Evaluating Risks of Dam-Reservoir Systems Using Efficient Importance Sampling

The occurrence frequency of failure events serve as critical indexes representing the safety status of dam-reservoir systems. Although overtopping is the most common failure mode with significant consequences, this type of event, in most cases, has a small probability. Estimation of such rare event risks for dam-reservoir systems with crude Monte Carlo (CMC) simulation techniques requires a prohibitively large number of trials, where significant computational resources are required to reach the satisfied estimation results. Otherwise, estimation of the disturbances would not be accurate enough. In order to reduce the computation expenses and improve the risk estimation efficiency, an importance sampling (IS) based simulation approach is proposed in this dissertation to address the overtopping risks of dam-reservoir systems. Deliverables of this study mainly include the following five aspects: 1) the reservoir inflow hydrograph model; 2) the dam-reservoir system operation model; 3) the CMC simulation framework; 4) the IS-based Monte Carlo (ISMC) simulation framework; and 5) the overtopping risk estimation comparison of both CMC and ISMC simulation. In a broader sense, this study meets the following three expectations: 1) to address the natural stochastic characteristics of the dam-reservoir system, such as the reservoir inflow rate; 2) to build up the fundamental CMC and ISMC simulation frameworks of the dam-reservoir system in order to estimate the overtopping risks; and 3) to compare the simulation results and the computational performance in order to demonstrate the ISMC simulation advantages. The estimation results of overtopping probability could be used to guide the future dam safety investigations and studies, and to supplement the conventional analyses in decision making on the dam-reservoir system improvements. At the same time, the proposed methodology of ISMC simulation is reasonably robust and proved to improve the overtopping risk estimation. The more accurate estimation, the smaller variance, and the reduced CPU time, expand the application of Monte Carlo (MC) technique on evaluating rare event risks for infrastructures.

An Exorbitant Privilege in the First Age of International Financial Integration

The exorbitant privilege literature analyzes the positive differential returns on net foreign assets enjoyed by the United States in the last quarter of the twentieth century as the issuer of the global reserve currency. In the first age of international financial integration (1870-1914), the global reserve currency of the period was the British pound sterling. Whether the United Kingdom enjoyed a similar privilege is analyzed with a new dataset, encompassing microdata on railroad and government financial securities. The use of microdata avoids the flaws that have plagued the US studies, particularly the use of incompatible aggregate variables. New measures of Britain’s net external position provide estimates on capital gains and dividend yields. As the issuer of the global reserve currency, Britain received average revenues of 13.4% of GDP from its international investment position. The country satisfied the necessary condition for the existence of an exorbitant privilege. Nonetheless, Britain’s case is slightly different from the American one. British external assets received higher returns than were paid on external liabilities for each class, but British invested mostly in securities with low profile of risk. The low return on its net external position meant that, for most of the time, Britain would not receive positive revenues from the rest of the world if it were a net debtor country, but this pattern changed after 1900. The finding supports the claim that, at least partially, exorbitant privilege is a general characteristic of the issuer of the global reserve currency and not unique to the late twentieth century US.