Sustainable Sediment Management in Reservoirs; An OvervieKeywords: Sediment, dams and reservoirs, sediment management, ... Introduction . Reservoirs are used to provide water supplyportable

Proceedings of the 5th NA International Conference on Industrial Engineering and Operations Management Detroit, Michigan, USA, August 10 - 14, 2020

© IEOM Society International

Sustainable Sediment Management in Reservoirs; An Overview

M. Nda

Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein Onn Malaysia.

Department of Civil Engineering, The Federal Polytechnic Bida,

Niger State, Nigeria [email protected]

M. S Adnan


Parit Raja, Johor Malaysia [email protected]

M. A. M Yusoff


Parit Raja, Johor Malaysia [email protected]

G. S Jiya

Department of Civil Engineering, School of Engineering Technology, The Federal Polytechnic Bida

Niger State, Nigeria [email protected]

I.Y Ebenehi

Department of Building Technology, School of Environmental Technology, Federal Polytechnic Bauchi

Bauchi State, Nigeria [email protected]

Abstract

Presently there is considerable demand for improved sustainable water supply systems globally, even though sedimentation is taking up the most needed reservoir capacity. Dams continuously trap most sediment transported by rivers in its reservoirs. This results in reduced reservoir storage capacity and useful life, and substantially altering the sediments due downstream leading to the riverbank and channel erosion as well as affecting the aquatic habitats. Several available techniques can be used to preserve and mitigate the effects on reservoirs downstream but not often put to use. This paper summarizes the importance of dams vis a vis its effects on rivers, and sedimentation effects on reservoirs. Lastly, the paper outlines a few techniques available for sustainable management of sediment in reservoirs and dams. Keywords: Sediment, dams and reservoirs, sediment management, sustainability

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1. Introduction Reservoirs are used to provide portable water supply, hydroelectricity, and flood control management services worldwide. They are particularly important in areas of the world with high hydrological variability, where both seasonally and annual amount of water flowing in rivers varies significantly. Storing enough water for use in these areas during extreme or multi-year droughts, and therefore maintaining the continuity of water and power supply requires very large storage volumes. Countries where hydropower is a major source of energy also have projects in both reservoir and run-of-river projects, where storage preservation is often a secondary goal that accounts for around 11 percent of all big dams (ICOLD, 2014). With an ever-increasing global population, there is an increasing demand for a more sustainable system of water supply. Given this demand, water storage capacity worldwide is decreasing steadily due to sedimentation in reservoirs (Annandale, 2016). Sedimentation in rivers and dams has been a topic of continuous discussion among researchers in recent years, due to its negative impact on water supply and hydroelectricity generation dams all over the world. Dams are characterized by low inflow water velocities, as sediment gradually deposits in the reservoir the ability to contain it declines and eventually contributes to reducing its operational capacity and storage while, substantially altering the sediment regime downstream causing sediment shortage suitable for maintaining channel form and riparian life. Several variables, such as the existence of the catchment materials, catchment slope, capacity inflow ratio, the reservoir trap efficiency (Te), the reservoir operational method, and climate change are identified as major causes of reservoir sedimentation (Adeogun et al., 2020). In several regions of the world, climate change is expected to enhance hydrological instability, thereby increasing the severity of both floods and droughts. This instability would increase the need for greater volumes of reservoirs to ensure stable water and power supplies and the much-needed control of floods. Climate change is also expected to increase the sediment concentration in many rivers, exacerbating the reservoir sedimentation threat (Annandale et al., 2016). Consequently, as dams and reservoirs approach the end of their initial design life, most operators are interested in maintaining the infrastructure and continuing to generate economic and social benefits, including water supply, hydropower, and flood mitigation control, even though the benefits are not as great as in the original project. Extending the life of the dam involves adopting a new design and operational standard that focuses on managing the reservoir and watershed system to balance sediment inflow and outflow to the degree that this is practical, thus giving the reservoir a much longer life. It is also important to plan, construct, and maintain a new dam and reservoir projects bearing in mind the long-term threat of reservoir sedimentation. Building and sustaining reservoir capacity is essential for dam projects to provide benefits in terms of irrigation, water supply, flood control, hydroelectricity supply, and other recreational use. Thus, this paper is aimed at providing an overview of dams and reservoirs, sedimentation effects, and reservoir sustainable management. 2. Dam and its reservoir Rainfall is considered a seasonal event at most locations. Also, there may be less rain or even completely dry in one location while another location some miles away receive rain in abundance at the same time. Dams are built to serve as water preservation structures; the reservoirs retain water during seasons with surplus rain event (wet periods) which can be put to use during dry periods when the natural flow is inadequate to meet the daily needs (O’Flynn, Phillips, & Toland, 2013; Phillips, 2013). Based on records available for 2014, there are 37,641 dams in the world with ≥15 m height. The majority of these dams were constructed after 1950 to serve a variety of purposes; to supply water for hydropower generation, for municipal and industrial uses, for irrigation, fishing, and recreation. Furthermore, dams are constructed to serve as flood control measures as well as to divert river flow to other locations far away. As the population rose rapidly and emerging national economies grow in the early 1950s up till the 1980s, many dams were constructed. This trend of dam construction slowed down afterward. However, in recent years building dams have picked up again particularly in areas with extreme climate conditions, areas prone to natural disasters related to water, and high variability between demand and supply of the resource (Lempérière, 2017).

