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Understanding Your Property :
-----------------------------------------------------------------------------------Puget
Sound Geology
Most of the Puget Sound geology
that influences coastal slope drainage and stability
is a result of past glaciations. Figure 2 shows the
general geology of the Puget Sound area. The glaciers
deposited the soil layers that make up your slope. A
common soil layer sequence found along coastal slopes
is shown on Figure 3 (below). The layers consist of
glacial till, sand and gravel, and silt/clay. Your shoreline
bank or bluff could be composed of one or more of these
soil layers. Some isolated areas of Puget Sound shorelines
consist predominantly of rock instead of soil.
In the construction
industry, glacial till is frequently referred to as
hardpan because it is very dense, making it difficult
to dig. Hardpan is a mixture of clay, silt, sand, and
gravel. Water infiltrates through this soil very slowly
and consequently water will often accumulate or "perch"
above the glacial till during and aftpugeter wet weather
periods.
Figure 3. Common soil layer sequence found
along coastal slopes.
Sand and gravel soils usually contribute to the stability
of a slope in the absence of water. However, water readily
flows through sand and gravel. Water can reduce the
stability of the slope when it accumulates above a soil
layer that is not as permeable as the sand and gravel.
Water that accumulates above the impermeable soil layer
and may flow laterally until it "daylights"
as seepage on the slope face.
In the Puget Sound
region, the impermeable soil layer can be a silt layer
within the sand and gravel unit or a silt/clay layer
located under the sand and gravel deposits. The impermeable
silt/clay layer was compacted under the weight of the
overlying soil units and more importantly, the weight
of the glaciers.
Today's shoreline
is a result of continuing geologic evolution since the
time of the glaciers. At the top of the slope is a weathered
zone (including topsoil) that is usually a few feet
thick. Landslide debris and deposits from erosion and
weathering can be found on the slope face and at the
toe (bottom) of the slope. The debris and deposits usually
consist of a jumbled mixture of all the geologic soil
layers that make up the slope as well as vegetation
that moved down the slope.
Surface and ground
waters influence slope erosion and stability. Each year
wet weather stresses many vulnerable properties to their
points of failure which causes severe erosion and landsliding
events around Puget Sound. These notable occurrences
can usually be traced to the following issues: recent
changes in the surface conditions around a property;
accumulated small slope stability weaknesses that go
undetected or unattended; or poor drainage system performance
on a property.
You have probably
observed some signs of slope distress during a wet weather
season. As a coastal property owner, you should be aware
of the role water plays in the short and long term stability
of coastal slopes. Excessive soil erosion and land movements
can create restoration costs and environmental impacts
costs. Each of these costs is avoidable.
Although this website
deals primarily with managing drainage issues along
coastal slopes, other factors also influence slope stability
and erosion of slopes. These factors include: subsurface
geological characteristics; vegetation management on
and above slopes; property modifications during property
development; and coastal marine processes acting at
the slope toe. Each of these factors should also be
considered in your drainage planning to provide a comprehensive
approach to slope stabilization and erosion control.
Other Ecology publications are available to help with
your planning. Refer to the links section for more information.
The main sections
are introduced on Figure 1. Each section builds on information
presented in previous sections. So, it is important
that you review each section of the website before skipping
directly to specific sections.
Three basic steps
can protect your slope against accelerated erosion and
landsliding.
First,
understand your property. It is not an extensive effort
to generally characterize your slope area and identify
the water movement around the slope.
Second,
identify problems and plan appropriate improvements
into your site. Take the opportunity during property
development to include drainage control with your landscaping
work. On each coastal property, there are typical site
constraints which must be considered. Identifying the
opportunities and constraints of your site are key goals
of your planning effort.
Third,
carefully construct and maintain your drainage system.
Taking the time to ensure that good materials and workmanship
are used on your property cannot be overemphasized.
Give your system periodic maintenance tune-ups.
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