Fjola Gudrun Sigtryggsdottir

The breaching of embankments have devastating consequences for the economic, human, cultural, and environmental assets. One of the most widely used approaches for understanding the characteristics of embankment breaching is through laboratory and field-scale experiments. Despite the advancements in instrumentation and measurement techniques of embankment breaching experiments, there is a lack of comprehensive documentation. In this review, the advancements and state-of-the-art instrumentation and measurement techniques employed in overtopping-induced embankment breaching of laboratory and field-scale experiments are discussed. The key parameters commonly measured in experimental modeling are breach morphological changes, reservoir and flow depth, velocity, breach outflow, and pore water pressure. Instrumentation for breach morphological change detection, including mechanical, photography, photogrammetry, electronic sensors, and laser technologies, are reviewed. The various flow velocity measuring techniques, such as Particle Tracking Velocimetry (PTV), Particle Imaging Velocimetry (PIV), acoustic, and radar-based techniques, are discussed. Instrumentation for water level, flow rate, and pore pressure measurements are also briefly documented. The challenges and constraints encountered during embankment breaching experiments are discussed. The review further suggests future perspectives in improving the accuracy of breach detection, velocity, and pore pressure measurement techniques. Additionally, improving scale effects by incorporating geotechnical factors is also recommended.

The 2021 Geldingadalir eruption in the Mt Fagradalsfjall Volcanic System within the Reykjanes Peninsula volcanic zone was the first eruption on the peninsula in about 800 years. Prior to the eruption, increased earthquake activity and signs of magma intrusion indicated a volcanic threat to populated areas and important infrastructures. Preliminary design principles were developed for protection works comprising lava barriers, earthen diversion barriers, or dams, respectively, for diversion or delaying lava flow. During the 2021 event, three dams were constructed from in situ earth material, along with a 300-m-long diversion barrier, accompanied by a short 35-m diversion barrier. The barriers constructed reflected the site conditions, available material, and equipment. The article describes the dam construction and the experience in securing the construction site, as well as diverting, containing, and delaying lava flow by the different barriers, for potentially reducing/delaying downstream effects. The importance of considering the lava type in the design of lava barriers is highlighted. ʻAʻā lava thickened considerably behind a barrier with influx of fresh lava under chilled outer crust, while sheets of pāhoehoe lava accumulated and eventually overtopped or bypassed it. Generally, pāhoehoe lava creeping slowly over barriers did not cause erosion or damage. However, pāhoehoe lava could cause failure on a downstream slope in the case of weak dam tops of loose material. The dams delayed lava flow, one of them by up to 16 days, and the diversion barriers diverted lava effectively. The lessons learned were valuable for constructing lava defences in subsequent eruptions closer to populated areas, starting in 2023.

Dam failures are examples of man-made disasters that have stimulated investigation into the processes related to the failure of different dam types. Embankment dam breaching during an overtopping event is one of the major modes of failure for this dam type, comprising both earthfill and rockfill dams. This paper presents the results of a series of laboratory tests on breach initiation and progression in rockfill dams. Especially eight breaching tests of 1 m-high 1:10 scale embankment dams constructed of scaled well-graded rockfill were conducted. Tests were performed with and without an impervious core and under different inflow discharges. Controlling instrumentation includes up to nine video cameras used for image analysis and photogrammetry. A previously little-used technique of dynamic 3D photogrammetry has been applied to prepare 3D models every 5 s throughout the breaching process, allowing us to track in detail breach development. These dynamic 3D models along with pressure sensor data, flow data, and side-view video are used to provide data on erosion rates throughout the breaching process. One important purpose of this research is to test methods of observing a rapidly changing morphology such as an embankment dam breach that can easily be scaled up to large-scale and prototype-scale tests. The resulting data sets are further intended for the verification of existing empirical and numerical models for slope stability and breach development as well as the development of new models.

There are increasing demands from dam safety regulations and guidelines to upgrade the rockfill dams, especially in Norway where over 180 large rockfill dams are present. To protect the hydraulic structure against overtopping events or leakages, it is important to use defence mechanisms such as a protective layer of riprap on the downstream slope. In this article, we display 9 experimental setups of riprap, conducted at the hydraulic laboratory of NTNU (Trondheim) and subjected to overtopping phenomena with increasing water discharge, until the complete failure of the model. These tests were performed on models with dumped and placed riprap, with or without toe support, with or without the downstream rockfill shoulder, and finally on models with a full dam profile. The models with downstream rockfill shoulder as well as with full dam profiles allowed for throughflow. The model behaviour during these experimental tests is described and discussed, according to their respective critical discharge values and associated failure mechanisms. Limitations are also discussed. The results bring to light the benefit of placed riprap compared to dumped riprap structures. As the results show a placed riprap can withstand a significantly higher overtopping discharge than a dumped riprap. Also, the use of toe support enables a significant increase of resistance against overtopping of placed riprap structures. However, toe supports have not proven any significant improvement in stability for dumped riprap structures. This research also puts forward that dumped riprap undergoes a surface erosion process with smaller slides. Placed riprap undergoes a sliding failure mechanism when unsupported at the toe, and a buckling deformation when supported.

