1. Flood Management in the Vaal- and Orange River System December 2010 - February 2011 Johan van Heerden & Brink du Plessis (Chief Eng.) D: HS Elias Nel.

1

Flood Management in the Vaal- and Orange Flood Management in the Vaal- and Orange River System River System

December 2010 - February 2011December 2010 - February 2011

Johan van Heerden & Brink du Plessis (Chief Eng.) D: HS

Elias Nel (Sen. Scientist) W/R Planning Systems

Department of Water Affairs

The “Flood Room” in Pretoria serves as an information

centre to collect and distribute rainfall and flow data. Management of extreme floods in the Vaal / Orange River System is done by Directorate: Hydrological Services of the Department of Water Affairs.

The main objectives of flood management are:

• Ensuring the safety of the structures (and therefore

human life)

• Minimisation of damages (by minimising flow in system)

• Ensuring that the dams are 100% full at the end of the flood

Vaal / Orange River System

Vaal / Orange System is approximately 49% the size of South Africa

Vaal- and Bloemhof Dams are equipped with spillway gates (no uncontrolled spillway)

Bloemhof Dam

Gariep Dam

Real time data collection points in the Upper Vaal

17 Gauging stations collect and transmit near real time flow and rainfall data to Pretoria

Liebenbergsvlei River at Frederiksdal

Liebenbergsvlei River at Frederiksdal

Real time data collection points in the Vaal / Orange River System

80 Gauging stations collecting near real time flow and / or rainfall data - available on the Hydrology real time Web Site

Routing times in Vaal / Orange System (days)

Flood warnings provide time for people to take action to protect their property as well as allowing emergency services to prepare and plan for evacuations

El-Nino: higher probability for a dryer period in the summer

rainfall areaLa Nina: higher probability for a wetter period in the

summer rainfall area 

Basically all high summer rainfall events, since 1980,

occurred in a La-Nina period but not all La-Nina events

produced high rainfallRoughly about a 80% correlation – must be

confirmed with SAWS – this is their terrain

Weather forecast

0

1000

2000

3000

4000

5000

15-Dec

19-Dec

23-Dec

27-Dec

01-Jan

05-Jan

09-Jan

13-Jan

17-Jan

21-Jan

26-Jan

30-Jan

03-Feb

07-Feb

11-Feb

15-Feb

20-Feb

24-Feb

28-Feb

03-Mar

07-Mar

11-Mar

Dis

char

ge

(m3 /s

)

0

20

40

60

80

100

120

140

160

180

200

Dam

Cap

acit

y (%

)

45

15

30

Inflow 3380 m3/s

Maximum dam capacity: 118.5%

Maximum outflow 2400 m3/s

Peak inflow

4765 m3/s

Rain

fall (mm

)

Vaal Dam flood event 15 December 1995 - 15 March 1996

Flood damage Feb 2011 ~ 3000 m3/s

Confluence of the Vaal- and Orange Rivers near Confluence of the Vaal- and Orange Rivers near Douglas Douglas

Lower Orange Lower Orange RiverRiver

5 200 m5 200 m33/s on 2011/01/12/s on 2011/01/12

DischargDischarge (me (m33/s)/s)

Date Date (days)(days)

Routing of 7 Routing of 7 January 2011 at January 2011 at

07:0007:00

2 800 m2 800 m33/s Vaal River /s Vaal River

2 400 m2 400 m33/s Orange /s Orange River River

Confluence of the Vaal- and Orange Rivers near Confluence of the Vaal- and Orange Rivers near Douglas Routing with all dams removedDouglas Routing with all dams removed

Lower Orange Lower Orange RiverRiver

Orange Orange RiverRiver

Vaal RiverVaal River

6 300 m6 300 m33/s/s

DischargDischarge (me (m33/s)/s)

Date Date (days)(days)

Return periods of 2011 floods

Vaal River: (first and second flood)Vaal Dam inflow 2 900 m3/s ~15 year; 1 650 m3/s <5 year(Regional Maximum flood (RMF): 14 300 m3/s)Bloemhof Dam inflow 3 600 m3/s ~20 year; 1 700 m3/s <5 year(RMF: 16 000 m3/s)

Orange River:Gariep Dam inflow 2 700 m3/s ~5 year; 5 300 m3/s ~10 year(RMF: 26 600 m3/s)Vanderkloof Dam inflow 2 700 m3/s ~15 yr; 3 400 m3/s <25 year(RMF: 22 600 m3/s)Orange at Upington 5 100 m3/s ~20 year; 5 100 m3/s ~20 year

Flood management: Bloemhof Dam (inflow, outflow and capacity)

Komati River at Mozambique border

GENERAL COMMENTS

When to start making use of the flood absorption capacity of the dam (Vaal Dam 26%)? (The volume of the 2011 flood at Vaal Dam was larger than the volume of the dam and at Bloemhof Dam larger than four times the volume of the dam)

Weather forecasts: Short, medium and long term weather forecasts, satellite and radar images and 24-hour rainfall figures play a major role in operating decisions during the flood

Thank you