Stormwater Systems
When properly designed and constructed, a drainage plan protects the environment, property owners, and neighboring properties from adverse impacts related to residential development.
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Seeking on-site areas that infiltrate well will lead to the most cost-effective designs. Effective siting identifies soil variability and includes doing some initial infiltration rate testing for planning phases.
Engineered Systems
• Engineered systems that filter storm water from parking lots and impervious surfaces, such as bioretention cells, filter strips, and tree box filters
• Engineered systems that retain (or store) storm water and slowly infiltrate water, such as sub-surface collection facilities under parking lots, bioretention cells, and infiltration trenches
• Pervious, permeable, and porous surfaces that allow drainage between impervious surfaces such as porous concrete, permeable pavers, or site furnishings made of recycled waste
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• Remove curbs and gutters from streets and parking areas to allow storm water to "sheet flow" into vegetated areas.
Low-tech Systems
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• Native or site-appropriate vegetation.
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• Low-tech vegetated areas that filter, direct, and retain storm water such as hedgerows, rain gardens, and bio-swales
• Pervious, permeable, and porous surfaces that help break up (disconnect) impervious surfaces such as porous concrete, permeable pavers, or site furnishings made of recycled waste
• Water collection systems such as subsurface collection facilities, cisterns, or rain barrels
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• Shape driveways, parking areas, and Landscape areas to allow storm water to "sheet flow" into vegetated areas.
Downspouts
can be directed to a water catchment system or directed to areas such as a rain garden, planted area, or gravel filled trenches, where it can be absorbed.
Rain dispersal systems spread out the flow of water from impervious surfaces.
Spreading out the water reduces the force hitting the soil, allowing it to slow down and be absorbed into the soil.
Rainwater Dispersion
Driveways, sidewalks, etc.
can be sloped to shed water into areas such as a rain garden, planted areas, etc
Rainwater Catchment
Also known as rainwater harvesting or rainwater collection, it is the simple act of collecting the rainwater that runs off the hardscapes on your site for beneficial use.
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Residents of Washington state may harvest rainwater without a permit as long as:
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it's used on the property from which it was collected
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it's collected on an existing rooftop
Washington state law even authorizes counties to reduce rates for
stormwater control facilities that utilize rainwater harvesting. Wash. Rev. Code §36.89.080.
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Rainwater collection is legal in the State of Washington
Passive methods for rainwater harvesting, include infiltration basins, bio-swales, etc. that slow or stop the flow of runoff across your site. These allow stormwater to infiltrate into the ground, hydrating soils and recharging groundwater.
Active
Active rainwater harvesting catches and stores the water in one or more containers, such as barrels or cisterns for later use. With active rainwater harvesting, you control when, where, and how the water is used.
Passive
Rainwater harvesting makes the most of your resources;
managing water that would, at best, be wasted and, at worse, be destructive, and directing it to where it can be a useful and cost-effective resource.
During the summer months it is estimated that nearly 40 percent of household water is used for lawn and garden maintenance. A rain barrel collects water and stores it for those times that you need it most — during the dry summer months. Using rain barrels potentially helps homeowners lower water bills, while also improving the vitality of plants, flowers, trees, and lawns.
The average rainfall of one inch within a 24-hour period can produce more than 700 gallons of water that runs off the roof of a typical house.
Harvesting rainwater doesn't have to be a big project. It can just be one rain barrel attached to one gutter that provides convenient water for the chicken yard or for watering a flowerbed without having to drag a hose, etc.
Small systems work fine and expanding your system is fairly easy. Start by deciding how much water you are hoping to store and where you want to locate it. Placing the barrel higher up than anywhere the water will be used allows the use gravity instead of a pump.
It is best if barrels block out any light that could enter them, to avoid growth of pathogens inside the water.
LID Landscaping (Rainscaping)
Plants are an Effective L.I.D. Resource.
The amount of area covered by plants affects the amount of water that will infiltrate the soil.
Greater impervious areas (like roads, roofs, and parking lots) result in greater amounts of water runoff.
Rainscaping is a way to manage water on your own site, by allowing the land and plants to slow rainwater down, controlling the quantity and flow of stormwater. Plants slow the flow of water and increase the permeability of the soil enabling it to soak up water and clean it.
