The Impact And Reflection Of Waves On A Multi-chamber Perforated Caisson

Benefiting from its good performance of wave dissipation and adaptability for poor sea conditions, the perforated caisson breakwater is widely used in coastal engineering to protect wharves and ports, whilst saving the construction cost in relatively deep water and causing less disturbance to the coastal water environments. This is why lots of researchers show their interests on the perforated caisson and conduct plenty of studies on it under a variety of conditions of the incident waves and the structure itself.From these massive researches, the properties of the perforated caisson in the scope of its reflection and the wave loads it bared have been well discussed. However, the influence of changing water depths on the partially perforated caisson, which is relevant because the water depth before the perforated caisson is varied in a certain range with the tidal process in the real sea, was scarcely highlighted in the past studies. On the other hand, the studies of the wave loads on vertical structures indicate that, when waves break against the coastal structures, they could abruptly transfer their momentum into the vertical structure so as to motivate the violent impact pressure on the structure and affect the stability and the integrity of it. The impact pressure could be more than tens of the maximum wave pressure which we expected in calm waves. The perforated caisson might stand the impact loads frequently for the wave and air is turbulently mixed in the wave absorbing chamber, but few studies were published before.This study is based on a series of physical model tests for a particular partially perforated caisson with three wave absorbing chambers under the conditions of different irregular waves in varied water depths. The main purposes are to discuss the change of the frequency-averaged reflection coefficient for different water depths and to establish a primitive understanding of wave impact loads on the perforated caisson.In order to consider the tidal process in real sea, we introduce the weighting function for each water depth and calculate the depth-averaged reflection coefficient of partially perforated caisson by producing the frequency-averaged reflection coefficient of each water depth with its weighting function. Detailed relations between the impact processes and the main influencing factors (including the relative width of wave absorbing chamber B/L\n, the relative water depth kd, the wave steep ness H1/3/L1/3 and the relative height of the cap of the wave absorbing chamber s/Hw1/3 etc.) are also discussed, respectively. The main conclusions are as follows:1) Generally, the perforated area is close to the designed tide level for partially perforated caisson. Faces of the perforated caisson, which are exposed to the waves, are easily suffering from the impact loads. In the test range, the vertical walls and cap of the wave absorbing chamber bear the similar level of maximum impact pressure, which is 1.5-2.0 times of the maximum impact pressure on the very front wall.2) The frequency-averaged reflection coeffient of the partially perforated caisson is closely related to the water depth, so that the reflection coeffient for a single water depth could not present the whole reflection characteristics of the partially perforated caisson. We, therefore, introduce the calculating method by using the weighting function of reflection coeffient to consider the varied water depths, which is much more realistic.3) The wave impacts are violent but unrepeatable. There is neither fixed time for the occurrence of wave impacts nor certain values of the maximum impact pressure.4) For the vertical walls, the wave impacts are more likely to occur in the vicinity of the still-water level with the greatest impact pressures, which is also generally acknowledged from the past investigations, and this "dangerous" domain shall move with the changing of still-water level under a complex way. Negative pressures accompany the impact prosses.5) For the cap of the wave absorbing chamber, the wave impacts do not exist at lower water depths, and shall become violent and frequent at higher water depths. The greatest impacts are more likely to occur in the vicinity where the cap meets the solid rear wall. Negative pressures accompany the the peak of wave pressures.