Violent breaking-wave impacts. Part 4: A detailed analysis and comparison of field and 1:4 scale measurements on sloping and vertical walls including the influence of air and scale effects

Abstract

Pressure and aeration measurements obtained under storm conditions on the steep-fronted masonry wall of a rubble-mound breakwater are analysed in detail, and the results compared with those obtained using a 1:4 scale freshwater model of the field test site. New insights are gained into the complex behaviour of the most violent impacts, with particular attention given to aeration and scale effects.

The existence in the field of both low-aeration (LA) and high-aeration (HA) impacts is confirmed and new parameters introduced to facilitate further analysis. Maximum pressures (P_max) up to 771 kPa are categorised and the magnitudes of the resultant impulses found to depend mainly on their durations. The alternate expansion and recompression (ERC) of air following a HA impact is shown to apply significant oscillatory pressures and forces to the wall. Information on the magnitude, period and damping of these oscillations is presented.

The model results are initially scaled in accordance with the Froude law. In conditions comparable to those in the field, the highest pressure on the sloping wall is again found to occur in HA impacts with P_max ≤ 3.17 MPa followed by ERC oscillations. Like those in the field, the oscillations at different elevations tend to come into phase with each other and can subject the wall to oscillatory forces of significant vertical extent. Both the initial excursion and the damping of the oscillations tend be greater in the model than in the field. The maximum forces on the wall also tend to be greater than those on the field breakwater, but the durations of the impulses tend to be shorter. This apparent trade-off between force and duration may indicate that the model is responding differently to the momentum flux of the incoming waves. P_max ≤ 5.42 MPa are obtained when the model wall is vertical.

Because the Froude law does not scale aeration effects correctly, model data are also scaled in accordance with the Bagnold-Mitsuyasu (B-M) law which increases the highest P_max for the vertical wall to 20.97 MPa. An alternative assessment of the ERC oscillations is also made on the assumption that the trapped-air pockets are geometrically similar to ones that could occur in the field.

Likely generic characteristics of violent wave-impacts are identified as are probable model scale-effects. Impact-pressure reduction curves derived from a numerical model are presented to emphasise the influence of entrained air on wave loading. Further work is recommended.