WC129 MarApr2023 - Flipbook - Page 25
by the concrete as a heat sink. This results
in less stress on the receiving streams.
These tangible benefits make the
investigation of underground stormwater
capacity worthwhile when planning new
residential neighbourhoods in high land
When properly designed, maintenance
costs for underground detention systems
are generally equivalent to or lower than
above ground stormwater management
ponds. With larger detention systems
where water quality is addressed off-line to
the underground detention storage via a
treatment train-based approach, maintenance is relatively straightforward and
consistent with the processes developed
by the manufacturers of these systems.
Any bioretention-based systems upstream
of the storage system that are part of the
treatment train approach are typically
maintained by urban park staff (such as
trash removal, vegetation control or minor
grading, repair, etc.).
Proprietary systems, such as oil-grit
Including underground stormwater management storage in the design of new builds can make the most of
separators or filter-based systems using
high land value locations.
cartridges or filter media, can often be
maintained by third-party services as well
and typically do not require confined space entry, except in rare
re-suspension with the use of bypass systems for heavy flows and
conditions. Maintenance can be completed in a single day with
hydraulic processes that allow removed sediment to accumulate
little mess or evidence left behind. In contrast, maintenance of a
in storage areas resistant to it. When combining gravity-based
surface stormwater management pond requires disturbance of the removal of larger sediments in a primary water quality treatment
pond’s surface layers with excavation equipment such as excavasystem (such as an OGS unit), and a filtration-based approach in
tors and/or dump trucks. The surface stormwater management
a secondary treatment system in a treatment train configuration
ponds then require a period of time to re-establish disturbed
targeted at smaller fractions of sediments, an increase in sediment
vegetation before full treatment performance is restored.
removal (and attached pollutants such as hydrocarbons or excess
As for water quality treatment, an underground-based system
nutrients) is provided.
with a treatment train-based water quality approach, depending
Without question, there are definite advantages to the introon site considerations, is likely better at removing fine particuduction of this type of underground stormwater collection syslates from the runoff stream via media filtration than a surface
tem where the business case makes sense, as many of the previous
stormwater management pond. Meanwhile, surface-based wet
challenges in doing so have been addressed through innovation
ponds operate on a gravity settling principle where sediment
and technology improvements.
drops out of solution by detention in a low-energy environment
after a designated period of time. This performance can be comUnderstanding the challenges
promised by sediment re-suspension from a number of factors
As important to understanding the benefits of underground
including neglected maintenance, increased frequency of incident stormwater collection is the appreciation of the challenges
rainfall, and inadequate design due to above ground constraints
that such a system presents. However, these challenges can
on the pond’s geometry.
typically be met with fairly simple solutions, which is helping
In contrast, underground systems with off-line water quality
to make underground storage such a viable option when land
treatment systems are often manufactured to avoid sediment
values are high.
WAT E R C A N A D A . N E T