The two 105MW hydro stations that make up the Theun Himboun station are run-of-river plants, so there have been no large dam structures built at the site. There is a small reservoir, however, and water level control is needed, as are discharge options in the event of heavy flooding, which happens in this area almost every rainy season. Five rubber gates have been used for this purpose.

Although rubber gates are familiar technology in many countries, including the US and Europe, this is the first time they have been used in Laos. The Theun Himboun Power Company has been established by Electricite du Lao, Nordic Hydropower and MDX Lao, with funding from the Asian Development Bank, and it plans to sell most of the station’s power output to Thailand. Electricite du Lao is said to be very happy with the performance of the gates.

The rubber gates, each 3m high and 32m wide, were installed in a three-month process on the crest of a ogee-shaped concrete weir by Japan-based Sumitomo Electric. The gates can be deflated or inflated individually so water flows can be controlled with precision.

Sumitomo has now supplied over a thousand rubber gates to water control projects worldwide. The gates are formed from high-strength nylon fabric and synthetic rubber, which are joined longitudinally in layers. The number of layers varies depending on the required gate height and the flow of the river. The tube body is attached to a concrete foundation. In low overflow conditions, or where there is minimal tailwater, the tube is attached with a single row of anchors. In areas where reverse flow is expected or where the flow is greater a double line of anchors is used. This increases the stability of the body and minimises vibration.

For sites such as Theun Himboun where water-borne debris is likely to be a problem polyethylene foam cushioning can be installed inside the rubber body of the gate. When the gate is deflated the cushioning, which can vary from 40mm to 360mm in thickness depending on the conditions, absorbs the impact of the debris. The gate can also be strengthened by attaching one or two layers of steel wire mesh coated with synthetic rubber to the outer surface of the body. This protects against cutting and penetration, including vandalism.

In use the gate can be filled with water or, as at the Laos site, with air. One benefit of the air-inflation system is that in dams of a suitable size it is possible to perform inspections while the dam is inflated, and an access door and airlock system is provided to allow entry for this purpose. Inflation and deflation can be carried out automatically: the gate inflates when the upstream water level falls below a preset value and the supply of water or air is then controlled to ensure the pressure inside the gate is maintained. The gates can also be controlled remotely. Where more direct control of the flow and discharge is required a gate can be used that is split into several chambers. The chambers on this type of gate can be individually adjusted, and deflated fully or partly, to increase discharge capacity as flow rate rises. Gates of this type are in use at the Cavendish dam in the US.

Sumitomo also offers its rubber gates for use as fish ways. Several narrow gates are in use for this purpose at the Nagara river in Japan where they control access to a series of fish passageways. Under development is a refinement of the gate in which a series of rubber bodies forms a stepway downstream of the gate.