Riverbank Failure Research Articles

Morphological changes of Gumara River channel over 50 years, upper Blue Nile basin, Ethiopia

Summary In response to anthropogenic disturbances, alluvial rivers adjust their geometry. The alluvial river channels in the upper Blue Nile basin have been disturbed by human-induced factors since a longtime. This paper examines channel adjustment along a 38-km stretch of the Gumara River which drains towards Lake Tana and then to the Blue Nile. Over a 50 years period, agriculture developed rapidly in the catchment and flooding of the alluvial plain has become more frequent in recent times. The objectives of this study were to document the changes in channel planform and cross-section of the Gumara River and to investigate whether the changes could have contributed to the frequent flooding or vice versa. Two sets of aerial photographs (1957 and 1980) were scanned, and then orthorectified. Recent channel planform information was extracted from SPOT images of 2006 and Google Earth. Channel planform and bed morphology (vertical changes) were determined for these nearly 50 years period. The vertical changes were determined based on aggradation along a permanent structure, historic information on river cross-sections at a hydrological gauging station, and field observations. The results indicate that the lower reach of Gumara near its mouth has undergone major planform changes. A delta with approx. 1.12 km 2 of emerged land was created between 1957 and 1980 and an additional 1 km 2 of land has been added between 1980 and 2006. The sinuosity of the river changed only slightly: negatively (−1.1% i.e. meandering decreased) for the period from 1957 to 1980 and positively (+3.0%) for the period 1980–2006. Comparison of cross-sections at the hydrological gauging station showed that the deepest point in the river bed aggraded by 2.91 m for the period 1963–2009. The importance of sediment deposition in the stream and on its banks is related to land degradation in the upper catchment, and to artificial rising of Lake Tana level that creates a backwater effect and sediment deposition in Gumara River. Direct anthropogenic impacts (irrigation activities and building of dykes along the river banks) have contributed to the huge deposition in the river bed. Where the abstraction of water for irrigation is intensive, seepage water through the banks has contributed to river bank failure. In general, this study showed that changes to the planform at the mouth of the river and to the riverbed level are substantial. Moreover, the study indicated that the flood carrying capacity of the Gumara River channel has diminished in recent times.

SEEPAGE FLOW-STABILITY ANALYSIS OF THE RIVERBANK OF SAIGON RIVER DUE TO RIVER WATER LEVEL FLUCTUATION

The Saigon River, which flows through the center of Ho Chi Minh City, is of critical importance for the development of the city as forms as the main water supply and drainage channel for the city. In recent years, riverbank erosion and failures have become more frequent along the Saigon River, causing flooding and damage to infrastructures near the river. A field investigation and numerical study has been undertaken by our research group to identify factors affecting the riverbank failure. In this paper, field investigation results obtained from multiple investigation points on the Saigon River are presented, followed by a comprehensive coupled finite element analysis of riverbank stability when subjected to river water level fluctuations. The river water level fluctuation has been identified as one of the main factors affecting the riverbank failure, i.e. removal of the balancing hydraulic forces acting on the riverbank during water drawdown.

Modeling Interactions between Riverbank Hydrology and Mass Failures

AbstractRiverbank retreat derives from a complex combination of various processes where a key role is played by the interactions of groundwater and surface water. Recent progress has been made in two main research areas, i.e., the effects of hydrological factors on mass failures, and the role of subsurface flow in seepage erosion and bank stability. This paper aims to review recent progress and identify the main knowledge gaps in modeling riverbank failures, with a particular focus on the interactions between mass failures, bank hydrology and other hydrological factors related to the river hydrograph. Changes in pore water pressures related to transient variations of surface water and groundwater are widely recognized as one of the most important factors controlling the onset and timing of bank instability. Inclusion of negative pore water pressures and groundwater flow modeling has greatly improved the capability to predict mass failures. Fluvial erosion has a fundamental role in deforming the bank profi...

