Extending the Life of Reservoirs: Sustainable Sediment Management for Dams and Run-of-River Hydropower

CHAPTER 1

Purpose and Application of This Book

Pravin Karki

Introduction

This book was developed to facilitate implementation of a programmatic approach using selected scientific methods for screening climate change and disaster risks, and integrating appropriate resilience measures into water, hydropower, and dam investment projects. As the World Bank Group steps up its activities in both the water and energy sectors, the risks of climate change and disasters need to be better understood and managed to ensure sustainable, resilient, and cost-effective outcomes. This increased awareness is particularly important for hydropower, water supply reservoir, and dam projects, given that climate change is projected to significantly affect water resources by changing mean annual river flows and hydrologic variability, thereby causing more extreme droughts and floods. For many countries, hydropower is now the largest source of affordable renewable energy (World Bank 2013b). This is especially true in regions like Sub-Saharan Africa, South Asia, and Southeast Asia, which are characterized by significant untapped hydropower potential and water shortages.

The World Bank Group’s engagement in hydropower and water supply projects of all sizes and types requires careful planning to ensure resilience against the uncertainties of climate change and disaster risk. Even though sedimentation poses the greatest threat to the sustainability of hydropower, most guidelines on water supply and dam projects provide virtually no direction on how to deal with the sediment problem. Current guidelines tend to focus on mitigating changes in flows associated with climate change and do not address sedimentation.

Recognizing the importance of creating and maintaining reservoir storage, the World Bank previously developed the reservoir conservation (RESCON) approach (Palmieri et al. 2003) to facilitate rapid identification of technically sound and economically optimal reservoir sediment management strategies. Since then, greater understanding of reservoir sediment management technology has emerged, leading the World Bank to invest further in upgrading the RESCON approach. This book complements the upgraded RESCON model by providing guidance on adopting sustainable sediment management practices for hydropower and water supply dam projects.

The World Bank’s Role in Sustainable Infrastructure Activities

The World Bank is intimately involved in the development of sustainable infrastructure worldwide. The International Development Association (IDA), in the period from July 1, 2014, to June 30, 2017 (IDA17), will place special emphasis on ensuring that development projects incorporate climate and disaster risk considerations and encompass a sharper focus on value for money through enhanced efforts to improve both results and cost-effectiveness (IDA 2014). In addition, in its 2013 report Building Resilience: Integrating Climate and Disaster Risk into Development (World Bank 2013a), the International Bank for Reconstruction and Development (IBRD) stressed that building climate resilience is critical for achieving the World Bank Group’s goals of ending extreme poverty and building shared prosperity. The report called for the international development community to build long-term resilience, reduce risk, and avoid rising future costs.

This book supports these goals by providing practical solutions for those who will be involved in the planning, design, construction, and operation and maintenance of dams, reservoirs, and hydropower plants so that the threat of climate change and the need for sustainable, cost-effective infrastructure are taken into consideration. Specifically, this book addresses the critical threat of sedimentation—a process that reduces the storage capacity of reservoirs and with it all the water supply, flood control, and hydropower benefits they provide, and that damages hydromechanical equipment leading to a loss in hydropower generation.

The Importance of Sediment Management for Ensuring the Sustainability of Reservoir and Run-of-River Projects

Reservoirs are used worldwide to provide reliable water supply, hydropower, and flood management services. They are particularly important in areas of the world with high hydrologic variability, where the amount of water flowing in rivers varies significantly both seasonally and from year to year. In these areas, storing enough water for use during severe or multiyear droughts, and thereby ensuring the reliability of water and power supply, requires very large reservoir storage volumes. Countries where hydropower is an important source of energy often have both reservoir and run-of-river (ROR) projects. ROR projects, where preservation of storage is often a secondary objective, represent about 11 percent of all large dams (ICOLD 2015). For the remainder of dam projects, creating and maintaining reservoir storage is crucial to providing irrigation, water supply, flood control, multiple-use, and hydropower benefits.

Sedimentation poses a significant threat to the longevity, usefulness, and sustainable operations of both storage reservoirs and ROR projects (Palmieri et al. 2003). Over time, sediment builds up in reservoirs and displaces usable storage volume, which in turn negatively affects hydropower generation, reduces the reliability of domestic and irrigation water supply and flood management services, and degrades aquatic habitat. In ROR projects, sedimentation damages turbines and leads to inefficiencies in power generation and costly repair. In short, sedimentation is a major factor influencing the sustainability of dams, reservoir storage, and all types of ROR projects.

Dams have traditionally been designed under the design life paradigm, which entails estimation of the sedimentation rate and trap efficiency, and provision of a sediment storage pool volume equivalent to the design life (typically 50 or 100 years). Under this paradigm, consequences beyond the design life are not addressed, leading to decommissioning.¹ In many regions, however, new reservoirs are both costly and difficult to construct because the best (least costly) dam sites have already been used, and because there is intense resistance to the flooding of additional lands due to competing land uses and social and environmental concerns. The cost of dam decommissioning may also be very high. Finally, deposition of sediment in reservoirs removes it from downstream river reaches, thereby causing erosion of those reaches and degradation of aquatic habitat.

