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Low Impact Development
​Low Impact Development (LID)
is a stormwater management approach that works with nature to manage rainwater as close to where it falls as possible.
What is Stormwater runoff?
Stormwater runoff is rain or snowmelt when it flows over land or paved surfaces and is not absorbed into the ground.
The Problem with Runoff
As water runoff flows across the ground's surfaces, it picks up what it touches. The further over land runoff travels, the more polluted it becomes.
Unless something intercepts it, the runoff carries dirt, chemicals, and other pollutants directly to our streams and waterways.
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Skagit County is subject to a National Pollutant Discharge Elimination System (NPDES) permit that can result in significant penalties upon the County and property owners if polluted water is discharged to ditches or streams. Stormwater management is governed by Skagit County Code Chapter 14.32, which received major revisions effective January 1, 2016. All land disturbing activity (except when vested before that date) must manage stormwater consistent with this chapter.
Stormwater Management Requirements
The other Problem; there's too much runoff
As we build more roads, houses, and other hard surfaces, the less water is able to be absorbed into the soil and underground aquafers, so more and more water is running across the surface. The increase of surface water Runoff can be a powerful thing, causing erosion and flooding damage.
Every site that is not perfectly flat and perfectly level is a watershed
(the land area down which all water flows).
All land on earth is a watershed
Q. Is it true that LID practices don't work in areas that receive large volume storms?
A. No. read more...
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.
Plants slow the flow of water and increase the permeability of the soil.
LID simply works
Low Impact Development manages water on your own site, focusing on slowing rainwater down, controlling the quantity and flow of stormwater, soaking it up, and using soil and plants to clean it. Slowing the water down limits erosion and runoff of polluted water, filters it, and keeps water usable onsite
When it rains in the forest, most of the water evaporates or is absorbed into the ground where it recharges groundwater or is taken up by the roots of the plants and trees. Allowing water to filter through the ground naturally, removes many of the pollutants in the water before it reaches our rivers, lakes, and marine waters.
Traditional Stormwater management (also called "gray infrastructure")
channels runoff from the site as quickly as possible. When our stormwater management process was originally created, the main concern was the potential damage to structures, so the focus was to direct runoff away from roads, buildings, and structures quickly.
Unfortunately, this type of management results in too much water, flowing along, picking up contaminants, flooding rivers, and polluting all our waterways. The gray infrastructure in many areas is aging, and it doesn't have the capacity to manage large volumes of stormwater.
Low-Impact Development (LID) (also called "green infrastructure")
is designed to mimic the natural hydrologic functions of a site.
Slowing the runoff, allows the water to filter into the soil which reduces the amount of runoff eroding land and picking up pollutants, which reduces the amount of pollutants that end up in our waterways.
The Solution
Slow the flow of water, so it can be soaked into and filtered through the soil.
Water is an important resource; we shouldn't let it run amuck.
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The Basics of LID
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Minimize site disturbance and protect native soils and vegetation (Don't remove native trees and shrubs unnecessarily. Do not disturb or compact soil unnecessarily.)
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Use on-site natural features (Let the site work for you.)
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Manage stormwater close to the source (don’t let the water leave the site).
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Distributed stormwater Best Management Practices (BMPs) (Use BMPs effectively throughout the site.)
The Good News!
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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, beautiful, and low maintenance additions to our homes that don't require any extra cost, just a different plan.
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WSU Catching Rain fact sheets
Low impact development promotes the view of rainwater as a resource to be preserved and protected, not a nuisance to be eliminated.
When implemented broadly, LID can mitigate the urban heat island effect, save energy, reduce air and water pollution, improve neighborhood aesthetics, increase groundwater recharge, and increase habitat for wildlife, such as birds and pollinators.
LID Parking Lot
LID is economical
LID is as cost-effective as—if not more cost-effective than—conventional approaches in part because of the long term savings in maintenance and repair. Not to mention the aesthetic benefits.
Raingardens attract dragonflies, frogs, and birds
Water in a properly designed rain garden will not last more than 2 days after most storms which is not long enough for mosquitoes to use, but it will be enjoyed by many of our wild friends.
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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.
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Why aren't more people using LID techniques?
I don't know! Maybe they haven't heard about them yet. We need to spread the word!
LID Techniques Can Be Applied at Any Development Stage
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• In undeveloped areas, an LID design can be incorporated in the early planning stages. Typical new construction LID techniques include protecting open spaces and natural areas such as wetlands, installing bioretention areas (vegetated depressions) and reducing the amount of pavement.
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• In developed areas, communities can add LID practices to solve problems and provide benefits such as being used to buffer structures from roads, enhance privacy among residences, and for an aesthetic site feature. Typical post-development LID practices range from directing roof drainage to an attractive rain garden to retrofitting streets with features that capture and infiltrate rainwater
LID Basics
Is LID required?
LID is the preferred approach to stormwater management countywide.
Inside the NPDES Permit Area and the Special Flood Hazard Area, LID techniques are required unless not feasible. When required, applicants must use the techniques in the Low Impact Development Technical Guidance Manual for Puget Sound (Puget Sound Partnership and WSU Extension, 2012) unless the Administrative Official determines the techniques as not feasible.
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.
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.
WSU "Catching Rain" fact sheets
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 Construction Techniques
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Conserve natural areas wherever possible (Don't remove native trees and shrubs unnecessarily.)
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Minimize the development impact on hydrology (Do not disturb or compact soil unnecessarily.
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Maintain runoff rate and duration from the site (don’t let the water leave the site).
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Distributed stormwater Best Management Practices (BMPs) (Use BMPs effectively throughout the site.)
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Implement pollution prevention and proper maintenance.
Help your landscape to work for you.
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Retain trees and other vegetation which intercept precipitation with the tree canopy, leaves, and roots.
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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.
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Disperse water by directing runoff from roofs, pavements, and similar impervious surfaces to rain catchments or planted areas that can benefit from the water.
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Create beautiful and useful outdoor spaces that limit lawn and other compacted areas while maintaining soil that can absorb water.
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Place driveways and parking areas thoughtfully to limit compacted soil and direct runoff to planted areas.
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Use natural mulch to improve soil's ability to absorb and filter water.
Site Planning Tips
to minimize 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:
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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.
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Slope paved areas to facilitate drainage to stormwater management areas.
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Reduce building footprints whenever possible. Utilize basements or taller structures with lofts or second stories to
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Orient buildings on slopes with long-axis along topographic contours to reduce grading requirements.
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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.
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
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.
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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
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Draw the general layout of buildings on your site (graph paper helps).
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Add impervious areas like the driveway, sidewalks, or parking areas.
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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.
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Locate the downspouts that drain water from your roof and mark them on your map. Note the rooflines and area draining to the downspout.
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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.
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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).
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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.
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.
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.
Rainwater collection is legal in the State of Washington
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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.
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.
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
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.
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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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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Turf block systems are pavers with empty spaces filled with soil and planted. Turf block systems are suitable for residential driveways.
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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