HomeMy WebLinkAbout701153016 Drainage Report
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STRATUM GROUP
1451 Grant Street, Bellingham, W A 98225
Phone (360) 714-9409
July 31, 2005
Michael Havens
Re: Clearing and Storm Water Impacts
Tax Parcel 701153016, Toandos Road
Jefferson County, Washington
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Dear Mr. Havens:
Clearing of trees from the site as described by you in your email to me will have no impact on
storm water. Most of the site is underlain by sand and gravel (advance glacial outwash and/or
transition beds). These soils are very well draining with very rapid permeability. Clearing of trees
from areas underlain by these well-drained soils will not lead to any increase in storm water flow
on the surface. The ravine on the north side of the propertY and the ravine south of the property
are dry ravines with no indication of flowing water at any time. These features appear to be
remnant outwash ravines and no longer transmit water even during extreme rainfall events.
The following recommendations are made to ensure that the clearing will not have any negative
impacts to storm water or to slope stability. The first recommendation is the same as the
recommendation I made in my initial report. The other recommendations are based on my
understanding of the geology of the site and slopes on the site.
Ground cover vegetation within the ISO-foot landslide buffer should not be disturbed.
Trees within the buffer may be thinned or limbed for view purposes. However, if the trees
are thinned, trees should be left on at least 30-foot centers. The mature trees on the slope
are hydrologically mature and as such will greatly reduce the risk of the upper slope from
becoming saturated. The trees also playa significant role in landslide deposit stability.
That is large wood debris greatly slows the rate at which erosion of a landslide debris pile
at the base of the bluff in the event that another landslide takes place.
No clearing of trees should take place within the pipe stem portion of the east end of the
property as this area is underlain by glacial till and clearing in this area may lead to an
increase in storm water flow. Clearing in this area may be appropriate subject to further
on-site review.
No clearing of tress should take place on the steep slope between the access road and the
southeast comer of the property without additional review as to the potential storm water
impacts. If this area is underlain by till (I did not look specifically at this slope), increased
run off down the slope could lead to erosion and slope stability problems.
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July 31, 2005
Tax Parcel 701153016, Toandos Road, Jefferson County, WA
Clearing and Storm Water Impacts
The clearing of between 3 and 5 acres with the above areas excluded will not in my opinion
increase the risk of another large slope failure on the shoreline bluff. While water plays a role in
the failures on the bluff, the main drivers of the bluff slope failures have been erosion of the toe
of the slope and saturation of the steep slope itself. Though the slide is very large, it is mot a
deep-seated failure driven by excess ground water recharge. The size of the failures appears large
simply because of the very high bluff slope.
I will forward this letter directly to Gregg Ballard for his review along with your email. I will
follow up with you after I return from vacation.
We appreciate the opportunity to be of service to YOlL Should you have any questions regarding
our reconnaissance please contact our office at (360) 714-9409.
Sincerely yours,
Stratum Group
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Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
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Daniel McSf-,2no I
Stratum Group
File:7.31.05
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