Understanding Your Property
The locations of Puget Sound coastlines were established by the retreat of the last period of glaciation in the Puget Sound lowlands. Coastlines are continually modified by coastal marine processes and by the gradual natural rise in the sea level. Today, Puget Sound coastlines boast a range of shore forms which include high bluffs and rock cliffs, tidal mud flats, and sand and gravel beaches. You are probably most familiar with the common Puget Sound bluffs and their associated sand and gravel beaches which have varying slope heights and beach widths. The glacial deposits which define the general geologic composition of coastal slopes are illustrated in Figure 3 (below). Glacial till is typically the most common geologic unit along Puget Sound coastlines and contributes heavily to the sands and gravels deposited on beaches. The effect of the slow rise in sea level over time in the Puget Sound area can be seen on Figure 4 (below).
Figure 4. Coastal Bluff Development. Left: Evolution of a gravel and cobble beach. The longshore transport diagrams illustrate the direction of material movement along the beach. Right: Sea level since the last glaciation (from Downing, 1983. Used with permission of the Washington Sea Grant Program).
Selecting a successful vegetation program requires that the landowner spend time understanding what is happening around their slope. This means that you need to start keeping a list, observing the behavior of your slope and noting deviations from that typical behavior. The sections in this chapter will help you learn to make use of your observations.
Observations made during periods of heavy precipitation, strong winds, freeze-thaw, heavy wave action, or other local slope movements are extremely important and empower you to help provide solutions to your situation. Ecology publication 93-31, Vegetation Management: A Guide for Puget Sound Bluff Property Owners, will also be helpful to you in performing your site evaluation - particularly as it relates to existing vegetation.
Your site observations should be grouped into the following general categories:
The Coastal Property Owner Checklist
provided in this online manual will allow you to start
organizing your observations. Additional information about your
slope site should also be added to the checklist as necessary.
The following discussion will aid you in completing your
Slope Angle and Height
To determine the angle of your slope, an easy method is illustrated in Figure 5. It is a simple device which can be constructed with a protractor and a yardstick. The protractor is fastened securely to a yardstick with a string and weight attached as shown. When the yardstick is held up and aligned with what appears to be the average slope of the land, the slope angle can be read directly from the protractor. This slope angle can then be converted to the appropriate horizontal/vertical ratio as shown. The quickest way for you to estimate slope height is to visually estimate the height of some nearby vertical structure on the slope (i.e. tree or bluff face) and then estimate how many tree heights would equal the overall slope height.
Figure 5. Slope Angle Determination
Site specific information noting soil types, thicknesses, and moisture conditions is very helpful to you when you select both plant species and planting techniques to be used in your program. Observing the types of vegetation already living on the slope is a good clue to slope soil conditions. The County Soil Conservation Service, Soil Conservation Districts and Cooperative Extension Agents are good sources of information and expertise on soil characteristics.
Puget Sound coastal weather is strongly influenced by changing topographic and atmospheric conditions. The degree of precipitation, available sunlight, temperatures, and wind can change radically from one coastline sector to another and consequently from site to site. These factors should be recognized in your site evaluations and planning effort which includes both planting and drainage control elements.
Water in and on the slope is the most common agent causing erosion and slope instability and should be adequately examined during the site evaluation and planning effort. Observations of your slope's drainage conditions should include a careful search for seepage from the slope face. Surface water runoff should also be investigated and is best observed during periods of heavy rainfall. Pipe discharges and uncontrolled drainages across poorly vegetated surface soil are typical slope erosion problems. Visual observation of surface water runoff problems include watching for thin layers of water flowing across the slope face. Surface water runoff contained in channels usually starts in small rills in the upper soil horizon and then through time erodes into gullies which downcut into the slope. Runoff which concentrates into channels creates greater problems for slope stabilization programs because of the magnitude of erosive forces.
Evaluation of the existing slope vegetation is the key final observation of the slope because it should confirm your previous observations. Use the presence of vegetation (or lack of it) to confirm or modify your site evaluation. Vegetation will exist in areas which favor its establishment. As you identify specific types of vegetation on your slope, you can consult other references to determine the range of environmental conditions (i.e. soils type and moisture, light preferences, etc.) which the observed plants tolerate. You should then be able to confirm your original observations. If your observations indicate that these species should not typically grow on your slope, you may want to perform your evaluations again or investigate whether the site has recently been disturbed or modified.
Loss of your slope's surface soil layers by the action of wind, water, and ice are what is referred to as soil erosion processes. Rainfall is the major factor along Puget Sound coastlines. However, wind and frost wedging do act upon some exposed slopes. The rates of erosion can be controlled by reducing the sources of runoff to the slope and/or by modifying the nature of the site conditions which influence runoff rates (e.g. soil type, slope length and steepness, ground roughness, type of vegetation covering the slope, and exposure to winds). A typical formation of rills and gullies is shown schematically on Figure 6.Wind and frost wedging erosion can be reduced by allowing plant foliage to cover exposed slope areas. The foliage creates a physical and thermal barrier which protects the surface soils.
Figure 6. Typical Puget Sound Coastal Slope Processes
When a downward movement of relatively intact masses of slope material occurs, it is called a mass movement (instead of erosion). Mass movements can take the form of landslides, earth/debris flows and slumps, and rock falls/earth topples.
Photo at right: Initial shallow-seated landsliding.
Figure 6 (above) illustrates these basic soil movement processes. These events have been lumped together by planning agencies and the public and are generally called landslides. For simplicity, mass movements will be referred to as landslides in this manual. Recognition of active or historical landslide areas is important. Should you believe landsliding has occured or that there is the potential for landsliding on or adjacent to your property, it would be prudent to secure the assistance of a professional geotechnical engineer prior to the site planning process. It is important to reiterate that vegetation can be used to control shallow seated landsliding which is common along coastal slopes. However, it does not address deep seated landslide events. Vegetation can also be a valuable component of biotechnical engineering design solutions to these deep seated potential landslide conditions. Consult a geotechnical/biotechnical engineer for design input on stabilization alternatives.
Should you recognize or suspect that your property or adjacent properties contain any of the following landforms or slope conditions, you should contact the appropriate government agencies to acquire assistance in these technical areas. You may also want to have consultations with technical experts in these areas:
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