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Based on the results of a recent study by geotechnical and
environmental consultant Shannon
and Wilson, Inc. (S&W), DPD will be reviewing new
development projects in the former peat bog area of Greenwood
with the goal of limiting the withdrawal of groundwater both
during construction and from permanent development.
Commissioned by Seattle
Public Utilities in 2003, the S&W study responds to
neighborhood concerns that property settlement in the area
was increasing.
During the study S&W developed a map that delineates the
former peat bog area and identifies the variable depth and
thickness of the peat. They also tested the peat for the ability
to re-introduce water into the substrata and placed monitoring
devices in several locations for a more long-term assessment
of groundwater flows and levels in the area.
The S&W study indicates that the withdrawal of ground
water from the peat area should be avoided, and water should
be re-introduced where feasible to maintain current water
levels.
One outcome of this study is that DPD is now requiring new
projects subject to the State
Environmental Policy Act (SEPA) to show how their design
will result in no net loss of groundwater. In the study area,
SEPA applies to developments above certain thresholds on properties
zoned commercial or multifamily along portions of North 85th
Street and Greenwood Avenue North.
Several options for meeting this requirement have been identified,
which include eliminating basements, developing water-tight
basements, or providing the re-introduction of water via such
methods as infiltration and newly permeable surfaces that
replace water lost through traditional drainage facilities.
DPD is researching whether Environmental
Critical Area (ECA) regulations might also apply, which
would extend the new groundwater requirement to single family
zoned properties, which constitute the majority of the peat
area.
In addition, Seattle Public Utilities plans to inspect and
clean out perforated catch basins that were placed in the
area in the past to re-introduce water into the substrata.
They are also investigating opportunities for other water
re-introduction features in other components of the utility,
where replacement of utilities is required and such a goal
is feasible, based on soils conditions.
Questions?
For more information about the S&W study, or on DPD's
review of projects affected, please contact:
Cliff Portman
DPD Principal Planner
cliff.portman@seattle.gov
(206) 684-5593
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