London, E.C.




CHAPTER I.

THE FORMATION OF TIDES AND CURRENTS.


It has often been stated that no two well-designed sewerage
schemes are alike, and although this truism is usually applied
to inland towns, it applies with far greater force to schemes
for coastal towns and towns situated on the banks of our large
rivers where the sewage is discharged into tidal waters. The
essence of good designing is that every detail shall be
carefully thought out with a view to meeting the special
conditions of the case to the best advantage, and at the least
possible expense, so that the maximum efficiency is combined
with the minimum cost. It will therefore be desirable to
consider the main conditions governing the design of schemes
for sea-coast towns before describing a few typical cases of
sea outfalls. Starting with the postulate that it is essential
for the sewage to be effectually and permanently disposed of
when it is discharged into tidal waters, we find that this
result is largely dependent on the nature of the currents,
which in their turn depend upon the rise and fall of the tide,
caused chiefly by the attraction of the moon, but also to a
less extent by the attraction of the sun. The subject of sewage
disposal in tidal waters, therefore, divides itself naturally