FLOODING IN SOUTH AFRICA THE URGENT ROLE OF MUNICIPAL STORMWATER MANAGEMENT

With the rainy season approaching in most of South Africa, communities across the country, and especially in flood-prone areas, face the threat of storms, overwhelmed infrastructure and floods.

The Western Cape has already experienced winter storms with torrential rains and high winds that caused devastating flooding. For municipalities, residents and businesses in other regions, their risks will soon increase as the summer rains start. Increasingly erratic weather patterns, rapid urbanisation and ageing infrastructure mean that the challenge of managing urban flooding has never been greater or more urgent, according to the Institute for Municipal Engineering of Southern Africa (IMESA). Key to addressing this challenge is stormwater management, better planning and putting an end

to the abuse of stormwater systems, IMESA states.

Municipalities must prepare now - fixing what they can, educating communities and designing systems that can withstand the storms to come.

Stormwater systems under immense strain

Much of South Africa’s municipal stormwater infrastructure was designed for an era of lower

population densities and less intense weather events. Today, many of these systems, which typically comprise closed piped networks, kerb inlets, grid inlets and other structures, are under immense strain. The primary function of these systems is to convey runoff safely during rains. However, in many cities and towns, they no longer function as intended. This is not only due to capacity constraints or ageing infrastructure but increasingly due to human interference, which IMESA refers to as "stormwater system abuse".

System abuse is a concerning cause of flooding

One of the most concerning trends is the intentional removal of manhole

covers to gain access to underground stormwater pipes, IMESA reports. “Once open, these systems are used to dispose of inappropriate materials ranging from builder’s rubble and household waste to clothing, tyres and even large rocks. These items cannot enter the system through normal kerb or grid inlets, so their presence is almost always due to deliberate misuse. The impact is severe. Blockages in the system prevent water from flowing freely, leading to localised flooding when it rains. Since many municipalities lack the resources for routine preventative maintenance, these blockages often remain undetected until it’s too late. They are only discovered when water rises and roads, homes and businesses are flooded.”

Recent floods serve as a wakeup call

IMESA asserts that the floods that swept through parts of the Western Cape earlier this year offered a stark reminder of the consequences of inadequate stormwater planning and maintenance. “In many areas, culverts overflowed, roads collapsed and informal settlements bore the brunt of the damage. But the Western Cape is not alone. Cities like Johannesburg, Durban and Tshwane have all experienced recurring flood events in recent years, some with tragic outcomes. As weather patterns become more unpredictable and rainfall intensity increases, these events are likely to become more frequent and severe,” the institute advises. “This puts added pressure on municipal engineers and planners to anticipate and mitigate the impacts of flooding, even as they contend with limited budgets, outdated data and aging infrastructure.”

Landmark document is a critical tool for South Africa

To help municipalities respond to the challenges, IMESA and the Water Research Commission (WRC) released the Best Practice Guideline for Design Flood Estimation in Municipal Areas. This landmark document is proving to be a vital tool in bridging the gap between academic expertise and onthe-ground implementation. Prior to its publication in 2023, there were no standardised design standards for urban flood estimation and risk assessment in South Africa.

Developed by a consortium of experts from South African universities and municipal practitioners, the guideline provides a comprehensive framework for flood estimation in urban catchments. It equips engineers with the tools and knowledge needed to assess stormwater infrastructure, estimate design floods and plan accordingly.

The key features of the guideline include multiple flood estimation methods; real-world data sources; an emphasis on catchment-based planning; and guidance on uncertainty and calibration. IMESA notes that the document is not just a technical manual, it is a call to action for municipalities to build institutional capacity, create reliable data systems and work across departments to develop longterm flood resilience.

Planning Beyond the 100-year flood line

Traditionally, flood planning in South Africa has focused on the 100-year flood line, which is widely used in development control. A flood line is a graphical representation of where the edge of the flood waters will reach for a given return period flood (e.g. a 1 in 100 year flood).

The flow is determined by rainfall and the characteristics of the catchment (i.e. the slope and extent of hardening within the catchment). IMESA explains that the term “1 in 100 year flood” is misleading because it leads people to believe that it happens only once every 100 years. “The truth is that an uncommonly big flood can happen any year. The term “100-year flood” is really a statistical designation, and there is a 1-in-100 chance that a flood this size will happen during any year. Another way of referencing a 1 in 100 year flood is that there is a 1% chance of this flood flow being exceeded in any given year. A 1 in 50 year flood will have a 2% chance of being exceeded in any one year.”

But planning for the 100-year flood is no longer enough, IMESA stresses. As the guideline points out, municipalities must also plan for smaller, more frequent events (such as 1-, 2-, 5-, 10-year events), as well as rare but catastrophic events (like the 1 000-year flood that is now considered in parts of the United Kingdom). “Planning for these varied scenarios requires municipalities to invest in better data, maintain accurate stormwater asset registers and, critically, to design systems that not only convey water but also manage it sustainably. This aligns with global shifts towards Water Sensitive Urban Design (WSUD) and Sustainable Drainage Systems (SuDS), which prioritise detention, retention and the environmental integration of stormwater infrastructure.”

To manage flood risk more effectively

The institute says that to manage flood risk more effectively, municipalities should prioritise stormwater maintenance budgets, enabling regular inspections and preventative cleaning.

“It is highly recommended that they use the IMESA-WRC Best Practice Guideline as a standard framework across all new developments and upgrades. Investing in local data collection, including rainfall, flood events and asset conditions is vital, along with inter-departmental collaboration, especially between planning and engineering teams. Educating the public about the d angers of system abuse and illegal dumping is crucial.

“Municipal infrastructure is a shared asset and communities can play a role in protecting it. When residents dump waste into stormwater systems - whether intentionally or unknowingly - they put their own homes and neighbours at risk,” warns IMESA. “Simple actions, like reporting missing manhole covers or illegal dumping, can go a long way in preventing localised flooding. Municipalities can support this by ensuring that reporting channels are open, accessible and responsive.”

Design for a more resilient future

Flooding is not a new problem in South Africa, but it is a growing one.

The IMESA-WRC Best Practice Guideline offers municipalities the tools to design for a more resilient future. But these tools must be used alongside a cultural shift that sees stormwater not as waste to be discarded, but as a critical part of our urban ecosystems, IMESA emphasises.

“As the summer rains approach, the time for preparation is now. We must fix what we can, educate where we needto and build the kind of systems and communities that can withstand the storms to come,” the institute concludes.

ABOUT IMESA

Since 1961, The Institute of Municipal Engineering of Southern Africa (IMESA) has been committed to the pursuit of excellence in all facets of infrastructure, pushing boundaries and driving innovation. Our unwavering dedication extends to the professionals who form the backbone of this industry.

IMESA creates a platform for the exchange of ideas and viewpoints on all aspects of municipal engineering with the aim of expanding the knowledge and best practices in all local government municipalities. This is made up of a community of pioneers, professionals and enthusiasts united by a singular purpose: to reshape the landscape of infrastructure engineering.

For media enquiries, please contact: Ingrid Botton Head Office Manager / Institute Secretary Institute of Municipal Engineering of Southern Africa (IMESA)

Tel: 031 266 3263

Email: admin@imesa.org.za Website: www.imesa.org.za