Study on river Dyke breach characteristics by overtoppingflow

Abstract

A large number of dykes (levees or banks or river embankments) have been built by humans or formed naturally along the rivers throughout the world. River dykes, considered as defense structure, are constructed to prevent flooding of valleys and their inhabitants and confine the flow of the river for higher and faster flow. In this context of work, we refer to dykes as to man-made earthen embankments built along a river parallel to the river flow and flow strikes the dykelength not perpendicularly. River Dykes is an important and effective measure to prevent floods. Themain consequences on the flood risk due to construction of river embankments or dykes are that, firstly, it increases the flood hazard reducing the lateral flow storage area and hence the flow capacity of peak discharges attenuation and, secondly, the amount of potential damages induced by flooding is dramatically increased, being the surrounding areas often urbanized. Therefore, although the existence of dyke lowers the probability of flooding but the consequences to personal safety and property are much higher should a dyke overtop or fail. In the context of study, Dyke failure is considered to be the situation where erosion or structural failure of the earthen embankment cause flood water to pass over or through the embankment in an increasingly uncontrolled manner (Morris et. al. 2009), leading to a hole or breach in the dyke.The problem of embankment failures has always been of great importance because of their disastrous effects.Most of the embankments are not designed to be overtopped hence are vulnerable and sensitive to overtopping flow. The failure of river embankment due to overtopping flow may cause widespread inundation and catastrophic damages of properties, even if a failure occurred at one spot.Recent climate change will lead to an increase in heavy rainfall events. The occurrence of river water level rise, which exceeds the design, is increasing. Dyke may fail by erosion due to overtopping, abrupt collapse of the dam body or progressive failure. However, in-depth knowledge of the mechanism of the dyke failures and measured data are still lacking. Dyke failure has been frequently studied as an earthen dam failure despite of their differences in geometry, dimensions and material properties. Very few models are developed for dyke failure that can treat the flow as both sediment flow and debris flow. In this context, an attempt is made to integrate separate models (i.e. seepage flow model, dam surface ii

erosion and flow model) to predict the lateral widening pattern and outflow hydrograph resulted from failure of dyke by overtopping. The seepage flow model calculates pore water pressure and moisture content inside the dam body. The model of the dam surface erosion and flow calculates dam surface erosion due to overflowing water.Infiltration process and role of suction are incorporated in the integrated model although this process is neglected in almost allavailable models. The model can predict both total discharge and sediment discharge hydrographs. The model is capable of treating all types of flows according to sediment concentration. Preliminary experimental studies are carried out to determine the initial understanding of the lateral widening and breach hydrograph using the dyke as the side using four types of sediments (sediment numbers 5, 6, 7 and 8). Within the range of test parameters, the dykebreach process is accelerated with increasing sediment d50. For coarse sediment, due to quick saturation of the dykebody, the collapse time decreases. The width of the breach increases at the top and toe section than the middle section which acts like a hinge point. At the final stage, the middle section also fails leading to whole dyke collapse. The breach discharge and sediment discharge shows the time lag and peak according to the sediment size. Finally, the time of failure curve is proposed for the pre determination of occurrence of peak breach dischargebasedon the pre-disaster parameters. Dyke surface erosion and flow model is developed for simulation of outflow hydrograph due to dyke failure by overtopping. The proposed model is tested for different experimental cases of dyke failure for overtoppingover partial pilot channel width. The model is able to reproduce the resulting hydrograph reasonably. The simulated overtopping time and dyke surface erosion at different time steps are also in good agreement with experiments. The incised channel is almost vertical for both simulations and experiments in small inflow discharge. In the case of larger inflow discharge and larger reservoir volume, slumping occurred at irregular time steps. The lateral widening pattern is somehow reproduced well with experimental values. Also, the scour pattern at the foundation of dyke for various scenarios with the variation of different sediment sizes due to overtopping arealso analyzedexperimentally. The scour pattern is also following a trend with regards to materials forming the dyke and the foundation. The finer the materials, more vulnerable to the scouring were observed. The proposed model is tested for experimental case of dykefailure due to overtopping. The model reproduced reasonably similar hydrograph as that ofexperiment. The numerical simulation and experimental results of lateral widening, sediment hydrograph etc. are also in good agreement.