Four remarks on the growth of channel networks

Networks fed by subsurface flow are a natural, but dynamically simple, system in which to consider general problems of network growth. Here we present four examples in which this dynamic simplicity can be used to develop a quantitative understanding of a natural kilometer-scale network of streams. In these four sections, we investigate the relation between the position of a spring in the network and the groundwater flux into it, the flow of water through streams and the stream shape, and the influence of solar radiation on the rate of sediment transport around the streams. In each case, a quantitative comparison is made between theory and observation.     

Area balancing as a test of models for the deep structure of mountain belts,with specific reference to the Alps

Basic concepts of structural restoration are applied to crustal cross-sections through mountain belts to explore large-scale tectonic models and deep structure. However, restored sections should account for variations in pre-orogenic crustal thicknesses. Crustal balancing approaches are reviewed and applied to two Alpine sections, coinciding with deep seismic experiments: NRP-20 East (Central Alps) and ECORS-CROP (Western Alps). Existing studies assume large (>300 km) orogenic contraction and only moderately thinned pre-orogenic crust. The resulting restored sections contain more crust than is imaged beneath the present-day Alps, the missing crust generally assumed to be subducted. Two kinematic modifications reduce the requirement for subduction: thinning and buoyancy-driven return flow of ultra-high-pressure metamorphic rocks during orogenesis; and pre-orogenic hyperextension. Using large stretching factors for the pre-orogenic crust negates crustal subduction on both Alpine transects. If the lower crust was approximately rigid, restorations of the Central Alps require strongly depth-heterogeneous stretching of upper and lower crust during Mesozoic rifting. Relaxing this requirement allows uniform lithospheric stretching, a corollary consistent with published subsidence estimates. Restorations make implicit statements on the form of pre-orogenic basins and the structure of continental margins incorporated into mountain belts that can in turn provide tests of tectonic models.     

Tectonic geomorphology of the Mississippi Valley between Osceola, Arkansas and Friars Point, Mississippi

A geomorphic study of the Mississippi River and its alluvial valley between Osceola, Arkansas and Friars Point, Mississippi has identified anomalous surface features that can be linked to known geological structures. For example, relatively recent deformation along Big Creek fault zone, White River fault zone, Bolivar-Mansfield tectonic zone, Blytheville Arch, Crittenden County fault zone, and Reelfoot Rift margins is suggested by river and topographic anomalies. Faults and plutons appear to affect drainage networks, and the morphology of Crowleys Ridge suggests significant fault control. The many anomalies probably reflect a fractured suballuvial surface. Although movement along these fractures will most likely occur in seismically active areas, the probability of movement elsewhere     

Impact of range‐parallel sediment transport on 2D thermo‐mechanical models of mountain belts: Application to the Kyrgyz Tien Shan

Evolving mountain belts dynamics is very sensitive to surface processes. The surface processes affect tectonics by enhancing crust exhumation and thermal weakening, and depositing soft yet cold sediments in surrounding basins. While 2D plane strain models approximate cylindrical tectonic structures well, simple 1D mass transfer cannot capture erosion–sedimentation complexity. The Eastern Kyrgyz Tien Shan, where structures, basins and exhumation rates are well constrained, is used here to illustrate this issue. Thermo‐mechanical models demonstrate that 1D transport cannot adjust both basin geometry and Apatite Fission Track exhumation ages. When out‐of‐plane sediment transfer is considered, the amount of evacuated sediment delays or accelerates the formation of new faults, affecting the relative timing of exhumation. For our case study, lateral drainage must evacuate 80% of the sediments to match the geological constraints, which is consistent with other source to sink analyses. This indicates that lateral drainage should not be neglected in regional 2D models.     

