Wang Kairong
Institute of Hydraulic research. Yellow River Conservancy Commission,
Zhengzhou 450003, china
LI Ping
Shandong Normal University, Jinan 250014, China
Wang Zhengang
Reconnaissance,
Planning, Design and Research Institute of Yellow River Conservancy Commission,
Zhengzhou 450003, China
Abstract:
Based on the data collected in the Yellow River Estuary, this paper gives a
discuss and an analysis to the mechanism and characters of Runoff Flood and
Ice-Flood Disasters (RFIFD) in the estuary. The result shows that, the enormous
sediment depositing in the adjacent area of the river mouth, the shape of river
channel both in vertical and horizontal section, and the sudden events in the
river mouth channel are the main factors charged for frequently happening of the
RFIFD. The effective measurements dealing with the disasters are to reduce the
amount of sediment discharge to the estuary, to renovate the river channel
constantly and to enlarge the sediment emptying ability of the river mouth.
Keywords:
water level, sediment discharge, ice-flood disaster prevention, yellow river
estuary
The Yellow River
Estuary always shifts its location with the lower reaches changing. The present
position is developed in the event that the Yellow River changed its lower
reaches by grabbing the Daqinghe River in 1855. It ranges in the scopes of 118°
10' -119°
15' E,37°
15' -38°
10' N. The economic development in the present delta area is seriously
restricted by the ineffective improvement to the frequent shift of the river
mouth channel. Since 1960', with the Shengli Oil-Field's developing in a large
scale, the river mouth channel got a father improvement, and the economy in this
area has rapidly developing. But the RFIFD is also the serious hidden trouble to
confine the regional economic develop. Therefore, it is very significant to
study the RFIFD's mechanism and the characters and put forward the
countermeasures on the analysis of related data in the Yellow River Estuary. It
may provide the important reference for the insurance of the estuarine
embankment prevention and regional industry/agriculture production, and also has
a actual value for understanding the river mouth change rule and the deltaic
programming and exploitation.
The prominent character of the Yellow River Estuary distinct from other estuaries is that the river has less water carrying huge sediment discharge; the suspended sediment is coarser; the water runoff is extremely different between flood and dried up seasons and the water flood is quickly forming and rising and dropping-down rapidly later. The statistic data in 1950 through 1999 indicate that, the annual average water amount and sediment amount discharged to the estuary (by Lijun hydrological station) is separately 342.6´l08 M3 and 8.7´l08 tons. That takes part in about 84% and 73% of amount entering to the lower reaches of the river. The water and sediment discharge is not well-proportioned with the seasons in a year. It accounts for about 61.3% and 85% of the yearly water and sediment transport. It is also very different between years. Taking sediment as a example, in the 50 years during 1950-1999, there 4 year that sediment discharge exceeds 18´l08 tons, while in another 5-years, its amount is only less than 3´l08 tons. The annual maximum/ minimum ratio of sediment discharge is 128: 1.
The marine energy such as wind wave or tide current in the in the Yellow River Estuary is relatively weaker. Here the tide range is only about 0-61-1.13m.The current velocity commonly reached Im/s. The gradient of river mouth channel is about 0.1 %o far larger than that of Changjiang River's 0.005‰. Hence, here is no tide current segment, and with a just very short tideway of 15-30km influenced by tide.
The Yellow River Estuary is a heavy accumulated estuary with large amount of sediment discharge under weaker marine dynamics. Its outstanding feature is frequently swing and bursting of the river mouth channel with a intense process of land progradation. Since 1855, The Yellow River lower ranch has undergone 10-time course change, up to 1985, the delta has got land area net increase in 2260km2.An integrated course evolution for the Yellow River mouth channel usually experiences three periods of wander and scattering, relative stability and branch-off and shifting into unstable status successively. The actual representation in river model shows in a circle process of meandering and dispersing then return to main channel in the earlier, straight singularity and then winding in the medium-term, and branching/swing-branch position upward-big splay and course change in later. The riverbed height and water level accordingly move up and down with the process.
The RFIFD in the Yellow River Estuary is affected by many respects such as the situation of water and sediment supply boundary form of the river channel, the scale and effect of renovation engineering, marine dynamic condition, climatic change etc. These influence factors interlace together and stochastically combine in temporal and special scale to results in various kinds of forming situation for RFIFD, and make the RFIFD happening in a complex and random way in the Yellow River Estuary.
