Stormwater management should be firmly on the urban agenda

Stormwater management

Chris Brooker, a consulting engineer specialising in water management

should be firmly on the urban agenda

Urbanisation increases the coverage of impervious surfaces, preventing water from infiltrating the ground and increasing the volume of run-off.

The impervious surfaces (such as roads) are spaced closely together and have their own stormwater drains. This means that the whole system responds rapidly to a high-intensity storm – causing frequent flash flooding. Compounding the issue is the fact that run-off often carries debris, chemicals, bacteria, eroded soil and other pollutants, and transports these into streams, rivers, lakes or wetlands,” explains Chris Brooker, a consulting engineer specialising in water management.

Urban stream syndrome Flash flooding can damage road infrastructure, stormwater drains and bridges, and degrades the morphology of water courses. Run-off increases the occurrence of flowing water courses – tearing up vegetation and causing soil erosion. Due to the high frequency of this run-off, the vegetation does not have an opportunity to repair itself or grow back. Stream banks become more vulnerable to erosion, as they are not protected by vegetation, creating more run-off and more erosion.

“The regular, rapid flow of water in water courses – where the rate of flow changes quickly – creates a negative ecological impact. There is increased shear stress on the beds and banks of a water course, and the living organisms do not have the time to find a place to hide before they are washed away. The movement of sediment affects the ecology, as organisms either have no place to live (as sediment has been washed away) or they are swamped by too much sediment (where it has been deposited),” says Brooker.

Pollution – ranging from shopping trolleys and mattresses to chemicals and microplastics – is carried to water courses via the run-off.

“Urbanisation has a major impact on water courses. Groundwater resources are reduced, as much less water is absorbed into the soil. There is the drying up of small springs that previously provided a perennial water source to flowing streams. Streams are becoming ephemeral in urban areas, unless they are fed by water leaking from municipal sewers and water supply pipes,” Brooker adds.

Stormwater infrastructure Stormwater management can mitigate the negative effects urbanisation has on the receiving system and the reduction of usable water.

“It is a multidisciplinary aesthetic and technical engineering design process – first to find the right mix of interventions suited to a particular location and then requiring careful mathematical calculations, meticulous specifications and properly supervised implementation,” maintains Brooker.

A number of technologies and infrastructure (grey and green) are

ABOUT CHRIS BROOKER

Chris Brooker is a professional engineer working as a specialist consulting engineer in the field of environmental water management. He enjoys river and wetland rehabilitation and renaturalisation, and actively promotes stormwater management and modelling.

With many years of experience in the computer modelling of stormwater management systems, Chris teaches numerous workshops and occasional courses, such as PCSWMM Workshops for the Computational Hydraulics International and the SA Roads Federation Urban Drainage Course. He is an external examiner and honorary senior lecturer in the Department of Civil and Environmental Engineering at Wits University, where he teaches part of the postgraduate ‘Design for the Environment’ course.

He gives advice and acts as an expert witness in High Court hearings and arbitrations.

available; however, none of them can be used in isolation. Grey infrastructure is traditional stormwater infrastructure, while green infrastructure mimics nature and captures rainwater where it falls. Some stormwater infrastructure options include: • Rainwater harvesting is recognised as a valuable water source. Storing and using rainwater reduces the demand on the water supply. Rainwater from the roof would typically be led into a tank and then pumped out for domestic use or garden irrigation. • Attenuation storage, while not changing the volume of the run-off, reduces the peak flow rate and can improve the quality of water released as it traps the sediment that is associated with many urban pollutants. • Permeable paving is starting to gain more favour as techniques have been found to reduce the frequency of blockages as well

as clean and maintain the paving – e.g. it has been found that the gaps between the block should be filled with 6 mm gravel instead of sand. • Bioswales initially fell into disrepute due to their incorrect construction. It has been found that, when constructing bioswales, the soil used in the surface layer as a growing medium and the plants need to be carefully selected. For instance, the granite soils commonly found in the north of Johannesburg do not work because they are clayey and self-compact, becoming impermeable.

Organic material that is mixed into the growing medium helps to hold water and supports the growth of microorganisms that purify the water.

The bacteria that live on the organic matter are able to absorb the nitrogen and phosphorous, as well as other pollutants from the stormwater run-off. • Rain gardens are created by digging a basin, planting it with plants that are happy with wet or dry soil, and adding a layer of mulch or rocks. They are bowlshaped and are designed to capture and absorb rainwater and promote infiltration into the soil. • Bioretention ponds are depressed vegetated or open water areas that capture and store stormwater run-off, and can work in combination with bioswales. • Green roofs have a layer of plant material that absorb water like a sponge. They capture water when it rains, slowly

releasing it through evaporation and plant use. Green roofs can significantly reduce the amount of rainwater that would otherwise run off an impervious roof surface. Brooker believes that a lot of green stormwater infrastructure initially failed in South Africa because the technology was adopted directly from American and European systems. “In Johannesburg specifically, the evaporation rate is double the rainfall rate. This means that the plants in the ground will die unless they are irrigated. Green roofs must have appropriate plants and the correct design. With proper planning and design, green infrastructure can be invaluable to urban areas.”

He adds that while green infrastructure often requires more frequent upkeep and maintenance than grey infrastructure, it is more resilient. “For example, it is much harder to increase the capacity of a pipe than to increase the depth of a channel or swale. Furthermore, nature-based systems are less vulnerable to climate change than engineered systems because they can be overloaded with minimal damage. There is an increasing acceptance of nature-based systems. This is because they offer a wider benefit to society in terms of aesthetics and ecology (attracting pollinators and bats that control insect pests), and mitigating the urban heat island associated with increasingly densely populated cities.”

CITY OF JOHANNESBURG’S (COJ) STORMWATER DESIGN MANUAL

The manual promotes the management of stormwater by sustainable drainage system (SuDS) with the objective of recognising the resource value of this water.

In essence, the by-laws require that development does not change the characteristics from the natural condition. This applies to all developments, but specifically those with areas exceeding 4 000 m2 . The parameters are peak flow rate, total volume of run-off, frequency of surface run-off, and water quality. These parameters must be determined for all recurrence intervals from less than one year to greater than 100 years, and for all storm durations.

Analysis using continuous modelling is therefore recommended; so, to this end, CoJ has purchased long-duration rainfall data sets that it will make available free of charge to engineers working on stormwater management in the metropolitan area.