Mathematical modeling of flooding due to river bank failure

Modeling of flooding events resulting from bank overflooding and levee breaching is of relevant social and environmental interest. Two-dimensional (2D) hydrodynamic models integrating the shallow water equations turn out to be very effective tools for the purpose at hand. Many of the available models also use 1D channel elements, fully coupled to the 2D model, to simulate the flow of small channels dissecting the urban and rural areas, and 1D elements, referred to as 1D-links, to efficiently model the flow over levees, road and rail embankments, bunds, the flow through control gates, either free or submerged, and the operation of other hydraulic structures. In this work we propose a physically-based 1D-link to model breach formation and evolution in fluvial levees, and levee failure due to either piping or overtopping. The proposed 1D-link is then embedded in a 1D–2D hydrodynamic model, thus accounting for critical feedbacks between breach formation and changes in the hydrodynamic flow field. The breach model also includes the possibility of simulating breach closure, an important feature particularly in the view of hydraulic risk assessment and management of the emergency. The model is applied to five different case studies and the results of the numerical simulations compare favorably with field observations displaying a good agreement in terms of urban and rural flooded areas, water levels within the channel, final breach widths, and water volumes flowed through the breach.

Study on the effects of vegetation density in reducing bed shear stress on the downstream slope of earthen embankment

River bank, coastal erosion and embankment failures are endemic and recurrent natural hazards, causing loss of lands and livelihoods along major rivers and coastlines. Overflow is the most damaging factor for earthen embankments on the downstream (d/s) slope due to the erosion of surface material which is caused by the high bed shear stress and supercritical flow over the entire d/s slope. Vegetation planted on d/s slope may have a considerable effect in reducing bed shear stress which makes erosion. However,the influence of vegetation for flow resistance as well as bed shear stress control has been studied a little. Therefore, the aim of this research is to introduce the use of natural resources like vetiver grass for controlling bed shear stress which actually causes embankment erosion starts from the high flow region that is, in supercritical flow state. A small-scale experiment is performed with four different densities of vegetal cover considering different discharges on d/s slope. Vegetative barrier affect the mean flow rate by induced drag force against hydraulic forces. Hydraulic characteristics and mechanism of vegetation in open channel during overflow are studied through flume experiment. Results demonstrated that vegetation markedly reduced the effective bed shear stresses which are the influencing factor for causing erosion on the d/s slope, toe and bed of embankment. Results also revealed a significant reduction of damage due to erosion and risk of failure from overtopping flow can be achieved by providing dense vegetation cover on the d/s slope of earthen embankment. Key words: Embankment, downstream slope, drag force, bed shear stress, vegetation density.

Spatio-temporal changes in river bank mass failures in the Lockyer Valley, Queensland, Australia

Wet-flow river bank failure processes are poorly understood relative to the more commonly studied processes of fluvial entrainment and gravity-induced mass failures. Using high resolution topographic data (LiDAR) and near coincident aerial photography, this study documents the downstream distribution of river bank mass failures which occurred as a result of a catastrophic flood in the Lockyer Valley in January 2011. In addition, this distribution is compared with wet flow mass failure features from previous large floods. The downstream analysis of these two temporal data sets indicated that they occur across a range of river lengths, catchment areas, bank heights and angles and do not appear to be scale-dependent or spatially restricted to certain downstream zones. The downstream trends of each bank failure distribution show limited spatial overlap with only 17% of wet flows common to both distributions. The modification of these features during the catastrophic flood of January 2011 also indicated that such features tend to form at some ‘optimum’ shape and show limited evidence of subsequent enlargement even when flow and energy conditions within the banks and channel were high. Elevation changes indicate that such features show evidence for infilling during subsequent floods. The preservation of these features in the landscape for a period of at least 150years suggests that the seepage processes dominant in their initial formation appear to have limited role in their continuing enlargement over time. No evidence of gully extension or headwall retreat is evident. It is estimated that at least 12 inundation events would be required to fill these failures based on the average net elevation change recorded for the 2011 event. Existing conceptual models of downstream bank erosion process zones may need to consider a wider array of mass failure processes to accommodate for wet flow failures.

