Browsing by Author "Tung, Yeou-Koung"

Now showing 1 - 13 of 13

Bivariate frequency analysis of rainfall intensity and duration for urban stormwater infrastructure design
(2017-10-01)
Jun, Changhyun
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Qin, Xiaosheng
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Gan, Thian Yew
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Tung, Yeou-Koung
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De Michele, Carlo
This study presents a storm-event based bivariate frequency analysis approach to determine design rainfalls in which, the number, intensity and duration of actual rainstorm events were considered. To derive more realistic design storms, the occurrence probability of an individual rainstorm event was determined from the joint distribution of storm intensity and duration through a copula model. Hourly rainfall data were used at three climate stations respectively located in Singapore, South Korea and Canada. It was found that the proposed approach could give a more realistic description of rainfall characteristics of rainstorm events and design rainfalls. As results, the design rainfall quantities from actual rainstorm events at the three studied sites are consistently lower than those obtained from the conventional rainfall depth-duration-frequency (DDF) method, especially for short-duration storms (such as 1-h). It results from occurrence probabilities of each rainstorm event and a different angle for rainfall frequency analysis, and could offer an alternative way of describing extreme rainfall properties and potentially help improve the hydrologic design of stormwater management facilities in urban areas. (C) 2017 Elsevier B.V. All rights reserved.
Dam overtopping risk assessment considering inspection program
(2008-04-01)
Kuo, Jan-Tai
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Hsu, Yung-Chia
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Tung, Yeou-Koung
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Yeh, Keh-Chia
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Wu, Jian-De
Safety inspection of large dams in Taiwan is conducted every 5 years. The practice does not take into consideration uncertainty of dam conditions. The goal of this study is to determine the optimal dam inspection interval under the consideration of overtopping risk incorporating uncertainty gate availability. In earlier studies, assessment of overtopping risk only considered the uncertainties in reservoir properties and natural randomness of hydrologic events without giving much thought to the availability of spillway gates. As a result, the overtopping risk could be underestimated. In this study, an innovative concept is proposed to evaluate dam overtopping by taking into account spillway gate availability. The framework consists of three parts: (1) evaluation of conditional overtopping risk for different numbers of malfunctioning spillway gates; (2) evaluation of spillway gate availability; and (3) dam inspection scheduling. Furthermore, considerations are given to overtopping risk, inspection cost, and dam break cost for determining the optimal inspection schedule. The methodology is applied to the Shihmen Reservoir in Taiwan and to evaluate its time-dependent overtopping risk. Results show that overtopping risk considering the availability of the spillway gates is higher than the one without considering the availability of the spillway gates.
Derivation of rainfall IDF relations by third-order polynomial normal transform
(2018-08-01)
You, Lingwan
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Tung, Yeou-Koung
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防災與水環境研究中心
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Disaster Prevention and Water Environment Research Center
Establishing the rainfall intensity-duration-frequency (IDF) relations by the conventional method, the use of parametric distribution models has the advantage of automatic compliance of monotonicity condition of rainfall intensity and frequency. However, fitting rainfall data to a distribution separately by individual duration may possibly produce undulation and crossover of IDF curves which does not comply physical reality. This frequently occurs when rainfall record length is relatively short which often is the case. To tackle this problem this study presents a methodological framework that integrates the third-order polynomial normal transform (TPNT) with the least squares (LS) method to establish rainfall IDF relations by simultaneously considering multi-duration rainfall data. The constraints to preserve the monotonicity and non-crossover in the IDF relations can be incorporated easily in the LS-based TPNT framework. Hourly rainfall data at Zhongli rain gauge station in Taiwan with 27-year record are used to establish rainfall IDF relations and to illustrate the proposed methodology. Numerical investigation indicates that the undulation and crossover behavior of IDF curves can be effectively circumvented by the proposed approach to establish reasonable IDF relations.
