西安高陵近代河漫滩沉积与洪水变化

 根据高陵渭河河漫滩沉积剖面中105个样品的颗粒与有机质分析,Hg与CaCO3含量测定和史书记载的年代资料,研究了西安高陵渭河距今约120 a来河漫滩沉积指示的洪水变化。结果表明,高陵渭河约120 a来至少发生了16次大洪水, 洪水规模存在一定差别, 各次洪水一般代表了当年降水量的明显增加; 高陵渭河河漫滩沉积中有机碳浓度、金属汞的含量、CaCO3含量变化显著,它们与颗粒大小之间有明显的相关性,沉积物粒度越细,其有机碳和重金属汞的含量越高,CaCO3含量越少。有机碳和重金属汞的含量变化能够间接指示洪水动力强弱变化和短暂降水量增加。汞含量变化表明,渭河汞污染发生时间相当晚。     

兰州黄河高漫滩沉积与洪水变化研究

根据兰州黄河高漫滩沉积剖面的观察和81个样品的粒度分析,研究兰州黄河马滩剖面沉积特征和当时的洪水变化。结果表明,马滩剖面明显分为15个沉积层,指示15次较大的洪水。这15个阶段河漫滩洪水深度和规模由大到小的变化顺序依次为第10>12>11>13>15>14>3>5>7>8>6>1>4>9>2阶段。据历史文献推算,马滩高漫滩沉积代表的15次洪水发生在170 a B.P.以来。兰州洪水沉积不连续,这15次洪水的每一次一般代表1 a雨季的短暂时间,两次洪水间隔时间通常为10 a左右,这10 a是没有洪水发生或洪水规模小的阶段,也是降水量正常或偏少的阶段。     

西安高陵渭河近120年来的洪水演变

西安高陵渭河漫滩沉积剖面发育,能较好地指示渭河洪水变化.沉积物以粗粉砂、细粉砂、极细砂及粘粒为主.粒度较粗层位指示较强的水动力条件和短暂的湿润气候或一定地区的暴雨增加,粒度较细的层位则代表较弱的水动力条件和短暂的湿润程度略低的气候,而在各洪水沉积层之间无洪水发生的阶段则是正常的气候阶段或偏干的气候阶段.高陵渭河近120年来的河漫滩沉积剖面可分为16层,代表了16个大的洪水阶段.高陵渭河近120年来的河漫滩沉积粒度成分比更早期的河漫滩沉积粒度偏细,反映出近120年来渭河洪水深度与规模在变小,这主要是气候变干造成的.     

渭河西安高陵耿镇历史时期古洪水研究

根据高陵白家嘴剖面和马坊滩剖面渭河古河漫滩沉积中147个样品的颗粒分析和史书记载的年代资料,研究了西安高陵耿镇渭河距今1 500—1 100 a间洪水深度、洪水规模和气候变化。粒度分析表明,高陵渭河距今1 500—1 100 a间厚约3.4 m的河漫滩沉积物至少包括了9个粗细明显不同的沉积层,清楚地反映出至少发生了9个大洪水阶段;距今1 500—1 100 a间 的洪水深度比现今明显大,白家嘴剖面中9个洪水阶段发生时河漫滩上的洪水深度均明显大于2 m,其中第2、4、6、8阶段的洪水规模比第1、3、5、7、9阶段更大;第2、4、6、8大洪水阶段是降水量的明显增加造成的,当时年降水量应为800~900 mm;第1、3、5、7、9洪水阶段主要是降水集中或降水量有一定的增加造成的。     

