Hydrological Processes and Water Management in Urban Areas (Proceedings of the Duisberg Symposium, April 1988). IAHS Publ. no. 198, 1990.

FLOOD PROTECTION SCHEME FOR THE CITY WASSERBURG A. INN

Werner Kraus Wasserwirtschaftsamt Rosenheim, D 8200 Rosenheim, Federal Republic of Germany

ABSTRACT

The city of Wasserburg A. Inn was often damaged by floods. So a protection scheme was to be constructed by the Wasserwirtschaftsamt Rosenheim. The medieval town is under a preservation order. It was difficult to design the flood protection wall in front of the city. The remainder of the protection dam was to be arranged as urban recreation area. A team of watershed management engineers developed the solution, including architectural problems, by cooperating with biologists and the City Council.

INTRODUCTION

The small town "Wasserburg a.Inn" in Upper Bavaria, Federal Republic of Germany, is situated on a peninsula created by the river Inn, which comes from Switzerland passing through the Tyrol to Bavaria at Kufstein and flows into the Danube at Passau. Only from the west is there access for a road and railway into the city.

As the name "Wasserburg" (water-castle) is telling us the town was always intimately linked to the watercourse, in both good and bad respects. The role of navigation on the river Inn meant a great deal for the city in former times, but also floods caused damage again and again. So a flood protection dam had to be built by the "Wasserwirtschaftsamt" (Watershed Management Authority) in Rosenheim.

From the "Brucktor", the gate at the end of the bridge and the entrance into the city, a flood protection wall of 170 m length is the start of the protection scheme. The largest part of the peninsula is protected by a one km long earth dam. It follows a 450 m length of bank, formed of earth in the lower part, but (due to restricted space) of wall in the upper part. Finally a dam of 330 m closes the gap up to the natural hills in the Northwest. In case of flood a pumping station lifts any infiltrated water from the city back into the river. There have been two demands in the planning and construction of the protection walls: The first part of the wall had to be adapted to the medieval shape of the town; and the earthdam should serve as recreation landscape around the city.

THE FLOOD PROTECTION WALL

In spite of wars and many bad floods Wasserburg succeeded in preserving its medieval structure almost completely. Due to special "Innstadt-Bauweise" the city looks very southern, nearly Italian, especially to visitors from Middle- and North-Germany. Characteristics of that special type of construction are frontwalls going up to the top of the

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houses hiding the roofs, and arbors along the streets. Like ancient cities there is no straight front, no plane wall in town. No arch is exactly like another, neither those of the arbors nor of the windowbays. The city of Wasserburg is under a preservation order.

One of the most beautiful and famous views of Wasserburg is the old-fashioned bridge with the ancient bridge-gate and the connecting medieval riverfront of houses. So it was not acceptable to build a single flood wall of total height equal to the 3.80 m necessary to confine flooding. Instead the wall was divided into two, a bund in the lower part and a protection wall above (Photo 1).

Between these two walls there is a footpath instead of the former riverwalk. To avoid monotony, the level of this footpath (which marks the upper border of the bund) is varied. Steps drop down to the footpath from the bridge-gate. After some distance there is a raised platform from where there is an outlook, where you can rest and where you can cross to the street behind the upper protection wall. Close to the end of the wall there is another change to cross from the footpath into the background of the wall, giving further relief to the construction. In the ground-plan the wall isn't straight. The platform projects out to the front and there are other corners and niches in the wall.

Corresponding to the medieval architecture of the city, the bunding (the lower part of the wall) echo on the arches of the arbors and windowbays. Stonecarved outlets for the drainage of the platform continue the horizontal line of the edge at the top of the concrete wall of the bunding. This edge, with the pillars of the hand rails and the coping of the upper protection wall are made of natural stone called "Nagelfluh", found 50 km south of Wasserburg close to the river-banks. The concrete of the bunding and protection wall is specially designed to match this beautiful and very often used stone. In the invitation for tenders several combinations of gravel of different size and colour, of colour additives to the cement and of surface structure were requested. A very rough surface similar to the natural stone Nagelfluh was chosen. The hand rails are constructed of metal painted in the green colour of the river Inn. Along the wall there are several gaps in the pavement of the riverwalk. Plants like wild vine and others have been planted to grow up along the wall. Benches for resting have been put where space allows. Furthermore there was one special point during the planning period: a model of this architecturally difficult wall was built at the scale 1:50 with three variations of the wall.

THE EARTH-DAM

Following on from the flood protection wall a dike in the form of a zoned earth dam is built, rising almost 3 m over the natural level of the riverbank.

The core of compacted puddled clay extends 2.5 m below the natural level. The land and waterside fill are both constructed of compacted gravel. On top of the dike there is a 2.50 m broad "way" to defend the dike in times of flood. On the landside bottom of the dike a drain was built to convey the infiltrating water.

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Flood protection scheme

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Photo 1. The design of the flood protection wall fits into the skyline of the medieval city.

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There were several technical points to the design and construction of the dike as a recreation area close to the city. Over the centuries, the peninsula of Wasserburg grew like the rings of a tree. From time to time in history a new suburb-street was added to the old city close to the new riverbank. But it always took several decades till the soil of the new embankment was strong enough to carry new houses and streets. So fortunately there was some space to provide not only a narrow dike construction but a varied park which protects against flood. First of all each of the older trees there had to be noted and incorporated into a general plan. Then a decision made of which trees could be cut down so the dike could be built between the others. The route dumpers had to go during the construction of the dike was also needed. Too heavy a compaction of soil in the root zone would kill trees like oak or linden trees. In all a line for the dike had to be found as a compromise between technical and ecological requirements.

The slope of the dike varies from metre to metre. There are little hollows in the slope, horizontal areas and smooth hills. Beside the "way" on the top of the dike a footpath follows along the dike, but it doesn't remain on one level (Photo 2). It climbs up to the top and swings down to the water. It divides into two and comes back together again. It is important that the footpath doesn't remain at an altitude where the eyes of a walker are at the same level as the horizontal surface of the top of the dike. To guard against this, little hills are added on the top surface of the dike aside from the way.

At suitable places along the footpath benches are installed, inviting walkers to rest. At other places huge stones, monolothic "litter" from the gravel pit, are arranged. Children climb the rocks and young people sit there.

Another point in designing the recreation park close to nature is the vegetation. There are old trees, groups of bushes, grassland and rough soil. Wet hollows, sealed by clay, are planted with reed, waterlilies and others, while a natural succession is allowed in rough dry habitats.

Also important are the various outlook opening views to the city and to the river, but with closed places nearby, closed by trees or bushes. Furthermore, there are several accesses to the Inn itself which enable people to go down onto the widespread banks and sandbars of the river during the lower water periods.

Today, two years after construction, you can see mothers there with their babies, grandparents watching their grandchildren playing, people walking and jogging. This is proof that the flood protection dam succeeded in fulfilling its technical purpose as well as giving an urban recreation area.

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Flood protection scheme

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Photo 2. The protection dike is arranged like a natural landscape along the river serving as urban recreation area.

CONCLUSION

The example Wasserburg a. Inn shows that it is possible to design a technical flood protection scheme in front of a medieval city which is under a preservation order without destroying impressive views. To reach this goal it is important to design with a team of watershed management engineers sensitive to problems of architecture, to cooperate with biologists, and to keep close contact with the administration and the council of the city. All design consideration must of necessity start together, simultaneously with those relating to technical solutions.

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