Spare That Shrub
  – Do Your Part To Control Erosion


Rain has long been the single word most identified with the Puget Sound region. In a watershed that consists largely of forest land and farmland, most rain soaks slowly into the ground and gradually drains to nearby surface waters or into the groundwater. As forests and hillsides are cleared and wetlands are filled for development, more of the water runs off instead of soaking into the ground. As the Puget Sound basin has developed, changes in land use have increased seasonal flooding.

Roofs, roads, parking lots, sidewalks, driveways and other hard surfaces are impervious to rain. Virtually all of the rain that falls on these surfaces has nowhere to go but downhill, fast. As a result, flooding increases in frequency and severity. A short, intense rainstorm that only slightly raised water levels in the past, now turns a stream into a torrent. The speed of water moving across the land and in the streams increases and accelerates soil erosion.

Much of the soil washed off vacant lots, cleared land, uncontrolled construction sites, and from road cuts is carried into streams and can eventually reach Puget Sound. In streams, this sediment smothers plants and animals that live in the shallows and clogs the clean gravel needed by spawning salmon. In bays and inlets of the Sound, this sediment smothers lingcod spawning grounds and eelgrass beds. This runoff may also include a variety of pollutants. Metals from downspouts and pipes, paints, brake linings, engine drippings and tires join nutrients from lawn fertilizers, detergents, animal wastes, and failing septic tanks. Bacteria and oxygen-demanding substances from farm animal and pet waste, failing septic systems and combined sewer outflows are carried downstream by the runoff. As if these weren’t enough, oil and grease from parking lots, roads, service stations and illegal disposal of waste oil into storm drains combines with other toxic chemicals that may be released from industrial and commercial businesses.

Dealing with Surface Runoff
What can be done to treat these rainy day blues? Natural wetlands slow the flow of runoff and filter pollutants from the water passing through them. Help to preserve wetlands in your neighborhood and throughout Puget Sound. When development occurs, insist that best management practices be used. These include detention ponds which allow pollutants to settle, infiltration devices which channel runoff through the soil layer, and grassy swales which use vegetation to filter runoff.

Reduce your use of impervious surfaces. Use spaced paving stones instead of concrete, ground cover instead of grass, and pervious asphalt instead of standard. Plant grass along and in drainage ditches or channels to slow the rate of runoff and help filter pollutants.
Where impermeable surfaces are used, divert rain from the paved surfaces onto grass or into vegetation to permit gradual absorption. Use grass-lined swales, berms, and basins to control runoff on your property, reduce its speed, and increase the time over which the runoff is released.
Preserve the established trees around your home and in your neighborhood. Plant new trees and shrubs to encourage excess rainwater to filter slowly into the soil. Plant and maintain a vegetated buffer strip at the base of steep slopes and along water bodies. Resod bare patches in your lawn as soon as possible and minimize bare soil in your garden to avoid erosion.
Install gravel trenches along driveways or patios to collect water and allow it to filter into the soil. To be most effective, trenches should be one foot wide by three feet deep. Grass swales, low areas in the lawn, can be used to move water from one area to another. If you build a new home, ask your builder to leave as much of the original vegetation as possible on site. Before you start construction grading, acquire a copy of the Associated General Contractors booklet, Waste Disposal and Erosion and Sediment Control Methods, by calling (206) 284 -0061. Read the booklet and share it with your builder.

Permeable Paving Surfaces
Because so many of our human landscape features are impervious, a few words about using paving surfaces that allow rainwater to soak into the ground seem in order. There are many materials that provide the durability of concrete while allowing rainwater to filter into ground.

Bricks, interlocking pavers, precast concrete lattice pavers, or flat stones make an attractive, durable walkway. If placed on well drained soil or on a sand or gravel bed, these modular pavers allow rainwater infiltration. To control weeds growing in the spaces between the pavers, consider Corsican mint or moss as a natural way to crowd out weeds and add beauty to the paved area.

Wood decks, usually installed for their functional good looks, can serve as a form of porous pavement. Redwood, cedar, or treated wood are as durable as most other paving surfaces. The spacing in the decking allows rainwater to drain directly onto the soil surface and soak into the ground. Maintaining the distance between the soil surface and the decking recommended by your county building department will minimize risk of wood rot.

New, porous materials are also becoming available. For example, porous asphalt is similar to conventional asphalt in durability and cost. You can use porous asphalt on your new driveway and encourage its use on streets and parking lots in your community. For specifics, call the Washington office of the Asphalt Institute at (859) 288-4960 or the Washington Department of Ecology at (360) 407-6000..

Streamside Erosion