Presented by Lecturer Assist. Kawther Hayder Abdulateef – Department of Civil Engineering Abstract Hydraulic structures, such as dams, canals, culverts, water tanks, and river bridge foundations, differ from conventional buildings in the nature of their loads and operating environment. They are subjected to permanent water pressures, dynamic loads, and continuous interaction between water, the structure, and the soil, which means their structural design requires a specialized approach. This article reviews the main loads acting on these structures, the treatment of water–structure interaction, the requirements for crack control and watertightness, and the risks of scour at foundations, and it presents practical design recommendations. Keywords: Hydraulic structures, hydrodynamic loads, fluid–structure interaction, crack control, scour, water tanks. 1. Introduction Hydraulic structures lie at the intersection of structural engineering and water resources engineering. They are distinguished by the fact that water is not a secondary load but, in many cases, the governing one. Moreover, their failure can have serious consequences that extend beyond the structure itself to the residential and agricultural areas downstream. For this reason, specialized codes impose stricter requirements than those adopted for ordinary buildings, whether in terms of safety factors, crack limits, or durability requirements (USACE, 2016; ACI Committee 350, 2020). This article aims to summarize the fundamental principles of the structural design of these structures and to link them to approved references and codes. 2. Loads Acting on Hydraulic Structures 2.1 Hydrostatic Pressure Water pressure increases linearly with depth, producing horizontal forces that cause sliding and overturning in massive structures such as gravity dams and retaining walls. These forces are calculated based on operating and maximum water levels, taking into account rapid drawdown conditions, which may result in unfavorable stability conditions (USBR, 1987). 2.2 Uplift Forces Water seeps through the foundation and construction joints, generating upward pressure that reduces the effective weight of the structure and lowers its safety factors against sliding and overturning. In practice, these forces are reduced by grout curtains, drainage wells, and internal drains, and the design must reflect the efficiency of these systems realistically. 2.3 Seismic Hydrodynamic Loads When an earthquake occurs, the water adjacent to the structure moves and generates additional pressures. Westergaard (1933) presented a simplified analytical solution that represents these pressures as an added mass of water moving with the structure, and this approach is still used in preliminary design. More recent methods, which account for structural flexibility, water compressibility, and interaction with the foundation, provide more realistic results in critical cases. 2.4 Sediments, Waves, and Other Loads Other loads include the pressure of sediments accumulated behind the structure, wave effects, and impacts from floating objects during floods. All possible load combinations must be examined at appropriate probability levels. 3. Crack Control and Watertightness 3.1 Particularities of Concrete in Aquatic Environments In water structures, it is not enough for concrete to achieve the required strength; it must also remain impermeable and resistant to the penetration of water and harmful substances throughout its service life. Therefore, cracks are the most critical threat to performance, because they open pathways for leakage and accelerate the corrosion of reinforcing steel. 3.2 Code Requirements Specialized codes such as ACI 350 rely on strict limits for crack widths and on modified load factors that reflect the behavior of structures exposed to liquids, with the aim of reducing steel stress and minimizing crack widths (ACI Committee 350, 2020). 3.3 Practical Practices Distributing reinforcement with smaller spacing and smaller diameters to improve crack control. Carefully designing expansion and contraction joints and waterstop details. Controlling the heat generated by hydration in large concrete masses to avoid early thermal cracking. Ensuring good concrete curing, especially in hot and dry climates. 4. Scour at Foundations and River Structures Local scour around bridge piers and the foundations of structures built in watercourses is one of the most common causes of failure. It occurs when the flow increases its capacity to transport the bed sediments surrounding the foundation. Specialized references provide equations for estimating scour depth as a function of velocity, depth, pier shape, and sediment properties (Arneson et al., 2012). Reducing this risk requires: Setting foundation levels below the maximum expected scour depth with a safety margin. Using protective measures such as riprap or anchored slabs. Conducting periodic monitoring of bed levels after major floods. Coordinating between the structural engineer and the hydraulic engineer to determine design flood scenarios. 5. Current Challenges These structures face challenges that are intensifying with climate change, including shifting flood patterns and the increasing frequency of extreme events, the aging of existing structures that were designed according to outdated hydrological assumptions, and the difficulty of obtaining reliable field data. 6. Conclusion and Recommendations Hydraulic structures cannot be designed in isolation from an understanding of water behavior and its interaction with them. The key recommendations are summarized as follows: Adopt comprehensive load combinations covering hydrostatic and dynamic loads and uplift forces. Use water–structure interaction models in critical cases rather than relying solely on simplified methods. Comply with specialized codes on crack control and watertightness. Study scour carefully when determining foundation levels and protection measures. Periodically review existing structures in light of updated climatic and hydrological data. References ACI Committee 350. (2020). Code Requirements for Environmental Engineering Concrete Structures (ACI 350-20) and Commentary. American Concrete Institute. Arneson, L. A., Zevenbergen, L. W., Lagasse, P. F., & Clopper, P. E. (2012). Evaluating Scour at Bridges (HEC-18, 5th ed., FHWA-HIF-12-003). Federal Highway Administration. U.S. Army Corps of Engineers (USACE). (2016). Strength Design for Reinforced-Concrete Hydraulic Structures (EM 1110-2-2104). U.S. Bureau of Reclamation (USBR). (1987). Design of Small Dams (3rd ed.). U.S. Government Printing Office. Westergaard, H. M. (1933). Water pressures on dams during earthquakes. Transactions of the American Society of Civil Engineers, 98, 418–433.