鲁北丹河下游洪水决口扇沉积岩芯粒度特征与沉积过程重建

洪水沉积物粒度是反演洪水特征及过程的关键参数,也是识别古洪水沉积层的重要前提和依据。通过对山东北部丹河2018年洪水淹没区考察,取得21cm长无扰动沉积物短钻DH2,室内进行粒度分析,并对实验结果进行组分拟合,获得如下主要结论:1)DH2钻孔粒度整体较细,以细粉砂为主;2)洪水决口扇沉积层与下伏土壤层相比,沉积物粒度整体变细,黏土和细粉砂含量略有增加,粗粉砂和砂含量大幅减少;3)分选系数和峰度均显示洪水沉积分选趋向良好,偏度指示颗粒物变细,水动力减弱,沉积环境稳定;4)决口扇沉积层粒度具有由细到粗三个组分,根据其含量变化特征,可将此次洪水过程划分为三个阶段,即洪水初期强水动力快速输入阶段、中期持续性稳定输入阶段、末期准静水环境下沉积阶段。研究结果有助于认识洪水层沉积特征及水动力过程,也为识别地层中古洪水沉积提供了有力参考。同时揭示丹河下游地区河流地貌-水文系统受到人类活动的负面影响,导致洪水水流不畅,滞流现象严重,对于开展河道整治和防洪减灾具有一定意义。

Grain-size characteristics of crevasse splays from the lower reaches of Dan River in northern Shandong province and reconstruction of sedimentary process

Grain size of flood sediments is a key parameter in the inversion of flood characteristics and process,and is also an important prerequisite and basis for identifying palaeoflood events.A 21 cm-long undisturbed sediment core DH2 was sampled based on the investigation of the flooded area of Dan River.Grain size was analyzed in the laboratory,the components of which were partitioned by using the log-normal distribution function method.The main conclusions are as follows:(1)The grain size of DH2 core is generally fine,mainly fine silt.(2)Compared with the underlying soil layer,the grain size of the sediments became finer as a whole,the content of clay and fine silt crevasse splays sediments increase slightly,and the content of coarse silt and sand decreased significantly.(3)Sorting coefficients and kurtosis both reflect the good sorting trend of deposits,and the skewness shows that the grain size become finer,which indicates that the hydrodynamic force is weakened and the sedimentary environment is stable.(4)Crevasse splays sediments consist of three grain size components from fine to coarse,according to the variation of content of which,the flood process can be divided into three stages:rapid input of strong hydrodynamic force in the early stage of the flood,continuous and stable input in the middle stage.The research results provide a powerful reference for identifying palaeoflood deposits in the strata,through which we can understand the hydrodynamic characteristics of floods and reconstruct the deposition process.At the same time,it reveals that the river geomorphology-hydrological system in the lower reaches of the Dan River is negatively affected by human activities,resulting in unsmooth flood flow and serious stagnation phenomenon,which has certain significance for the development of river improvement and flood prevention and disaster reduction.