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 Vaal-Wilge
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 Vaal-Wilge Flood Monitoring

DELPORT DU PREEZ AND ASSOCIATES

CONSULTING ENGINEERS AND PROJECT MANAGERS

VAAL DAM FLOOD MONITORING SYSTEM

Index: Introduction, Vaal dam, Purpose, Planning, Project team, Description, Facilities, Facts, Power supply, Surge protection, Further developments

INTRODUCTION

Vaal dam is a vital part in the water supply system of the Pretoria-Witwatersrand-Vereeniging region. Water for Vaal dam is collected by the Vaal and Wilge rivers, with the Vaal River catchment area in the Mphumalanga and the Wilge river catchment area in the Eastern parts of the Free State.

Management of this Water Resource includes the following aspects :

  • Monitoring water supply to the major dams.
  • Exercising flood control.
  • Maintaining water supply to downstream communities.
  • Plan alternatives to accommodate future growth.
  • A flow monitoring system is one of the important tools to help the management with the day-to-day operation of the system, as well as providing records for long term planning. In order to plan the release of water from the Sterkfontein dam it is necessary that water flow in all rivers feeding the Vaal Dam be known.

Data for Hydrological history compilation is collected using standalone-recording stations situated at strategic points throughout the catchment areas. Data is collected from these stations at weekly intervals, digitized and processed for inclusion in the National Hydrological Data Bank in Pretoria.

For flood warning and control purposes it is however, necessary that real-time data be made available at the Flood Control Centre in Pretoria, from where control of dams is co-ordinated on a countrywide basis. The information is also required at Vaal dam, where the actual control is exercised via the operation of gates.

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VAAL DAM

This dam is situated at Deneysville near Vereeniging and some interesting points can be noted :

  • Full storage capacity: 2 529 million cubic metres.
  • Full storage level: 22.460 metres of water.
  • Full flood level: 24.390 metres of water.
  • Full flood capacity: 126% of full storage capacity.
  • Water level at the foot of the crest gates: 8.295 metres
  • Crest gates: 60 units (6 metres high and 7,78 metres wide).
  • Crest gate discharge: 200 cubic metres per second at 2340m. 12 000 cubic metres per second total.
  • The peak flood during the 1974 season: 3 000 cubic metres per second.
  • Vaal dam is designed for a possible maximum flood of 25 000 cubic metres per second.

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PURPOSE OF THE TELEMETRY SYSTEM

 The telemetry system was primarily designed to provide early warning of conditions that may cause a flood and thereafter to provide an accurate record of the situation existing at various points in the catchment area. The information collected is used to perform flood routing by taking the hydrographs obtained form the various river measuring stations and forecasting the propagation of the flood wave to the stations further downstream. Where rivers and tributaries join en route, the hydrographs are combined and the flood wave propagated further downstream. The combined effects of all these flood waves that will arrive at the various points are analyzed and steps are then taken to either delay or deflect one or more of the flood waves to reduce damage or alternatively to evacuate endangered areas. This process also provides an indication of the volume of water that can be expected to flow into Vaal dam. By having the information available at an early stage, a 10 to 20 hour period will become available to taksteps to attenuate the flood wave by the release of water in controlled volumes from Vaal dam and still have a reasonable chance to end the flood with a full dam.

Other uses of the telemetry system are :

  • Collecting water flow statistics.
  • Assist with day-to-day resource management (monitoring of water released from Sterkfontein and Grootdraai dams and pumped from the Highveld via Geelhoutboom pump station.)

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PLANNING OF THE TELEMETRY SYSTEM

Planning and design of a flow and flood monitoring telemetry system commenced early in 1985 and tenders were invited at the end of 1985.

The contractor was appointed in January 1986 and commissioning of the system was undertaken in June 1987. The network started functioning in December 1986.

PROJECT TEAM

Employer: the Department of Water Affairs and Forestry

 User division: the Operations Division, Vaal River Region

Flood control: the Director, Hydrology, Pretoria

Consulting engineers: Delport du Preez and Associates

Contractor: ABB Brown Boveri South-Africa

Sub-contractor: Prodesign (Pty) Ltd

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DESCRIPTION OF THE SYSTEM

15 River measuring stations are placed at carefully selected positions along the rivers and major tributaries, with 7 stations in the Vaal River area, 5 stations in the Wilge River area and 3 stations covering the rivers downstream of Vaal Dam.

