormwater and stabilize them. Finally, tree canopies shade and cool paved areas. The following characteristics will determine how effectively a tree performs the functions described above: Persistent foliage Canopy spread Longevity Growth rate Drought tolerance Tolerance to saturated soils Resistance to urban pollutants (both air and water borne) Tolerance to poor soils Root pattern and depth Bark texture Foliage texture Branching texture Canopy density

Soil volume, density, and compost, along with appropriate irrigation the first three years, are important to tree performance. Other aspects that influence how street trees perform

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S A N M A T E O C O U N T Y W I D E W A T E R P O L L U T I O N P R E V E N T I O N P R O G R A M

include resistance to exposure (wind and heat) and resistance to disease and pest infestations.

When planting trees, particularly along streets where space is limited and roots may damage hard surfaces, consider the use of structural soils. Structural soil is a new medium, consisting of a stone skeleton structure for strength and clay soil for water retention, which allows urban trees to grow under pavement. The structural soil system creates a load-bearing matrix with voids filled with soil and air, essential for tree health. This allows for greater tree growth, better overall heath of trees, and reduced pavement uplifting by tree roots. The voids that benefit the tree roots also provide increased stormwater storage capacity, allowing tree pits in paved areas to serve as a series of small detention basins. See www.asla.org/latis/pdf/Structural_soils_update081202.pdf for more information on structural soils.

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4.2 Using Self-Treating Areas Some portions of your site may provide “self-treatment” if properly designed, drained and maintained. Such areas may include conserved natural spaces, landscaped areas, green roofs, and areas paved with turf block. These areas are considered “self-treating” because infiltration and natural processes that occur in these areas remove pollutants from stormwater.

As long as the self-treating areas are not used to receive runoff from other impervious areas on the site, and are installed and maintained without the use of pesticides or quick-release synthetic fertilizers, your drainage design may route the runoff from self-treating areas directly to the storm drain system or other receiving water. Consult Appendix B for guidance on using Bay-friendly landscaping and integrated pest management to avoid the use of pesticides and quick-release synthetic fertilizers. Stormwater runoff from the self-treating areas is kept separate from the runoff from paved and roofed areas of the site, which requires treatment.

Even vegetated areas will generate some runoff. If the runoff from self-treating areas is not kept separate from the runoff from impervious surfaces, your stormwater treatment measure must be hydraulically sized to treat runoff from both the self-treating areas and the impervious areas. Figure 4-1 compares the size of the stormwater treatment measure that would be required to treat the runoff from a site, depending on whether or not the runoff from a self-treating area discharges directly to the storm drain system or other receiving water. In the first (upper) sequence, runoff from the self-treating area is directed to the stormwater

treatment measure. In the second (lower) sequence, runoff from the self-treating area bypasses the treatment measure and flows directly to the storm drain system or other receiving water, resulting in a smaller volume of stormwater that will require treatment. This results in a smaller stormwater treatment measure.

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Figure 4-1: Self-Treating Area Usage (Source, BASMAA, 2003)

Figure 4-2 compares the conventional drainage approach to the self-treating area approach. The conventional approach combines stormwater runoff from landscaped areas with the runoff from impervious surfaces. Assuming the parking lot storm drain leads to a treatment measure, in the conventional approach, the treatment measure will need to be sized to treat runoff from the entire site. The self-treating area approach routes runoff from appropriately designed and maintained landscaped areas directly to the storm drain system. In this approach, the treatment measure is sized to treat only the runoff from impervious areas.

Conventional Drainage Approach Self-Treating Area Approach

Figure 4-2: Commercial/Industrial Site Compared to Same Site with Self-Treating Areas (Source, BASMAA 2003)

Figure 4-3 (below) shows an example site in which the runoff from impervious areas must flow to the stormwater treatment measure before discharging to the storm drain, while runoff from the self-treating area may discharge directly to the storm drain. This is allowable because the self-treating area does not accept runoff from the impervious areas on the site.

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Figure 4-3: Schematic Drainage Plan for Site with a Self-Treating Area (Source: ACCWP, 2006)

4.3 Reducing the Size of Impervious Areas A variety of project features can be designed so that they result in a smaller “footprint” of impervious surface. These techniques generally need to be incorporated very early in the project design. A number of techniques for reducing impervious surfaces are described below.

