water Article Classification of Management Alternatives to Combat Reservoir Sedimentation Gregory L. Morris GLM Engineering PSC, San Juan, PR 00907, USA; [email protected] Received: 9 January 2020; Accepted: 13 March 2020; Published: 19 March 2020 Abstract: Sedimentation is steadily depleting reservoir capacity worldwide, threatening the reliability of water supplies, flood control, hydropower energy and other benefits that form the basis of today’s water-intensive society. The strategies available to combat reservoir sedimentation may be classed into four broad categories. Three proactive categories seek to improve the sediment balance across reservoirs by: (a) reducing sediment yield from the watershed, (b) routing sediment-laden flows around or through the storage pool, and (c) removing deposited sediment following deposition. The fourth category (d) consists of strategies that adapt to capacity loss, without addressing the sediment balance. Successful management will typically combine multiple strategies. This paper presents a comprehensive classification of both proactive and adaptive strategies, consistent with current international practice. Functional descriptions and examples are given for each strategy, and criteria are provided to differentiate between them when there is potential for ambiguity. The classification categories can be used as a checklist of strategies to consider in evaluating sediment management alternatives for new designs as well as remedial work at existing sediment-challenged reservoirs. This will also help practitioners to more clearly describe and communicate the nature of their management activities. Widespread application of both active and adaptive strategies is required to bring sedimentation under control to sustain benefits of water storage for today’s and future generations. Keywords: reservoir sedimentation; reservoir sustainability; water supply; sediment bypass; sediment management; sustainable hydropower 1. Introduction Dam construction creates a quiescent pool that interrupts the transport of sediment to the ocean. Reservoirs have already trapped more than 100 billion metric tons of sediment, representing 26% of the global sediment delivery to the ocean [1]. In regulated basins, more than 50% of the sediment flux may be trapped. In highly developed river basins such as the Colorado and the Nile virtually all sediment is trapped in reservoirs [2]. Progressive sedimentation together with a low rate of new reservoir construction has resulted in a declining reservoir storage volumes, and when population growth is factored in the per capita storage is declining rapidly [3]. For example, per capita reservoir storage in the USA peaked at 45,000 m3/capita in 1975, but by 2019 had declined to 27,000 m3/capita, a 40% reduction [4]. Total global capacity peaked around 2006, and global per capacity has been in decline since the 1980s [5]. Without effective action many reservoirs will see their function substantially impaired by sedimentation before reaching 100 years of age. For example, India’s Central Water Commission [6] published survey data from 243 large reservoirs—of which, 141 had been impounding for at least 50 years. In this group, 25% had already lost more than 30% of their original capacity. At 50 sites for which design sedimentation rates were given, 63% of the sites (representing 45% of the storage volume) were experiencing volume loss rates more than double the predicted rate. Water 2020, 12, 861; doi:10.3390/w12030861 www.mdpi.com/journal/water Water 2020, 12, 861x FOR PEER REVIEW 2 of 24 Declining reservoir capacity directly threatens our ability to provide reliable water supplies for both Decliningagricultural reservoir and urban capacity use, directly and also threatens interferes our abilitywith toother provide uses reliableincluding water flood supplies control, for hydropower,both agricultural navigation and urban and use, fisheries and also interferes[7]. The im withpacts other of uses sediment including trapping flood control, are not hydropower, limited to reservoirs,navigation but and also fisheries extend [7]. downstream The impacts to of the sediment coastline. trapping River arechannels not limited deprived to reservoirs, of their sediment but also load,extend and downstream particularly to thethe coastline.load of coarse River channelsmaterial deprivedthat helps of define their sediment channel load,morphology, and particularly can be heavilythe load impacted of coarse materialby channel that incision, helps define dewatering channel of morphology, riparian areas, can beaccelerated heavily impacted bank erosion, by channel and habitatincision, loss dewatering [8]. Sediment of riparian trapping areas, in accelerated reservoirs, bank together erosion, with