Challenges of reservoir planning and management in a changing world

Considering Hydrological Change in Reservoir Planning and Management Proceedings of H09, IAHS-IAPSO-IASPEI Assembly, Gothenburg, Sweden, July 2013 (IAHS Publ. 362, 2013).

Copyright 2013 IAHS Press

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Challenges of reservoir planning and management in a changing world ALBERTO VIGLIONE1, ALBERTO MONTANARI2 & GÜNTER BLÖSCHL1,3 1 Institute of Hydraulic Engineering and Water Resources Management, Vienna University of Technology, Vienna, Austria [email protected] 2 Dipartimento di Ingegneria Civile, Ambientale e dei Materiali, University of Bologna, Italy 3 Centre for Water Resource Systems, Vienna University of Technology, Vienna, Italy Abstract Possible shifts of the rainfall regime in the next decades may have important implications for reservoir planning and management. Also, land use change may alter the inflow regime to reservoirs, including the water and sediment dynamics. This paper reviews challenges to reservoir planning and management, and emerging approaches to address these issues in a changing world. Changing persistence of streamflow may increase the uncertainties of sizing reservoir storage, and changing floods may increase the uncertainties associated with spillway design. Short term reservoir operation can be supported by ensemble forecasts and robust operation models to assess the uncertainties of management alternatives. Reservoir sedimentation is a major issue in many parts of the world which may be exacerbated by changes in catchment land use. It is argued that robust bottom-up approaches are needed that are vulnerability or resilience centred to complement the traditional top-down (scenario) approaches to reservoir planning and management. Key words reservoirs; flood estimation; climate change; runoff processes; long range dependence; sediment; reservoir operation and control; sustainability INTRODUCTION

Reservoirs are an essential part of water resource systems in many parts of the world. Hydrological fluxes are highly variable in space and time, and reservoir storage helps match the supply and water demand. Reservoirs are also essential components of flood protection schemes, hydropower operation, and they are used for recreation and navigation. There are millions of reservoirs around the world, about 50 000 of them have been classified as large dams (i.e. over 15 m high) by the International Commission on Large Dams (Fig. 1). The water in these reservoirs amounts to about 10% of the annual river runoff, clearly a storage component of global significance (Jacquot, 2009). While the importance of reservoirs is increasing due to increasing pressures on the water resources, there are recent challenges brought about by potentially increasing hydrological variability and possibly negative side-effects of reservoirs. This paper briefly reviews these new challenges. We will first consider two alternatives to risk assessment and management and then

Fig. 1 Large reservoir around the world by country (Lehner et al., 2011).

Alberto Viglione et al.

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discuss the challenges of reservoir planning (including reservoir sizing and spillway design) and reservoir management (including flood management, sediment management), in each case proposing emerging methods for addressing them. APPROACHES TO RISK ASSESSMENT AND MANAGEMENT

Prudent reservoir planning and management strives to maximise benefits, and minimise costs and risks involved in the operation of the reservoir, in addition to other societal goals. Important risks of reservoir planning and management are related to hydrological characteristics. Specifically:

– The risk of not meeting the goals of the reservoir (negative environmental impact, incompatibility of multiple uses, insufficient capacity, lifespan shorter than expected)

– The risk related to floods (dam break, downstream inundations)

There are two approaches to addressing the prioritisation of risk management measures in reservoir planning and management (Fig. 2, Blöschl et al., 2013a). The first, termed the top-down approach, starts from the climate forcing, cascading down information to the people affected by the management of a reservoir. The second, termed bottom-up approach, starts from the people affected and explores possibilities of risk management.

Fig. 2 Risk assessment options for the example of climate impacts on flood risk downstream of reservoirs. Left: Traditional top-down approach to hydrological risk assessment based on scenarios. Right: Bottom-up approach to hydrological risk assessment that is vulnerability or resilience centred. Grey arrows indicate less dependence than black arrows (Blöschl et al., 2013a).