Slowing the water down limits erosion and keeps water usable onsite.
Plants Reduce Energy & Maintenance Costs
​Shade from trees keeps homes and yards cooler in the summer.
In the winter, Trees and shrubs slow the wind and reduce wind chill.
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Ground covering plants reduce the amount of water evaporating from the soil which therefore requires less water.
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Soil effectively covered by plants, shades out weed seeds, so requires less maintenance than landscape with exposed soil.
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Plants protect the soil from wind and water erosion and reduce the amount of pollutants that enter our waterways.
Turf grass vs other vegetation
While turf grass is better than pavement at allowing water to infiltrate into the soil, research has demonstrated that areas covered with turf grass control much less water than other vegetation. Therefore, in keeping with the goals of nonstructural LID s, the amount of lawns and other grass areas at land development sites should be minimized. The use of plants can provide a low-maintenance alternative to turf grass, resulting in lower fertilizer and water needs. The use of native ground cover, shrubs, and trees instead of turf grass can create infiltration characteristics similar to those of natural areas.
Consider reducing the size of excessive lawns. Changes can be made gradually over several seasons.
An easy way to start is by expanding areas already established with shrubs and trees.
Expand the width and include ground covers, xeriscape plantings, perennial flower beds, and/ or tiered shrub plantings.
Convert areas that are difficult to mow, such as corners, edges, and under trees.
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Vegetated areas provide a pervious surface for groundwater recharge and can remove pollutants from the runoff flowing through it. Installing or maintaining a strip of plants at the edge of an area, whose purpose is to slow surface water runoff, to assist in infiltration, and prevent soil erosion is an effective and easy way to maintain a healthy landscape.
Hedgerow, Filter strip, or Vegetated buffer
There are many ways plants can be effectively used in the landscape, but plant selection and proper planting and care are vital.
Select plants. Include native plant species that provide food and habitat for wildlife and insect pollinators. Native plants are naturally adapted to local growing conditions. They require less water, fertilizer, and maintenance. Native plants are also known to be very effective in managing storm water because many species have deep root systems which stabilize soil and facilitate the infiltration of storm water runoff. These plants evolved and adapted to the local climate and growing conditions. Adding even just a few native plants to your landscape can go a long way towards supporting wildlife.
Select a mix of plants with different foliage, texture, and flowers that bloom at different times for season.
New plantings require extra care during the first 1-3 years.
Make sure new plants receive a deep watering 1 or 2 times per week for the first several months and then at least once per week for the first year.
Good maintenance while the garden is becoming established is important. Pull weeds while they're young, before they've gone to seed. Replace any dead plants to fill in holes. A full coverage of plants, like mulch, helps maintain soil moisture, prevent erosion, and reduce weeds.
Once native plants are established, they need little maintenance to perform well and look good.
If the garden is near the road, consider sight lines and setbacks
Edging (such as pavers, stones, etc.) can facilitate access for maintenance and provide separation from lawn and other landscaped areas.
To maintain access to the middle of the garden for weeding and other tasks. A few strategically placed flat rocks or pavers can allow access without compacting the soil or leaving room for weeds.
The preservation of existing natural vegetated areas is a nonstructural LID-BMP that should be considered
throughout the design of a land development. Ideally, the more natural area to be preserved, the better.
Using plants as a nonstructural LID technique can significantly reduce the impact of rainwater.
Vegetated areas provide a pervious surface for groundwater recharge, particularly during dormant or non-growing seasons. In addition, vegetation can remove pollutants from the runoff flowing through it.
Vegetative filters and buffers can be created by preserving existing vegetated areas over which runoff will flow or by planting new vegetation.
Vegetative filters located immediately downstream of impervious surfaces such as roadways and parking lots can achieve pollutant removal, groundwater recharge, and runoff volume reduction.
Vegetated buffers adjacent to streams, creeks, and other waterways and water bodies can also help mitigate thermal runoff impacts, provide wildlife habitat, and increase site aesthetics.
A rain garden is a landscaped depression in the land, with soil designed to help rainwater runoff from a roof, driveway or other impervious surfaces to soak into the ground and be filtered.
