Geoscience Reference
In-Depth Information
nature of the peat itself, in terms of its ability to retain and store water, is important
and determines the relative height of the water table.
Drainage is mostly radial (due to the dome's convex surface) and diffuse (i.e. near-
surface flow occurs rather than channel flow). Surface runoff concentrates in shallow
depressions, from where small rivulets are formed that turn into larger streams and
finally into navigable streams.
Peat land can be divided into several catchments areas. The boundaries of these
catchments areas change over time as the elevation of the peat surface changes because
of the accumulation of organic matter or subsidence caused by oxidation.
Natural peat swamps are important water catchments and control systems. They
function as aquifers by absorbing and storing water during wet periods and releasing
this water slowly when the rainfall is low. As a result they help reduce flood peaks
and provide water in dry periods. Coastal peat lands are a buffer between marine
and freshwater systems, maintaining a balance between them and preventing excessive
saline intrusion into coastal lands. They also protect offshore fisheries from onshore
pollution (Andriesse, 1988). Groundwater can be successfully extracted from peat
aquifers and treated to conform to World Health Organization standards for potable
water, making peat swamps an important area for water supply.
The physical and chemical properties of peat depend on the species composition
of the peat-forming plant communities and their nutritional and hydrological require-
ments. In its natural state, tropical peat land (for instance) is covered with a variant of
tropical evergreen forest known as peat swamp forest. Peat land vegetation is specially
and variably adapted to a waterlogged, fluctuating water table, with adaptations such
as stilt roots, buttresses and sclerophyllous leaves. However, compared with mixed
dipterocarp forest, the number of tree species found in peat land/swamps is rather lim-
ited. Notable examples of highly commercial timber trees found in the tropical peat
swamp of Malaysia and Indonesia are Ramin, Meranti and Nyatoh. Anderson (1961)
divided tropical peat forest into six distinct communities, designated PC1 to PC6, as
shown in Table 8.2. PC1 (mixed peat swamp forest) is found at the edges of the peat
land or swamp. This is followed by PC2 (Alan forest), PC3 (Alan Bunga forest) and
PC4 (Padang Alan forest). At the centre of a highly developed tropical peat swamp,
PC5 and the highly stunted vegetation of PC6 (Padang Paya forest) are found. These
forest associations, particularly PC3 and PC6, can be easily recognized from aerial
photographs.
Water draining from the tropical peat land, as mentioned before, is highly acidic
(pH 3-4.5), is low in inorganic ions and oxygen, and has high concentration of humic
acids that give it a characteristic 'black water' appearance.
Over-drainage can cause serious ground subsidence problem in peat areas. Tai
and Lee (2003) proposed a concept of groundwater monitoring over dry seasons to
study historic low groundwater levels to facilitate efforts to minimize risks of ground
subsidence due to over-drainage. Standpipe piezometers can be used for this purpose.
Figure 8.1 shows an example of ground water monitoring.
From this record, a programme of ground water monitoring works can be devised.
This applies to both the construction of new drains and to dewatering works during
construction activities. The effect of lowering ground water levels to beyond their
limiting values on ground subsidence can be determined by estimating the loss of
buoyancy in the dewatered soils. In the Sibu case, for example, the lowering of 1.4
