A new wind-formulation for hydrodynamic models

A new wind-formulation for hydrodynamic models

Bokhorst, J.S.

Stelling, G.S. (mentor)
Pietrzak, J.D. (mentor)
van Mazijk, A. (mentor)
Dollee, A.W. (mentor)

Civil Engineering and Geosciences

2003-08

Storms that come ashore across wide, shallow, continental shelves cause extreme changes in the sea level at the coast; these are called storm surges. Basically, these surges can be divided into two types: surges due to mid-latitude storms and surges caused by tropical cyclones. Due to lack of warning and insufficient preparation, these surges can often cause high numbers of casualties. Therefore a lot of effort is put into trying to forecast storm surges. Over the last decades scientists have been developing computer-models with which they try to simulate these surges and predict the water levels. Though currently a lot of attention is being paid to find a correct and physically justifiable formulation for the wind drag coefficient in these models, all wind formulations are dependent on the squared wind velocity, while they omit the influence of the flowing water itself. However, it seems more logical that the wind stress should be dependent on the wind velocity relative to the flow velocity. In this MSc-thesis, a new wind formulation for hydrodynamic models is proposed, in which the wind stress term is not only dependent on the squared wind velocity, but on the wind velocity relative to the water. This should result in a physically more realistic simulation. In very shallow water the new formulation clearly leads to different results: 1D experiments show that the eigenfrequencies damp out much faster when the new formulation is used. Since the flow velocity is included in the wind stress, this leads to extra friction. In 2D experiments the new formulation leads to smaller wind set-up on very shallow areas surrounded by channels (e.g. tidal flats). This is because in a stationary situation a constant flow remains, due to which the wind stress term becomes smaller. The new formulation also leads to more (numerical) stability, especially in cases of flooding and drying. The new formulation introduces a factor a, which is the relation between the wind velocity at the surface and the wind at 10 m height (usurface=au10). The effect of this parameter is that the influence of the flow velocity on the wind is reinforced. The results of the 1D and 2D experiments suggest that further research should be carried out for the case of 3D flows. The effects of the new formulation will probably be even stronger in this case, since the flow velocity at the surface is usually larger than the depth-averaged velocity

http://resolver.tudelft.nl/uuid:b2bf02a8-f721-42a2-9a0a-dc53a298a124

TU Delft, Faculty of Civil Engineering and Geosciences, Hydraulic Engineering

Student theses

master thesis

(c) 2003 J.S. Bokhorst