Mechanisms of riverbank failure along the Arno River,central Italy

Riverbanks along the Arno River have been investigated with the aims of de?ning the main mechanisms of failure and retreat, their spatial distribution, and their causes. Geomorphological aspects were investigated by a reconnaissance of riverbank processes, for a number (26) of representative sites. Laboratory and in situ tests were then performed on a selected number of riverbanks (15). Based on the material characteristics, six main typologies of riverbanks have been de?ned, with homogeneous ?ne‐grained and composite banks representing the most frequent types. Slab‐type failures are the most frequent mechanism observed on ?ne‐grained banks, while cantilever failures prevail on composite banks. The role of river stage and related pore water pressure distributions in triggering the main observed mechanisms of failure has been investigated using two different types of stability analysis. The ?rst was conducted for 15 riverbanks, using the limit equilibrium method and considering simpli?ed hypotheses for pore water pressure distribution (annulment of negative pore pressures in the portion of the bank between low water stage and peak stage). Stability conditions and predicted mechanisms of failure are shown to be in reasonably good agreement with ?eld observations. Three riverbanks, representative of the main alluvial reaches of the river, were then selected for a more detailed bank stability analysis, consisting of: (a) de?nition of characteristic hydrographs of the reach with different return periods; (b) modelling of saturated and unsaturated ?ow using ?nite element seepage analysis; and (c) stability analysis with the limit equilibrium method, by adopting pore water pressure values derived from the seepage analysis. The results are compared to those obtained from the previous simpli?ed analysis, and are used to investigate the different responses, in terms of stability, to different hydrological and riverbank conditions. Copyright © 2003 John Wiley & Sons, Ltd.     

Monitoring and modelling of pore water pressure changes and riverbank stability during flow events

Pore water pressures (positive and negative) were monitored for four years (1996–1999) using a series of tensiometer‐piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to ?ow events and their effects on bank stability. The saturated/unsaturated ?ow was modelled using a ?nite element seepage analysis, for the main ?ow events occurring during the four‐year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern‐Price), using pore water pressure distributions obtained by the seepage analysis. The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of ?ow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the con?ning pressure of the river during the initial drawdown were responsible for triggering the mass failure. Results deriving from the seepage and stability analysis of nine selected ?ow events were then used to investigate the role of the ?ow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main one. Copyright © 2004 John Wiley & Sons, Ltd.     

Signal crayfish burrowing,bank retreat and sediment supply to rivers: A biophysical sediment budget

Burrowing into riverbanks by animals transfers sediment directly into river channels and has been hypothesised to accelerate bank erosion and promote mass failure. A field monitoring study on two UK rivers invaded by signal crayfish (Pacifastacus leniusculus) assessed the impact of burrowing on bank erosion processes. Erosion pins were installed in 17 riverbanks across a gradient of crayfish burrow densities and monitored for 22 months. Bank retreat increased significantly with crayfish burrow density. At the bank scale (<6 m river length), high crayfish burrow densities were associated with accelerated bank retreat of up to 253% and more than a doubling of the area of bank collapse compared with banks without burrows. Direct sediment supply by burrowing activity contributed 0.2% and 0.6% of total sediment at the reach (1.1 km) and local bank (<6 m) scales. However, accelerated bank retreat caused by burrows contributed 12.2% and 29.8% of the total sediment supply at the reach and bank scales. Together, burrowing and the associated acceleration of retreat and collapse supplied an additional 25.4 t km⁻¹ a⁻¹ of floodplain sediments at one site, demonstrating the substantial impact that signal crayfish can have on fine sediment supply. For the first time, an empirical relation linking animal burrow characteristics to riverbank retreat is presented. The study adds to a small number of sediment budget studies that compare sediment fluxes driven by biotic and abiotic energy but is unique in isolating and measuring the substantial interactive effect of the acceleration of abiotic bank erosion facilitated by biotic activity. Biotic energy expended through burrowing represents an energy surcharge to the river system that can augment sediment erosion by geophysical mechanisms.     

