Prospective Chapter: Dam Design Challenges under a Clam for Sustainability on a Modified Catchments

Abstract

Dams play a crucial role in water management and have both positive and negative impacts on river catchments. One of the significant concerns is the distortion of natural flow discharge and sediment dynamics downstream of a dam. This distortion affects the ecosystem and the river’s natural habitat, and it is essential to address these implications for sustainable water management. The aim of this study is to develop a comprehensive understanding of the impacts of dam construction on river catchments and devise effective strategies to mitigate the negative consequences. To achieve this, it needs to consider the modification caused by dam construction in hydrological simulations to improve downstream hydrologic predictions. Hydrological modeling applications have been instrumental in the design and management of hydraulic structures like dams. These models help estimate essential variables and provide insights into the flow and sediment dynamics within a river catchment. However, to ensure accurate predictions, it is crucial to incorporate the modifications caused by dam in these simulations. A combined hydrologic and hydraulic routing application based on a model background was proposed to consider the modification of a catchment. The study made through this method shows a good improvement for instantaneous flows on a modified catchment

1. Introduction

Hydraulic structures are commonly built worldwide to facilitate the efficient use of water for various purposes. Dams, weirs, barrages, culverts, and dikes are different types of hydraulic structures that can be used to divert water from a river, store water for multiple uses, protect an area from a flood, etc. These structures are often built across or along a river and can significantly alter the flow dynamics and sediment flow in a river. The changes in the flow and sediment due to dam construction need to be well understood to prevent downstream flooding events [1, 2, 3, 4]. Moreover, climate change effects make water resources management, specifically dam operation, a challenging task. This imposes ambiguity on the storage and release of water timely to flood management tasks.

Dam design and operation require accurate estimates of discharges and sediment flows in a river basin, which can be achieved through hydrologic modeling. Hydrologic modeling involves simulating the rainfall-runoff process and making assumptions about the natural river basin catchment [5, 6]. However, it is rare to find a natural river basin catchment due to the development and use of water resources for various purposes. As a result, hydrologic models must be applied to modified catchments in practice. Adding hydraulic structures to a river basin can further complicate the alteration of flow and sediment flow, potentially leading to more floods downstream. Therefore, it is essential to understand the background and assumptions of a hydrologic model before applying it to a specific case. Understanding routing methods and application conditions can improve the precision of solving riverine system problems and help develop more accurate hydrologic models for modified catchments [7, 8].

According to recent news, China has demolished an Earth dike/dam in a Yangtze tributary river and blasted holes in the Chuhe River dam to manage a flood disaster and reduce pressure on the three Gorges dam (Figure 1) [9]. The three Gorges dam was designed to control the flooding of the Yangtze River, generate electricity, and become a symbol of national pride, but this summer’s record rains have revealed its limited ability to control floods. The flooding has caused damage worth 7 billion dollars and forced the evacuation of 1.8 million people in central and southern regions of China. The events have revealed the need for further assessments of outflow prediction under modified hydrologic catchments, considering hydrologic characteristic modification due to dam construction and the effect of climate change under the current situation [10].

Figure 1.

(a) Blasted holes in the Chuhe River dam and (b) Dike demolished due to extreme flooding (Yangtze tributary river, China, July 2020).

The selection of flow routing methods depends on the catchment’s physical hydraulic structure arrangements. Hydrologic routing methods are more appropriate in the upstream part of the catchment with no hydraulic structure presence, while hydraulic routing methods are more applicable in the downstream part of a catchment with modified flow dynamics due to the existence of hydraulic structures [11, 12]. A modified catchment study requires an understanding of the background assumptions and suitability of the routing methods to forecast hydrologic events accurately. A combination of hydrologic and hydraulic routing methods can be used to predict hydrologic variables under a modified catchment, such as those resulting from dam construction. Ref. [13] have suggested the use of a combined hydrologic and hydraulic method of routing for such scenarios.

