David Stephenson

Direct optimization of Muskingum routing coefficients: An extension to the paper by Gill, M.A., 1978. Flood routing by the Muskingum method. J. Hydrol., 36: 353–363

Abstract A direct method of calibrating the Muskingum routing equation is explained. Linear programming techniques are used to minimize the absolute value of the error. The error is defined as the difference between an observed and a routed outflow hydrograph. An alternative but more complicated objective function would be to minimize the sum of the squares of the errors at observation points. The linear method could be applied in conjunction with Gills segmented curve method for non linear storage relationships.

Radiotracer evaluation of groundwater dispersion in a multi-layered aquifer

Groundwater flow near the Koeberg Nuclear Power Station was investigated by Stephenson in order to evaluate the extent of seawater intrusion around the foundations. In particular he studied the movement of the interface between fresh groundwater and seawater. For the purpose of such study it has been found necessary to determine the dispersion coefficient and confirm the velocity of fresh groundwater. This is carried out using131 I as tracer. Measurement of time distributions of tracer concentration in a series of boreholes permits the development of a mathematical model which appears to describe quite satisfactorily groundwater flow as well as intermixing in several layers. Dispersion across the interface has been anticipated and the dispersion coefficients for each separate layer and for the whole aquifer are determined from a fit of the model to the experimental data.

Integrated flood plain management strategy for the Vaal

Owing to development pressure for land bordering the Vaal river, in South Africa. Rand Water are revising their development policy. Restrictions were in place to avoid flooding and obstructing the flood flow of the river. Relaxation of the regulations will permit controlled development along the river. A flood hazard-risk index was developed to indicate where development could be permitted. An economic comparison of costs and benefits supports the relaxation.

Water demand theory and projections in South Africa

Abstract This study estimates Rand Waters water demand in South Africa for 2000 to 2020. It indicates lower growth in water consumption in the foreseeable future than in the past century. The reduced growth is due to: Economic slowdown (international and South African); Lower standard of living/residences for the “white” population; Corresponding decrease in formal business growth; Limitations of services in built up areas; Increased water costs; Improved loss control; HIV/Aids; Unwillingness of informal residents to contribute for services. This is offset by: Migration of poor to cities and creation of peri-urban growth; Increased standards for previously disadvantaged people; Improved health facilities; Growth in informal economy; New political freedom. A net growth of 2 percent annually is forecast. A statistical approach is adopted to obtain confidence limits for individual sectors. A decomposed demographic model is developed and used to predict water demand until 2020. A wider uncertainty in future consumption is foreseen. Conservative planning is recommended for future water resource projects, e.g. low capital, and operating intensive schemes are preferable. A methodology for planning in the face of uncertainty and low growth is developed.

Radiotracer dispersion tests in a fissured aquifer

Abstract Coastal sites were to be investigated for possible construction of a power station. Aquifer geohydrological parameters were to be established on a macro scale for purposes of estimating sea water intrusion and interface mixing. Laboratory tests, pump tests and packer tests were conducted to make successively better estimates of the aquifer properties. Site tests were then conducted to determine dispersivities. The holes were in groups of five, with a central injection hole and four surrounding holes on generally north-south and east-west axes at distances ranging from 3 to 16 m from the central hole. The radiotracer was selected after conducting laboratory adsorption tests. Injections of 51 Cr were made, followed by continuous pumping of water into the central hole. Radiotracer concentrations were determined by inserting a probe in the surrounding holes at intervals over the depth of the hole. The concentrations were corrected for background and decay and plotted against time. A mathematical model of the process was calibrated to yield the aquifer dispersivities at different depths. The values were found to be depth and direction dependent. Aquifer characteristics in saturated zones, i.e. transmissivity and dispersivity, can be quite effectively determined in situ by means of artificial tracers, provided the behaviour of such tracers is representative of that of the water in the aquifer, and the scale of the field experiment is comparable with the largest expected fissures and channels in the formations. Because radioisotopic tracers can be detected at extremely low concentrations, they have been increasingly used in such type of studies and this method was consequently selected for use in the work.

Computer analysis justifies desalination

Abstract Whereas it is recognized that desalination is not economically justifiable in many water supply situations, this paper describes how by careful re-planning of the water distribution system in a gold mine, the installation of a desalination plant underground can be shown to be economic despite cheaper fresh water being available on the surface. The advantage is largely in the reduction in pumping costs of return water together with the cost of the fresh water. The optimum distribution pattern to meet quality requirements can be selected using a computer program developed for this purpose. The program receives data on minimum flow requirements and quality constraints and adjusts flows around closed loops in a way which meets operating requirements whilst minimizing total distribution costs.

Depiction of friction damping in numerical flow modelling

Abstract The use of numerical modelling for prediction of water levels and flows is introduced. Models are classified and simplifications appropriate to various configurations are suggested to minimise programming effort and computational capacity and time. The use of animated colour graphics is advocated for depiction of results, and for setting up the numerical scheme for solving the hydraulic differential equations. The effect of friction in damping waves and improving numerical stability is demonstrated with the assistance of a micro computer.

Estimate of dispersion in an unsaturated aquifer

Abstract The Nuclear Development Corporation of South Africa (Pty) Ltd. (NUCOR) is constructing a low-level radioactive waste disposal site near Springbok in Namaqualand, an arid region to the west of South Africa. A groundwater model was developed which required site-specific data and this work describes procedures developed to assess the dispersivity of the soil in the vicinity of the proposed site. Preliminary laboratory tests, carried out using a sodium chloride solution, indicated the order of magnitude of the dispersivity for saturated soil at various levels. This enabled site tests to be designed. The site tests were done by injecting a pulse of scandium-46 into a hole and monitoring the displacement of the radioactive cloud as it moved down under gravity and spread laterally. A mathematical model was developed to predict the behaviour of the cloud and calibration of the model yielded vertical and horizontal dispersivities. The dispersion of radioactivity at the cloud front was assumed to occur in unsaturated medium while the continuously injected water behind the radioactivity was assumed to disperse in a saturated medium. Thus monitoring the concentration of both yielded approximate values for the effective dispersivities in unsaturated and saturated media.

Demand Management, Price and Reliability

This chapter addresses the water supply issue from the consumers’ point of view, whereas the previous chapter was written for the engineer, economist or planner concerned with establishing water supply. It also assists in establishing a pricing policy. Demand management refers to the control of use of water, but equally well requires consumer interaction rather than autocratic imposition of a supply rate or price.

Asset Management and Information Technology

The operation of a water authority should, like any other organization, be run on sound business principles. Even though profitability may not be the sole driver, a sound economic basis is required. This requires good control of finances, physical installations and human resources, as well as exhibiting a good profile to convince the public, the controlling authorities and the consumers of the direction of the water company.