Sedimentation in Mangrove Forests

Keita Furukawa(1,2) and Eric Wolanski(2)

e.wolanski@aims.gov.au

Port and Harbour Research Institute, 1-1,3 Chome, Nagase, Yokosuka 239, Japan
Australian Institute of Marine Science, PMB No.3, Townsville MC, Queensland 4810, Australia

JOURNAL

Mangroves and Salt Marshes (submitted on Feb., 1996)

KEYWORDS

Dense Vegetation, Flocculation, Flow Modelling, Tidal Pumping, Turbulence

ABSTRACT

The tidal currents in mangrove forests are impeded by the friction caused by the high vegetation density and they are also complex comprising eddies, jets and stagnation zones. The sediment particles carried in suspension into the forest during tidal inundation are cohesive, mainly clay and fine silt, and form large flocs. These flocs remain in suspension as a result of the turbulence created by the flow around the vegetation. The turbulent intensity is largest for trees forming a complex matrix of roots such as Rhizophora sp. and smallest for single trees such as Ceriops sp. The flocs settle in the forest around slack high tide. At ebb tides the water currents are too small to re-entrain this sediment. Hence the inundation of coastal mangrove forests at tidal frequency works as a pump preferentially transporting fine, cohesive sediment from coastal waters to the mangroves. Mangroves are thus not just opportunistic trees colonising mud banks but actively contribute to the creation of mud banks.

(Full manuscript can be obtained by writing to Keita Furukara)