Construction of the first components of the Wroclaw floodway system in Poland, one of the largest flood protection systems in Europe started in 2011. The project includes large scale improvements to the system of river channels and flood defenses which provide protection from the floodwaters of the River Odra that flows through Wroclaw. The goal of the project is to reduce the city’s flood risk to a probability of less than a 1000-year event.
Relief channels are designed to re-direct some of the flow at peak river levels by using an off-take structure (normally a side weir) and a canal to an area where water can be safely discharged without adverse impacts. A ‘normally dry’ relief channel is at a higher level than the main channel and only carries flow in flood conditions. There may, therefore, be safety issues during sudden flooding as people in the area will have become accustomed to using the area for other purposes. A ‘normally wet’ channel carries some flow at all times, but in both cases there will be a downstream impact of the diverted flow. In an urban context, floodways act as relief channels and may be a significant component in the ‘major’ drainage system, as discussed in a Case Study below
Case Study: Modernization of the Wroclaw floodway system
The city’s present floodway system dates to 1923 and has a capacity of approximately 2,400 cubic meters per second, corresponding to a 200-year return period. In 1997, the largest flood event ever recorded in Wroclaw flooded about 35 percent of the city, causing major damage and widespread disruption along the valley of the Odra, this flood was variously estimated as a 1 in 200 to in 1000 year event.
The estimated total flow upstream of the city in this event was over 3,500 cubic meters per second, almost 50 percent greater than the capacity of the city’s existing floodway system. Widespread flooding was caused by the breaching and overtopping of the flood defense embankments; in addition a diversion structure to the River Widawa was destroyed, together with the training embankments. After the floods of 1997, a range of responses to the problem of flooding in Wroclaw, and along the River Odra valley were investigated. Changes in responsibility for flood protection came in 1999 so that the governors of large strong provinces have no reason to wait for central directives. The feasibility study identified a need for the following complementary and interdependent projects mounting to an estimated cost of over $400 million:
- The Bukow Polder completed in 2002 at a cost of $51million
- A 185 million cubic meter on-line flood storage polder (the Raciborz Polder) to be constructed 200 kilometers upstream of the city, this to be extended to 320 million cubic meters once gravel extraction is completed in the area.
- The capacity of the diversion structure and channel to the River Widawa to be increased to 300 cubic meters per second, in combination with improvements to embankments along the River Odra. This will by increasing the conveyance of the channel rather than raising defenses and will require the removal of large amounts of material and bridge widening and strengthening
- Improved forecasting and warning systems
In order to assess the highly complex impacts of each of the proposed improvements, their interaction with each other and with other components of the flood protection scheme, a hydrodynamic computer model of the floodway system has been developed to:
- Test the effect of each component of the project
- Enable ‘fine tuning’ of the design where necessary
- Understand and allow for the effect of uncertainties in key design parameters.
The models have concluded that the combination of the three measures will be sufficient to manage a 1 in 1000 year flood and protect 2.5 million inhabitants of towns and villages as well as the city of Wroclaw. The construction of the Raciborz polder would pose a risk of occasional flooding of inhabitants that are concentrated primarily in two villages (240 families). Under Polish regulations, this risk is unacceptable and therefore, the inhabitants are to be moved and the land/property within the polder acquired by the State. This resettlement plan has delayed the completion of the polder.
This example illustrates the complexity of negotiating a new integrated system of flood control. The defenses have been designed at a catchment level and involve an international water body (the river Oder). A full range of evaluation reports such as EIA and safeguarding reports were required. Structures downstream of Wroclaw have also had to be altered to accommodate extra flow, but areas downstream will also benefit from the increased attenuation in the polders.