Water storage dams that provide water to satisfy the different uses such as agriculture, industry, drinking, tourism, etc. are usually designed based on three main capacities. The first is the dead storageallocated for trappingsediments, the second is the live storage used to store water during flood seasons and rainy years in order to be utilized throughout the year and during dry periods, the third is for the high flood protection storage that protects the people from potentialrisks and protect the dam from collapse.
Taking Aswan High Dam as an example, the dead storage is in the range of 30 billion cubic meters, the live storage is 90 billion cubic meters, and the flood protection storage is estimatedat 40 billion cubic meters. Therefore, all the water discharge outlets are situated above the dead storage. It is worth noting that Aswan High Dam Operation Protocol stipulates that before the start of the annualflood season specifically on the first of August each year, the water level in Lake Nasser shall not exceed the live storage maximum level in order to provide sufficient capacity to face any new flood.
With regard to hydroelectric dams, the amount of generated electricity is directly proportional to the amount of water discharged from the turbine’s tunnels (Water Discharge), and to the depth of the water stored above the turbine level (Water head). Therefore, these dams aim to maintain the highest water storage level to be able togenerate the maximum amount of electricity, and sometimes a portion of the flood protection storage is used to increase the amount of storedwater in order to increase power generation.
The Cameroonian Dam (Lagdo Dam), which is located in West Africa near the Nigerian border, is an example of the violations that may occur during the filling and operationofhydropower damwith the aim to optimize the hydropower production. In 2012, Cameroon had witnessed heavy and intensive rainfall that caused high floods in the Niger River where this dam is located. The officials in charge of the Lagdo dam operation opened its gates in orderto ensure itssafetyandreleasedthe stored water to be able toaccommodate the new flood.
Unfortunately, the released water was drained quickly towards neighbouring Nigeria causing flash floods, killing 431 Nigerians, destroying more than 150,000 hectares of agricultural land, and migrating 1.3 million Nigerians.This situation was repeated this year during the current month, as the dam region in Cameroon was subjected to intensiveand prolonged rainfall, so the dam authority discharged large amount of the dam storage to accommodate this year’s floods, which cause huge damageinlarge areas of agricultural land downstream the dam in Nigeria.
Another example from the east of Africa is Owen falls dam located near Victoria Lake's outlet in Uganda. Due to intensive and prolonged rainfall this year, in addition to the decision of the authority in charge of Owen Dam management to maintain high level in the lake to optimize energy production, the water level in Lake Victoria has overflowedand flooded the north of Uganda. Accordingly, Owen Dam management authority was forced to release huge amount of stored water causing devastating flooding inSouth Sudan that swept away its plains and villages.
The Grand Ethiopian Renaissance Dam (GERD) has a capacity of 74 billion cubic meters, but there are not any available official data regarding the capacity of its dead storage, live storage and flood protection storage, noting that GERD is much larger than the Cameroonian Lagdo Dam. Moreover, GERD has 13 Francis turbines, generating 375 MW each.
It is necessary for the safety of the dam to put in place a telemetric communication system that linksrainfall and flood monitoring stations and link them to the dam site using satellites systems. This information should be shared with the downstream countries to assist them in their water management as well asflood risk management.
The hydrological data available for the Blue Nileindicates thatits maximum daily discharge during the flood period (July/August) is estimatedat 800 million cubic meters, its maximum monthly discharge is estimated at 24 billion cubic meters, and its maximum annual discharge in the range of 70 billion cubic meters. Whereas the average annual discharge of the Blue Nile is about 50 billion, therefore, the live storage in the dam should not be less than the average value of the annual flow, and the flood protection storage capacity should not be less than the maximum monthly discharge of the Blue Nile.
The total capacity of GERD is 74billion cubic meters, which is sufficient to fulfil the live storage and flood protection storage requirements. However, it is obvious that there is no clear consideration regarding the dead storage capacity allocated to store sediments, the Blue Nile is known by its high erosion rates, and the huge quantities of sedimentation that the river carries annually to Sudan and Egypt.
It is worth noting that Egypt has allocated 30 billion cubic meters of Aswan High Dam storage capacity to the dead storage to accommodate the quantities of sedimentsof the Blue Nile. The limited dead storage capacity of GERD may be due to Ethiopia’s plans concerning the construction of other dams along the Blue Nile that will retain these sediments and generate hydropower.
Once GERD is operational, the sedimentary materials will start to be accumulated upstream the dam occupying part of the dam's storage capacity. This part will increase annually causing a continual decrease of the live storage capacity, consequently the power production will be reduced, on the other hand it might affect the flood protection storage capacity, increasing the risks of floods in the near future in the region in general and in Sudan in particular.
Concerning GERD construction progress, which has not completed yet, as it may take an additional year to complete the middle part of the dam,Ethiopia is trying to complete the implementation of the transmission line (Chinese funded), which will transfer the bulk of the dam's electricity to the Ethiopian territories.
It is to note that Egypt will not import electricity from Ethiopia as a result of its hostile positions, while Sudan desires to buy from the generated electricity but it has not started yet the construction of thetransmission line that will transfer the generated electricity from the dam site to the Sudanese territories, which may require 3-4 years to be implemented.
One of the basics in hydropower dams is that if one of the dam's turbines is notoperated, the tunnel or part of the tunnel designated for it will be closedaswater is not allowed to pass through it, otherwise the turbine will be damaged and destroyed.Therefore, the amount of water released from GERD at the beginning of operation will be limited, but in case the dam experiences high floods during this period, water will start to be accumulatedupstream the dam and the water level will rise until it begins to flow through the high flood emergency spillway at the top of the dam, threatening the safety of the dam asit will become like a large waterfall and the water flows through the spillway at an approximate height of 150 meters causing severe problems to Sudan.
When Ethiopia completes the construction of its transmission line, followed by Sudan'scompletionto its electrical transmission line from the dam site to its territories, and over the years, the sediments will start to be accumulatedupstream the dam affecting the flood protection storage capacity. If the water storage reaches 65 billion cubic meters in the beginning of August and the new flood season reaches 14 billion cubic meters, accordingly the dam will be filled and forced to discharge an amount of 500 million cubic meters per day downstream the river for a whole month.
In case the dam is completely filled with water that starts to overflow through the emergency spillway, and if this flood lasts for a week or two, and the dam was completely unable to store one additional drop of water, then the Blue Nile region in the beloved Sudan will be drowned and water structures will collapse causing death and damage, and the terrified management of GERD will fear its collapse, therefore they will release the excess water until the discharge of the dam's water reaches more than one billion cubic meters per day.
Some may wonder what is new for Sudan about the Blue Nile floods since itis already accustomed to it. Andwhy Sudan is the one threatened by therisks of the Blue Nile floods as a result of GERD construction while the opposite is supposed to happen? The truth is that before GERD Sudan used to unload its dams, but after GERD construction, and according to many studies, Sudan will not need to empty its dams before the flood season, hence Sudan can use these dams to increase its power generation capacity.
Whereas, before GERD the problem was confined to the Blue Nile flood, but after the dam operation there will be a probability of error occurrencewhile managing the dam during floods (Release huge amounts water) as a dam safety measure, which may increase the risks for Sudan. Furthermore, the water of the Blue Nile will not be loaded with sediments, as was the case before GERD, which increases its speed and erosion threats to canals and other installations.
*The writer is Egypt's former Minister of Water Resources and Irrigation and Professor of Water Resources, Faculty of Engineering, Cairo University