HomeMy WebLinkAboutPN_Project_Narrative_181022_v1.pdf1
Clark Close
From:Bean, Tom <Tom.Bean@kingcounty.gov>
Sent:Friday, October 12, 2018 4:35 PM
To:Clark Close
Subject:Geotechnical exploration at Black River Pump Station
Attachments:BRPS_ProposedBoringLocations_DRAFT_20180827_v2.pdf; Renton Exemption Request
signed 20181012162111.pdf
Clark,
I am writing to request help with navigation of the City of Renton’s permit process for geotechnical exploration that we
propose at our existing Black River Pump Station (BRPS). The BRPS is a surface water pump station that lifts all flow from
the Springbrook Creek system to the level of the Green/Duwamish River in the remaining Black River channel. The pump
station is also an integral part of the Green River levee system that protects the City of Renton and the surrounding area
from Green River inundation during floods and high tides.
My office is beginning the design process for extensive BRPS improvements on behalf of the King County Flood Control
District. One of our first major design issues is to determine the risk of seismic damage to the pump station, which has
shallow spread footings. The Black River channel bottom has been identified as a seismic liquefaction hazard area, so we
wish to determine the vulnerability to possible seismic damage at BRPS and to evaluate opportunities for remedial work
that might provide cost-effective improvements in BRPS resilience.
To that end, our design consultants propose to pursue two drilled explorations of the strata beneath the pump station,
one on each bank adjacent to the pump house. Both of the drilled explorations will be on King County property (PIN
377920-0090) within the existing fenced yard around the BRPS. Truck-mounted drill equipment will be used, and no
trees or other vegetation will be disturbed.
Both of the drilled explorations will extend roughly 100 feet below ground surface, which is roughly at 28.5 feet
(NAVD88) for both drill sites. This surface elevation is roughly two (2) feet above the best available base flood elevation
per the 2010 preliminary DFIRM (panel 53033C0976K). The existing effective 1995 FIRM (panel 53033C0976F) shows a
much lower base flood elevation; both drill sites are roughly six feet above the existing effective BFE. Both sites are
within the buffers of the Black River channel and identified wetlands, but we anticipate no adverse impact from this
proposed work because, as I’ve said, we’ll be within the existing cleared yard.
I have attached the City of Renton’s Critical Areas Exemption form, along with a sketch that shows the proposed drilling
locations. Please note that the test pits shown on the sketch are not proposed at this time.
I would appreciate your advice as to how I should best proceed. Should I request a pre-application conference? Or
provide additional supporting information? How can we help the City approve this simple proposal? Our hope is to
complete this drilling in early November.
Thank you,
Tom
______________________________
Tom Bean, P.E.
Engineering Special Projects Lead
River and Floodplain Management Section
King County Water and Land Resources Division
201 S. Jackson Street, Suite 600
Seattle, WA 98104