Floods all over the world are caused by several factors.
Floods can be defined as the overflow of river water into the flood plains. A river's main job is to channel water into the sea or ocean, apart from cutting down it sides of the mountains to accomodate the stream flow.
And we now have to discuss what cause this overflow of river water. According to nature, water springs from the mountain tops and cuts a valley on the surface of the earth. At the upper course, the river is fierce and powerful and cuts deep into the rocks to form a very deep v shaped valley. In the middle course, the river is wider, slower and cuts a wider v-shaped valley. As the river moves nearer the sea,w here it will pour out its volume into a huge body of wa ter, it meanders and swings like a snake , criss crossing the deltaic region to form a huge flood plain.
A river is dynamic. It has a life of its own. And nature evolves itself into new forms, new shapes, sometimes beyond the control of man. But nature will definitely take its course.
Left alone, a river runs its course, its high tide, its low tide,its over flow and its low and slow flow. But man wants to be a master. And this is where the nature of a river will become unruly. Man wants to tame a river. But does it really want to be tamed on man's terms?
When man removes too much forest cover through rapid logging, runoff will be too rapid and the poor river cannot contain the heavy torrents and floods occur.
when erosion is too rapid, sedimentation and siltation of the river takes place. And the river overflows naturally.
The normal high tide will flood the deltaic regions without doubt.
Too much concretization in urban areas will also mean that run off will be too rapid and the river cannot channel the volume of water into the sea rapidly enough. So the excessive volume of water will only top up the river flow, forming an overflow. Flood results.
We humans must learn to create a balance with nature. And when man play God, he will have to face unpredictable consequences.
Floods have been a regular occurence in all parts of Sarawak and in particular Sibu. They are not strange or unique to us. We have been having huge floods sicne 1963.
We just have to something about it, quickly and sincerely because too many people will be affected and too much property will be destroyed. Our projections must be accurate, our designs must be intelligent and our lives and livelihood must be preserved.
We have learnt a great deal of history and geography in school. I am taking a leaf out of that education to share with you here:
In the Minjiang River of the Sichua Province, the annual flux up to 15 billion cubic meters, and about 40 million tons of sands and stones were washed down from upper reaches. Flood burst out frequently, in 256 BC, the governor of the Shu Prefecture (now as Sichuan area) organized local people to build the greatest project, and till now it works very well.
The Sichuan people summarized its experience in flood control and water usage as: "Harnessing Shoals Deep, Building Dams Lower" or "Keep the weirs low and the sluices deep".
Fish Mouth (Yuzui in Chinese),is located in the upper middle (the heart point close to the bend of the river) of Minjiang River . The Fish Mouth dyke divides the river into 2 flows: the inner river (D) and outer river (E), with wide for outer river (with little higher river bottom) to keep more flux (60%) into outer river if rainy season and keep few flux (40%) into outer river if non-rainy season, as well as more sands and stones into outer river, narrow for inner river (with little deeper river bottom) to keep certain amount of flux to into inner river.
Bottle Mouth (Baobingkou), the bottom width 14.3M, the top width 28.9M, height 18.8M, water width 19M at lower water and 23M at its flood water. It divides and controls the water again to keep water into irrigated canals for Sichuan irrigation.
Feishayan (drainage dam, training banks),has width of 240M with 2 meters height. It is about 710M to Yuzui and 200M to Baopingkou. During flood season, the water flows to the bank (upper) and then returns to this dam, and carry sands with stones across the dam into outer river with large flux. And this dam was built with bamboo cages filled with cobblestones more than 2000 years ago.
Actually there is no dam in this project, but only a lower dyke to keep the water flowing naturally with different direction to different way. It is believed that no one should ever break the flow with a high dam. Every year there is a annual reconstruction (just as: carry the sands out of the river bottom). Thus there has been no environment problem such as Dam breaking up,sedimentation and erosion etc.
Here a second article to share with you.
Dujiangyan Irrigation Project of China
Dujiangyan Irrigation Project is time-honored water conservation works. It is 56 kilometers (34.8miles) away west of Chengdu at Dujiangyan city, lying in the middle reach of Minjiang River, the longest tributary of Yangtze River.
In old days, Minjiang River surged out down Mt. Minshan, pushing toward Chengdu plain. When it came to flatland, the speed slowed down abruptly. Thus the watercourse filled up with silt became vulnerable to flood, the people living on the plain suffered a lot. Around BC 250 during Warring States Period, Libing, a governor of Shu prefecture (present Sichuan province) in Qin state, together with his son, directed the construction of Dujiangyan. The governor gave up the old way of dam building to catch floodwater. Instead, he employed a new method by water channeling and diversion to harness Minjiang River and built the whole works up mainly in two parts: the headwork and the irrigation system.
The project effectively put the flood under control. Up to now, the whole system still functions perfectly, serving over two thousand years for flood prevention, irrigation, shipping and wood drifting. It contributes a lot to the richness of Chengdu plain and its reputation as the Land of Abundance.
On November 29th, 2000, Dujiangyan was listed on world cultural heritages by UNESCO.
Headwork of Dujiangya Irrigation System
The headwork consists of three projects: Yuzui, Feishayan, Baopingkou.
Yuzui: It is a long and narrow dyke built in the center of the Minjiang River, dividing Minjiang into the inner river and the outer. Uniquely governor Libing designed it in a shape of fish mouth, in order to receive least water resistance. In average, 40 percent of river' runoff goes into the inner river in flood season, 60 percent into the outer, and vice versa in dry season. The inner river diverts water into Chengdu plain through Baopingkou. And the outer is the main flow, which carries off 80 percent of silt.
Baopingkou: It is the main diversion gate to draw in water for irrigation in shape of bottleneck. In construction of irrigation system, governor Libing had a canal cut through Mt. Yulei toward Chengdu plain. Baopingkou marks the inlet of the man-made river. It works for conducting water and controlling the volume of inflowing water.
Feishayan: It is spillway for releasing flood and silt from inner river to outer river. When the volume of water in inner river goes beyond the upper limit of influx at Baopingkou, excessive water will flow over Feishayan to outer river. At the same time, the eddy force of overflowing water helps take along the silt and sand, which in other cases would settle to the bottom. In ancient times, with no cement in use, Feishayan spillway was originally made of piles of bamboo cages filled with cobblestones. If there happened unusual big flood, Feishayan could collapse by itself, leaving water channel much clearer. Now it has been concrete work.
Anlan Suspension Bridge
Anlan Suspension Bridge, also called Couple's Bridge, spans 500 meters (1640.4feet) long over both the inner and outer river, right above Yuzui dyke. It used to be secured by thick bamboo rope. Although it is reinforced by tight steel wire now, visitors still can have fun by swinging back and forth on the bridge while walking across.
It was built to commemorate Libing and his son, who made great contributions to enable people a better life. Besides, Libing was an outstanding hydraulics engineer. He invented the dyke of fish-mouth style, erected a stone man amid river as water gauge for long-time observation of water in different seasons, and buried a stone rhino on bottom of the inner river as titer for measuring the concentration of silt and sand when dredging for annual maintenance. He ever summed up his experience of water-control: dredging the sand deeper, building the dam lower. The eight words are inscribed on a wall inside the temple.
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Tuesday, December 18, 2007
Floods all over the world are caused by several factors.
Memoir by I Am Sarawakiana at 10:21 PM