1

Flood Management in the Vaal- and Orange River System December 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

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 area La 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 rainfall Roughly about a 80% correlation – must be confirmed with SAWS – this is their terrain Weather forecast

Vaal Dam flood event 15 December March 1996

Flood damage Feb 2011 ~ 3000 m 3 /s

Confluence of the Vaal- and Orange Rivers near Douglas Lower Orange River m 3 /s on 2011/01/12 Discharge (m 3 /s) Date (days) Routing of 7 January 2011 at 07: m 3 /s Vaal River m 3 /s Orange River

Confluence of the Vaal- and Orange Rivers near Douglas Routing with all dams removed Lower Orange River Orange River Vaal River m 3 /s Discharge (m 3 /s) Date (days)

Return periods of 2011 floods Vaal River: (first and second flood) Vaal Dam inflow m 3 /s ~15 year; m 3 /s <5 year (Regional Maximum flood (RMF): m 3 /s) Bloemhof Dam inflow m 3 /s ~20 year; m 3 /s <5 year (RMF: m 3 /s) Orange River: Gariep Dam inflow m 3 /s ~5 year; m 3 /s ~10 year (RMF: m 3 /s) Vanderkloof Dam inflow m 3 /s ~15 yr; m 3 /s <25 year (RMF: m 3 /s) Orange at Upington m 3 /s ~20 year; m 3 /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