Analysis Of Consolidation Settlement And Its Contribution To Topographical Change In The Modern Yellow River Subaqueous Delta

The modern Yellow River delta was formed since1855when a major switch tookplace at Tongwaxiang and plenty of suspended loads from the Yellow River appearedto deposit within the modern active delta in the Bohai Sea. The modern Yellow Riverdelta has only one hundred years history and it is one of the youngest deltas in theworld.There are many domestic and international researches of subsidence in land area,and researches of the Yellow River subaqueous delta are mostly focus on sedimentinput, erosion of the coastline, erosion and deposition of the seabed. Consolidationsettlement of the subaqueous delta is barely. Studies of the Yellow River delta erosionare based on water depth measurement; nevertheless, the morphology of the delta iscontrolled by a variety of factors, such as suspended sediment load, coastalengineering and hydrodynamic environment. Most important of these factors areconsolidation settlement due to fast deposition rate, loosened, high water content, andlow density.In this paper, the consolidation settlements of the modern Yellow Riversubaqueous delta are studied based on the drilling data and laboratory experimentresults of the drillings in the modern Yellow River subaqueous delta, analyzing thevertical distribution characteristics of sediments, forecasting the final consolidationsettlement and settlement rate based on one-dimensional consolidation theory andPlaxis mathematical model. Finally, calculate the characteristic of consolidationsettlement and its contribution to the underwater topography of the Yellow River deltaquantitatively, in order to prevent or reduce the geological disaster which caused byconsolidation settlement, and provide important parameters for marine engineering.Characteristics of current, suspended sediment concentration and surface sedimentare systematically analyzed on the basis of measured and collected data. Results show that the tidal in the study area is complex and the offshore area is mainly irregularsemidiurnal tide and the flood duration is less than ebb duration. The maximummeasured flow velocity is at134cm/s, and the ebb flow velocity is greater than risingtide. The suspended sediment concentration in the study area is between12.14mg/Land251.1mg/L, and the suspended sediment concentration in autumn is obviouslyhigher than the concentration in summer. Clayey silt, sandy silt and silty sand are thedominant surface sediments type in the study area, and the grain size of surfacesediments are relatively coarse. The surface sediments of Chengbei are significantlycoarsened than Gudong area correspondingly with varying physical and mechanicalproperties because of river diversion and depositional environment changes.Based on field notes, particle-size and geotechnical analysis data, stratum above60meters in the study area can be divided into two large units. The sediments above10~15meters are delta and Holocene neritic deposits with feature of high watercontent, high void ratio and high liquidity. The sediments below are consisted of thelate Pleistocene deposits alternating by continental and marine layers. The sedimentcomposition, physical and mechanical properties are quite different between the upperand lower strata. The drill hole can be divided into3layers and6sublayers referringto shallow stratigraphy profiles and extremely shallow stratigraphy profiles.Based on one-dimensional consolidation theory and different drilling depth, thefinal consolidation settlement of each drill hole was between0.6and3.0meters, andthe mean settlement of unit depth was at1.5~3.5centimeters per meter. Fine-grainedsediments’ contribution to the final consolidation settlement amount is significantlygreater than the coarse-grained sediments, and the lower stratum’s contribution isgreater than the upper stratum. The consolidation degree increased with the increaseof time, and its rate tapered off varied with time. The consolidation degree of differentdrillings needed about15~20years to reach90%and the average sedimentation ratewithin overlying50m strata is at5~12cm/a.Based on Plaxis mathematical model and drilling depth, the final consolidationsettlement of each drill hole is between0.54and2.16meters, and the mean settlementof unit depth is at1.35~3.58centimeters per meter. The consolidation settlement amounts obtained by Plaxis model are smaller than obtained by one-dimensionalconsolidation theory (Terzaghi K,1943).After the Yellow River diversioned in1976, topographical variation of theabandoned Yellow River estuary near Chengbei area experienced rapid erosion period(maximum erosion rate is1.2m/a), slow erosion period (maximum erosion rate is0.5m/a), adjustment stage near equilibrium between scour and erosion.1976~1996,the seabed are mainly erosion in the whole of Chengbei and only have a small rangeof deposition in the north side of the study area. There is a large range of erosioncenter in the north of Feiyantan, and the maximum intensity of erosion is at8.0meters.2000~2004, a scour trenches formed from the northwest to southeast between4and7meters isobaths in Gudong area, and the erosion depth is between0.5and2.4m.In the erosion center of Chengbei area and the north side of the center, erosionintensity under the actual hydrodynamic is lower than the changes of the measureddepth. The contribution of consolidation settlement to the topographical changes inthe erosion center is between20and43.8%, and the contribution of consolidationsettlement to the topographical changes in the north side of the erosion center isbetween30.1and86.5%. For the balance area of erosion and deposition anddeposition area, the actual deposition amount under actual hydrodynamic is muchlarger than the changes of water depth obtained by measured data. The contribution ofconsolidation settlement to the topographical changes in the balance area of erosionand deposition is between46.5and50%, and the contribution of consolidationsettlement to the topographical changes in the deposition area is between30.7and41.5%. Additional, the contribution of consolidation settlement to the topographicalchanges of KD34in Gudong area is between68.9and89.5%.