Xia Xi-he Lu Cai-xia
Yangtze River Scientific Research Institute, Wuhan:430010，
China Tel: 027-82820547
Abstract: The frequent bank failure in Middle and Lower
Yangtze River caused the big damage to the flood control, and the people’s
lives and property along the both banks. On the basis of the features of the
river bank failure, this paper analyzes the damage produced by the bank failure
and proposes the measure about bank failure harnessing.
Keywords: bank failure, disaster, bank revetment
The Yangtze River is the largest one in our country. The main stem channel at the Yangtze River down from Yichang is the Middle and Lower Reaches, with the total length of 1893m, which flows through six provinces or city including Hubei, Hunan, Jiangxi, Anhui, Jiangshu, Shanghai. In history, the people along both banks were often subjected to the disaster caused by flood and channel change. The recorded burst of the Jingjiang Big Dyke amounted to 19 times from A.C 1644 to 1949. Every time thousands and thousands of people died from the disaster. Particularly the catastrophic flood in Yangtze River in 1931, caused the disaster area of 130,000 km2 and death number of 145,000. The bank failure caused by the unstable river is one important reason of generating disaster in Middle and Lower Yangtze River. According to the preliminary statistics, the total length of the river bank line of Middle and Lower Yangtze main stem is about more than 4,249 km, among which the length of the bank failure amounts to 1,520 km. It is 35.7 percent of the total length of the river bank line. After the harnessing over 40 years, According to the inadequate statistics the length of the bank protection has been about 1,200 km, and the riprap quantity has been 68,000,000 m3 in the main stem of Middle and Lower Yangtze River by the end of 1994. The construction of the bank revetment works has preliminarily controlled bank caving reaches changing greatly. The capacity of the riverbank resisting flood has been improves greatly. As a whole, the main stem of Middle and Lower Yangtze River basically reaches to the stability. However, the bank caving of local reaches exists. The bank failure not only seriously threatens the dyke protection in the Middle and Lower Reaches, but also has important effect on the navigation and the development of the economic construction along both banks.
The reaches of the Yangtze River down from Zhicheng belong to alluvial stream. Most banks consists of the loose deposition having the features with the binary facies texture, namely, an upper stratum of cohesive soil with flood-plain facies, and a lower stratum of medium fine sand with bed facies. When the larger flood generates, the banks were often scoured seriously by flow and the bank slope becomes steep, then the larger bank failure generates. The main form of the bank failure is arc-shaped bank caving, and also there is strip-shaped bank sloughing. Because the areas near the river mouth are affected greatly by the wind and wave, the washing bank caving often occurs. In general, the intensity of the arc-shaped bank caving is the highest and the probable collapsed land area once is several 100,000 m2; the intensity of the strip-shaped bank sloughing is smaller than the arc-shaped bank caving, and the intensity of the washing bank failure is much smaller. The collapsing range in the bank failure reaches with higher intensity amounts to several ten meters to over one hundred meters. The most intensive bank caving is in the xiajingjiang, for example, the maximum annual caving width reached to over 600 m in the Liuhejia Reaches in 1962. There were several times serious bank caving at flood season in Middle and Lower Reaches of the Yangtze River such as the bank caving occurring in the Sima river mouth of Jiangdu county in Jiangshu province lasted 63 hours. Its entrance caving length was 330 m, and the inner length was 350 m, and the maximum scouring depth was 25.4 m. The collapsed area was 115,000 m2. The strip-shaped bank sloughing with 210 m length and 5~10 m width generated at the Zongyang Big Dyke in Anhui province on June 4,1998. Then the bank failure developed continuously and formed a 135 m long and 40~50m inner long big arc-shaped bank caving by Oct. At Beimenkou in the Shishou Reach of Middle Yangtze River, during the period of 1998 flood, the serious bank caving occurred too. Two times of bank caving occurred on June, 13, whose length was 130m, and the inner length was 30~60 m. The same place reoccurred arc-shaped bank caving on Oct, 14, and the inner length was over 100 m. Obviously the intensity of the bank caving was great.
