Sand Dikes Research Articles

Breaching of sand dikes: assessment of 1D and 2D numerical models against laboratory experiments

Dike failures often induce highly damaging floods. Improved understanding of the inherent mechanisms is important to predict and prevent dike failures. This research focuses on sand dike failure by overtopping. During the experiments, the evolution of nine sections was monitored using laser-sheet while the water level in the upstream reservoir was captured using ultrasonic gauges. The impact of the grain size was studied by comparing a fine and a coarser sand. Even though the failure mechanisms look similar in both cases, some differences could be observed in the evolution of the geometry of the dike. Finally, the failure was simulated with 1D and 2D models. The 1D model shows good results for the initial stage of the dike evolution and in predicting the evolution of the width of the breach. The 2D model predicts the morphological evolution of the dike more accurately but slightly overestimates the widening of the breach.

Cascading hazards of a major Bengal basin earthquake and abrupt avulsion of the Ganges River

Earthquakes present severe hazards for people and economies and can be primary drivers of landscape change yet their impact to river-channel networks remains poorly known. Here we show evidence for an abrupt earthquake-triggered avulsion of the Ganges River at ~2.5 ka leading to relocation of the mainstem channel belt in the Bengal delta. This is recorded in freshly discovered sedimentary archives of an immense relict channel and a paleo-earthquake of sufficient magnitude to cause major liquefaction and generate large, decimeter-scale sand dikes >180 km from the nearest seismogenic source region. Precise luminescence ages of channel sand, channel fill, and breached and partially liquefied floodplain deposits support coeval timing of the avulsion and earthquake. Evidence for reorganization of the river-channel network in the world’s largest delta broadens the risk posed by seismic events in the region and their recognition as geomorphic agents in this and other tectonically active lowlands. The recurrence of comparable earthquake-triggered ground liquefaction and a channel avulsion would be catastrophic for any of the heavily populated, large river basins and deltas along the Himalayan arc (e.g., Indus, Ganges, Brahmaputra, Ayeyarwady). The compounding effects of climate change and human impacts heighten and extend the vulnerability of many lowlands worldwide to such cascading hazards.

Tracking surface and subsurface deformation associated with groundwater dynamics following the 2019 Mirpur earthquake

The Mirpur Mw 5.8 earthquake on September 24, 2019, produced extensive liquefaction-induced surface deformation (LISD) in the surrounding villages. Due to the complexity of seismic hazards and the occurrence of their effects on a large spatial scale, the resulting surface, and subsurface deformation are often poorly resolved. To cover spatially extended LISD, the PSInSAR technique provided subsidence and uplift rate values ranging from −110 to +145 mm/yr consistent with the spatial distribution of the mapped liquefaction features. The most prominent surface change occurred in Abdupur and Sang villages. GPR measurements were conducted to map the near-surface cracks produced by transported liquified sand into the shallow subsurface layers and other liquefaction features (elevated groundwater table, conductive clay pockets, fractures, sand dikes, and water-enriched zones). Thus, the GPR survey assisted in the reconstruction of these structural and hydrogeological features on the near surface. In addition, the highly vulnerable zones were identified and mapped using space- and ground-based remote sensing measurements supported by the field observations. The results highlight the effectiveness of the proposed novel approach for detailed assessment of the coseismic liquefaction-induced deformation on- and near-ground surfaces by identifying areas prone to failure during earthquakes and thereby can help with hazard mitigation.

