WC133 NovDec 2023 - Magazine - Page 28
STORMWATER
pressure, or “head,” to push it through the
biofilter media layer below. When the water level rises above the berm, it is evenly
Thanks to green infrastructure, Peel Region was able
distributed and flows into the larger infilto successfully execute the project within a limited
tration basin.
This portion of the infiltration basin
footprint while still meeting the design objectives.
includes landscape plantings to promote
the biological uptake of pollutants, while
below the surface treatment layers is an
infiltration gallery comprised of clear
stone layer wrapped in geotextile. After
area of 3.74 hectares. Fletcher’s Creek, a known habitat for the
flowing through the clear stone layer, stormwater enters the surfederally and provincially identified species at risk–Redside Dace
rounding native soils and continues through to the environment
fish–is the ultimate downstream receiver.
as it naturally would. Any stormwater that does not infiltrate is
Previous performance monitoring showed that the facility
collected by perforated pipes that discharge to the downstream
experienced longer drawdown times than originally designed and watercourse via an outlet-control manhole.
less than optimal water quality treatment. Significant sediment
To manage fluctuating water levels and reduce pressure on the
accumulation within the facility was also observed, as it had not
basin’s surface, especially during larger storm events, the design
been serviced since construction.
includes an overflow along the basin’s edge. The first flush of
In essence, the original facility design was ill-equipped to meet stormwater undergoes initial treatment, passes over the biofilter
the contemporary challenges of stormwater management, lacking media, and then flows into the infiltration gallery. By the time
both size and thermal mitigation measures. To address these
water in the facility rises to the level of the overflow outlet, it is
issues, a facility retrofit was deemed necessary.
considered relatively clean and enters directly into the infiltration
Potential retrofit options were evaluated to meet four projgallery.
ect objectives: flood control with a 100 per cent reduction in
An important design feature is that the infiltration basin was
peak flows from the 2- to 100-year storm events; the ability to
intentionally oversized five times the required storage capacity.
infiltrate 100 per cent of a 27-millimetre precipitation event;
This excess volume provides ample opportunity for runoff from
enhanced water quality with at least 80 per cent removal of total
all storms to gather in the infiltration gallery where it comes in
suspended solids; and thermal mitigation.
contact with a layer of clear stones, which help cool the water as
it passes through.
Reimagining stormwater management
Specifically selected vegetation complements the basin’s design.
The preferred option was an innovative solution that mimics the
Drought- and salt-resistant plants thrive in the sandy soils of the
natural movement of precipitation into the surrounding environ- biofilter media, ensuring they survive even during the driest parts
ment by incorporating multiple design features in a treatment
of summer.
train approach. Instead of maintaining a wet pond design and
digging deeper to provide thermal mitigation, the facility was
Performance and recognition
to now operate as a dry infiltration facility with pre-treatment
Post-construction performance monitoring conducted by the
features to improve the longevity of the infiltration facility. An oil Region indicates the facility is performing well, with an avergrit separator provides water quality treatment to remove heavier
age peak flow reduction of 98 per cent and volume reduction
sediments, while a membrane filtration unit provides enhanced
of 86 per cent from the facility inlet to the outlet. Notably, the
water quality treatment before discharge into the main basin.
temperature of the outgoing water is, on average, 2.6°C coolThe main basin is a hybrid infiltration system with two stages
er, achieving the thermal mitigation target. Thanks to green
of treatment. The first stage is a hardwood mulched area at the
infrastructure, Peel Region was able to successfully execute the
pond inlet, underlined with a layer of specially designed biofilter
project within a limited footprint while still meeting the design
media (including sand, fines, and organic matter) to promote
objectives. Its innovative approach has earned the Bovaird facility
infiltration, that slows down stormwater entering into the treatretrofit project’s innovative design acknowledgment as a signifiment area and settles out sediment. A strategically placed berm
cant advancement in the delivery of stormwater management to
runs along the main basin, serving to disperse the flow of incom- protect sensitive fisheries and their habitat, and has been recoging stormwater. This design enables the stormwater to accumunized with the 2021 Ontario Public Works Association Project of
late within the mulched inlet area where it builds up enough
the Year Award in the technical innovation category.
28
WATER C AN ADA • NOV EMBER/ DECEMBER 2023
WAT E R C A N A D A . N E T
