Moddergat River rehabilitation and Flood Management Project Cape Town, South Africa


	Urbanisation in the world Urbanization process is a domain of developing countries. One of the results of the extremely high densities of fast-growing populations is "wild development", leading to such negative social and ecological effects as: [...] + more Water cycle in urban areas Combined effects of urbanization, industrialization, and population growth greatly modify landscapes and thus the continuous circulation of water within catchments and the Earth's hydrosphere - the hydrological cycle [...] + more Urban Aquatic Habitats Aquatic habitats are water bodies supporting aquatic life. Increased temperatures of effluents, greater discharges of water, pollutants and waste, and changes in water bodies morphology impact all the basic habitats characteristics. [...] + more Urban Aquatic Habitats Management A balanced approach addressing the pressing issues in urban environment is not common in the water service sector [...] + more Ecohydrology for Urban Aquatic Habitats From the point of view of environmental science, urban environment can be considered as a highly condensed anthropogenic system, which is organised for efficient flow of water, matter, energy and information [...] + more

MODDERGAT RIVER REHABILITATION AND FLOOD MANAGEMENT PROJECT
CAPE TOWN, SOUTH AFRICA
Case Study Leader: Elizabeth DAY

University of Cape Town
Freshwater Research Unit
Rondebosch, 7700; South Africa

The Moddergat River is located within the urban edge of Cape Town (South Africa), in the Cape Floral Kingdom. The city has a population of around 3,5 million and a Mediterranean climate. Under natural conditions, the river would have been a diffuse, seasonal wetland. Major impacts on the system include channelisation, urban and agricultural runoff, invasive terrestrial and wetland species, changes in natural hydrological conditions and a poor water quality. Improvements / rehabilitation measures addressed flood control, public safety and security, amenity and ecological functioning. Major Actions implemented were soft engineering measures in the riverine area (gabion weirs, landscaping and replanting) and widening of floodplain.

Background

The residential areas of Firgrove and Macassar on the Cape Flats, Cape Town, are prone to flooding by the Moddergat River. This is largely the result of alterations in natural channel morphology, encroachment by buildings and other structures into the floodplain, and constriction of the channel by culverts. An initial hydrological study recommended hard canalisation as the most suitable engineering option to protect affected areas from flooding. However, as well as being associated with prohibitive costs, canalisation is also less popular because of its negative ecological and aesthetic characteristics. A revised, softer treatment of the stormwater problems was devised by a multi-disciplinary team.

Key aquatic habitat issues in urban water management

At the start of the project, the river in the study area was impacted by the factors listed below (CCA Environmental 1999a; Day and Ractliffe 2002; Day et al. 2005):

Encroachment of buildings and infilling into the floodplain,
Extensive erosion,
Channelisation, including straightening of the channel and steepening of banks,
Loss of virtually all indigenous riparian vegetation,
Encroachment of alien vegetation into the river margins,
Nutrient-enrichment, and elevated concentrations of faecal coliform bacteria.

In addition, dense growths of the invasive bulrush Typha capensis in places along the channel constituted both a safety hazard (the channel was hard to climb out of) and a security threat (reedbeds in this area often harbour criminal elements and facilitate attacks on passers-by).

The residential areas of Firgrove and Macassar on the Cape Flats were prone to flooding by the Moddergat River, a small tributary of the Eerste River. This was largely the result of alterations in natural channel morphology, catchment hardening, encroachment by buildings and other structures into the floodplain and constriction of the channel by culverts. An initial hydrological study recommended hard canalisation as the most suitable engineering option to protect affected areas from flooding. However, as well as being associated with prohibitive costs, canalisation is also less popular because of its negative ecological impacts and aesthetic characteristics. Therefore a revised soft-engineering treatment of the stormwater channel was devised by a multi-disciplinary team.

Objectives of the Case Study

The primary objective of the project was to address flood control, by accommodating the 1:50 year flood; with secondary objectives including improving the amenity value along the river, preventing erosion of the river bed and banks and, where possible, improving ecological function.

The project design involved the use of so-called "soft" engineering methods and comprised the following aspects (CCA Environmental. 1999b):

A macro-channel with a capacity designed to accommodate the estimated 1:50 year flood volume, with allowance for colonisation of the low flow channel section by dense reed beds,
A low-gradient trapezoidal channel, in which the low-flow channel was lined with loose river boulders,
Installation of gabion weir energy dissipaters at intervals along the channel,
Landscaping and planting (mainly grassing) of the flood channel, so as to create an attractive and safe amenity, useful for playing, walking and ball kicking,
Planting of the low flow channel edges with hardy, indigenous plant species,
Safeguarding against erosion, particularly in the early stages of the project before establishment of vegetation, by laying a high density polyethylene mesh along the inner edge of the high flow channel, effectively protecting the intersection between the low and high flow channels.

Stakeholders

The City of Cape Town (land owner; flood control and safety and security concerns),
Geustyn Loubser Streicher Inc (design engineers),
CCA Environmental, CNdeV landscape architects - public process, overseeing of landscaping implementation, environmental monitoring.

See References
For more details, results, concussions and recommendations of this case study, see the publication: Wagner, I. , Marshalek, J. and Breil, P. (eds). 2007. Aquatic Habitats in Sustainable Urban Water Management: Science, Policy and Practice. Taylor and Francis/Balkema: Leiden.