Structure & functional principle
Rubber dams are hydrostatic structures provided with a bladder of textile-reinforced rubber screwed to the dam structure which creates a tight inner space.
The pumps and valves are controlled from a power house. The appliances required for water-filled systems are located in underground shafts and for air-filled systems in an over ground power house. Using the respective control system the retention set point is adhered to reliably. The duration of Inflating and/or deflating is adjustable.
The filling medium is supplied to the rubber dam by pipes connected with the control system. As soon as the rising water level in the retention zone exceeds the retention set point, the bladder is gradually deflated until complete depletion, thus clearing the full dam opening. Vice-versa, as soon as the water level decreases the bladder is inflated. In this way the upstream water level is kept constant at the specified retention set point.
The choice of the appropriate filling medium depends on the requirements of the individual project. A number of criteria have to be considered at the planning stage.
Das Füllmedium wird dem Schlauchwehr durch Rohrleitungen zugeführt, die mit dem Reguliersystem in Verbindung stehen. Bei steigendem Wasser im Staubereich wird der Schlauch bei Überschreitung des Stauzieles sukzessive entleert, bis er schließlich vollständig entleert die gesamte Wehröffnung freigibt. Umgekehrt wird der Schlauch bei fallendem Wasserstand wieder aufgerichtet und hält auf diese Weise den Oberwasserspiegel auf ein vorgegebenes Stauziel konstant.
Die Entscheidung über das jeweils geeignete Füllmedium richtet sich nach den Anforderungen des Projektes. Dabei müssen verschiedene Kriterien im Planungsstadium untersucht werden.
| Air-filled rubber dams | Water-filled rubber dams |
|---|---|
| Operation | |
| Even overtopping when operating at 95%-100% of the maximum bladder height | Even overtopping in every operating mode |
| V-shaped dent is formed when bladder is deflated below 95% to 90% of the maximum bladder height | |
| Spans can be controlled individually | Spans can be controlled individually |
| Setting times for inflation and deflation within 5 to 30 minutes (depending on size) | Setting times for inflation and deflation from 1 to 4 hours (depending on size) |
| The filling medium is ambient air | The filling medium is generally river water (measures for improving the water quality might be required) |
| No icing of filling medium in winter | Efficient measures against icing required in the cold season |
| Calculation of discharge is possible for 95% - 100% of the max. bladder height | Calculation of discharge is possible because of the Floecksmühle system for measuring the bladder height (in the range of 25% - 100% of the max. bladder height) |
| Longer service life of the dam in hot regions due to less heating of the membrane | |
| Lower loss of filling medium in the case of damage to the bladder | |
| Dimension of dam structure | |
| Width of depleted bladder: approx. 1.8 x bladder height | Width of depleted bladder: approx. 2.4 x bladder height |
| Small pipe diameters | Larger pipe diameters |
| Control system of compact design requires little space, especially in costly underground structure | Control system must be located in shafts next to the rubber dam |
| Control system not required to be located directly next to the rubber dam | |
Air filled rubber dams
The bladder of this type of rubber dam is filled with air. The dam is kept erect only by the air pressure inside the bladder. For inflating the dam, the compressor located in the power house on the riverside feeds air into the bladder through a control pipe.
The inflating bladder is completely air-tight, including the side wall sections, impounding the upstream water as the filling proceeds.
The deflation process is carried out by electrically or pneumatically controlled valves. Additionally, all valves can be operated manually. In the case of small dams the entire control system may be housed in a thermally insulated and heated control cabinet.
The rubber dam is equipped with a number of sensors ensuring safe operation, precise setting and permanent monitoring of the dam operation. The sensors measure the water level in the retention area and the internal pressure of the bladder. Besides, the interior of the bladder is monitored for possible condensate formation or intrusion of outside water. The internal pressure of the bladder is measured separately for each span. Therefore separate measuring pipes are provided leading to the control house.
Up to a deflation of approx. 95% to 90% of the maximum bladder height, air-filled dams behave in a way similar to water-filled dams. When depleted further, a V-shaped dent forms involving higher overtopping of the dam. As a consequence, air-filled dams are not overtopped as evenly as water-filled dams. This may have advantages as well as disadvantages depending on the site location. The control system developed by Floecksmühle ensures strict observance of the retention set point also for air-filled rubber dams.
As a general rule, rubber dams may be used in locations where the downstream water is retained. When there is no downstream retention, the air-filled bladder is fixed to the dam body with one upstream fixing line. If the retained downstream water reaches up to the non-overtopped dam, a second fixing line may be necessary.
Water-filled rubber dams
In water-filled rubber dams the water pressure in the bladder is created to the principle of communicating vessels as a water column in a regulating shaft connected with the inside of the bladder by a pipeline (inlet/outlet pipe). The height of the water column in the regulating shaft determines the internal pressure and thus the height of the inflated bladder.
The water bag – and thus also the bladder – are filled by one or more filling pump(s). The water is taken from the supply shaft which is generally connected with the upstream water by one or more pipes. The bladder is inflated by feeding water into the regulating shaft. As soon as the pressure inside the bladder exceeds the sum of external forces (bladder weight and water load), the bladder starts to prop up. The inflated bladder retains the water in the retention area.
The crest of water-filled rubber dams is always horizontal, even when filled only in part. Therefore the overtopping water flows very evenly across the dam.
The measured parameters include the water level in the retention area and on the downstream side as well as the bladder height if required. As a consequence, water-filled dams make it possible to calculate the discharge rate in a certain range. In this way the discharge rate may be accounted for in the control system as a control parameter.