Therefore, as dams and reservoirs approach the end of their original design lives, most owners are interested in maintaining the infrastructure and continuing to generate economic and social benefits, including water supply, hydropower, and flood control, even if the benefits are not as large as in the original project. Extending the dam’s life entails adopting a new design and operational paradigm that focuses on managing the reservoir and watershed system to bring sediment inflow and outflow into balance to the degree that doing so is practical, thereby giving the reservoir a greatly extended or even indefinite life.

Climate change is projected to increase hydrologic variability in many parts of the world, increasing the intensity of both floods and droughts. This variability will increase the need for larger reservoir volumes to ensure reliable water and power supplies and much-needed flood control. Climate change is also expected to increase sediment loads in many rivers, amplifying the threat of reservoir sedimentation. Therefore, it is essential that new dam and reservoir projects be designed, built, and maintained with the long-term threat of reservoir sedimentation in mind, and that existing projects be converted to sustainable use insofar as is possible. This perspective is consistent with the World Bank’s efforts to develop climate screening tools.²

Solutions and Recommendations for Successful Sediment Management

The sustained threat of reservoir sedimentation and the anticipated increase in demand for large reservoir projects as a result of the effects of climate change oblige governments to assume a leadership role in sustainable development by investing in projects with lasting benefits and ensuring that investments made in the near term incorporate sediment management measures that will reduce future maintenance costs and ensure the long-term functionality of dams and hydropower infrastructure.

The World Bank often relies on client countries to hire consulting firms to address long-term sustainability of reservoir and ROR projects. However, it is the duty of World Bank staff to ensure that the consultants fulfill this role and that the goals of sustainable development are never compromised. This book calls on engineers and economists to incorporate sediment management measures into the early phases of project planning as part of a sustainable management approach. Specific sediment management techniques outlined herein include

• Reducing upstream sediment yield through erosion control and upstream sediment trapping,

• Managing flows during periods of high sediment yield to minimize trapping in reservoirs, and

• Removing sediment already deposited in reservoirs using a variety of techniques.

Purpose, Uses, and Organization of This Book

Purpose and Uses

The purpose of this book is twofold: (1) to illustrate why incorporating sediment management into dam projects is important and (2) to provide information on specific sediment management strategies that can be undertaken in projects as part of a sustainable sediment management approach. One of the key messages of this book is that incorporating sediment management into the planning and design phases of dam projects is essential for ensuring that the benefits of reservoir storage are sustained over the long term. Without sediment management, reservoir storage space is eventually lost, and it is extremely difficult—if not impossible—to reclaim it. Reservoir storage space is a key factor of production for water and renewable energy supply, and it is becoming increasingly important as climate change–related stresses increase and suitable storage sites become increasingly scarce. As a result, it is essential that projects incorporate sediment management at the outset as an integral part of their configuration to ensure lasting benefits.

This book aims to present techniques for sediment management in a manner that is accessible to a nontechnical audience. It is written primarily for World Bank Group team leaders, planners, government officials, and developers who are involved in the planning, design, construction, and operation and maintenance of dams, reservoirs, and hydropower plants. The book is neither an engineering manual nor an economic analysis manual; it is designed to fill the gap between general summaries, which are not useful in the practical sense when it comes to project planning and design, and detailed manuals, which are referenced throughout the book for further information.

This book is intended to inform readers of approaches to sustainable development of water resource infrastructure that will allow them to confidently review proposed projects. In particular, the Checklist for Sediment Management provided in appendix A highlights recommendations based on sediment problems that typically arise in projects. It is important to note, however, that sediment management strategies for specific projects must be tailored to site-specific conditions and limitations.

Organization

The remainder of this book is organized into nine chapters and an appendix:

• Chapter 2: Climate Change, Sediment Management and Sustainable Development discusses the importance of reservoir sediment management for preserving reservoir storage and illustrates how it contributes to satisfying the tenets of sustainable development.

• Chapter 3: Overview of Sedimentation Issues discusses the importance of reservoir storage and the impacts of reservoir sedimentation up- and downstream of dams.

• Chapter 4: Sediment Yield provides an overview of sediment yield, describes the important factors that determine the magnitude of sediment yield, and presents ways to estimate sediment yield.

• Chapter 5: Patterns of Sediment Transport and Deposition describes techniques for estimating the amount of sediment that will be deposited in a reservoir.

• Chapter 6: Sedimentation Monitoring discusses sedimentation monitoring procedures, bathymetric mapping of sediment, and estimation of sediment bulk density.

• Chapter 7: Sediment Management Techniques presents an overview of activities to combat reservoir sedimentation.

