Hydrodynamics of vegetated compound channels

Abstract

Mangroves are an interesting species of vegetation, surviving and thriving at the interface of land and water, in the inter tidal brackish coastal waters between the mean sea level and mean high water. Mangroves are a highly productive and complex ecosystem, providing numerous services and goods to people and marine environment. Mangroves are home to a large variety of underwater animals. Mangrove wood is valuable because it is resistant to rot and insects and can be harvested for pulp or charcoal production. Most importantly, complex roots, stems and canopies of mangroves provide effective protective means for coastal and estuarine regions. Waves and tidal flows are significantly slowed down as they make their way into and through the roots, stems and canopies of the mangrove forest. Nutrients and sediments can be deposited, providing necessary conditions for a sustainable development of the mangrove ecological system in particular and a stable coastal area in general. However, despite the important role of mangroves along the Mekong delta estuary, a large part of the mangrove forests has been converted into fish farms. In many regions, only a narrow strip of mangroves remained and, as a consequence, mangrove forests degraded and banks and shorelines experience severe erosion. Although numerous attempts were implemented to restore the mangroves and to enhance the river bank stability, these were not really successful. A possible explanation may be that knowledge about the hydrodynamics and exchange processes in and around the mangrove forest vegetation area is still not yet rigorously researched. In order to understand the dynamics of mangroves, the remaining width of mangrove forests and erosion (accretion) rate were observed, collected and analysed in terms of a morphological perspective. It is found that the river bank erosion appears to relate to the width of mangrove forest. The larger the width of the mangroves, the less erosion of the river bank and vice versa. In this context, the concept “Squeeze Phenomenon,” explaining the degradation of mangroves together with the erosion of the river bank, is introduced. Based on a schematised numerical model, changes in hydrodynamics and exchange processes caused by the limited width of the forest are proposed to be the fundamental reason for the “Squeeze Phenomenon.”…