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Alternative Site Layout Techniques Check with your local jurisdiction about its policies regarding the following site design measures:

Reduce building footprints by using compact, multi-story structures, as allowed by local zoning regulations.

Cluster buildings to reduce the length of streets and driveways, minimize land disturbance, and protect natural areas.

Design narrow streets and driveways, as allowed by the local jurisdiction.

Using sidewalks on only one side of the street may be appropriate in areas with little pedestrian and vehicular traffic, as allowed by the local jurisdiction.

Minimize Surface Parking Areas A variety of techniques can be used to minimize surface parking areas, in terms of the number and size of parking spaces, as allowed by the local jurisdiction. These solutions focus on either reducing the demand for parking, maximizing efficient use of parking space,

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or implementing design solutions to reduce the amount of impervious surface per parking space.

Reduce parking demand by separating the cost of parking from the cost of housing or leasable space. This allows the buyer or tenant to choose how much parking they actually need and are willing to pay for.

Maximize efficient use of parking space with shared parking that serves different land uses that have different times of peak demand. For example, an office use with demand peaks during the day can share parking with restaurants, where demand is greatest during the evening, and to some extent residential uses, where demand peaks are in the evenings, nights and on weekends.

Parking structures can be an efficient way to reduce the amount of impervious surface needed for parking. Structured parking can be integrated with usable space in buildings that also house office or residential space, or include ground-floor retail lining the street. Shared parking strategies can work very well with structured parking.

Parking lifts are another way to reduce the amount of impervious surface needed for parking. A parking lift stacks two to three cars using a mechanical lift for each surface space. They can be operated manually by residents or employees, or by a valet or parking attendant. With proper training for residents, employers, or parking attendants, this strategy can be a practical way to double or triple the parking capacity given a set amount of land.

Another way to maximize the efficient use of parking area is valet parking, where attendants park cars much closer than individual drivers would in the same amount of parking space.

Figure 4-4: Parking Lifts in Parking Garage, Berkeley (photograph courtesy of City of Berkeley)

4.4 On-Site Water Storage Water storage systems collect rainwater from roofs and other impervious building surfaces, and store it so it may be released soon after a storm, or used for irrigation and other non-potable uses. As allowed by the local jurisdiction, these systems can be used to reduce the amount of stormwater that must be treated while attenuating peak flows and possibly conserving potable water. Water storage systems in proximity to the building may be subject to approval by the building official. The use of waterproofing as defined in the building code may be required for some systems, and the municipality may require periodic inspection. Check with municipal staff for the local jurisdiction’s requirements.

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Rainwater storage systems connect to a building’s gutters and downspouts and convey the water to storage vessels, such as rain barrels or above- or below-ground cisterns. In “metered detention and discharge,” the collected stormwater is slowly released into the landscape beds in the hours following the storm, at a rate that allows for better filtration and is less taxing to the community storm drain. As allowed by the local jurisdiction, harvested rainwater may be used for toilet flushing, car washing, washing machines, and chlorinated swimming pools. For rainwater to serve as useful irrigation in the Bay Area, it may need to be stored until the dry season, which would require more storage capacity. Water storage systems should include preventative measures for pollutants and mosquito control. The initial rainfall of any storm often picks up the most pollutants from dust, bird droppings and other particles that accumulate on the roof surface between rain events. A roof washer device separates the dirtier, early rainfall and diverts it so that it does not mix with the cleaner runoff that follows. A roof washer will, though a simple valve design, automatically divert the first 0.02 inches of rainfall per 24-hour period per square foot of roof area away from the rainwater harvesting storage tanks or cisterns. In projects that must meet Provision C.3 requirements, the diverted water would be routed to a hydraulically-sized treatment measure. Roof washers should be installed in such a way that they will be easily accessible for regular maintenance. Also, water storage facilities should be equipped with covers with tight seals, to reduce mosquito-breeding risk.

Figure 4-5 Water may be stored in rain barrel for later use.

Although most roofing materials are compatible with rainwater storage and harvesting systems, rainwater should not be collected from roofs with redwood, cedar, or treated wood shingles or shakes, which may pollute water and soil by leaching toxic materials when wet. In addition, food-producing gardens should not be watered with rainwater from roofs with asphalt shingles, tar, or other materials that may adversely affect food for human consumption.

