SA Affordable Housing November - December 2018 // Issue: 73 | Page 6

NEWS
stormwater surveys undertaken by Graham McGill of Graham
McGill Consulting.
The channel forms part of a system which has been
designed to ensure that stormwater flows are managed and
dispensed in a controlled manner. Several tributaries and
stormwater pipes which previously flowed into several
evaporation ponds, situated on the land earmarked for the
new housing estate, now drain into the main channel,
Channel 1 and into two tributary channels 2 and 3.
The unlined section of the main channel was designed to
accommodate a 1:100 year flood (50.2m³/s) whereas the
lined section can accommodate a 1:5 (18m³/s) year flood.
The deployment of the blocks and mini gabion dams,
which were built into the downstream section of Channel 1,
reduce flow-rates and allow the SUDS system to better
filtrate any runoff. The other major component of the system
is a large attenuation pond which was excavated at the
bottom end of Channel 1.
1
The pond further reduces the flow rate and prevents surges
by releasing water gradually through a 450mm outlet pipe at
the far end of the pond. However, when water volumes are
such that the pond fills to capacity, the overflow pushes back
into the main channel at the entrance to the pond.
In addition, a secondary and smaller attenuation pond has
been excavated immediately above some culverts which
have been installed under a main tarred road adjacent to the
estate. Grass has been planted on the floors of both ponds to
minimize dust during dry periods and encourage water
infiltration.
The main channel is composed of four material layers: the
sub-soil which was compacted with rollers to 93% MOD
AASHTO; a layer of Grade A2 geofabric, which prevent the
blocks from settling too much and allows filtration while
preventing the washout of locally sourced bedding sand
(20-30mm); and the grass blocks.
The blocks are being filled with a bio-charcoal material
and mushroom spores, the former to absorb pollutants in the
water and the latter to recharge the charcoal and keep it fit
for purpose.
Specific plants are currently being grown in a nursery to
be planted in and around the blocks, these are chosen for
their water purification properties, as the storm water from
this channel ultimately ends in the Berg river. The channel
banks above the blocks are planted with grass.
The main channel, the bulk earthworks, including the
attenuation ponds and other channels, were constructed by
JVZ Construction.
1. The mini gabion dams also allow the system to better filtrate
any runoff.
2. The completed channel was finished off by planting specific
plants that contribute to water purification.
2
4
NOVEMBER - DECEMBER 2018
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