泾阳泾河段全新世剖面元素地球化学特征与洪水变化

根据泾阳泾河一级阶地前缘沉积剖面(JYc)中285个样品的元素、化合物含量和粒度分析,对泾阳泾河段全新世剖面元素地球化学特征与洪水变化进行研究,结果表明,元素Mn、Ni、Cu、Zn、Ga、Pb、Sc、Ti、Nb、Th、U、Rb、V、Ba、Co和化合物Al2O3、Fe2O3、K2O含量在剖面各层中变化明显,分辨率高,能够清晰地指示洪水以及降水量的变化;JYc剖面分为41个沉积层,指示了41次大小不同的洪水和41个降水较多的年份;沉积物粒度细,元素Mn、Ni、Cu、Zn、Ga、Pb、Sc、Ti、Nb、Th、U、Rb、V和化合物Al2O3、Fe2O3、K2O含量高,元素Ba、Co含量低,指示沉积时洪水强度小,洪水位低,降雨量少;沉积物粒度粗,元素Mn、Ni、Cu、Zn、Ga、Pb、Sc、Ti、Nb、Th、U、Rb、V和化合物Al2O3、Fe2O3、K2O含量低,元素Ba、Co含量高,指示沉积时洪水强度大,洪水水位高,降雨量多.泾河泾阳段洪水代表的是黄土高原泾河流域的洪水,特大洪水与大洪水,来自整个泾河流域或泾河流域大部分地区,较小的洪水来自泾河流域小部分地区.泾河流域发生洪水的原因有3种:1.年降水量明显增加造成;2.年降水量没有增加,而是由短时暴雨造成;3.年降水量没有增加,而是由于降水集中、持续时间长造成的洪水.泾河泾阳段JYc剖面的洪水沉积主要是由中度涝灾、大涝灾、特大涝灾形成的,轻度涝灾一般不会造成洪水灾害.     

西藏安多剖面沉积物粒度特征及环境意义

对西藏安多剖面的3个层位沉积物与两个现代流动沙丘表层沉积物粒度组成、传统粒度参数及体积分维值进行了对比分析,探讨其粒度特征以及环境指示意义。结果表明：安多剖面的3层代表了3个不同的气候阶段,第一层（0～172 cm）沉积动力为风,其中在0.44～1.08 m深度处是一次风力加强的阶段,第二层（172～224 cm）、第三层（224～232 cm）均在6 Φ附近出现次峰,悬浮组分明显增加,推断为远距离输送的粉砂级颗粒沉降,表明这两个阶段风力有所减弱;安多剖面颗粒比研究区现代流动沙丘颗粒细,表明剖面可能为固定或半固定沙丘沉积;本研究区样品的体积分维值在0.215～1.77,虽然没有实际的物理意义,但作为粒度参数指标,在指示环境变化上是有效的,并且与粘粒、粉砂含量呈现明显的正相关关系,与细砂呈现明显的负相关关系。     

汉江上游沉积记录的东汉时期古洪水事件考证研究

总结近年来在汉江上游古洪水研究成果的基础之上,采用文献资料分析和洪水模拟计算方法,对汉江上游6个沉积剖面纪录的东汉时期古洪水事件进行了年代考证和洪水模拟计算。结果表明,通过分析文献记载的汉江上游东汉时期洪水影响范围、强度和程度,以及结合洪痕沉积规律,在时间上考证认为这6个沉积剖面记录的东汉时期古洪水事件可能是东汉建安二年（197年）九月的一次特大洪水事件。选取合适的河槽横断面和糙率系数,采用HEC-RAS模型对距离较近的4个沉积面记录的东汉时期古洪水事件进行洪水模拟计算,模拟洪水位与各剖面依据古洪水SWD恢复的洪水位误差在-0.18%~0.25%,而且模拟的1983年洪水位与剖面及其附近发现的1983年洪痕水位误差小于0.25%,说明洪水模拟计算选取的河槽横断面和水文参数准确、可靠,从洪水模拟计算的角度也说明了汉江上游沉积记录的东汉时期古洪水事件可能为一次特大洪水事件。该研究结果不仅延长了汉江上游洪水序列,而且也为汉江上游水利工程建设、水资源管理和防洪减灾等提供重要的水文资料。     

汉江上游白河段万年尺度洪水水文学研究

通过对汉江上游的考察,在多个地点发现古洪水滞流沉积层夹在古土壤层与坡积石渣土层或坡积角砾层中。选择白河段JJTZ剖面进行沉积学和水文学研究。对于采集的全新世古洪水滞流沉积样品,进行沉积学分析,与2010年现代洪水滞流沉积物进行对比,结合其宏观特征确认其为汉江洪水悬移质沉积物,粒度成分分类为细沙质粉沙。它们记录了汉江上游特大古洪水事件。通过地层学对比分析和OSL测年断代,确定其发生在全新世中期-晚期转折阶段,即3 200～2 800 a B.P.。利用沉积学和古水文学原理恢复古洪水洪峰水位,采用比降法计算出两次古洪水洪峰流量为40 180 m3/s和49 170 m3/s。该结果大大延长了洪水水文数据序列,从而能够建立汉江上游万年尺度洪水流量-频率关系,对于揭示汉江上游水文过程对于全球变化的响应规律具有重要的科学意义,对于防洪减灾和水资源水能源工程建设具有重要的现实意义。     

