Safeguarding the Flow

Published: 01 May 2017 |  Water Sewage and Effluent

"Specifically in urban areas, our rivers have become unstable due to the increase in cumulative run-off from the developed catchments," said SRK partner and principal civil engineering technologist Murray Sim. ''A more regional approach is now required to solve the problem, to ensure the complete river system is taken into account."

Johannesburg's longest river, the Braamfontein Spruit, is a case in point and is the subject of a study by SRK on behalf of the Johannesburg Roads Agency. The study into the state of the Braamfontein Spruit will establish what needs to be done to improve the natural habitat and surrounding infrastructure, and will recommend rehabilitation and management measures to safeguard lives, property, and public assets.

Sim said one needs to understand the key drivers behind the river's instability, such as hard surface areas replacing natural vegetation, due to the increasing infrastructure required for a development. As development shrinks the areas of permeable ground, more stormwater discharges into the river during rainstorms. This raises the levels of energy in the river    levels that exceed its naturaI capacity causing erosion and instability of the banks, excessive loss of soil, and the deposition of this soil at man-made structures and bends in the river.

''This resuIts in siltation of the river bed, which decreases the flow capacity and increases the flood levels as the river morphology changes," he said. ''Debris from the river banks and within the catchment makes the situation worse by blocking the watercourse at the crossings, restricting the flow, and resulting in further silt deposition."

He highlighted under-designed road crossings and formalised drainage structures as further aggravating factors, as these restrict water flow. ''These structures may have been appropriately designed in the past, but the 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 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 ruraI 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 of using appropriate technology to preserve the environment, enhance living standards, and improve the quality of life.

The BMPs have two main focal areas of alternative technology: Low impact developments (LIDs); and Sustainable urban drainage systems (SUDs), where interventions to stabilise rivers could range from the use of natural materials like rocks and gabions, to naturally vegetated banks and weirs designed specially to dissipate energy. Water quality can be enhanced by preserving, extending, or creating urban wetland areas.

''These best management practices integrate town planning and stormwater planning to reach the optimal solution," said Sim.

Magic of Modelling

By working with highly regarded experts such as Professor Gerrit Basson of Stellenbosch University, SRK Consulting uses complex modelling techniques to devise river rehabilitation measures.

By developing a hydrodynamic river model, current and future river instability and erosion issues can be determined;· also, long-term changes in the  river morphology can be predicted. The modelling helps engineers to quantify problems and to identify sustainable solutions - reducing potential liability claims on municipalities and even loss of life.

Among the innovations pursued by SRK is the application of 3-D visualisation models to the field of river rehabilitation and management. The 3-D visualization can be used in this new context to assist stakeholders like municipal managers and members of the public to understand how river rehabilitation will happen, and what its result will be. This visualization technology, therefore, will be an important communication and learning tool in the public engagement phase that must comprise part of the environmental permitting process.

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