Based on current research, the assumption that the construction of a dam improves downstream flow and reduces flooding events needs to be reconsidered [14]. While dams were conventionally thought to mitigate flood risks, studies have shown that they can actually increase the likelihood of flooding under certain circumstances [10, 15]. In fact, the outflow from a dam during emergency situations can cause significant impacts on downstream areas [16]. However, it is important to note that the impact of a dam on downstream areas depends on the hydro-geomorphic landscape context and the length of time beavers can sustain disturbance at a given site [16, 17]. Therefore, river management and restoration practices should take into account the impacts of dams and other factors, such as climate change, to minimize flood damage in downstream areas.

The chapter provides an in-depth understanding of the impact of modified catchment properties on river basins. It highlights the changes that result from the construction of a dam in a specific catchment and presents a practical example of how to study a modified catchment using a combined hydrologic and hydraulic routing approach. This approach can enhance the prediction of hydrologic variables in a river catchment by applying routing methods that consider the upstream and downstream conditions. The chapter also discusses the challenges of using hydrologic models under modified catchment conditions and explores options for studying modified catchments. Understanding the impact of modified catchment properties is crucial for effective design and management existing and new dam development.

2. Research methodology and approach

The research project focuses on practical experience, observation, and data analysis in various river basins in Ethiopia. In this study, the flow in the Awash River basin was simulated using a combined hydrologic and hydraulic model. The upstream highland areas, where physical river features did not dominate the flow, were modeled using the SWAT model, which employs a hydrologic routing method. On the other hand, the middle and downstream sections, where river dynamics processes dominate, were best suited for the hydrologic routing method (Figure 2). To conduct this research, input data for the hydrologic model (SWAT) and hydraulic model (HEC-RAS) were collected both locally and through the use of GIS and remote sensing applications (source: https://asf.alaska.edu/). Additionally, specific cases of dam development were considered to evaluate the sustainability of past, current, and future dam development challenges in the river basin. This study aims to provide valuable knowledge and understanding that can inform decision-making regarding dam development and overall river basin management.

Figure 2.

Flow discharge variations upstream and downstream of a dam (Koka Dam, Ethiopia).

The main points to suggest a combined hydrologic and hydraulic routing method for accurately predicting flow and sediment in modified catchments due to dam construction are as follows: In the upstream highland areas, where the usual rainfall-runoff relationships are applicable, the hydrologic routing method is most suitable. However, in the downstream areas, if there are any modifications such as dam construction, the traditional rainfall-runoff application is not suitable due to the changes in flow caused by the regulated dam outflow [18]. To address these changes, the hydraulic routing methods can handle them effectively by considering the dam outflow as the upstream boundary condition and incorporating any additional lateral flows. This approach was applied in a research study conducted on the Awash River basin, which underwent modifications due to dam construction. By employing a combined hydrologic and hydraulic routing method, accurate predictions of flow and sediment can be made in modified catchments, thereby aiding in the effective management of dam construction and its impact on the surrounding environment.

3. Hydrologic modification and challenge on a dam design

Dam construction has a significant impact on catchment flow discharge and sediment dynamics [19, 20]. The main function of a dam is to regulate the flow discharge of a river throughout the year, except in cases of flood protection where the reservoir needs to be emptied before a flood event. As a result, dam construction modifies the flow discharge at both the upstream and downstream parts of the dam [21]. Figure 2 shows a variation in discharge hydrograph at an upstream and downstream of a Koka Dam on Awash River in Ethiopia. The importance of a dam is more significant in tropical regions, where even perennial rivers with high discharges can be emptied during dry periods. In such regions, excessive floods during rainy seasons support the construction of dams to use the excess water during dry periods. Understanding the impact of dam construction on downstream areas is crucial, and solutions can be sought through different approaches if the modifications are well understood.

