Cross-shore profile development during storm conditions: Analysis of field data gathered at Petten sea defence

Cross-shore profile development during storm conditions: Analysis of field data gathered at Petten sea defence

Dal Corso, D.

Civil Engineering and Geosciences

Hydraulic Engineering

2002-06-01

The majority of population in the Netherlands live well below the mean sea level. At some locations people and infrastructures are only protected from the sea by a narrow stretch of sandy beaches and dunes, so reinforcement works will be necessary to prevent a break through during storm surges. Therefore it is necessary to study problems correlated with the failure mechanisms of protecting structures and in particular our objective of study is wave overtopping due to extreme conditions, which can cause structure's collapse.Design criteria for flood protection works are based commonly on an estimate of some extreme condition, usually a combination of surface level elevation and wave characteristics. The cross-shore profile in front of the structures may change during storm surges and its evolution in time affects the propagation of waves towards the flood protection works. Hence, the wave conditions to which the structure is exposed may be different from what one would expect on the basis of the cross-shore profile that is encountered during moderate conditions. To deal with this, it is necessary to gain insight into how the cross-shore profile evolves during a storm. Within this framework it has been simulated, for eight observed storms, the crossshore profile evolution at the sea defence at Petten. The results obtained with the simulations have been evaluated in terms of general characteristics and tendencies of profile evolution and changes of wave conditions. The different computations show in some cases an increase in the wave height at the sea defence and in some cases a decrease. This phenomenon depends from the increase during a storm of the crest level of the offshore bar (breaker bar that corresponds to moderate wave conditions). In fact, if this increase lead to a crest level (below MWL) comparable with the wave height in the same area, a relevant fraction of the waves will break. This causes an energy dissipation that results in a smaller (with respect to the wave height used as input in the computations) wave height at the sea defence. On the contrary, if the increase of the crest level is not enough to cause wave breaking, the waves on the bar will only undergo a shoaling effect without losing energy. The result of this is the fact that at the dike there will be a bigger wave height, as there has not been energy dissipation caused by breaking. Regarding bottom variations, it can be seen that in some zones there is erosion and in some others accretion. Most of the computations show the formation of a scour hole at the toe of the dike of an average depth of 2.5 metres. The sediment eroded in this zone will then settle seaward, leading to the formation of some bars in the range of ISO metres. The offshore bar shifts horizontally in the offshore direction and increases its crest level. At an average depth of -11 metres all the changes in bottom profile become negligible (at the scales of the considered storms) with respect to what happens closer to the shore. This point means that, from the cross-shore coordinate that characterises it seaward, all the mechanisms that have influence in sediment transport are in balance. In fact it doesn't mean that there the offshore-directed sediment transport rate is zero, as we calculated its value, which is about 80 nr'.

erosion
bed profile
storm surge

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Student theses

master thesis

(c) 2002 Corso, D. Dal