FAUNA AND FLORA O F MANGROVE SWAMPS
83
probably always true. Van Bemmelen (1949) records extensive shifts
of the coastline of Sumatra, Java and Borneo as the result of accretion
in mangrove areas. Van Steenis (in Ding Hou, 1958) commented that
Junghuhn in 1853 had pointed out that “ the mangrove follows the
silting up of a coastal area rather than precedes and initiates the
accumulation of mud or other soil and that it establishes itself merely
on accrescent coasts ”.
Palembang in south-eastern Sumatra was one of the ports visited
by Marco Polo in 1292; it was still a coastal or river-mouth port 400
years ago, it is now 50 km inland. The rate of accretion is therefore
around 125 m per annum. Near Indramaju in north-western Java it is
108 m per year ; in the Bodri delta an accretion of around 200 m per
annum is threatening to close the port of Semarang (East Java). A
similar rate of accretion is reported for the retreat of the sea from
Hanoi in North Viet Nam (dat)a from ~7an Bemmelen, 1949). Anderson
(1964) records that the Baram and Limbang flood plains in Sarawak
have been extended seawards a t an estimated rate of 27.8 m (90 ft) per
annum since the sea reached its present level there some 5 400 years
ago.
The silt which forms the mud so characteristic of mangrove areas
has its origin in the load brought down by rivers. Guilcher (1963)
comments that the mass of material in suspension carried to and fro
by the rising and falling tide may be derived from the river or from
material being resorted by wave action in the shallows just off shore.
At Majunga in Madagascar Lafond (1957) showed that the mud was
lateritic, derived from the deforested slopes of’the escarpment drained
by the rivers; in Malaya and Indonesia the source is again from the
rivers (van Bemmelen, 1949; Schuster, 1952; Carter, 1959). In some
areas with a limestone hinterland or with a coral reef nearby the intertidal deposits are formed of marl (Schuster, 1952 ; Davis, 1940).
These materials all settle out a t the slack of the tide. The current
velocity of the tide falls off rapidly as it goes farther into the mangal
and the water can no longer support its load, which then falls out.
Much of the material settles in the seaward fringe, but due to a time
lag between the moment a t which the current is no longer able to carry
its load and the moment when the material reaches the bottom, some
of the material may be carried farther into the swamp. According to
Guilcher (1963), this “ settling lag effect ” may carry fine silt even to
the inner reaches of the marsh. Guilcher goes on to write that “ the
minimum velocity required to erode a sediment after it has been
deposited is higher than the maximum velocity at which these same
particles can settle : this ‘ scour lag ’ favours an excess of deposition ”.
83
probably always true. Van Bemmelen (1949) records extensive shifts
of the coastline of Sumatra, Java and Borneo as the result of accretion
in mangrove areas. Van Steenis (in Ding Hou, 1958) commented that
Junghuhn in 1853 had pointed out that “ the mangrove follows the
silting up of a coastal area rather than precedes and initiates the
accumulation of mud or other soil and that it establishes itself merely
on accrescent coasts ”.
Palembang in south-eastern Sumatra was one of the ports visited
by Marco Polo in 1292; it was still a coastal or river-mouth port 400
years ago, it is now 50 km inland. The rate of accretion is therefore
around 125 m per annum. Near Indramaju in north-western Java it is
108 m per year ; in the Bodri delta an accretion of around 200 m per
annum is threatening to close the port of Semarang (East Java). A
similar rate of accretion is reported for the retreat of the sea from
Hanoi in North Viet Nam (dat)a from ~7an Bemmelen, 1949). Anderson
(1964) records that the Baram and Limbang flood plains in Sarawak
have been extended seawards a t an estimated rate of 27.8 m (90 ft) per
annum since the sea reached its present level there some 5 400 years
ago.
The silt which forms the mud so characteristic of mangrove areas
has its origin in the load brought down by rivers. Guilcher (1963)
comments that the mass of material in suspension carried to and fro
by the rising and falling tide may be derived from the river or from
material being resorted by wave action in the shallows just off shore.
At Majunga in Madagascar Lafond (1957) showed that the mud was
lateritic, derived from the deforested slopes of’the escarpment drained
by the rivers; in Malaya and Indonesia the source is again from the
rivers (van Bemmelen, 1949; Schuster, 1952; Carter, 1959). In some
areas with a limestone hinterland or with a coral reef nearby the intertidal deposits are formed of marl (Schuster, 1952 ; Davis, 1940).
These materials all settle out a t the slack of the tide. The current
velocity of the tide falls off rapidly as it goes farther into the mangal
and the water can no longer support its load, which then falls out.
Much of the material settles in the seaward fringe, but due to a time
lag between the moment a t which the current is no longer able to carry
its load and the moment when the material reaches the bottom, some
of the material may be carried farther into the swamp. According to
Guilcher (1963), this “ settling lag effect ” may carry fine silt even to
the inner reaches of the marsh. Guilcher goes on to write that “ the
minimum velocity required to erode a sediment after it has been
deposited is higher than the maximum velocity at which these same
particles can settle : this ‘ scour lag ’ favours an excess of deposition ”.