The influence of sedimentary architecture on the formation of earthquake-induced liquefaction features: A case study in the New Madrid seismic zone

This study is aimed at understanding how the arrangement of fluvial sedimentary deposits and their physical properties influences the formation and location of liquefaction features resulting from earthquake strong ground motions. The study site is located in the Mississippi River floodplain in the New Madrid seismic zone of the central United States, an area that hosts abundant liquefaction deposits resulting from both historic and prehistoric earthquake sequences. We combine satellite imagery, electrical resistivity tomography, soil textural analyses and ditch exposures of liquefaction features to develop a model of how the spatial distribution of sedimentary deposits, such as point bars, overbank, levee, backswamp and channel-fill, contribute to pore pressure build-up and subsequent development of liquefaction features, such as sand dikes and sand blows, as water and entrained sediment forcefully flow toward the surface. The model helps to explain (1) the abundance of earthquake-induced liquefaction features present in the Mississippi River valley, where fine- to medium-grained saturated sands are interbedded with or overlain by fine-grained portions of channel-fill, levee, overbank or backswamp deposits, and (2) the concentration of liquefaction features along the margins of abandoned-channel fill, point bar and overbank deposits, as seen in other locations worldwide. This understanding can contribute to identifying areas where liquefaction may be a threat to engineered structures and limiting the extent of areas in which detailed engineering geology measurements (e.g., in situ testing) may be necessary.

Static Liquefaction Causes the Flow Failure of a Tailings Dam: A Case Study of El Descargador, Cartagena–La Unión Mining Region, SE Spain (October 1963)

The aim of this work is to investigate the causes of the El Descargador tailings dam failure, at the mine district Cartagena–La Unión (SE Spain), in October 1963. Dam stability back analyses have been carried out by applying a geotechnical and geophysical approach. The failure occurred in the form of several landslides in five different points along the dam structure. The rise in the pore pressure and the steeped slopes of the tails, scaling up to 40° in some sectors, were the main causal preparatory factors. Here we propose that static liquefaction is the most plausible cause of the tailings dam flow failure. The presence of sand dikes and sand volcanoes with atypical stratigraphic architecture, both in the lagoon and at the surfaces exposed in the landslide areas, as well as the evidence of conspicuous sand fraction on the surface support the occurrence of the liquefaction processes. Major landslides were located near the drainage pipe and the flow directions were controlled by its position. Our results reveal that the liquefaction processes were triggered and aggravated by the poor drainage capability of the tailings dam structure.

Synthetic modeling and field GPR survey to understand the near-surface deformation induced by coseismic groundwater dynamics following the 2019 Mirpur earthquake in Pakistan

The Mirpur earthquake of Mw 5.8 on 24 September 2019 severely damaged villages in the epicentral area. The earthquake-triggered ground dynamics produced widespread liquefaction features on the surface (e.g., fractured road structure and sand boils) and in the shallow subsurface (e.g., elevated groundwater table, fractures, and sand dikes). Due to complexity of liquefaction-induced deformation and timing of field surveys, the structural and hydrogeologic features that develop in the near surface during seismic events are often poorly resolved on ground-penetrating-radar (GPR) profiles. To track and delineate the internal architecture and coseismic changes in groundwater levels and/or pressures due to liquefaction, a GPR survey was carried out soon after the Mirpur earthquake. The sudden pore water pressure changes in the shallow aquifer system caused a rise in the water table via upward groundwater movement and fracturing followed by rapid infiltration. This cascade phenomenon has adverse effects on soil, water resources, civil infrastructure, and public health and safety. To better understand induced changes in the physical properties of road infrastructure and the shallow subsurface, we performed synthetic modeling of electromagnetic wave propagation by employing the finite-difference time-domain method. Based on the available information at multiple scales (borehole and geophysical), subsurface physical models representing the most common and important coseismic features were considered for synthetic modeling. Likewise, synthetic tests of paved road were conducted to simulate the radar response of structural discontinuities (fractures) developed at different angles. To validate synthetic results by identifying a similar dynamic response of the induced hydrogeologic changes in the form of coseismic liquefaction features, a field GPR survey was conducted. Our results clearly demonstrate that the GPR technique is a potential candidate for hydrogeophysical detection and characterization of features of interest in the case of paved roads as well as agricultural and residential areas. Additionally, the successful application of GPR proves it to be a cost-effective, high-resolution, and speedy tool for better assessment of hydrogeologic hazard, socioeconomic consequences, and mitigation in the Mirpur area. The proposed approach may find applications in other seismically active regions around the world.

