Breach Dimensions Research Articles

Impact of Dam Materials and Hydraulic Properties on Developing the Breaching Dimensions

Dam safety is an important issue as it directly influences environmental and life losses. Embankment material and embankment core are important factors that contribute to determining the size and hydraulic characteristics of the breach that occurs due to dam failure. The current topic is to present the analysis of the impact of the dam materials properties and hydraulic characteristics on breach dimensions due to overtopping failure using 2D-HEC-RAS software with the aid of RAS-Mapper. The analysis was made by Digital Elevation Model (DEM) conversed to a Triangular Irregular Network (TIN), then the 2D area of the dam was determined and the storage area boundary was calculated. Mosul dam in Iraq, considered one of the big dams that consist of different materials in the core and body, was selected as a case study to simulate the dam breach development due to overtopping failure. Six scenarios of dam failure were carried out for three types of dam materials (dam with corewall, concrete-face dam, and homogeneous zoned fill dam), each with high and medium erodibility. All scenarios were made for the reservoir elevation of 341 m which is considered the probable elevation for failure occurrence. The results showed that the minimum and maximum values of breach width were found to be 542 m and 1118 m, respectively in both cases of concrete-face dam type with a high erodibility and homogeneous zoned fill dam with medium erodibility rate. It has been illustrated that the medium erodibility in each scenario gives a smaller width for a long time, especially in homogeneous zoned fill dam type which is a good indication in engineering design. The analysis of sensitivity was carried out to examine the impact of pool volume at failure on breaching time and width of the breach. It showed that the breach width is more sensitive to reservoir fill capacity and there is a linear relationship between water volume reduction ratios and the breach width. Finally, tests of the six cases reveal that the proposed model is sensitive to soil conditions.

Stability of a Tidal Marsh Under Very High Flow Velocities and Implications for Nature-Based Flood Defense

Nature-based strategies, such as wave attenuation by tidal marshes, are increasingly proposed as a complement to mitigate the risks of failure of engineered flood defense structures such as levees. However, recent analysis of historic coastal storms revealed smaller dike breach dimensions if there were natural, high tidal marshes in front of the dikes. Since tidal marshes naturally only experience weak flow velocities (~0-0.3 ms-1 during normal spring tides), we lack direct observations on the stability of tidal marsh sediments and vegetation under extreme flow velocities (order of several ms-1) as may occur when a dike behind a marsh breaches. As a first approximation, the stability of a tidal marsh sediment bed and winter-state vegetation under high flow velocities were tested in a flume. Marsh monoliths were excavated from Phragmites australis marshes in front of a dike along the Scheldt estuary (Dutch-Belgian border area) and installed in a 10 m long flume test section. Both sediment bed and vegetation responses were quantified over 6 experimental runs under high flow velocities up to 1.75 ms-1 and water depth up to 0.35 m for 2 hours. These tests showed that even after a cumulative 12 hours exposure to high flow velocities, erosion was limited to as little as a few millimeters. Manual removal of the aboveground vegetation did not enhance the erosion either. Present findings may be related to the strongly consolidated, clay- and silt-rich sediment and P. australis root system in this experiment. During the flow exposure, the P. australis stems were strongly bent by the water flow, but the majority of all shoots recovered rapidly when the flow had stopped. Although present results may not be blindly extrapolated to all other marsh types, they do provide a strong first indication that marshes can remain stable under high flow conditions, and confirm the potential of well-developed tidal marshes as a valuable extra natural barrier reducing flood discharges towards the hinterland, following a dike breach. These outcomes promote the consideration to implement tidal marshes as part of the overall flood defense and to rethink dike strengthening in the future.

The importance of marshes providing soil stabilization to resist fast-flow erosion in case of a dike breach.

