Chyan Deng Jan

Movements of a sphere rolling down an inclined plane

Released from rest on an inclined smooth plane in a stationary fluid, a sphere accelerates along the plane under the influence of gravity and eventually reaches a terminal velocity. The variations ...

Experiments on Discharge Equations of Compound Broad-Crested Weirs

A linear combination of traditional discharge equations for simple rectangular and/or triangular weirs is proposed to describe the discharge equations of compound broad-crested (CBC) weirs. The CBC weirs are composed of rectangular, triangular, and/or truncated triangular weirs. Dimensionless discharge equations have been also derived. Laboratory experiments on discharge relations for flows over four CBC weirs were conducted in this study in order to calibrate the proposed discharge equations. The experiments were carried out under the conditions of the H1 ∕ H2 -ratio of water heads above upper and lower crests less than 0.54, and a dimensionless discharge less than 2.174. The result shows that the differences between the calculated discharges by the proposed equations and the measured ones are less than 3% for flows over these CBC weirs under the present experimental conditions.

RAINFALL CONDITIONS FOR THE INITIATION OF DEBRIS FLOWS DURING TYPHOON MORAKOT IN THE CHEN-YU-LAN WATERSHED IN CENTRAL TAIWAN

In the August of 2009, Typhoon Morakot brought heavy rainfall and caused several landslides, floods and debris flows in central and southern Taiwan. This work presents the critical rainfall conditions for the initiation of debris flows in the Chen-Yu-Lan water - shed of central Taiwan during the action of Typhoon Morakot, where had many severe debris-flows events during the actions of Typhoon Herb in 1996 and Ty- phoon Toraji in 2001. Thirty-seven debris flow events induced by the heavy rainfalls brought by Typhoon Morakot in the Chen-Yu-Lan watershed were iden- tified by the examination of aerial photographs and field investigations. The rainfall conditions, including the rainfall intensity, the cumulative rainfall, the rain- fall intensity-duration curve and the return period of the rainfall, for debris-flows initiation were analyzed and compared with the rainfall conditions of histori- cal debris-flows events in the watershed. The average rainfall intensity that triggered abundant debris flows in the watershed were determined and the relation be- tween the density of debris-flows occurrence and the rainfall characteristics were also presented.

Modified McCormicks model for equilibrium shorelines behind a detached breakwater

Abstract Experimental data of equilibrium shorelines behind a detached breakwater obtained by previous investigators were collected and re-reviewed to investigate the major parameters affecting the shoreline shapes. The result shows that the equilibrium shorelines depend not only on the breakwater length B and the distance of the breakwater from the initial shoreline S, but also on the incident wave steepness H0/L0, beach slope Sb and the sand size D50. Most of equilibrium shorelines behind a detached breakwater could be approximately described by a couple of elliptic curves as proposed by McCormick (Ports, Coastal & Ocean Engineering ASCE 119, 1993, p. 657). However, after re-examination, this paper shows that the dimensionless semiminor axis b/S and the dimensionless distance G/b in the McCormick’s elliptic-curve model should be modified, as compared with the available experimental data. The modified expressions of b/S and G/b were proposed, and the performances of the modified expressions were also detaily examined in this paper.

Hydraulic Jumps in an Inclined Rectangular Chute Contraction

Hydraulic jumps in an inclined rectangular chute contraction were studied in this paper. Theoretical equations for the sequent-area and sequent-depth ratios for hydraulic jumps in the contraction were developed considering the effects of contracting width and sloping bottom. The equation of the sequent-area ratio (instead of the sequent-depth ratio) has the same form as the traditional Belanger equation for the sequent-depth ratio for hydraulic jumps in a horizontal rectangular constant-width channel (HRCW channel). A modified approach Froude number M , including the effects of contracting width and sloping bottom, was introduced to replace the approach Froude number F1 in analyzing hydraulic jumps in the contraction. Laboratory experiments of hydraulic jumps in inclined contractions were also conducted to verify the theoretical sequent-area ratio and also to develop the empirical equations of the location of the start of the jump, flow depth of the start of the jump, length of the jump, and energy loss o...