Rockfill dams are hydraulic structures of major importance. They can be exposed to extreme flood events, in turn leading to overtopping. These phenomena erode and affect structural and geotechnical integrity, which in turn can cause dam breach. Ripraps are broadly used for rockfill dam protection against such erosion processes. For steep slopes, as the one considered in this study (S = 1:1.5, vertical: horizontal), understanding the riprap behavior during overtopping is an important issue to improve dam design and reinforcement techniques. In this work, datasets are obtained from five experimental models of placed riprap built on a rock filter layer in a flume, at the hydraulic laboratory of the Norwegian University of Science and Technology, Trondheim. The riprap stones were placed in an interlocking pattern with a metallic support at the toe. The models were subjected to successive and incremental overtopping discharges until their complete failure. A laser traverse system was used to measure the coordinates (3D) of individual marked riprap stones between each discharge increase. Six load cells located at the toe measured the imposed loads during the entire procedure. From the total load values, two different types of load contributions could be distinguished: the self-weight of the stones and the hydraulic load depending on the discharge level of the overflow. This article highlights the strong relation in each of the five tests between riprap stone displacements, axial reaction load values measured at the toe and overtopping discharges. Moreover, as demonstrated in previous works, a buckling deformation of the riprap layer was observed and described. The results demonstrate that as the hydraulic load induces 2D deformations of the riprap, a larger part of the riprap weight is supported at the toe. Thus, the measured axial load during overtopping arises both from the hydraulic load and from the load imparted due to the compaction of the riprap layer. This compaction effect induces an even greater load than the one imposed due to the hydraulic contribution. The results from this study are finally put into perspective with the Norwegian Water Resources and Energy Directorate recommendations for full scale dams and suggest the great resistance of supported riprap at the toe.

The rockfill toe structure situated within the downstream slope of rockfill dams is an integral part of a defense mechanism safeguarding the dam structure in throughflow situations. Recent studies have concluded that the rockfill toe structure can have significant impacts on throughflow development and stability of rockfill dams under scenarios of accidental throughflow caused by overtopping of the dam core. The ability to numerically model the effect of various toe configurations on flow through rockfill dams can support the design of effective toe drainage structures for rockfill dams. Development and calibration of a reliable numerical modeling tool in this regard has been challenging owing to lack of availability of extensive datasets from physical modeling investigations. This study further employs datasets gathered by a recent physical modeling study investigating the effects of various toe configurations on throughflow development in rockfill dam models. A commercial numerical seepage modeling tool with an option for non-Darcy flow was calibrated against the datasets with good calibration metrics. The study is novel in providing a rare report on the usage of this option. The calibrated tool can further be employed to carry out a wide array of simulations to arrive at an ideal design for a toe structure for rockfill dams and for assessment of hydraulic performance of toe structures.

Rockfill dams must be equipped with defence mechanisms to counteract the destabilizing effects of throughflow forces under accidental leakage scenarios. A key component of the rockfill dam overtopping system is the rockfill dam toe, constructed in tandem with the downstream rockfill shoulder. Quantitative descriptions of effects of different toe configurations on throughflow hydraulic properties of rockfill dams are currently unavailable in international literature. To address this, experimental investigations were conducted on 1 m high model rockfill dams with disparate toe configurations. Investigation outcomes describe the effects of internal, external and combined toe configurations on pore-pressure distributions within rockfill dam models subjected to throughflow conditions. Research outcomes provide vital information which can facilitate effective decision-making with regards to rockfill dam design. The accumulated data sets could also enable development, calibration and validation of numerical design tools and dam breach models.

A novel approach quantitatively describing two-dimensional displacements of placed riprap stones exposed to overtopping flows is developed and validated, coupling experimental data with Euler’s buckling theory for columns. Past studies investigating the failure mechanisms of placed ripraps with toe support have analysed unidirectional displacements of riprap stones parallel to the slope. 2D description of this process is currently not available in the literature. To address this issue, datasets obtained from a physical modelling investigation conducted on model ripraps constructed with angular stones on a slope of 1:1.5 (vertical : horizontal) were further analysed along with additional experimental data. Investigation results demonstrated that placed ripraps supported at the toe underwent progressive deformations in 2D when exposed to overtopping flows. The deformation behaviour closely resembled buckling of a slender-long column pinned at one end and free at the other and this ultimately led to total riprap failure at the upstream section of the riprap.