The construction of a dam can significantly alter the flow discharge of the river. The flow discharge of the river before the dam has a small base flow and higher peaks, whereas the modified flow has relatively lower peak flow and higher values of base flow. The hydrologic models can effectively simulate the upstream part of the dam since it has the usual hydrologic property of the rainfall-runoff property. However, the downstream part of the basin was modified due to the existence of the dam, and the outflow of the dam depends entirely on the dam’s operation. The hydrologic simulation at a downstream need to consider the modified outflow and other relevant lateral flows joining the river. However, hydrologic models cannot simulate such conditions under normal trends, and some adjustment has to be made in the simulation processes [22]. The complication due to dam construction worsens when there are many numbers of dams to downstream reaches. The analysis of the flow downstream of the dam is not a similar trend to the usual hydrologic processes, and different modeling approaches are needed to handle the outflow from the dam and the usual rainfall-runoff hydrologic processes in common. One such approach is to subdivide the total catchment into sub-catchments of each dam and adopt a different modeling approach.

The modification of an individual catchment can be evaluated through hydrological modeling [6]. In the case of dam 1, there is no modification from the usual hydrologic analysis. However, the uppermost dams, such as dams 2 and 3, pose a challenge as the hydrologic discharge values need to be sought for the downstream. According to Figure 3, the catchment for dams 2 and 3 has a rainfall-runoff relationship that is not similar to the usual catchment due to the dams’ existence on a specific river reach. The discharge components were modified downstream of dam 1, to the outflow from dam 1, and the usual rainfall runoff for the catchment between downstream of dam 1 and upstream of dam 2. There is no direct modeling application to predict the flow discharge downstream of dams 2 and 3, but available models need to be improved to handle such modifications for direct hydrological modeling applications. The data requirement and computational efficiency must be considered when seeking options to modify existing models. Further research is required to solve such bottleneck problems in hydrological modeling [11, 16].

Figure 3.

Typical dam arrangement in a catchment for hydrologic characteristics evaluation.

4. Dam and sustainability at the downstream due to catchment modification

The design and management of dams, including operation, is being challenged by climate change and catchment modification due to development works [23]. It is crucial to understand the effects of climate change on catchments and the sustainability of water systems. The usual modeling approach for dam design and management is inadequate in managing water systems under current climate dynamics [24]. To ensure the sustainability of the system, all-natural and man-made changes in the environment must be considered [25, 26]. As most of the best dam sites have already been developed, recent dam development works will be carried out under modified catchments due to existing dams upstream. Therefore, the sustainability of dam systems must consider the actual conditions of the upstream basin and the operation of existing dams [24]. The sustainable development of a dam in a transboundary river, such as the Blue Nile River basin faces various social, political, and economic challenges. The old-fashioned agreements made during the colonial era have hindered sustainable development efforts. Sustainable dam development requires an understanding of the environmental dynamics, political conditions, and socioeconomic factors of the basin. Engineering aspects of development have been prioritized over sustainability, and training and development policies need to be reevaluated to align with sustainable development concepts.

The Blue Nile basin is facing significant challenges due to the construction of the Grand Ethiopian Renaissance Dam and conflicting interests among the riparian countries [27]. Egypt has historically viewed control over the Nile as a national security issue, with 86 percent of its water consumption coming from the Blue Nile River, and is willing to go to war to prevent tampering with the waters [28, 29]. Ethiopia argues that the dam will not significantly affect the flow of water into the Nile and will benefit the region as a source of affordable electric power and drought mitigation. Talks chaired by President Cyril Ramaphosa of South Africa have resolved several issues, but there is still no agreement on the role the dam will play in mitigating droughts. Managing the dam in the long term is key to reaching an agreement.

The sustainable development of dams requires a comprehensive understanding of the hydrologic condition in the basin, technological advancements, and mutual understanding toward common development goals [23]. Engineers, policy makers, and technologists must work together to promote sustainable development through dam construction. In Figure 3, the dams were developed at different periods and were not linked based on catchment modification or climate change. As a result, there were high flooding and drought problems in the upper part of the Blue Nile basin. Although each dam design and operation look good individually, the actual operation and management cannot be run separately as they are within a single catchment. Therefore, future dam development must address these challenges to achieve sustainable development [23] and prioritize water as a key determinant in all aspects of social, economic, and environmental development [30, 31].

