Research On Sediment Transport And Coastal Profile Shaping Processes Of An Abandoned Subdelta

Since terminal reach of the Yellow River shifted from Diaokouhe channel to the Qingshuigou channel in 1976, the northern Yellow River Delta had been rapidly eroded owing to the lack of the sediment supply. Thereafter, many oil wells of the Shengli Oil Field located in Yellow River Delta had been seriously destroyed for the coastline retreat, which was a result of directly great economic loss. Moreover, the coastal engineerings were also damaged, and the area of coastal wetland reduced constantly, soil salinizaion aggravated and the ecological function of the littoral zone is weakened. Thus, the data including the repeatly measured coastal profiles lasted several decades, hydrology, suspended sediment, surficial sediments and a core sample of 30 m depth were obtained form the study area in the April 2004. Subsequently, from the view point of dynamics, the sediment resistance and the changes of the coastal profiles, respectively, these data were analyzed by using the mechanism analyses, empirical orthogonal function (EOF) and BP artificial neural network to discusse the mechanism of the intense erosion and profiles shaping processes of the northern Yellow River Delta. In addition, the changes of the different profile types were also predicted. The conclusions were shown as follows:(1) The characteristics of the flow and wave located at the nearshore zone of the northern Yellow River Delta: (a) the friction velocity of tidal flow gradually decreases landward and the curve of the friction velocity of tidal flow vs spatial distance was appeared as a linear distribution with a maxmum value range of 1.3～2.7cm/s and the average value range of 0.8～1.8 cm/s. In addition, the characteristic of the tidal flow is similar to that of the friction velocity; (b) the friction velocity of wave increases landward with sharply strengthened trends in the wave broken zone. The value of the wave friction velocity with a wave height below 0.5m could not exceed 2.5cm/s, and the maximum wave friction velocity with a wave height over 4 m could exceed 20cm/s; (c) the process of wave propagation is strongly influenced by the topographical factors.(2) Closely correlation between the sediment concentration and flow velocity: the lagged phase for the high sediment concentration to the high flow velocity is about 1 hour owing to the sediment resuspended. The spreaded models for the suspended sediment is that the water body with high sediment concentration is upwards from the bottom.,and the high energy resuspending environment located at about the area of 5m depth. Moreover, Most of the surficial sediments consist of silt.The trend of the medium grain size (D₅₀) is diminished, and sorting coefficient is poor from the bank to seaeard area of below 10 m isobaths. The values of the sorting coefficient become well at area of over 10 m isobath. The deposition in the present sdudy area could be divided into 3 zones: The composition of the sediment grain size is rather coarse in the nearshore without tidal creeks and fine in the nearshore with tidal creeks; and the characristic of the sorting is worse and the composition of the grain-size is fine in the study area of over 10 m water depth.(3) Deposition centre located at outside of river mouth before1976, and the strong erosion areaoccurred in the same area after 1976. The degree of the erosion is strong in the eastern and weak in the western. Since the northern Yellow River delta was abandoned. The changes of the coastal profiles experienced three periods which could be termed as 'rapid erosion - slow eroding modulate - fluctuate triggering change'. The shape parameter A and F may be suggested to reflect the shape change of the profiles. The changes of the profiles were rather obvious when the the value of the shape parameter had a distinct changed trends. Based on the profile shape parameter and developing characteristics, the profiles could be divided into 3 types as follows: (a) the dynamic equilibrium type: change rate of the parameter A is 1.20～1.26 and parameter F is 1.19-1.38. The siltup was occurred around the profile with a concave appeareance located at the seawards of shallow water where the delta front slope was un-developed; In addition, although minor erosion was happed in the shallow water, the appeareance of the profile was stable after 1976; (b) the strong silting and weak erosion type: The profile consist of delta platform, delta front slope and pro-delta shelf. The change rates of parameter A and F are 1.39～1.46 and 1.59-1.80 respectively before 1976. After terminal reach shifted in 1976, change rates of parameter A and F are 0.84～1.87 and 0.66～1.71, respectively. The changed trend of the profile configuration was from the symbol "S" to a linetype. However, it is strong erosion in the shallow water; (c) the weak silting and strong erosion type: Taking the year 1976 as the node, before 1976 change rates of parameter A and F are 1.07～1.29 and 1.16～1.46 respectively. It also has the three parts that were delta platform, delta front slope and pro-delta shelf, and the delta front slope is the broad. After 1976, Change rates of parameter A and F are 0.56～1.67 and 0.45～1.67 respectively. This types of the profile has minor accretion coccured in the transition between the the delta frnt slope and sea bed, and erosion focused on the delta front slope.(4) The developed mechanism of strong erosion type profile: The development characteristic of topographical is decided by hydrodynamic force and sediment factor. Firstly, during the initiative abandoned stage, the weak sediment resistance of newly incompact deposition and the fluctuant coast line are the main reasons for erosion and retreation of the profile with a high speed. Secondly, the main erosion action force was changed from tidal to co-action of tidal and wave after the newly incompact deposition disappeared. The wave plays a role of lifting the sand, and the flow transports sediment mainly. The frition velocity of co-action of wave and flow reduced gradually after 1976; Due to the effect of the sorting and compaction, the retreating speed of the profile was slow down. Finally, due to the frequently swing of terminal reach of the Yellow River, the sea region experienced the two sedimentary environments that are mouth bar type and side bay type of mouth bar, which has different sediment resistance and different erosion rate during the different periods (1985～1989).(5) The prediction of profile change in the short-time period. The first and second eigenfunctions of EOF could explain the main change characteristic, one shows the changes of the shallow water zone and the another shows deep water zone. The first eigenfunction of profiles CS1-CS5 could reflect the changes in the deep water zone, the second eigenfunction reflect the changes of shallow water zone profiles CS6～CS8 are opposite. The eigenfunction thatshows the deep water zone represents the tidal action; the eigenfunction that shows shallow water zone represents the wave action. From profile CS1 to CS8, the change of the dominant actions is transferred from tidal flow to wave gradually. By EOF method, the development of profiles in the short term could be forcasted. Moreover, BP artificial neural network is used to predict the development of space phase and time phase seperately. The results show that the BP artificial neural network method can be applied to predict the changes of the profile while the changed trend of the profile is obvious.(6) The differences between analysis methods of hydrology. The vertical average characteristic value error between nation criterion and traditional hydrology method is below 8%; while calculating per wide flux of tide period, the error could be ignored. The traditional hydrology method is availiable to deal with interaction of water dynamic force and sediment, and the nation criterion method could be adapt to analyse the tidal flow and direction of net flux.