Dam spills can have disastrous consequences for the people living downstream. The case study from Volta River Authority, Ghana looks at how geospatial technology can be used effectively to monitor and thus minimize the impact of large dam spills
Volta River Authority (VRA) was established in 1961, with the aim to generate and supply electrical energy for industrial, commercial and domestic use in Ghana. The VRA currently operates two hydro power plants with total installed capacity of 1020 MW at Akosombo and 160 MW at Kpong. The VRA also operates a 330 MW Combined Cycle Thermal Power Plant at Takoradi and a 220 MW simple cycle thermal plant at the same site, developed through a joint-venture partnership between VRA and CMS Energy of Michigan, USA.
As part of its Dam Safety Assessment (DSA), the VRA in 2010 developed an Emergency Preparedness Action Plan (EPP) and an Environmental Management Plan for the Akosombo and Kpong Dams to mitigate the impacts caused by the existence and operation of the dams and to keep stakeholders aware of their roles and responsibilities during emergency situations such as flooding during dam spilling, to keep the loss of life and property to the barest minimum.
In view of this, the VRA deployed geospatial technology to accurately analyze and delineate areas likely to be affected by flooding during dam spilling or dam break.
The Akosombo and Kpong Dams are large dams according to ICOLD (International Commission on Large Dams) definition. The mere existence of large dams containing reservoirs just upstream of populated areas is always a reason to be alert. In order to be able to warn and evacuate people living within the reach of any potential flood wave, an Emergency Preparedness Plan (EPP) should be prepared. It should include among others; information to people potentially at risk and the delineation of areas possibly affected. The plans have to be prepared to cope with the possibility of unusual spills, exceptional flood conditions, operational errors and the unlikely case of a dam break.
The EPP specifies the roles and responsibilities of stakeholders when emergency situations occur that eventually could lead to the failure of the dam, or when unexpected operational flow release threatens downstream life, property or economic activities.
The approach involved the application of geospatial tools in capturing data for the preparation of orthophotos and digital terrain models and using various hydraulic models to delineate the extent of the areas likely to be flooded during dam spilling. Inundation maps are prepared based on GIS analysis of hydraulic models.
Orthophoto of study area
Data Capturing and Analysis of Results
Aerial photography of the potentially affected area was undertaken using LiDAR technology. GPS survey was adopted for the measurement of the coordinates of ground base stations. Geospatial software was then deployed for the preparation of orthophotos and digital terrain models. Based on hydraulic analysis; extent of flooding, inundation and evacuation maps for the visualization of the effects of the flooding were prepared. Using the digital terrain model some areas of high grounds were identified as “safe havens” for evacuation. Hydraulic model runs were made to map the resulting inundations. Based on the modeling results maps were prepared using appropriate software.
Use of Inundation Maps
The inundation maps which depict the extent of inundation by each hydraulic scenario shall be used by various stakeholders for planning their course of action in case a disaster happens, and to raise awareness of the potentially affected people.
By adding daily monitored information, the reservoir level, the speed of rise and including the possible consequences downstream, VRA makes a final decision whether to spill or otherwise.
As the VRA always tries to follow a predetermined rule curve for power production, or at least the upper reservoir control curve in case of abundant inflow, it is known couple of days or even weeks in advance when spilling is going to occur, how much and for how long. This enables effective preparation in advance to control and mitigate any effects to the potentially affected communities.
Dam breaks cannot be predicted, however, with the production of appropriate inundation maps showing the delineation of evacuation zones, with identified “safe havens” the loss of life can be minimized to the barest minimum.
In case of evacuation as a result of excessive spill, warning time can be in the order of days or weeks. However, in case of evacuation as a result of dam break, warning time is very unpredictable and might be as short as hours. A dam break is classified as very high emergency due to the uncertainty with regard to warning time. Safe Havens have been identified for each emergency situation in the potentially affected communities.
