1. flood management in the vaal- and orange river system december 2010 - february 2011 johan van...
TRANSCRIPT
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