The sharply rising and declining flood peak, the ice-flood and the storm surge are the main factors which result in the RFIFD in a sudden happening and rapidly developing manner. Taking the ice-flood in January, 1955 as an example, under the situation of the related lower air temperature in the estuary segment area at that time, melted /thawing upper river segment carry large amount of floating ice to the estuary, that causes the serious ice block to stuck up a 24km long ice-bar in the reach between Wangzhuang and Wuzhuang. The stuck amount of the ice reached 1200x1 O^'m3. The water level in front of the ice bar raised quickly in a rate of 0.9m/h. that resulted in more than ten hole appearing in the levee around the Wuzhuang of Lijin county, and at last the bank bursting.
In case the RFIFD occurring in the Yellow River Estuary, the aftereffect is extremely serious. On July 15,1997 when the peak runoffin Lijin Hydrolical Station reached 3180 m3/s. the corresponding water level rose to 14.12m, 0.36m higher than that in 1958 with a runoff flux of 104000 m3/s. This made the river levee in dangerous status for 27830m long and the water depth at the bank base was 0.5-3.0m, and there are 2 points on danger-protecting engineering and 9 points on the control-guiding shoal-protectional engineering. Along lower reaches of the river happened 81 bank burst and 11 shoal-overing. 5853.5ha2 farmland were submerged in the flood. The economic loss reached 12 millions RMBYUAN. In February, 1985, and December, 1987, the estuary area taken place 2 times of ice jams in the ice-flood season, and total 600000 Mu farmland was inundated during these flood disasters, approximately ten thousand peoples were besieged , traffic in Shengli Oil field was partly intermitted, communication fall in failure, borer halted and the directive economic loss reached more than 30million Yuan.
The forming case of RFIFD in the Yellow River Estuary is related to these aspects as fellow:
First, enormous sediments depositing in the estuary directly results in accumulated heighten of the riverbed, river mouth channel extension, and main channel shrink. That heavily influences weaken the water and sediment discharge ability of the river mouth. The field data measuring in 1950-1985 shows that, the amount of sediment accumulated in the river mouth channel and the adjacent marine areas exceed 250´108 tons, which accounts for 77% of the total sediment into the Estuary. The only 1/3 proportion of the sediment discharge were transported outward to the shelf area. This large amount sediment depositing in the river mouth channel makes the river ability of water and sediment discharge get lower and lower. Compared to that in May, 1984, the area of the main trough below standard water level at each section of Qing 1 to Qing 6 of the river mouth channel reduced by 38%-25% in October viz. the flood-passing section of the main trough minished by 1/3-1/4.The equal-shoal flux before the flood at the Lijin section had minished from 6000 m3/s inl985 to about 3400 m3/s in 1997, increasing the chances of disaster during the middle and small floods, resulted in the abnormally increase of the water level of flood peak.. As for the water level of the river mouth channel, from 1958 when the max flood peak flux had come forth to 1996, the average water level of the 3000 m3/s flux had increased accumulatively by 3.39m at Lijin station, and by 3.06m at the Nol dam water level station at its lower reaches. The continuous increase of water level, besides may lead the river mouth to sway and change its course bringing some submerge loss, aggravates the press on the flood control forming latent danger of flood disaster.
Second, the ice flood, apart from the summer and autumn floods, is one of the main latent danger to threaten safety of the embankment produced by the differences of the latitude and shape of the high and lower reaches. The near mouth channel of Yellow River, an important part of the lower reach, is narrow and full of curves above the course-changing point. Below the course-changing point, the river mouth channel is wide, shallow, and characteristic of disorder. The floating ice discharging down to the sea, arisen by the tide, masses in the river mouth to form ice blocks, so the chance and severity of ice flood in the near mouth reach are far larger than in the other segments of the river. According to statistics, during 1950-1985,32 years saw the phenomena of ice blocks, and the blocks happened first in the near mouth reach 26 times, 23 of which happened in the segment down Lijin accounting for 72% of the total ice blocks, making the high water level of ice flood happen every two years. As ice jams and ice bars in the river channel blocked and affected the normal discharge of the water and ice from the high reaches, the water couldn't make its way out and amassed in the high reaches of the ice bars, resulting in the rocketing water level, so ice flood disasters were caused.
