The Basics of LID

  • Minimize site disturbance and reduce impervious surfaces where feasible (don’t pave over the whole site if you don’t need to).

  • Protecting and restoring native soils and vegetation (Don't remove native trees and shrubs unnecessarily. Do not disturb or compact soil unnecessarily.)

  • Manage stormwater close to the source (don’t let the water leave the site).

  • Scatter Integrated Management Practices (IMPs) throughout your site that infiltrate, store, evaporate, and/or detain runoff close to the source.

 
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The Good News!

Not only do these techniques improve water quality, restore ground water reserves, and create a healthier yard, but many of these techniques are easily accomplished and beautiful additions to our homes that don't require any extra cost, just a different plan.  

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

•    Remove curbs and gutters from streets and parking areas to allow storm water to "sheet flow" into vegetated areas.

Low-tech Systems

•    Native or site-appropriate vegetation.

•    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

•    Shape driveways, parking areas, and Landscape areas to allow storm water to "sheet flow" into vegetated areas.

When properly designed and constructed, a drainage plan protects the environment, property owners, and neighboring properties from adverse impacts related to residential development.

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. 

 

NOTE:

All development within the special flood hazard areas (SFHA) must incorporate low impact development techniques where feasible to minimize or avoid stormwater effects. With various elements of low impact development (LID), most projects on parcels ½ acre in size or larger in rural areas can often meet these requirements by using dispersion as follows:

LID  Dispersion worksheet

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LID Construction Techniques

  • Conserve natural areas wherever possible (don’t pave over the whole site if you don’t need to).

  • Minimize the development impact on hydrology (Don't remove native trees and shrubs unnecessarily. Do not disturb or compact soil unnecessarily.

  • Maintain runoff rate and duration from the site (don’t let the water leave the site).

  • Scatter integrated management practices (IMPs) throughout your site that infiltrate, store, evaporate, and/or detain runoff close to the source.

  • Implement pollution prevention and proper maintenance.

To minimize the detrimental impacts of stormwater runoff associated with development, appropriate management is necessary.

Help your landscape to work for you.

  • Retain trees and other vegetation which intercept precipitation with the tree canopy, leaves, and roots. 

  • Grade only as much of the land as needed so soil, terrain, and plants can slow runoff and hold water until it is absorbed into the soil. 

  • Disperse water by directing runoff from roofs, pavements, and similar impervious surfaces to rain catchments or planted areas that can benefit from the water. 

  • Create beautiful and useful outdoor spaces that limit lawn and other compacted areas while maintaining soil that can absorb water. 

  • Place driveways and parking areas thoughtfully to limit compacted soil and direct runoff to planted areas.

  • Use natural mulch to improve soil's ability to absorb and filter water.

Site Planning Tips 
to minimizes the impact the construction project will have on the patterns of water flow and vegetated areas of the site and help facilitate stormwater infiltration on the property: 

  • Place structures as close to the public access point as possible to minimize road/driveway length. Minimize paved parking areas and utilize porous paving options wherever possible. 

  • Slope paved areas to facilitate drainage to stormwater management areas. 

  • Reduce building footprints whenever possible. Utilize basements or taller structures with lofts or second stories to
    achieve square footage goals. 

  • Orient buildings on slopes with long-axis along topographic contours to reduce grading requirements. 

  • Set clearing limits that give maximum protection to soils and vegetation while allowing reasonable areas for equipment to maneuver on the site. Delineate the areas both on the construction plans and on the ground with temporary fencing or taping. 

Rainwater Dispersion

Depending on the soil type that you have, your water movement will be affected in certain ways. If your soil is predominantly sand, you are not likely to have flood issues, but may struggle to retain water, because sand’s large particles let water through easily. If you have predominantly clay soil, your soil’s tiny particles hold water very easily, become waterlogged quickly, and then tend to let water run off the surface thereafter.

 

 

Developing a Drainage Plan 

1. Conduct a site inventory to determine existing patterns of water movement and vegetated areas on your site. Consider ways your proposed development can avoid impacts to them. 


2. Obtain an accurate topographic map for the site to use as a basis for the drainage plan. This can be as simple as a map that denotes flat areas, sloped areas with approximate percent grade, and drainage paths. Topographic information for your site can be found on the Skagit County imap. 

