Rubber dams are hydrostatic structures in which a bladder made of fabric-reinforced rubber is screwed with clamping plates to the dam structure which creates a sealed inner space.

The filling medium can be water or air. In both cases, a balance between the internal pressure forces and the external water load is established at the membrane of the rubber dam bladder. By varying the filling volume, the height of the rubber dam bladder or the storage and discharge section can be changed and thus adapted to the requirements of the dam.

The pumps and valves are controlled from a control house. The appliances required for water-filled systems are located in underground shafts and for air-filled systems in an overground control 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 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 most suitable filling medium depends on the requirements of the project. Several criteria need to be examined during the planning stage.

With this type of weir, the rubber dam is filled with air. The dam is held in raised position only by the air pressure inside. When the rubber dam is inflated, the blower located in the control room on the bank blows air into the rubber dam via a regulating pipe.

The inflated 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 outdoor 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 only. If the retained downstream water reaches up to the non-overtopped dam, a second fixing line may be necessary.

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 control shaft connected with the inside of the bladder by a pipeline (regulation pipe). The height of the water column in the control shaft determines the internal pressure and thus the height of the inflated bladder.

The control shaft can be solid or height-adjustable. The height-adjustable control shaft is a combination of a base pipe and a so-called water bag.

The filling of control shaft – and thus also the inside of the bladder – is carried out 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 control shaft. As soon as the pressure inside the bladder exceeds the sum of external forces (bladder weight and water load), the bladder starts to raise. 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.