Effect of uncertainties on probabilistic-based design capacity of hydrosystems
(2018-02-01)
Tung, Yeou-Koung
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防災與水環境研究中心
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Disaster Prevention and Water Environment Research Center
Hydrosystems engineering designs involve analysis of hydrometric data (e.g., rainfall, floods) and use of hydrologic/hydraulic models, all of which contribute various degrees of uncertainty to the design process. Uncertainties in hydrosystem designs can be generally categorized into aleatory and epistemic types. The former arises from the natural randomness of hydrologic processes whereas the latter are due to knowledge deficiency in model formulation and model parameter specification. This study shows that the presence of epistemic uncertainties induces uncertainty in determining the design capacity. Hence, the designer needs to quantify the uncertainty features of design capacity to determine the capacity with a stipulated performance reliability under the design condition. Using detention basin design as an example, the study illustrates a methodological framework by considering aleatory uncertainty from rainfall and epistemic uncertainties from the runoff coefficient, curve number, and sampling error in design rainfall magnitude. The effects of including different items of uncertainty and performance reliability on the design detention capacity are examined. A numerical example shows that the mean value of the design capacity of the detention basin increases with the design return period and this relation is found to be practically the same regardless of the uncertainty types considered. The standard deviation associated with the design capacity, when subject to epistemic uncertainty, increases with both design frequency and items of epistemic uncertainty involved. It is found that the epistemic uncertainty due to sampling error in rainfall quantiles should not be ignored. Even with a sample size of 80 (relatively large for a hydrologic application) the inclusion of sampling error in rainfall quantiles resulted in a standard deviation about 2.5 times higher than that considering only the uncertainty of the runoff coefficient and curve number. Furthermore, the presence of epistemic uncertainties in the design would result in underestimation of the annual failure probability of the hydrosystem and has a discounting effect on the anticipated design return period. (C) 2017 Elsevier B.V. All rights reserved.
Evaluation of dam overtopping probability induced by flood and wind
(2011-01-01)
Hsu, Yung-Chia
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Tung, Yeou-Koung
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Kuo, Jan-Tai
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防災與水環境研究中心
;
Disaster Prevention and Water Environment Research Center
This study develops a probability-based methodology to evaluate dam overtopping probability that accounts for the uncertainties arising from wind speed and peak flood. A wind speed frequency model and flood frequency analysis, including various distribution types and uncertainties in their parameters, are presented. Furthermore, dam overtopping probabilities based on monthly maximum (MMax) series models are compared with those of the annual maximum (AMax) series models. An efficient sampling scheme, which is a combination of importance sampling (IS) and Latin Hypercube sampling (LHS) methods, is proposed to generate samples of peak flow rate and wind speed especially for rare events. Reservoir routing, which incorporates operation rules, wind setup, and run-up, is used to evaluate dam overtopping probability.
Incorporating Daily Rainfall to Derive At-Site Hourly Depth-Duration-Frequency Relationships
(2009-09-01)
Wu, Shiang-Jen
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Tung, Yeou-Koung
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Yang, Jinn-Chuang
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土木工程學系
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Department of Civil Engineering
Two models are herein presented to derive at-site hourly based rainfall depth-duration-frequency (DDF) relationships by incorporating daily rainfall information. The model development is motivated by the desire to utilize daily rainfall data that are in existence prior to the conversion from a manned gauge to an automatic one for improving the establishment of hourly based rainfall DDF relation at the site. The performance of the proposed models is assessed by comparing the rainfall DDF relationships derived from the conventional method using solely annual maximum rainfalls of various durations. Hourly rainfall data at Hong Kong Observatory over the period of 1884-1990 are used to demonstrate the application of the two proposed models and to examine their performance. Results from the numerical experiments show that the two proposed models, which incorporate daily rainfall data, are capable of producing more accurate and reliable rainfall DDF relationships than the conventional method solely on the basis of annual maximum hourly rainfalls.
On the statistical analysis of rainstorm events between historical (1777-1907) and modern (1961-2010) periods in Seoul, Korea
(1970-01-01)
Jun, Changhyun
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Qin, Xiaosheng
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Tung, Yeou-Koung
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De Michele, Carlo
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防災與水環境研究中心
In this study, an event-based approach for developing precipitation indices of extreme hydrological events was proposed to analyse characteristics of rainstorm events over long-term periods. Statistical properties of hourly rainfall data were summarized from the retrieved rainstorm events between historical (1777-1907) and modern (1961-2010) periods at Seoul rain gauge station, Korea. The trends and variations of rainstorm events reflecting rainfall depth/duration/intensity thresholds and annual maximum rainstorm events with the largest rainfall depths in these two periods were analysed and compared. The study results indicated that, at a 5% significance level, there were differences in the means and variances of the number of rainstorm events and the average rainfall depth/intensity per storm. For the historical period, the average rainfall depth, duration and intensity per storm with total rainfall durations had a temporal linear trend with slopes of 0.011 mm year(-1), 0.027 hr year(-1), and -0.009 mm hr(-1) year(-1), respectively; whereas, a general decreasing trend was detected during the modern period with slopes of -0.218 mm year(-1), -0.076 hr year(-1), and - 0.010 mm hr(-1) year(-1), respectively. In terms of annual maximum rainfall depth, no significant temporal trend was found in the historical period but an increasing trend (with a slope of 0.710 mm year(-1)) was detected for the modern period. This study was the first attempt to investigate the variability of rainfall characteristics in Seoul, Korea with high-resolution historical data dating back to almost 250 years ago, using a storm-event-based concept which defined precipitation indices with a holistic consideration of the number of events and their rainfall depth, duration, and intensity.
