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Drivers, Impacts and Mitigation of Increased Sedimentation in the Hydropower Reservoirs of East Africa

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Drivers, Impacts and Mitigation of Increased Sedimentation in the Hydropower Reservoirs of East Africa land Review Drivers, Impacts and Mitigation of Increased Sedimentation in the Hydropower Reservoirs of East Africa Aloyce Amasi 1,*, Maarten Wynants 2 , William Blake 2 and Kelvin Mtei 1 1 School of Material, Energy, Water and Environmental Science, The Nelson Mandela African Institution of Science and Technology, Arusha P.O. BOX 477, Tanzania; [email protected] 2 School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus Plymouth, Devon PL4 8AA, UK; [email protected] (M.W.); [email protected] (W.B.) * Correspondence: [email protected]; Tel.: +255-766-877920 Abstract: Hydropower reservoirs are essential for the climate-neutral development of East Africa. Hydropower production, however, is threatened by human activities that lead to a decrease in water storage capacity of reservoirs. Land use/land cover and climatic changes are driving accelerated soil erosion in semi-arid East Africa, which ultimately increases reservoir sedimentation and decreases energy production. Sediment delivery dynamics at the catchment scale are complex, involving the interaction of multiple factors and processes on different spatial and temporal scales. A lack of under- standing of these processes and their interactions may impede the efficiency of sediment mitigation and control strategies. A deep understanding of the processes of erosion and connectivity of the land to river channel, as well as storage of eroded material within hillslopes and floodplains, and sediment accumulation in the reservoirs supports selection of future dam locations and sustainable management of reservoirs. The sediment budget approach can provide such a holistic perspective by accounting for the various sediment sources, transport, sinks, and redistribution when the sediment Citation: Amasi, A.; Wynants, M.; is routed through that catchment. Constructing sediment budgets is challenging, but the potential Blake, W.; Mtei, K. Drivers, Impacts and Mitigation of Increased for integrating a number of different techniques offers new opportunities to collect the required Sedimentation in the Hydropower information. In East Africa, the spatial planning of dams is mainly dominated by political and Reservoirs of East Africa. Land 2021, financial motives, and impacts of land use and climate on the sediment transport dynamics are not 10, 638. https://doi.org/10.3390/ adequately considered. Production of sediment budgets under different scenarios of land use and land10060638 climate change should be an essential step when deciding the location and management strategies for dams. Selection of new hydroelectric reservoir sites must consider long-term scientific data on Academic Editor: Ioannis climate change, and the sediment budget components for sustainable land management planning, N. Daliakopoulos hydropower sustainability. Received: 28 April 2021 Keywords: climate change; land use change; hydropower; reservoir; sedimentation; sediment delivery Accepted: 12 June 2021 Published: 16 June 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Hydropower reservoirs are essential for producing climate-neutral energy [1] and iations. ensuring long-term energy stability for economic growth in developing countries [2]. In addition, they provide other essential economic and ecological resources, such as irriga- tion and drinking water sources for agriculture and livestock, recreational spaces, and fishing habitats [1–5]. The hydropower industry and its share of power production in East Africa are expanding linearly, while the East African population and its energy de- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. mands are growing exponentially [6]. Despite the key socioeconomic services they offer, This article is an open access article hydropower reservoirs are currently threatened by changing water supply and sediment distributed under the terms and transport dynamics in wider catchments [6,7]. Unsustainable land use and climate changes conditions of the Creative Commons increase soil erosion and sediment delivery rates, resulting in accelerating reservoir sedi- Attribution (CC BY) license (https:// mentation [8]. Consequently, water storage capacity is decreasing, and energy production creativecommons.org/licenses/by/ capacity is declining [8]. Moreover, increased sedimentation can cause flooding that may 4.0/). disrupt the local infrastructure. Among their longer-term negative impacts, mega-projects, Land 2021, 10, 638. https://doi.org/10.3390/land10060638 https://www.mdpi.com/journal/land Land 2021, 10, 638 2 of 22 such as hydropower constructions, could also