Study On The Characteristics Of Land Subsidence In The Mordern Yellow (Huanghe) River Delta

In allusion to land subsidence in the modern Yellow (Huanghe) River delta, this paper addresses the recent characteristic, the factors and its effects to delta of this problem, in light of leveling data, nearby tide gauge records, the soil mechanics parameters of the seafloor and the delta sediments, remote sensing interpretation, results published in literatures analyzed integrated with GIS tools and so on.The results of repeat leveling from 1964 to 2007 covering the whole delta and the SAR data covering southern of delta from 2007 to 2008 acquired by the ENVISAT calculated using Differential Interferometric Synthetic Aperture Radar(D-InSAR)show the variability of the surface depression in the study area. The rate of subsidence has been decreasing from area near vertex of delta to the coastal zone and some place is even rising, particularly in the early formed area. What is the different in the coastal is that the rate of subsidence is still high and can sustain a long time. This is because the land subsidence in the delta is mainly caused by the consolidation of sediments if there is no human activity. Especially in the newly formed area, Qingshuigou sub-delta, the rate of subsidence was more than 50 mm/a in the begining and now is still higher than 10 mm/a. When the newly sediments discharged by the Yellow (Huanghe) River landed on the subaquatic delta, its load can drive the concolidation of the silt, which is on the surface of the Bohai seafloor. The rate of consolidation of the silt will be decreasing quickly in the first years, but still a long time in the later time with a slow rate. Consolidation of soft soil, which is caused by its weight in the natural state, is the main proportion of the ground settlement in the plain area, account for 40-90 % of the total settlement, while that of the upside silt is only 3-12 % or 30-50 %. As a result, characteristic of distribution of the Land subsidence is controlled by the thickness and the depth. Calculation of the consolidation rate show that the consolidation maybe approach to complete recently, but it will be still the main factor if there is superfluity load by human activity.From the result of relative sea level change in the delta by a stochastic dynamical prediction model, the rates of land subsidence is about 3.42 mm/a, which is mostly resulted in diastrophism and will continue a long time. The rate caused by tectonic is about 2.0 mm/a, while the other is caused by isostasy as a result of burden of sediments after 1855.With an accumulating land subsidence, it can change the surface slope, consequently influenced the riverbed and estuary evolution. Additionally it increased the rise of the relative sea level, as a result changed the models of coastal erosion and forced the coastline retreat landward or narrowed the tidal flats at an accelerating rate with the strengthened hydrodynamic action.