不同植被类型的坡面径流侵蚀产沙试验研究

利用野外模拟降雨试验,研究不同植被类型的侵蚀产沙、径流、地表糙度和入渗规律以及相关关系。结果表明,植被对水沙调控作用大小依次为:植物根系,整个坡面植被,坡面下部植被和坡面上部植被。林草地具有蓄水减沙的水土保持措施功效,该机制通过植物根系对水沙的调控作用实现;草地具有直接拦沙的水土保持功效,该机制通过地表植被对水沙的调控作用体现。累计产沙量随累计径流量呈幂函数递增趋势,具有很好的相关性;坡面径流产沙过程分为发育期、活跃期和稳定期3个阶段。植被覆盖度越大,地表糙度变化越小。     

Geometry of two glacial valleys in the northern Pyrenees estimated using gravity data

We estimate using gravity data the thickness of post-glacial unconsolidated sediment filling two major glacial valleys in northern Pyrenees: the Gave de Pau valley between Pierrefitte-Nestalas and Lourdes, and the Garonne valley between Saint-Béat and Barbazan. One hundred and eighty-four new gravity data complete 74 measurements obtained from the International Gravimetric Bureau database. Negative residual anomalies resulting from the presence of small-density unconsolidated sediment approach 4 mgal in both the Gave de Pau and the Garonne valleys. Estimating the sediment thickness requires knowing the density contrast between Quaternary sediments and the underlying bedrock. Supposing this density contrast is 600 kg/m³, the maximum estimated thickness of post-glacial sediment is ∼ 230 and 300 meters, and the volume of sediment is 2.1 and 3.2 km³ in the Gave de Pau and Garonne valleys, respectively. In both valleys, the depth of Quaternary sediment suddenly increases at the confluence between two major glacial valleys (Gave de Pau – Gave de Cauterets, and Garonne – Pique confluences). Overdeepened basins are less deep downstream when approaching terminal moraines (Lourdes and Barbazan area), illustrating that the efficiency of glacial erosion depends on the ice flux flowing through valleys.     

Late Quaternary dynamics of the Tisza River: Evidence of climatic and tectonic controls

The focus of this study is to understand a dramatic avulsion event on the Tisza River. During the Late Pleistocene the river course switched by about 80 km from the east to west of the Great Hungarian Plain (GHP) through the Záhony bend to its present meander belt. The aim of this study is to date this Záhony avulsion: based upon radiocarbon and pollen samples from six cores in the Polgár study area, situated in the west of the GHP at the middle course of the Tisza River. In addition, a grain size composition and heavy mineral analysis has been performed. The results of these analyses reveal a sequence of paleochannels and have been plotted on a high-resolution digital elevation model, illustrating the paleochannel form and age relationships.

The study suggests that the age of this major avulsion event is significantly older than it was previously supposed. Instead of 10 to 11 ka it is, according to our new data, 16–18 ka, and definitely predates the Last Glacial Maximum (LGM). The river cut into the previous, contiguous surface and formed at least one climatic terrace, then drifted gradually westward as a response to a tilt or the differential subsidence of tectonic origin.     

安第斯与冈底斯成矿带斑岩铜矿床矿物学和成矿斑岩地球化学特征对比

在总结安第斯和冈底斯斑岩铜矿床地质矿物学特征的基础上,通过对2个成矿带与斑岩铜矿床有关的岩浆岩地球化学特征的对比分析,探讨了2种构造环境下形成的斑岩铜矿床含矿斑岩与成矿过程的异同点。安第斯成矿带的斑岩铜矿床形成于洋壳俯冲陆缘弧环境,成矿时代主要集中在始新世晚期—渐新世(43~31Ma)和中新世中期—上新世(12~4Ma),金属组合包括Cu-Mo和Cu-Au,含矿斑岩的SiO_2含量变化范围较大,岩性从中性到酸性,以钙碱性-高钾钙碱性系列为主,少部分具有典型埃达克岩地球化学特征,而大多数安第斯含矿斑岩具有正常岛弧系列火山岩的地球化学特征。冈底斯成矿带斑岩铜矿床主要发育于陆-陆碰撞环境,成矿时代为中新世(20~12Ma),金属组合为Cu-Mo,缺乏Cu-Au组合,含矿斑岩岩性以酸性为主,且主要为高钾钙碱性-钾玄质系列岩浆岩,具有典型埃达克岩的地球化学特征。安第斯成矿带含矿斑岩的形成很可能是板片释放流体交代楔形地幔,经部分熔融与MASH过程的产物,并不是直接源于洋壳的部分熔融;而冈底斯成矿带含矿斑岩成因可能是早期洋壳多次俯冲形成俯冲增生弧,之后在陆陆碰撞过程中经历缩短加厚,与深部构造动力学机制发生变化时的部分熔融有关。     