Coupling effect of seepage flow and river flow on the bank failure

On the basis of the generalized physical model of the riverbank, the experiments were conducted to study the mechanisms of riverbank failure under the coupling effect of seepage flow and river flow. The experimental setup was specially designed, as well as test point location, parameters and procedures, and the main influencing factors were analyzed affecting riverbank failure based on the failure types, the variations of pore water pressure and soil displacement. The results indicated that the coupling effect has different influences on the bank failure in three aspects: the failure type, the process and the extent. In addition, the river flow played a more important role than the seepage flow in the coupling effect on the bank failure.

River Embankment and Bank Failure: A Study on Geotechnical Characteristics and Stability Analysis

Problem statement: Embankment failure and riverbank erosion are common problem in Bangladesh. Almost every year earthen embankments and riverbanks are facing problems like erosion, breaching or retirements. Among many reasons the major causes are considered due to the use of geotechnically unstable materials, improper method of construction, seepage and sliding. In this study the problem is considered geotechnical point of view where the geotechnical properties of failed Jamuna river embankment material and Padma riverbank material were investigated. Moreover, stability analysis technique of embankment has been reviewed through a case study of Manu river embankement. Approach: Sample materials were collected during field investigation and tested at laboratory according to Japanese Industrial Standard (JIS). Limit equilibrium stability analysis and steady state seepage analysis was conducted for Manu river embankment to review the existing design procedure of embankment. Results: Study results reveal that the soil of Jamuna river embankment is not well graded sand and the permeability is found minimum of 1.29×10-5 cm sec−1 (at w = 24%) which increases rapidly in submerge condition. The maximum strength is found 51.8 kN m−2 which is not preferable as embankment material. Moreover, the slope is not well protected that makes the embankment vulnerable to erosion. In contrast, the soil of Padma riverbank contains mostly sand with 25% clay content. Both permeability and strength of bank material decrease rapidly with the increase of water content. Nevertheless, tension crack and toe erosion also accelerate the mass failure mechanism of riverbank. From case study the Factor of Safety (FS) is found overestimated of about 22-24% in stability analysis of embankment in usual practice. Conclusion: Embankment soil needs to be improved geotechnically to minimize mass failure. Geo-bags, cement composites with reinforcement could be used for slope protection. To obtain reliable factor of safety seepage analysis is recommended in designing stable embankment.

REPRODUCIBILITY OF SMALL SCALE HYDRAULIC EXPERIMENT ON CROSS-LEVEE BREACH BY OVERFLOW

River bank failure causes a heavy disaster. Many experiments had been conducted. Full-scale hydraulic experiment is very few, because of it has a limit to conduct in terms of space and budget. Therefore, many experiments were conducted in small scale and they need to be compared with the actual phenomenon. The large scale hydraulic experiment on river bank failure was conducted at Chiyoda experiment flume in 2008. We conducted small scale experiments on river bank failure and examined the reproducibility of the small scale experiments by comparison with the Chiyoda experiment. Fine-grained soil strongly influences the widening process of bank failure. In order to reproduce the results of experiment at Chiyoda experimental flume on dike failure, it is necessary to remove the fine material from the material of scaled dikes. And it is found that the time scale of widening process of bank failure follows the Froude similarity law.