Risk analysis for flood-control structure under consideration of uncertainties in design flood
(2011-07-01)
Wu, Shiang-Jen
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Yang, Jinn-Chuang
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Tung, Yeou-Koung
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土木工程學系
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Department of Civil Engineering
This study presents a risk analysis model to evaluate the failure risk for the flood-control structures in the Keelung River due to the uncertainties in the hydrological and hydraulic analysis, including hydrologic, hydraulic, and geomorphologic uncertainty factors. This study defines failure risk as the overtopping probability of the maximum water level exceeding the levee crown, and the proposed risk analysis model integrates with the advanced first-order and second-moment (AFOSM) method to calculate the overtopping probability of levee system. The proposed model is used to evaluate the effects of the freeboard and flood-diversion channel on the flood-control ability of the levees in the Keelung River, which were designed based on the 3-day, 200-year design rainfall event. The numerical experiments indicate that the hydrologic uncertainty factors have more effect on the estimated maximum water level than hydraulic and geomorphologic uncertainty factors. In addition, the freeboard and the flood-diversion channel can effectively reduce the overtopping probability so as to significantly enhance the flood-control capacity of the levee system in the Keelung River. Eventually, the proposed risk analysis successfully quantifies the overtopping risk of the levee system under a scenario, the increase in the average 200-year rainfall amount due to climate change, and the results could be useful when planning to upgrade the existing levee system.
Stochastic generation of hourly rainstorm events
(2006-12-01)
Wu, Shiang-Jen
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Tung, Yeou-Koung
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Yang, Jinn-Chuang
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土木工程學系
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Department of Civil Engineering
Occurrence of rainstorm events can be characterized by the number of events, storm duration, rainfall depth, inter-event time and temporal variation of rainfall within a rainstorm event. This paper presents a Monte-Carlo based stochastic hourly rainfall generation model considering correlated non-normal random rainstorm characteristics, as well as dependence of various rainstorm patterns on rainfall depth, duration, and season. The proposed model was verified by comparing the derived rainfall depth-duration-frequency relations from the simulated rainfall sequences with those from observed annual maximum rainfalls based on the hourly rainfall data at the Hong Kong Observatory over the period of 1884-1990. Through numerical experiments, the proposed model was found to be capable of capturing the essential statistical features of rainstorm characteristics and those of annual extreme rainstorm events according to the available data.
Stochastically optimal considering groundwater management land subsidence
(2007-11-01)
Chang, Yin-Lung
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Tsai, Tung-Lin
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Yang, Jinn-Chuang
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Tung, Yeou-Koung
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土木工程學系
This paper presents a stochastic groundwater management model explicitly considering land subsidence. Through the use of response matrix technique and one-dimensional consolidation equation, a deterministic management model is first developed. By Latin hypercube sampling technique, along with numerical subsurface flow simulation, statistical features of unit response coefficients due to random hydrogeologic parameters, including hydraulic conductivity (K) and Lame constants (mu and lambda), are quantified. The first-ordervariance-estimation method is adopted to analyze the uncertainties of drawdown and land subsidence based on which the concept of chance-con strained programming is applied to transfer the original deterministic management model into its stochastic form. The stochastic management model enables the determination of optimal total pumpage subject to the constraints that drawdown and land subsidence do not exceed the allowable values with a specified reliability. A hypothetical example is utilized to demonstrate the applicability of the stochastic model to five cases in which various levels of parameter uncertainty are considered. The results indicate that joint consideration of drawdown and land subsidence is essential, and the proposed stochastic management model can be generally applied for regional groundwater resources management in conjunction with controlling land subsidence.