Evidence of Seismic-Related Liquefaction Processes within the Volcanic Record of the Campi Flegrei Caldera (Italy)

We report the occurrence of several sand liquefaction structures, such as sand dikes, in the stratigraphic record of the Campi Flegrei volcano, located both inside and outside the caldera. Five sites were analyzed within the caldera and two outside. The grain size analysis of the sand fillings indicates that these deposits are very fine-to-coarse sands generally poorly sorted. All of the granulometry curves fall within the field of the liquefiable, loose sediments. Frequently, dikes are characterized by two fillings: a rim showing poorly sorted finer sands and a core with extremely poorly sorted coarser sediments. We suggest that seismic-related liquefaction processes triggered the injection of these sand dikes during unrest episodes in the last 15 kyr. In particular, the sand dikes located outside the caldera, characterized by larger thicknesses and lengths, mark an important extensional episode, probably associated with the caldera formation during the Neapolitan Yellow Tuff eruption at 15 ka. Furthermore, liquefaction structures within the caldera are related to the seismic activity, probably occurring during the Agnano–Monte Spina caldera formation and the volcano-tectonic ground deformation, predating Epoch 3b (4.3 ka) and the Monte Nuovo (1538 CE) eruptions. This study highlights that these seismic-related liquefaction structures are common within the volcanic record of the Campi Flegrei, suggesting that the sand source can be both the widespread marine succession underlying the Epoch 3 deposits in the caldera central sector and the primary ash layers extensively present in the volcanic record.

The Liquefaction Record of Past Earthquakes in the Central Virginia Seismic Zone, Eastern United States

AbstractFollowing the 2011 moment magnitude, M 5.7 Mineral, Virginia, earthquake, we conducted a search for paleoliquefaction features and found 41 sand dikes, sand sills, and soft-sediment deformation features at 24 sites exposed in cutbanks along several rivers: (1) the South Anna River, where paleoliquefaction features were found in the epicentral area of the Mineral earthquake and farther downstream to the southeast; (2) the Mattaponi and Pamunkey Rivers east of the Fall Line, where liquefiable sediments are more common than in the epicentral area; and (3) the James River and Rivanna River–Stigger Creek, where a few sand dikes were found in the 1990s. Liquefaction features are grouped into two age categories based on dating of host sediment in which they occur and weathering characteristics of the features. A younger generation of features that formed during the past 350 yr are small, few in number, and appear to be limited to the James and Pamunkey Rivers. Though there are large uncertainties in their locations and magnitudes, one or more preinstrumental earthquakes, including the 1758, 1774, and 1875 events, likely caused these features. An older generation of liquefaction features that formed between 350 and 2800 yr ago are larger, more numerous, and more broadly distributed than the younger generation of features. Several earthquakes could account for the regional distribution of paleoliquefaction features, including one event of M 6.25–6.5 near Holly Grove, or two events of M 6.0 near Mineral and M 6.25 near Ashland. Amplification of ground motions in Coastal Plain sediment might have contributed to liquefaction along the Mattaponi and Pamunkey Rivers.

Seismically-Induced Soft Sediment Deformation Structures In and Around Chamoli, Garhwal Himalaya, India

Soft-sediment deformation structures were identified along Nandakini River valley near Chamoli village of Garhwal Lesser Himalaya. The deformation, in each outcrop, is restricted to a single stratigraphic layer bounded by undeformed flat-lying layers, eliminating the influence of slope failure. The structures are multiple liquefaction features like sand dikes, flame structures, pear-drop disturbance and downward warping of beds. The preferred interpretation is that the deformations resulted from earthquake-induced liquefaction and their existence adjacent to the Nandaprayag Fault can be related to episodic seismicity that occurred along this fault. However, more data and dating technique is needed to constrain the timing and distance to the causative seismic source. It is possible that more than one earthquake has effected the deformations.

