Floods as Hazard
Floods are a natural behaviour of streams. Size of an unmodified stream channel is directly related to the quantity of water that it usually carries (more frequent, moderate flows). Most of the time river stage is below the channel banks. Times of higher discharge overflow banks leading to floods. In Bank-full Stage, water fills the channel to the level of the bank top and the Bank-full Discharge is the water discharge when the water level is at the tops of the stream banks. Recurrence interval for bank-full discharge can have varying periods and higher discharge flood events occur much less frequently.
Recent events especially hurricanes of the Caribbean region and notably IVAN, have caused greater damages to property, lives and housing. Floods were also caused by the incessant and torrential rains across the Americas.
Major Flood Disasters
1.Hwnag Ho River, China 1887: estimated 900,000 fatalities
2. Johnstown, Pennsylvania 1889 Dam failed: 2,200 fatalities
3. Yangtze, China 1911: estimated 100,000 fatalities
4. Los Angeles, CA 1928 St. Francis Dam fails
5. Yangtze, China 1931: estimated 200,000 fatalities
6. Huang He, China 1938: estimated 900,000 fatalities
Floods in India
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 behaviour 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.
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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