The “top-down” approach is designed to represent the main processes causing the hazard (such as a flood or a drought) and its consequences. In the case of climate impacts (Fig. 2), for example, the “top-down” approach starts at the global scale and cascades information from emission scenarios of future societies to simulated climates using global and regional climate models, to runoff using hydrological models, to inundations using hydrodynamic models and to damage costs using economic models (Merz et al., 2010). The top-down approach is motivated by an economic paradigm. Risk is therefore defined from an economic perspective, specifying hazard as the occurrence probability of a flood (or drought) event, the vulnerability as the damage cost in monetary terms, and (economic) risk as the product of the two. This leads to an optimisation problem that aims at identifying the most economic management strategy. The top-down approach has been widely used to asses the impact on reservoir management of future climate changes as predicted by global circulation models (GCM) of the atmosphere (Carpenter & Georgakakos, 2001; Yao & Georgakakos, 2001; Christensen & Lettenmaier, 2007). To obtain a precautionary

Challenges of reservoir planning and management in a changing world

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estimate of risk in the presence of relevant uncertainty in the GCM scenarios that are used at the top of the modelling cascade, safety factors are generally introduced in the assessment of the future climate, rainfall–runoff modelling and risk analysis. Therefore the top-down approach typically leads to oversized (and therefore economically intensive) solutions. The “bottom-up” approach (Fig. 2), in contrast, puts at the top of the modelling chain an assessment of the resilience of the reservoir and related social systems with respect to extreme events. Being based on actual facts and not future predictions, the resilience assessment is generally supported by less uncertain information with respect to the top-down approach. In fact, the bottom-up approach starts at the local scale of individuals, households and communities and explores the factors and conditions (including reservoir design and management and climate forcing behaviours) that enable successful coping with hydrological extremes (Wilby & Dessai, 2010). Therefore, the bottom-up approach starts from the analysis of the current situation and investigates to what extent it can cope with climate change, rather than identifying which future solutions (often not economically feasible) are needed for mitigating the effects of predicted (and therefore uncertain) perturbations. The modelling chain that is used in the two approaches (rainfall–runoff modelling, reservoir management) is the same, as well as the possible solutions (management strategies, increase of the reservoir volume) that can be identified with the same optimisation procedures. The distinguishing behaviour of the bottom-up approach is that the most uncertain information is used at the end, rather than the top, of the modelling exercise, while the current situation is put front stage. Therefore, the bottom-up approach is motivated by a social paradigm. The main goal is to enhance resilience (the ability to copy and recover after an event) of the system associated with reservoirs and the downstream river reaches rather than designing solutions that might be economically unfeasible. While the top-down approach is currently more popular, realisation of the importance of the bottom-up approach is emerging (European Commission, 2009, p. 13). In fact, being the bottom-up approach aimed to reduce vulnerability and enhance resilience, it has particular value in countries where the vulnerabilities to floods and droughts are high while the economical resources are limited, such as in the developing world (Di Baldassarre et al., 2010; UNDP, 2011). In essence, the two approaches are two alternative ways of dealing with uncertainty associated with reservoir planning and management in a changing world. The following sections therefore focus on the uncertainties involved in reservoir planning and management, their sources and methods for dealing with them. UNCERTAINTY IN RESERVOIR PLANNING

Reservoir sizing: inflow volumes and long term persistence of streamflow

One of the key design criteria of reservoirs is the active storage volume as it will determine the risk with which the objectives (e.g. flood control, power production, irrigation) are not met at a particular time of the year. The required storage volume depends both on the future inflows to the reservoir and the future demand. It has been clear for a long time that the issue of reservoir sizing is intimately linked to the long term persistence characteristics of streamflow (Hurst, 1951; Klemeš et al., 1981) as, for the same distribution of inflow and the same water resources objectives, increasing long term persistence will require increasingly larger storage volumes. Also, increased long term persistence will likely increase the uncertainty of the design estimates (Koutsoyiannis & Montanari, 2007). The realisation that streamflow is dramatically changing in many locations due to water abstractions, land use change and climate change has spurred a renewed interest in the issue of long term persistence and the possible drivers (Koutsoyiannis et al., 2009; Blöschl & Montanari, 2010). Numerous long streamflow records are available now that can be analysed for their dependence structure. There is a range of studies supporting that long term persistence (as expressed by a Hurst coefficient H > 0.5) occurs in essentially all streamflow records examined (e.g. Montanari et al., 1999a,b, 2003; Mudelsee, 2007; Ehsanzadeh & Adamowski, 2010). A recent study by Szolgayova