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4.5 Alternative Pervious Surfaces It may be possible to use alternative, relatively pervious, surfaces for some features of your project. Green roofs can be used in place of traditional roof surfaces. Patios, sidewalks, or lightly traveled driveways and parking areas can be constructed of

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pervious surfaces, such as crushed aggregate, turf block, unit pavers, pervious asphalt, or pervious concrete. Each of these alternative surfaces will have a lower coefficient of runoff, or “C-factor,” than standard roofing or paving surfaces, which means they generate less stormwater runoff requiring treatment. The C-factor describes the amount of runoff that is generated by different types of surfaces. For example, a C-factor of 0.25 for natural stone without grout (BASMAA 2003) means that about 25 percent of the rainfall will become runoff. Surfaces that produce higher volumes of runoff, such as standard asphalt or concrete, are represented by higher C-factors, while surfaces that produce lower volumes of runoff, such as green roofs or pervious concrete, have lower C-factors. Typical C-factors for a variety of surfaces are shown in Table 5-3 in Section 5.1, which describes how treatment measures are sized using a composite C-factor, taking into consideration the percentage of the various types surfaces used in your project.

Figure 4-6: Unit pavers used in parking lot, Palo Alto. (photo courtesy of City of Palo Alto)

Sections 4.6 through 4.8 provide technical guidance for green roofs and pervious paving with underdrain systems.

Figure 4-7: Installation of notched unit pavers in Portland, Oregon. Water infiltrates through notches between pavers. (Photo courtesy of City of Portland)

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4.6 Green Roof Best Uses For innovative

architecture Urban centers

Advantages Minimizes roof runoff Reduces “heat

island” effect Absorbs sound Provides bird habitat Low maintenance

costs Limitations Sloped roofs require

steps Non-traditional

design High installation costs

Figure 4-8: Parking Lot with Turf-Covered Roof, Google building,

Mountain View

A Green roof can be either extensive, with a 3"-7" lightweight substrate and a few types of low-profile, low-maintenance plants, or intensive with a thicker (8" to 48") substrate, more varied plantings, and a more garden-like appearance. The extensive installation shown in Figure 4-10, at Gap Headquarters in San Bruno, has experienced relatively few problems after nearly a decade in use. Green roofs may be considered “self-treating areas” and may drain directly to the storm drain, as allowed by the municipality. Design and Sizing Guidelines Extensive green roof systems contain several layers of protective materials to convey

water away from the roof deck. Starting from the bottom up, a waterproof membrane is installed, followed by a root barrier, a layer of insulation (optional), a drainage layer, a filter fabric for fine soils, the engineered growing medium or soil substrate, and the plant material.

Design and installation is typically completed by an established vendor. Slopes should not exceed 15%. Follow manufacturer recommendations for slope, treatment width, and maintenance

requirements. Either grass or a diverse selection of other low growing, drought tolerant, native

vegetation should be specified. Vegetation whose growing season corresponds to the wet season is preferred. See Appendix B for planting guidance.

Green roof shall be free of gullies or rills.

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Maintenance Installations require inspection at least semiannually and may or may not require

irrigation in the Bay Area’s semi-arid climate. See www.greenroofs.com for information about and more examples of green roofs.

Figure4-9: Intensive Green Roof at Kaiser Center Parking Garage, Oakland.

Figure 4-10: Extensive Green Roof at Gap Headquarters, San Bruno (William McDonough &

Partners)

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http://www.greenroofs.com/


4.7 Pervious Paving Best uses Parking areas Common areas Lawn/landscape buffers Pathways

Advantages Flow attenuation Removes fine

particulates Reduces need for

treatment

Limitations Needs periodic cleaning Weeds Use only in lightly

trafficked areas Higher installation costs

Pervious concrete

Standard concrete sidewalk

Figure 4-11: Pervious Concrete Parking Lot, Emeryville, California

Pervious paving is used for areas with light vehicle loading and lightly trafficked areas, such as automobile parking areas. Table 4-1 shows possible applications for different types of pervious paving. The term pervious paving describes a system comprised of a load-bearing, durable surface together with an underlying layered structure that temporarily stores water prior to infiltration or drainage to a controlled outlet. The surface is porous such that water infiltrates across the entire surface of the material (e.g., crushed aggregate, porous concrete and porous asphalt). Pervious paving is considered a site design measure, and its use can help reduce the size of stormwater treatment measures, but it is not a treatment measure. See Section 4.9 for guidance on how to combine pervious paving with stormwater treatment measures.

