Effect of adjustable flaps on river surf waves at abrupt drops

Abstract

Surfing is a popular water sport in regions with ocean access. In countries without ocean access, standing river waves occurring in flood situations are often employed for surfing. The hydraulic conditions involving high flow velocities and large recirculation zones then impose a risk of injury to the surfers. As an alternative, artificial surf wave conditions are provided at a number of river locations in Europe and worldwide. In most locations, a wave-type hydraulic jump is generated at a bottom drop. Based on the experiences with those engineered surf waves, adjustable installations demonstrate a positive effect on the surf wave characteristics. Physical laboratory experiments were conducted to quantify the effect of adjustable flaps on the generated wave characteristics at abrupt bottom drops. Whereas the wave-jump at a plain bottom drop without flap is very sensitive to the tailwater flow depth, adjustable flaps prevent the change of flow type from the favored wave-jump to non-surfable conditions for a wider range of downstream flow depths. Depending on the approach flow conditions, flaps significantly increase the wave height. However, adjustable flaps may also implicate additional limitations to the parameter range. Flaps with a small angle led to a wave height reduction, as compared to the plain bottom drop without flap. This paper details the above descriptions, and selected wave-type flow features are discussed. The research thus contributes to the general process understanding of surfable river waves and safer surf conditions, as compared to the common use of standing waves in rivers at flood situations.