Oscillatory characteristics of young mangroves exposed to short-period waves

Ultragravity waves with a period of <1 s are often ignored in coastal engineering because they have little effect on coastal structures. In this study, we demonstrate that even though the height of short-period waves is small, the waves cause young mangroves to oscillate greatly. Indoor and onsite mangrove growth tests, oscillation tests, wave experiments, and theoretical analyses were conducted on a mangrove species (Kandelia obovata) to determine the oscillatory characteristics of young mangroves when exposed to small waves. Natural period of oscillation of the young mangrove shoots was less than 0.5 s. Their resonance occurred when the wave period was a multiple of the natural period of oscillation of the mangrove shoots. Because of resonance, the horizontal acceleration of the mangrove leaves was calculated to reach twice the acceleration of gravity. If these waves act for a long time, young leaves can fall. A test performed on a mangrove forest in Amami Island in Japan revealed that although the mangrove heights grew rapidly, their stem thickness did not change substantially. Thus, the young mangroves need to gain rigidity in a short period to withstand short-period waves. To increase the survival rate of the young mangroves, short-period waves, whose period is close to the natural period of oscillation of the mangrove stems, could be avoided by installing a small breakwater. However, stem oscillations may positively affect leaf photosynthesis because small waves have the same effect as a gentle breeze and can promote the growth of young mangroves. Therefore, further studies on ultragravity waves and the growth of young mangroves are required.