An extended hydrological classification for mangrove rehabilitation projects: a case study in Vietnam.

Abstract

Abstract Mangrove rehabilitation projects often fail to achieve their goals because hydrological aspects are not taken into account. This is understandable since the only hydrological tool available for such projects is the classification by Watson (1928), developed for mangrove forests in regions with a semi-diurnal tidal regime and a gradually rising elevation. Therefore, the possibilities for application seem rather limited. This study evaluates the Watson classification for mangrove forests in Vietnam, with planted and natural mangrove forests and rather irregular hydrological characteristics. During two fieldwork periods in spring 2004 and 2007, tidal regime, elevation profile, water levels in open water and in the mangrove forest, inundation characteristics and vegetation were measured. In Can Gio mangrove forest, the tidal regime is irregular semi-diurnal, which results in a highly variable level and frequency of high and low tides. This irregular tidal regime influences the inundation frequency strongly. Elevation measurements in the mangrove forests revealed a rather regular, gradually rising elevation profile in the forest close to the main creeks, and ridge-like structures and slightly lower basins behind such ridges deeper in the forest. The inundation characteristics in the zone with gradual rising profile were in line with expectations based on elevation. But in the zone with the irregular (micro) topography, direct surface discharge from the basins was impeded and therefore such basins were inundated considerably longer than estimated from their elevation. As a result, the inundation classes according to the Watson classification did not correspond very well with observed hydrological conditions. Therefore, an extended classification was developed that corresponded better to the actual hydrological conditions because it included the effects of an irregular tidal regime and an irregular elevation profile. This extended classification was tested on other locations in Can Gio. It can be concluded that the extended hydrological classification is a promising tool for predicting the natural development of the mangrove vegetation and determining the need for active management in the forest. Future mangrove rehabilitation projects might be more successful in restoring natural mangrove forests with a high biodiversity using this extended hydrological classification.