Sedimentary deformation features as evidence for paleoseismic events in the middle Eocene in the Dongying Depression of the southern Bohai Bay Basin, eastern China

Cores from the middle Eocene sediments of the Dongying Depression of the southern Bohai Bay Basin in east China yield various kinds of sedimentary deformation structures. They include microfolds, load casts, flame structures, ball-and-pillow structures, load-cast ripples, pinch-and-swell structures, boudinage structures, sand dikes, microfaults, and cataclastic breccias. Gravity flows, including turbidites and debris flows, also occur in the study area. The deformation layers can be divided into plastic deformation and brittle deformation types. These develop in a succession composed mainly of dark finely laminated and massive mudstone interbedded with thin sandstones and carbonate rocks that accumulated in a low-energy semi-deep to deep lacustrine environment in a tectonically active setting. Considering the facies attributes in the study area, intrinsically possible trigger mechanisms such as rapid sediment loading and storm currents are absent. Thus, the sedimentary deformation features should be induced by seismic activity, and the same with the gravity flows. These seismites are interpreted to have originated from earthquakes with magnitudes exceeding M 5.6. Basin-controlling faults in the north border of Dongying Depression give rise to the occurrence of seismites. The increasing occurrence frequency of seismites from Es4u to Es3l corresponds with the increase in the activity velocity of the basin-controlling faults. Compared with the seismites developed in other fault depressions in the Bohai Bay Basin, seismites developed in the Dongying Depression are relatively smaller scale and are dominated by microfault layers, relatively more cohesive sediments, and greater distance between seismites and active faults results in the occurrence of these features.

Experimental Study on Sand Dike Breaching by Wave Overtopping

A series of large-scale experiments are conducted to investigate the breaching process of sand dikes under wave overtopping. The dikes are constructed with uniform sand, covered with a thin clay layer on the upstream slope to prevent seepage during breaching. Wave height, wave period and dike slopes are systematically varied to examine their effects on the breaching process. Morphological development of the breach is recorded using a 3D laser scanner. The results indicate that erosion starts at the downstream edge of dike crest and the downstream dike slope gradually flattens. The breach cross-section is observed to be of a trapezoidal shape. Breach flow increases at an accelerating rate after initial overtopping until a peak flow is attained. The breach dimension and peak flow rate increase with increasing wave height, wave period, and upstream and downstream dike slopes. A dimensionless dike breach parameter is proposed to establish regression relations for the breach geometry and flow characteristics.

Coupled hydro-mechanical evolution of fracture permeability in sand injectite intrusions

Sandstone “injectite” intrusions are generally developed by the fluidization of weakly cemented sandstones and their subsequent injection into fractured reservoirs. In this work, a continuum coupled hydro-mechanical model TOUGH-FLAC3D is applied to simulate the discrete fracture network in large-scale sand injectite complexes. A sand production constitutive model is incorporated to consider the formation of sand through plastic deformation and its influence on evolution of fracture permeability. Overpressures in the fluidized sand slurry drives the injection with sand dikes intruded upwards, typically into previously low permeability “tight” mudstone formations. The contrast in poroelastic properties of the underlying weak sandstone and overlying injectite receptor directly affects the evolution of fracture aperture both during and after intrusion. Fluid drainage into the unconsolidated matrix may reduce the extent of fracture aperture growth, through the formation of shear bands. The results of this work have broad implications related to the emplacement of sandstone intrusions and subsequent hydrocarbon accumulation, maturation and then production.

