Urban drainage

Switzerland has a reliable urban drainage infrastructure. Drainage plans help to maintain and develop this service. As a result of climate change, new approaches to dealing with rainwater are also necessary, e.g. 'sponge cities'.

Switzerland has a comprehensive, efficient and reliable drainage infrastructure. A General Drainage Plan (GEP) is required to ensure that these remain functional and up to date going forward. Climate change is resulting in more frequent heavy precipitation, heat waves and dry periods, so we need to rethink the way we deal with rainwater. The 'sponge city' model provides a promising approach. It allows large quantities of water to be stored in urban areas during heavy precipitation and then slowly released into the environment. This relieves urban drainage systems and has a positive impact on the microclimate in these areas.

The General Drainage Plan GEP

The General Drainage Plan (GEP) was anchored in the Waters Protection Act in 1991. The aim of the GEP was for each commune to know the condition of its drainage infrastructure and the water bodies affected by it, and to draw up a concept for how and when to remedy shortcomings. The GEP is updated on an ongoing basis. Where appropriate, there are also waste water authority drainage plans (VGEP) or regional drainage plans (REP), which deal with the drainage of local catchment areas, because it makes sense to coordinate water protection measures in a specific hydrological area.

Today, the GEP is the central planning instrument for urban drainage, which ensures the maintenance of the drainage infrastructure on the one hand and efficient water protection on the other. In order to prevent the (value) decay of the multi-billion investments made in the past, the GEP has transformed the underground, invisible and uncontrolled drainage systems into a modern, visible and controlled infrastructure management. However, the GEP is much more than infrastructure management. It also considers water protection legal requirements. For example, the drainage plan includes the condition of the water bodies, extraneous water, hazard prevention and insurance options. It then also defines measures to avoid adverse impacts on water bodies.

The GEP was launched 30 years ago. While this may seem like a long time, the idea is still a relatively new one in the context of lawmaking and implementation. Its positive effects of the GEP can already be felt and the need for such a plan has been confirmed. Today, 94% of Swiss municipalities have a GEP and 4% are in the process of drawing one up. Existing shortcomings, in particular with regard to the drainage plan, the condition of the pipes and special structures, and the accumulation of extraneous water have been identified and the action to be taken defined.

The GEP has proven to be a valuable a planning instrument. Some of the challenges now to be addressed by the GEP are: implementation of the measures defined; groundwater protection zones threatened by the expansion of urban areas; digitalisation and standardisation of data management.

Sponge cities

The climate is changing and so are precipitation patterns. Extreme events such as heavy precipitation and prolonged periods of heat and drought are becoming more frequent. This has an impact on drainage systems in Switzerland's towns and cities, where more than 60% of the ground area is sealed over, so water cannot easily drain away. The sponge city model can mitigate this negative impact. It provides the greatest possible benefit for waters protection with the least possible damage to property during flood events. It also creates a more balanced climate in urban areas.

Like a sponge, the city of the future will store as much rainwater as possible during heavy precipitation, and then slowly release it again. This brings multiple benefits. During heavy rain events, sewerage systems come under extreme pressure. This results in flooding, and waste water may be discharged from the sewerage system or waste water treatment plants into water bodies. With more water retention and storage possibilities in urban areas, such as temporary ponds and green spaces on squares and roofs, the rain can then flow out of the urban area more slowly, seep into the ground, be absorbed by plants or evaporate. Larger areas covered in vegetation also counteract the urban heat island effect. A sponge city experiences less flooding and less pollution and has a more balanced climate.

Initial steps have already been made to promote the notion of a sponge city by the Swiss Water Association (VSA), which has drawn up guidelines on integral rainwater management, or good precipitation waste water management (Abwasserbewirtschaftung bei Regenwetter (2019)).

Drainage on transport routes

Considerable loads of pollutants can enter groundwater or surface waters during rainy weather from heavily frequented traffic routes, especially roads. Various publications regulate the implementation of the legal requirements for the disposal of this waste water.

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Last modification 04.10.2021

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