•Headward erosion during the breaching process of landslide dams is developed.•A dam break model with six modules based on failure mechanism is proposed.•An empirical model to estimate the breach ...depth of landslide dams is given.•The diversion channel is an effective measure to reduce hazards due to dam break.
Landslide dams often fail shortly after the formation, resulting in huge risks to the downstream residents and properties. Fast and accurate forecasting of the breaching process and discharge hydrograph is important for a decision making to mitigate the damages. Morphological data indicate that the longitudinal length of most landslide dams is much larger than their transverse length. Additionally, headward erosion is more intense in landslide dams than manmade dams during the overtopping failure process. However, most existing models neglect this phenomenon. The objective of this study is to build up a dam-break model considering the headward erosion process, assuming that the change rate of downstream slope-angle is consistent with the rate of breach vertical erosion. And a dam-break numerical model to simulate the overtopping failure process of landslide dams considering the headward erosion was developed based on the theories of hydraulics, hydrology, soil mechanics, sediment dynamics, and etc. Nevertheless, the breach final bottom elevation, which can only be measured after the dam failure, is directly applied in currently available dam break models. To overcome this problem and for the prediction purpose of a newly formed landslide dam, we proposed an empirical model to estimate the breach final bottom elevations of high, medium and low erodibility landslide dams based on the statistical analysis of worldwide landslide dam failure cases collected recently. To verify the effectiveness of proposed dam-break numerical model, the ‘11.03′ Baige landslide dam which locates at the border of Tibet Autonomous Region and Sichuan province was selected as a case study. Results of this study show that the relative errors in the simulated peak discharge, time to the peak, and the total flood volume are 1.59%, 3.50% and 0.53%, respectively, which meet the accuracy requirements of hydrological forecasting. The relative error in the prediction of breach final bottom elevation is 4.28%, which indicates that the accuracy of the simulation is satisfactory. Finally, a sensitivity analysis indicates that the downstream slope affects the prediction of the breach and flood hydrograph significantly, and the excavation of diversion channel is an effective measure to reduce the peak discharge of dam overtopping failure.
In this study, overtopping-induced levee breach experiments were conducted to explore the effect of differences in levee structure on levee breach characteristics, considering the heterogeneity of ...the levee material and the flow outside the levee. A homogeneous levee composed of silica sand No. 7 and a heterogeneous levee composed of silica sand No. 5 and No. 7 overlapped by two levees were subjected to overtopping-induced levee breaches in the presence of flow outside the dike. Comparison of these processes revealed that the downstream progress of erosion is more plausible than the upstream side due to flow outside the levee. The shape of the breach opening was different during the levee breach process between the levee made of a heterogeneous structure and that made of a single material. Moreover, the rate of widening of the breach opening on the back slope increased at an earlier stage in the former case than in the latter, resulting in a larger overflow rate. To the best of our knowledge, few studies have simultaneously considered the heterogeneous structure of an actual levee and the flow outside the levee.
•We evaluated the performance of satellite-based precipitation estimates (SPEs).•Soil and Water Assessment Tool (SWAT) was used to simulate river discharge.•After the implementation of ...bias-correction, all SPEs revealed improved results.•We find the precipitation elasticity of the study area is ∼1.5.•This study supports the application of SPEs for hydrological modelling.
West Rapti River (WRR) basin, Western Nepal.
Hydrologic modeling requires an accurate precipitation data at a high spatial resolution, which is often limited in many regions of the globe. As a complement to the ground (gauge) precipitation data, satellite-based precipitation estimates (SPEs) appear useful. At first, this study evaluated performance of three different SPEs, namely i) CHIRPS, ii) PERSIANN-CCS, and iii) IMERG, with respect to gauge data using different event detection and quantification indices. Soil Water Assessment Tool (SWAT), a semi-distributed hydrologic model, was used to simulate the river discharge. We then analysed precipitation elasticity, as a first kind of such study in Nepalese river basin, by scaling the precipitation input in both positive and negative directions (ranging from -20 % to +20 %) in order to explore basin response on likely alteration of precipitation. A non-parametric precipitation elasticity was finally computed for three different cases: 1) observed river discharge, 2) gauge-based simulated river discharge, and 3) SPEs-based simulated river discharge.
