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.

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. 

  • Direct water 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.

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

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. 

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. 

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.

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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.  

Even better

Many of these techniques don't require any extra costs, just a different plan.