The total length of
the river bank line of the main stem of Middle and Lower Yangtze River is about
more than 4,249 km, and among which the length of the bank failure is 1,520 km,
which is 35.7 percent of the total length of the river bank line. During the
period of 1998 flood, the dangerous situation of bank failure occurred at over
330 places in Middle and Lower Reaches, among which the seriously dangerous
situation occurred at 56 spots. The bank caving mostly occurred at the old
dangerous places of the bank caving. In old days because of the low standard of
bank protection works, and the inadequate range of bank revetments, or defending
points and controlling line, retaining spaces, and without the normal
maintenance and consolidation after the construction of the revetments, and the
protected bank damaged seriously, etc, the old dangerous places of the bank
caving often can’t resist the great flood and cause future development of bank
failure. For example, during the period of 1998 flood, there occurred the
seriously dangerous situation at the Yuweizhou of Shishou Reach, Laozhoutou of
the Zongyang Dyke, Xiaojiangba of Wuwei Big Dyke, Long Menkou and Liuwei Curve
Bend of Zhenyang Reach, and the Sima Curve Bend of Yangzhong Reach, etc.
The bank failure at the main stem of Middle and Lower Yangtze River has intimate relation with river regime of the upper Yangtze reaches. For example, in Xiangjiazhou shoal cutting and bend bypassing occurred in Shishou Reach of Middle Yangtze River in 1994, the bank failure occurred violently at Beimenkou in Shishou, Yuweizhou, etc. The mainstream line of Xiangjiazhou moved towards south in 1996 and flowed into the south branch of Xinshengtan. The mainstream line moved towards north at flood season in 1998 and resumed the north branch of Xinshengtan, and the 2,000 m long bank line collapsed violently from Jiahekou to Xiangjiazhou in Shishou Reach. The protected works were damaged seriously. At the same time, because of the change of the upper river regime at Upper River, the direct scouring point moved down and the intensive bank failure occurred continuously at Beimenkou and Yuweizhou. For instance, the shape of the Yizheng Curve Bend in Lower Yangtze River was always stable. However, because of the influence of the continuous flood in recent years particularly after 1995, the thalweg moved towards right at the exit of the Nanjing River, and the effect of ski-jump water jets at Sanjiangkou improved, and the bend often occurred dangerous situation of bank caving at flood season. The length of the maximum bank failure amounted to over 200 m and the inner length was 70~80 m.
In gentle, the cohesive soil stratum in Low Yangtze River can be thinner and the capacity of resisting the flow scouring of the flow is weaker. When the flood occurs at flood season, the bank scoured directly by the mainstream tends to generate bank failure. For example, the head of the Hechangzhou scoured directly by the mainstream retreated nearly 600 m in Zhenyang reach at flood season in1983. In 1998 flood, there occurred seven times of seriously dangerous situation of bank failure in Hechangzhou, among which the maximum bank caving length was 280 m and the inner length was 250 m. The length s of the collapsed dyke and the area of collapsed land was over 14.83 acre.
Because the water level of Changjiang falls and the mainstream flows along the thalweg after the flood season, and the scouring intensity to the bank toe has been improved, and the soil condition of shoal bank is bad, and the bank long-time soaked in flood season, whose shear drag reduces, the bank caving occurs frequently and violently during the water level falling period after flood season such as the bank caving occurring at Mahudi in Jiangxi province on Jan,8, 1996, the bank caving at beimenkou in Shishou Reach on Oct,14,1998 and so on. The intensity of bank caving all was higher.
The bank failure is one important reason of the disasters in Middle and Lower Yangtze River. It not only threatens the safety of the flood control of the main dykes, but also affects the smooth of the navigation in Yangtze River. It is very inconvenient to economic develop and people’s lives and property safety along both sides of the river. So the situation of the bank caving must be paid much attention.
The bank failure in Middle and Lower Reaches of the Yangtze River brings great damage to both sides of the river. During the past years the bank failure has made the annual flood control situation in the big and medium cities close to the Yangtze river intense and passive such as Wuhan, Shashi, Jiujiang, Anqing, Maansan, Nanjing, Zhenjiang and so on. The flood control dyke and the industry and agriculture facilities tend to be in danger along the Yangtze River. the length of dyke with no shoal or narrow shoals in Jingjiang Dyke is 35 kilometer. The elevation of the dyke is over 10 m. The situation of flood control is austere, because the bank failure caused by the flow directly scouring threatens the safety of flood control of Jingjiang Big Dyke. The bank failure occurred during flood season at Qijiayuan Revetment Stretch,made dyke body collapsed and almost caused the collapse of the whole big dyke in 1949. The inner length of bank caving in Jiepai Reach close to Xiangjiang was 1.5 kilometer in 1949~1967. The damaged acres were 4,119 acre. The houses owed by the citizen close to the river were retreated by 26 times. The total length of the retreating dyke was near 34.5 kilometer.