Bioretention Cells, Bioswales, and some Hedgerows are sometimes referred to as "Rain Gardens", because they are basically doing the same thing in different situations. For example, a "Bioswale" is basically a kind of rain garden where water is slowed and filtered, but much of the water is directed to another location.
Bioretention is a more complex rain garden with drainage systems and amended soils.
Hedgerows are not technically rain gardens but can be used to slow surface water as it heads down slope, as an edge to a Bioswale, or as check dam
Curb Cut Extension
Curb Cut Raingarden
Curb Cut Inlet
A Bioretention cell (strip or trench) is more complex rain garden with engineered drainage systems in a slightly recessed landscaped area constructed with a specialized soil mixture, an aggregate base, an underdrain, and site-appropriate plants.
Bioretention
A bioswale is a slightly recessed landscaped area constructed downstream of a runoff source. At the beginning of a rain event, a bioswale absorbs and filters water runoff. Once the soil-plant mixture below the channel becomes saturated, the swale acts as a conveyance structure to a bioretention cell, wetland, or infiltration area.
There is a range of designs for these systems. Some swales are designed to filter pollutants and promote infiltration and others are designed with a geo-textile layer that stores the runoff for slow release into depressed open areas or an infiltration zone.
Bioswale
Rain gardens are designed to be self-sufficient. Some weeding and watering will be needed in the first two years, and perhaps some thinning in later years as the plants mature, but a well-planned raingarden can be maintained with little effort after the plants are established.
Include native plants!
Native plants eliminate the need for fertilizers and pesticides, and require little or no supplemental water. These plants evolved and are adapted to the local climate and growing conditions.
Native plants are important to wildlife including bees, butterflies, and birds which are adapted to using native plants as a source of food and shelter. Adding even just a few native plants to your landscape can go a long way towards supporting wildlife.
Rain gardens function only as well as they are designed. Planning is needed as well as knowledge of soil and local weather conditions. Take the time to plan effectively.
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Note: Don’t locate a rain garden within 10 feet of a building foundation, near the edge of a steep slope or bluff, in low spots that do not drain well, where groundwater is within one foot of the bottom of the finished rain garden, over a septic drain field or tank, over shallow utilities (call before you dig), or in areas that would require disturbing healthy native soils and vegetation. For more in depth information, see the online books below.
There are many ways plants can be effectively used in the landscape, but plant selection and proper planting and care are vital.
When planting near the road, consider sight lines and setbacks
Anatomy of a Rain garden
RAIN GARDEN DESIGN TIPS
Make it part of the landscape. It should work together with and be visually and functionally integrated into the rest of the landscape.
Choose a shape. Consider all the rules of composition, screening, and circulation—not just the rule that says to put a rain garden in a low spot 10 feet from the house.
Consider style. A rain garden can be as formal or as wild as you like, but pay attention to how it looks with your home’s facade.
Integrate with other gardens. Consider making a depression within a perennial bed or shrub border (especially if space is tight and you don’t have room for a larger rain garden that stands alone).
Create repetition. Put in more than one rain garden for repetition and continuity. If it works with your overall design, create a little rain garden for each downspout. Or add other water features around your yard such as a fountain, birdbath, or waterfall to repeat the water theme, which is another way to lend cohesion to your landscape.
Consider aesthetics and function. Make your rain garden more attractive and user-friendly. Use decorative stones as edging, create an adjacent seating area, build an attractive pathway, or add other hardscape and accessories to make your rain garden more visually appealing.
Select plants. Include native plant species that provide food and habitat for wildlife and insect pollinators. Rain garden plants should be able to tolerate moisture as well as intermittent dry spells. Include sedges, grasses, and rushes with deep root systems that will help water seep into the soil. Select a mix of plants with different foliage, texture, and flowers that bloom at different times for season-long interest. Add marginal plants that are more drought-tolerant around the perimeter. Plant shrubs in groups of 3 to 5 plants and small plants in "drifts" for greater visual impact.