Liquefaction structures induced by the M5.7 earthquake on May 28, 2018 in Songyuan, Jilin Province, NE China and research implication

An earthquake of magnitude M5.7 occurred in Yamutu village, Songyuan City, Jilin Province, NE China (45°16′12″N/124°42′35″E) on May 28, 2018, with a focal depth of 13 km. The epicenter is located at the intersection of the Fuyu/Songyuan-Zhaodong Fault, Second Songhua River Fault and Fuyu North Fault which lies northwest of Tancheng-Lujiang Fault (Tan-Lu Fault). The earthquake-induced widespread liquefaction structures and ground surface fissures within 3 km from the epicenter, caused serious disasters to the local surroundings. The visible liquefied structures include sand volcanoes, liquefied sand mounds, sand dikes and sand sills. Sand volcanoes can be divided into sand volcano with a crater, sand volcano without a crater and water volcano (no sand). Other soft-sediment deformation structures (SSDS) induced by the earthquake include deformation lamination, load and flame structures, deformation folds, dish structures, convolute bedding and water-escape structures. The formation process of the sand volcanoes comprises three stages: (1) building up excess pore-fluid pressure in the liquefied layer, (2) cracking of the low-permeable overlying layer, and (3) mixture of sand-water venting out of the ground surface. During the upward movement, the liquefied sand is injected into the low-permeable layer to form sand veins, sand sills and various types of deformation structures. Vertical distribution of seismic liquefaction structure can be divided into four zones: the thoroughly liquefied zone, the lower liquefied zone with SSDS, the upper liquefied zone with SSDS, and the ground surface liquefied zone. The liquefaction occurred at a burial depth of 2–5 m, and the thickness of liquefied sand is 2 m. NE-SW (35°–215°) trending compressive stress is possibly the seismogenic trigger of the Songyuan M5.7 earthquake that caused the fault (Fuyu/Songyuan-Zhaodong Fault) to reactivate. The study of the Songyuan seismic liquefaction structures gives insight into the prediction of modern earthquakes and disaster-prone areas. Meanwhile it provides abundant basic material for studying earthquake-induced SSDS in both ancient and modern sediments. The research is obviously of great significance to reveal that the northern Tan-Lu Fault has entered a stage of active seismic activity since the twenty-first century.

Radial interpolation of GPS and leveling data of ground deformation in a resurgent caldera: application to Campi Flegrei (Italy)

This study presents a new method, called the Radial Interpolation Method, to interpolate data characterized by an approximately radial pattern around a relatively constrained central zone, such as the ground deformation patterns shown in many active volcanic areas. The method enables the fast production of short-term deformation maps on the base of spatially sparse ground deformation measurements and can provide uncertainty quantification on the interpolated values, fundamental for hazard assessment purposes and deformation source reconstruction. The presented approach is not dependent on a priori assumptions about the geometry, location and physical properties of the source, except for the requirement of a locally radial pattern, i.e., allowing multiple centers of symmetry. We test the new method on a synthetic point source example, and then, we apply the method to selected time intervals of real geodetic data collected at the Campi Flegrei caldera during the last 39 years, including examples of leveling, Geodetic Precise Traversing measurements and Global Positioning System. The maps of horizontal displacement, calculated inland, show maximum values lying along a semicircular annular region with a radius of about 2–3 km in size. This semi-annular area is marked by mesoscale structures such as faults, sand dikes and fractures. The maps of vertical displacement describe a linear relation between the maximum vertical uplift measured and the volume variation. The multiplicative factor in the linear relation is about 0.3 × 106 m3/cm if we estimate the proportion of the ΔV that is captured by the GPS network onland and we use this to estimate the full ΔV. In this case, the 95% confidence interval on K because of linear regression is ± 5%. Finally, we briefly discuss how the new method could be used for the production of short-term vent opening maps on the base of real-time geodetic measurements of the horizontal and vertical displacements.