and in-channel habitat loss mining [8]. Sediment [9], impact trapping the fluvialin reservoirs, sediment together balance with and in-channeleven contribute mining to coastal [9], impact erosion the [10,11]. fluvial sediment balance and even contributeBecause to coastalerosion erosionwill never [10, 11stop,]. and reservoir capacity is limited, sediment trapping is only a transitoryBecause phenomenon. erosion will The never equilibrium stop, and between reservoir sedi capacityment inflow is limited, and outflow sediment will trappingbe restored is after only thea transitory reservoir phenomenon. becomes filled The and equilibrium storage benefits between are sedimentlost, or it inflow may be and achieved outflow while will besustaining restored reservoirafter the reservoirfunctions becomes through filled sediment and storage management benefits ac aretions. lost, To or sustain it may belong-term achieved reservoir while sustaining capacity isreservoir a management functions decision through (Figur sedimente 1). Despite management knowledge actions. of Tosediment sustain management long-term reservoir alternatives capacity for reservoirsis a management prior to decision the dam (Figure construction1). Despite boom knowledge that began of in sediment the 1950s management [12,13], nearly alternatives all reservoirs for worldwidereservoirs prior have tobeen the designed dam construction on the basis boom of thatthe “life began of inreservoir” the 1950s pa [12radigm.,13], nearly Sediment all reservoirs inflows worldwidehave been calculated have been using designed a 50 onto 100 the year basis planning of the “life horizon of reservoir” and the paradigm. corresponding Sediment sediment inflows storage have volumebeen calculated allocated using in the a 50 storage to 100 year pool. planning No consideration horizon and was the correspondinggiven to sedimentation sediment storage consequences volume beyondallocated this in the planning storage pool.horizon. No considerationReservoir desi wasgn givenand operation to sedimentation without consequences a long-term beyond sediment this managementplanning horizon. strategy Reservoir is not designa sustainable and operation approach, without and ano long-term longer represents sediment managementan engineering strategy best- practice.is not a sustainable approach, and no longer represents an engineering best-practice. Figure 1.1.Alternative Alternative transitions transitions from pre-impoundmentfrom pre-impoun todment post-impoundment to post-impoundment sediment equilibrium. sediment equilibrium.To maintain long-termTo maintain reservoir long-term storage reservoir is a management storage is a management decision. decision. Most civil civil and and hydraulic hydraulic infrastructure infrastructure such such as ashighways, highways, buildings buildings and and mechanical mechanical systems systems can becan replaced be replaced or reconstructed or reconstructed after afterthey theyhave haveaged. aged. However, However, dams damscannot cannot be reconstructed be reconstructed to serve to theirserve intended their intended purpose purpose once oncethe storage the storage pool pool becomes becomes sedimented sedimented because because the the removal removal of of large sediment volumesvolumes is rarelyis rarely economically economically feasible. feasible. Sites suitableSites suitable for new damfor new and reservoirdam and construction reservoir constructionare limited by are topography, limited by geology, topography, hydrology, geology, and competinghydrology, landand uses.competing Today’s land reservoirs uses. Today’s already reservoirsoccupy the already best sites occupy and represent the best sites a unique and represent and limited a unique resource and [7 ,limited14]. resource [7,14]. Today, wewe have an inventory of aging reservoirs reservoirs with with steadily steadily growing growing sedimentation sedimentation problems. problems. This underscores the need to analyze the sedimentationsedimentation status and managementmanagement potential for all reservoirs, and to begin aggressive aggressive implementation implementation of of technologies technologies to to sustain sustain reservoir reservoir function function [15]. [15]. Given the importance of today’s reservoirs in sust sustainingaining our our society, society, and th thee impossibility of simply replacing today’s inventory of dams with new construction, Morris and Fan [14] [14] noted, “ Whereas the twentieth century focused on the construction of new da dams,ms, the twenty-first twenty-first century will necessarily focus on combating sedimentation to to extend the life of existing infrastructure. The The ta tasksk will will be be greatly facilitated if we start today. ” Reservoir function cannot be sustained withou