(Converted into web page from Ecology publication #93-31:Vegetation Management: Guide for Puget Sound Bluff Property Owners)
This website has been prepared to provide property owners and others with information about the role, benefits, and management of existing vegetation common to steep, often unstable shore sites in the Puget Sound area. It will also identify and discuss the limitations of plant cover under some conditions. The focus of this guide is on vegetation management during site development with an emphasis on reducing the hazard of surface and mass soil erosion (landslides).
The subject of vegetative restoration of slopes is discussed in a companion website, Slope Stabilization and Erosion Control Using Vegetation. Issues regarding sea level rise, beach nourishment, regulatory management of shorelands and other important topics are likewise not addressed here.
A Word of Caution
There is a lack of detailed research on vegetation management for Puget Sound bluff sites. The information and recommendations provided here have been gathered from a variety of published and unpublished sources in forestry, fisheries, geology, horticulture, soil science, and arboriculture. Many of the observations and suggestions are based on the experience of the author and from conversations with researchers and land managers from the United States and Canada.
This guide is not intended as a substitute for professional assistance. Readers are advised to become familiar with any federal, state, county and/or municipal ordinances that may apply to development of shoreline sites. Neither the author nor the Washington State Department of Ecology assumes responsibility for any results or consequences that arise from the treatments or techniques mentioned in this guide.
Readers who have additional information, pertinent bibliographic citations, or management suggestions are invited to submit their comments to Ecology, attention the Shorelands and Environmental Assistance program.
A note about navigation:
Imagine you have just bought the property in Illustration 1. You are going to build your dream house here. Note the stand of trees on the uplands, the brush and trees growing on the crest, and the scattered growth on the face of the bluff. The information in the guide will help the following unfortunate scenario from happening to you.
Heavy equipment clears the brush and small trees from the uplands. Trees on the bluff top are cut; their stumps and roots are pulled and pushed over the crest. Clearing debris are piled and burned or join the stumps over the bluff edge. Trees on the slope and crest are removed or topped to open up the view. The top of the bluff is graded to remove topographic irregularities and allow free access to the edge.
The home is sited as close to the crest as possible to obtain the most dramatic panorama. The septic system is installed. Excavations for foundation footings are dug. Trenches for water, power, and waste lines are dug. Roof and footing drains are installed. Construction of the residence is begun. The house takes shape quickly. As construction proceeds, a stairway is built to the beach and more trees and brush are removed from the slope.
The area surrounding the house has been repeatedly scraped, graded, and subjected to traffic. Soil has become compacted and fairly impervious to water. It is doubtful that it will support a lawn. A landscaper is called in. Topsoil is brought in and the lawn is installed. Flower beds are built and ornamental trees are planted. In neglected corners of the clearing thickets of alder, thistle, and Scot's broom grow in the disturbed soil.
After several years some irritating problems begin to worry you. The lawn dries out in the summer and requires frequent watering. In the winter the yard is soggy and puddled. The few trees left on the bluff top have blown down, died, or the tops of some have broken in the wind. The brush below the crest has grown too tall to see over and young alders have begun to obscure the view. The trees that were topped are also in the way again and make you nervous when the wind blows. The trees you planted don't seem to be doing well; they are brown and dead-looking on the seaward side.
A tree trimmer tops the trees again and cuts the brush and alder so your view is back. He mentions that some of the old stumps from the initial view-clearing are becoming undermined by erosion and the rootwads that were pushed over the edge made it difficult to work. They have been sliding downslope and have caused some small landslides. He also remarks that in places the edge of the bluff is undermined and seems unsafe. He notices that there are several patches of bare ground and signs of mudslides. You are surprised and concerned. You don't remember seeing bare spots the last time you used the stairway to the beach, though you have not been down there since a washout made it unsafe.
After the tree trimmer's visit you decide to call a geologist. Her investigations indicate that the slope shows signs of serious surface erosion, soil slumpage and the potential of a landslide. She also notes the undermined crest and suggests it be fenced off from use. She says that bluff retreat has accelerated and advises that perhaps the house be moved further back from the edge in the near future. You are understandably unhappy and wonder how your dream house could become such a nightmare.
The scenario above is rather dismal. While often the situation is not this bleak, these problems nevertheless occur all too often in the Puget Sound area. Many of the problems property owners experience in regard to surface erosion and slope failure can be attributed to ill-advised clearing of vegetation. It can sometimes take years for the consequences to become evident. Thus it is crucial that property owners understand the role of vegetation in the shoreline environment and how proper management and planning during development of shore and bluff sites can benefit the land and protect your investment.
Vegetation management should be incorporated into your site development plans before you begin construction. This requires that you understand the role of vegetative cover and its ability to protect a site in relation to topography, drainage patterns, soil type, and natural shore processes such as wave attack. Also, before you alter the shoreline environment, it is wise to first learn how it was formed and the processes that are continually shaping it.
Keep in mind that vegetation alone cannot protect against erosion in all cases. Vegetation cannot withstand wave attack at the toe of a slope, nor will it prove effective in stabilizing a slope already subject to deep-seated mass soil movements. If you suspect problems of this nature, seek the services of a geologist who is familiar with conducting geotechnical site investigations before you build.
Could the difficulties our hypothetical homeowner suffer have been avoided? What could have been done differently? Would careful clearing and tree trimming rather than removal have made a difference? There are no "cookbook" recipes for maintaining the stability of dynamic shorelands, but a knowledgeable property owner is less likely to make mistakes that could have been avoided. The purpose of this website is to give you the the resources to make informed choices.
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