珠江网河水沙分配变化及其对伶仃洋水沙场的影响

采用二维ECOMSED模型对径流来水来沙变化前后的1978年和1999年伶仃洋洪水期水动力场和悬沙场进行模拟,结果表明:伶仃洋水动力场整体增强,但不同地貌单元水动力变化具有明显差异.分流比变化后洪水期伶仃洋西槽、东槽和中滩水动力增强,涨潮和落潮流速普遍增大0.1m/s;西部各口门区和西滩涨潮流势减弱,径流优势更加明显,但西滩三个口门水道及滩面潮沟落潮流速增大约0.1 m/s;东滩水动力特征变化不大.悬沙浓度亦整体升高,平均增加了0.02~0.05 kg/m3,悬沙高值区范围扩大,0.10 kg/m3等值线由西槽西侧明显向东推进到沿东槽南北向分布,径流来沙的影响范围东扩.     

水电开发对河流水沙年内分配的影响分析

以中国西南重要的国际河流澜沧江为研究对象,借鉴"基尼系数"的构建思路,建立了水沙年内分配均匀度计算模型,并通过下游水沙与上游天然水沙和区域降水的关系,建立了河流水沙年内分配均匀度回归还原模型。还原结果与实测对比表明:漫湾、大朝山电站建设以来,澜沧江径流年内分配均匀度年均升高1.14%,变幅在-7.32%~10.81%之间,输沙年内分配均匀度年均降低0.23%,变幅在-18.98%~26.76%之间;漫湾、大朝山电站同时运行期间,澜沧江径流年内分配均匀度年均升高3.90%,输沙年内分配均匀度年均降低0.42%;大朝山水电站建设后,澜沧江径流年内分配均匀度升高而输沙年内分配变化不明显,说明大朝山水电建设后河流水沙关系发生了变化,但变化较小。该成果不仅能为相关研究提供方法借鉴,也为水电开发对澜沧江水沙年内分配影响程度的科学评估提供了新的依据。     

黄河下游洪水的泥沙输移特征

研究了黄河下游1950-1960年、1969-1985年144次洪水的泥沙输移特征.结果表明:泥沙输移比(SDR)随场次洪水平均含沙量和平均来沙系数的增大而迅速减小;存在着一个使泥沙的输移比达到最大值的最优洪水流量级(4000m3/s左右);场次洪水泥沙输移比与场次洪水最大含沙量之间存在着负相关,当最大含沙量(C_max)>300kg/m3时,泥沙输移比(SDR)<0.50,说明高含沙洪水的输移比是很低的.上中游不同源区的洪水对下游的SDR有显著的差异.来自河口镇以上清水区洪水的SDR大多数大于0.60;来自多沙细沙区洪水的SDR都大于0.50;来自多沙粗沙区洪水的SDR则小于0.50.黄河下游SDR与来自不同来源区洪水的搭配关系有关,SDR随来自粗泥沙区来沙量比例的增大而增大,达到一个峰值,与之相对应的粗泥沙区沙量百分比为50%;对于细泥沙区来沙量比例而言,情形类似,与SDR峰值相对应的细泥沙区来沙量百分比为40%.     