Plants for shady zone 1
Slough sedge (Carex obnupta)
Small-fruited bulrush (Scirpus microcarpus)
May Lily (Maianthemum dilatatum)
Pacific waterleaf (Hydrophyllum tenuipes)
Ferns Lady fern (Athyrium filix-femina)
Deer fern (Blechnum spicant)
Goat's beard (Aruncus dioicus)
Red-twig dogwood (Cornus sericea)
Black twinberry (Lonicera involucrata)​
Plants for sunny zone 1
Dagger-leaf rush (Juncus ensifolius), Taper-Tipped rush (Juncus acuminatus)
Cascade penstemon (Penstemon serrulatus)
Henderson’s checker-mallow (Sidalcea hendersonii)
Rocky Mountain Iris (Iris missouriensis)
Red-twig dogwood (Cornus sericea)
Pacific ninebark (Physocarpus capitatus)
Black twinberry (Lonicera involucrata)
Pacific crabapple (Malus fusca)
Plants in zone 1 need to be able to tolerate wet conditions and seasonal flooding.
Plants for shady zone 2
May Lily (Maianthemum dilatatum)
Oregon wood sorrel (Oxalis oregana)
Sword fern (Polystichum munitum)
Lady fern (Athyrium filix-femina)
Low Oregon Grape (Mahonia repens)
Snowberry (Symphoricarpos albus)
Salal (Gaultheria shallon)
Western Pussy Willow (Salix scouleriana)
Cascara (Frangula purshiana)
Plants for sunny zone 2
Daylily (Hemerocallis spp.)
Giant camas (Camassia leichtlinii)
Henderson's Checker Mallow (Sidalcea hendersonii)
Douglas Iris(Iris douglasiana)
Red-twig dogwood (Cornus sericea)
Snowberry (Symphoricarpos albus)
Western Pussy Willow (Salix scouleriana)
Tall Oregon grape (Mahonia aquifolium)
Pacific crabapple (Malus fusca)
Plants in zone 2 need to be able to tolerate moist to occasionally flooding conditions.
Plants for shady zone 3
Inside-out flower (Vancouveria hexandra)
Western bleeding heart (Dicentra formosa)
Sword fern (Polystichum munitum)
Evergreen huckleberry (Vaccinium ovatum)
Low Oregon grape (Mahonia nervosa)
Rhododendron macrophyllum
Vine maple (Acer circinatum),
Cascara (Frangula purshiana)
Indian plum (Oemleria cerasiformis)
Plants for sunny zone 3
Black-eyed Susan (Rudbeckia hirta)
Cooley's Hedge-nettle (Stachys cooleyae)
Oceanspray (Holodiscus discolor),
Red-flowering currant (Ribes sanguineum)
Snowberry (Symphoricarpos albus)
Oregon Boxleaf (Paxistima myrsinites)
Tall Oregon grape (Mahonia aquifolium)
Serviceberry (Amelanchier alnifolia), Oceanspray (Holodiscus discolor)
Mock Orange, Philadelphus lewisii
Plants in zone 3 need to be able to tolerate moist to dry conditions.
New plantings require extra care during the first 1-3 years.
Make sure new plants receive a deep watering 1 or 2 times per week for the first several months and then at least once per week for the first year.
Good maintenance while the garden is becoming established is important.
Apply a good layer of mulch to maintain soil moisture and reduce weeds.
Once Rain gardens are established, they need little maintenance to perform well and look good.
Rain Garden Maintenance
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After heavy storms, check the inflow and overflow areas to make sure they are still intact and can continue to carry water into and out of the rain garden.
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At least twice a year, check around the inlet and overflow areas for debris build-up such as leaves, sticks, and other items.
Keep the Flow
Water flowing into the rain garden can carry with it various types of debris that can clog the soil mix and slow drainage.
Fast flowing water can also slowly eat away at your soil layer, washing it away and damaging your garden.
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Maintain a cover of decorative rock around the inlet and overflow area to protect the soil.
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Look for areas where water may not be soaking into the ground. This may be due to fine sediment or compaction of the soil.
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Remove sediment that may be building up and rake the soil surface. If you suspect compaction, break up and loosen the soil when it is not saturated.
Maintain soil coverage​
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Mulch provides multiple benefits for rain gardens by helping to:
Keep the soil moist.
Replenish organic material in the soil.
Prevent erosion.
Discourage weeds.
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Every year check the mulch layer and, if needed, apply enough to maintain a layer of shredded or chipped wood mulch that is about 3 inches deep all throughout your rain garden—on the bottom, the sides, and around the perimeter.
Maintain a healthy cover of plants
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Replace any dead plants to fill in holes.