Storm event-based frequency analysis method
(2018-06-01)
Jun, Changhyun
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Qin, Xiaosheng
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Tung, Yeou-Koung
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De Michele, Carlo
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防災與水環境研究中心
In this study, a storm event-based frequency analysis method was proposed to mitigate the limitations of conventional rainfall depth-duration-frequency (DDF) analysis. The proposed method takes the number, rainfall depth, and duration of rainstorm events into consideration and is advantageous in estimation of more realistic rainfall quantiles for a given return period. For the purpose of hydraulics design, the rainfall depth thresholds are incorporated to retrieve the rainstorm events for estimating design rainfalls. The proposed method was tested against the observed rainfall data from 1961 to 2010 at Seoul, Korea and the computed rainfall quantiles were compared with those estimated using the conventional frequency analysis method. The study results indicated that the conventional method was likely to overestimate the rainfall quantiles for short rainfall durations. It represented that the conventional method could reflect rainfall characteristics of actual rainstorm events if longer durations (like 24 hours) were considered for estimation of design rainfalls.
Third-Order Polynomial Normal Transform Applied to Multivariate Hydrologic Extremes
(2019-03-08)
Tung, Yeou-Koung
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You, Lingwan
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Yoo, Chulsang
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防災與水環境研究中心
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Disaster Prevention and Water Environment Research Center
Hydro-infrastructural systems (e.g., flood control dams, stormwater detention basins, and seawalls) are designed to protect the public against the adverse impacts of various hydrologic extremes (e.g., floods, droughts, and storm surges). In their design and safety evaluation, the characteristics of concerned hydrologic extremes affecting the hydrosystem performance often are described by several interrelated random variables-not just one-that need to be considered simultaneously. These multiple random variables, in practical problems, have a mixture of non-normal distributions of which the joint distribution function is difficult to establish. To tackle problems involving multivariate non-normal variables, one frequently adopted approach is to transform non-normal variables from their original domain to multivariate normal space under which a large wealth of established theories can be utilized. This study presents a framework for practical normal transform based on the third-order polynomial in the context of a multivariate setting. Especially, the study focuses on multivariate third-order polynomial normal transform (TPNT) with explicit consideration of sampling errors in sample L-moments and correlation coefficients. For illustration, the modeling framework is applied to establish an at-site rainfall intensity-duration-frequency (IDF) relationship. Annual maximum rainfall data analyzed contain seven durations (1-72 h) with 27 years of useable records. Numerical application shows that the proposed modeling framework can produce reasonable rainfall IDF relationships by simultaneously treating several correlated rainfall data series and is a viable tool in dealing with multivariate data with a mixture of non-normal distributions.
Uncertainty analysis and risk-based design of detention basin without damage function
(2017-05-01)
Tung, Yeou-Koung
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防災與水環境研究中心
;
Disaster Prevention and Water Environment Research Center
Risk-based analysis provides an economically defensible framework for determining the optimal design of hydrosystems with the minimum total cost including project cost (installation plus operation/maintenance/repair) and failure-induced expected damage cost. However, failure-related damage function with good quality may not be widely available in practical applications for assessing annual expected damage cost. In addition to aleatory uncertainty representing natural randomness of hydrologic events, there exists a variety of epistemic uncertainties due to knowledge deficiency from the use of inadequate models, inaccurate model parameters, etc. The presence of epistemic uncertainties could affect the loads and capacity of hydrosystem facilities which, in turn, would affect the value of failure-induced physical performance indicators. Using detention basin design as an example, this paper presents a systematic framework to integrate aleatory and epistemic uncertainties for the risk-based design under the condition of no monetary damage function. For illustration, aleatory uncertainty due to randomness of rainfall intensity and epistemic uncertainties caused by runoff coefficient and curve number are considered in risk-based design of an example detention basin. Plain Language Summary Due to the presence of uncertainties from natural randomness of hydrologic events and knowledge deficiency, risk-based procedure provides a economically and logically defensible framework for determining the optimal design of hydrosystems with the minimum total cost including project cost and failure induced damage cost. However, such framework requires failure related damage information with good quality, which may not be widely available in practical applications. To extend the generality and usefulness the risk-based procedure, this paper presents a viable framework to integrate aleatory and epistemic uncertainties in risk-based design under the condition of without monetary damage information. Design of stormwater detention basin is used as an example to illustrate the methodology.