Effect of offshore wave boundary conditions on longshore variability of extreme wave runup

Abstract:

While previous studies have generally addressed the longshore variability of extreme wave runup on the beach and have linked it to the surf zone morphology and shoreline bed slopes, the effect of the offshore boundary conditions have not been studied broadly. The phase resolving Boussinesq model, FUNWAVE-TVD, is used in this study because of its capability in simulating infragravity waves in the nearshore and the resultant extreme wave runup. Using a longshore uniform barred beach, it is found that the existing directional wave maker in FUNWAVE-TVD generates a longshore varying wave field averaged over hundreds of waves, which can cause longshore variation of extreme runup. The reason for this phenomenon can be traced back to the energy spectrum discretization techniques in phase-resolving numerical models generating wave components with the same frequency and different wave directions. Individual waves with equal frequency and different directions, the so-called twin waves, drive this alongshore variation of wave height. To generate a statistically longshore uniform wave field, a new wave maker is implemented in FUNWAVE-TVD generating wave components having distinct wave frequencies. Three different numerical experiments are carried out. Initially, the longshore variability of the extreme runup using the existing directional wave maker with numerous twin waves is investigated. Then, the same process is carried out using the new directional wave maker that does not generate any twin waves. Finally, using the new wave maker, a certain number of twin waves are added at the offshore boundary to probe their effect on the longshore variability of wave runup. The last experiment provides insight on the aleatory uncertainty of real world conditions. The numerical experiments shed light on the sources of longshore varying extreme wave runup apart from the morphological variation in the nearshore.