• Chapter 8: Sediment Management at Run-of-River Headworks describes basic concepts to consider in the design or rehabilitation of run-of-river headworks with regard to sediment management.

• Chapter 9: Reservoir Sustainability Best Practices Guidance summarizes sediment management strategies that will provide a higher level of assurance that the project operation can be sustained indefinitely.

• Appendix A: Checklist for Sediment Management is for use by project proponents to help ensure that projects adhere to the recommendations put forth throughout the book. The checklist is divided into three sections: sediment yield, sustainable sediment management measures, and sediment patterns and impacts.

Notes

1. Three examples are the San Clemente Dam (California, United States), the Matilija Dam (California, United States), and the Camaré (Pedregal) Dam (República Bolivariana de Venezuela).

2. World Bank Group. 2015. Climate & Disaster Risk Screening Tools (http://climatescreeningtools.worldbank.org).

References

ICOLD (International Commission on Large Dams). 2015. Register of Dams. http://www.icold-cigb.org/GB/World_register/general_synthesis.asp.

IDA (International Development Association). 2014. Report from the Executive Directors of the International Development Association to the Board of Governors 2014. Washington, DC.

Palmieri, A., F. Shah, G. W. Annandale, and A. Dinar. 2003. Reservoir Conservation: The RESCON Approach. Washington, DC: World Bank.

World Bank. 2013a. Building Resilience: Integrating Climate and Disaster Risk into Development. Washington, DC: World Bank. https://openknowledge.worldbank.org/handle/10986/16639.

———. 2013b. Toward a Sustainable Energy Future for All: Directions for the World Bank Group’s Energy Sector. World Bank, Washington, DC.

World Commission on Environment and Development. 1987. Report of the World Commission on Environment and Development: Our Common Future. United Nations, New York.

CHAPTER 2

Climate Change, Sediment Management, and Sustainable Development

George W. Annandale

Introduction

Dam projects can generally be divided into run-of-river and storage projects. Run-of-river projects (figure 2.1, panel a), often used for hydropower generation, usually have small active storage volumes and large dead storage volumes. The objective is to maximize the head¹ and have just enough storage to satisfy peaking demands.

Figure 2.1 Active Storage Features of Run-of-River and Storage Reservoirs

Storage projects (figure 2.1, panel b), in contrast, have large active storage volumes and small dead storage volumes. The active storage contains a large amount of water for irrigation and water supply and, in the case of flood management projects, is used to attenuate floods. Storage may also be used for hydropower generation. In such cases, the head used for power generation can vary, which will affect the efficiency of power production but increase the reliability of power supply (Annandale 2015).

Sediment management objectives in these two types of projects differ. For run-of-river projects, sediment management aims to improve operational efficiency. If sediment is not removed from run-of-river facilities before it enters the turbines, it may cause abrasion and clog the cooling water intakes of the electromechanical equipment, which increase operation and maintenance costs and diminish the amount of power that can be generated.

Sediment depositing in the dead storage space in run-of-river projects does not affect operational efficiency, although it may result in increased amounts of sediment entering the turbines. Sediment depositing in the active storage volume may diminish peaking ability, which, although undesirable, is often not addressed in project design (that is, designs have not historically allowed for removal of deposited sediment from the active storage).

The objective of sediment management in storage projects is to ensure project longevity for storing large amounts of water for use during droughts. Such storage also provides the opportunity to attenuate floods. Failure in the past to design dams that include sediment management has resulted in the current net reduction of reservoir storage space worldwide. Annandale (2013) estimates that net reservoir storage has been decreasing since about 2000, and per capita storage worldwide is now at levels last seen in 1965.

In general, this book gives less attention to preserving storage space in run-of-river projects compared with storage projects. The reason is that reservoir storage is the most important function of dam projects, considering that run-of-river projects represent only 11 percent of all large dams (ICOLD 2015). Storage reservoirs deliver reliable water supply, irrigation, flood control, and hydropower services. Therefore, losing storage to sedimentation reduces the services provided by dams (Annandale 2013, 2015).

If storage is so important, why is reservoir sediment management to retain reservoir storage not routinely considered when designing dams? The answer is that most designers and economists rely on outdated design paradigms and are not familiar with modern reservoir sediment management techniques.

This book focuses on informing the reader of the basic concepts underpinning sustainable sediment management strategies. The purpose is to emphasize the importance of managing sediment to prevent or minimize storage loss. The book lists and categorizes reservoir sediment management techniques to encourage changes to design paradigms. It does not provide detailed instructions on how to design reservoir sediment management systems, nor on how to execute economic analysis. The key principles of integrated engineering and economic analysis of sediment management are well covered elsewhere; in particular, see Palmieri et al. (2003), which is extensively referenced here. Economic analysis is critically important, and a discussion of basic concepts is included for the non-specialist