Sediment dynamics and morphological stability of estuarine mangrove swamps in Sherbro Bay, West Africa

The relationship between the circulation of fine suspensions in a mangrove-colonised estuarine complex, short-term mangrove substrate accretion and medium-term mangrove swamp geomorphic evolution was studied in Sherbro Bay, an infilling microtidal to mesotidal embayment in southern Sierra Leone formed behind Holocene sandy beach ridge barriers. The estuarine reaches of streams debouching into the bay have relatively high suspension concentrations (SC) of silts and clays that ranged, during the survey period in 1991, from 0.8 to 40 g l-1, in association with the constitution of turbidity maxima (5–50 g l-1) at the interface between freshwater drainage and the saltwater intrusion. However, the surveyed mangrove swamp plots showed much lower suspension concentrations (0.09–0.6 g l-1), and low albeit minimum rates of accretion (1.1–3 mm a-1), monitored between 1980 and 1989. These low rates are attributed mainly to the short duration of tidal flooding due to a high substrate elevation within the tidal range, and to the low settling potential of the very fine-grained suspensions circulating over the swamps. A comparison of two sets of aerial photographs (1954–1960 and 1976) showed that these swamps and their channel networks are also characterised by a remarkable medium-term morphodynamic stability. This stability and the low accretion suggest that the swamps and their channel networks are largely in equilibrium with the hydrology, notably flooding duration, and sediment loads. Settling out of mud from the turbidity zones contributes to higher accretion rates (up to 12 mm a-1, not taking into account the effects of shallow subsidence) on parts of the mudflats fronting the mangroves which are flooded for longer periods. Some of the mud is deposited over sandy channel bars in the main bay channel and the rest exported seaward. It is tentatively suggested that an increase in the duration of tidal flooding generated by global sea-level rise, at the presently predicted rates, could lead to higher sedimentation in the mangrove swamps, thus enhancing accretion and the chances of mangrove survival.