Discharge Measurements at Natural Controls in Western Cape Rivers
Report no 1270/1/04
Dec 2004

EXECUTIVE SUMMARY

This study set out to explore the possibilities and accuracy of flow measurement at natural controls, focusing specifically on Western Cape cobble-bed rivers. These rivers are regarded as being difficult in terms of flow measurement, mostly due to their large scale roughness and very turbulent and uneven flows.

The aim was to determine a system through which various types of natural controls could be calibrated, by establishing a relationship between the discharge coefficient and the physical characteristics of each control type. This was achieved by conducting an extensive field investigation, focusing on the identification and gauging of both critical and uniform natural controls. Two types of critical controls were identified and investigated, namely step-pool controls and horizontal constriction controls as well as one type of uniform flow control, namely the plane bed control.

Step-pool controls were found to be very robust controls which provided efficient critical controls for a wide range of flows. Horizontal constriction controls proved to be reasonably accurate measurement sites and, because of their physical characteristics, they are able to measure the full range of flows from low to flood flows. Uniform controls are widely used for flow measurements on deep rivers, but such measurements become highly unreliable when flow depths are small and bed roughness high as is the case with cobble-bed rivers. Under conditions of high roughness, generally speaking, critical controls can be calibrated more accurately than uniform controls and are therefore to be preferred.

The field study was followed by a series of laboratory tests, focusing on horizontal constriction controls. Calibration equations were derived by which the discharge coefficient value can be determined for a given constriction ratio and upstream energy head.

Results from both fieldwork and laboratory work were analysed and integrated to determine a calibration system applicable to natural controls in Western Cape cobble- and boulder-bed rivers. Reasonably accurate values of the discharge coefficients for different types of natural controls were established, which should be widely applicable.

Additionally, guidelines were drawn up for the measurement of flow at natural controls.

Discharge measurement at natural controls in the Western Cape was found to be a viable and reasonably accurate way of determining river discharge. It proves to be an environmentally acceptable way of measurement, using the natural river characteristics rather than seriously interfering with them or damaging them. These advantages make discharge measurement at natural controls a promising means of determining river discharge under suitable conditions.

Discharge measurement can be undertaken at suitable natural controls in rivers where a lower degree of accuracy (± 10% in terms of discharge) than that which is achievable with man-made weirs and flumes, is acceptable.

Natural controls may be utilized in environmentally sensitive areas and their accuracies can be improved by modification and stabilization of the river topography whilst maintaining a “natural look”.

Natural controls can either be calibrated by means of discharge coefficients, such as those in this report, or in situ by means of temporary structures in the river, which can be removed once the stage discharge relationship has been established at the control.

Natural controls can also be utilized and adapted to serve as fishways.

Given the complex flow patterns which are encountered at fishways, natural control should only be calibrated by persons with the necessary knowledge and experience in the field of hydraulics.

Further research on discharge measurement at natural controls in rivers needs to concentrate on the application of the principles and coefficients in prototype (field) situations.

This report forms one of the outcomes of WRC Project No 1270 : FLOW MEASUREMENT AT NATURAL RIVER CONTROLS AND THE PROVISION OF FISHWAYS. The other outcome is a WRC report titled PRELIMINARY GUIDELINES FOR THE PLANNING AND DESIGN OF FISHWAYS IN SOUTH AFRICA.

Natural or slightly modified natural controls form preferred fishways whilst natural controls may cause drowning out of man-made flow gauging structures. The two reports therefore supplement each other.