Restoring mangroves with structures

Abstract

Mangrove forests effectively function as natural flood defences, and their deforestation has exposed millions of people worldwide to coastal erosion and flooding. Since mangroves require a stable sedimentary environment, stopping coastal erosion is a necessary step for their restoration. Bamboo structures have thus been built to induce accretion at the coast by attenuating waves. However, these structures often fail to rehabilitate mangroves, likely due to the lack of guidelines for their design.
This thesis investigates the effect of structures formed by bamboo poles on waves, currents, and sediment transport, to develop physics based models for structure design. These effects were studied through flume experiments with scaled structure prototypes, field experiments in Demak (Indonesia), 1D morphodynamic modelling (with the model XMgrove, calibrated with field measurements), and remote sensing.
Models to predict structure performance were developed for waves and currents. Flume experiments showed ways to optimize structure designs. For instance, wave dissipation per pole is maximum for dense rows of poles with large spacing in the wave direction. Modelling scenarios with XMgrove suggest that the optimal structure location is site-dependent, and that subsidence rates in Demak may be too high to be counteracted with structures. A large-scale method to find potential restoration sites was also developed and applied in Bangladesh.
As such, the physics-based tools, together with the mapping method presented in this thesis, open up the path to optimize and generalize mangrove restoration efforts.