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Hydraulic Boundary Conditions
Published in Krystiaim W. Pilarczyk, Dikes Aimd Revetments, 2017
The waterlevels on the oceans continue also in shallow water. Some of the effects are amplified. But also typical near-shore effects may occur, like wind set-up. The friction of the wind causes a force on the water surface; this creates the ocean currents. But if the sea is relatively shallow, no return-current can be formed in deeper water, and the surface water will rise near the shoreline. This can happen during a storm, but it can also be caused by a very long lasting wind system, like a monsoon or a trade-wind. The effects are more significant in shallow seas like the South Chinese Sea, the North Sea of the northern Gulf of Bengal. In this last case set-up values of 6-9 m were observed. In the Netherlands the wind set-up along the coast of the North Sea can be in the order of 3.5 m (with a probability of 1:10000).
Modelling of coastal and nearshore structures and processes
Published in P. Novak, V. Guinot, A. Jeffrey, D.E. Reeve, Hydraulic Modelling – an Introduction, 2010
P. Novak, V. Guinot, A. Jeffrey, D.E. Reeve
Dynamic surge refers to propagating Kelvin wave disturbances that are triggered by moving storm systems. To forecast this component of surge it is necessary to use numerical prediction with, for example, the shallow- water equations as described in Chapter 11. Another component of surge is wind set-up. Wind blowing over the surface of the sea induces a surface stress. This force is balanced by a gradient in the sea surface, as discussed in Chapter 11, equation (11.27): () dηwdx=τsρwgd=C10W2ρwgd
Coastal water level variations
Published in Dominic Reeve, Andrew Chadwick, Christopher Fleming, Coastal Engineering, 2018
Dominic Reeve, Andrew Chadwick, Christopher Fleming
A storm surge is a deviation of the still water level from its tidal value. Surges are predominantly caused by storms with their associated low atmospheric pressure and strong surface winds. Surges can be geographically confined where their effects are local or, more problematically they can be dynamic and propagate like a long-period wave, affecting large regions over many hours. Surges can be both positive or negative (the water level is higher or lower than otherwise expected, respectively). Positive surges are of particular importance in coastal engineering as flood defences and coastal protection structures must be designed to take surges into account. Referring to Section 4.4 it is clear that surge is equivalent to the residual computed in harmonic analysis. There are usually several components to any particular surge. These include: The ‘inverse barometer effect’: The inverse barometer effect is a static response of the sea to atmospheric pressure. Where there is high surface atmospheric pressure this acts to push the sea level down slightly, and, conversely, where there is low atmospheric pressure this acts to raise the sea level slightly. The equation for estimating this static effect isza=0.01×(1013−pa) where za is the static rise in sea level in metres and pa is the surface atmospheric pressure in millibars. Wind set-up: When the wind blows across a length of open water it exerts a stress on the water surface. This produces surface waves. If the wind blows in one direction consistently and strongly, then the stress alters the mean slope of the water surface, creating a pressure force counteracting the imposed stress. This is essentially a balance between the wind stress and the pressure gradient force. Here, we use a simplified form of the equations to illustrate the physics of wind set-up.
Prediction of nontidal sea level variations in the Persian Gulf using data assimilation techniques
Published in Coastal Engineering Journal, 2018
Naghmeh Afshar-Kaveh, Abbas Ghaheri, Vahid Chegini, Mostafa Nazarali
As stated before there are three main factors affecting NTR of each location, which are wind velocity, atmospheric pressure changes and cross-shore variation of water depth. Ideally, atmospheric pressure acts inversely with NTR as the higher pressure causes the sea level to drop and vice versa. The wind setup has direct correlation with power of cross-shore wind velocity and inverse correlation with water depth. All of these factors must be quantified in the weight function equation of DA schemes. The mentioned factors are shown in Figure 10.