XII. CHANGES IN TIDAL DATUM PLANES 
Implications in Assumption of Constancy of Tidal Datums 
In the use of tidal datums as planes of reference for elevations it is implied that 
such datums at any given place remain constant over relatively long periods of time. 
Underlying this implied constancy are the tacit assumptions of coastal stability and 
constancy of hydrographic features. If changes take place in the relative elevation of 
land to sea or in the hydrogrpahic features of the body of water on which the given place 
is situated, changes will also take place in the tidal datum planes, which are fixed by 
reference to local bench marks. 
With regard to periods of time measured in thousands of years, local changes in 
relative elevation of land to sea of considerable magnitude have been fully demonstrated. 
But for the lesser periods of time involved in everyday affairs any such changes, as a 
general rule, are so small that with respect to tidal datum planes they may be disre- 
garded and coastal stability taken for granted for a number of years. 
The changes in hydrographic features that bring in their train changes in datum 
planes are those that affect the local tidal régime. With regard to such changes in hy- 
drographic features, distinction must be made between the open coast and inland bodies 
of tidal water. While the open coast is at all times under attack by wave and current 
and thus subject to change, such changes are relatively slight and only rarely bring 
about changes in the rise and fall of the tide, even over a period of a number of years. 
Hence, along the open coast, it may be assumed that tidal datum planes remain con- 
stant for periods covering many years. 
But in inland bodies of tidal water changes in hydrographic features are as a rule 
followed by changes in the tidal régime, which are reflected by changes in the tidal 
datum planes. The changes that may be expected under different conditions will be 
discussed briefly in this section. But it will be of advantage to consider in this connec- 
tion the changes in tidal datums that result from changes in relative elevation of land 
to sea. 
Changes Due to Change in Relative Elevation of Land to Sea 
If a coast is undergoing a slow gradual subsidence the first effect would obviously 
be an apparent elevation of all the tidal datum planes with respect to local bench marks 
by the same amount, this being the amount of the subsidence. That is, if after a num- 
ber of years the subsidence of the coast in question amounted to one-tenth of a foot, 
mean sea level, half-tide and also the low-water and the high-water planes would, with 
respect to the local bench marks, stand one-tenth of a foot higher than at the beginning 
of the period. 
But if the subsidence becomes sufficient to alter materially the hydrographic fea- 
tures of the coast, changes would ensue in the tidal régime along the coast, and as a 
consequence the different datums would change differently, as the following considera- 
tions will make clear. 
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