HomeMy WebLinkAboutCAP 2007-10-23 Item 2C - Update - Tukwila Pond Water Quality Improvement Feasibility Study and Design
City of Tukwila
Steven M. Mullet, Mayor
Department of Community De1Ielopment
Steve Lancaster, Director
MEMORANDUM
Subject:
Mayor Mullet
Jack Pace, Acting Director, DCD \/!.1
October 18, 2007 ~ \
Tukwila Pond, Update on Water Quality Improvement Feasibility Study and Design
To:
From:
Date:
ISSUE
Briefing on status of project and presentation of conceptual designs for water quality improvements to
Tukwila Pond.
BACKGROUND
Design of the improvements to Tukwila Pond, as outlined in the Council-approved Tukwila Pond
Concept Plan are currently underway through a contract with Robert Droll Landscape Architect, being
managed by Parks and Recreation and through a second contract with Tetra Tech for feasibility and
design of \vater quality improvements, being managed by DCD. The concept plan calls for improvements
for public access and amenities to and around the pond, as well as wildlife protection and enhancement,
and identifies the need to improve water quality.
The water quality consultant, TetraTech has developed a series of conceptual design drawings for three of
the four elements of the water quality improvement program. Before proceeding to detailed design, staff
would like the consultant to present the concepts to the Council. The three concept drawings are attached
and show:
~ Hydroponic Docks/Islands. These are floating docks or islands that would be planted with
wetland vegetation to absorb nutrients (especially phosphorus) from the water column. They
would also provide nesting or browsing habitat for waterfowl and help to shade parts of the
pond, thus helping reduce temperatures underneath.
~ Circulation! Aeration! Alum Injection. This involves pumping an alum flocculate into the
pond to bind phosphorus into the sediment and keep it out of the water column. It also
involves aeration to circulate water in the pond and thereby improve the oxygen content.
~ Southwest Pond Area Planting. This element involves planting emergent wetland plants in
the shallow southwest area of the pond. The vegetation would help remove phosphorus from
the water column, provide some shading, and provide habitat benefits.
S. Whiting H:rrukwiIa Pond Plan/WQ Update CAP Info memo
6300 Southcenter Boulevard, Suite #100 e Tukwila, Washington 98188 · Phone: 206-431-3670 · Fax: 206-431-3665
A fourth element, dilution of the pond water by bringing in cleaner water, is still under evaluation by the
consultant. This alternative \vould lower phosphorus concentrations in the pond by flushing clean water
with low phosphorus concentrations into the system. Several dilution/flushing options are being
evaluated for their cost and effectiveness.
Detailed designs will be developed in November and finalized in December. Other tasks under this
contract include development of cost estimates, an operations and maintenance plan, a contingency plan,
and permit documents.
RECO:MMENDATION
Forward this issue to the COW for a presentation by the consultant, TetraTech.
S. Whiting H:ffukwila Pond Plan/WQ Update CAP Info memo
PUBLIC WORKS DEPT.
-ENGINEERING-STREETS-WATER-SEWER-PARKS-BUILDING-
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1420 50, Avenue, Salta 550
Solna, WA 98101
Phone (206)728.9655
Fax (206) 728-9670
TETRA TECH TUKWILA POND LAKE
ISLAND SECTION
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ISLAND MODULE PLAN
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VEGETATED FLOATING ISLAND CONCEPT
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TUKWILA POND LAKE
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CIRCULATION AND ALUM INJECTION CONCEPT
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12/12/2007
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tukwila pond lake map