Hydraulic structures such as dams, regulators, weirs, canals, waterways and culverts are among the structures most exposed to the effects of water, because they operate in constant contact with it and withstand both steady and varying forces throughout their service life. Their design requires a precise understanding of these effects, since a defect in a single element can lead to the failure of the entire structure. The first of these effects is hydrostatic pressure generated by the water stored behind the structure, which influences its stability against sliding and overturning. It is accompanied by uplift pressure resulting from water seeping beneath the foundations, which reduces the effective weight of the structure and weakens its resistance. Seepage also creates a risk of internal erosion, known as piping, in which water gradually carries away soil particles and forms channels that weaken the foundation and may end in sudden collapse. In waterways and structures carrying high-velocity flow, scour appears downstream of weirs, spillways and stilling basin floors, as the water washes sediment away from around the foundations. When high-velocity flow runs over concrete surfaces, cavitation may occur, in which vapour bubbles form and collapse near the surface, causing pitting and local damage. Suspended sediment also causes abrasion, which gradually reduces the thickness of the concrete, and sediment accumulation (sedimentation) in reservoirs and canals leads to a loss of part of their storage and conveyance capacity. The damage is not limited to hydraulic effects. Water carrying sulfates and chlorides attacks concrete chemically and accelerates reinforcement corrosion, a problem that is aggravated in saline soils and where groundwater levels are high. To limit these risks, several measures are used: cutoff walls, filters and drainage networks to reduce uplift pressure and seepage; stilling basins to dissipate flow energy before it returns to the channel; aeration of spillway surfaces to limit cavitation; and riprap to protect banks and beds. Piezometers and other monitoring equipment are used to observe the behaviour of the structure, together with physical models and computational fluid dynamics (CFD) modelling to verify the design. In conclusion, understanding the effect of water on hydraulic structures at the design stage, together with good construction, periodic monitoring and preventive maintenance, is the basis for ensuring their safety and the continued efficiency of their operation throughout their service life. Presented by Assistant Lecturer Farah Hameed Abdul-Khudhair Department of Civil Engineering Source: Novak, P., Moffat, A.I.B., Nalluri, C. & Narayanan, R. Hydraulic Structures, 4th ed., Taylor & Francis, 2007.