The vestige of retreat by the bank caving can be seen obviously at satellite picture in Lower Yangtze River at Huaining in Anhui province in 1931 and1954. The Madian dyke at Huaining has been retreated and rebuilt for three times for the damage of bank caving since 1950s. The total length of retreat is 350 m. The bank line of the Huikou reach of Tongma Big Dyke and the Daguai reach of Wuwei Big Dyke has been retreated continuously 2 kilometer in the past years. Continuously there are 10 kilometer reaches, which has no shoal or narrow shoals closing to the thalweg because of the bank caving. This will threaten the safety of the important dyke protection in Yangtze River.
The bank failure makes the river width added, the sediment deposited and the navigation water depth decreased in the straight river. The shallow shoals counteracting navigation tend to be formed at falling water season when the scouring can’t form the thalweg. The bank failure caused many bend channels to move in the meandering Xia Jingjiang. There produced over ten places of the shoals prohibiting navigation, and the navigation channels changed fast. The most serious is in Shishou bend, about 10 kilometer long bank line has collapsed violently at Xiangjiazhou since 1960s and finally could be flown through until Jun, 11,1994, and formed an 1.2 kilometer wide new entrance. Now the mainstream sluices down the left bank close to the new entrance and directly get to the Beimenkou district in Shishou city. The docks and shoal lands near the river at Beimenkou collapsed continuously. A large quantity of sediment was carried and moved down and caused a large quantity of deposition of sand at Nianzhiwan Reach in the lower river. The flow couldn’t accumulate and flow into the thalweg at the dry season. The rapid deposition in navigation channel caused two times of interrupting navigation out of shallowness in February ~ March, among which the maximum value of interrupting navigation is 22 days.
The bank failure is one important reason of river regime changing. The bank failure induced the shoals to move down and the mainstream to sway in the straight channels; The bank caving makes the directly scoured point by the flow move down in the bend channels, and forms the situation of a bend changing causing other bends changing; The bank failure often makes the branch channel develop slowly or change instantly in the braided channels. It can induce the main and branch channel to develop alternately at the serious region, etc. The river regime change not only seriously affects the normal operation of many factories, enterprises and the facilities close to the river along both sides of the river, but also confines the economic development along the river. For example, the head of the Baguazhou in Baguazhou branch channel in Nanjing Reach retreated continuously (the retreat length was 135 m in 1982~1983). This made the diffluent ratio at left branch decrease continuously and affected the development of the big and medium enterprises at left branch such as Yangzhishihua Corporation, Nanjing Thermoelectric Factory, Nanjing steel Factory and so on. The retreat of the head of the Hechangzhou in the Zhenyang reach and the development of the bank caving at left branch of the Hechangzhou induced the transposal of main and branch channel, which affected seriously the normal operation of basic measure of big and medium industry and agriculture such as Jianbi Electric Factory at right branch channel, Jinghang Cannel and so on. Furthermore the bank caving can bring serious problem of deposition. For example, the retreat of bank failure at Qiba, Xibatou, and the head of Baguazhou at Nanjing Reach, caused respectively the deposition of Meisan Steel corporation, Nanjing Refinery, the dock and the water catchment of Nanjing Steel Factory. Because the bank line of Liuwei Bend in Zhenyang Reach retreated violently (the maximum range attains 2.5 kilometer), it made the channel undercutting towards south change into towards north. The Zhenjiang port couldn’t operate because of the deposition. Now it has to move.
According to the preliminary statistics, the 1,2356 acre good field are damaged annually along the river in Lower Yangtze River since 1950s. The more serious bank failure was in Huaining Guanzhou in 1950s ~ 1970s, which caused collapsed land of 12 km2 and the damaged good field of nearly 280 acre. The dam, the sluice and the houses of the citizens were collapsed into the river, and the homesteads of thousands of peasants were ruined. The bank line at Liuwei Bend at Zhijiang county in 1950s ~1970s retreated nearly 2.5 kilometer. The area of the damaged soil was 3130 acre. The Liuwei Port was removed time after time. The total area of the damaged soil at Longtan bend in Nanjing city in 1954 ~1981 was 3492 acre. The damage of the bank caving occurring at Mahudi at Jiujiang Reach on Jan, 8,1996 was much serious since 1980s. The length of the bank failure attained 1.2 kilometer. The maximum longitudinal depth was 240 m. It caused the great disaster with 96 collapsed houses, and 24 persons died from the accidence, 5 persons injured seriously because of the accidence.
