Ecosystem Services Assessment
Case Studies:
Silwermine River: cost-benefit of restoration
Candice Haskins, Martin Thompson & Julia Wood
28 February 2012
Silvermine River - locality
Source above Silvermine Dam
in Slivermine Nature Reserve
on Ou Kaapse Weg
Middle reaches through
SANParks land and
Clovelly Country Club
Lower reaches flow past
Clovelly / Fish Hoek
residential area
Outlet under Main Rd and
Railway – Clovelly Beach
Before….
Historic meandering constrained and
river flowed in an incised channel
Mouth dynamics constricted by road &
rail way crossings (19th century)
Little flood capacity – impacted Fish
Hoek and Clovelly properties
Little habitat diversity, invasive spp.
Original plan (~1969) was to canalise river in a concrete-lined canal
Greater environmental awareness in the late 1980’s in Cape Town’s
stormwater engineering sector resulted in alternatives being considered
Decided on “softer” approach using gabions, wider floodplain, mosaic of
habitats, re-vegetation with indigenous species, boardwalks and wooden
bridges
Lower Silvermine River Flood Control
Scheme - Approach
EIA with specialist studies, public participation, and Environmental
Liaison Committee
Improving the ecological functioning was achieved by studying the
original state of the river and attempting to rehabilitate as much as
possible
Careful shaping of the banks and creation of areas of differing depth
was done in consultation with on-site freshwater ecologist > habitat
diversity improved
Locally indigenous species were sourced
Job creation – maintenance of vegetation in establishment phase
Continued community involvement (Riverine Rovers) – litter, alien
vegetation, education and awareness, monitoring (miniSASS and
report incidents)
After…
Adequate flood attenuation capacity
a
Diverse habitats (a, b, c)
Seasonal and permanent wetland areas
b
Biotic diversity
Improved amenity / recreational value
c
New development – residential properties set back
Gardens with open fencing face onto the wetland
Appreciation of aesthetic value of the project
Improved vigilance
Walkways create
recreational and
learning opportunities:

Diversity of habitats

Diversity of fauna and
flora

Urban stormwater
management – flood
control and WQ
Lower Silvermine River Flood Control Scheme
In 2003 the project was awarded the SAICE
Annual Award for Excellence was also a Green
Trust Award finalist
•
2004: Study on the cost benefits of various rehabilitation projects in
Cape Town was conducted and the Silvermine River Flood Control
Scheme was included (van Zyl, Leiman &Jansen )
•
Local residents & estate agents interviewed
•
Property values increased
•
Area improved aesthetically
•
Increase in recreation opportunities
•
Flooding reduced
•
Biodiversity increased
The study concluded:“Out of the three case studies, the Lower Silvermine River upgrade
project stands out as an example of how benefits can be maximised if
attention is paid to restoring ecosystems and creating aesthetically
pleasing spaces with recreational opportunities. The cost benefit
analysis confirms the desirability of the project even under
conservative assumptions.”
Now…. un-intended consequences
Phragmites reed beds – invasive, out-compete
indigenous species, expansion into open water areas,
reduction in habitat and biotic diversity
Reed and silt management
“mentioned” in management
plan (Southern Waters 2000)
> to prevent obstruction of
flow and ensure public
access and safety along
walkways
Not enough detail
Maintenance?
NEMA requirements!
COSTS!
“Lessons for Cape Town”
Beliefs:
 Rehabilitation is desirable to improve the ecological state and resilience of
degraded urban systems
 Ecological objectives can be compatible with engineering requirements
But do altered / rehabilitated aquatic systems perform like un-impacted
ones?
Perhaps the urban requirement for stasis is not necessarily the same
as the stable / equilibrium state that would occur under natural
conditions?
“Maintenance” is required to ensure the system still functions in a manner
that people perceive it should – delivering “goods and services”
Are “urban goods and services” the same goods and services offered
under natural conditions?
“Lessons for Cape Town” (cont.)
Applying some of these thoughts to the Silwermine case:
Sediment retention areas (stilling basin where energy is dissipated)
included for pollution management > requires machine interventions to
remove sediment. Access point included
Sediment build-up often leads to shallower waters and therefore reed
growth is inevitable
Reed growth traps more sediment – i.e. reeds create environment more
conducive to spreading further!
Effective reed management often requires machine intervention
Machine intervention viewed as “undesirable / heavy handed”
Are maintenance requirements, methods and costs adequately
considered? Do we have capacity to perform this function?
In the urban setting, can we “let nature be natural?” (Martin Thompson)