Morphological Change of the Yellow River Estuary and its Training.doc

此文档收集于网络，如有侵权，请联系网站删除Morphological Change of the Yellow River Estuary and its TrainingWang Wan ZhanYellow River Institute of Hydraulic Research, Zhengzhou, ChinaAbstractThe paper presents the fundamental features of the morphological changes of the Yellow River estuary and the deltaic shore due to the river feedings, tides, waves and other factors, and gives the basic information of the training. The major morphological process, among others, with the estuarine course, is that a terraced-shaped profile is formed due to tidal level fluctuation, river flow and sediment. The major process with the Yellow River deltaic shore is that it not only extends or retreats due to river sediment, tides and waves, but also tends to adjust its shoreline to be perpendicular to the strong waves from northeast. 此文档仅供学习与交流1. Basic information of the Yellow River EstuaryThe Lower Yellow River used to shift frequently on the Northern China Floodplains and on the area north of the Huai River. It has alternately changed its course 9 times on the modern delta since 1855 when it switched its course northwards into Bo Sea (Fig.1). It had its name as Shengxiangou River (S.R.) in 1953-64, Diaokou River (D.R.) in 1964-1976 and Qingshuigou River (Q.R.) since 1976(Table 1). Generally, the Yellow River estuarine courses each tend to evolve through three stages, namely, from multiple channels, to single channel, and to avulsion. It is found that the course running with a single channel will be favorable for sediment transport to the sea, and it is taken as the training target. 1.1 Regimes of Flow and Sediment coming to the estuary The Yellow River estuary sees great variations in annual flow and sediment load, with long- term averaged sediment concentration as high as 25 kg/m3. It is generally accepted that larger sediment load coming to the estuary contributes to a shorter life span of the estuarine course, but one will find it is not true for D.R., which ran with a single channel for as short as four years before starting to avulse in 1972. 1.2 Tides, wave climate, salinityThere are two tidal amphidromic points with the constituent M2 within the Bo Sea, one close to the mouth of S.R. and the other close to the coastal city of Qinghuangdao. The tidal range varies from 0.0-2.0 m with maximum local tidal current speed 1.84m/s measured at flow surface, near a protruding river mouth. Water bordering the delta receives a full directional spectrum of waves with the strong waves coming from NE, which have the significant wave heights about 4m. The NE strong waves have been found playing a significant role in changing the local morphology 2.Annual net LST load is estimated around 83 000 m3 moving southeastwards on the eastern deltaic shore in 1985 2, 90% of it was contributed by the strong NE waves（Fig.2）. Similarly, the net LST is found 200 214 m3, moving westwards on the northern deltaic shore.Annual net LST load is estimated around 83 000 m3 on the eastern deltaic shore in 1985 2, 90% of it contributed by the strong NE waves（Fig.2）. Similarly, the net LST is found 200 214 m3, moving westwards on the northern deltaic shore. Salinity wedge is found to intrude the estuarine channel only 2 km during flood seasons and about 10 km during dry seasons. Temperature in the sea changes with seasons. Unfortunately, no research has been done on the effect of the salinity or temperature on the local current, sediment transport and the morphological changes.2. Morphological changes 2.1 Deltaic shore changesRatios of the sediment load transported to the deep sea to the load at Lijin (about 100 km upstream of the river, see Fig.1 for its location, the nearest hydrometric station to the river mouth) are found 38% during S.R. running and 21% during Q.R running respectively. The estuary created 1.9 km2 of land per 0.1 billion t of river sediment load during S.R. running, and 3.5 km2 during Q.R. running. The both index indicate a strong current field near the S.R mouth and a weak one near Q.R. mouth. The deltaic shore has been extending seawards at a rate of 0.2-0.7 km/a since 1855 while the shoreline bordering the D.R. has been retreating since the river was abandoned in 1976, which is caused not only by the decrease in river sediment load, but also by the planting of a stretch of sea dikes, the latter to protect the local Feiyantan oil field from shore erosion. Correlation between measured data of the river sediment load and change in the shore areas leads to the conclusion that the whole deltaic shore would be in a dynamic equilibrium state if the river empted 0.2-0.4 billion t of sediment into the sea. The eastern shoreline had been turning clockwise before 1996 so that it tended to be perpendicular to the NE approaching waves (Fig.2), decreasing LST rate gradient 2. 