The construction of the bank protection works in Middle and Lower Reaches of the Yangtze River can be traced back to the bank revetment with riprap and spur of Jingjiang Big Dyke in 1465. The riprap and spur system of Jingjiang Big Dyke was of original scale until the last of 18 century and the early of 20 century. In addition, there were only a few spurs and pile stones in Middle and Lower Stretch. After the foundation of New China, the harnessing river works was established unprecedentedly in the Middle and Lower Yangtze River. The establishment of the bank protection works developed rapidly. At 50’s, a large scale of riprap and mattress revetments were carried out at the places such as Shashi and Haoxie Reach of Hubei Jingjiang, Qingsan Town of Wuhan city, Andingjie,Yuxikou, Maansan of Anhui province, Nanjingxiaguan, Pukou, Dachang Town, Haimenqinglong Port of Jiangshu province. The riprap revetments were adopted generally after 60’s, which included the bank revetmentand consolidation of Jingjiang Big Dyke, the river regime control of the cutoffs in Xia Jingjiang, the revetment of the dykes close to the Xiangjiang , the consolidation of dangerous works at Wuhan city, the revetment at Yong-an Dyke at Jiujiang, the revetment consolidation of Tongma and Wuwei Big Dyke, the control of river regime in braided river such as Maandan, Nanjing, Zhenyang and so on. At the same time, the river dyke which gave priority to the spur system and seawall works have been construcing at Changjiang Mouth. A revetment system of the main stem channel has been preliminarily formed in Middle and Lower Yangtze River by 70’s.
According to the inadequate statistics, the length of the protected bank line has been nearly 1200 kilometer at the main channel in the Middle and Lower Reaches of the Yangtze River till 1994. The riprap quantity was 68,000,000 m3. The mattress area was 4,100,000 m2, The established spur number was 685. The construction of the bank revetments improved the stability of the dyke bank. As a constituent of the flood control, it safeguarded the safety of the flood control of the large plain in the Middle and Lower Reach, particularly including the commercial food and cotton bases such as Jianghan Plain, Dongtinghu Region, Poyanghu Region, Chaohu Drainage, Taihu Drainage and so on, and the safety of the important big and medium cities along the river such as Shanghai, Nanjing, Wuhan, Nantong, Zhenjiang, Maansan, Wuhu, Anqing, Jiujiang, Shashi and so on. At the same time, considering the request of the every departments of the national economics, stabilizing and improving Changjiang and accelerating the economic development along the river, the bank revetments in the Middle and Lower Reaches of the Yangtze River will play greater and better comprehensive effectiveness from now on.
According to the plan shape, the form of the bank revetments can be categorized into three types: the direct continuous bank revetment, the bank protection works with the spur system, the revetment with dyke. To each type, according to the different structure and material, it can be classified into riprap, mattress, sinking-pillow, sinking-cage, plastic mattress, concrete mattress, etc. In the seawall works of the Chanjiang mouth, there are ferroconcrete caisson-pattern and pile fence-pattern dykes, pile stone, the longitudinal dam of the abnormal nub style and the traditional pile stone works.
For the present, the development tendency of the revetments in the Middle and Lower Reaches of the Yangtze River is to adopt the direct continuous bank revetment. The practice shows, that it is useful to the works stability, particularly to the important cities, the port docks, irrigation channel mouth, or the cannel mouth, and the important dykes with narrow outer shoal. At the same time, the pile stones and the ripraping of confining position are the adopted gently building material and the construction method hitherto. At river mouth reach, when the bank shoal is mainly affected by the tide, adopting the dyke to protect the shoal and form shoal can gain good effect. When the bank bar close to the sea is mainly affected by wind and wave, we should use the longitudinal dam to resist the tide. So the dyke protection of river and sea can be protected.
The bank caving is a basic form of the fluvial process in the Middle and Lower Yangtze River. It often occurs at the reaches where the capacity of resisting the flow scouring is weak. Because the occurrence of the bank failure is paroxysmal and the intensity is high, the disaster induced by it is also out of estimation. According to the present research, the bank caving directly threatens the safety of flood control of Yangtze main dykes and affects the smooth of the Yangtze navigation, and causes the worse change of the river regime and brings the serious harm to the economic development and the safety of the people’s lives and property along both sides of the river. The construction of the bank revetments in Middle and Lower Yangtze River since 50’s, not only improves the stability of the dyke and bank, but also accelerates the economic development along the river and gains the great economic and social effectiveness.
[1] The Changjiang Water Resource Commission, “The Third Collection of Bank revetment Work in Middle and Lower Reaches of Yangtze River”, vol.3, 1983,9.
[2] The Changjiang Water Resource Commission, “The Fifth Symposium on River Training Works and Management Experience of Middle and Lower Yangtze River”, 1995,12.