Table 4-1: Types of Pervious Paving and Possible Applications

Paver Type Description Possible Applications Porous Asphalt Open-graded asphalt concrete over an

open-graded aggregate base, over a draining soil. Contains very little fine aggregate (dust or sand) and is comprised almost entirely of stone aggregate and asphalt binder; surface void content of 12-20%.

Low traffic use, such as parking lots, travel lanes, parking stalls. Surface may be too rough for bicycle path.

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Table 4-1: Types of Pervious Paving and Possible Applications

Paver Type Description Possible Applications Pervious Concrete A discontinuous mixture of coarse

aggregate, hydraulic cement and other cementitious materials, admixtures, and water which has a surface void content of 15-25% allowing water to pass through.

Sidewalks and patios, low traffic volume and low speed (less than 30 mph limit) bikeways, streets, travel lanes, parking stalls, and residential driveways.

Source: Design Guidelines for Permeable Pavements, Redwood City

Design and Sizing Guidelines The design of each layer of the pavement must be determined by the likely traffic loadings and the layer’s required operational life. To provide satisfactory performance, the following criteria shall be considered: The sub-grade shall be able to sustain traffic loading without excessive deformation. The granular capping and base layers shall give sufficient load-bearing to provide an

adequate construction platform and base for the overlying pavement layers. The base aggregate particles shall be selected based on strength and durability when

saturated and subjected to wetting and drying. To allow for subsurface water storage, the base must be open graded, crushed stone (not pea gravel), meaning that the particles are of a limited size range, with no fines, so that small particles do not choke the voids between large particles.

The sub-grade shall be either ungraded in-situ material with a percolation rate of 5-inches per hour, backfilled with coarser fill material, or installed with an underdrain that will remove detained flows within the pervious paving and base.

The pavement materials shall not crack or suffer excessive rutting under the influence of traffic. This is controlled by the horizontal tensile stress at the base of these layers.

Pervious pavements require a single size grading to give open voids. The choice of materials is therefore a compromise between stiffness, permeability and storage capacity.

Runoff coefficients for pervious pavements are listed on the Estimated Runoff Coefficients for Various Surfaces During Small Storms (1.5 to 2-year storm), Table 5-3 in Chapter 5. Pervious pavements by themselves do not treat runoff. However, because of the higher infiltration rates compared to an impervious surface, the size of downstream stormwater treatment measures can be reduced.

Design calculations for the base shall quantify the following: Type of soil, type of fill if used, permeability of base, k-values (psi/cubic inch) Compressibility (clay and silt contents, organics, muck) Traffic loading (in 18,000 lb. single axle loads) Drainage routing of detained flows within the pervious pavement and base

(infiltration through minimum 5-inch per hour base into in-situ soils, or collection in underdrain if percolation rate cannot be met with in-situ soils)

Design shall be reviewed by manufacturer or National Ready Mixed Concrete Association (NRMCA) (www.nrmca.org).

Depth to groundwater shall be at least 10 feet from bottom of base.

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Permeable pavements must be laid on a relatively flat slope, generally 5% or flatter. If permeable pavements are laid on steep slopes, the open graded crushed aggregate base may tend to migrate downhill, causing the surface to deform.

Installation shall be by contractors familiar with pervious paving installation. Only contractors with certification from NRMCA should be considered. More information can be found at www.concreteparking.org.

Figure 4-12: Surface view of parking lot with pervious paving in lightly-trafficked areas. (Source: Bay Area Stormwater Management Agencies Association [BASMAA], Start at the Source, 1999)

Impervious aisle Permeable stalls

Figure 4-13: Pervious Concrete Installation. (Source: BASMAA, 1999). Depth of pervious concrete will vary with type of usage.

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Maintenance Standards for Ongoing Maintenance and Upkeep: Keep landscaped areas well maintained. Prevent soil from washing onto the pavement. The surface of the pervious pavement

shall be vacuum cleaned using commercially available sweeping machines at the following times: End of winter (April) Mid-summer (July / August) After autumn leaf-fall (November)

Inspect outlets yearly, preferably before the wet season. Remove accumulated trash and debris.

