Su-Chin Chen

Two-layer shallow water computation of mud flow intrusions into quiescent water

The present computational study addresses two-layer shallow flows in which the superposed layers differ in velocity, density and rheology. The geomorphological phenomena motivating this model are confluence problems in which mud and debris surges slump into upland lakes and rivers. Specifically, the flows of interest are assumed to be sharply stratified, with a clear water layer flowing over a moving layer of mud modelled as a Herschel–Bulkley fluid.A finite volume computational scheme suitable for the simulation of such flows is presented and applied to various validation cases. The scheme extends to two-layer flows the robust method of Harten, Lax and Van Leer. Special care is devoted to the following numerical issues: the treatment of pressure along interfaces, the ”contact points“ at which the thickness of either layer can vanish, and the finite yield stress characterizing the mud rheology. Results are presented for the intrusion of mud surges into shallow quiescent water.

Large-Scale Desiccation of the Aral Sea due to Over-Exploitation after 1960

The Aral Sea was one of the largest lakes in the world before it started to shrink in the 1960s due to water withdrawal for agricultural irrigation. Precipitation decreased from 9.4 km3 in 1960 to 3.2 km3 in 2009, and annual river inflow into the Aral Sea decreased from 31.5 km3 in 1998 to 5.2 km3 in 2009. Comparison on the hydrological data of the Aral Sea between 1960 and 2009 showed the evaporation, water surface area, and water volume decreased by 90%, 80%, and 88%, respectively. This study employs the observed values of water volume, precipitation, runoff, evaporation, and salinity to estimate water volume and salinity from 1960 to 2009, and the efficiency coefficients for predicted water volume and salinity are 0.975 and 0.974, respectively. Regression equations calculated from the observed data are used to predict precipitation, runoff, evaporation, and salinity from 2010 to 2021, and the results are then applied in the estimation of water volume and salinity. Our estimates suggest that salinity will increase to around 200 g/L and water volume will decrease to around 83 km3 in 2021.

Debris flow disaster prevention and mitigation of non-structural strategies in Taiwan

Taiwan has disadvantageous conditions for sediment-related disasters such as debris flows. The construction of engineering structures is an effective strategy for reducing debris flow disasters. However, it is impossible to construct engineering structures in all debris flow areas in a short period. Therefore, the government aims to gradually develop non-structural preventive strategies, including evacuation planning, debris flow disaster emergency action system, disaster resistant community program, recruitment of debris flow professional volunteers, debris flow warning systems, and land management strategies, to mitigate disasters and secure the safety of residents. This review describes the processes and effects of recent debris flow non-structural preventive strategies in Taiwan. The average number of casualties prior to the year 2000 was far higher than the corresponding number after 2000 because debris flow evacuation drills have been promoted since 2000 and the debris flow disaster emergency action system has been progressively improved since 2002. Furthermore, the changes in risks caused by debris flow disasters before and after the implementation of non-structural preventive strategies were used to explain the effectiveness of these strategies at the community level. The results showed that software-based non-structural preventive strategies can effectively reduce the casualties caused by debris flows at both the national and community levels.

Sediment removal efficiency of siphon dredging with wedge-type suction head and float tank.

Abstract Siphon dredging with a float tank and different siphon suction heads, including the plain-type and wedge-type with/without side holes, through flume experiments were used to investigate the reservoir sediment removal efficiency. The experiment revealed maximum suction pressure and velocity when the distance from the suction head to the bed was about 25% of suction head diameter. Suction pressure decreased rapidly as the distance from the suction head to the bed increased to 50% and 100% of suction head diameter. Suction pressure achieved by wedge-type suction head was only 16.9–17.6% of that around plain-type suction head, and the velocity around wedge-type suction head was 64.7–68.4% of that around plain-type suction head. However the plain-type suction head was easily clogged doe to its long bobbing period. The average bobbing period for the plain-type suction head was about 105–263% that of the wedge-type suction head. The sediment removal efficiency was achieved when float tank diameter was 3–4 times that of the suction head. Flow discharge and sediment removal increased as the suction head diameter and side hole diameter increased, respectively. The most efficient side hole area was an area within 15% of suction head area. Totally, the optimal sediment removal efficiency was the wedge angle of 20° with 3 side holes. The efficiency of sediment dredging by siphon suction with a suction tube passing through the dam bottom was better than that with suction passing over the top of the dam. The wedge-type siphon suction with a float tank is preferable for sediment dredging in small reservoirs because it is inexpensive and efficient.

An experimental method to verify soil conservation by check dams on the Loess Plateau, China

A successful experiment with a physical model requires necessary conditions of similarity. This study presents an experimental method with a semi-scale physical model. The model is used to monitor and verify soil conservation by check dams in a small watershed on the Loess Plateau of China. During experiments, the model–prototype ratio of geomorphic variables was kept constant under each rainfall event. Consequently, experimental data are available for verification of soil erosion processes in the field and for predicting soil loss in a model watershed with check dams. Thus, it can predict the amount of soil loss in a catchment. This study also mentions four criteria: similarities of watershed geometry, grain size and bare land, Froude number (Fr) for rainfall event, and soil erosion in downscaled models. The efficacy of the proposed method was confirmed using these criteria in two different downscaled model experiments. The B-Model, a large scale model, simulates watershed prototype. The two small scale models, Da and Db, have different erosion rates, but are the same size. These two models simulate hydraulic processes in the B-Model. Experiment results show that while soil loss in the small scale models was converted by multiplying the soil loss scale number, it was very close to that of the B-Model. Obviously, with a semi-scale physical model, experiments are available to verify and predict soil loss in a small watershed area with check dam system on the Loess Plateau, China.