2.2 Morphological changes to the river course2.2.1 Channel length to the mouth The estuarine channel extension rates of S.R., D.R. and Q.R. are found 3.45 km/a in 1953-60, 2.68 km/a in 1964-76, and 2.0km/a in 1976-95, respectively. The Q.R. channel length has been found relatively constant in the recent years, due to mild river sediment load each year.2.2.2 Changes in channel morphologies (a) Changes in longitudinal channel profile (Fig.3): The upper part of the estuarine channel is found easily eroded for high flow and deposited for low flow, which is, to quite an extent, similar to the lower part of the Lower Yellow River while the middle and lower estuarine parts are found liable to be deposited no matter either for high flow or low flow, due to effect of tidal level fluctuation on the river flow. Further, the middle estuarine part has a continuously decreasing slope while the lower estuarine part keeps a constant and steep slope. Consequently, the longitudinal profile of the estuary gradually evolves into a terrace-shaped one, which consists of a topset and a foreset. The process is, more or less, similar to the process in the backwater zone of a sediment-laden reservoir when a underwater deltaic entity forms3. (b) Changes in river patterns and lateral profiles: With its slope decreasing, the middle part evolves from straight to meandering channel while the lower part remains straight and wandering. In the same period the lateral slopes of the local floodplains have been increasing. When the slope of the middle part decreases to a critical extent, the first channel avulsion will take place, always starting from the site where the middle and the lower parts (namely, the topset and the foreset) meet. Then channel avulsion will move upstream until the deltaic apex. In addition, the mild-sloped part is liable to be overflowed and be ice jammed 3. 3. Controversial issues relevant 3.1 How is the terrace-shaped profile formed?Is the longitudinal terrace-shaped profile formed by tidal fluctuation as said above, or by retarding effect of the river mouth bar on current and sediment transport seawards or anything else?It is found, by analysis of the measured data, that there always exists a transverse gully formed just upstream of the river mouth bar (Fig.4). More importantly, somewhere within the gully there always exists a maximum sediment transport seawards4, which means the river mouth bar has resistant action on current and sediment transport seawards as expected, but just downstream of the maximum sediment transport site, which, of course, is much far downstream of the topset of the terrace.3.2 What is the effect of man-made removal of the river mouth bar ? Some Chinese hydraulic professionals hold that man-made removal of the river bar doesnt have any effect on its immediate upstream while others think to the contrary.3.3 What is effect of the estuarine extension on its upstream channel?The question comes from the fact that the lower Yellow River bed and the estuarine bed have risen by a similar margin of about 2-3 m since 1949. Some hydraulic professionals hold that it is the estuarine extension that has caused the whole Lower Yellow River bed (i.e., 900 km-long upstream channel) to rise by an identical margin5 . Others hold that that the effect cover about 220 km upstream6. And the last group hold that the effect covers quite shorter, (i.e. 70 km, at most) upstream 7. The great differences are caused by the complexity of the morphological changes of the Lower Yellow River and the estuary and unavailability of a high-precision numerical model for them. 4Training of the estuaryIn order to protect the delta from the river floods, Yellow River Conservancy Commission (YRCC) applies structural and non-structural measures. Structural measures include river dikes, flow guiding groins (both built along the upper and middle parts of the estuarine course leaving the lower part running freely without any man-made constraints), channel dredging, and reservoirs built in the middle Yellow River, etc. Nonstructural measures include flood forecast and water regime forecast, ice forecast by using numerical models, physical scale models and data analysis as the tools, and others. The major philosophy for the estuarine training is to keep the estuarine course as short as possible, say, by planned shifting the estuarine course.Research on the deltaic ecological protection has been going under the frame of Sino-EU Cooperation Program since about 3 years ago. One of the protection measures is by providing enough water to the deltaic system. 5ConclusionsInfluenced by the river flow, sediment, tides, waves and others, the Yellow River estuarine course tends to extend seawards and then avulse. Further, the estuarine extension seawards make its upstream channel rise. Clear philosophy for the flood control has been made, which is by keeping the estuarine channel as short as possible while the deltaic ecological protection is under research, one of the initial results is by providing enough water to the deltaic system A master plan for the deltaic social and economic development should be based on the river flow conditions and the law governing morphological changes, which, to the disappointment, unfortunately, has not been fully understood by the local governments and the local people. They have been trying to occupy more deltaic land for social and economic developments, consequently leaving a decreasing area for future estuarine courses to run and shift. Fig.1 The Yellow River courses on the modern delta and coastlinesTable 1 Average flow and sediment loadRiverPeriodFlowvolumeSedimentloada108m3108tS.R.*1953-6345911.8D.R.*1964-7643311.0Q.R.*1976-952526.3S.R.*1953-5947614.5D.R.*1968-723488.6Q.R.*1980-952455.8Note:* denotes the estuarine course running for a whole life Fig. 2 LST and the Yellow River deltaic shorespan while * the course running with a single channelFig.4 Effect of the river bar on sediment transportFig.3 Morphological chan