Paleoliquefaction Studies and the Evaluation of Seismic Hazard

Recent and historical studies of earthquake-induced liquefaction, as well as paleoliquefaction studies, demonstrate the potential usefulness of liquefaction data in the assessment of the earthquake potential of seismic sources. Paleoliquefaction studies, along with other paleoseismology studies, supplement historical and instrumental seismicity and provide information about the long-term behavior of earthquake sources. Paleoliquefaction studies focus on soft-sediment deformation features, including sand blows and sand dikes, which result from strong ground shaking. Most paleoliquefaction studies have been conducted in intraplate geologic settings, but a few such studies have been carried out in interplate settings. Paleoliquefaction studies provide information about timing, location, magnitude, and recurrence of large paleoearthquakes, particularly those with moment magnitude, M, greater than 6 during the past 50,000 years. This review paper presents background information on earthquake-induced liquefaction and resulting soft-sediment deformation features that may be preserved in the geologic record, best practices used in paleoliquefaction studies, and application of paleoliquefaction data in earthquake source characterization. The paper concludes with two examples of regional paleoliquefaction studies—in the Charleston seismic zone and the New Madrid seismic zone in the southeastern and central United States, respectively—which contributed to seismic source models used in earthquake hazard assessment.

Seismically Induced Soft‐Sediment Deformation Phenomena During the Volcano‐Tectonic Activity of Campi Flegrei Caldera (Southern Italy) in the Last 15 kyr

AbstractWe report, for the first time, evidence of seismically induced soft‐sediment deformations in the central area of the active Campi Flegrei caldera (southern Italy). We analyzed the marine‐transitional and continental sequences located along the coastal La Starza cliffs and several stratigraphic logs exposed during the excavation of a 1‐km‐long tunnel in the Pozzuoli area. The successions host several soft‐sediment structures including sand dikes and sand volcanoes, which are largely dated within the 4.55‐ to 4.28‐kyr BP interval. The volcano‐sedimentary sequence, deposited within the Campi Flegrei caldera in the last 15 kyr, is schematically formed by the superposition of three layers with different rheological behaviors; from the base progressing upward we recognize (1) a massive tuff, (2) marine‐transitional sands of the La Starza unit, and (3) a dominance of continental volcanoclastics. We envisage that during unrest episodes of the volcano, which included ground deformation and seismic activity, the whole volcano‐sedimentary pile was deformed through brittle mechanisms with the formation of normal faults. However, the intermediate layer, when subject to seismic shaking, behaved locally as a viscous material facilitating liquefaction processes and lateral spreading deformation. Furthermore, new geophysical, stratigraphic, and structural surveys allowed us to model the deformation evolution of this area over the last 15 kyr. The evidence of seismically induced soft‐sediment deformation within the volcano‐sedimentary record suggests that moderate earthquakes could occur during future volcano‐seismic unrests. Consequently, liquefaction and related gravitational mass movements must be considered as a hazard during these unrest and volcanic crises.

Soft-sediment Deformation Structures and Sand Body Architecture in the Chang 6 Oil Member of the Upper Triassic Yanchang Formation, Southwestern Ordos Basin, China

Soft-sediment deformation (SSD) structures of the Upper Triassic Yanchang Formation are laterally widespread in the Ordos Basin. In the Huachi-Qingyang (H-Q) area of the Ordos Basin, the Chang6 oil member of the Upper Triassic Yanchang Formation is among the most significant Mesozoic oil-bearing strata. It is characterized by the development of reservoir sand bodies. During the depositional evolution of the Chang6 oil member, SSD structures induced by paleo-seismic events developed in the H-Q area in the middle of the basin. The SSD structures developed in the sand bodies of the Chang6 oil member are mainly ball-and-pillow structures, fold structures, sand dikes, irregular convolute stratifications and synsedimentary faults. The architecture of the sand bodies resulted from paleo-seismic events and gravity slumping and mainly include two types of structures: 1) SSD structures driven by paleo-seismic events with normal sedimentation (delta front sand body) (SN-SSD) and 2) SSD structures driven by paleo-seismic events with turbidites (formed by delta-front slumping and re-distribution due to seismic action) (ST-SSD). As a consequence, genetic models of the sand bodies formed by different sedimentation processes are established.

