Author(s): Purwanto Bekti Santoso; Hitoshi Tanaka; Susumu Kanayama; Mitsuru Takasaki; Hiroto Yamaji
Linked Author(s): Hitoshi Tanaka
Keywords: Nagatsura-ura Lagoon; Mid-layer intrusion; Breaking wave; Turbulence model
Abstract: Mid-layer intrusion in Nagatsura-ura Lagoon, located at the southern end of Sanriku Coast, Japan, plays very important roles in determining the lagoon’s water quality. Depth of the midlayer intrusion, which depends on inflowing salinity and the lagoon stratification state, affects the corresponding lagoon’s water quality. This paper discusses reproduction of the mid-layer intrusion considering the effect of wind, which is responsible for enhancing near surface turbulence kinetic energy through the breaking wave. Having nearly closed shape, with only one small entrance for the water exchange, the lagoon is expected to behave similar to a small lake when it experiences wind forcing over its surface. Wind forcing is responsible for developing wave fields. Due to limited horizontal extend of the lagoon area, the fetches have insufficient length to allow the waves to be fully developed. The waves are underdeveloped and therefore short, steep, and of high frequencies and, as a result, those waves break more frequently. Breaking waves provide enhancement of turbulence kinetic energy close to the water surface; this turbulence is responsible for mixing the momentum down through the water, thus determining the shape of the near-surface current profile. The QETE (quasiequilibrium turbulent energy) model, using both conventional and wind enhanced breaking wave boundary condition, was used for turbulence closure modelling in numerical simulation of three-dimensional hydrodynamic model. Furthermore, parameterization of turbulence using constant eddy viscosity was also simulated. The simulation was then verified using measured flow velocity data of ADCP (Acoustic Doppler Current Profiler) installed at bottom of a fix location in the lagoon. Comparisons among three parameterizations of turbulence closure showed that the inclusion of breaking wave effect on the turbulent closure formulation gave improvement on resulted vertical profile of velocity and the predictive skill.