Salt marshes provide valuable ecosystem services including coastal protection by reducing wave loading on dikes and seawalls. If the topsoil is erosion resistant to fast‐flowing water, it may also reduce breach depth if a dike fails. In this experiment, we quantified the topsoil erosion resistance from marshes and bare tidal flats with different soil types to understand the extent to which they can help reduce breach depth. Intact soil samples were collected from 11 locations in the Netherlands at different tidal elevations and then exposed for 3 h to 2.3 m/s currents. To the samples that remained stable after flow exposure, an artificial crack was made to test their stability following soil disturbance. All samples from the tidal flats were completely eroded, regardless of sediment type. In contrast, all samples from well‐established marsh plateaus were stable as long as no disturbances were made, including those with sandy subsoils. After creating artificial cracks, samples with a thin cohesive top layer on top of sandy subsoil collapsed, while marshes with silty subsoils remained stable. Pioneer marshes on sandy substrate without a cohesive top layer were the only vegetated soils that completely eroded. The lower erosion of marshes with either sandy or silty soils compared to bare tidal flats was best explained by the presence of a top layer with belowground biomass, high organic content, high water content, and low bulk density. When analyzing the erodibility of marshes only, fine root density was the best predictor of erosion resistance. This study demonstrates the importance of preserving, restoring, or creating salt marshes, to obtain a topsoil that is erosion resistant under fast‐flowing water, which helps reduce breach dimensions if a dike fails. The probability of topsoil erosion in established marshes with sandy subsoil is higher than in silty marshes. A silty layer of cohesive sediment on top of the sand provides extra erosion resistance as long as it does not break. Pioneer marshes that have not developed a cohesive top layer are erosion sensitive, especially in sandy soils. For future marsh creations, using fine‐grained sediments or a mixture of sand with silt or clay is recommended.

Modelling early medieval flood-induced breaching of a coversand ridge in the IJssel valley, Rhine delta, the Netherlands

Connecting geologically mapped data to numerical modelling can help in understanding river landscape evolution. This study focuses on flood-induced breaching of the coversand ridge in the IJssel valley floodplain (Rhine delta, the Netherlands). The development of the breach would explain why this river branch came into existence in early medieval times. Prior to the breaching, the coversand ridge formed a barrier for the Rhine River to discharge towards the north. A palaeoflood model was coupled to a local sediment transport model to predict sediment transport rates in two competing coversand ridge breaches. The results show that the breach at the current IJssel River location was expected to expand earlier and faster in time for various upstream discharge waves and initial breach dimensions. This provides quantitative argumentation for the IJssel river avulsion case to have been triggered by the breaching of the coversand ridge during a large palaeoflood.

Modelling of Flood Inundation due to Levee Breaches: Sensitivity of Flood Inundation against Breach Process Parameters

This paper analyses the sensitivity of flood inundation due to river levee breach against breach process parameters using the 1996 Awash River levee breach case at Wonji, Ethiopia. A parametric levee breach model integrated into the 2D hydrodynamic numerical model Telemac-2D is used to simulate a levee breach flood event at Wonji, Ethiopia. Levee breach process parameters are systemically varied to find out their effect on the flood inundation. The analysis of the model results shows that the flood inundation is sensitive to the final breach dimensions and breach location. However, the parameters describing the levee breach development have negligible influence on the flood inundation. This implies that final breach dimension and breach location in an event of levee breach are the most important and decisive parameters affecting the resulting inundation of the flood plain, and as such should be given due consideration when creating flood inundation maps due to levee breach.

A comparative study of embankment breach parameters in different breach locations

In general, when an embankment breach occurs, the breach expands laterally due to erosion of both breach sides. However, when the breach occurs close to a solid wall, the erosion occurs primarily on one side. This paper presents a study on the comparative parameters of the breaches that occurred at the middle of the embankment width, and at the location adjacent to a solid wall. A physical model is constructed such that the breaches could be observed. The downstream slopes of the embankment model were set at 1:1, 1:1.5, and 1:2. Cohesive soil was used to construct the embankments with the bulk density distributions ranging from 1.4 to 1.7 g/cm3. The experiments were conducted by allowing water to flow over the embankment at different flow rates. The breach dimensions were measured and transformed into dimensionless breach parameters. These dimensionless breach parameters were analysed to ascertain their relation with relevant dimensionless flow parameters. The results reveal that for the same embankment slope and discharge rate, the breach parameters change depending on the location of the breach, except for the parameter of the soil loss volume. The distance between the breach location and the solid wall affects the values of the maximum breach width, the maximum breach depth, and the maximum scour depth. There is shown that breach depth and scour depth for breaches adjacent to the solid wall are deeper than those of the breaches occurring in the middle section of the embankment. In addition to these findings, the empirical relation among these dimensionless breach parameters obtained from other case studies are also discussed.

