Investigation of model and parameter uncertainty in water quality models using a random walk method

A mathematical model to predict the effect of chemical spills in the Forth estuary in Scotland has been in use for many years. The model, based on the random walk method, predicts chemical concentrations in the estuary waters and estimates the elapsed time before the dilution is sufficient to render the spill harmless (making use of a toxicity measure such as the LC50 or a water quality standard). The model gives a deterministic result without any estimate of the uncertainty. Field studies using tracer dyes to measure the horizontal and vertical mixing rates in the estuary show that these rates vary over time. The literature on turbulent diffusion includes modelling applications using different parameterisations of the mixing process. This paper investigates the uncertainties in predicted concentrations due to model parameterisation of horizontal mixing and due to the variability in the measured mixing rates determined from surveys in the estuary. Estimates of the range of concentrations for a specific spill scenario are presented.The study shows that model formulation and parameter uncertainty are both important factors in estimating the uncertainty in model predictions. The uncertainty caused by the variations with time found in the measured mixing rates is found to be of similar magnitude to the differences in concentration resulting from using three different methods for modelling the horizontal mixing in the estuary. Uncertainties associated with model formulation could be reduced if a small number of longer timescale (e.g. 24 h) dispersion experiments were available. In addition, further data from short-term (3 h) dispersion experiments would give a better understanding of the distribution of mixing coefficients and how the mixing relates to other parameters such as tidal range and wind speed and direction.     

Rapid transit system simulation with interactive graphics display

A mathematical model of a rapid transit system has been developed. The model employs computer techniques to simulate the physical system. To further enhance the results and gain management confidence in the reliability and validity of the design of the model, a TV‐like display of the computer results enables the viewer to “see” the performance of the model as it happens. The results displayed thus include: The track layout, time (simulation), train position on the track, and the number of passengers riding on each train and waiting at each platform or station. The computer printout of results is limited to summary type data, relying on the display for detailed evaluation and analysis.     

Book reviews

Road haulage licensing and EC Transport Policy. By K. J. Button. (Aldershot: Gower Publishing Co., 1984.) [Pp.127.] £14.50.

Bicycles and Public Transportation: New Links to Suburban Transit Markets. By Michael A. Replogle. (Bicycle Federation, Washington DC, 1983.) [Pp. 171.] US$15.95.

Urban Transport in ASEAN. By V. Setty Pendakur. (Institute of Southeast Asian Studies — Research Notes and Discussions Papers Series No. 43, 1984.) [Pp. 65] $4.50 ISBN 9971–902–67–2.

The World Railway System. By Bernard de Fontgalland (Cambridge Univerisity Press, 1984.) [Pp. 209.] £16.00.     

Congestion and emissions mitigation: A comparison of capacity,demand, and vehicle based strategies

Capacity, demand, and vehicle based emissions reduction strategies are compared for several pollutants employing aggregate US congestion and vehicle fleet condition data. We find that congestion mitigation does not inevitably lead to reduced emissions; the net effect of mitigation depends on the balance of induced travel demand and increased vehicle efficiency that in turn depend on the pollutant, congestion level, and fleet composition. In the long run, capacity-based congestion improvements within certain speed intervals can reasonably be expected to increase emissions of CO₂e, CO, and NO_x through increased vehicle travel volume. Better opportunities for emissions reductions exist for HC and PM_2.5 emissions, and on more heavily congested arterials. Advanced-efficiency vehicles with emissions rates that are less sensitive to congestion than conventional vehicles generate less emissions co-benefits from congestion mitigation.     

大型基础设施系统设计中的风险及概率

在大型基础设施系统中,不可避免地会出现重大的不确定性,因而,无法确保已建系统的安全性和可靠性.有功能失效甚至破坏的可能,同时也有相应的规避风险的措施.从实用角度出发,不确定性可以分为两大类:数据型(又称偶然型)不确定性和知识型(又称认知型)不确定性.前者可以通过对数据的观察来评估,而后者则需要主观判断.系统地分析了这两...     

Balancing the economic and environmental performance of maritime transportation

This paper looks at the implications of various maritime emissions reductions policies for maritime logistics. There can be important trade-offs that have to be made between the environmental benefits associated with such measures as reduction in steaming speed and change in the number of vessels in the fleet, and more conventional logistics attributes such as in-transit inventory holdings.     

Distribution of plankton lipids and their role in the biological transformation of Antarctic primary production

Production and transfer of lipid through the Antarctic food web is reviewed for the Indian Ocean sector. The slow settling fine particles showed a marked inter-annual variability in biochemical composition with an increase in lipid content as % organic carbon. Comparison of the fatty acid spectra of different size categories of organic particles indicated that fine particles are dominated by saturated, monoenoic and branched acids, while larger material (50–100 μm, 200–500 μm net collected fractions) displayed a signature dominated by polyunsaturated acids. Zooplankton taxa displayed different strategies of lipid accumulation. Lipid content was highest in Thysanoessa macrura females and copepodite stages of Calanus propinquus. Relatively low levels were recorded for juveniles and male stages of euphausiids. Reserve lipids varied with species: C. propinquus showed equal content of triglycerides and wax esters, T. macrura showed a dominance of wax esters and Euphausia superba and Themisto gaudichaudii accumulated only triglycerides. Computed as carbon equivalent and integrated over 200 m, lipids in slow settling particles represented 22.6% of annual primary production. Similar computation with mesozooplankton and E. superba data on biomass and population structure from several summer cruises indicated values of carbon accumulation as lipid reserves and egg production of 4.2 and 0.1% of annual primary production for copepods and 4.4 and 3.8% for E. superba. When all trophic levels are considered, the overall mean exceeded 30% of annual primary production.     

Intercity rail services: A nested logit stated choice analysis of pricing options

There is a renewed interest in intercity and long-distance rail services in many countries, with both new high-speed rail services and improvements to conventional rail under review. The current study reports on an investigation using a stated choice experiment, of the demand for sleeper services between Sydney, Northern New South Wales (NSW) and Brisbane in Australia, a 12 to 14 hour trip, just after a decision by the NSW government to temporarily suspend sleeper services and introduce seating service only, pending an inquiry into the demand for such loss-making services under alternative price and service levels. A matrix of direct fare elasticities within the rail mode and between rail and competing modes are obtained for concession and non-concession travellers from a nested logit model. The empirical evidence extends our knowledge of the sensitivity of the long distance passenger market to a range of rails fares, distinguishing between classes of fares and levels of service.     

焊轨机的发展前景

本文介绍了能够解决现代钢轨,包括电炉钢钢轨,在使用线路焊轨机进行焊接工作时发生焊接问题的新工艺、新技术和新装备。