What is a Stormwater Pond?
Stormwater management devices are put in place to minimise
adverse effects associated with runoff from urban development.
Stormwater ponds are used for both water quantity and quality
control. These ponds detain runoff and discharge it at a specified
rate reducing the potential for flooding and stream erosion by
slowing the rate of stormwater discharged to the receiving
environment.
A treatment pond can be an amenity to an urban development by
using it to enhance landscape quality. Ponds can blend into natural
environments or create reserve settings in highly urbanised areas.
The element of water, along with the planting of native vegetation
surrounding the pond can add value to a development and provide
habitat for local fauna.
Key Design Features of a Stormwater Pond
There are a number of essential components, which should be
taken into consideration when designing a stormwater pond.
Stormwater ponds are deigned to contain a permanent pool of water
known as the water quality volume. Water flows into the through the
pond thereby providing time for contaminants to settle out of the
water column. The Auckland Regional Council sets a treatment
standard for stormwater ponds to remove 75% of total suspended
solids entering the ponds. Design calculations are also performed
to limit the post-development discharge rates for the 2 and 10 year
frequency storms or 100 year storm where there are known downstream
flooding issues. Calculating accurate orifice sizes will limit
discharge rates to match pre-development peak discharge release
rates.
Stormwater ponds should be constructed with safety features,
including a reverse slope bench or slope break and depth of no more
than 2m. A reverse slope bench will sit 300mm above the normal
water level and extend at least 3m from the waters edge. To further
improve the safety, dense vegetation should be planted on the
surrounds of the pond. The emergency spillway releases water from
the stormwater pond at a capacity greater than the pond outlet. The
spillway allows for excess water to be released from the pond
during storms. They should preferably be placed in natural ground
and designed to convey at least the 100-year storm with a freeboard
of at least 300mm. The sediment forebay is located at the inlet end
of the stormwater pond and is designed to capture coarse sediment.
This area of the pond (>15%) accumulates the greatest quantity
of coarse sediment and therefore requires regular cleaning out.
Minimising flow velocities will prevent sediment from being
re-suspended within the pond. Debris screens are used in ponds to
trap rubbish and organic debris to prevent blockages of the inlet
or outlet structures.
Cache is a painted 200ml sistema container