Typhoon-dominated Influence on Wood Debris Distribution and Transportation in a High Gradient Headwater Catchment

Wood debris is an important component of mountain streams. It causes serious damage and renders difficulty of water resource management in Taiwan. In this study, the quantity of wood debris and variation of migratory wood debris during flood events were examined. The downstream of Gaoshan Creek and Qijiawan Creek, located at Central Taiwan, was selected as the study area. The distribution and dynamic of wood debris in a high gradient headwater catchment were quantified using field surveys. A formula of critical depth for wood debris entrainment was used to evaluate the wood debris migration during three flooding events. In the study area, wood abundance and unit volume increased downstream, and wood density decreased downstream within a channel network. The channel morphology, riparian vegetation, and wood debris characteristics were found to influence the wood storage. As a result, the wood debris has an irregular accumulative distribution in the steep stream, and it migrates easily in the stream because of a high flow discharge. Strong relationships between the channel width and wood debris variables are discovered. Moreover, wood debris has a tendency to accumulate at sites with low stream power and wood debris dams, topographical notches, and unique geological structures. Our findings assist in the understanding of the effects of channel characteristics on distributions of wood debris in steep stream systems.

Effects of submerged flexible vegetation and solid structure bars on channel bed scour

Abstract The effects of different submerged obstacle longitudinal bars with different arrangement densities on the flow profile and morphology of a scour hole were investigated under clear water conditions. Acoustic Doppler velocimetry (ADV) data were applied to plot the vertical distributions of three-dimensional velocities and turbulent contours. The experimental results indicate that arrangement density (also can represent porosity), structural material (flexible or solid), and the sidewall effect are the main factors impacting turbulent kinetic energy and the morphology of scour holes. For flexible vegetation, the maximum turbulent kinetic energy near the bed surface increased with the arrangement density. For the same structure, the depth and the magnitude of the lateral expansion of the scour hole also increased with the arrangement density. The flexible vegetation reduced the depth of the scour hole because of deflection and arrangement density. The larger volumes of scour found in the upstream and middle sections of solid structures compare well to those in flexible vegetation. The deflection of porous flexible vegetation transported the turbulent kinetic energy downstream, reduced the turbulent kinetic energy near the sediment bed, and increased the stability of the bars. Flexible vegetation bars are able to protect the bank and the bed of a river under normal conditions, making them a good alternative design in the management and restoration of rivers.

Characteristics and interpretation of the seismic signal of a field-scale landslide dam failure experiment

Outburst floods caused by breaches of landslide dams may cause serious damages and loss of lives in downstream areas; for this reason the study of the dynamic of the process is of particular interest for hazard and risk assessment. In this paper we report a field-scale landslide dam failure experiment conducted in Nantou County, in the central of Taiwan. The seismic signal generated during the dam failure was monitored using a broadband seismometer and the signal was used to study the dam failure process. We used the short-time Fourier transform (STFT) to obtain the time–frequency characteristics of the signal and analyzed the correlation between the power spectrum density (PSD) of the signal and the water level. The results indicate that the seismic signal generated during the process consisted of three components: a low-frequency band (0–1.5 Hz), an intermediate-frequency band (1.5–10 Hz) and a high-frequency band (10–45 Hz). We obtained the characteristics of each frequency band and the variations of the signal in various stages of the landslide dam failure process. We determined the cause for the signal changes in each frequency band and its relationship with the dam failure process. The PSD sediment flux estimation model was used to interpret the causes of variations in the signal energy before the dam failure and the clockwise hysteresis during the failure. Our results show that the seismic signal reflects the physical characteristics of the landslide dam failure process. The method and equipment used in this study may be used to monitor landslide dams and providing early warnings for dam failures.

Long-Term Impact of Extra Sediment on Notches and Incised Meanders in the Hoshe River, Taiwan

The extra sediment load induced by typhoons and rainstorms in the Heshe River, Taiwan, are the principal reason for severe sediment-related disasters. The total sediment load during Typhoon Morakot in 2009 was 31 × 106 m3, accounting for 95% of the annual sediment discharge. Large amounts of sediment load entered the Hoshe River, causing the braiding index (BI) to increase. Subsequently, the BI became positively correlated with the channel width in the Hoshe River. The specific typhoon and rainstorm events decreased after Typhoon Morakot, the sediment input decreased, inducing the fluvial morphology of the braided river to develop into a meandering river. The extra sediment load induced the deposition depth to increase and produce a headward deposition in the main channel and its tributaries. In addition, the river bend and the topographical notch restrained the sediment from moving downstream and being stored locally, indirectly increasing the erosion density of the river banks from 2.5 to 10.5 times.

Simulating Runoff Using the Method of Characteristics with Unsteady Rainfall Events

This investigation presents the solution of kinematic wave equations for overland flow, in which the lateral term is determined from unsteady rainfall and the infiltration φ index model [1], in which the rate of abstractions is constant, yielding an excess rainfall hyetograph with a total depth that equals the depth of direct runoff over the watershed. Lateral inflow is inferred by the unit step function to represent an unsteady rainfall event; the solution uses the method of characteristics. Part of the discharge hydrograph that satisfies the boundary condition is semi-analytically solved, by a reduced, simple procedure, which does not require the use of numerical method such as finite difference. The analysis presented herein this investigation deals with both rising and falling stages. Example calculations, water surface profiles, and discharge hydrographs are also presented.