The design, construction, and operation of a dam must take into account the current environmental conditions and other relevant factors in the basin. To evaluate the climate dynamics and ensure the sustainability of the dam in the future, it is essential to consider the ideal hydrologic catchment (Figure 3 dam 1) and the built dam (Figure 4) [32]. The modification of the catchment by upstream dams, such as dam 1, impacts the design of downstream dams, making usual hydrologic modeling insufficient. Additional information about the dam and spillway is needed to design the downstream dam below dam 1. The downstream flow is influenced by factors such as dam storage, dam operation, and spillway releases. Furthermore, climate change can affect downstream areas by either increasing or reducing the flow based on rainfall patterns [32, 33].

Figure 4.

Major dams in upper Nile Basin before the Sudanese’s capital.

The case highlighted in Figure 3 illustrates a significant challenge faced in the upstream areas of catchments, particularly in transboundary rivers. Such cases require organized interaction and management. An example of this challenge is the Roseries dam in Sudan, which was built a long time ago without considering the upstream modifications. The construction of the Grand Ethiopian Renaissance Dam (GERD) upstream poses a sustainability issue for older dams, such as Roseries. The operation of the upstream dam affects the downstream flow, and recent observations suggest increased flood events in downstream areas. Addressing these challenges requires a new approach to sustainability that minimizes the environmental impact of development and considers the effects of climate change [34, 35, 36].

5. Evaluation of dam sustainability under the current development trends

The need for dam development is increasing worldwide due to the importance of water in achieving development goals. To ensure sustainability, it is crucial to reconcile the need for water with environmental challenges, such as catchment modification. Water resource development can be more sustainable if it is based on natural water boundaries rather than political boundaries [34, 37]. Achieving sustainable development requires considering the catchment boundary as most hydrologic information can be estimated at this level [38]. However, planning water resources and dam development at a catchment scale is challenging due to various factors, including socioeconomic, political, and technological conditions.

These conditions must be carefully considered to achieve sustainable and effective dam development [37, 39]. In developing countries, considering development work at a catchment scale poses challenges due to the demand for development, required knowledge, and technologies [40]. Many studies have been carried out based on local information and cost-effective technologies, which creates limitations for sustainable dam management [41]. However, cooperation for sustainable development is crucial not only for dam construction and management but also for addressing global development needs and environmental changes. To achieve sustainable development, an integrated approach that understands the changes and challenges is necessary, promoting mutual development beyond political boundaries [41, 42]. This approach aims to maximize benefits and minimize the effects of current environmental dynamics.

6. Evaluation of the routing method under a modified catchment

Hydrologic routing methods play a crucial role in flow prediction for hydrologic studies. When selecting a routing method, factors, such as data requirements and computational efficiency, are important considerations [43, 44]. Additionally, the physical characteristics of the river system and modifications due to hydraulic structures should be taken into account. In cases where a modified river basin is being studied, additional considerations are necessary to improve prediction accuracy. One limitation of hydrologic routing methods is that they do not fully account for flow channels as the primary means of flow transportation. In reality, most flow in a river system occurs within well-defined channels, particularly in downstream reaches. The flow in these downstream reaches is significantly influenced by river cross sections and roughness. Therefore, it is crucial to consider the detailed river basin system and flow dynamics processes when working with real river systems. Hydrologic routing methods are typically applied to the upstream part of a river, where the flow is fast-moving, and the channel is shallow and steep. These conditions align well with the assumptions made in hydrologic routing. However, as the river progresses toward the middle and downstream reaches, the flow becomes slower and the channel becomes deeper and more well-defined. This presents an opportunity for the construction of hydraulic structures, such as dams, which can modify the river system. The presence of hydraulic structures introduces distortions to the natural river system, particularly in the middle and downstream areas. To accurately represent these modifications in a routing approach, a combination of hydrologic and hydraulic routing can be employed. This integrated approach takes into account the real river system and the impact of hydraulic structures, leading to a more appropriate modeling application for river management. Moreover, this combined routing approach can also serve as a starting point for improving existing hydrologic models to better suit catchments that have been modified by the presence of hydraulic structures. By understanding the conditions of the river system and the effects of these structures, hydrologic models can be refined and adapted to effectively manage river systems.

