IN the historic city of Wroclaw in south-western Poland, work is under way to create one of the largest flood protection systems in Europe. Since October 2009 Halcrow Group Ltd have been working in partnership with Scott Wilson and BRL Ingenierie on the detailed design of large scale improvements to the ageing system of river channels and flood defences which provide protection from the floodwaters of the River Odra that flows through the heart of the city. The goal of the Wroclaw Regional Water Management Authority’s project is to deliver a robust, reliable protection system to reduce the risk of flooding to the city’s population of 630,000 to a probability of less than 0.1% in any year.

Rising in the mountains of the Czech Republic, the River Odra has a catchment area of over 20,000km2 at Wroclaw. The residents of Wroclaw have a long history of fighting floods from the Odra. The construction of flood embankments along the Odra river valley dates back at least 160 years following severe floods in 1854. Major floods in 1897, 1902 and 1903 triggered the development of flood storage reservoirs and channel canalisation works all along the Odra and a system of large scale flood defences in Wroclaw itself.

The present Wroclaw Floodway System dates from around 1923 and provides a high level of protection to the city. The estimated system capacity of approximately 2400m3/sec corresponds to the 0.5% annual probability flood. At the front line of the system are two flood diversion structures on the River Odra at Bartoszowice, at the upstream limit of the city. Up to 140m3/sec is transferred from the Odra via a sheet-pile spillway and diversion channel to the River Widawa, which flows around the eastern boundary of the city. Around 700m3/sec can be diverted into the main Flood Channel which runs through the eastern suburbs of Wroclaw. The River Odra itself flows through the centre of the city where it divides into two main channels, the Old Odra and the City Odra. The City Odra divides into a network of picturesque channels and islands as it flows through the historic Old Town. Up to around 700m3/sec of flood flow can pass through the City Odra. The remaining flow is diverted into the Old Odra channel via an inflatable crested weir and re-joins the Flood Channel on the eastern side of the city.

The River Odra has long been an important transportation route for coal and other goods. Although the volume of shipping has declined in recent years, the river remains heavily regulated for navigation purposes. In Wroclaw, water levels are controlled by five separate gated weirs or barrages with associated ship locks. Two dedicated navigation canals run alongside the flood channel. In extreme flood events these structures constrict flood flows, increasing water levels in the channels and the risk of flooding. At least twenty major bridges span the river and flood channels. The piers and abutments of these structures further impede the passage of floods. Flow through the city centre is also restricted by the weirs and gates at two low head, river hydroelectric power stations on the City Odra.

System integrity

The integrity of the floodway system and other river structures was severely tested in 1997. On the night of the 11 July the largest flood wave ever recorded in Wroclaw arrived at the city. The estimated total flow upstream of the city was over 3500m3/sec, around 50% greater than the capacity of the city’s substantial floodway system. Over the next two days breaching and overtopping of the flood defence embankments resulted in widespread flooding of the western and eastern parts of the city. The diversion structure to the River Widawa was destroyed together with training embankments resulting in flooding of the eastern districts of the city. The historic Old Town was saved from flooding only through the voluntary action of thousands of the city’s residents in building temporary defences from sand bags. Overall, around 35% of the city area was flooded. The flood resulted in massive damages and widespread disruption all along the River Odra valley and in other parts of Poland. Over 2500 towns and villages were flooded, 480 bridges were destroyed, 5000km of roads and railway were seriously damaged and 54 people lost their lives. Nationally, estimates of damages run into several billion US$.

