inflatable dams can be filled with water, air or both. they are low pressure—typically 4 to 10 psi. the present trend suggests an increased use of air-filled membranes because they can be deflated or inflated more rapidly, and they are little affected by freezing conditions. characteristic dimensions cover typically lengths of about 100 m with specially-made membranes up to 200-m wide, dam heights usually less than 5-m but some special designs might be up to 10-m high. the membrane is usually deflated for large overflows. it is however common practice to allow small spillages over the inflated dam. during overflows greater than 20% over-topping, vibrations might result from fluid-structure interactions, and the instabilities might damage or destroy the rubber membrane. several failures were experienced (e.g. chanson 1996). in practice, a deflector (i.e. fin) is installed on the downstream face of the rubber dam to project the nappe away from the membrane, hence preventing rubber membrane vibrations.