An analytical method to estimate failure plane angle and tension crack depth for use in riverbank stability analyses

Of the many different types of riverbank failure, the planar failure mechanism is perhaps the most common, being associated with steep, relatively low banks composed of cohesive sediments. To reliably analyze the stability of riverbanks against planar failures many parameters (including the failure plane angle and the depth of the tension crack) have to be determined. In this paper we introduce a new analytical method to estimate the failure plane angle. This is achieved by employing a combination of field and laboratory data to first derive a set of empirical curves to estimate the tension crack depth. The estimated tension crack depth is then used in a bank stability analysis, from which the failure plane angle can be evaluated. Based on observations from field and laboratory tests, we found that the new model provides a mean relative error ( MRE) of 4%, 23%, and 27% between calculated and observed values of failure plane angle, tension crack depth, and bank-top retreat, respectively. Based on a comparison of these errors with those derived from a range of existing equations, we show that this represents a considerable improvement relative to prior studies. The new model can therefore be used to determine the geometrical parameters of the failed blocks of riverbanks subject to planar failure.

Conjunction effect of stream water level and groundwater flow for riverbank stability analysis

Variations in pore-water pressure determined by the groundwater table within a riverbank have been investigated and recognized as an essential factor in determining riverbank stability with respect to mass failure. However, the effect of pore-water pressure is taken into account for most of the existing riverbank stability models under some simplified assumptions, and the limitations of predicting ability may arise. To avoid the unrealistic estimation of pore-water pressure distribution, the new approach proposed here is to couple riverbank stability with groundwater flow modeling, and apply this to tackle the conjunction effect between river stage and groundwater table. Moreover, riverbank material characteristics and the influence of infiltration can be taken into consideration via groundwater flow modeling. Two hypothetical examples, stage rising and stage falling, are used to investigate the capabilities of the present study and two representative methods. The simulated results show that riverbank failure is triggered particularly during the falling stage, which has been pointed out by other researchers as well. Furthermore, the riverbank material characteristics predominantly control the occurrence of failure and should be considered regarding assessment of riverbank stability. Additionally, the effects of parameters indicate that riverbanks with soil properties of low permeability or high specific yield with great infiltration intensity during the falling stage have a tendency to riverbank failure.

Mechanisms of riverbank failure along the Arno River, central Italy

AbstractRiverbanks along the Arno River have been investigated with the aims of defining 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 defined, with homogeneous fine‐grained and composite banks representing the most frequent types. Slab‐type failures are the most frequent mechanism observed on fine‐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 first was conducted for 15 riverbanks, using the limit equilibrium method and considering simplified 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 field 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) definition of characteristic hydrographs of the reach with different return periods; (b) modelling of saturated and unsaturated flow using finite 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 simplified 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.

Did Nile flooding sink two ancient cities?

The discovery of the two cities of Herakleion and East Canopus under the waters of the Bay of Abu Qir (east of Alexandria, Egypt) stirred worldwide attention when it was first announced in the summer of 2000. Their disappearance some 1,250 years ago has been ascribed by Stanley, Goddio and Schnepp to a strong Nile flood that caused riverbank failure and the destruction of the two cities, rather than to the action of earthquakes, as was first proposed when the ruins were discovered. But I believe that this interpretation is flawed, because no flood could have reached the Abu Qir Bay at the time of the disappearance of the two cities, as the Canopic branch of the Nile, along whose banks they were situated, had dried to a trickle more than 200 years earlier.

Prediction of tension crack location and riverbank erosion hazards along destabilized channels

AbstractRiverbank erosion, associated sedimentation and land loss hazards are a land management problem of global significance and many attempts to predict the onset of riverbank instability have been made. Recently, Osman and Thorne (1988) have presented a Culmann‐type analysis of the stability of steep, cohesive riverbanks; this has the potential to be a considerable improvement over previous bank stability theories, which do not account for bank geometry changes due to toe scour and lateral erosion. However, in this paper it is shown that the existing Osman‐Thorne model does not properly incorporate the influence of tension cracking on bank stability since the location of the tension crack on the floodplain is indirectly determined via calculation or arbitrary specification of the tension crack depth. Furthermore, accurate determination of tension crack location is essential to the calculation of the geometry of riverbank failure blocks and hence prediction of land loss and bank sediment yield associated with riverbank instability and channel widening. In this paper, a rational, physically based method to predict the location of tension cracks on the floodplain behind the eroding bank face is presented and tested. A case study is used to illustrate the computational procedure required to apply the model. Improved estimates of failure block geometry using the new method may potentially be applied to improve predictions of bank retreat and floodplain land loss along river channels destabilized as a result of environmental change.

