Kenji Kawaike

Experiment and simulation of turbulent flow in local scour around a spur dyke

Abstract The turbulent flow in the local scour hole around a single non-submerged spur dyke is investigated with both experimental and numerical methods. The experiments are conducted under clear-water scour regime with an impermeable spur dyke. The scour geometry and flow velocities are measured in details with a high-resolution laser displacement meter, electro-magnetic velocimetries and PIV (Particle image velocimetry). A 3D non-linear k -ɛ model is developed to simulate the complex local flow field around the scour area. The numerical model is formulated using FVM (Finite volume method) on a collocated unstructured mesh, capable of resolving complex geometries and boundaries. It is found that the simulation results are reasonably consistent with those of the experimental measurements. Based on the study results, the nature of the flow structure around a spur dyke with local scour hole is analyzed.

Modeling Flow Exchanges between a Street and an Underground Drainage Pipe during Urban Floods

Modelling floods in urban areas generally requires the modelling of both surface and subsurface flows, along with the exchange discharges between these flows. These latter exchanges are studied on an experimental facility at the Ujigawa Open Laboratory of Kyoto University representing a typical urban drainage system with a single street connected through inlets to a drainage pipe located underneath. An exchange model is developed for the cases in which exchange flows are pressurized, applying a head balance between the surface and subsurface layers and using common head loss formulae. Results of the exchange model agree with experimental measurements and show that, for this experimental setup, the exchanges cannot be modelled by fitting a simple and commonly used orifice-type equation. A hydrodynamic model coupling the one-dimensional (1D) and two-dimensional (2D) shallow water equations for, respectively, the pipe and the street flows and including the analytical exchange model is used to simulate the experimental flows. The numerical model can simulate the global characteristics of both street and pipe flows but shows some discrepancies concerning local perturbations of street flow because of the exchange processes.

Driftwood deposition from debris flows at slit-check dams and fans

Experience shows that debris flows containing large woody debris (driftwood) can be more damaging than debris flows without driftwood. In this study, the deposition process of debris flows carrying driftwood was investigated using numerical simulations and flume experiments. Debris-flow trapping due to driftwood jamming in a slit-check dam was also investigated. A numerical model was developed with an interacting combination of Eulerian expression of the debris flow and Lagrangian expression of the driftwood, in which the fluctuating coordinates and rotation of the driftwood were treated stochastically. The calculated shapes and thicknesses of a debris-flow fan and the positions and orientations of the deposited driftwood on a debris-flow fan were consistent with experimental flume results. The jamming of driftwood in a slit-check dam was evaluated based on geometry and probability. The simulated results of outflow discharge and the proportion of driftwood passed through the slit-check dam also agreed with the experimental results.

Effects of hydraulic structures on river morphological processes

Abstract Study of hydraulic structures such as groins and bandal-like structures can provide valuable information on their influences on morphological processes in natural rivers. These structures usually used for bank protection and formation of deep navigation channel can locally create complex flow patterns, reduce flow velocities and also increase the flood levels. Most of the previous studies are focused on structures like groins under non-submerged flow condition. However, the recent demand of nature friendly low cost and sustainable methods for river bank protection and channel formation leads to the necessity of study different type of structures like bandal-like structures. In this context, this study investigates the flow characteristics and sediment transport process influenced by bandal-like structures through laboratory experiments. The experiments were carried out under live-bed scour condition with sediment supplied from the inlet for two submergence (non-submerged and submerged) conditions. The experimental measurements contribute to better understand the mechanism of deposition/erosion process around different type of hydraulic structures. The performance of bandal-like structures considering the erosion around the structures, the deposition near the bank and the formation of deep main channel show promising results compared with conventional structures such as groins (impermeable and permeable ones).

Numerical modelling of river embankment failure due to overtopping flow considering infiltration effects

This paper reports laboratory experiments and numerical simulations of river embankment failure due to overtopping flow for different sediment sizes and different saturation conditions of embankment body. The effects of saturation and sediment size of embankment materials on the erosion process are discussed based on the results of the laboratory experiments. A numerical model is proposed to simulate the erosion process of embankments by overtopping flows. The proposed model considered the effects of infiltration process and resisting shear stress due to suction of unsaturated sediment. To simulate the embankment erosion phenomenon, the numerical model consists of four modules: two-dimensional (2D) shallow-water flow, seepage flow, sediment transport using a non-equilibrium model framework, and 2D slope stability. The validity of the developed model is tested using experimental data on embankment erosion. The numerical results on progressive embankment erosion agree well with the results of the sandy river embankment experiments.