IMERG proved to be superior among three SPEs. All SPEs showed improved results after implementation of different levels of bias-correction where daily precipitation data were corrected using linear correction factors computed at a mean monthly scale. Computed correction factors are replicable to nearby basins. Precipitation elasticity of the study area ranged from +1.3 to +2.0 (approximately +1.5) which indicates that a 1.0 % change in precipitation will result in 1.5 % change in river discharge.
AbstractThis paper presents a hybrid Euler-Lagrange model for bed-material load considering transition between the bed load and suspended load. The hydrodynamic model and suspended sediment transport ...model are solved in the Eulerian grid. A Lagrangian model integrating the near-bed grain trajectory and the momentum equations is employed to predict the motion of the bed load. Sediment exchange among stationary bed, bed load, and suspended load are modeled by considering the deterministic nature of the motion of individual bed load particles and the stochastic nature of the behavior of particle groups. The numerical model was applied to two types of laboratory experiments: (1) local scour upstream of a slit weir and (2) sediment release from a dam gate where an open channel flow and a closed conduit flow appear. All validation cases show that the model is able to reproduce temporal variation of the three-dimensional bed geometry around a weir-type structure with sufficient accuracy. Numerical results imply that computing the transition process from the bed load motion into suspension has a key role in three-dimensional simulation of nonequilibrium transport of bed load and suspended load upstream of a weir-type structure.
We propose a methodology to estimate the spatial distribution of the probability of sediment deposition due to debris flow from rainfall data by combining the probability prediction of erosion volume ...based on an ordinal logistic regression and a sediment transport simulation. By using the Receiver Operating Characteristic (ROC) curve and Area Under Curve (AUC) we have selected the best combination of shortand long-term rainfall indices used as explanatory variables in the ordinal logistic model. The results showed that the regression model using 60-minute and 48-hour rainfall indices performed well and that the regression model using three events improved the predictability of local disasters in 2014. Furthermore, we performed Monte Carlo debris-flow simulations using rainfall data from 2014 using the model. We confirmed that the spatial distribution of disaster probability is consistent with the actual damage.
This paper focuses on finding efficient solutions for the design of a highly permeable pile spur (or slit type) dike field used in morphologically dynamic alluvial rivers. To test the suitability of ...different arrangements of this type of permeable pile spur dike field, laboratory experiments were conducted, and a three-dimensional multiphase numerical model was developed and applied, based on the experimental conditions. Three different angles to the approach flow and two types of individual pile position arrangements were tested. The results show that by using a series of slit-type spurs, the approach velocity of the flow can be considerably reduced within the spur dike zone. Using different sets of angles and installation positions, this type of permeable spur dike can be used more efficiently than traditional dikes. Notably, this type of spur dike can reduce the longitudinal velocity, turbulence intensity, and bed shear stress in the near-bank area. Additionally, the deflection of the permeable spur produces more transverse flow to the opposite bank. Arranging the piles in staggered grid positions among different spurs in a spur dike field improves functionality in terms of creating a quasi-uniform turbulence zone while simultaneously reducing the bed shear stress. Finally, the efficacy of the slit-type permeable spur dike field as a solution to the riverbank erosion problem is numerically tested in a reach of a braided river, the Brahmaputra–Jamuna River, and a comparison is made with a conventional spur dike field. The results indicate that the proposed structure ensures the smooth passing of flow compared with that for the conventional impermeable spur structure by producing a lower level of scouring (low bed shear stress) and flow intensification.
