HomeMy WebLinkAboutTree Removal Hartstene Island Bluffs - GEO General - 10/12/2011 ..j
A Division of The Bradley Groin, Inc.
PO Sox 122e7, Olyrn!:)ia WA 98508-2267
?hone 360-3--7-7333 = ,-Av 350-367- 307
12 October 2011 RECEIVED
Mrs. Michelle Bell
587 East Point Drive West 44" vV
Shelton, Washington 98584
Subject: Effect on the erosion of the marine bluff by removal of small diameter
trees from the top of the marine bluff to maintain the view for properties
at 581, 585, and 587 East Point Drive West in the Hartstene Pointe
development, Hartstene Island, Mason County, Washington.
Dear Mrs. Bell:
As requested, we met with you in order to evaluate the probable effect of the removal
of small diameter trees from the top of bluff on the bluff stability and present active
erosional processes. From our field conversation, we understand that removal of trees
less than six inch in diameter at breast height is permitted by the home owners
association in order to maintain views. Trees larger than six inch diameter typically can
be limbed to maintain views. We also understand that a forester has also consulted
with you in the past on tree removal.
The view from your and adjacent properties is being reduced by the growth of small
diameter trees. The trees in question are growing between the present top of bluff and
the community walking trail in front of your and adjacent properties. In order to
remove these trees, a geotechnical opinion is required to evaluate the possible affects
on the present erosion rate of the bluff.
The Coastal Zone Atlas for Mason County, Map MA 14 presents information of site
geology and an opinion of slope stability for the marine bluff at this site. This map
indicates the bluff to be formed by about 10 feet of Vashon age subglacial till (Qvt) at
the top of the bluff, 35 feet of Pleistocene gravels older than Vashon age till (Qpg) and at
the toe of the bluff an exposure of about 10 feet of Pre-Fraser drift that is
undifferentiated. Visual examination of the bluff face is in conformance with the geologic
profile presented in this map. The bluff and area immediately upslope of the top of bluff
is indicated as unstable. The upland area above the bluff is considered to be stable. We
are in agreement with this opinion.
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Site geology controls the rate of erosion of the marine bluff. Erosion of the toe of bluff
by marine process is not considered to be a major erosional agent at this site. The
active erosional agents working on the marine bluff are sheet washing of loosened soils
by water flowing over the bluff face. The source of this water is wind driven rains and
flows of upslope storm water over the bluff. Ice wedging and ice plucking are active
during periods of subfreezing weather. Soils cropping out in the bluff face are
overconsolidated due to loading by the Vashon lobe of the Cordilleran glacier as it
moved into the Puget Sound Lowland during the Fraser glaciation in late Wisconsinan
time. The overconsolidation of these soils is the result of the Vashon ice sheet loading
these soils. The ice sheet is thought to have been from one-half to one mile thick in this
area.
The rate of retreat of the top of the bluff is slow in this location. We estimate that the
average rate of retreat of the top of the bluff is probably about six inches over a 30 to
50 year period. Dicing of soils from the bluff face is the most obvious event. These
dicing events are caused by the slow erosion of the bluff face creating overhanging soil
mass at the top of the bluff that also supports vegetation. When sufficient material has
been removed by erosion, these overhanging soils masses fail by gravity and carry soil
and vegetation to the toe of the bluff. These events are random in occurrence, with
apparent stability returning between events.
We find trees growing out over the top of the bank. These!ttrees can cause loosening of
the upper soils of the bluff by wind action working the root ball of these trees. When
these trees fail and fall to the toe of the bluff, the root ball can wrench soils from the
bluff. Our opinion is that reduced potential damage to the top of the bluff can be
L developed by their removal as long as the root ball is retained. Even when the
overhanging tree is removed, long-term erosional process will eventually cause the root
ball to fall to the toe of the bluff.
We do not believe that the removal of small diameter trees from the area between the
top of the marine bluff and the walking path would have any adverse influence on the
rate of the natural erosion and headward retreat of the bluff. We do recommend that
the removal of these trees be conducted in a manner that minimizes damage to the low
growing vegetation. Trees felled should be removed to upland areas of disposal to aid in
the protection of the low growing vegetation. The roots of the low growing vegetation
aid in protection of the soils from sheet washing. The stumps of the removed trees
should be left in place.
�In summary, removal of trees in the area between the top of the marine bluff and the
walking trail will not have any adverse affect on the present bluff erosional processes.
The bluff will continue to erode at the observed rates due to geologic processes not
influenced by trees in the upland area. During tree removal, care must be exercised to
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protect the low growing vegetation from excessive damage. Any areas of exposed
mineral soils created by the tree removal must be protected from sheet washing. To
stabilize exposed mineral soils, replanting or other erosion control methods may be
required.
If you have any additional geotechnical questions on the opinions and recommendations
presented in this report, please contact us at our Olympia office.
Cordially,
BRADLEY-NOBLE GEOTECHNICAL SERVICES
David C. Strong, L.E.G.
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