Kuils River corridor plan unlocks development

The upgrading of a 2 km long section of the Kuils River in Cape Town has transformed a dysfunctional watercourse into a vibrant ecosystem and serves as an excellent example of the execution of the city’s Liveable Urban Waterways Programme in practice.

New gabion drop structure

Undertaken by Redefine Properties and VDMV Property Holdings, in collaboration with the City of Cape Town, the Kuils River upgrade project paved the way for the Stikland Industrial Precinct development, which is situated within the suburbs of Bellville and Brackenfell. But first a comprehensive River Corridor Plan needed to be developed.

Periodic, aggressive clearing of invasive riverine vegetation had contributed to the destabilisation of the river channel, putting sections of the city’s water and sewerage infrastructure at risk. Over the years, adjacent properties had also encroached on the river, with illegal infilling of the floodplain. The subsequent degradation of the river channel resulted in frequent flooding.

These and other factors posed a major stumbling block to the future development of the Stikland Industrial Precinct, as this affected section of the Kuils River traverses the precinct site. Therefore, it was in everyone’s interest to implement a sustainable and environmentally beneficial solution.

The developers commissioned a multidisciplinary project team that included Zutari (previously Aurecon South Africa) to provide hydraulic engineering design consulting services, together with a freshwater ecologist, as well as environmental, civil and landscaping consultants. The River Corridor Plan was workshopped through several iterations with the relevant Stormwater, Parks, Environmental and Maintenance departments of the City of Cape Town, prior to being approved and adopted for implementation.

The river corridor was initially 100 m wide; however, it was reduced in size to 50 m to allow for additional developable area, while also allowing sufficient space for river improvements.

Integrated elements

Key features of the river upgrade include: • A 13 m wide, trapezoidal, riprap-lined, low-flow channel, which conveys the oneyear flow. The use of riprap allows for the establishment of aquatic plants and organisms within the river channel, improving biodiversity and water quality. • A 50 m wide, terraced, trapezoidal, grasslined floodplain, which contains the 100year flood. Various species of indigenous vegetation were also strategically positioned and planted on the floodplain. • Buried Reno-mattress erosion protection on the bends of the terraced floodplain, which prevents lateral movement of the river. • A 150 m long and 3.5 m high gabion retaining wall on the west bank to enable reclamation of the floodplain area. • A 50 m long, 1 m high gabion drop structure across the low-flow channel and floodplain,

which protects a concrete-encased sewer pipe crossing the river. The drop structure also serves to reduce the longitudinal grade of the channel and floodplain to keep flow velocities within the limits appropriate for the riprap and grass lining. The stilling basin downstream of the drop structure also functions as a silt trap. • A new road bridge over the low-flow channel and floodplain, conveying the 100-year flood without raising upstream flood levels. • 2.4 ha seasonal and permanently constructed wetlands to compensate for the loss of wetland habitat in the development area. The wetlands are designed to be fed by the river during occasional overtopping of a lowered riverbank in this area. They also serve as stormwater detention ponds and compensating flood storage facilities. • Swales on the floodplain for the treatment of stormwater run-off from the development site. All of the above, including the development of a stormwater management plan for the development site itself, were undertaken in compliance with the City’s Management of Urban Stormwater Impacts and Floodplain and River Corridor Management policies.

The river upgrade was split into two contracts. The first entailed the river works, which were designed by Zutari and implemented by contractor Burger & Wallace. The second

contract involved the landscaping of the river, which was designed by Greenspiration (landscape architect) and implemented by Cape Contours (landscape contractor).

“The challenge was to achieve a balance between designing a river cross section and slope that wasn’t too flat or too steep,” explains Matthew Phillips, Zutari’s design engineer and assistant resident engineer for the project.

“The cross section also needed to include a low-flow channel that would have capacity for the annual flows (one-year recurrence interval), after which the flow would spill out onto the terraced, grassed floodplains,” Phillips continues.

The levels of the terraced portions were designed such that the 10- and 20-year floods would be contained within the first terrace. The 50- and 100-year floods would then be contained within the second terrace.

Natural aesthetics

A key focus of the design effort was on creating a natural sustainable riverine ecosystem that would be aesthetically pleasing for the local communities and visitors. To this end, the use of concrete and unsightly concrete structures was minimised.

“The planting of indigenous vegetation along the river corridor and within the offset wetland provides a very aesthetically pleasing natural look, which, to the naked eye, hides the complex engineering design,” says Phillips.

A conventional river upgrade design would consider minimising the space taken up by the river by canalising it with the use of concrete rectangular sections. “Although, this approach would result in the most hydraulically efficient design and unlock additional development area, it would offer very little in terms of sustainability. Maintenance of the river would require heavy machinery; dumping would also be prevalent, as there is little ownership of concrete canals compared with natural-looking rivers,” Phillips continues.

Keeping it green

Grassed riverbanks provide good protection when the vegetation is established and maintained. However, if a drought occurs and the grassed banks aren’t looked after, the river is at risk of erosion in lateral and longitudinal directions. For this reason, the team was forward-thinking in designing lateral and longitudinal erosion protection. This was achieved by including buried Reno mattresses on the outside bends of the river for the lateral erosion protection, and gabions buried at regular intervals to prevent the cross-sectional shape and slope of the river from changing.

The downstream end of the river upgrade required careful consideration to ensure a smooth flow transition. The chosen point for this was at an older gabion drop structure approximately 180 m downstream from an existing bridge crossing the Kuils River. During the conceptual design phases of the project, it was identified that this bridge was a hydraulic control that would cause flooding in the area. For this reason, it was demolished, with a new bridge constructed at the low-flow channel and floodplain section.

Public and private sector collaboration

The upgraded river corridor and gabion drop structure after landscaping

The existing bridge that was demolished during construction of the river upgrade. Note the existing gabion drop structure in the foreground, which was a key tie-in point for the river upgrade

The Kuils River upgrade is an excellent example of a project that embodies the vision of Cape Town’s Liveable Urban Waterways Programme and transitioning towards a watersensitive city by 2040. It also exemplifies what can be done when the public and private sectors work together to achieve a solution, which is ultimately beneficial to all parties. If it were not for the excellence of the river upgrade project, none of the development within the Stikland Industrial Precinct would have been able to materialise.

CONTRACTORS

River contractor Burger & Wallace Construction Landscape contractor Cape Contours

PROFESSIONAL TEAM

Project manager Johan Botha Consulting Engineers River engineer Zutari Landscaping architect Greenspiration Environmental consultant Gibb Freshwater ecologist Blue Science Health and safety agent Safe Working Practice