长江中游沿江牛轭湖沉积及其环境意义——以长江荆江段天鹅洲、中洲子为例

以长江中游荆江段天鹅洲和中洲子(牛轭湖)为对象,通过对其沉积物柱样进行210Pb、137Cs、粒度、磁学测试分析,建立了牛轭湖沉积的基本序列、沉积特点与年代学框架。研究表明:1牛轭湖沉积为A-B层,A层(下层)为河相沉积层和河/湖过渡层,B层(上层)为湖相沉积层,沉积物粒径在剖面上呈"粗—细—粗"变化特点。2对于近现代牛轭湖,笼统运用放射性核素(如210Pb)比活度计算的平均沉积速率推测沉积物年代,会产生较大误差,需运用其他沉积特征参数和近现代环境变化事件记录进行多重校核。3牛轭湖沉积可以较好地记录河流流域洪水事件,可为重建古洪水事件提供借鉴。牛轭湖沉积较其他湖泊沉积更为复杂,相关研究尚待继续深入。     

江汉平原近3000年来古洪水事件的沉积记录

洪水发生规律是洪灾预报的前提,已有的历史洪水记录时间尺度不足以认识和把握洪水的出现规律。因此,利用地质记录延长洪水时间序列,从地质记录中认识洪水的发生规律,显得非常重要和必要。江汉平原江陵地区的洪水地质记录表明,近3000年来,该区共发生了18次特大洪水漫滩事件。这些古洪水的频发与湿冷气候相对应。功率谱分析发现古洪水漫滩事件包含26、31、36、43、52、79、110年这样的周期。分析表明江陵地区古洪水发生的驱动因子可能是太阳活动。     

长江下游南京-镇江河段河漫滩粒度特征

分析长江南京-镇江河段3个现代河漫滩沉积孔上部100 cm的粒度特征,探讨宁镇河段现代河漫滩的沉积环境。结果表明：GB孔、ZR孔和ZH孔沉积水动力较小,沉积物颗粒较细,均以粉砂为主; ZR孔的砂含量最多、沉积粒径最大、粒径和砂含量由底层向表层增加;ZH孔分选系数呈明显的阶段性特征。研究认为：河流比降、河势、分汊河床演变以及滩面植被是影响宁镇河段河漫滩粒度特征的重要因素。     

黄河中游马莲河历史与现代洪痕沉积与水文学研究

通过对马莲河流域深入调查研究,在马莲河下游峡谷峭壁发现了一组4个显著的洪痕。洪痕沉积物分析看出,马莲河洪水悬移质泥沙以粉沙为主,粘粒和沙粒含量都较低,磁化率值也很低。将这些数据与流域内马家原剖面黄土土壤的的分析结果对比,可知马莲河现代洪痕沉积物是暴雨洪水对于流域内黄土和土壤侵蚀、搬运沉积形成。采用面积－比降法对该组洪痕所记录的4次洪水事件进行了洪峰流量恢复计算。表明高于常水位3.3 m、5.5 m、6.7 m洪痕洪峰流量分别为1 170m³/s、4 060 m³/s、5 690 m³/s。它们与雨落坪水文站观测2005、2003、1977年的洪水洪峰流量值相当,误差小于5 %。最高洪痕高出常水位10～12 m,计算恢复其洪峰流量为13 980 m³/s,洪水发生年代为1841年（清代道光21年）。这些研究结果对于马莲河流域的防洪减灾、水利水电工程设计和水土流失防治具有重要科学意义。     