流滑型崩岸河岸侧蚀模式初探

流滑型崩岸是冲积河流河岸常见的一种崩塌形式,其发展速度快、破坏力强;该类崩岸主要取决于水流的直接冲刷,河岸崩退过程中水面与岸顶高差较小.依据此类崩岸的特征,基于水流冲刷分析,在基础层面上初步探讨并建立了流滑型崩岸的河岸侧蚀模式.结合水槽试验与土工离心模拟试验,对该模式进行了分析研究,初步率定了模式中的河岸侧蚀系数,计算...     

渗流作用下的江河岸坡冲刷变形分析

岸坡地形随河流上游来水来沙、河势变化等因素而不断变化,直接影响岸坡稳定性。江河水流、岸坡渗流均对岸坡水下地形变化有很大的影响。通过分析泥沙受力条件和冲刷特性,应用在室内水槽进行的不同泥沙粒径在多种水流条件下的冲刷率试验研究成果,在岸坡泥沙受力分析基础上推导了崩岸冲刷率公式,并以算例具体说明江河水流和岸坡渗流对其冲刷变形过程的影响。     

长江江都嘶马段岸崩灾害的形成机理与防治对策

江岸稳定性取决于最大崩塌临界机制的驱动力和抵抗力的平衡,二者的平衡又受控于岸边坡形态、岩土组成、水流浪涌特征、植被覆盖等。崩岸是长江嘶马弯道床演变的一个重要方面。通过对嘶马弯道遥感解译图像资料进行对比分析,查清研究区江岸岸线特征及其变迁历程。为了揭示北岸崩塌产生的原因,通过野外踏勘、测量与钻探等技术方法查明该区地质基础条件,并基于实时动态定位技术(Real Time Kinematic,RTK)的水下测量数据建立典型剖面,分析凸岸和凹岸的水流运动特征及其对边岸的影响。从地质基础和水动力条件2个方面揭示嘶马段江岸坍塌形成机理,即具有上细下粗二元结构的边岸地层是岸崩形成的基础,丁坝回流和弯道螺旋流的掏蚀作用是岸崩形成的重要条件。并基于江岸稳定性现状及其发展趋势,提出相应的江岸整治措施。     

1977年以来黄河临河段河岸冲淤变化及河道萎缩速率

不同时段河道的侧向侵蚀/加积面积变化的定量研究可以揭示河道的变化特征。以黄河临河段213 km长的河段为研究对象,利用1977年以来19个时段基于遥感影像绘制的河道平面形态图的面积变化来估算其4个亚河段（S1、S2、S3和S4）在不同时段的河岸侧向侵蚀/加积面积以及全河段的河道平均萎缩速率。研究结果表明,黄河临河段左右河岸在37年间都表现为侧向净加积,其4个亚河段左岸的侧向累计加积面积分别为33.16 km²、49.59 km²、29.52 km²和30.85 km²,其中1995-2000年的加积面积分别占到其总加积面积的85.5%、51.2%、47.2%和104.6%;右岸的侧向累计加积面积分别为30.83 km²、8.74 km²、26.44 km²和18.76 km²,而1995-2000年的加积面积分别占到其总加积面积的57.2%、111.9%、65.7%和61.6%。该河段河道面积1977-2001年具有减小趋势,2001年之后河道表现为侧向侵蚀、加积的交替变化,1977-2014年间河道平均萎缩速率为6.16 km²/yr。该河段河道平面形态值最明显的变化也发生在1990s,与1995年相比,2000年的河道长度增加了5.8%,河道面积减少了39.4%,河道平均宽度减小了42.8%,弯曲系数增加了6.6%。黄河临河段河道形态剧烈变化及河道严重萎缩都发生在1990s,这主要是黄河上游刘家峡和龙羊峡水库联合运行导致汛期水沙量大量减小所致,4个亚河段的河岸冲淤变化还受到局部河岸物质结构、护岸工程及水动力差异的影响。随着2000年后河流综合管理措施的调整,黄河临河段河道的上述变化趋势明显弱化,河流健康程度有所好转。     