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Like mulch, full coverage of plants provides multiple benefits for rain gardens by helping to:
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Keep the soil moist.
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Replenish organic material in the soil.
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Prevent erosion.
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Discourage weeds
Edging (such as pavers, stones, etc.) can facilitate access for maintenance and provide separation from lawn and other landscaped areas.
To maintain access to the middle of the garden for weeding and other tasks. A few strategically placed flat rocks or pavers can allow access without compacting the soil or leaving room for weeds.
Weeding
Rain gardens will still soak up and filter rain water even if they are full of weeds. However, the plants in the rain garden may not grow as well with all the competition. Soils in rain gardens have good structure, so weeds should be easy to pull by hand, especially in the spring when the soil is moist and the weeds are small.
Watering
Plants will need to be watered every few days until established (about 4 weeks). For the first year, most plants need deep watering during the dry summer season to establish healthy root systems. After two or three years the native plants in your rain garden will need little or no watering, except for during times of drought.
Fertilizing
Do not apply fertilizers
to your rain garden. The rain garden soil mix provides plenty of nutrients and the native plants in your garden are well suited to local growing conditions, so extra fertilizing is not needed.
Permeable Pavement
Permeable pavement is a type of pavement with a porous surface that is composed of concrete, open pore pavers or asphalt with an underlying stone reservoir. It allows water to run through it rather than accumulate on it or run off of it. The water slowly infiltrates the soil below or is drained via a drain tile. The stone or gravel acts as a natural filter and clears the water of pollutants. It is important to note that one size does not fit all - there are many pros and cons for use of each type of permeable pavements
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Porous Asphalt and Pervious Concrete are like conventional asphalt and concrete but with less fine aggregate content leaving open spaces for water to pass through and soak into the ground. Porous asphalt and pervious concrete are the most suitable for large areas including residential driveways and parking lots. More...
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Permeable paver systems have gaps between the pavers that allow water to pass. A layer of gravel under the paver system acts as a reservoir, holding rainwater while it soaks into the ground. Pervious paver systems are the most versatile type of permeable pavement and are suitable for residential driveways, patios, and parking lots.​
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Turf block systems are pavers with empty spaces filled with soil and planted. Turf block systems are suitable for residential driveways.
When to Call a Professional
Call a professional designer if you have more vehicle traffic than a residential driveway.
Also, if your soil infiltration rate is less than 2 inches per hour, you will need to hire a designer to help you.
Summary of Permeable Pavement Design Requirements
These site and design requirements can help you decide if permeable pavement might be appropriate for your project.
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Permeable pavement is allowed on surfaces with slopes no greater than 5 percent.
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Underlying soils should have a minimum infiltration rate of 2 inches per hour.
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There are no setback requirements for permeable pavement.
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There must be 5 feet between the high groundwater level and the excavated bottom.
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The subgrade next to structures should slope away from the structures.
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Use a minimum of 6 inches of washed, crushed 2- to ¾-inch or No. 57 rock under concrete or asphalt.
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Consult the Stormwater Management Manual regarding required edge restraints.
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For best results, keep in mind the following construction considerations:
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Protect the subgrade from over-compaction during excavation.
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Do not excavate or compact the native subgrade in wet conditions.
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Consider the sequence of construction activities to protect the subgrade from traffic.
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Protect the paving from construction traffic and sediment after installation.
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Special Maintenance Considerations
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Prevent Clogging of Pavement Surface with Sediment
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Vacuum pavement twice per year
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Maintain planted areas adjacent to pavement
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Immediately clean any soil deposited on pavement
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Do not allow construction staging, soil/mulch storage, etc. on unprotected pavement surface
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Clean inlets draining to the subsurface bed twice per year
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Snow/Ice Removal
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Porous pavement systems generally perform better and require less treatment than standard pavements
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Do not apply abrasives such as sand or cinders on or adjacent to porous pavement
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Snow plowing is fine but should be done carefully (i.e. set the blade slightly higher than usual)
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Salt application is acceptable, although more environmentally-benign deicers are preferable
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Repairs
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Surface should never be seal-coated
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Damaged areas less than 50 sq. ft. can be patched with porous or standard asphalt
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Larger areas should be patched with an approved porous asphalt
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