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Rehabilitation of the weir at Ahausen reservoir

K. Dickel, P. Klein and A. Nielinger , Ruhrverband, Germany

The weir for the Ahausen reservoir was built by the Ruhr Reservoirs Association in 1938. Since the completion of the Bigge dam in 1965, its main task has been to compensate the flow of the Bigge river. After more than 60 years of operation, the fish-belly flap gates used for spilling floods had become corroded and therefore needed to be replaced. To stabilize the structure, ten anchors were installed in the dam. The rehabilitation was carried out successfully in 1999, with a construction time of only seven months.


The Ahausen reservoir is in the southwest of the Sauerland below the town of Attendorn in the valley of the Bigge (Fig. 1). The reservoir was first impounded in 1938 to produce energy and to regulate the Bigge river below the plant.



To meet the increasing water demand of the population and industry as a result of the expansion of the Ruhrgebiet, the Ruhrtalsperrenverein (today Ruhrverband) began to plan and construct the Bigge dam about 7 km upstream of the Ahausen reservoir, which was commissioned in 1965.

Since that time the reservoir has also served as a re-regulation reservoir to control the water discharge resulting from the operation of the Bigge power station, which is mainly used for peak energy production. The function as a re-regulation reservoir currently also includes water discharge control of the Bigge reservoir system within the reservoir system of the River Ruhr. In times of low natural flow, the Bigge reservoir system supplies about one third of the necessary water for all the reservoirs to raise the low water level of the Ruhr. The Ahausen therefore serves as a supra-regional water balancing reservoir and therefore has great economic importance.
The catchment area is about 359 km2, and includes the catchment area of the Bigge dam, which is located upstream, as well. The Ahausen reservoir extends to an area of 54 ha with an average annual inflow of 295 x 106 m3 and a storage capacity of 2.09 x 106 m3.
The dam is a 165 m-long earth structure with an impermeable core and a combined weir with two stationary weir sills and two weir gates, each equipped with one flap gate (Fig. 2). The control equipment is installed in the control pier, which is between the two weir gates. The powerhouse is arranged between the earth dam and the weir. It contains two turbines with a total discharge capacity of 22 m3/s. The water discharge from the reservoir is mainly done by the turbines.

Once the discharge exceeds the turbines’ capacity, the bottom outlet is used. This is also placed inside the weir pier and has a capacity of 32 m3/s. Flood waters are discharged over the weir gates. The height of the upper edge of the flap gates above dam base is up to 12.3 m.
The power station outlet, bottom outlet and the spillway all lead to a common stilling basin.


1. Reasons for refurbishment

Because of the head of water (> 5 m) and storage capacity (> 100 000 m3) the Ahausen weir is considered to be a full size dam in accordance with the water law of the state of Northrhine-Westfalia.
Corresponding to the DVWK Leaflet 231/1995, such dams have to be inspected in detail about every 10 years. The dam at the Ahausen reservoir was initially inspected in 1995/1996. After more than 60 years of operation, an intensive examination programme demonstrated some obvious defects, which have been known for years. Important components of the flap gates had become corroded and because of this, the material thickness was reduced. Moreover, corrosion had caused leakage in the flap gates’ sealing area.
Limited by the mechanical construction of the counter balance drive, it was difficult to control the gates when the reservoir level was low. In addition, there was a danger that friction forces could cause a sudden lowering of the flap gates if used in the future.
Boring samples gave further information about material characteristics, which were needed for stability examinations.
Computations to determine stability showed that the safety factors according to DIN 19700 and DIN 19702 could not be fulfilled.

These conditions did not correspond to the established technical standards and safety regulations.


2. Scope of construction work

The aspects mentioned above for re-establishing the functional efficiency of the facility have required a series of single steps (Figs. 3 and 4).




These steps affect the weir gates and the weir pier above all, and can be summarized as follows:

  • Dismantling of the old flap gates; weir crest demolition in the area of the base of the flap gates (about 3.3 m).
  • Complete demolition of the considerably matured reinforced concrete weir footbridge and replacement by a new steel structure.
  • Redevelopment of the weir structure to its previous height with the installation of a base for the new flap gates.
  • To increase stability, fitting of 10 rock anchors with 750 kN load capacity per anchor.
  • Delivery and installation of the new fish-belly flap gates; replacement of the original drive by hydraulic cylinder drive including electro-hydraulic equipment.
  • Installation of lateral support shields which could be heated.
  • Installation of a de-icing facility.
  • Patching of concrete and brickwork.


3. Prerequisites for the rehabilitation

The redevelopment measures required considerable lowering of the reservoir level. This limited the possibilities of water and energy-economy related tasks considerably, although it was the most important boundary condition for the construction project. The function as a re-regulating reservoir for the power station at the Bigge dam upstream had to be guaranteed.



Thus the only time period for construction was between March and September.
The possibility to split the work over two years did not seem practical, because this would have unnecessarily increased the construction costs. However, the double superstructure and dismantling of the construction site infrastructure would have been expensive.
Because of the limiting conditions explained above, a site road was built behind the weir (see photo below left). Thus the flood danger was reduced for about two months, and the construction progress was simplifed.
The stilling basin was filled with coarse-grained material as far as was necessary. The bottom outlet nevertheless had to be used for water discharge; therefore concrete components were arranged to form a culvert.
The reservoir level had to be lowered at certain times to prevent water from flowing through the turbines.

