Journal of The School of Marine Science and Technology,Vol.8 No.1
Water Exchange in Lake Sanaru “Tidal Brackish Lake Left Far from Seashore”
Kuniaki YASUDA, Misato IGA and Shozo SAWAMOTO
Abstract
Characteristics of tide by measuring temporal change of surface level in Lake Sanaru which is tidal brackish lake
left far from seashore were investigated. Tidal amplitude attenuates and phase becomes late while tidal wave
propagates to upstream on the long river where cross-sectional area is small. Consequently, tidal range in Lake
Sanaru attenuates in 1/3 or more of that in Lake Hamana,and tide in Lake Sanaru is delayed from 3 to 5 hours than
that in Lake Hamana. The strength of tidal water exchange and mean residence time of fresh water which flowed
into Lake Sanaru during precipitation are estimated.The water exchange ratio by semi-diurnal tide between Lake
Sanaru and Lake Hamana is approximately 0.13,and the mean ratio of exchanged water volume to capacity of Lake
Sanaru is about 2%. In addition, mean residence time to take for fresh water to be discharged from Lake Sanaru
by tidal water exchange is estimated to be from 22 to 26 days.It is supposed that tidal water exchange between Lake
Sanaru and Lake Hamana is controlled or suppressed by mixing or dilution process in lower Shin River. Effect of
suppression by the physical process in lower Shin River intensifies exclusiveness of Lake Sanaru, therefore, it is
difficult for the pollutant in Lake Sanaru to be discharged from the lake and is easy to stay in the lake.