Thwake Dam’s vantage position a blessing to Lower Eastern region

By Eng. Samuel A.O Alima, EBS
Water Secretary and Project Engineer

Issue Number 15 of the Advancing Open Science Journal, through a write-up titled; Dam Siting: A Review, authors Yang Wang, Yonghzong Tian and Yan Cao, submit that dams are key for hydraulic projects, but not all dam construction processes are based on a scientific and systematic approach to decision making. 
As a researcher and an engineer who has been involved in conceptualization of critical water reservoirs in the country, and in the light of the existing scientific literature, many scholars have actually analyzed the optimal location for dam construction. These studies have specific factors to determine the appropriate location and show variability in different purposes of dams, for example: irrigation, power generation, water supply, and flood control. I want to agree with these submissions.  
Actually, worldwide, most major dams are typically situated in locations with a combination of factors including a large river with significant water flow, a narrow gorge or valley to create a natural reservoir, suitable geology for dam construction (stable rock formations), and access to infrastructure for building and maintenance. 
When the Government of Kenya toyed with the idea of constructing Thwake Multipurpose Dam - only second after Masinga Dam - several considerations came into play. 
Thwake Dam was identified by the State and the African Development Bank (AfDB) as a strategic facility to supply water to the semi-arid greater Makueni County and the adjoining areas downstream. 
Following various feasibility studies of covering the entire region, it was established that water is a priority to all the residents in terms of livelihoods and social requirements. The overall ecosystem was also considered as a key factor that would transform the region and heavily benefit the surrounding communities. 
The dam was therefore proposed to be located immediately downstream (1km) from the confluence of Athi and Thwake rivers in Mavindini Division (on the Makueni side) and Kanyangi Division (on the Kitui side) while the flow back was to extend into Kathulumbi Division of Mbooni Location to the northwest of the dam site.
According to an Environmental Social Impact Assessment (ESIA) report by the AfDB done before the construction started in 2018, Thwake project site was seen as generally suitable with exceptional river water resource and suitable point of confluence of major rivers. 
“Based on this information, several sites in the area were explored for suitability of dam construction. The proposed site was selected following extensive studies taking cognizance of each of the various site attributes, namely environmental, social and sustainability, technology and commercial consideration, including the strength and stability of the water flows in the area, proven technology, benign environmental setting, low population density, security of the area, water availability and road accessibility,” reads the ESIA report in part.
For many years, rresidents drawn from the rural parts of the greater Makueni and Kitui counties and the Lower Eastern region in general, have been forced to walk distances of up to 10km daily in search of water for livestock and domestic use. 
It is even a pity that the little water found is mostly of inferior quality effectively risking the people’s health and hygiene. This is clearly depicted in the reported cases of water borne ailments including diarrhea, intestinal worms, bilharzias and skin problems. 
The ongoing construction of Thwake Multipurpose Dam, once completed, will be a necessary intervention that will address water scarcity and in turn open up the areas for development and improve the socio-economic status of the involved areas. 
The dam will further open up the areas for development in terms of activities such as fishing, tourism and the fact that the dam axis may provide a causeway that will open up the area by providing social and economic access between Makueni and Kitui counties. The overall ecosystem of the project area stands to get transformed to the benefit of the communities
Consequently, sound justification and analysis conclude that the current location of the dam appears to be the most suitable location because of factors such as low density of the population of the area and the steep to vertical configuration of the banks of the river in the area of the reservoir resulting in less impact than if the bank gradient was smoother.
The dam axis is located downstream of the confluence of Thwake and Tana Rivers giving it a huge advantage of sustainable and constant water supply. Thwake dam falls within the Athi Catchment drained by the Athi River basin and its tributaries. The hydrology is, therefore, influenced by the flows from Nairobi’s river system (Nairobi, Ngong, Mbagathi, Ruaka, Ruiru, etc.) in the upper reaches of the catchment that spans as far as Ngong hills, Kikuyu Plateaus and the lower slopes of the Aberdares. Nearer the project area, are notable rivers (most of them seasonal) including Thwake, Tawa, Kaiti, Iuani and Kalawa rivers upstream of the site while Kiboko, Makindu, Muoni, Kiangini, Mbanya, MtitoAndei, Kibwezi, Kambu and Thange rivers discharge into Athi River downstream.