often causes loss of life and property, and involuntary resettlement which could further lead to poverty [9,10]. By confining sedi- ments to reservoirs, dams also hinder sediment transfer to the downstream river system, which subsequently lacks the sediment input required for maintaining channel shape and preserving the aquatic habitats [7]. In addition, sedimentation in reservoirs can add com- pressional forces to the dam structure, thereby exceeding the normal hydrostatic design, while clogging of water intake also hinders the production of energy [11]. Dynamics of sediment availability in a catchment are complex in time and space, and depend mainly on the climate, geology, topography, soil types, land cover, and land use [12]. The rapid expansion of agricultural land area with respect to population increase in Eastern Africa has led to an increase in the rates of soil erosion from large areas [13,14]. In upstream catchments, fluvial processes are susceptible to land use and land cover changes on the basin scale, resulting in robust landscape reactions by modifying processes of soil erosion, sediment transport, and deposition [15]. Conversely, natural climate variability and climatic changes in East Africa affect the hydrological cycle and, in turn, production capacity [16]. In addition, increased runoff and gully incision also lead to an increase in sediment connectivity and sediment supply, leading to rapid transport of eroded sediment to downstream sinks [17]. Increased erosion following land use or climate change and rapid downstream transport of eroded sediment is thus the biggest threat for the sustainability of reservoirs [8,18]. All these factors ultimately influence downstream siltation and sedimentation problems in dams/reservoirs [8,19]. While unsustainable land use, climate change, and natural climate variability influ- ence sediment transport [19], the processes by which they change catchment hydrology are nonlinear in semi-arid East Africa, where the spatial and temporal dynamics of sediment connectivity are not well understood [20]. Such dynamics are often neglected in reservoir planning [21]. Sediment budgets as a functional reservoir management tool have rarely been established at the catchment scale in East Africa [22]. In this context, some pressing questions remain regarding hydropower management now and in the future. Are dam and reservoir systems managed in the same way the planners and designers intended [23] concerning managing sediment accumulation? Are there any consequences of the construc- tion and operation of the dam that were not foreseen by the designers [23]? What are the processes and features controlling sediment connectivity and sediment supply to reservoir sink zones? What are the best techniques to assess reservoir sedimentation rates? What approaches can reduce the quantity of sediment incoming to the reservoirs from upstream? What degree of the induced climate change variations in rainfall and temperature affect sediment delivery dynamics, and can these be mitigated? These unknowns need to be answered and integrated into decision making for endorsements at early planning stages of future hydropower dams. Informed policy decisions and innovative mitigation solutions are required to move hydropower towards sustainable practices and meet the rising energy demands while ensuring water availability in East Africa. This review presents an overview of reservoir siltation issues in East Africa, followed by a detailed description of the driving processes behind observed increases in sediment delivery. Subsequently, different methods to evalu- ate and quantify source siltation of hydropower reservoirs are discussed, with emphasis on their strengths and weaknesses. Finally, we give an overview of the mitigation options for reservoir sedimentation, emphasizing different techniques for (1) reducing the influx of sediments, (2) managing and evacuating sediments from reservoirs, and (3) replacing lost storage of the reservoirs. On this basis, this paper aims to provide a blueprint for sustainable catchment and reservoir management in East Africa. 2. Review Approach on Hydropower Development in East Africa This review offers an insight of the scale of reservoir siltation issues in East Africa, with a subsequent detailed description of the driving processes behind observed increases in sediment delivery to hydropower reservoirs. Lack of information on potential sedi- Land 2021, 10, x FOR PEER REVIEW 3 of 23 2. Review Approach on Hydropower Development in East Africa This review offers an insight of the scale of reservoir siltation issues in East Africa, with a subsequent detailed description of the driving processes behind observed increases Land 2021, 10, 638 in sediment delivery to hydropower reservoirs. Lack of information on potential3
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