Three-Dimensional Simulation of Local Scour around a Weir-Type Structure: Hybrid Euler-Lagrange Model for Bed-Material Load

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 transpo...

Underground Inundation Analysis by Integrated Urban Flood Model

Recent urban flood induces inundation into underground space and causes extensive damage. An inundation flow model is developed which can treat inundation in both surface and underground space. We combine a horizontally 2-D inundation flow model based on unstructured meshes with an underground inundation model by use of pond model. A runoff model based on the kinematic wave model is also incorporated. This model enables us to predict underground inundation by imposing rainfall condition in an urban river basin. This model is applied to Sannomiya area and Sannomiya underground mall in Kobe City, Japan. As a result, it is found that the underground mall there is likely to be dangerous by inundation due to heavy rainfall such as that observed in Tokyo in September 2005. It is also found that setting of step at each entrance is one of the effective measures to reduce the risk at underground inundation.

Hyper-resolution 1D-2D urban flood modelling using LiDAR data and hybrid parallelization

Abstract Coupled 1D-2D modelling is a widely used approach to predict water movement in complicated surface and subsurface drainage systems in urban or peri-urban areas. In this study, a hybrid parallel code, H12, is developed for 1D-2D coupled urban flood modelling. Hybrid-1D-2D, or H12, enables street-resolving hyper-resolution simulation over a large area by combining Open Multi-Processing (OpenMP) and Message Passing Interface (MPI) parallelization. Variable grid sizing is adopted for detailed geometric representation of urban surfaces as well as efficient computation. To assess the capability of H12, simulation experiments were carried for the Johnson Creek Catchment (∼40 km2) in Arlington, Texas. The LiDAR-derived digital elevation model (DEM) and detailed land cover map at 1-m resolution are used to represent the terrain and urban features in flood modelling. Hybrid parallelization achieves up to a 79-fold reduction in simulation time compared to the serial run and is more efficient than either OpenMP or MPI alone especially in hyper-resolution simulations.

Event-Consequence Chain of climate change-induced salinity intrusion in Sundarbans mangrove socioecological system, Bangladesh

The Sundarbans is an active biogenic coast entitled to world heritage site. The most notable feature of this Sundarbans is that it is located in a sea-influenced region but depends on nutrient-enriched freshwater flow from the upstream rivers. With its intrinsic transboundary ecological functions, the Sundarbans also provides buffer for lives and livelihood opportunity for millions of people living in its immediate vicinity. Woodcutter, honey collector, fisher, fuel wood collector, and snail and oyster collector are the major livelihood groups directly depending on Sundarbans. Society–ecosystem interrelationship and dependence on hydromorphologic system make this deltaic mangrove a unique socioecological system and also very vulnerable to climate change-induced salinity intrusion. Mangrove ecosystem health with its productivity is deteriorating due to climate change-induced rising salinity in major rivers of the Sundarbans. This change in ecological productivity ultimately affects the forest-dependent livelihoods. This chapter presents an analysis of evolving nature of salinity intrusion hazard developing in the mangrove socioecological system. The evolving nature of salinity intrusion was analyzed through developing Event-Consequence Chain for each of the five livelihood capitals (Natural, Financial, Physical, Human, and Social) of a Sundarbans-dependent livelihood group. Besides, the control options were identified with the aim of unplugging the pathways between two successive events (or sequential consequences) or of lingering the evolving nature. The overall study was carried out through participatory approach using Shared Learning Dialogue method to ensure engagement of the local community. This research discusses the sequential events evolving with salinity intrusion from hydrologic system to its reach—the livelihood system. At the end, it presents an indicative adaptive strategy developed based on identified control stages of the hazard chains for climate change resilient governance of the mangrove socioecological system that clearly calls for delta-based approach of management.

Field application studies of the effect of an underground storage chamber using an urban inundation model

Recently, urban inundation disasters resulting from torrential rain have led to serious problems in many countries worldwide. To mitigate the damage of urban inundation, various structural strategies have been carried out, one of which is the creation of underground storage chambers attached to sewerage systems. In this strategy, part of the storm water within a sewer pipe is diverted over the side weir into the storage chamber. The function of this storage chamber is to manage the storm water runoff at a developed site to prevent flooding, and it is essential for providing a temporary storage area for excess storm water. In this study, the differences in the effects of the storage chamber using an urban inundation model due to several conditions such as the rainfall type, changes in the weir shape, installation locations that have different areal features, and changes in the storage capacity volume are verified. Lastly, a suitable installation site is verified via assessments of the inundation damage costs. The results show that the criteria for the degree of mitigation that can be expected in these conditions.