Erosion-based analysis of breaching of Baige landslide dams on the Jinsha River, China, in 2018

The Yangtze River is one of the most important rivers in China due to its large basin size, the large population along the river, and the numerous large dams and reservoirs on the river. The Jinsha River, the upper reach of the Yangtze River, was dammed twice recently at Baige, Tibet, one on 10 October 2018 and the other on 3 November 2018 (UTC + 8). Accordingly, two large landslide dams, 61 m and 96 m in height to the lowest dam crest, were formed in a 3-week interval. Due to the large inflow rates at the time of damming, the barrier lake level rose rapidly, posing huge risks to the downstream residents and properties. In managing the landslide dam risk, one of the important tasks is to predict the dam breaching flood beforehand. This paper focuses on rapid prediction of the dam breaching hydrograph and breach geometric parameters of the two landslide dams. The predictions were made timely before the breaching of the two landslide dams using both erosion-based empirical equations and numerical simulation and were refined based on detailed field investigation at the site after breaching. Comprehensive field investigations were conducted to determine the geological structures of the landslide dams, characterize the erodibility of dam materials, and measure the final beach dimensions. The simulated dam breaching processes, outflow hydrographs, lake water level changes, and final breach dimensions were validated by field observations. Compared with the hypothetical scenario without a diversion channel on the second landslide dam, a diversion channel 15 m in depth successfully lowered the peak flood discharge by about one third and helped to mitigate the flood risk significantly. The analysis outcome serves as basis for warning and evacuation of the downstream residents and making appropriate engineering risk mitigation plans.

Peak flood estimation under overtopping and piping conditions at Vahdat Dam, Kurdistan Iran

Estimation of maximum discharge and time of transfer dam break wave are among the main factors in provision of emergency action plan and safety assessment of dams. Most dam breaks begin with a breach in the body of the dam or flood overtopping. Estimation of maximum discharge for determining affected areas and also determination of hazards due to the dam break are essential issues. In this research, the dam break of Vahdat Dam located in Kurdistan, Iran, and in neighborhood of Sanandaj City was simulated by integrating ArcMap and HEC-RAS software and field observations. The necessary data was collected from the corresponding organizations. The Manning coefficient of the Gheshlagh River at downstream of the dam was estimated using the Manning coefficient relationships and field observations and measurements. The breach dimensions were determined using the conventional empirical relationships. The dam breaks in both overtopping and piping states were simulated. The maximum discharge values obtained from the modeling and empirical relationships were compared. The results indicated the substantial effect of selecting break types on the hydraulic characteristics of dam break wave in the river and downstream areas. While in the case of break due to the breach, selection of the methods for calculation of breach dimensions would affect the break and these relationships in various conditions would give different breach dimensions.

Unveiling the consequences of your breach growth model choice

Within the frame work of the realisation of the ‘Sigmaplan’ for the river Schelde in Flanders (Belgium), a large-scale dike breaching experiment following overflow was held at Lillo (Antwerp) in 2012. The outcomes of the breach test serve to unveil the impact of a chosen breach growth model, to set application limits, to come up with guidelines for proper selection and usage of the model to be applied. Breach growth models are used to predict the breach dimensions and to estimate the flow through the breach. All assessed models pretty well succeed in this. However, starting from various premises and taking into account a (limited) set of different breaching mechanisms, the use of today’s state-of-the-art breach growth models is not entirely trouble free

Impact of managed realignment design on estuarine water levels

Large realignment sites near the mouth of an estuary have the potential to raise water levels throughout the estuary. This increase is produced by the newly created intertidal area drawing additional water into the estuary on the flood tide. Although some of this additional water enters the realignment site, a proportion also bypasses the realignment site and continues upstream, thus increasing water levels in these areas of the estuary. The modelling results described in this paper suggest that an important determinant on estuary water levels is the rate at which the realignment site fills. Schemes that fill more slowly are predicted to have less impact than those that fill more rapidly, even if high water levels within the schemes are similar. For the schemes tested in this paper, the rate at which the site flooded appeared to depend less on breach dimensions and more on the width of the site that the flood waters had to cross in order to cause full inundation. It is proposed that those options with the shortest distances result in the flood tidal waters experiencing less frictional losses. These options showed a more rapid rise in water levels within the site, and produced a correspondingly larger increase in water levels within the estuary.