Modeling diagnosis of suspended sediment transport in tidal estuarine system

A three-dimensional, time-dependent hydrodynamic and suspended sediment transport model was performed and applied to the Danshuei River estuarine system and adjacent coastal sea in northern Taiwan. The model was validated with observed time-series salinity in 2001, and with salinity and suspended sediment distributions in 2002. The predicted results quantitatively agreed with the measured data. A local turbidity maximum was found in the bottom water of the Kuan-Du station. The validated model then was conducted with no salinity gradient, no sediment supply from the sediment bed, wind stress, and different freshwater discharges from upstream boundaries to comprehend the influences on suspended sediment dynamics in the Danshuei River estuarine system. The results reveal that concentrations of the turbidity maximum simulated without salinity gradient are higher than those of the turbidity maximum simulated with salinity gradient at the Kuan-Du station. Without bottom resuspension process, the estuarine turbidity maximum zone at the Kuan-Du station vanishes. This suggests that bottom sediment resuspension is a very important sediment source to the formation of estuarine turbidity maximum. The wind stress with northeast and southwest directions may contribute to decrease the suspended sediment concentration. When the freshwater discharges increase at the upstream boundaries, the limits of salt intrusion pushes downriver toward river mouth. Suspended sediment concentrations increase at the upriver reaches in the Danshuei River to Tahan Stream, while decrease at Kuan-Du station.     

Late Miocene fluvial sediment transport from the southern Appalachian Mountains to southern Florida: An example of an old mountain belt sediment production surge

Past geomorphological models assume that erosion of sediments from old mountain belts occurred at a relatively constant rate, based on comparatively uniform isostatic adjustment caused by unloading. Late Miocene strata of the south‐eastern United States provide an example of pulsed tectonism resulting in a surge in siliciclastic sediment production and transport. Regional tectonism (uplift of the southern Appalachian Mountains) and climatic conditions during the Late Miocene resulted in the long‐distance (up to 1000 km) fluvial transport of coarse siliciclastic sediments onto a stable carbonate platform in southern Florida. The sediments are unusual in that they are significantly coarser than marine‐transported sands in southern Florida, with discoidal quartz and quartzite clasts up to 40 mm in diameter locally present, and have relatively high potassium feldspar contents (up to 16% in some sample fractions), whereas feldspar is rare in modern Florida beach sands. It is suggested that previously documented rejuvenation of the southern Appalachian Mountains during the Middle to Late Miocene time, coupled with the Messenian sea‐level low, generated the increased rate of sediment production and necessary hydraulic gradient to allow rapid transport of coarse sediments. Tectonic influence on the river pathway in Florida, as well as in the southern Appalachian Mountains, may have maintained the river on the narrow carbonate platform. The Florida Platform during the Late Miocene must also have had a sufficiently wet climate to cause episodic transport of the coarse sediments. Siliciclastic sediment transport on the Florida Platform during the Late Miocene greatly differed from Pleistocene to modern conditions, which are dominated by the transport of fine‐grained sands by longshore marine processes.     

输沙能力模式在坡面非恒定输沙过程中的应用

在坡面土壤侵蚀输沙计算中,可选用的输沙能力模式较多,但这些模式多运用于恒定输沙,在非恒定输沙中运用不多。建立了坡面土壤侵蚀非恒定输沙数学模型,模型中的输沙能力运用水流切应力、水流功率、单位水流功率3种模式,对模型过程采用有限差分格式离散求解。根据实测水沙资料进行模型参数率定,运用3种输沙能力模式于不同降雨强度、不同坡度的非恒定坡面输沙过程中。结果表明:在坡面非恒定输沙计算中,不同输沙能力模式对计算结果有明显影响,在降雨强度较小时,单位水流功率模式结果较其他两个模式为好,而在雨强较大时,切应力模式计算结果较好。     

Modeling the influence of settling velocity on cohesive sediment transport in Tanshui River estuary