Investigating solutions

In the years since the disastrous events of 1997, a range of solutions to the problem of flooding in Wroclaw and along the River Odra valley have been investigated. The preferred components for the modernization of the Wroclaw Floodway System were identified in a feasibility study undertaken by Jacobs in 2005 and three complementary and interdependent projects are now underway, together forming one of the largest flood protection schemes in Europe. A 185Mm3 on-line flood storage polder will be constructed at Raciborz, 200km upstream of Wroclaw, by a team comprising CES Consulting Engineers, Ekosystem and Royal Haskoning for the Gliwice Regional Water Management Authority. The capacity of the diversion structure and channel to the River Widawa will be increased to 300m3/sec in combination with improvements to embankments along the River Odra in a project for the Lower Silesia Board of Land Reclamation and Water Facilities in Wroclaw by a team including Grontmij and Sogreah consultants. The Raciborz polder will provide protection to the towns and villages along the River Odra valley upstream of Wroclaw but will also attenuate the flood hydrograph in the River Odra at Wroclaw, reducing the 0.1% probability flow by up to around 30%. The improved diversion structure to the River Widawa will allow up to 10% of the 0.1% probability flow to be transferred away from Wroclaw.

The overall objective of the modernization of the Wroclaw Floodway System is to allow the safe passage of the 0.1% annual probability flood in the River Odra, including a 15% safety margin, and meet the legal freeboard requirements for first class flood defence structures under Polish law. A freeboard of 0.1m is required for concrete structures and 0.3m for earth embankments. Allowing for the attenuation provided by the Raciborz polder, this ‘control flood’ is currently estimated as 3100m3/sec upstream of Wroclaw compared to the current system capacity of around 2400m3/sec. Providing sufficient additional system capacity is the key challenge for the team.

The objective will be met largely by increasing the conveyance of the system rather than raising the levels of flood defence walls and embankments. Flood water levels in the channels will be reduced by increasing the capacity of the channels and reducing the obstruction to flow at structures. Around 4Mm3 of material will be excavated from the Odra, the Flood Channel and the Old Odra. The transport and disposal of this material will be a major exercise. The navigation facilities of the river itself will be used to transport much of the material.

The flow area under seven major road and rail bridges will be increased to reduce the afflux at these structures. The integrity of the bridge foundations and of the structures as a whole will need to be maintained both during construction and on completion, requiring specialist skills. The capacity of the Rozanka gated weir complex in the Old Odra will be increased by adding an additional weir gate and the capacity of the Redzin barrage just downstream of Wroclaw will also be increased. The permanent, fixed crest weir at the hydroelectric power station in the centre of the city will be replaced with an adjustable flap gate structure. This will be lowered during floods to reduce the upstream water levels. The City Navigation channel flood gates, lock and the channel itself will be modified to allow flood water to pass along the channel. At present the channel is closed during floods but with these improvements will be able to provide an additional flood flow route through the city. Repairs to existing flood defence walls and embankments will be made and limited lengths of new defences will be constructed at critical locations.

Complex system

The complexity of the floodway system means that assessing the impact of each of the proposed improvements and their interaction with each other and the two other components of the River Odra flood protection scheme is not straightforward. To provide confidence in the performance of the scheme a hydrodynamic computer model of the floodway system is being developed. This will allow the effect of each component of the project to be tested during detailed design and prior to construction, enabling fine tuning of the design if needed. The model will also be used to understand and allow for the effect of uncertainties in key design parameters on the project. On completion of the works the model will be used by the Wroclaw Regional Water Management Authority to assist in the future management and operation of the system and potentially for flood forecasting.

At the end of May 2010, just a few months into the project, devastating floods once again swept across much of Poland. The flow in the River Odra at Wroclaw peaked at around 2000m3/sec, close to the current capacity of the system. In Wroclaw the flow was contained within the Floodway system although emergency reinforcement of many kilometres of embankments was necessary to prevent breaching of the flood defences. In the Odra valley upstream of Wroclaw, towns and villages were inundated. The flood did however provide the team with a valuable opportunity to collect measurements of flood water levels in the channels at key points around the city for verification of the model of the system.

In addition to the engineering design and supervision of the construction works, the team are also responsible for obtaining the necessary environmental permits and acquiring land both for temporary use during construction and for permanent acquisition, no small task due to the large number of working sites and private landowners involved.

Construction of the first components of the scheme is due to start in the middle of 2011 and work will continue for two to three years. The team will supervise construction and provide support during the initial maintenance period following completion. The project is funded by the Polish Government, the World Bank and the European Union Cohesion Fund.

For further information contact Gareth Heatley, Halcrow Group Ltd,