Geotechnical and environmental problems of the Niger delta

This paper examines the geotechnical and environmental problems arising from the peculiar characteristics (topography, geology, soil types including their properties and their areal distribution) of the Niger Delta area. The area is characterised by the widespread and irregular distribution of weak soils whose strength is further reduced by the presence of expansive clays. This causes foundation problems in roads, houses and embankments. Excavations are unstable because they are either made in very permeable sands which are subject to enormous seepage pressures or in weak, fat clays of low shear strength. Environmental problems include flooding, poor water quality and excessive river bank erosion and recession. While the surface waters are very turbid due to high clay and silt content the ground waters are very acidic (pH 4.5–5.5) and contain high proportions of nitrates, ammonia and chlorides which contaminate aquifers especially in the coastal and central portions of the delta area. River bank failure and recession of up to 2 to 3 m occur annually and continue to pose an enormous threat to the villages along the river banks.

Analysis of stratigraphic control on river bank failure : Okagbue, C O; Abam, T K S Engng Geol V22, N3, April 1986, P231–245

Analysis of stratigraphic control on river bank failure : Okagbue, C O; Abam, T K S Engng Geol V22, N3, April 1986, P231–245

An analysis of stratigraphic control on river bank failure

Most of the rivers of the Niger Delta flow through alluvial deposits whose general stratigraphy is that of cohesionless sand overlain by cohesive silty clay. Erosion and recession of the river banks have constituted a threat to many villages along the various creeks in the delta area. Analyses of the recessional mechanisms indicate that bank failure is initiated by a fluvial erosion of the lower cohesionless bank layer at a much faster rate than the upper cohesive bank layer. This leads to development of overhangs of the upper cohesive materials. The failure mechanism of the river bank is complex, ranging from slip failures in high banks to shearing and toppling failures in low banks. The use of the common slip circle model for stability evaluation appears to be adequate only for high banks. Results obtained for the low banks suggest that the use of this model of stability analyses for low banks may yield misleading results.

Control of 1973 Mississippi River Flood

In 1973 a major flood occurred on the Mississippi River. Much of the alluvial valley of the river between Alton, Ill. and the Gulf of Mexico is protected against floods by Federally constructed flood control works. As some of these systems were not complete at the onset of the flood, a major flood fight was required to safeguard the valley. Many miles of levee had to be raised to provide proper protection. Other problems requiring flood-fight efforts were seepage under levees, scour at levees and structures, and riverbank failures. Even though major emergency efforts were required, the Federal flood control systems were effective and of great value in controlling the flood. The projects prevented the inundation of 26,000 sq miles (67,500 km²) of land and reduced flood damages by over –14,000,000,000. However, the losses sustained indicate the need to complete authorized works and possibly expand existing projects or initiate new projects in some areas, or both.

Mississippi River Bank Failure, Fort Jackson, Louisiana

Failures in Deltaic Plain sediments have resulted in bank recession in a slowly growing bend-way of the Mississippi River near Fort Jackson, Louisiana. Hydrographic surveys dating back to the early 1930's show that failures occur at a rate of one per 12–15-year period along this bend. Each failure destroys an area about 150–200 feet wide and 1000 feet long, thus threatening the levee on the south bank of the river. Comparison of borings made in the laded and unfailed parts of the riverbank indicates that deep-seated shear failures take place, resulting in lateral wedge-type failures initiated by progressive over-steepening of the bank slope by river erosion. Evidence of physical disturbances in the cohesive, fine-grained deltaic sediments in the unfailed bank suggests that incipient weaknesses may contribute to failures along this bend.