Cause analysis for a new type of devastating flash flood

This work introduces an unprecedented flash flood that resulted in nine casualties in Shimen Valley, China, 2015. Through field survey and numerical simulation the causes of the disaster are systematically analyzed, finding that the intense storm, terrain features, and the large woody debris (LWD) played important roles. The intense storm induced fast runoff and, in turn, high discharges as a result of the steep catchment surfaces and channels. The flood flushed LWD and boulders downstream until blockage occurred in a contraction section, forming a debris lake. When the debris dam broke, a dam break wave rapidly propagated to the valley mouth, washing people away. After considering the disaster-inducing factors, measures for preventing similar floods are proposed. The analysis presented herein should help others manage flash floods in mountain areas. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/nh.2019.091 ://iwaponline.com/hr/article-pdf/51/1/1/758970/nh0510001.pdf Jingming Hou (corresponding author) Bingyao Li Yu Tong Liping Ma State Key Laboratory of Eco-hydraulics in Northwest Arid Region of China, Xi’an University of Technology, No. 5 Jinhua Road, Xi’an, 710048, China E-mail: jingming.hou@xaut.edu.cn James Ball School of Civil and Environmental Engineering, University of Technology Sydney, Sydney, NSW, 2006, Australia Hui Luo Meteorological Bureau of Shaanxi Province, No. 102-1 Weiyang Road, Xi’an 710015, China Qiuhua Liang School of Architecture, Building and Civil Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK Junqiang Xia State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China


INTRODUCTION
The field survey discovered that a barrier lake was formed at a narrow section of the valley by debris transported by the flash flood. More and more water was stored in the reservoir until it was full. When the water level was high enough to overtop the embankment, a dam break occurred and the wave began to propagate downstream the channel, causing severe flash flooding. Figure 3 shows an aerial view of the study area. It should be noted that the scene of the accident site is different in Figures 2 and 3, as a concrete channel was built after the flood event to raise the capacity of the flow conveyance channel.
According to the field investigation on March 15, 2018, the flash flood was relevant to the extreme rainfall, the terrain features, and the vegetation conditions of the catchment, so these three main reasons leading to the new flood are analyzed in detail.

Heavy storm
A flash flood is normally caused by heavy rainfall in a short time, usually less than 6 hours (National Oceanic and Atmospheric Administration, NOAA, version 2.60). The catchment is located on the north slope of the Qinling Mountains, an important geographical border between the north and south of China, i.e., the transitional zone between subtropical and temperate zones. This area is reported by He et al. () to be a region with high-frequency heavy rain.
In order to understand the precipitation process of the event, the hydrography of the study area is required. As there is no rain gauge in the valley, the closest rain gauge, referred to as Yinzhen, was selected to represent the storm at the Shimen Velley (9.5 km from the valley, as plotted in Figure 4). The hyetograph at Yinzhen rain gauge is illustrated in Figure 5, indicating the rainfall increased sharply from 17:00 to 18:00, on August 3, 2015. The total rainfall was 144.8 mm in 5 hours and reached 126.6 mm in the first 2 hours. According to the IDF curves of Xi'an City in the form of a Chicago storm type, this rainfall is in a return period of around 1,000 years. Such intense rainfall would be expected to produce large runoff and, in turn, lead to severe flooding.

Terrain features
The terrain features also play a significant role in generating a severe flood. In this work, a high-resolution digital      Although no direct evidence is available, a witness living in the valley mouth reported that there was no water in the channel after about half an hour; the discharge suddenly increased once the storm started. The period with low discharge indicated there was a high likelihood that a dam had formed in the upstream reaches and that this dam blocked the main flow.
As plotted in Figures 6 and 7, the side slopes of the catchment are considerably steep. In some cross sections, the slope could reach 60 and is prone to induce fast hydrological responses; that is, the surface runoff moves quickly to the channels. The channel slope in the catchment is also very abrupt, as shown in Figure 8, with an average value of 1:5 (horizontal to vertical). The rapidly collected water in the channel will be transported efficiently to the catchment outlet and, therefore, is likely to lead to flash floods. This high-velocity flow will sweep the channel and flush boulders and trees downstream.
The dam break flow would also be expedited in the steep channel and the water move to the mouth of

