[A-OS19] 沿岸域における混合，渦，内部波に関わる諸現象

[AOS19-06] 混合状態に影響される浅水湖の循環と水交換過程の評価

*増永 英治¹、北村 立実² (1.茨城大学、2.茨城県霞ケ浦環境科学センター)

キーワード：混合、湖、沿岸域、水交換

Understanding water transport processes is a critical issue to manage water environments in lakes and reservoirs. This study presents water circulation in a continuous lake system of Lake Kasumigaura and associated water exchange processes investigated with a high-resolution numerical simulator, SUNATNS. The study area of Lake Kasumiagura mainly consists of two lakes, West Lake (Nishiura) and North Lake (Kitaura). The surface area of West Lake (172 km²) is much larger than North Lake (36 km²). The model was forced by river discharges, surface height at the downstream end of the lake system and wind forcing. Model results showed good agreements with observed flow field observed by ADCPs at four locations. The correlation coefficient of the velocity magnitude ranged 0.34–0.75 (p < 0.01). Differences between time averaged velocity magnitude from the observations and model were less than 10%. Passive tracer was released with river discharges to investigate transport and water exchange. When water in a lake is completely mixed, the concentration of initially released tracer is explained by a continuously stirred tank reactor (CSTR) theory, C₀exp(-t/T_f) (where C₀ is the initial tracer concentration, t is time and T_f is the flushing time scale; T_f = V/Q; V is the lake volume and Q is the river discharge). The modeled tracer concentration was lower than the CSTR theory. The concentration was 90% and 68% of the CSTR theory at T_f in West and East Lakes, respectively, which indicates that mixing in larger West Lake is stronger than that in smaller North Lake. Large scale eddies/motions exhibited in West Lake due to wind forcing leading to enhanced mixing. This study suggest that mixing induced by winds in lakes significantly influence water exchanges in lakes and reservoirs.