The river measuring stations record the following information:

  • Water level (2 methods) Temperature
  • Water flow
  • Battery voltage
  • Rain precipitation
  • Intrusion alarms
  • Maintenance alarms

3 Measuring stations are located on the Vaal dam, recording the following additional parameters :

  • Average wind speed
  • Average wind direction

4 Main radio repeater stations form the communication backbone, with a dual UHF radio system and PSTN back-up. Two terminal repeater stations can also act as manually operated network controllers. All 4 stations are equipped with telemetry facilities and also collected rainfall data.

7 Secondary repeater stations complete the radio communication network to all remote stations.

The network control master station is situated at Vaal dam control centre. A 40 day data history file is maintained for each outstation in the network and colour graphics facilities are available for data base manipulation and processing. The master station data base is automatically updated at 15 minutes intervals. The day-to-day operation of the network is controlled from this station.

Secondary master stations are operated in Pretoria and Standerton respectively. These master stations maintain duplicate copies of the data base and updating is done on a daily basis, using the PSTN facilities.

The Pretoria master station is connected via a LAN to Govnet and assist with flood routing functions.

The Standerton master station is located at the Departmental workshops and provides daily logs to the maintenance staff, listing faults, failures and maintenance alarms. Alarms counters are also provided.

Network communication is based on UHF radio carrier. 5 Double frequency channels in the 450 - 470 MHz band are employed. Telephone facilities provide access to the network controllers at Frankfort and Standerton. In case of failure of the Vaal dam master station of the main radio network, outstation data can be obtained from the two other master stations, ensuring continued operation.

Telephone access to the Vaal dam master station from the other master stations can be obtained through any one of three points :

  • PSTN directly to Vaal dam.
  • PSTN to Standerton and via radio to Vaal dam.
  • PSTN to Fankfort and via radio to Vaal dam.

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FACILITIES OFFERED BY THE NETWORK

All telemetry stations are of the intelligent type using microprocessors. Outstations are equipped with alpha-numeric keyboard and display facilities and operator interface is via menu driven routines.

The operator is able to completely interrogate the outstation from the local keyboard/display and all parameters can be set and calibration routines performed at the outstation. All changes are reported to the master station.

Alpha-numeric text messages can be transferred between outstation and master station operators. All outstation parameters can also be downloaded from the master station.

Each outstation can store data and alarms collected over a period of 96 hours. This ensures the maintenance of the master station fault is repaired within the 96 hour period, no data will be lost. This also applies to master station faults.

An engineers voice channel is available between the master station and any outstation in the network.

Each outstation provides water level as well as river flow readings. Flow is calculated from level readings using floating point arithmetic and conversion formulae with 5 sets of conversion constants, all downloadable from the Pretoria of Vaal dam master stations.

Operator access to the outstation is passkey protected.

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SOME INTERESTING FACTS

  • Area covered by the radio network: 16 300 square km.

 Accumulated length of the radio links: 590 km.

 Total number of radio links: 28.

 Number of PSTN links: 8.

 Number of radio transmitters/receivers: 40.

 Longest radio route for the Wilge River area: 213 km.

 Longest radio route for the Vaal River area: 137 km.

 Number of data blocks transferred per 24 hours: 2 112.

 Number of measurements collected per 24 hours: 17 000.

 Number of measurements stored at the master: 680 000.

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POWER SUPPLY UNITS

The major source of power for the outstations and repeater stations is solar power.

Each outstation is equipped with an 80 watt solar panel. The main battery is a Willard tubular plate type with a 75 Ah capacity. Emergency power is provided by a sealed 6,5 Ah battery. Standby battery capacity is approximately 1 week.

The Pretoria and Vaal dam master stations are equipped with uniterruptable power supply units. At Vaal dam the capacity is 24 hours using a 960 Ah battery bank. At Pretoria the standby capacity is 4 hours.

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SURGE PROTECTION

Extensive steps were taken to limit damage caused by lightning and other static discharges.

All input and output circuits are routed via surge protection units. SABS and CSIR tests were performed on these units to ensure proper functioning.

Common bonding of all equipment was performed.

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FURTHER DEVELOPMENTS

In 1997 the system was upgraded in incorporated into the Bloemhof Dam flood monitoring system and the two systems had been extended to provide country wide coverage of mayor rivers. Stations are now operating in Kwazulu Natal, Mpumalanga, Northern Province, Free state, North west and Northern Cape. A total of 70 stations are now in operation using Meteosat and land based radio communication systems in an integrated flood and flow monitoring network.

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Page prepared by: P. du Preez.

©-1997, 1998, 2001. DPA CC

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