HEALTH AND SANITATION
storm drains as a stormwater conveyance system, where development
density, topography, and soils permit.
Dry swales
Dry swales, also known as bioswales, are essentially bioretention cells
that are shallower, configured as linear channels, and covered with turf
or other surface material (other than mulch and ornamental plants).
The dry swale is a soil filter system that temporarily stores and then
filters the desired design storm volume. Dry swales rely on a premixed
soil media filter below the channel that is similar to that used
for bioretention.
If soils are extremely permeable, runoff infiltrates into underlying soils.
In most cases, however, the runoff treated by the soil media flows into
an underdrain, which conveys treated runoff back to the conveyance
system further downstream. The underdrain system consists of a
perforated pipe within a gravel layer on the bottom of the swale,
beneath the filter media. Dry swales may appear as simple grass
channels with the same shape and turf cover, while others may have
more elaborate landscaping. Swales can be planted with turf grass, tall
meadow grasses, decorative herbaceous cover, or trees.
SELECTING THE APPROPRIATE OVERFLOW DISCHARGE LOCATION
Selection of an overflow discharge location must not only include a
comparison of the advantages and disadvantages discussed above,
but also a number of other factors, which include:
• Stormwater management requirements – in some cases,
overflow from the rainwater tank may need to be handled in
accordance with special stormwater management requirements.
These requirements may be imposed by a municipality or various
conservation authorities for buildings located in an
environmentally sensitive area, or in an area where the existing
storm sewer infrastructure does not have sufficient capacity
to accept additional stormwater flows, or for a variety of
other reasons.
• Applicable provincial/national regulations and municipal bylaws
– even if there are no special stormwater management
requirements, provincial regulations and municipal bylaws
may still restrict the locations where rainwater overflows can
be discharged.
• Tank location – the location of the storage tank can also have
an impact on the overflow discharge location selected. Overflow
handling is simplest with above ground tanks, since overflows
can typically be discharged to grade.
• Site conditions – site conditions, such as topography, space
availability and accessibility, and the existence of other
buried services, also affect the selection of an overflow
discharge location. PA
In the next article we look at overflow design and maintenance.
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