Obtaining some figures from [Taylor et al] and [Hutchison 1976]...
Here are some estimates relevant parameters.
The surface area of the lake A is approximately  and the volume
 and the volume  is
approximately
 is
approximately  , therefore the L is about 1m. Huizenga's report says a head
difference of about 0.3m is needed to compensate for evaporation rate at
, therefore the L is about 1m. Huizenga's report says a head
difference of about 0.3m is needed to compensate for evaporation rate at  .
The head difference is I/k where
.
The head difference is I/k where  thus we can estimate
 thus we can estimate  .
. 
How transient are transients? The time constant for the transients is typically -k/A
thus the half-life is :- 

Which implies... 

Thus  is 36 days.
 is 36 days. 
When the flow is in steady state from sea to lake with  , the time taken
for the concentration to double
, the time taken
for the concentration to double  , (assuming no fresh water inflow) is...
, (assuming no fresh water inflow) is... 

Solving for  ...
... 

Assuming the lake is initially at sea water salinity ( ), then the
concentration doubling time is
), then the
concentration doubling time is  days.
 days. 
When the flow is steady state from lake to sea with  . The half-life for
the salt concentration
. The half-life for
the salt concentration  is...
 is... 

Thus  days.
 days. 
How much water is needed to make up the head loss of 0.3m?  . Given that the
Full Supply Capacity of the anticipated new dams is
. Given that the
Full Supply Capacity of the anticipated new dams is  it is unlikely
that any compensation release of water would flush the system.
 it is unlikely
that any compensation release of water would flush the system.