The study presented in Figure 5 demonstrated the application of a hydrologic and hydraulic routing method. The results of the modeling output indicated that the combined approach performed better in predicting flow discharge and sediment in the study basin compared to using an individual routing method [45]. This highlights the effectiveness of utilizing both hydrologic and hydraulic routing methods together in enhancing the accuracy of predictions in hydrological studies [46]. By integrating information on channel cross-section geometry, the combined method was able to simulate river discharge, flow depth, and inundation width more accurately.

Figure 5.

Flow modeling with hydrologic and hydraulic modeling.

6.1 Flow discharge modeling to a modified catchment

The modification caused by the construction of a dam can have a significant impact on the hydrograph. In Figure 4, the effects of the dam on the hydrograph are depicted. The upstream portion of the catchment is ideal for applying rainfall-runoff modeling techniques to estimate the inflow to the dam. Hydrologic routing methods can be used to determine the flow pattern and volume of water entering the dam. However, applying the usual rainfall-runoff modeling approach becomes challenging in the downstream catchment below dam 1. In this case, it is necessary to categorize the system based on the specific observations of the modified catchment caused by the dam construction. It is commonly believed that dam construction modifies the downstream flow discharge condition by reducing peak flows and increasing base flow, resulting in a more uniform flow distribution throughout the year. However, this perception may not hold true in all cases. When dams are constructed for purposes such as irrigation, water supply, or general water abstraction, this perception may be accurate. However, for dams constructed for hydropower development and flood protection, the situation is different. Under worst-case scenarios, such as when reservoirs are full and spillways are working at full capacity, the downstream reach can experience high floods that are much higher than the flow without dam conditions. Therefore, the evaluation of dam construction needs to take into account the purpose of the dam and its practical conditions [16, 47].

The general procedure for predicting discharge under a modified catchment involves several steps. Firstly, the upstream part of the catchment, which is upstream of the dam, can be simulated using any hydrologic model. Secondly, the downstream areas below the dam need to be divided into two major components: the outflow from the dam and a rainfall-runoff component at the sub-basin downstream of the dam. Additionally, if there are any additional rivers or streams, the lateral flow should also be considered. The outflow from the dam varies based on the dam’s operation and purpose, so understanding the operation plan and actual outflow hydrograph is crucial for accurate prediction. The flow discharge below the dam can be estimated using rainfall-runoff relationships, and the water flow at the sub-basin level can be estimated in the same way. However, applying a hydrologic model downstream of the dam can be challenging, so a hydraulic model is suggested as an upstream boundary condition to provide the modified outflow from the dam and any additional inputs to the downstream areas. A combined hydrologic and hydraulic modeling approach can be used, taking into account the processes involved. An example application of this approach was executed in the Awash River basin in Ethiopia, where a combined SWAT and HEC RAS model was used to simulate the modified flow discharge under a dam construction. The model setup and results showed that the combined model performed well in simulating the modified flow discharge [48, 49].

7. Future direction on sustainable dam design

The design and management of dams require a comprehensive understanding of environmental dynamics and development at a catchment scale [50]. To achieve sustainable development, it is crucial to address the technological and knowledge requirements [51]. Additionally, future development in a catchment scale should be a key focus. Cooperation, technological advancements, common planning, and the development of a database for hydrologic information collection and sharing are essential for future development. These measures will facilitate the exchange of information and promote mutual understanding among countries for sustainable development [50, 52]. According to research, organizing hydrological information at a catchment scale, rather than locally, is crucial. It is necessary to establish and strengthen river basin organizations with immediate interaction at both local and large scales. The construction of early dams has significantly altered the hydrological characteristics of river basins[53]. Additionally, climate change exacerbates these issues, posing a threat to downstream areas where dam design and management are challenging. To address these challenges, river basin organizations should be organized at a catchment scale, and cooperation on a large scale should be pursued through a comprehensive understanding of the current conditions. This approach will facilitate the design and management of new dams downstream of existing ones [54].