3. Find out the soil type on your project site in order to determine which stormwater management techniques will be applicable for your site

Site Assessment

  • Draw the general layout of buildings on your site (graph paper helps).

  • Add impervious areas like the driveway, sidewalks, or parking areas.

  • Measure the length and width, then multiply the two together to get the area. Estimate hard, or impervious areas where water runs off and note the measurements on the map.

  • Locate the downspouts that drain water from your roof and mark them on your map. Note the rooflines and area draining to the downspout.

  • Look at other impervious surfaces on your site. Try to figure out where runoff from these areas goes. If it isn’t raining, use a hose. Use arrows to note on your map the direction the water flows. 

  • Look at other surfaces of your property and mark any noticeable hills and dips. Note areas that stay wet and muddy. Note areas where water soaks in or are soft (lawns, planting beds, trees).

  • Soil type has a lot to do with how well rainwater soaks into the ground. Sandy, loamy soil soaks up water very quickly. Heavier soils with clay don’t soak up water as well. 

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Evaluate Your Site Assessment Map

The goal is to direct stormwater towards water storage areas or those surfaces which soak up rain

such as vegetated surfaces like rain gardens or other garden areas.  If the space is too small, a rain harvesting practice, such as a rain barrel can be used. You may need to reroute drainage systems to get water to where there is enough space to install a particular practice. What is possible depends onsite conditions, set back requirements, sizing, and soil constraints. 

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Sketch Some Ideas and Take Action
In this Example of Plan, they have decided to install rain barrels at 3 downspouts and a rain garden at the fourth.  Along with directing rain water across driveway into second rain garden, they plan to install gutters and downspouts on garage. One drains to rain barrel, on drains to rain garden. The more area where excess lawn is replaced with flowers, shrubs, and trees, the more water will be absorbed into the ground in a useful way.

Low impact development promotes the view of rainwater as a resource to be preserved and protected, not a nuisance to be eliminated. 

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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Rainwater collection is legal in the State of Washington

Residents of Washington state may harvest rainwater without a permit as long as:

  • it's used on the property from which it was collected

  • 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.

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.

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Active

Passive

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.

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. 

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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.

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Permeable Pavement

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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. 

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Summary of Permeable Pavement Design Requirements

These site and design requirements can help you decide if permeable pavement might be appropriate for your project.

  • Permeable pavement is allowed on surfaces with slopes no greater than 5 percent.

  • Underlying soils should have a minimum infiltration rate of 2 inches per hour.

  • There are no setback requirements for permeable pavement.  

  • There must be 5 feet between the high groundwater level and the excavated bottom.   

  • The subgrade next to structures should slope away from the structures.

  • Use a minimum of 6 inches of washed, crushed 2- to ¾-inch or No. 57 rock under concrete or asphalt.

  • Consult the Stormwater Management Manual regarding required edge restraints.

For best results, keep in mind the following construction considerations:

  • Protect the subgrade from over-compaction during excavation.  

  • Do not excavate or compact the native subgrade in wet conditions.  

  • Consider the sequence of construction activities to protect the subgrade from traffic.

  • Protect the paving from construction traffic and sediment after installation.   

 

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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. 

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Special Maintenance Considerations

  • Prevent Clogging of Pavement Surface with Sediment

    • Vacuum pavement twice per year

    • Maintain planted areas adjacent to pavement

    • Immediately clean any soil deposited on pavement

    • Do not allow construction staging, soil/mulch storage, etc. on unprotected pavement surface

    • Clean inlets draining to the subsurface bed twice per year

  • Snow/Ice Removal

    • Porous pavement systems generally perform better and require less treatment than standard pavements

    • Do not apply abrasives such as sand or cinders on or adjacent to porous pavement

    • Snow plowing is fine but should be done carefully (i.e. set the blade slightly higher than usual)

    • Salt application is acceptable, although more environmentally-benign deicers are preferable

  • Repairs

    • Surface should never be seal-coated

    • Damaged areas less than 50 sq. ft. can be patched with porous or standard asphalt

    • Larger areas should be patched with an approved porous asphalt