Settling velocities of suspended cohesive sediment in estuaries vary over a range of several orders in magnitude. Variations in the suspended sediment concentration are often considered as the principal cause. Turbulence and the suspended sediment concentration, as well as other factors such as salinity, dissolved organic substances, flocculation ability, and the rate of floc growth affect setting velocities. A laterally–averaged finite difference model for hydrodynamics and cohesive sediment transport is developed and applied in the Tanshui River estuary, Taiwan. The model has been calibrated and verified with water surface elevation, longitudinal velocity, salinity, and cohesive sediment measured. The overall performance of the model is in qualitative agreement with the available data. The model is used to investigate the influence of settling velocity on cohesive sediment transport dynamics. The simulation indicates that the turbidity maximum zone is near Kuan–Du. When settling velocities increase the surface cohesive sediment concentration at Kuan–Du station trends to decrease and bottom cohesive sediment concentration increases. Both surface and bottom cohesive sediment concentrations decrease at Taipei Bridge and Pa–Ling Bridge. This implies that suspended sediment advected seaward and deposited. There is consequently a net seaward flux of suspended sediment near surface, and a net landward flux near the bed.     

黄河下游河道输沙水量及计算方法研究

根据黄河下游1950年以来的水沙、河道冲淤及洪水观测资料,系统分析了黄河下游主要控制站输沙水量与来沙量、洪水量级、水沙搭配、区间引水引沙及河道允许淤积度等因子间的相互关系。在探讨泥沙输移规律和机理的基础上,引入水沙搭配参数,建立了适用于黄河下游主要控制站汛期及洪水期计算输沙水量的数学表达式,量化了水沙条件及河道允许淤积度变化对河道输沙水量的影响程度。该研究对维持黄河健康生命及黄河水资源的规划利用具有重要的理论和实际意义。     

黄河下游河道边界条件影响输沙效率研究述评

如何高效输沙始终是黄河下游河道治理的一个根本问题,其中河道边界条件对泥沙输送具有重要的影响,这源于两者之间存在的耦合作用。对现有河道边界条件影响输沙效率相关研究进展及存在的问题进行了系统的总结与分析。河道边界条件不仅影响河道空间沿程输沙效率,而且河道边界条件的演化还决定了输沙效率的时间调整过程。这种影响在不同输沙计算方法(水文学方法、水动力学方法)中的表现形式有所不同,但反映物理本质相同,彼此之间存在一定的联系。能否反映水沙运动与河床变形之间的相互作用及耦合特性,这是判断输沙模拟有效性的关键所在。河道输沙与河床变形作为泥沙赋存状态同一问题的两个方面,提出了从河床演变学角度研究输沙效率的问题,为相关研究的开展提供思路与参考。     

三门峡库区一维非恒定非均匀泥沙输移数学模型

黄河是高含沙河流,含沙量季节差异显著。因此研究黄河干流的非恒定水沙输移规律以及库区泥沙的淤积问题极为重要。应用圣维南方程组以及非恒定泥沙连续方程建立了非恒定非均匀泥沙含沙量计算公式,并根据沙量平衡方程推求出三门峡库区河底高程的变化规律。使用已有的资料对模型进行的验证表明:模型计算与实测资料符合良好,该模型具有较好的应用前景。     

长江上游近期水沙变化特点及其趋势分析

对长江上游主要控制站水沙量进行了统计分析。近期长江上游水沙变化特点主要表现为:与1990年前相比,1991-2002年宜昌站年均减少沙量约1.3亿t;金沙江屏山站年均增加沙量约0.35亿t,其占宜昌站的百分数由51.8%增加到71.9%;嘉陵江北碚站年均减少沙量约0.985亿t,占宜昌站的百分数由25.7%减小至9.1%。采用滑动平均法以及Spearman秩次相关检验和线性回归检验等方法,对屏山、北碚和宜昌等3站水沙变化趋势进行了分析,分析结果表明:宜昌站径流量无趋势性变化,但输沙量有减小趋势;北碚站水沙减小趋势均较为明显;屏山站径流量无趋势变化,但输沙量有一定的增大趋势。另外还对影响长江上游水沙变化的降水量、水土保持措施、水库拦沙以及人类活动增沙等主要因素进行了分析。