Natural levee deposition during the 2005 flood of the Saskatchewan River

In summer 2005, a controlled flood of the Saskatchewan River (east-central Saskatchewan, Canada) resulted in general floodplain inundation and extensive natural levee deposition along a 60-km reach extending from 40 km below the E.B. Campbell dam to Cumberland Lake. Levee crests along channel banks were inundated for up to 7 weeks in some areas of the floodplain. New deposits on levee crests varied from 0 to 70 cm in thickness, displaying large variations both along reach and in opposing sites across channels. Mean grain sizes, mainly silt and very fine sand, likewise varied considerably among sample sites.Pre- and post-flood surveys of channel cross sections along the flooded reach permitted assessments of relationships between channel-area changes and patterns of levee sedimentation in this system in which virtually all new flood sediment was derived by channel scour. Results show that both net deposition and net erosion occurred within the channel cross sections, but that on average, net channel enlargement of 4.2% prevailed over the entire survey reach when weighted by cross-section size. Over the 60-km flooded reach, zones of thick levee deposition occur at or just downstream of two areas of major channel enlargement, and an intermediate zone of thin levee deposits is associated with an intermediate area of net channel aggradation. This bimodal distribution of flood-deposit thickness is inferred to have resulted from differences in sediment supply produced locally by the different extents of channel-perimeter erosion. Two other factors—(1) position of interfacial zones between clear floodbasin water and turbid channel water, and (2) difference in pre-flood levee heights—contributed to the poor correspondence in thickness and grain size between opposing levees at some sites. Additional features of the new levee deposits, including increases in transverse slopes, abrupt basinward fining, and paucity of deposition in distal areas due to clear floodbasin waters, are characteristic of strong front loading that results when suspended sediment production is restricted to channel erosion processes.     

Diagenesis in Okavango fan and adjacent dune deposits with implications for the record of palaeo-environmental change in Makgadikgadi–Okavango–Zambezi basin, northern Botswana

This work considers the spatial distribution and ages of western MOZ basin siliclastic sediments prior to providing insights into the diagenesis of degraded dune and alluvial fan sands. Previously published and new TL/OSL ages imply that extensive over-washing of dune sands took place at least 100 ka ago while ages on Okavango floodplains imply that the fan was formed ca. 40 ka and has since undergone periods of higher and lower flood regimes. Sediment analyses indicate that both dune and fan sands contain a diagenetic matrix of clay-enhanced amorphous silica (CEAS) which bonds weakly formed aggregates. The time of formation of diagenetic matrix products is inconclusive but may have been accelerated during or shortly after events dated using OSL/TL techniques. Hence earlier dune over-washing may have led to greater porewater of an acidic to near neutral nature which in turn promoted smectite formation and silicic acid precipitation > 100 000 years ago. The relatively abundant CEAS matrix in floodplain sands implies more recent semi-continuous flood events again of an acidic-near neutral nature leading to the formation of smectite. In this case the floodplain sediments are dated as having been deposited around 40 and 11 ka, when porewater content may have accelerated clay formation and silica dissolution. The dual nature of the CEAS in the islands reflects a changing environment from smectite-dominated flooding events to sepiolite-dominated desiccation events. Flooding may also correspond to TL/OSL ages over the past 40 000 years which contributed to accelerated CEAS formation. The sepiolite is associated with a Ca-rich matrix implying desiccation which may relate to drying events over the 40 000 year period or to riparian tree root pumping and selective salt accumulation. This work shows that sedimentation in incipient rifts is complex and rarely explained totally in terms of primary depositional events. The implications of different stages of sand diagenesis may be significant in enhancing palaeo-environmental interpretations in semi-arid fluvial environments.     