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Dams are made of several components each with its unique characteristics and function. A detailed diagram showing the components is presented as Figure 1. with a brief description the component and basic used terms as follows; (i) full reservoir level (FRL) is the highest water level to which the water surface will rise during the normal operating condition. (ii) maximum water level (MWL) is the maximum level to which the water surface will rise when the design flood passes the spillway. Besides, the maximum water level is higher than the full reservoir level so that some surcharge storage is available between the two levels to absorb flood. (iii) minimum pool level (MPL) is the lowest level up to which the water is withdrawn from the reservoir under ordinary conditions. The minimum pool level generally corresponds to the elevation of the lowest outlet (sluiceway) of the dam. (iv) useful storage is the volume of water stored between the FRL and the MPL. (v) surcharge storage is the volume of water stored above the full reservoir level up to the maximum water level. (vi) dead storage this is referred to as the volume of water held below the minimum pool level. (vii) valley storage is referred to as the volume of water held by the natural river channel in its valley up to the top of its banks before the construction of the reservoir (Arora, 2002).

Figure 1. Schema of a dam and its components (Arora, 2002)

Dam design, inspection, and maintenance are based on its classification criteria. Thus, dams are classified by their size and their hazard criteria. Dam classification by size is determined by the height of the dam and its storage capacity. The hazard potential determines the damages that may affect future or present development in the event the dam suddenly releases a large volume of water due to its operations or misuse downstream (Mehta et al., 2012). The classification by size is divided into three (small, intermediate, and large) and the details of this form of rating are provided in Table 1.

Table 1. Dam Classification by Size (Mehta et al., 2012)

Dam Type Height (m)

Storage Capacity (mcm)

Small 7.5 – 12 0.5 – 10

Medium 12 – 30 10 – 60

Large >30 >60

Reservoirs formed by large dams can store about 6600 km3 volume of water annually or approximately 15% of annual continental runoff. Estimates are available to show that the amount of reservoirs capacities and the total flooded area

