WC129 MarApr2023 - Flipbook - Page 26
STORMWATER
Underground water storage has the potential to lose dissolved
oxygen via stagnation. This can lead to anoxic conditions in underground water stored for any appreciable period of time where
water chemistry shifts to the production of noxious odours such
as hydrogen sulfide (the “rotten egg” smell) and the growth of
cyanobacteria.
The key to mitigating these impacts involve the regular changing of any standing volume of water stored in the facility so it
does not experience stagnation or ensuring the storage volume
of the underground facility fully empties regularly after precipitation events. Another mitigation approach involves ensuring
stormwater is well-saturated with dissolved oxygen before it is
stored underground for any appreciable period of time. The
approach will vary with project size with smaller systems benefiting from more rapid turnover, while larger systems should be
designed to fully empty between storm events.
Environmental impacts
As for environmental impacts, surface-based stormwater
management ponds are not a natural feature and they are
normally considered as necessary infrastructure by municipalities.
While they resemble natural features and allow for natural,
open-space-based experiences for pedestrians, their required
functions are not typically compatible with the establishment
of habitat-based improvements. Their open water nature allows
for some facultative use of them by waterfowl, but are typically
inconsistent with supporting natural flora and fauna, thus are
rarely if ever configured for this function and as a result are not
usually added to the inventory of natural heritage features or
considered in urban planning processes.
Open-air wet ponds possess other social issues that impact
the environment. One example is their use as a disposal
destination for unwanted aquatic pets, such as goldfish. While
it may seem to be a kindness to release these animals to the
wild to fend for themselves, the effect of this action is to
introduce an invasive species to aquatic systems where they
aren’t present naturally. Goldfish in particular act as bottom
feeders, stirring up sediment in receiving streams, which can
impact habitats for sight-feeders such as the endangered Red
Side Dace. In storm ponds, goldfish burrow into the sides of
the ponds, eroding their edges and increasing the maintenance
burden for municipalities. Substituting open air wet ponds
with underground systems removes the temptation to dispose
of these creatures in this fashion, and avoids the unanticipated,
associated environmental impacts.
Large underground detention systems are developed and
configured to resemble vault-style systems, with regular repeating
structural units providing a large unit storage volume. When
combined together they can create massive amounts of storage
suitable for flow control of large upstream development areas.
They are always structurally configured to support the weight of
soil, park land, or whatever is planned above them via reinforced
concrete columns, culvert rows, or other approaches, often with
a healthy factor of safety. For single site applications, typically
smaller than two hectares, similar performance may be achieved
via smaller systems that utilize structural plastic (similar to milk
crates or arch based half-pipes) that provide a lower relative
water storage volume and material cost. However, for larger
areas that would historically require a stormwater management
pond (drainage areas larger than five-to-10 hectares), concretebased systems are typically used. The economics of land value
in a metropolitan setting is shifting to allow this approach to be
economically viable compared to previous decades when it would
have been considered prohibitively expensive.
SUBSCRIBE TO THE EMAGAZINE FOR
FREE AND NEVER MISS ANOTHER ISSUE OF
Charles Bois
is a Partner with
Miller Thomson, LLP.
Michelle Eng
is an Articled Student with
Miller Thomson, LLP.
The Strategy puts an
emphasis on providing
You asked for it, and we heard you. Water education
Canada is
excited
to
and
outreach
unveil a brand new interactive eMagazine option
that:management
around water
by collaborating with various
Is easy to share; no download necessary
levels of governments
Links to related articles
Indigenous
and non Enables immediate engagementand
with
advertisers
Indigenous groups.
Includes video and other rich media
Cloud-based – access your issue anywhere
SUBSCRIBE HERE: WaterCanada.net/subscribe
26
WATER C AN ADA • M ARCH/APRIL 2023
WAT E R C A N A D A . N E T
