Using real time data and a flow prediction model to assist in the operation of the lower Orange River

R. S. McKenzie1, Kerry A. Fair2, Andrew R. Craig3
BKS Consulting Engineers, PO Box 3173 Pretoria, South Africa, 0001.
Tel. : +27-12-421 3500,
Fax +27-12-421 3501,
E-mail: 1ronniem@bks.co.za, 2kerryf@bks.co.za, 3andrewc@bks.co.za

A hydraulic model is being used to simulate 1400 km of the Orange River, the largest river in southern Africa, south of the Zambezi and one of South Africa's most important water resources. The system will be used as a decision support tool in the operation of the river so that the supply of water to the lower Orange River can be guaranteed. By incorporating real time telemetry flow and stage data into the model the forecast flows in the river are improved.

Vanderkloof Dam is the last storage structure on the Orange River and after water is released from the dam there is no controlling structure for the remaining 1400 km of river. In this river reach, the annual average evaporation of 3000 mm far exceeds the annual average rainfall of only 50 mm with the result that the net runoff from the catchment is usually insignificant. Therefore, under normal hydrological conditions, the users in this reach of river are dependent on the releases made from Vanderkloof Dam. Furthermore, it can take up to 8 weeks for releases from the dam to reach the most downstream users with the result that there can be no immediate relief from any shortages which might be experienced. Due to the nature of these demands any water shortages would have a major impact, both economic and environmental. In order to manage the Orange River, the Water Research Commission of South Africa, in association with the South African Department of Water Affairs and Forestry, appointed BKS (Pty) Ltd to provide a tool to assist the operators of Vanderkloof Dam in releasing the correct volume of water to meet the downstream requirements without excessive spillage into the ocean.

As part of the World Hydrological Cycle Observing System, the South African Department of Water Affairs, a (SADC) HYCOS member, has set up 6 real time telemetry flow gauging stations on the Orange River, downstream of Vanderkloof Dam. In addition to these stations, flow data will be transmitted from several other stations by means of a cellular telephone communication system. This continuously monitored stage data will be incorporated into a real time simulation of the river so that the pre-calibrated model can be used to make predictions of the river conditions at any point on the river. Starting with the actual stage and flow conditions as given by the real time recorded data, various scenarios of discharges from Vanderkloof Dam can be modelled and the effects monitored. In this way the model will also act as a decision support tool for the operators of Vanderkloof Dam. By incorporating the real time data in the simulations, the need to model the actual runoff from storms in the catchment is reduced as the model is continuously being updated to actual conditions.

The software used for the model was developed to automatically download the real time stage data at intervals which match the transmission times of the stations. The hydraulic modelling software had to be robust enough to cope with sudden changes in stage at the monitoring points. An adaptive timestepping strategy was also introduced to overcome the computational problems caused by the very steep hydrographs resulting from the hydro-power releases from Vanderkloof Dam.

It is intended that, in the near future, the results of the simulations will be available on the Internet allowing any interested parties access to the information. Other logical extensions to the system include linking the software to meteorological and rainfall runoff models to be used for both catchment management and flood forecasting purposes.