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in the world to be between 6000 km3 to 400,000 km3 respectively (M. Nda et al., 2019). Thus, humanmade reservoirs store quite a large part of the global freshwater, and the proportion is still progressing as new dams are still under construction. Asia, Central, and South America and Oceania constitute regions with the largest increase in large dam construction. With India with the highest number and some still under construction. The development of new structures is slow in developed countries where they have stabilized the population and most such projects already in place, compared to developing countries (Hoeinghaus, 2017). However, the existence of reservoirs is seriously challenged with storage impairment due to sedimentation. According to Palmeri et al., (2003), storage capacity and storage loss rate of reservoirs worldwide amounts to around 7,000 km3 and between 0.5 and 1 percent per annum. This rate of loss is equivalent to adding about 50 km3 of storage per year worldwide, with an estimated replacement expense of about US$ 13 billion in 2003. Thus, estimating a reservoir's current storage capacity requires the deployment of several known techniques from field surveys to applying physical and empirical models. But one of the most commonly used reservoir storage assessment technique is to continuously take survey measurements (bathymetric survey). The results of the surveys will provide an avenue for comparison of results collected over time. Thus revealing the change in the available reservoir storage capacity. This results will be a required data in predicting sedimentation, useful for the reservoir maintenance and operation (George et al., 2017). 3. Sedimentation Rivers can be considered as a combination of flowing water as well as sediment. A substantial amount of water is delivered into the rivers by rains (in the form of heavy precipitation or runoff). Rainfall carries along much eroded soil and other materials (debris) from the surrounding landscape, some are little tiny particles while other are larger particles or boulders called sediments (Milliman & Meade, 1983; Newbury & Bates, 2017). Sediment is most of the time transported by water and referred to as the fluvial process, for example, river channel deposits and sandbars. Another means of sediment transport is by wind or glaciers, which is known as the aeolian process, an example of this form of transportation and deposition is sand dunes and desert losses (Muhammad et al., 2019). The process in which sediments or particles are moved from its point of origin by natural or human enhanced means and deposited in another location different from its source in water bodies or on land surface is known as sedimentation (Baldwin et al., 2017; Xia et al., 2016). 3.1 Sedimentation effects downstream a dam The decrease in the volume of sediment transported downstream by water flow is primarily caused by the regulated flow associated with dams and other similar structures in rivers (Adnan et al., 2018). Substantial, large materials carried by water are trapped upstream the structure with mainly most of the smaller particles falling out to the crest allowing just suspended sediment to be transported downstream (East et al., 2015). The changes caused by flow regulation and sediment flux are responsible for both spatial and temporal variations in river morphology downstream. The river responds to changes according to the transport capacity and sediment available for transport to attain a balance in the system (Lai et al., 2014). The river tends to erode its banks and beds in a bid to achieve equilibrium when the conveyance capacity exceeds sediment supply. Adjustment in the morphology of a system downstream of a dam structure varies in response to some conditions, and a lot of likely outcomes will result. The river channel downstream of the dam is characterized by erosion and degradation when the river transport capacity exceeds sediment load (Botter et al., 2013). Also, there will be channel incision and erosion at the banks due to rapid hydraulic head change caused by increased flow velocity and reduced sediment. This incision and degradation downstream have the effect of reducing the channel slope, and eventually, the transport capacity of the system as a new equilibrium condition is reached (East et al., 2015). Downstream erosion and degradation are sometimes seen to differently erode channel banks, coarse materials associated with the regime before construction of the dam may be more resistant to erosion than finer channel bank deposits. Furthermore, erosion may occur downstream of a dam, while aggradation of sediment can happen further down in response to different channel conditions and an increased sediment load from upstream erosion (Moges &

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Bhole, 2016). Consequently, the two-primary effect of dam concerning changes in sediment supply and transport capacity is; sediment shortage or sediment surplus with the most common downstream river segments been sediment shortage (Schmidt & Wilcock, 2008). 3.2 Sedimentation effects upstream a dam The effect of putting up a dam across a river causes near still water hydraulic conditions and reduced flow velocity in the system. This effect causes the settling out of sediment that is transported from upstream the dam, which occurs with time, concerning sediment size, settling velocity, and the incoming flow velocity. Further upstream in the system, it is found that coarser materials drop out in suspension and the fines are being transported and deposited further down. Catchment sediment transport characteristics determine the amount of sediment retained behind a dam. Land use, runoff, rainfall intensity and depth, geology, topography, cleared area for agricultural purposes as well as urban development are the influencing factors (Csiki & Rhoads, 2014). The two significant geomorphological changes likely to emanate when regulating the flow regime of a river system with a dam are; reduction in the storage capacity and flow capabilities upstream the dam. These changes are caused by the infilling of sediment behind the structure. Another problem that may arise upstream the reservoir is the formation of in-stream delta deposits, which can cause a significant reduction in the stream capacity (Anderson et al., 2015). Figure 2 provides a basic understanding of conventional upstream impacts, showing that total storage capacity for the reservoir is the summation of the active and dead storage. Dead storage is situated below the low water level defined by the elevation of the lowest outlet, while active storage includes water that can be released for power generation or water supply, or can be reserved for flood mitigation purposes in the case of flood control dams. Hydropower generation is adversely affected by sedimentation in two facets; the amount of power generated and the increase in turbine maintenance needs. This is because sedimentation hampers the reservoir active storage leading to reduced power generation. Also, the presence of sediment flowing through the turbines largely made from hard minerals will result in abrasion requiring maintenance. Similarly, water supply needs will face shortages due to reduced reservoir storage caused by sedimentation (Annandale et al., 2016) .

