ETHIOPIAN DAM: SALVATION TO SUDANESE FLOODING

The Ethiopian Dam currently under construction on the Abbay river, otherwise known as the Grand Ethiopian Renaissance Dam (GERD), has great benefits for Ethiopia as well as the two downstream countries: Sudan and Egypt. One major benefit is that it mitigates flooding in Sudan. Notwithstanding that, in recent weeks the Sudanese Foreign Minister Mariam al-Sadiq al-Mahdi has been quoted to the effect as saying that the dam will endanger the livelihood of over 20 million Sudanese people. As will be demonstrated in this article, this position is entirely contrary to the facts on the ground. The dam does in fact eliminate flooding which has been a major problem in the Sudan including in its capital city, Khartoum.

Case in point: Sudan was hit hard in 2020 with the Nile River outflowing its banks. A lot of people were displaced, and the extent of property damage was enormous. According to United Nations Office of the Coordination of Humanitarian Affairs (OCHA)’s report issued on 10^th of September 2020, over half a million people were affected.  More than 111,000 houses were destroyed or damaged.  The flood also damaged 1,700 hectares (ha) of agricultural land, 179 facilities (schools, health centers, and government offices), and 359 shops and warehouses. Furthermore, it killed 5,500 head of livestock. The monetary value of the damage has not been quantified but it could run into billions of dollars. As shown in Table 1, devastating flooding events have been recorded in 1946, 1958, 1969, 1988 and 1998.

Flooding is, therefore, a repeat occurrence in Sudan.  Each of these flooding events is associated with heavy rainfall in the highlands of Ethiopia, particularly in the Abbay watershed. GERD, by virtue of its location in the valley of the Abbay river upstream of the flat topography of Sudan, is a textbook example of a flood control reservoir, even though the primary objective of the dam is power generation.

        Table 1. Peak flows at El Diem stream gage station ranked from highest to lowest – Flooding along the bank of the Blue Nile in Sudan commences when flow exceeds approximately 7,000 cms.                               

The Abbay river (referred to as the Blue Nile in the Sudan) receives 80 % its water flow in the months of July through October.  During this period, the water level fluctuates greatly and recorded as high as 11,000 cubic meters per second (cms) and as low as 800 cms. Sudan has been flooded repeatedly during the rainy season. In the remaining months, the flow is on the average less than 250 cms.  The GERD modulates these fluctuations and provides for a steady flow of water that is much more suitable for irrigation and drinking.

When the flow of the Nile exceeds approximately 7,000 cms, flooding commences in most areas along the banks of the river within the Sudan. Once the flood water is out of the bank, it spreads fast in the flat terrain of the adjacent areas, affecting farmlands, villages, and cities. In 2020, the peak flow of the Nile recorded at the El Diem stream gage station near the Sudan-Ethiopia border was over 11,000 cms.  The results of a stream flow frequency analyses to determine the frequency of occurrence and the probability of exceedance using 95-years stream flow records from the El Diem stream gaging location are shown in Figure 1. They indicate that the 7,000 cms flow has a return period of less than 10 years. The probability that 7,000 cms flow is equal or exceeded in the coming 10 years is about 70%.  This means that, without taking measures of flood mitigation, the probability of getting flooded in any given year within the next 10 years is 70%.  This is a very likely scenario as flooding in Sudan is a repeated occurrence, past record attest to this fact.

Figure 1: Frequency analyses using data from El Diem Stream Gaging Station 

GERD Eliminates Flooding in Sudan

One Sudanese official stated several years ago “As Aswan dam is to Egypt; GERD is to Sudan”. This is a correct depiction of the benefits of the GERD to the Sudan. An interesting question is this: Could the GERD have enabled Sudan to avoid the 2020 flood or similar magnitude of future flood had it been completed sooner? Since the 2020 flood has a return period of approximately 100-years (Figure 1), the answer to this question depends on the operation of the reservoir. Even though, the primary objective of the GERD is to generate hydropower, it does have an ancillary benefit of reducing or eliminating flooding in the downstream countries: Sudan and Egypt.

The amount of water storage in the reservoir at the onset of the rainy season is critical to prevent downstream flooding. To demonstrate whether GERD eliminates flooding in Sudan, a reservoir simulation analysis was conducted using the inflow data from 2020 and planned Ethiopian reservoir operation scenarios. The GERD has a capacity of 74 billion cubic meters (bcm). The normal operating range of the GERD reservoir is between 625 to 640 cms and the minimum operating level of 610 cms.  If the reservoir is below elevation 610 m there is more storage to hold the flood water and release it in a controlled manner.

For this analysis, two scenarios were considered. The initial reservoir level was assumed at elevation 610 m and 625 m at the onset of the rainy season, June 28, 2020. The release from the reservoir was set not to exceed 6,000 cms to eliminate overbank flooding in Sudan. The goal was to assess if there is enough storage capacity in the reservoir to hold the 2020 flow of the river without exceeding an elevation of 640 m which may trigger additional release through the gated spillway or the emergency spillway. The results are shown in Figures 1 & 2.

Figure 2: GERD reservoir level for the two scenarios

Figure 2 demonstrates that, in both scenarios, the water levels in the reservoir would have remained below 640 m even at the peak period of water inflow in the month of September. The GERD could have stored all the water flow in the river with a release of less than 6,000 cms and without triggering flooding downstream.

 Figure 3: Inflow to the reservoir and releases for the two scenarios

Figure 3 shows that to prevent the water level in the reservoir rising above 610 or 625 m, which would trigger additional release through the gated spillway or the emergency spillway, which in turn would cause overbank flooding in the Sudan, only 6,000 cms of water would need to be released from the reservoir.

Therefore, the 2020 flooding in the Sudan with all the damages and the suffering of the population would have been avoided had the GERD be completed. Similarly, any potential future flooding due to inflows into the reservoir of up to 11,053 cms will be prevented by the GERD.

Based on these analyses, one wonders why a high-level Sudanese official would make a statement against the interest of the Sudanese people without offering alternative plans for alleviating the repeated floods that devastate the Sudan. It is up to the Sudanese people to decide what is good for them. Otherwise, it is obvious that the GERD, without costing the Sudanese people a penny, will solve the age-old problem of flooding in the Sudan. Therefore, it is reasonable to expect that the Sudanese people would embrace the GERD and thank the Ethiopian people for finally ridding them of their chronic flood problems.

Dr. Gabriel Alem is a water resources Technical Director with a global engineering company, WSP.  He has over 30 years of experience in water resources planning, hydrology, hydraulics, and feasibility studies for flood mitigations and, simulation of reservation.