The spacing is an important consideration in spur dikes design. In meandering channels, the flow field and sediment transport are affected by both the spur dikes and channel sinuosity. The paper ...employed a 3D model to investigate the effect of the spacing between spur dikes on bed deformation and flow in meandering channels under different sinuosity. The results showed that severe local scour occurred near both the most upstream spur dike and spur dikes downstream of the meander apex. As the spacing increased, complete scour holes emerged starting from the downstream spur dikes, the potential bank erosion became larger, and the flow near spur tips became more turbulent with a large velocity gradient. The maximum local scour depth increased with the spacing and decreased with the channel sinuosity in most cases. The non-uniform spacing could be used to lower the construction cost, and different sinuosity required different spacing arrangements. Based on the simulation results, the performance of different spacing was quantitatively evaluated simply by considering four factors of bank protection, structural stability, construction cost, and aquatic habitat.
Predictive simulation of concurrent debris flows using only pre‐disaster information has been difficult, partly because of problems faced in predicting debris‐flow initiation locations (i.e., slope ...failure). However, because catchment topography has convergent characteristics with all channels in it joining each other as they flow downstream, damage to downstream areas could be predicted using relatively inaccurate initiation points. Based on this hypothesis, this study uses debris‐flow initiation points generated randomly with statistical slope failure prediction and performs a many‐case simulation across numerous initiation points to quantify the effect of slope‐failure locations in terms of deviations in the predicted water level and topographic change. This paper presents the results of 2D simulations based on a conventional debris‐flow model that was run on a supercomputer to realise simulations of many cases. The obtained relative standard deviation was found to decrease as the debris flow and sediment‐laden flood approached the downstream area, indicating that the predictability of the inundation and topographic change can be decided from the terrain characteristics.
Highly accurate air-borne Digital Elevation Models (DEMs) are not commonly available in most regions of the world, particularly developing countries like Nepal. Better topographic data are required ...to assess catchment hydrology, flood/inundation propagation, and sediment transport/deposition. Ground-based surveys (i.e., topographic and bathymetry) and Light Detection and Ranging (LIDAR) are expensive in terms of cost and time. We rely mostly on space-borne DEMs when the area of interest is quite large. And, for river cross-sections at specific regions, ground-based surveys are conducted at some regular intervals. The ALOS World 3 D − 30 m (AW3D30), ASTER Global DEM Version 3 (ASTER GDEM v3), SRTM-30, and Carto DEM v3 are some of the popular and latest space-borne DEMs that have been made available to the general public free of charge at a spatial resolution of 30 m. The availability of multiple space-borne DEMs may likely puzzle the practitioners on the selection of appropriate products. Here in this study, we evaluate the vertical accuracy of different space-borne DEMs with respect to the ground-based survey points (2891 points) at river cross-sections and ground-based indicative spot heights (8102 points) across the West Rapti River basin, Nepal. Performance metrics, like mean error and root mean square error (RMSE), were used to quantify deviation with respect to observed elevation. The AW3D30 showed the lowest RMSE value (i.e., 3.41 m) in the lower region of the study area. We numerically simulated the inundation propagation based on four selected DEMs using a calibrated two-dimensional runoff inundation model, and we find the AW3D30 outperforms other DEMs. All four DEMs showed a similar tendency on a catchment scale with respect to the information of indicative spot heights, suggesting interchangeable applications of all four DEMs in hydrologic analyses. However, precise ground-based observations across the catchment are needed for assuring the applicability.
Heavy rain and river flooding due to Typhoon No. 19 in October 2019 led to overflow and a dike breach on the left bank of the Chikuma River that caused large-scale inundation damage in Nagano City, ...Japan. To devise countermeasures, an inundation analysis model is an important tool. In this study, an inundation analysis model was developed to examine the inundation water behavior. The calculated inundation water depth and inundation area showed good agreement with the observed inundation water depth and the inundated area, confirming the validity of the analysis model. In addition, temporal changes of the inundation state were calculated considering the drainage process. However, the sewerage system, waterway, and drainage pump car were not taken into consideration in this analysis, and future issues for model improvement were also revealed. In addition, an analysis model with a 2 m grid was developed in the dike breach site, and the inundation water flow on roads and the fluid force around houses were obtained after taking into consideration the effect of houses. In paticular, the calculated value of the specific force exerted on damaged houses was very high. Moreover, it was proposed that house hazard should be evaluated while taking into consideration the loss of houses around the dike.
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