Figure 2. Reservoir storage loss due to sedimentation (Annandale et al., 2016)

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4. Reservoir sustainable management Most reservoirs impound sediment along with water, as a flowing river conveys both water and particles(sediment). Dams can be designed and built to allow passage of sediment through the dam or around the reservoir, using several tested techniques, each specific to a variety of conditions. Sediment management techniques are most times not used as they should be in several reservoirs. This could be because, policymakers and the operators of dams do not know the several possible approaches of management, nor have they been shown to be effective and the appropriate conditions of applicability. As a result, the opportunity presented for sustaining reservoirs and substantially reducing its effects downstream over a long period is missed (Kondolf et al., 2014). The reservoir sustainability approach must consider a sediment management strategy to either directly focus on sediment mitigation or makes a plan to provide sufficient monetary resources to do so later on. Else, a sediment-laden reservoir expected to achieve its benefits will end up becoming an economic burden for future generations (Wang et al., 2015). Besides, Kondolf et al., 2014 present a holistic classification of how sustainability in reservoir capacity can be achieved using appropriate sediment management strategies. The strategies the authors proposed came up under three broad headings; (i) sediment yield reduction at catchment level, (ii) sediment deposition minimization and, (iii) volume recover or increase. This classification is further elaborated based on the three heading in Figure 3.

Figure 3. Reservoir sediment management strategy classification (Kondolf et al, 2014)

On the other hand, there are several sediment management techniques available that will be vital in extending the reservoir life span and reduce the effect of sediment starvation at the downstream segments of rivers due to sediment impoundment. The following subsections present some available techniques according to (Muhammad Nda et al., 2017; Palmeri et al., 2003). 4.1 Flushing This is a technique that ensures the increases of flow velocities in a reservoir to such an extent that deposited sediment is remobilized and transported in the dam via low-level outlets. It is distinct from sluicing, which is meant to divert incoming upstream sediment through the reservoir. There are two available approaches; complete and partial

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drawdown flushing. Instances when reservoirs are empty i.e during flooding and are considered as complete drawdown flushing. Whereas, in scenarios when the water level in the reservoir is reduced partially are referred to as partial drawdown flushing. 4.2 Sluicing In this technique, a substantial portion of the incoming sediment load is allowed to pass through the reservoir and dam before the sediment particles can settle, thus reducing the reservoir's trap efficiency. In most cases, this is accomplished by operating the reservoir at a lower level during the flood season to maintain sufficient sediment conveying capacity (turbulent and colloidal) through the reservoir. Higher flow velocity and higher sediment transport capacity in the water flowing through the reservoir result from the operation of the reservoir at these lower levels. The increased sediment conveying capacity of the water flowing through the reservoir reduces the deposited sediment volume. The pool level in the reservoir is increased during the flood season, to store fairly clear water. The success of sluicing operations depends primarily on the availability of excess precipitation, on the sediment grain size and the morphology of the reservoir. For certain cases, the combination of sluicing and flushing is used. 4.3 Density current venting Density currents can sometimes develop due to varied conditions, causing more sediment to be conveyed to the dam. Such currents occur because the density of sediment ladened water flowing into the reservoir is higher than the density of clear water impounded in the reservoir. This higher density, is responsible for increased viscosity and consequent reduction of turbulence intensities leading to a reduction in a coherent current with a high sediment concentration that falls beneath the clear water. Low-level gates installation will enhance the discharge of sediment current downstream through the dam. In so doing the reservoir will be relieved of sediment deposit thus mitigating storage loss. Density current venting is a good sediment management technique where sediment-laden flows are released without necessarily lowering of the reservoir level as required in flushing operations. 4.4 Mechanical removal This is achieved using two conventional dredging techniques i.e. dry excavation and the hydro suction removal system (HSRS). The dry excavation in some cases referred to as trucking is used at a time the reservoir level is lowered especially during the dry season when the river flow has decreased and can be adequately controlled without interfering with the excavation works. The sediment is excavated and conveyed for disposal utilizing traditional earth moving equipment. The operating cost of sediment excavation and disposal is quite enormous therefore, the technique is generally more appropriate to be used for relatively small deposits and may also be suitable for flood control reservoirs. Conversely, HSRS is different from the traditional dredging as the dam hydraulic head used as the source of energy for dredging rather than using pumps powered by electricity or diesel. As such, where there is sufficient head available, the operating costs are substantially lower than those of traditional dredging. The system is made up of a barge that regulates the suction and discharges pipe flow, which can be used to move the pipe's suction end-around. The upstream section of the pipe is situated at the reservoir's sediment level and its downstream section is normally placed over the dam to remove sediment and water downstream. The pipe is arranged to effectively produce a siphon, and the suction is used to remove sediment at the upstream section of the siphon. This technique can be used in relatively small reservoirs no longer than around 3 km and relies on dam and reservoir level as well. 4.5 Dredging This management technique of removing deposited sediment from underwater is termed dredging. Dredging is a highly specialized operation that is mostly used in ports, rivers, and estuaries to clear navigation channels. However, the technique is also frequently used in reservoirs. Dredging of sediments is widely used to recover lost reservoir storage from sediment deposits. Although, the use of conventional hydraulic dredging is generally not advisable to be used as its usually very expensive. The cost of this technique may amount to more than what is required to replace the reservoir storage or the entire dam structure.

