Hydrobiologia 247: 141-161, 1992. V. Jaccarini & E. Martens (eds), The Ecology of Mangrove and Related Ecosystems. 0 1992 Kluwer Academic Publishers. Printed in Belgium. 141 Hydrodynamics of mangrove swamps and their coastal waters Eric Wolanski Australian Institute of Marine Science, P.M.B. No. 3, Townsville M. C., Qld. 4810, Australia Abstract Mangrove swamps help control the tidal hydrodynamics of many tropical estuaries. They generate an asymmetry of the tidal currents in both the tidal creeks and the mangrove swamps. This results in self-scouring of the tidal channels. Mangrove land reclamation results in siltation of the channel. Man- grove swamps control the flushing rates of the estuaries through the lateral trapping effect. Lateral trapping leads to the aggregation of mangrove litter along slick lines. Evapotranspiration plays a role in the hot dry season by forming a salinity maximum zone which isolates the estuary from the coastal waters for several months of the year. In the absence of runoff, evapotranspiration in the hot dry season gen- erates an inverse estuarine circulation which can trap high salinity mangrove water, and mangrove de- tritus, along the bottom of a mangrove creek. This bottom layer can become anaerobic. Groundwater flow appears to play a key role in the nutrient budget of mangrove creeks, exporting salt left behind by evapotranspiration, and inhibiting runoff after rainfall. Particulates and dissolved nutrients outwelled from mangrove swamps to coastal waters are retained in a coastal boundary layer. This coastal boundary layer water can be trapped along the shore for long periods if the coast is straight and mangrove-fringed and the coastal waters are shallow. Headlands inhibit coastal trapping because they enhance mixing. Nutrient-rich coastal boundary layer waters may be ejected offshore as tidal jets peeling off headlands and locally enriching offshore waters. Introduction A typical mangrove swamp is Coral Creek (Fig. 1). It is 5 km long, with fairly steep banks, and maximum depth decreasing with distance from the coast. The cross-sections can be repre- sented by polygons (Fig. lb). The fringing man- groves are typically 100 to 300 m wide on either side of the creek. The surface of the substrate slopes gently upwards from the creek, with ele- vation near the creek of about 0.3 m above mean sea level, and maximum elevation rarely exceed- ing 1.3 m. When many mangrove creeks exist one next to the other, they form a large mangrove swamp. Hinchinbrook Channel (Fig. 2) is an example of such a vast mangrove swamp. The channel is 44 km long channel with 109 km* of open water surrounded by 164 km2 of fringing mangrove swamps. Since the mangrove creeks draining the swamp are oriented perpendicular to the Chan- nel, the swamp is up to 5 km wide from the dry land to the edge of the Channel. Tidal circulation in a creek-mangrove swamp system The barotropic tidal circulation in the Coral Creek tidal creek-mangrove system was first modeled by Wolanski et al. (1980). They noticed that peak tidal velocities often exceed 1 m s - ’ in the tidal creek but never exceeded 0.07 m s - ’ in the heavily vegetated mangrove swamps 50 m from