This operation could ideally be carried out from the site road, for instance the removal of dismantled components as well as the delivery of all the construction materials, components, tools and machinery.
After extensive planning, in co-operation with the consulting engineers, for the concrete and steel construction, who also dealt with the hydraulic engineering aspects, the job was put out to tender. The placing of contracts was done in association with a main contractor.

It had to be ensured that both construction activities (concrete and steel structures which needed to be refurbished), were synchronized well. This turned out to be particularly necessary because of the limited construction period available. The project required good co-ordination between the individual operators, and an optimized schedule and timing.
To encourage good construction progress from the beginning, a contractual penalty of up to 10 per cent of the contract sum was to be imposed on the prime contractor, if he took longer than the specified construction time. This resulted in the work schedule assuming high priority for all parties involved.



4. Sequence of work

After the invitation to tender and placing of contracts, the construction work began at the end of February 1999. But work had to be interrupted for five working days as a result of a flood in the first week of March.
Fortunately, this was to be the only interruption caused by a flood during the whole construction period.
First, the access road was constructed and the stilling basin was partially filled with coarse-grained rock material. To guarantee the operation of the bottom outlet, a culvert made of concrete components was constructed.




Thus a flow of up to 18 m3/s could be discharged without damage during the whole construction time.
The demolition of the old flap gates was carried out in just a few days using flame cutters. The dismantling works on the weir structure could then be started in the third week of March. The excavation of the new lateral support shield chambers was very difficult and time-consuming. To reduce the possibilities of damage to the original reinforcement in the walls, it was necessary to proceed cautiously and to use quite small equipment.

After the construction joints had been cleaned, the bottom and side walls of the old structure were plugged. A connection between the old and new weir structure enabled power to be transmitted between the two.
After the first placement of concrete, anchor works on the weir crest could begin. The drilling equipment (with a dead weight of 5 t, see photo, left) was lifted with a truck crane onto the weir sill. The boring of 10 drill holes 146 mm in diameter and up to 24 m deep was carried out in only six weeks. The anchor heights were defined according to test cores . Rock anchors were then installed and injected. This was followed by the second placing of concrete on the weir crest.

During this work on the weir structure inside the weir pier, the footing for the cylinder was established and the cylinder was installed.
Then the reinforced concrete weir footbridge was removed, after being notched at the base. After the load had been taken over by a 160 t mobile crane, it was separated from its base using a rope saw.

Crushing was done at the bottom. In the area of the weir, the flap supports and lateral support shields were prepared for installation and filled with concrete. At the same time the abutments for the new weir foot bridge were constructed.
After hardening of the concrete and execution of quality controls, the 10 anchors were tightened to 750 kN each (see photo below left).

On 24 August both gates, with a length of 14 m each, were installed one after the other using a 120 t crane, as well as the weir footbridges with a length of up to 22 m (see photo below). The required accuracy for the installation of the lateral support shields, the flap gates and the bridge base proved to be consistent. All constructional elements fitted together according to plan, so that the difficult stage of installation was achieved without delay.
The flap gates and weir footbridges were delivered already finished with a protective coating. The railings for the footbridges had previously been installed, so the bridges could be used for transportation during the rest of the construction period.
The concrete patching of the weir’s upstream face and the masonry refurbishment of the downstream face were carried out simultaneously in August and September.

The final work for the project, the installation of the hydraulic cylinders up to the completed piping, the installation of the hydraulic facility and the electrical installations had to be done in September.

The trial run for the hydraulics and electricity was carried out on time. The weir site was ready for water management and energy production according to the contract requirements on 1 October 1999.

5. State of the works

The plant was put back into service as scheduled at the end of September. The limited time available for the project made it critical to adhere to a rigorous and optimized construction schedule, from the specification of materials and equipment, through the contract stage, to the implementation of the project itself. Above all, the excellent teamwork of all the people involved in the construction ensured good progress.

The costs for the whole project totalled about DM 3.75 million (about US$ 1.6 million). The construction work is now complete. The Federal State of Northrhine-Westfalia, which subsidized the rehabilitation work, also required a study and measures to improve the ecological situation of the Ahausen reservoir.

Currently the flora and fauna in the reservoir are being studied so that suggestions can be drawn up for their improvement.



Karl Dickel obtained his civil engineer degree at the Advanced Technical College of Siegen, Germany, in 1976.
From 1976 until 1977 he worked in the Wastewater Treatment Department of the Lippe River Association. After three years with an engineering consulting company in Ellwangen, Germany, he joined the Wasterwater Treatment Department of the Ruhr River Association, Essen, Germany, in 1980. Since 1987 he has been the Head of the Bigge Reservoir Operation Unit.

Peter Klein graduated in civil engineering from the University of Bochum, Germany, in 1990. After a two year probationary period at the District Government of Detmold, Germany, he joined the Ruhr River Association in 1992 as engineer in the Reservoir Division. Since 1994 he has been Deputy Head and since 1996 Head of the Reservoir Operation and Maintenance Department.

Antje Nielinger graduated in civil engineering from the University of Hannover, Germany, in 1994. After two years at the District Government of Hannover, Germany, she joined the Ruhr River Association in 1997 as engineer in the Reservoir Division. Since 2001 she has been Deputy Head of the Reservoir Operation and Maintenance Department.

Ruhrverband (Ruhr River Association), Kronprinzenstr. 37, D - 45128 Essen, Germany.

K. Dickel P. Klein A. Nielinger

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