When vacuum cleaning is conducted, inspect pervious paving for any signs of hydraulic failure.

As needed maintenance: If routine cleaning does not restore infiltration rates, then reconstruction of part of the

pervious surface may be required. The surface area affected by hydraulic failure should be lifted, if possible, for inspection

of the internal materials to identify the location and extent of the blockage. Surface materials should be lifted and replaced after brush cleaning. Geotextiles may

need complete replacement. Sub-surface layers may need cleaning and replacing. Removed silts may need to be disposed of as controlled waste.

Figure 4-14: Porous Asphalt Installation (Source: BASMAA, 1999)

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4.8 Turf Block and Permeable Joint Pavers

Best Uses Parking areas Common areas Lawn/landscape buffers Pathways

Advantages Flow attenuation Removes soluble and fine

particulates Reduces need for

treatment Limitations Periodic cleaning required Weeds Use only in lightly-

trafficked areas Higher installation costs

Figure 4-15: Turf Block and Pave Mat (Source: Georgia Stormwater

Handbook)

Turf block and permeable joint pavers (pavers) are used for areas with light vehicle loading, such as driveways, low-volume streets, street shoulders, and parking stalls (Table 4-2). The terms turf block and permeable joint pavers describe systems comprised of a load-bearing, durable surface together with a pervious soil that temporarily stores water, with overflow conveyed to an outlet. The turf block surface is constructed of impermeable blocks separated by spaces and joints, filled with soil and planted with turf, through which the water can drain. Alternately, the spaces and joints of turf block may be filled with gravel. Permeable joint pavers may be impermeable bricks, cobbles, natural stone, or modular unit concrete pavers with permeable joints to allow runoff to percolate to subsurface layers. Some pavers are designed with notched corners (Figure 4-21) to facilitate infiltration. Where soil permeability is low, an underdrain system connected to the storm drain system may be needed. Areas of turf block may be considered “self-treating areas,” and may drain directly to the storm drain system if they do not receive runoff from impervious areas, as allowed by the municipality. Turf block and unit pavers are considered site design measures, and their use can help reduce the size of treatment measures, by infiltrating or temporarily storing runoff, but they are not treatment measures. Section 4.9 offers guidance on combining pervious paving with stormwater treatment measures.

Table 4-2: Permeable Joint Paver Types and Possible Applications

Paver Type

Description Possible Applications

Brick Solid unit paver laid on a permeable base with sand joints.

Driveways, walkways, patios, public sidewalks, plazas, low volume streets

Natural Stone

Laid on pervious surface area in random pattern with wide sand, gravel, or soil joints (from 1/2 to 4 inches).

Driveways, walkways, patios, sidewalks, plazas, low-use parking stalls

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Table 4-2: Permeable Joint Paver Types and Possible Applications

Paver Type

Description Possible Applications

Turf Blocks

Open celled unit paver filled with soil and planted with turf. Sometimes the cells are filled with crushed rock only.

Areas of low flow traffic and infrequent parking, residential driveways and overflow parking areas, emergency access roads, utility roads, street shoulders, and outer edges of commercial and retail parking lots where low-use spaces are located.

Unit Pavers

Discrete units set in a pattern on a prepared base. Typically made of precast concrete in shapes that form interlocking patterns, some unit paver shapes form patterns that include an open cell to increase permeability. Solid unit pavers are made of impermeable materials, but can be spaced to expose a permeable joint set on a permeable base.

Parking stalls, private driveways, walkways, patios, low volume streets, and travel lanes, and bikeways.

Source: Design Guidelines for Permeable Pavements, Redwood City

Design and Sizing Guidelines The design of each layer of the pavement must be determined by the likely traffic loadings and their required operational life. To provide satisfactory performance, the following criteria shall be considered: The subgrade shall be able to sustain traffic loading without excessive deformation. The turf block or permeable joint pavers shall give sufficient load-bearing to provide an

adequate support for loading. The paver materials should not crack or suffer excessive breakage under the influence

of traffic. Both turf block and pavers require a single size, grading base to provide open voids.