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et al. (2013) analysed the factors controlling the magnitude of the Hurst coefficient from mean daily runoff time series in Europe. They tested a number of estimation methods, and all methods yielded estimates of H > 0.5. The results for the periodogram estimator are shown in Fig. 3. As can be seen from the figure, there is a tendency for smaller H in the North of Europe than in Central and Southern Europe, where climate warming is more significant. With increasing climate change it is therefore possible that the Hurst coefficient also increases, which would have important implications for reservoir sizing. The regression analysis of Szolgayova et al., (2013) indeed showed that H is positively correlated with air temperature and negatively correlated with a seasonality index (the maximum Pardé coefficient). A positive dependence of H on air temperature could be physically justified by a greater role of evaporation processes, and therefore temperature, in warm countries. Given that temperature is positively correlated, it is plausible that increasing temperature implies increasing long term persistence. Clearly, changes in the long term persistence are one of the challenges of reservoir management in a changing world. Their impact can be simulated in the reservoir modelling chain by appropriately modifying the rainfall–runoff model structure and/or parameters, by using either the top-down or bottom up approach.

Fig. 3 Estimated Hurst coefficients H of daily runoff for all estimation methods in European river basins based on the periodogram method. From Szolgayova et al. (2013).

Spillway design – flood inflows

Numerous recent floods around the world have raised the concerns that floods, and consequently flood risks, have increased (Kundzewicz, 2012). This is extremely important for the sizing of spillways of reservoirs as, for a given spillway size, increasing floods in the upstream catchment would translate directly into increased risk of dam failure. In order to understand changes in flood risk it is important to be clear about the physical causes of any such changes. The most obvious changes occur in the river system such as removal of flood plains and river training. For spillway design, however, other changes are often more important. For example, land use has been changed massively in many countries of the world due to urbanization and deforestation. Wetlands have been drained and agricultural practices have changed dramatically with the advent of heavy mechanical equipment. These changes will affect evaporation, infiltration into the soil and water storage in the soil (Salazar et al., 2012). Equally important, as floods driven by rainfall and snowmelt, or any changes in climatic variables such as precipitation and air temperature may


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induces changes in the flood magnitudes and the timing of floods within the year. Such changes may be both due to natural climate variability at different time scales and anthropogenic climate change and will affect the flood regime (Peel & Blöschl, 2011). When estimating future design floods the goal is to minimise the uncertainty induced by such changes. For this reason, the use of the bottom-up approach might be preferable. The key to reliably estimating floods in a changing world is to account for the runoff generation processes, as highlighted in the flood frequency hydrology concept of Merz & Blöschl (2008a,b) and Viglione et al. (2013). The main idea of this concept is to combine a range of sources of information including systematic flood data, spatial information expansion (regional flood data), temporal information expansion (e.g. historical floods) and causal information expansion (flood generation processes). The flood generation processes are usually considered at the event scale, e.g. by examining how the runoff coefficients change with the event magnitude, or by examining the effects of catchment characteristics on the shape of the flood frequency curve (Rogger et al., 2012). However, it is important to note that a changing world may entail more subtle long-term changes associated with the co-evolution of catchments in the interplay of climate, vegetation, soil, landscape and runoff dynamics. Figure 4 shows an example that illustrates the results of such co-evolutionary processes of flood response of catchments. The Gurk catchment has a dampened flood response while the Buwe catchment is very flashy. The catchment sizes do not differ much (432 and 184 km2) and annual precipitation and elevation are also similar. However, they are different in their landforms. The landscape of the Gurk is mountainous with flat valley bottoms and well rounded hills, while the landscape of the Buwe region shows deeply incised channels. They are also different in the flood producing storm types and the geology. The storm events in the Gurk are mainly synoptic, there is large subsurface

Fig. 4 Two hydrologically contrasting regions in Austria. Left: Gurk region (hydrographs of Glan at Zollfeld , 432 km2); Right: Buwe region (hydrographs of Raab at Mitterdorf, 184 km2). From Gaál et al. (2012) and Blöschl et al. (2013b).


