感潮河川における海塩水逆流区間の解析法

抄録

Stommel classified salt intrusion into rivers into (1) negligible mixing, (2) moderate mixing and (3) intense mixing.
However, this classification stresses only the vertical distribution of salinity and it is not effective for showing the over-all picture of intrusion of salinity into rivers. In view of this, the author classified the forms of intrusion of salinity as follows:
(1) Salt Water Wedge Type-The point of origin of the salt water wedge is at the mouth ofthe river.
(2) Upstream Flow Type Sea Water Intrusion-Sea water flows upstream from the tidal area. This type is shown in Fig. 1, in which the sea water wedge is at the upstream side of the upstream flow area of sea water.
(3) Completely Mixing Type-There is always a fixed salt concentration in the vertical direction and this concentration becomes lower the farther it is upstream from the mouth of the river.
Sea water wedge is present in rivers in which there is a small fluctuation in sea level and the point of origin is at the river mouth. However, in case of shallow rivers which have large fluctuation in sea level, sea water flows upstream and sea water wedge is present at the tip of the sea water which flowed upstream. In the completely mixing type, salinity is always uniform in the vertical direction.
The author analyzed the distance of this upstream flow of sea water from the mouth of the river. It is necessary to know the flow rate and depth of water at a certain point at a certain time in the river. The author used the characteristic theory for analyzing the hydraulic characteristics of rivers.
The fluctuation of water level in rivers moves by the speed of long waves but the sea water itself flows upstream at a mean flow rate. Also, the dffusion velocity in the direction of flow becomes somewhat faster than the mean flow rate.
Therefore, the distance the sea water flows upstream can be calculated from the following. l_s=tΣt₀u(1+k_s) Δt
equation:
where l_s: distance the sea water flows upstream (m)
u: velocity of Upstream flow (m/s)
k_s: ratio of diffusion velocity of salinity in the flow direction and mean flow rate
Δt: a short time (sec)
The above theory was verified by obser ations at Nagara river, Mie Prefecture and Kiso river.