Floating or elevated roads

Road transport infrastructure prone to flooding needs to be flood-proofed to reduce the potential negative impacts of these events on transport routes. This helps ensure the connectivity of the road network, accessibility to affected areas, or, if necessary, evacuation from flooded regions. Available options to reduce the negative impacts of flooding include use of appropriate design and materials, structural protection measures (flood protection barriers) and regular careful maintenance. Floating and elevated roads are also viable alternatives, which also provides additional benefits besides climate-proofing of road transportation.

Floating roads are roads that float on water or are built on a very unstable substrate, like peat. These roads can be used both as temporary and permanent solutions in areas where standard roads are difficult or even impossible to build due to unfavourable natural conditions. Floating roads were for example used in Scotland to ensure access to windfarm areas lying on peat bogs. They can be used in both permanently or intermittently flooded areas and in flood-prone zones, supporting disaster risk management and climate change adaptation. Floating roads are more flexible than bridges and can also be used as a bypass in case of road blockages due to reasons other than flooding, e.g. due to roadwork on a bridge or on a road along a waterway. This measure can be quickly assembled and easily relocated. Moreover, floating roads take less space than traditional alternatives. Floating roads built on water are placed on floating pontoons, so they can move accordingly to the change in the water level. On unstable solid substrates, two layers of geogrid are used as structural elements of the road.

Contrary to floating roads, elevated or flyover roads are normally used in the road network. Due to their design elevation, they can ensure connection to the wider road network also in case of flooding events. Elevated roads can look like a  bridge, but they are usually longer and rise above the ground for their entire length. Elevated roads can also be built for other reasons besides flood protection, such as: optimization of the traffic flow (e.g. in urban areas, to avoid too many road crossings), construction on hilly terrain impeding ground level roads or minimization of direct environmental impacts on precious ecosystems (although elevated roads can still significantly alter the local landscape). An elevated road can also be constructed on top of an embankment; in this case it is also called a causeway, like for example the Afsluitdijk waterflow between the IJsselmeer and the Wadden Sea. Building elevated roads is usually more expensive than building ground-level roads. Typically, the increased resilience to climate change is a co-benefit and not the main reason for their construction.

The main actors involved in the design, construction and maintenance of floating or elevated roads are similar to those involved in the development of other road transport infrastructure. They include territorial planners and urban developers, national or subnational administration bodies responsible for road network, and construction companies and institutions and organisations in charge of environmental protection. The involvement of research institutions is also highly relevant both for technological aspects and to deal with climate change vulnerability and risk analyses. Citizens and users of the transport infrastructures must be correctly informed on the location of floating and elevated roads and their use in case of flooding or any other emergency event. 

Key success factors include securing adequate funding, gaining support from administrative bodies responsible for transport infrastructure, and engaging stakeholders, including transport and environment experts, in the process of designing and constructing elevated or floating roads. 

Major limiting factors are related to conflicts with other land uses (agricultural, residential and recreational uses), mismatch with regional transportation strategies and insufficient financial resources to ensure the proper maintenance of the road infrastructure after the construction. As for any other road infrastructure, environmental impacts (e.g. noise and air pollution, destruction or fragmentation of habitats, etc.) during construction and operation phases and related mitigation measures must be carefully taken in consideration in the assessment of the alternatives. Impacts on ecosystems can be lower than those due to ground level roads, while elevated roads can more significantly affect the integrity of the landscape.  

Floating roads are less expensive than bridges, while elevated roads on top of an embankment are generally cheaper than constructing a bridge-like road. After construction, both floating and elevated roads do not need more maintenance than any other road. 

Elevated roads are effective against storm water runoff related flooding because they are normally situated higher than water levels. Floating roads and related accessing ramps can adjust to the fluctuating water level. In case of floods, the most important benefits of these infrastructures include: ensuring the accessibility to the area for rescue services, making the connection to the overall road network climate proof and therefore preserving the mobility of inhabitants, enabling the evacuation from the area when needed and necessary. Elevated roads in the urban areas also serve for redirecting the traffic beyond densely populated areas and therefore help in keeping cyclists and pedestrians safe on roads. As for the rest of the road network (see also the adaptation option Climate proofed standards for road design, construction and maintenance), elevated and floating roads are important for the transport of goods and services and hence for the operations of commercial firms and industrial producers.

Local, regional and national authorities are responsible for the construction and management of floating and elevated roads. The legal aspects related to the design, building and maintenance of elevated and floating roads are very similar to those of standard road transport infrastructure. 

The preparatory phase for the construction of floating or elevated roads includes several expert analyses (including a cost-benefit evaluation), designing and dimensioning, administrative processes including environmental impact assessment and evaluation focused on climate change proofing. This phase lasts approximately 1-2 years. The construction phase duration varies between several months to several years depending on the scope, size and complexity of the construction. 

If state-of-the-art technologies are used in construction and a proper and regular maintenance of the infrastructure is ensured, elevated and floating roads can last for decades. However, over time there might be a need to partially rebuild or adapt the roads to better cope with changing transportation needs or climate change.