Nitrate transport in the unsaturated zone: a case study of the riverbank filtration system Karany,Czech Republic

Nitrate transport in the unsaturated zone of a riverbank filtration (RBF) system in Karany, Czech Republic, was studied. Previous study of the system estimated RBF recharge as 60% riverbank filtrate and 40% local groundwater contaminated by nitrates. Nitrate concentrations observed in RBF recently cannot be explained by simple groundwater contamination and a new conception of groundwater recharge is suggested. A two‐component model based on water ¹⁸O data modelled recharge of local groundwater. One component of groundwater recharge is rainfall and irrigation water moving through the unsaturated zone of the Quaternary sediments in piston flow. The second component is groundwater from the Cretaceous deposits with a free water table. Both the components of groundwater recharge have different nitrate concentrations, and resulting contamination of groundwater depends on the participation of water from Quaternary and Cretaceous deposits. Nitrates' origins and their mixing in the subsurface were traced by ¹⁵N data. Nitrate transport from the unsaturated zone is important and time variable source of groundwater contamination. Copyright © 2011 John Wiley & Sons, Ltd.     

Groundwater flow path determination during riverbank filtration affected by groundwater exploitation: a case study of Liao River,Northeast China

The groundwater flow path plays an important role in maintaining hydrological and ecological quality and security, which are important in the comprehensive management and use of both groundwater and surface water. In this study, an integrated multi-tracer-constrained framework was used to determine the groundwater flow path. The results show that there are shallow and deep flow paths in riverbank filtration, controlled by the different permeabilities of riverbed sediments and aquifers at different depths. The contribution of river water to shallow groundwater is less than that to deep groundwater because of the low permeability of the riverbed sediment in the dense muddy layer in the shallow slope of the river valley. This contribution decreases with increasing distance from the Liao River. The shallow groundwater quality is better than the deep groundwater quality because of its longer residence time.     

Modelling of the retreat process of composite riverbank in the Jingjiang Reach using the improved BSTEM

Bank retreat in the Jingjiang Reach is closely related not only to the near‐bank intensity of fluvial erosion but also to the composition and mechanical properties of bank soils. Therefore, it is necessary to correctly simulate bank retreat to determine the characteristics of fluvial processes in the Jingjiang Reach. The current version of bank stability and toe erosion model (5.4) was improved to predict riverbank retreat, by inputting a dynamic water table, and calculating the approximation of the distribution of dynamic pore water pressure in the soil near the river bank face, and considering the depositional form of the failed blocks, which is assumedly based on a triangular distribution, with the slope approximately equalling the stable submerged bank slope and half of collapsed volume deposited in the bank‐toe region. The degrees of riverbank stability at Jing34 were calculated using the improved bank stability and toe erosion model. The results indicate the following trends: (a) the degrees of riverbank stability were high during the dry season and the rising stage, which led to minimal bank failure, and (b) the stability degrees were low during the flood season and the recession stage, with the events of bank collapse occurring frequently, which belonged to a stage of intensive bank erosion. Considering the effects of bank‐toe erosion, water table lag, and the depositional form of the collapsed bank soil, the bank‐retreat process was simulated at the right riverbank of Jing34. The model‐predicted results exhibit close agreement with the measured data, including the total bank‐retreat width and the collapsed bank profile. A sensitivity analysis was conducted to determine the quantitative effects of toe erosion and water table lag on the degree of bank stability. The calculated results for toe erosion indicate that the amount of toe erosion was largest during the flood season, which was a primary reason for bank failure. The influence of water table lag on the degree of stability demonstrates that water table lag was an important cause of bank failure during the recession stage.