Least‐cost analysis and the O and M problem

A major problem affecting public projects in many developing countries is that of inadequate standards of operation and maintenance (O and M). There are various reasons but the overriding one is the shortage of public funds and foreign exchange. Despite the efforts now being made to alleviate the problem it is likely to continue to be a major constraint which should be taken fully into account in the planning and analysis of public projects.

In the planning of irrigation, water supply, power and other projects a choice often exists between technical alternatives with high capital costs and low recurrent costs and others with the opposite pattern of expenditure. Apart from technical factors the final selection is usually based on least‐cost economic analysis, using the conventional discounting methodology whereby all costs, both capital and recurrent, are discounted at what is taken to be the opportunity cost of capital (usually 8 to 15%). These relatively high discount rates favour low capital — high recurrent cost alternatives, even though in practice these are often not desirable, because of the O and M difficulties that will occur.

In many developing countries capital funds for public projects are readily available, from foreign aid, whereas recurrent finance, which is normally provided by the government, with little foreign assistance, is much scarcer. It is argued that in such cases the use of a single discount rate for all types of expenditure is unrealistic. A better . approach would be to apply different interest rates for different types of expenditure, to take account of their differing opportunity costs. In the situation of capital abundance and recurrent funds scarcity, for example, capital costs would be valued differently from recurrent costs. This would be done by means of shadow pricing, the costs for the various types of expenditure being adjusted to take account of their relative opportunity cost, in the same way as adjustments are made for unskilled labour, foreign exchange and other factors in conventional economic analysis.     

Some numerical properties of explicit solutions to the one‐dimensional conservation equation

Abstract:

One‐dimensional unsteady‐state advection‐dispersion models are useful in practice for predicting time‐varying concentrations in streams for phenomena such as chemical spills or combined sewer overflows. The challenge in modeling such phenomena, which demonstrate sharp concentration gradients, is to ensure that the numerical dispersion, an artificial erroneous smearing of concentrations, not overshadow the true dispersion, which, while of significance due to sharp gradients, is ordinarily small compared to advection as a transport mechanism. This paper revisits stability criteria and numerical dispersion properties of explicit Lagrangian and Eulerian solutions of the one‐dimensional advection‐dispersion equation. It is shown that the Leith method of correcting for numerical dispersion (which was developed prior to the recent increase in popularity of Lagrangian methods) has similar stability restrictions on the step sizes as the latter when the zero‐overshoot criterion for stability is used. For uncorrected Eulerian formulations, the stable regions for Dx and Dt depend on the approximation of the advection term. For the forward and central difference approximations, the region is bounded, while for the backward difference formulation, Dx is unbounded. For Lagrangian formulations, the stable region has no upper bound, but has a lower bound on Dx. Correcting numerical dispersion in Eulerian formulations not only results in substantially better accuracy but changes the stable regions for Dx and Dt. The stable regions for corrected Eulerian formulations ‐ indeed, the formulations themselves ‐ are independent of the type of approximation of the first derivative, and are unbounded, although they become increasingly narrow as the true dispersion decreases. As the true dispersion approaches zero, the stable region approaches a line which corresponds to a Courant number of one. Thus, in the limit where the dispersion approaches zero, corrected Eulerian formulations, like Lagrangian formulations, must use a Courant number of one, but corrected Eulerian formulations do not have the lower bound on step size.     

Optimal Design of Barrage Profile on Anisotropic Soil Using Multi‐Objective Optimization Approach

Weirs and barrages are costly hydraulic diversion structures; therefore, any attempt to improve their design is a worthy contribution. Diversion structures, such as weir or barrage, may be designed on permeable formations considering homogenous soil properties. But in reality, soil properties are hardly homogeneous. In this paper, an approach is described to determine an economically efficient barrage profile by considering soil’s anisotropic behaviour. Hydraulic conductivity is considered to be an anisotropic soil property. An optimization-based methodology is developed to obtain the optimal barrage profile. The minimization of the material cost and minimization of the exit gradient is considered for multi-objective formulation. The multi-objective formulation is solved using NSGA-II, and a Pareto optimal front is obtained for different degrees of anisotropy. The flow interaction under a diversion structure in anisotropic soil is incorporated using the Modified Lane theory and is embedded in optimization formulation. The developed methodology is illustrated with a barrage profile as a hydraulic structure. A parametric study is carried out to study the effects of varying barrage design elements on the barrage’s optimum material cost.

     

Multi‐objective risk assessment of freshwater inflow on ecosystem sustainability in San Antonio bay,Texas

Abstract:

San Antonio Bay is located on the coast of Texas between Galveston Bay and Corpus Christi Bay and is the primary bay in the Guadalupe Estuary. Three rivers feed San Antonio Bay from two river basins, including the Blanco and Guadalupe Rivers in the Guadalupe River Basin and the San Antonio River in the San Antonio River Basin. The Canyon Reservoir regulates the flow of fresh water in the middle and lower reaches of the Guadalupe River. These inflows are a primary regulator of salinity and, thus, the productivity of commercially important estuarine species. Increasing demand for water has prompted plans for an increased diversion of 49.3 million m³ (40,000 acre‐feet) from the reservoir. An additional amount of 61.6 million m³ (50,000 acre‐feet) from the mouth of the river is to be pumped back to San Antonio to relieve over‐pumping of the Edwards Aquifer. Because the Guadalupe River Basin contributes 58.1 percent of the freshwater inflow to the estuary, it is not known what the impact of these actions will have on the ecological integrity of the San Antonio Bay. Water resource management in the San Antonio Basin consists of decision making under risk and uncertainty related to randomness in the critical parameters such as the salinity in the bay, biological productivity, and total flow into the bay. The aim of this study is to investigate the trade‐offs between the competing objectives of maximizing biological productivity in the bay and minimizing flow using Stochastic Compromise Programming (SCP). The SCP model solves a multi‐objective function subject to constraints that must be maintained at three different prescribed levels of probability providing a global set of solutions for the water resource management problem under input uncertainty. The SCP model provides information on the trade‐offs among the objective function value, tolerance values of the constraint at the prescribed levels of probability, which could be valuable to policy makers in risk assessment. Solutions were found using three distance functions. Model outputs may suggest the minimum amount of freshwater needed to maximize biological productivity of the bay at specified risk level for assessing the impact of upcoming diversion program. Results indicate that current flows in the Guadalupe River are of sufficient volume to satisfy harvest requirements.