INTERACTIONS BETWEEN WAVES, BANK EROSION AND EMERGENT VEGETATION - ANEXPERIMENTAL-STUDY IN A WAVE TANK

Emergent vegetation development, wave extinction and soil erosion are strongly interrelated processes in exposed riparian zones. The above-g round parts of the vegetation reduce wave energy, while the below-grou nd parts strengthen the soil. On the other hand, vegetation develop me nt may be restricted as a result of wave stress. Interactions between waves, soil erosion, and emergent vegetation were studied during three consecutive years. Two helophyte species, Phragmites australis (Cav.) Trin. ex Steudel and Scirpus lacustris L., were planted in separate b ank sections on two types of sediment, sand and silty sand, in a wave tank. Regular waves were transmitted through 4 m wide bank sections wi th and without helophytes growing on a horizontal part. Bank profiles, wave transmission patterns and vegetation parameters were measured af ter exposure to waves with a height of 10 cm (Year 1) and 23 cm (Years 2 and 3). Both 10 cm and 23 cm waves affected bank profiles. Erosion of the banks occurred due to downslope transport of sediment. Soil ero sion patterns closely reflected the patterns of standing waves over th e horizontal part of the bank. Emergent vegetation influenced the eros ive impact of waves by both sediment reinforcement and wave attenuatio n. A smaller amount of net erosion was measured in the wave-exposed se ctions covered by vegetation than in the unplanted sections. The stand s of Scirpus lacustris were damaged due to uprooting of rhizome parts by 23 cm waves, followed by increased erosion of the soil. No damage o ccurred to the Phragmites australis stands. The greatest wave attenuat ion (measured as relative wave height reduction) was measured in the f ully developed vegetation in August of each year in both types of vege tation.