Controls on patterns of liquefaction in a coastal dune environment, Christchurch, New Zealand

Liquefaction features in the geological record are important off-fault markers of moderate to large (>5 Mw) paleoearthquakes. The study of contemporary liquefaction features provides a better understanding of where to find past (paleo) liquefaction features which, if correctly identified and dated, can provide information on the occurrence, magnitude and timing of past earthquakes. This is particularly important in areas with blind active faults. This paper describes liquefaction features in the coastal setting of Christchurch (South Island of New Zealand), and explores the role of liquefaction and fluidization in the surface soil profile and their role in controlling surface ejection. The paper also compares the styles of liquefaction surface manifestation in the alluvial and coastal settings, and the role played by the sedimentary architecture in both environments. This analysis contributes to our understanding of which of the two environments provides a better target for paleoliquefaction studies, and which geomorphic setting within those environments is most likely to host paleoliquefaction features. The coastal setting (in particular, young coastal areas, <300 years old) is especially prone to liquefaction because near surface soils are dominated by well-sorted sandy aeolian deposits and a shallow water table exists. Fluidization within the near surface sandy layer that liquefied and in horizons above, due to upward moving pore water, resulted in a very disrupted soil stratigraphy, making it difficult to identify paleoliquefaction features. Therefore, young coastal areas are not the best target for identifying individual paleoliquefaction events. In comparison, the alluvial setting is characterized by sandy point-bar and channel deposits capped by cohesive overbank and abandoned-channel deposits. As a consequence, sand dikes that form in the alluvial settings are well defined, with preservation of the original soil stratigraphy and distinct cross cutting relationships. These conditions make the alluvial setting, along with older coastal deposits (>1000 years old), a better target for paleoliquefaction studies.

Overtopping induced failure of noncohesive, homogeneous fluvial dikes

AbstractAccurate predictions of breach characteristics are necessary to reliably estimate the outflow hydrograph and the resulting inundation close to fluvial dikes. Laboratory experiments on the breaching of fluvial sand dikes were performed, considering a flow parallel to the dike axis. The breach was triggered by overtopping the dike crest. A detailed monitoring of the transient evolution of the breach geometry was conducted, providing key insights into the gradual and complex processes involved in fluvial dike failure. The breach develops in two phases: (1) the breach becomes gradually wider and deeper eroding on the downstream side along the main channel and (2) breach widening controlled by side slope failures, continuing in the downstream direction only. Increasing the inflow discharge in the main channel, the breach formation time decreases significantly and the erosion occurs preferentially on the downstream side. The downstream boundary condition has a strong influence on the breach geometry and the resulting outflow hydrograph.

Retarding the breaching process of dikes

Dikes constructed from sand generally have a sand core and clay layers on the slopes and the crest to protect the core against erosion. In extreme hydraulic conditions, several failure mechanisms can lead to destruction of the clay layers, exposing the sand core to water. When water overtops the dike and the protective cover of the land-side slope or the crest fails, water flows over the core and erodes the sand. The dike starts to breach, and eventually the land behind the dike is flooded. The rate at which the dimensions of the breach grow influences the rate of inundation of the polder. Reduction of the inundation rate may be achieved by retarding the breaching process. This may reduce the number of casualties, resulting in increasing safety for the inhabitants. In order to achieve, for instance, a safety level ten times higher, mortality has to decrease by a factor of 10. The breaching process can be retarded by reducing the erosion velocity of the sand core. Experiments were executed to investigate the effect on the erosion velocity of adding bentonite to sand. The results of these experiments showed a significant reduction of the permeability and erosion velocity of the sand–bentonite mixtures compared with those of pure sand. The effect of adding bentonite on the breaching process was investigated by applying the Bres model (breach erosion in sand dikes model) to a sand dike tested in a large-scale field experiment (Zwin'94). It was found that adding a small percentage of bentonite reduces the rate of breach growth and the inundation rate in the polder. For the Zwin’94 dike it was determined that 5·4% of bentonite is sufficient to reduce the inundation rate below a threshold value of 0·5 m/h, leading to a significant increase in safety.