Stormwater management - a renewable resource instead of flash flooding

Managing periods of rainfall deficit and heavy rainfall have been a double challenge in recent years. In Hungary, no dramatic changes in annual rainfall indicators have been recorded, but the distribution of rainfall has become more uneven. Increasingly, periods of no rainfall are followed by short periods of locally significant rainfall, causing flash floods and urban flooding.

Water rushing over the paving stones

In the literature, a flash flood event is defined as a mostly small surge in smaller watercourses within less than 6 hours of rainfall.
Flash floods are often triggered by successive thunderstorms, which are a concomitant phenomenon of global warming. One reason for more intense rainfall is that warmer air can store more moisture. It is calculated that air can retain 7 percent more moisture for every 1℃ of warming.

Although the warmer air is storing more rainfall, the longer periods of drought mean that the dry soil can absorb less and less of the precipitation. Much of the rainwater runs off almost unhindered at the surface and does not have time to reach the deeper aquifers in the soil.
Another problem is the felling of shrubs and trees. The disappearance of green vegetation could slow down runoff, but in its absence, water can easily wash away the topsoil, causing erosion and mudflow.
In urban areas, undersized and poorly maintained stormwater drainage networks, high urban density, and a large number of paved surfaces also significantly reduce the infiltration of rainwater into the surface and deeper layers of the soil.

Beyond the capacity of the network

Urban stormwater management is an increasingly pressing issue. In cities, especially because of the high population and wealth concentration, the extent of damage can become significant.
Intense, localised rainfall, flooding and flash floods overload sewage systems, and sewage treatment plants are unable to handle the sudden surge of water, which can be discharged untreated into the watercourses. Another problem and risk is that water can be contaminated by run-off from heating, urban transport, industrial activity,
and discharges from the cultivation of private gardens.

Stormwater: a new perspective, economic benefits

From a different perspective, however, stormwater offers untapped potential for municipalities and the economy as a whole. For example, water collected from rooftops, which does not require treatment, could cover a significant part of the water use of residential buildings. It could, among other things, replace the use of potable water for flushing toilets and watering. The integration of urban water management - in terms of planning, development and operation - could also provide a solution to this challenge.

Aligning grey, green and blue infrastructure can bring a new approach to the way decision-makers think about and manage stormwater. Together, permeable green spaces, urban rain gardens, ponds, green roofs and green facades, drainage ditches, stormwater drainage networks, sewers, wastewater treatment plants can form an effective urban stormwater management system that prioritises water quality and protection.

One battery, many functions

Instead of building a single large sewer, the use of many small, step-by-step solutions can make a difference in the management of stormwater in settlements.
Retaining and using rainwater, prolonging run-off times and minimising run-off are all essential elements of complex stormwater management.
One of the main characteristics of blue and green structure elements - urban ponds, rain gardens, parks, roof gardens, green roofs, etc. - is that they model the natural cycle of water. Their role in the water management system is to capture, store, evaporate and purify rainwater. But, while reducing stormwater runoff, they also provide habitat for plants and animals, cool the environment, and provide recreation for people.

Sponge Town

Several international examples of successful stormwater management can offer solutions for our country. One such example is the sponge city concept, which uses a complex approach - rain gardens, permeable pavements, water catchments, public parks that can be used as temporary flood reservoirs - to manage rainwater in a given area, even during periods of high rainfall and drought. There are many good examples, but the key issue is to adapt them to the climatic, topographical and infrastructural conditions of the city in question, which requires the coordinated work of experts from many disciplines.
Rainwater management is not simply a matter of draining off water in a controlled way, but also of promoting water retention and the use of rainwater.

Greening and permeable pavements

The interventions are designed to relieve the sewerage network and the treatment plants, so it is essential to start retaining water as far upstream as possible in the catchment area,
In our country, siltation can be a cost-effective solution due to the favourable soil conditions. This requires sufficient time for the water to seep into the ground and a flat permeable area, such as a lawn, but rain gardens and simple grass ditches are also an alternative. In these options, flood-tolerant plants, which filter and purify rainwater through their roots before it reaches the groundwater, are a key consideration. Where paving is unavoidable, permeable pavements offer a solution. The range of materials available to the construction industry is growing.
The use of underground silt pits and gravel
silt pits under large spaces and car parks is also becoming more common. However, the fact that these devices do not purify rainwater requires considerable attention.

Store, store, store!

In case of intense rainfall, especially from paved surfaces, a significant amount of water runs off without silting. Another important aspect of water management is water storage - surface or underground, short or long-term solutions.

Short-term reservoirs can be a priority in areas where overloading of the sewer system needs to be avoided. These deeper areas, usually covered with turf, will help to lengthen the run-off time.
In Western Europe, sports fields and community spaces with temporary emergency storage functions can also be found.
Reservoir ponds, rainwater harvesting tanks, green roofs or a combination of these can also
help to recycle rainwater. For example, watering parks and sidewalks at the right time can significantly reduce the heat in our cities.

From a source of danger to a resource

Flash flooding, locally heavy rainfall, is not a "weather bug", but a planning problem and opportunity. Urban planners and the construction industry can play a key role in exploiting this by applying the blue-green-grey infrastructure toolkit in a coherent way. Heavy rain is not a risk, but a local resource - in a more resilient, livable urban fabric.
In the rest of this article, we will present some of the implemented projects that can be well integrated into the stormwater management of any municipality.