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storage (highly permeable rock) and the surface flow paths are tortuous, thus resulting in dampened floods. Because of this, there is little erosion and increased potential for soil development, which in turn tends to dampen flood response. However, in the Buwe region, floods are produced by convective storms with partial coverage of catchments and the soils are shallow, thus resulting in flash floods. Because of this, there is high erosion which reduces soil depth and increases the efficiency of the drainage network, which in turn tends to increase the flashiness of flood response. This example illustrates that design flood estimation for reservoirs in a changing world will have to go beyond the traditional estimation procedures to account for the co-evolution of climate, vegetation, soils and landforms. To adapt spillway management to cope with increasing future floods one needs to explore alternative management options to maximise reservoir resilience within the bottom-up approach. Given the maximum discharge that a given spillway may release, and the maximum river flow that can be released downstream, the target is to identify the optimal management of the spillway itself to cope with increasing floods. Solutions often imply a seasonal reduction of the reservoir storage that should be based to seasonal flood frequency analysis, a topic that has not been explored much in the literature (Baratti et al., 2012). If the spillway is to be designed, the vulnerability of the system can be reduced by using free overflow spillways rather than funnel-like morning glory spillways since the former are more robust against hydraulic overload. UNCERTAINTY IN RESERVOIR MANAGEMENT

Flood management and reservoir operation in a changing world

Managing reservoir releases in a prudent way can significantly enhance their efficiency and therefore may offer a more resilient approach to cope with climate change and increasing floods. In reservoir operation, generally conflicts exist between different objectives, such as hydropower generation versus flood control. A key issue is to assess how the tradeoffs between them co-evolve with society. The assessment of the capability of the reservoir to cope with future perturbations should not be limited to assessing the effects of climate change. Societal changes should also be considered, along with their feedbacks on hydrology and reservoir operation (Montanari et al., 2013). These targets can be more readily achieved through the bottom-up approach in view of its societal focus. A wide range of reservoir operation concepts have been designed over the years from simple operating rules to sophisticated optimisation models such as linear programming, dynamic programming and nonlinear programming, and simulation models (Rippl, 1883; Hazen, 1914; Yeh, 1985; Klemeš 1987; Wurbs, 1993; Kritskiy & Menkel, 1995; Labadie, 2004). Typically, the objective function is found by cost-benefit analyses and a paradigm shift is therefore needed to include societal changes and resilience. Particularly appealing are models that are robust to non-optimal releases. For example, Celeste & Billib (2012) proposes a reservoir operation model based on implicit stochastic optimization in which the release policy is guided by the forecast of the mean inflow for a given future horizon rather than by the prediction of the current-month inflow. They found the method to be more efficient in identifying low-vulnerability release policies than more traditional approaches. In all methods, the crux is the uncertainty in the future inflows and the future demands. Inflow forecasts are therefore often complemented by ensemble forecasts that represent a range of future inflows over the subsequent hours or days. These are usually considered estimates of the forecast uncertainty (Zhao et al., 2012). Typically, such ensemble forecasts are based on quantitative precipitation forecasts to obtain forecast lead times of a couple of days (e.g. Nester et al., 2012). These forecasts can then be used to account for the uncertainty in reservoir operation. Reszler et al. (2008) developed a reservoir operation model based on a set of rules that represent the main objectives of reservoir operation, in particular, the trade-off between hydropower generation and flood control. In the model, the forecast uncertainty is represented by an explicit forecast error. Figure 5 shows an example of a simulation for the Ottenstein reservoir in Austria based on inflow forecasts. Depending on whether priority is given to hydropower production or flood mitigation,


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different reservoir operation strategies are estimated by the model. When priority is given to hydropower production (case H in the figure), the simulations show a rapid gate opening and a steep reservoir level rise (upper panel). The model indicates that, without this rapid operation, critical water levels may be exceeded over the next 48 h. This leads to a steep rise in the released discharge (lower panel) during peak inflow. In this case, the reservoir is full at the end of the event. In contrast, when priority is given to flood mitigation (case F in the figure), the simulations show an earlier gate opening and a more moderate reservoir level rise (upper panel). The gates are simulated to stay open during peak inflow and lake retention is operative so that a significant peak reduction occurs (lower panel). However, in this scenario the reservoir is not quite full at the end of the event. This type of trade off is being made possible by ensemble inflow forecasts to the reservoirs. The system of Reszler et al. (2008) has been in operational use since 2006 and is also used for training the operating personnel.

Fig. 5 Simulated gate operation and releases from the Ottenstein reservoir, Austria for two operation alternatives: (H) priority on Hydropower production; (F) priority on Flood mitigation. From Reszler et al. (2008.)