8. Result and discussion

The sustainable development of river basins, especially large ones and those that span multiple countries, faces significant challenges due to new development, environmental modifications, and climate change. To address these challenges and ensure peaceful development, cooperation is crucial in various aspects such as data collection, technological use, database development, and information exchange between countries [55, 56, 57]. It is important to have knowledge about the hydrologic conditions in the basin, utilize technological advancements, and foster mutual understanding for common development goals [52]. To achieve sustainable development through dam construction, it is necessary for engineers, policymakers, and technologists to collaborate and work together [55]. Capacity building in these areas is essential for the future direction of sustainable development [58].

The simulation model output in Figure 5 was an application of a combined hydrologic and hydraulic modeling routing through SWAT and HEC-RAS model for practical application on Awash River basin in Ethiopia. In most researches because of the challenge in simulation model, the modification was not considered. In this research, the modification was considered as a major issue, and the basin was subdivided at the upstream and downstream based on the favorable condition on the model to simulate the physical condition in the basin. The upstream basin, which was not modified, was simulated with the usual hydrologic routing approach, whereas the downstream part was simulated using the outflow from the dam as a major input with hydraulic routing with HEC-RAS model. The model output in figure indicates a good simulation result with good performance indicator of R2 and NSE, 0.75 and 0.78, respectively at station two downstream of the dam (Figure 5). Further downstream the flow output was not good as it did not consider the lateral flow.

The combined hydraulic and hydrologic modeling approach is indeed a valuable tool for analyzing and managing the impact of dam modifications on hydrologic flow variations. However, this approach also has its limitations. One major limitation is the resolution limitation on the downstream reach in the hydraulic routing model. This can affect the accuracy of the results as the model may not capture detailed flow patterns in the downstream section. Additionally, the direct consideration of lateral flow can be challenging in the downstream section. Nonetheless, by thoroughly understanding the models and their capabilities, it is possible to incorporate lateral flow as an input in the downstream section, thereby improving the accuracy and usefulness of the modeling approach.

9. Conclusion

To ensure the sustainable development of a river basin, it is crucial to consider the modification of the catchment caused by dam construction. This case study focuses on two scenarios: the already developed sites and new developments. The already developed site, illustrated in Figure 4 in the Nile basin between Ethiopia, Sudan, and Egypt, necessitates an integrated basin information management approach and further research to effectively address the challenges posed by dam construction. To achieve this, it is essential to seek cooperation at a basin level rather than being restricted by regional boundaries. Strengthening data sharing and information exchange between upstream and downstream stakeholders is critical to finding a sustainable solution based on solid ground and satellite-based information.

The impact of new dam developments on downstream areas is significant, particularly due to modifications made to the dam. To effectively manage the entire river system, it is crucial to consider integrated development within the river basin. This becomes even more challenging for transboundary rivers, where urgent intervention and cooperation at a river basin level are necessary for sustainable development [59]. Climate conditions and human interference further complicate dam development in large basins, often leading to conflicts. Even small rivers are facing issues, such as water shortage due to unregulated water use, as the demand for development surpasses the available resources [59].To address these challenges, enhancing the hydrologic and hydraulic routing model application is imperative, improving its resolution and capabilities. This will help create a comprehensive and unified model for better management of dam development and its impact on downstream areas [60].

Acknowledgments

The author acknowledges Metrological Agency and Ministry of Water Resources, Energy and Irrigation of Ethiopia, for a cooperation on a data collection work.