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5. Conclusion Reservoirs and downstream sections of rivers are faced with serious consequences due to sediment impoundment. The sustainability of electricity and water supply is grossly threatened by the accumulation of sediment behind the dam resulting in a reduction of storage capacity and impairment of its day to day functions. The existence of inadequate suitable dam locations presents a serious challenge, thus the need to properly consider adequate sediment management in dam planning. Most dams are designed with projections to accumulate sediment in its reservoirs for 50 – 100 years without any adverse effect on its operation. However, several dams (water supply and hydroelectricity generation) are faced with serious operational impairment due to sediment accumulation before the designed projected lifespan. Based on existing techniques reviewed in this paper it is opined that sediment management strategies be considered in the siting, designing, planning, and operation of a dam. Besides, the possibility of designing future dams with sediment bypass through or around the reservoir will be a very viable option where applicable. Also, existing dams should be revisited to ensure workable sediment management practices are put in place to guarantee sustainability. References Adeogun, A. G., Ibitoye, B. A., Salami, A. W., & Ihagh, G. T., Sustainable management of erosion prone areas of

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Biographies Engr. Muhammad Nda is a Registered Engineer with COREN and a Ph.D. Research Fellow in Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein Onn Malaysia. He earned his B.Eng. Civil Engineering and M.Eng. Water Resources and Environmental Engineering from the Federal University of Technology Minna, Niger State-Nigeria. He has published and serve as a reviewer for international journals and conferences. His research interest is in sedimentation, erosion, river morphology, climate change, and environmental sustainability. Dr. Mohd Shalahuddin Bin Adnan is the Head of Department, Academic Development and Training Center, and a Senior Lecturer at Faculty of Civil Engineering and Built Environment Universiti Tun Hussein Onn Malaysia. Dr. Mohd Shalahuddin holds a Bachelor of Engineering from Universiti Sains Malaysia and a Master of Engineering Geology from Gadjah Maja University in Indonesia. He proceeded to Kyushu University in Japan and obtain a Ph.D. in Urban and Environmental Engineering. He has taught many courses and supervised many research thesis at both undergraduate and postgraduate levels. He has published several research papers and conferences. Dr. Mohd Shalahuddin has served as head and member of several research grants. His research interests are river restoration, improvement of the river environment, sedimentation and erosion, flood mitigation, and climate change. Dr. Mohd Azlan Bin Mohd Yusoff is a Lecturer at Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein Onn Malaysia. He holds a Doctor of Philosophy in Hydroinformatics, a Bachelor and Masters Degree from Universiti Sains Malaysia. He has taught many courses and supervised many research thesis at both undergraduate and postgraduate levels. He has published several research papers and conferences. Dr. Azlan has served and headed many research projects for the Universiti and Government MDA’s. His research interests are urban drainage, sedimentation and scour, physical modelling and water resources soft computing techniques. Engr. Gideon Shabako Jiya is a lecturer at the Civil Engineering Department, School of Engineering Technology, The Federal Polytechnic Bida, Nigeria. He holds a B.Eng. in Civil Engineering from the Federal University of Technology Minna, Nigeria, and M.Eng. in Highway materials and Transportation Engineering from the University of Ilorin Nigeria. Engr. Jiya's research interests are Public transportation system, Highway and pavement materials, and computer-aided transportation system.

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Dr. Ibrahim Yakubu Ebenehi is a lecturer in the Building Technology Department, School of Environmental Technology, The Federal Polytechnic Bauchi, Nigeria. had à BSc in Building, and MSc in Construction Management from the University of Jos, Nigeria, and a Ph.D. in Real Estate & Facilities Management from Universiti Tun Hussein Onn, Malaysia. Dr. Ebenehi's research involves fire safety management in Higher Education Institutions, post-disaster housing reconstruction, sustainable construction management as well as facilities management.

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Documents

Sustainable Sediment Management in Reservoirs; An OvervieKeywords: Sediment, dams and reservoirs, sediment management, ... Introduction . Reservoirs are used to provide water supplyportable