2.5 million euros for flood protection

Renovation of the flood control basin for Fautenbach

In recent decades experts have increasingly revised their outlook concerning projections for heavy rain events. This is because the HW100 – a high water event of such a magnitude that it is only to be expected once every 100 years – in use until now, and on which many plans are based, now occurs much more frequently. Consequently, damage to private property and public facilities occurs much more frequently and is much worse.
Indeed this not only affects the large German waterways such as the Rhine and the Elbe, but also their tributaries.

Achern (Ortenaukreis, Baden)

To the south of Achern the inconspicuous Fautenbach flows, coming from mid-Black Forest through the community of the same name, and continuing north-west towards the Rhine. Used as mill stream once upon a time, at times this small stream develops into a rapidly flowing river when the rain in its catchment area of 11 km² is heavier than usual and the water level swells several metres. This threatens the community of Fautenbach; there is even a kindergarten directly on the banks of the stream.
Following a large, long-lasting flood in 1978 an earth embankment was built at the beginning of the 1980s to protect the small valley, drainage was limited by a regulating pipe which allows the passage of a maximum of 6000 l/s, and the newly resulting basin of 250,000 m² in the direction of the mountains was designated as flood area with agricultural use. For amounts of water exceeding a HW20 (a high-water event expected on average once every 20 years) there was a regulated spillway over the dam.

This construction was to be completely replaced by a sluiceway structure with chute, which was to be equipped to withstand a HW100 based on new assessments.

Concrete construction and dam

The renovation of the flood control basin, for which 2.5 million euros are estimated, was commissioned by the city of Achern. The project is supported by the state of Baden-Württemberg which, according to figures of the regional council of Freiburg, approved an amount of more than 550 million euros for the construction and renovation of embankments and dams over a total length of 1100 km.
The contractor and company doing the construction work is Huber Bau-GmbH & Co. KG (Ottenhöfen). Responsible for construction management is Zink Ingenieure (Lauf); consulting engineers VBI are RS Ingenieure (Achern). The supplier and advisor for the concrete formwork is PASCHAL-Werk G. Maier GmbH (Steinach).

Earth excavations began in June 2011. Here a portion of the dam had to be displaced; the concrete construction is being built into the resulting space in the dam, and spans this distance completely. Also the stream was intercepted and redirected, and an emergency dam was erected around the construction site. All excavations may only be carried out during dry weather in order not to endanger the stability of the hillside.
The actual construction site is concentrated in an area of 250 x 220 m. There are layers of quaternary, sand-gravel and to some extent cohesive sediments.

Concrete work and formwork

Building work on the shell of the concrete construction that is to regulate the flow of water began in November 2011 with the formation of a bottom slab in the form of a chute with a thickness of 80 cm: the first 17.20 m have a downhill slope of 4%. For this the company’s own MODULAR UNIVERSAL FORMWORK was used.

In spring 2012 the work on the regulating construction began. This is 42 m in length, 12.55 m wide and 9.75 m high, measured from the bottom slab. The two outer wings have walls which are vertical on the inside and taper on the outside. These walls are 1.15 m thick at the bottom, 1.09 m in the middle and 0.5 m at the top. A total of 45 elements with LOGO.3 formwork with the dimensions of 2.70 x 2.40 m as well as some smaller raising elements, with an overall formwork area of roughly 350 m², as well as a half-side of an advance section were used.
The outer formwork was inclined and this way created an offset of 3 to 4 cm in the tie points. Even though this meant that the tie rods did not lie exactly horizontally, it was still possible to join these reliably without any problem despite the thick reinforcement, without having to use the familiar PASCHAL fillers (which can adapt up to 1.0 cm to formwork in the system, without any fillers by site, in order to meet requirements).
Since the formwork was set up in full phase height, there was a dead angle for the concrete vibrators at the base of the tapering walls. An external vibrator was used here.

Side wall: The walls are tapered; the visible sloping areas have a trapezoid shape. 350 m² of LOGO.3 wall formwork from PASCHAL was used, with three phases per side each. In addition a half phase ran in advance.

In the upper area a sill was set up from wall to wall, perpendicular to the direction of flow, which will hold the technical equipment.

Around 420 m³ C35/45 concrete was used for the foundation, and for the structure itself 680³; the reinforcement weighs around 50, or respectively, 110 tonnes.

Viktor Kern, foreman and master mason, who has worked at Huber-Bau GmbH for 32 years, is very satisfied: “The formwork work went briskly and completely problem-free. The specialist advisors are always there when one needs them, and solve any problems that arise quickly and competently!"

The 42 m long culvert construction is situated in the middle of the dam. It is 9.75 m high and 12.55 m wide. Integrated pressure sensors will constantly carry out measurements. When flooding occurs the opening will be closed and limited by valves to 6 m³/s.

Outlook

The structure is to be ready for operation in December 2012. Not only will it contribute to protecting Fautenbach much better than before, it will also help keep down the flooding along the Rhine. This will reduce or even prevent a considerable amount of economic damage.