India is prone
to floods, just as any other country in the world. What is worse however is
that when rivers in one part are in spate, the land in other parts could be
parched. Floods and droughts always occur at one and the same time, making life
difficult for the people on both the counts. Floods are a common, everyday
occurrence and in floodplains more so than anywhere. The India Meteorology
Department uses a simple classification of floods, and defines that rainfall in
excess of 75 per cent causes very severe floods, 50 per cent severe floods and
25 per cent moderate floods. The State of Assam, for example, located in the
north-east region of India, has become a multi-disaster prone area. Due to
deforestation in the upper catchment areas of the rivers and the lack of proper
maintenance of dams and protective embankments, the region is becoming more and
more vulnerable to flooding. In the last two years, the monsoon floods have
become a nightmare to thousands of resource-poor people living at the side of
the mighty Brahmaputra river. Every year, the river is becoming wider and
wider, consuming vast areas of fertile land and human habitations. With the
advent of the monsoon in June 2004, for example, there was heavy rain in the
entire region, including Bhutan. When the Kuriso Dam in Bhutan was in danger of
overflowing in early July the authorities released unexpectedly large amounts
of water, which caused an increase in the level of the Brahmaputra River. In
early July, it again started to rain heavily in this area and water levels in
the rivers started rising again. From July 10, 2004 onwards, the rivers started
overflowing their banks and more and more areas became inundated resulting in a
major flood. At several places, breaches developed in the embankments and
torrents of water swept through villages and washed away a large number of
houses. Twenty three districts have been severely affected by the floods.
Goalpara and Dhubri districts are among the worst affected. Bongaigaon, is also
badly affected.
Choosing Flood
Hazard Categories:
It is necessary to divide the
floodplain into flood hazard categories that reflect the flood behavior across
the floodplain. CSIRO (2000) refers to the degree of flood hazard as being a
function of:
- · the size (magnitude) of flooding;
- · depth and velocity (speed of flowing water);
- · rate of floodwater rise;
- · duration of flooding;
- · evacuation problems;
- · effective flood access;
- · size of population at risk;
- · land use;
- · flood awareness/readiness;
- · Effective flood warning time.
There are four degrees of flood
hazard: low, medium, high and extreme. The categorisation of the floodplain is
largely qualitative using the above factors. For example, medium hazard is
where adults could wade safely, but children and elderly may have difficulty,
evacuation is possible by a sedan, there is ample time for flood warning and
evacuation and evacuation routes remain trafficable for at least twice as long
for the required evacuation time. A key factor in the case of evacuation from
an area is the water depth and the velocity along the evacuation route; that
is, the stability of pedestrians wading through flood waters or vehicles
driving along flooded roads. There are some estimation procedures available for
stability estimation, but further research is required across a broader range
of conditions.
Recommended
Approach
In considering the application of
flood related issues to the specific flood characteristics of the lower
Johnstone River floodplain, it is noted that:
- · duration of flooding is universally long (in the order of days) across the floodplain;
- · warning times can be short (~ 6 hrs);
- · rates of floodwater rise are reasonably fast; and
- · Flood-awareness is generally high and does not vary significantly across the floodplain.
The four parameters are not
significantly variable across the floodplain to warrant specific treatment and
are therefore not used to define variations in the flood hazard, but should be
included in development control measures. The flood hazard is therefore defined
on the remaining, varying characteristics of:
- · the size of the flood;
- · depth and velocity of floodwaters; and
- · evacuation and access.
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