Sediment management

A key concern when operating reservoirs is to maintain the active storage to achieve their design goals. However, throughout the world reservoirs are being threatened by sedimentation. For example, in China sediment deposition in reservoirs has caused the loss of 66% of the total reservoir capacity (Wang & Hu, 2009). Reservoir sustainability is also an increasing issue in many other parts of the world (e.g. Graf et al., 2010; Haregeweyn et al., 2012). Reservoirs are already strongly affecting the sediment loads of rivers on the global scale due to the efficient trapping of sediments (Walling, 2012) and these loads are retained in the reservoirs thus reducing the active storage. While there are a number of potential sedimentation control strategies, including storing the clear water and releasing the turbid water, releasing turbidity currents, draw-down flushing and empty flushing and dredging (Wang & Hu, 2009), in many countries they are not feasible because of environmental concerns. Knowledge of the estimated lifespan of reservoirs is therefore critically important. The main control on reservoir sustainability is the sediment input to the reservoir and this is notoriously difficult to predict. Similar to the case of floods, key to reliably estimating sediment


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yields in a changing world is to account for the sediment production processes and the factors controlling them. While there is a wide diversity of the temporal evolution of sediment loads around the world (Walling & Fang, 2003) there are clear factors that one would expect to change the erosion and sediment yield. More intense rainfall may enhance erosion and thus sediment production. More generally, erosion mechanisms would be expected to change in a transient climate (Nearing, 2001). There are a range of feedback mechanisms associated with soil erosion/depletion and soil formation related to both climate and runoff generation (Blöschl et al., 2007). Some of the feedback loops are disrupted by human intervention (e.g. vegetation dynamics, soil formation). In arid environments erosion may be much more sensitive to land cover change than is runoff, and land degradation may be a major driver of erosion and therefore sedimentation of reservoirs (Grauso et al., 2008; Haregeweyn et al., 2012). There are a multitude of other factors, depending on the environmental setting. In the northern Rocky Mountains, for example, climate change is expected to increase sediment yield primarily through changes in temperature and hydrology that promote vegetation disturbances (i.e. wildfire, insect/pathogen outbreak, drought-related die-off; Goode et al., 2012). In alpine areas, melting of permafrost may increase the sediment yield of catchments. As Graf et al. (2010) noted, future surprise changes in sedimentation delivery and reservoir filling are to be expected. In the case of sediment management of reservoirs, a bottom-up approach of reducing vulnerability and enhancing resilience is therefore of particular value, especially in countries where the vulnerabilities to erosion tends to be high such as in arid and tropical environments. Such approaches may include local strategies to improved land management. CONCLUSIONS

Reservoir planning and management in a changing world poses many challenges. While it is not always clear what hydrological changes are exactly to be expected for a particular reservoir, increases in the uncertainties of hydrological estimates are likely. A changing world is inherently difficult to predict, so robust planning and management systems are becoming increasingly important. This downward approach of reducing vulnerability and enhancing resilience is particularly appealing in the light of unexpected events (Taleb, 2007; Blöschl et al. 2013a). The bottom-up approach has more potential to prepare for unexpected events in reservoir planning and management than the top-down approach as the focus is on reducing the vulnerability of the system by robust methods. Using free overflow spillways rather than funnel-like morning glory spillways allows one to obtain more robustness against hydraulic overload. This solution may not be optimum in an economic sense but may be more robust than alternative approaches to extreme events with large consequences. In highly vulnerable settings as they occur in developing countries it is particularly important to perform bottom-up risk analyses, i.e. to start from policy options and their risks, rather than to perform top-down modelling. While the importance of the traditional top-down (scenario) approaches to reservoir planning and management is undisputed, they should be complemented by robust approaches, reducing the vulnerability and increasing the resilience of the system in a changing world. Acknowledgements Funding from the European Research Council (Advanced Grant 291152-FloodChange) and the Austrian Academy of Sciences (ISDR Mountain Floods) is gratefully acknowledged. REFERENCES Baratti, E., Montanari, A., Castellarin, A., Salinas, J. L., Viglione, A. & Bezzi, A. (2012) Estimating the flood frequency

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Challenges of reservoir planning and management in a changing world

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