Mangrove colonization on tidal flats causes straightened tidal channels and consequent changes in the hydrodynamic gradient and siltation potential

Abstract

A healthy mangrove ecosystem includes diverse landscape structures, such as tidal flats, tidal channels, and areas with circulating waters, in addition to mangrove stands. The complex structure of mangrove forests affects the hydrodynamics and sediment transport behaviour of tidal channels. Understanding the influence of the mangrove invasion of tidal flats on the pattern and stability of tidal channels is essential. In this study, two types of remote sensing images, Google Earth images and aerial photographs, were collected to analyze the relationship between mangrove colonization and changes in tidal channel patterns. After applying binary image processing, these two kinds of images show similar abilities to discriminate the locations, extents, and boundaries of mangroves and tidal channels. We found that the mangrove area was inversely proportional to the tidal channel sinuosity and width. The tidal channels exhibited a meandering pattern with a wider width before the mangroves invaded the tidal flats. After the expansion of the mangroves, the tidal channels gradually transformed into a straight shape with a narrower width. After the mangroves developed into forests, the tidal channels maintained a straight and stable pattern. Since mangroves promote siltation and increase the elevation of the surrounding mudflats, the habitat suitability for mangroves in the neighbouring tidal flat areas may vary. These processes may help expand mangrove habitats, thereby compressing the area of flats and changing the shape of tidal channels. Due to tidal current effects, the unit stream power of a straight tidal channel is approximately twice that of a meandering channel, indicating that straight tidal channels have a stronger anti-siltation capability. Our research also found that the tidal channels may return to a meandering pattern when mangroves are degraded or die and their area decreases. This study provides key evidence that mangroves affect tidal channel types and hydrodynamic characteristics, thus providing a useful reference for restoring and managing estuarine mangrove ecosystems.

Keywords: Hydrodynamics; Mangroves; Remote sensing images; Tidal channels; Unit stream power.