Study on Salt Water Intrusion of Estuary in Compound Channel

在区域发展中，应考虑发展与保护自然资源的平衡。珍贵的自然环境必须得到保护。青森县的小川原湖是一个大型咸淡水湖泊，渔业最主要的水生生物是日本Corbicula japonica。高濑河将湖与太平洋连接起来。这条河是一条复合河道的潮汐河。为了了解河口混合特性，本研究进行了现场测量和室内试验。湖水的盐度可以根据实地资料估计出来。利用扩散方程得到了横向混合系数，并与复合通道中的混合系数进行了比较。这一结果与常规研究结果一致。混合系数与产物密度、电流参数和通道截面参数有关。结果表明：k_y/u_*h值与横向混合系数、剪切速度、水深、淡水与盐水相对水位差、泄洪道水深、主河道水深的乘积β和d/ d有关。该结果与实测数据所需的k_y/u_*h值一致。由此发现，高濑河河口受到复合河道的影响。湖水的水质和流量与蛤蜊的栖息地有很大的关系。自2002年8月起，对该湖进行了为期3年的水质野外观测。从水质环境的角度来看，它们的产卵和孵化在湖的北侧进行，并通过水流扩散到整个湖。

In the regional development, a balance of development and preserving of natural resource should be considered. The precious natural environment has to be maintained. The Lake Ogawara in Aomori Prefecture is one of large brackish lake where the most dominant water organism for fishery is Corbicula japonica. Takase River connects the lake with the Pacific Ocean. The river is a tidal river with the compound channel. Filed measurement and laboratory experiments were carried out in order to understand mixing characteristics of river estuary in this research. Salinities of the lake were able to estimate from field data. The transverse mixing coefficients were obtained from these results using diffusion equation, and compared with those in compound channel. This result agreed with the conventional research. The mixing coefficients were related to the product density current parameter and cross-section of channel parameter. It was showed that the values of k_y/u_*h were related to the product β and d/D (k_y : transverse mixing coefficient, u_* : shear velocity, h : water depth, β : relative water level difference of fresh water and salt water, d : water depth of flood channel, D : water depth of main channel ). This result agreed with the values of k_y/u_*h required field measurement data. Therefore, it was found that Takase River estuary was influenced by compound channel from these results.The water quality and the flow of the lake is very related to the habitation of the clams. The field observation of water quality was carried out for this lake in the period for three years from August 2002. From a viewpoint of water quality environment, egg-laying and hatching of them are performed on the north side of the lake, and they diffuse throughout the lake by the flow.