Burnaby Lake Regional Nature Park
comprises 280 hectares in the Still Creek - Brunette River
Watershed in Burnaby, B.C. The park has high ecological value,
and consists of wetlands, low-land forest and Burnaby Lake.
Numerous creeks flow into the lake, which acts as a sink for
sediments transported by the creeks from surrounding urban
areas. In 1972/73 the lake was dredged to facilitate the
rowing course for the 1973 Canada Summer Games.
Since the 1973 dredging, the lake has
continued to fill with sediments. The City of Burnaby
commissioned a study in 1997 to assess the environmental
implications of dredging the lake for environmental
rejuvenation. The study recommended that the City undertake a
pilot-scale dredging program, prior to initiating any
full-scale dredging operations, in order to collect more
information that would allow a better understanding of the
technical feasibility, economics, and environmental
implications of dredging Burnaby Lake. To this end, the City
retained the consulting team of Associated Engineering, Enkon
Environmental, and Hay & Company Consultants in the spring
of 1999 to execute a pilot-scale dredging program. The main
objective of this program was to conduct a short duration, but
high production capacity, sediment dredging and processing
operation that would allow for sufficient data to be
collected. Associated Engineering was responsible for
evaluation of sediment dewatering technologies, completion of
a contractor prequalification process, tender documents,
contract administration, and report preparation. Technical
feasibility of a potential full-scale dredging program was
assessed, considering method, feasibility and impacts of
dredging, dredging rates, environmental impacts, process and
implications for on-site sediment dewatering, and off-site
solids disposal constraints. Trimax Residuals Management Inc.,
retained as the project contractor, was responsible for the
dredging, sediment dewatering, and water treatment operations.
"The
project", relates Project Manager Larry Martin,
"determined, from a technical perspective, that
dredging of the lake is feasible and can meet the
requirements of the Federal Department of Fisheries and
Oceans and the Provincial Ministry of Environment Lands and
Parks. The project findings have only recently been released
and the client must now look at project cost estimates,
restrictive dredging windows imposed by the DFO and
extensive public hearings before determining the next course
of action".
The pilot
dredging project removed approximately 2500 cubic meters of
in-situ sediment from the lake with an IMS Versi-DredgeŽ
auger suction dredge. The sediment-water slurry was then
pumped to the dewatering site where the coarse organic
material was separated from the slurry with an inclined screen
and screw. The fine-grained solids were separated from the
slurry using centrifuges to form a product with a solids
content ranging from 25% to 40%. The centrate was treated to
reduce the total suspended solids concentrations (TSS) to
levels below those imposed by the Federal Department of
Fisheries and Oceans (DFO), and the treated effluent was then
discharged back to Burnaby Lake via Still Creek.
A silt curtain
encircling the dredging site proved successful in preventing
sediment migration down the lake. Lake water samples were
collected on a daily basis and analyzed for turbidity, TSS,
pH, conductivity and dissolved oxygen in order to determine
dredging impacts. Dewatered sediment samples were tested for
metals and hydrocarbons. To confirm that residual polymer
(used to enhance the solids capture of the centrifuges) in the
effluent would not be toxic to aquatic life, bioassay toxicity
tests were performed on Daphnia magna and Rainbow Trout
organisms.
The dredging
and sediment processing methodologies and technologies used in
the pilot-scale project successfully demonstrated the
technical feasibility of this type of project. Similarly, in
terms of environmental impacts to Burnaby Lake, the
pilot-scale project was successful in mitigating potential
impacts to allowable levels.
Associated
Engineering personnel involved in the project include Larry
Martin (Project Manager) and Dean Shiskowski.
