Manda Hinsch, SRK partner
and principal scientist.
Murray Sim, SRK partner
and principal civil
engineering technologist.
18
Encroachment of rivers by formal and informal urban development reduces water quality through
litter and nutrients; it also destabilises riverbanks and leads to siltation, raising the flood risk.
instability of the river systems and the
increase in peak flows have meant that
these structures are now inadequate,”
said Sim.
The steady rise in flood levels
inevitably leads to the flooding of houses
and other developments close to the
riverbanks, causing substantial damage
to property and even loss of life. The
flooding of river crossings like road
bridges or weirs is particularly dangerous;
high volumes of traffic in cities like
Johannesburg raise the risk of vehicles in
motion being washed off these crossings
by a river in flood.
“It is important to start reducing the
flood peaks and minimising the energy
potential in the outer catchment by
promoting attenuation facilities,” he said.
“This can be done through introducing
strategically placed ponds, dams, and
wetlands; these large permeable areas
will encourage groundwater recharge and
help reduce peak flows.”
These mitigation measures can
potentially minimise future impacts and
counteract years of urban development.
“Uncontrolled changes to the run-off
levels in the catchment also lead to a
general environmental degradation of
the river ecosystem through the loss of
soil cover and vegetation,” said Manda
Hinsch, SRK partner and principal
scientist. “Biodiversity in rivers is also
undermined when water quality is affected
by human settlements and related
development, including sewage treatment
plants that are not fully functional or are
poorly maintained.”
Hinsch said wastewater remains an
issue in many parts of South Africa, where
the Department of Water and Sanitation’s
Green Drop certification programme
Water Sewage & Effluent May/June 2017
has been trying to raise the standard of
wastewater treatment by municipalities.
“Data from the department indicates that
in only one out of three municipalities
is the quality of discharged water from
wastewater treatment plants of an
acceptable quality,” she said. She also
warned that river system instability is not
limited to urban environments.
“In rural areas, the natural base
flow of many rivers has been affected by
other changes in their catchments, such
as dam construction and water transfer
schemes,” said Hinsch. “Water quality
in rural rivers is also at risk of nutrient
enrichment from chemical fertilisers
on farmlands, or run-off from cattle
feed-lots.”
For current Department of Water and
Sanitation (DWS) data on Green Drop
compliance, visit https://www.dwa.gov.
za/dir_ws/gds/GDS/Default.aspx
The DWS uses a number of variables
to award Green Drop status to
wastewater treatment works, which
include operational standards,
maintenance, and level of staff training
associated with the capacity of
the plant.
Sustainable technologies
In response to intense urbanisation and
its impacts on rivers, a multidisciplinary
best management practices (BMPs)
approach has been developed and
successfully applied in the US, Australia
and Europe, and it guides SRK’s strategy