Terrace aggradation during the 1978 flood on Powder River, Montana, USA

Flood processes no longer actively increase the planform area of terraces. Instead, lateral erosion decreases the area. However, infrequent extreme floods continue episodic aggradation of terraces surfaces. We quantify this type of evolution of terraces by an extreme flood in May 1978 on Powder River in southeastern Montana. Within an 89-km study reach of the river, we (1) determine a sediment budget for each geomorphic feature, (2) interpret the stratigraphy of the newly deposited sediment, and (3) discuss the essential role of vegetation in the depositional processes.Peak flood discharge was about 930 m³ s^− 1, which lasted about eight days. During this time, the flood transported 8.2 million tons of sediment into and 4.5 million tons out of the study reach. The masses of sediment transferred between features or eroded from one feature and redeposited on the same feature exceeded the mass transported out of the reach. The flood inundated the floodplain and some of the remnants of two terraces along the river. Lateral erosion decreased the planform area of the lower of the two terraces (~ 2.7 m above the riverbed) by 3.2% and that of the higher terrace (~ 3.5 m above the riverbed) by 4.1%. However, overbank aggradation, on average, raised the lower terrace by 0.16 m and the higher terrace by 0.063 m.Vegetation controlled the type, thickness, and stratigraphy of the aggradation on terrace surfaces. Two characteristic overbank deposits were common: coarsening-upward sequences and lee dunes. Grass caused the deposition of the coarsening-upward sequences, which had 0.02 to 0.07 m of mud at the base, and in some cases, the deposits coarsened upwards to coarse sand on the top. Lee dunes, composed of fine and very fine sand, were deposited in the wake zone downstream from the trees. The characteristic morphology of the dunes can be used to estimate some flood variables such as suspended-sediment particle size, minimum depth, and critical shear velocity. Information about depositional processes during extreme floods is rare, and therefore, the results from this study aid in interpreting the record of terrace stratigraphy along other rivers.     

泾河近代洪水沉积物粒度特征分析

泾河是渭河的第一大支流,也是渭河下游和黄河潼关以下洪水的主要来源,洪枯变化量大,通过采集泾河S-剖面27个河漫滩样品,用Mastersizer 2000型激光粒度仪对其0.1~1 000 μm范围内的粒度进行分析,并根据Folk和Ward的概率公式计算出各种粒度参数。结果显示,泾河S-剖面沉积物指示了至少8次洪水的信息。剖面粒度组成以粉砂为主,其中粗粉砂不仅含量高,而且含量变化很好的对应了洪水变化阶段,可以作为泾河洪水大小的标志性粒级。     

新疆喀拉库姆沙漠新月形沙丘不同部位粒度特征

对新疆叶尔羌河流域绿洲边缘喀拉库姆沙漠高度在3～12 m的新月形沙丘进行了粒度分析。结果表明:表层和下层沉积物均以细砂和极细砂为主,二者含量达82%以上。表层沉积物较下层稍粗,平均粒径分别为2.09 Φ和2.16 Φ。下层沉积物较表层分选性好,σ均值分别为0.48和0.62。表层和下层沉积物偏度均为正偏,SK均值分别为0.21和0.24。峰态均为尖窄,Kg均值分别为1.18和1.11。表层和下层沉积物平均粒径均表现为丘顶最粗,背风坡坡脚最细,由两侧坡脚至丘顶平均粒径变粗,平均粒径与其他粒度特征参数之间均存在良好的相关性。分析表明,喀拉库姆沙漠沙丘沉积物粒度特征除受物源这一背景制约外,还与区域构造、沙漠地貌、植被、气候和较为复杂的风沙动力学过程等众多因素有关。     

新仙女木期黄河晋陕峡谷古风成沙层年代及其物质来源

通过对黄河晋陕峡谷壶口至龙门段开展广泛细致的野外考察,发现了典型的晚更新世以来黄土—土壤层夹古风成沙层剖面—北桑峪剖面（BSY）。结合不同类型沉积物的野外宏观特征、磁化率、粒度成分、石英颗粒表面特征和地球化学元素的对比分析,确定黄河晋陕峡谷壶口至龙门段BSY剖面中所夹古风成沙层的性质为中沙质细沙,是在干旱多风环境下形成的沙质沉积物。基于光释光（OSL）测年数据,确定其沉积年代为12.5-11.6 ka,记录了黄河中游发生在末次冰消期的极端干旱事件,其与相邻区域沙漠/黄土过渡带中湖沼沉积和风成黄土—古土壤剖面记录的新仙女木事件相对应。该剖面中古风成沙沉积物的物质来源,主要来自黄河晋陕峡谷的古河床相沙层物质。这是因为该河段处于干旱半干旱季风气候区,在新仙女木时期,黄河晋陕峡谷处于极端干冷的环境之中,冬季风强盛、夏季风衰弱,黄河水位下降明显,河漫滩和江心洲将大面积出露,大量的沙物质在风力作用下向岸边输移,成为河谷两侧缓坡台地上古风成沙沉积层出现的主要沙源地。这个研究成果对于进一步揭示黄河中游流域极端干旱事件发生的时间性规律及其与季风气候变化的关系,具有重要的科学意义。