Third, storm tides and local resistance of the channel to the discharging flow as well as other events may also cause the flood disaster in the river mouth. Take storm tides for example. Once storm tides take place, great patches of land will be submerged and tide water will go against the river amassing with water of flow from the high reach and cause the water level to rocket, so disasters come into being. For example, on April 5,1964,in the Yellow River delta especially in the region west of Shenxiangou storm tides took place. Arisen by the tide water, the water in the river mouth channel overflowed and approached the beaches, and affected the region near Luojiawuzi. The water levels at Yihaoba, Luojiawuzi and Chayi stations rose by 0.36m, 0.20m and 0.99m separately at the flux of 3240 m3/s at Lijin station on April 5 compared with the water level at the same flux in the same year.
Start with the following to control the flood hazard of Yellow River Estuary:
First, water and sediments are the leading factors to cause RFIFD and sediment is the sticking point. So an important way to control RFIFD is to improve conditions of the water and sediments discharged into the estuary, preserve the water and soil of the middle and high reach regions actively, strengthen efforts to discharge flood and silt to reduce amount of the sediment discharged into the estuary, alleviate filiup of the channel, optimize distribution of the water resource of Yellow River, and utilize reservoirs to distribute water and sediment to avoid the harmful conditions of water and sediment discharge. As for digging silt in the estuary, since problems of the amount of silt, position to dig, location to stack, the effect of digging silt, economic rationality and technical feasibility to dig silt haven't been tackled, pursuit should be made cautiously.
Second, before preservation of water and soil gains substantial effects, the amount of sediments discharged into the estuary can't be reduced fundamentally. As a result, the necessary way to control RFIFD now is to adopt effective engineering measures to increase the amount of sediments discharged from the estuary into the sea. Aiming at the point, many special measures are made and realized, such as to tighten water to suspend silt by constructing bars, draw silt in the estuary, dilute the water of Yellow River by Xiaoqing River and choose sea area of high marine dynamics as the position of the river mouth to increase the ability to discharge sediments, etc, but the measures are restricted by the characters of the developments of the river mouth itself, so their effects are not substantial and the measures should be perfected or altered.
Third, it is a relatively effective measure to deal with the rise of the channel caused by silt to improve standards to prevent RFIFD by heightening and strengthening bars and by rebuilding defending works. But heightening defending constructions increases probability and severity of the hazards, and costs increase dramatically as the bars are higher, so potential harms and expenses must be considered fully. It is necessary to construct a few temporary defending constructions and small constructions to stuff branches to strengthen the main channel, but the scale should be small to avoid the expense from the change of the river's course. Distributing flood and discharging silt or distributing flood to the sea make various benefits, for example, distributing the flood water to minish the runoff may alleviate the runoff flood and ice flood thereat, but the actual position and flow course should be chosen carefully.
Fourth, it is an effective means to control RFIFD to make necessary impairments improve the channel shape in order to increase the ability of the river mouth channel to discharge water and sediments. Viewed from present conditions, the trend of the flow to wander, the narrow channel, and the broad and shallow river bed tending to vanish have become important factors cause RFIFD, which must be tackled by adopting relating channel repair engineering measures.
Fifth, as for the ice flood, make full use of the middle and high reach reservoirs to modulate water amount: before the ice flood, increase the amount of discharge to heighten the ice cover, so as to enhance capability to pass water and reduce the length and time of the frozen up river; during the ice flood, reduce water discharged into the river to avoid loss; or adopt measures to break and take away ice bars and ice covers, and utilize channel broadening constructions to discharge backwater.
Sixth, the storm tide hazard of the Yellow River estuary is characteristic of strong outburst, short duration and great loss, so it is the chief way to alleviate the hazard to improve the observation and study on the storm tide, make effective preventive measures, and build and improve the tide-preventing constructions in the region.
At last, enhance non-engineering measures to minish the hazard, such as make scientific and normative tide-preventing ordinance and regulations, improve hydrological forecast of the river, the standard of flood prevention communications and the standard of flood prevention construction management, and enhance flood prevention power.
The fundamental aims to harness the river mouth are to ensure flood prevention, stabilize the flow course, make full use of the water and sediment resources, and completely promote the national economy to develop quickly under lasting development principles. Since there are abundant oil, land, beach, aquatic product stockbreeding and other resources, and the region is immediately to Japan and Korea , east of Liaodong peninsula, west to Beijing and Tianjin, an important tache of the around Bo Sea economic circle, and has potential geological advantage, relative decision-making departments and officers must attach enough importance to making practical flood prevention and disaster-minishing measures according the reality of the Yellow River Estuary.
References
[l] Wang Kairong. 1993.The abnormal high water level of the Yellow River Estuary. The collection of the seventh national ocean engineering science conference, P251-258, the Ocean Press, Beijing.