Abstract
At a river mouth, as the density of salt water is higher than that of fresh water, sea water goes under the river water, and a salt wedge is formed. H. G. FARM A r and G, . MORCAN showed the following equation for the length of salt wedge.
λAKα=(n2*)2 (3-2n2)/6-α[n2*/(1-n2*)+log(1-n2*)]
where λ=L/H0 (L is the length of salt wedge, and H0 is the depth at the end of salt wedge.)
α=U02/εgH0 (U0 is the velosity of fresh water, and g is the acceleration of gravity.)
ε=(ρ2-ρ1)/ρ2 (ρ1 and ρ2 are the fresh and salt water densities.)
K=τi/ρ2U02 (τi is the shear at the fresh-salt water interface.)
n2*=1-_??_
By applying the above-mentioned equation, the author computed the lengths of salt wedges in the rivers flowing into Tokyo Bay under the condition that the salt wedges lengthen most. However, as this equation was derived from the assumption that the river bed is horizontal, the values obtained here by computation were corrected in accordance with the gradient of each river bed. This equation can be applied only in case that a salt wedge is clearly formed. Results of the research for some rivers show that clear salt wedges are formed at the minimum tidal range. The length of salt wedge is maximum when the quantity of river flow is minimum. Therefore, this study was performed under the conditions of dry season and neap tide.
The conclusions obtained are as follows: 1. The lengths of salt wedges are related with the gradient of river bed. The lengths gained by computation decrease as the gradient of river bed increases. 2. The rates of effects which the gradient of river bed has an influence on the lengths of salt wedges differ from place to place. Those rates are lower on the east coast than on the north and west coasts of Tokyo Bay.
