Approximate Solution of Water Level Change in Brackish Lakes

Abstract

In a brackish lake connected with an open sea through a narrow channel, the change in the lake water level is controlled by hydraulic characteristics of the channel and geometrical form of the lake, and some changes in the geometry of the channel or lake will bring about large effects on the water level change. Then if we have a plan for dredging or reclamation of the channel or lake, we have to estimate the change in the lake water level caused with the planned project in advance.
For the purpose of such planning, at first we have to get some approximate equations to connect the water flux rate through the channel with the water level difference between at both ends of the channel. From the past studies, we can use three kinds of relationship of water flux rate and water level difference as following.
(1) Simple linear correlation assumption between water flux rate and water level difference.
(2) Square root correlation derived from hydraulic theory.
(3) S-letter type empirical correlation at Ohashi River observed by ISHITOBI (1994).
We have compared the three numerical solutions of the changes in lake water levels which are calculated from the above three different correlations using the in situ record of water level changes in Lake Shinji and Lake Nakaumi, and our solutions have shown that three different correlations brought about almost same results of the calculated water level change if we select the appropriate water flux coefficient at the case of small amplitude of tidal oscillation.
Then we can estimate the water level change in brackish lakes approximately using the simplest linear correlation.