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These notes are internal primarily for those engineers who,having a general knowledge of sewerage, are called upon toprepare a scheme for a sea coast town, or are desirous of beingable to meet such a call when made. Although many details ofthe subject have been dealt with separately in other volumes,the writer has a very vivid recollection of the difficulties heexperienced in collecting the knowledge he required when he wasfirst called on to prepare such a scheme, particularly withregard to taking and recording current and tidal observations,and it is in the hope that it might be helpful to others in asimilar difficulty to have all the information then obtained,and that subsequently gained on other schemes, brought togetherwithin a small compass that this book has written.
60, Queen Victoria St,London, E.C.
It has often been stated that no two well-designed sewerageschemes are alike, and although this truism is usually appliedto inland towns, it applies with far greater force to schemesfor coastal towns and towns situated on the banks of our largerivers where the sewage is discharged into tidal waters. Theessence of good designing is that every detail shall becarefully thought out with a view to meeting the specialconditions of the case to the best advantage, and at the leastpossible expense, so that the maximum efficiency is combinedwith the minimum cost. It will therefore be desirable toconsider the main conditions governing the design of schemesfor sea-coast towns before describing a few typical cases ofsea outfalls. Starting with the postulate that it is essentialfor the sewage to be effectually and permanently disposed ofwhen it is discharged into tidal waters, we find that thisresult 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 aless extent by the attraction of the sun. The subject of sewagedisposal in tidal waters, therefore, divides itself naturallyinto two parts: first, the consideration of the tides andcurrents; and, secondly, the design of the works.
The tidal attraction is primarily due to the natural effect ofgravity, whereby the attraction between two bodies is in directproportion to the product of their respective masses and ininverse proportion to the square of their distance apart; butas the tide-producing effect of the sun and moon is adifferent