Dam-break flood simulation under various likely scenarios and mapping using GIS: Case of a proposed dam on River Yamuna, India

The precision modeling of dam break floods can lead to formulation of proper emergency action plan to minimize flood impacts within the economic lifetime of the assets. Application of GIS techniques in integration with hydrological modeling for mapping of the flood inundated areas can play a momentous role in further minimizing the risk and likely damages. In the present study, dam break analysis using DAMBRK model was performed under various likely scenarios. Probable Maximum Flood (PMF) calculated for a return period of 1000 years using deterministic approach was adopted for dam break analysis of the proposed dam under various combinations of breach dimensions. The available downstream river cross-sections data sets were used as input in the model to generate the downstream flood profile. Dam break flow depths generated by the DAMBRK model under various combinations of structural failure are subsequently plotted on Digital Elevation Model (DEM) of the downstream of dam site to map the likely affected area. The simulation results reveals that in one particular case the flood without dam may be more intense if a rainfall of significant intensity takes place.

Landslide dam failure and flood hydraulics. Part II: coupled mathematical modelling

A coupled 2D mathematical modelling study of landslide dam failure and flood is presented, complementing our experimental investigation presented in the companion paper. The model is built upon the shallow water hydrodynamic equations. The governing equations are numerically solved using the total-variation-diminishing version of the second-order weighted-average-flux method along with the HLLC (Harten, Lax and van Leer with Contact wave restored) approximate Riemann solver. Two parameters related to bed-load sediment transport and critical slope stability are calibrated using the measured stage hydrographs from two runs of the flume experiments. The calibrated model is then applied to other independent runs of the experiments featuring different inflow discharges, dam geometry, dam composition and initial breach dimensions. It is found to be able to satisfactorily reproduce the measured stage hydrographs and the widening of initial breach. The experimental observation of the prime role of the inflow discharge and initial breach in dictating the dam failure process and flood is unequivocally resolved, along with the impacts of dam geometry as well the content of cohesive clay and gravel in the dam. Interestingly, the downstream peak discharge and stage of the flood are substantially reduced by initial breach, which clearly exemplifies its role in modulating the flooding.

Assessing breaching risk in coastal gravel barriers

This paper presents a method to assess the breaching risk to coastal barriers under a no-active-intervention (NAI) scenario. The approach categorises breaches as small high-level breaches, which operate periodically; to larger permanent tidal inlets with water flowing in/out on each tide. Assessing the risk of breaching involves four stages that consider the most probable location of a breach; the possible breach dimensions; the permanence of the breach; and the timeline of breach formation. The most probable location for a breach to form is assessed by considering the past and future shoreline behaviour, the susceptibility of beach profiles to overwashing, the current defence standard and probability of defence failure. The potential permanence and size of the breach is evaluated by considering a stability rating based on the ratio of the tidal prism to total annual littoral drift. The potential cross-sectional area of a breach is calculated by considering the relationship between the cross-sectional area and tidal prism. The results from this analysis are brought together in a written description of breach formation over time. The gravel barrier between Aldeburgh and Orford Ness on the Suffolk coast in the UK is used as a case study. The approach described within this paper is generic and can be used to assess the breaching risk for other vulnerable coastal barrier features under an NAI scenario.

Analytical Model of Incipient Breaching of Coastal Barriers

A mathematical model is formulated to describe incipient breaching of coastal barrier islands. The model is based on the assumptions of idealized breach morphology and is intended to describe the growth of breaches prior to possible closure by longshore sediment transport. The two coupled, non-linear equations governing breach width and depth are solved analytically for special cases. The analytical solutions explicitly exhibit an exponential behavior in breach dimensions and reveal that the macroscale process of breach growth is controlled by seven variables: initial width and depth of the breach, equilibrium width and depth of the breach, width of the barrier island, and maximum or initial net sediment transport rates at the bottom and sides of the breach. The literature is reviewed to compile general properties of coastal breaches, and sensitivity testing shows the model to be compatible with those observations. The model is applied to simulate the 1980 breach at Moriches Inlet, New York, and reasonable agreement is found.