Discharge Coefficient for a Water Flow through a Bottom Orifice of a Conical Hopper

Discharge coefficients for water flow through a vertical, circular orifice at the bottom of a conical hopper were experimentally studied in the present paper. The conical hopper consists of a cylindrical hopper of inside diameter of 48 cm and a bottom cone of side slope of 45°. Experiments were carried out under different orifice diameters and water heads. The dependence of the discharge coefficient on the orifice diameter and water head was analyzed, and then an empirical relation was developed by using a dimensional analysis and a regression analysis. The results show that the larger orifice diameter or higher water head have a smaller discharge coefficient and the orifice diameter plays more significant influence on the discharge coefficient than the water head does. The discharge coefficient of water flow through a bottom orifice is larger than that through a sidewall orifice under the similar conditions of the water head, orifice diameter, and hopper size.

Reliability analysis of design discharge for mountainous gully flow

A probabilistic equation of design discharge for a mountainous gully was derived in terms of reliability probability. Its derivation was based on the rational formula, the rainfall-intensity-duration formula, and the First-Order Second-Moment method (FOSM) by considering the uncertainty of parameters, such as the annual rainfall, the runoff coefficient, the watershed area, the sediment concentration, the length and velocity of overland flow, and the horizontal length and elevation difference of a gully, involved in the formulas. The probabilistic equation was also applied to the probabilistic design discharge for a debris-flow-prone gully in Taiwan.

A Web-Based Decision Support System for Debris Flow Disaster Management in Taiwan

Due to the fragile geologic structure and torrential rainfall brought by typhoons, debris flows have become one of the most serious disasters in recent years in Taiwan. The Soil and Water Conservation Bureau (SWCB), Council of Agriculture, is the national level department in charge of the debris flow disaster management according to the Disaster Prevention and Protection Act. In this paper, a web-based decision support system—Debris-flow Disaster Management Information System established by SWCB is introduced. By the way of advanced information techniques and user-friendly interface through the internet, the system provides the state-of-art debris flow knowledge and real-time debris flow warning information to various agencies of debris flow disaster management as well as to the general public. A rainfall-based debris flow warning model is also introduced herein. This warning model has effectively helped us in the debris flow management, including the people evacuation, risk warning and debris flow knowledge education.

Analysis and prediction of riverbed changes using empirical orthogonal functions

A two-dimensional empirical eigenfunction model is proposed to analyze and predict the variations of riverbed profile. This model consists of two along and cross-river eigenfunctions and one temporal eigenfunction. These eigenfunctions can be generated based on a series of measured riverbed profiles. The proposed model is applied to analyze the riverbed variations of the downstream of Chou-Shui River in Taiwan. The prediction of riverbed variations is also conducted by using the generated eigenfunctions with the help of the Markov process model.A dimensionless effective flow discharge is assigned to be the forcing parameter in the predicting model, which controls the riverbed changes. The results show that the proposed method can reasonably analyze and predict the riverbed variations.

Advances of Geo-Disaster Mitigation Technologies in Taiwan

Taiwan is located in the typhoon district of the Pacific Ocean and is hit by typhoons averaged 3.6 times annually. Many large scale hillside development projects were undertaken in the past decades. These areas have become very vulnerable to natural hazard and usually are subject to heavy damage under slope-land disasters. The loss of people’s lives and properties are severe. The average death toll was 8.26 casualties per event from 1981 to 1999. Starting from 1992, huge resources had been spent in the research of debris flows and landslides. After the assessment and warning system started on 2000, the number of casualties decreased to an averaged 2.23 per event between 2000 and 2008 and the average decreased as time progress. There were 22 large landslides or debris flows in 2008, but the average casualty was only 0.55 per event. This paper describes the technology advances from 2000 to 2010 in Taiwan. The most important advances are in the assessment technology, warning technology and mitigation concepts. The assessment method started with pure empirical formula, then improved to delimiting hazard zone numerically. The most recent change is to include social vulnerability in assessment. The warning system started with very local monitoring to nationwide warning with fine resolution quantitative precipitation estimation with multiple sensor radar system. The warning time has improved to more than 24 h before the disaster. The concept for mitigation changes from traditional structure countermeasures to ecological means. However the most fundamental change is the shifting emphasis from structure mitigation to non-structure mitigation with planning, education and drills.