Inflatable rubber dam 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,^[1] and the instabilities might damage or destroy the rubber membrane. Several failures were experienced . In practice, a deflector (i.e. fin) is installed on the downstream face of the rubber dam to project the nape away from the membrane, hence preventing rubber membrane vibrations.