Thick vegetation and boulders
In this disaster, the debris consisted mainly of boulders, rock fragments, logs, sticks, branches, and other wood that fell  into the channel (see Figure 9). Figure  The boulders, as shown in Figure 9, are another kind of debris source. Once the flood velocity is adequate to carry the big stones, they will move downstream, mixed with the LWDs. When they arrive at the contraction section, the debris gathered and a barrier lake was formed with the carrying action of the water. The water will be stored until the dam cannot host the water. In this area, the thick trees play an important role for building the dam, since the trees worked as pillars to trap the coming debris.
In summary, the heavy storm, the terrain features, and where t is time, x and y are the Cartesian coordinates; q is the vector of conserved flow variables containing h, q x and q y , which are the water depth and the unit-width discharges in the x-and y-directions, respectively; q x ¼ uh and q y ¼ vh; u and v are the depth-averaged velocities in the x-and y-directions, respectively; z b is the bed elevation; f and g are the flux vectors in the x-and y-directions, respectively; S is the source vector; i is the source or sink of mass caused by rainfall and infiltration; and c f is the bed

Simulation for Malpasset dam break
The Malpasset dam was located in the Reyran river valley in southern France, and collapsed in 1959 after extremely heavy rain. Figure 11 shows the DEM of the Malpasset and its floodplain. The locations of dam and survey points P are also plotted in the same figure. When simulating the   In this work, the computed water depth at gauges 3 and 8 are selected to compare with the measured data in Figure 14. This illustrates that the simulation results are in good agreement with the measured ones. From the hydrograph, the water depths are captured accurately in terms of peak discharge and the recession process. To quantitatively analyze the model performance, the Nash-Sutcliffe efficiency coefficient (NSE) is introduced as: where Q t o is the measured data at the time t; Q t m is the simulated results at the time t; Q o denotes the average of the measured data.
The NSE at gauges 3 and 8 are 0.950 and 0.955, respectively, indicating that the model performs well in simulating flash flood.

Dam break flood simulation
The proposed hydrodynamic model is used to model the process of the dam break flood. According to the field investigation, a barrier lake with a water depth of around 7 m was formed in the upstream reach about 200 m from the accident site. As the real dam break process is unknown, a sudden breach of the dam, in order to reflect the most dangerous scenario, was assumed to produce the dam break waves. The initial conditions of the water and bed elevation are shown in Figure 15. To account for   Figure 16 shows that the highest predicted depth is nearly 1.8 m. Thus, the simulation result is close to the measured water level.

LESSONS LEARNED FROM THE EVENT
The flood event considered herein is a type of rare event causing a catastrophic result (nine people lost their lives).
To avoid similar disasters, analysis of the precipitation, terrain, land cover effects, and human behavior can be used for development of mitigation plans: • Apart from the heavy storm, the terrain characteristics are one of the main reasons for formation of the debris  • Logs, sticks, and branches are scattered over the channel and make an enormous contribution to the LWD forming the debris dam. The timely cleansing of the woody debris in the catchment, especially in the channel, is required as a priority. For example, an annual patrol can be arranged by the local authority and some big logs could be cut into pieces to avoid the river clogging before the rainy season.
• Since there are two houses at the valley mouth, a contraction occurs at this point in the river, and the flow will be concentrated through the gap between the houses. A nozzle effect takes place, and the velocity will be increased; the intensified kinetic energy and shear stress will flush objects away. Therefore, buildings should not be planned at the valley mouth or enough space should be left for flood routing.
• A road along the river is located next to the valley mouth/ outlet. A flood will cross this road into the river.
However, there are no protecting measures by the riverside; people, therefore, will be prone to being swept into the river. If protecting measures are implemented, the victims will be intercepted and thus prevented from being drowned. The protecting measures, e.g., guard rails (Figure 22), should be designed to convey the water but intercept people, and also host the force arising from debris.

CONCLUSION
In this paper, an unprecedented flash flood leading to nine casualties in Shimen Valley is presented. The causes of the   Since it is an ungauged catchment, detailed hydrological and hydraulic data are not available. To systematically and quantitatively analyze similar flood events, future work is planned to install rain-gauge and discharge meters in the catchment and the long-term data collected can help investigate the mechanism in detail.