The choice of materials is thus a compromise between stiffness, permeability and storage capacity.

The uniformly graded single size material cannot be compacted and is liable to move when construction traffic passes over it. This effect can be reduced by the use of angular crushed rock material with a high surface friction.

The base shall be sized for strength and durability of the aggregate particles when saturated and subjected to wetting and drying. To allow for subsurface water storage, the base must be open graded, crushed stone (not pea gravel), meaning that the particles are of a limited size range, with no fines, so that small particles do not choke the voids between large particles. If subsurface water storage is not an objective, uncompacted soil with a sand bed to support the turf block or paver may be considered. The base should be reviewed by manufacturer of turf blocks or pavers. Check with the local jurisdiction regarding any local requirements for the base layer.

Runoff coefficients for turf blocks and permeable joint pavers are presented in Table 5-3 in Chapter 5. Because of the higher infiltration rates and localized storm water storage compared to an impervious surface, the size of downstream stormwater treatment measures can be reduced.

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Figure 4-16: Profile of Brick Paver Installation (BASMAA, 1999)

Figure 4-17: Profile of Natural Stone Paver Installation (BASMAA, 1999)

Figure 4-18: Profile of Turf Block Installation (BASMAA, 1999)

Figure 4-19: Profile of Unit Paver Installation (BASMAA, 1999)

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MAINTENANCE A maintenance plan shall be provided. Standards for Ongoing Maintenance and Upkeep : Keep landscaped areas well

maintained The surface of the unplanted

turf block and permeable joint pavers shall be vacuum cleaned using commercially available sweeping machines at the following times: End of winter (April) Mid-summer (July /

August) After autumn leaf-fall

(November) Planted turf block can be

mowed, as needed. Inspect outlets yearly,

preferably before the wet season. Remove accumulated trash and debris.

Figure 4-20: Unit Pavers in Private Road, Redwood City

When vacuum cleaning is conducted, inspect turf block and pavers for any signs of hydraulic failure.

As needed maintenance: If routine cleaning does not restore infiltration rates, then reconstruction of the part of

pervious surface not infiltrating is required.

The surface area affected by hydraulic failure should be lifted, if possible, for inspection of the internal materials to identify the location and extent of the blockage.

Surface materials should be lifted and replaced if damaged by brush (or abrasive) cleaning.

Sub-surface layers may need periodic cleaning and replacing.

Figure 4-21: Notched pavers (Source: Unigroup-usa.org). This photo is for example purposes only. It is not an endorsement of any proprietary product.

Deposits may need to be disposed of as controlled waste.

Replace permeable joint materials as necessary.

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4.9 Combining Pervious Paving with Stormwater Treatment

Best Uses Parking areas Sidewalks Pathways

Advantages Flow attenuation Provides detention Removes sediment Increases time of

concentration Limitations Needs periodic

cleaning by vacuuming, not by sweeping or blowing.

Removes dissolved solids, metals, nutrients

Use in lightly trafficked areas only

Higher installation costs

Figure 4-22: The City of Menlo Park used permeable concrete for parking stalls and standard paving in the drive aisles in this public parking lot. Pervious paving is considered a site design measure, and it's use can help reduce the size of treatment measures, but it is not considered a stormwater treatment measure comparable to a vegetated swale, bio-retention area or structural treatment device, due to the limitations mentioned above. Consequently, it is recommended that pervious paving be used in conjunction with a vegetated swale, bio-retention area or similar treatment measure to at least provide treatment of the surface flows that exceed the pavement's infiltration capacity. In addition, it is recommended to avoid or minimize the amount of runoff flowing onto pervious pavement from impervious surfaces to avoid overloading its capacity. Design and Sizing Guidelines Refer to Section 4.7, Pervious Paving technical guidance, for this information. Refer to Sections 6.1, Vegetated Swales, and 6.6, Bio-retention Areas, for this information.

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Maintenance

Refer to Section 4.7, Pervious Paving technical guidance, for this information. Refer to Sections 6.1, Vegetated Swales, and 6.6, Bio-retention Areas, for this information.

San Mateo CountywideStormwater Pollution

Prevention Program

Figure 4-23: Permeable Pavement Drainage and Overflow

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site design measures - flowstobay...synthetic fertilizers, your drainage design may route the runoff...

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