BREAKING WAVES HEIGHT FORECASTING AT THE SHORELINE OF PANCENG SUB-DISTRICT GRESIK DISTRICT

Shoreline is a line where the edge of the land and the sea met. Various amount of shore construction was built around the shoreline. The safety of surrounding community near the shoreline should be considered before the government planning on near shore and shoreline facility. Therefore, breaking waves height become important because of the impact that it brought to the shoreline, such as the destructive energy that breaking waves generates which would wipe anything that existed alongshore. Using the data collected from BMKG (Meteorology, Climatology, and Geophysics Council), maximum wind stress at the Panceng Sub-District was 17,2 m/s. Average effective fetch at Panceng Sub-District was 331,68 km with 5 m significant wave height and 11,2 s period. Breaking wave height at Panceng Sub-District is 5,5 m.


INTRODUCTION
Shoreline is well-defined as a line where the edge of the land and the sea met [1]. Fishermen port and many other shore building was build alongshore, which mean knowing the wave height near shore become an important things before planning on building a construction near the shoreline. Interaction between waves and the shoreline is causing a phenomenon that known as breaking waves [2]. Waves with smaller depth compared with its wavelength will break individually when their height reached 0,8 times higher than the depth of the water [3].
Breaking wave generates turbulence that may harm nearby objects [4]. Fishing boat that is docking in the fishing port could be aground or even flipped. Beach visitors can't swim if the waves is too high, and if they don't think about it, they may be drowned by the turbulence inside the sea that caused by the breaking waves. The energy of waves also causing sediment transport inside the littoral zone that end up in siltation or sedimentation and disrupting fishermen's activities [5] [6]. Therefore, breaking wave forecasting is very important for near shore and shoreline planning, not only for construction purpose, but for tourism and many other comercial purpose. The safety of the surrounding community can also be guaranteed by the breaking waves forecasting.

RESEARCH METHODS
In order to forecast a breaking waves height in this paper, there is several step [7][8]: 1. Wind Data used in this paper are collected from BMKG (Meteorology, Climatology, and Geophysics Council) official website and was a maximum wind speed from year 2017-2021. The data provided was measured in Tuban Meteorology Station (-6.82290 lattitude, 111.99177 longitude) and must be corrected with these few stages. a. Height Correction [9] U(10) = U(z)( Significant wave height (HS), wave period (Tm), also minimum wave duration value could be obtained by using the Deep-sea significant wave prediction nomogram with effective average fetch (Feff) and wind stress factor (UA) value that been calculated before [14].

Breaking Waves Height
Breaking waves was a complicated system. Even in a few moments earlier the waves broke, the form of the waves is not sinusoidal again. The energy gathered from the wind would be reduced when the waves broke. Part of the energy were repressed into the ocean and the quantity depends on the slope of the coast, the smaller the angle of the beach slope, the smaller energy is reversed.
Most energy reduced as heat in mixing small scale of water foam and sand [15]. Breaking waves height (Hb) gained from the chart below.

RESULTS AND DISCUSSIONS
Refers to wind data collected from BMKG, it can be seen that the dominant wind direction is from the northeast, thus, the wind direction used for the next calculation is from the northeast with a maximum wind speed of 10 m/s.  From the wind stress and average effective fetch gained before, using deep-sea significant wave prediction nomogram, the significant waves height and period could be obtained. Finally using the breaking wave height chart, the breaking waves height could be calculated with significant wave height and significant wave period that is previously computed.

CONCLUSION
Based on the calculation of breaking waves height at the shore of Panceng Sub-District -Gresik District, the height of breaking waves is 5,5 m.