Part 2 (1/2)
Until proficiency is attained, two copies of the alphabet should be kept by each observer for reference, one for dispatching a ed in alphabetical order and the other far reading ashould be used when standing against a dark background, and the blue one when on the skyline or against a light background
The conditions in tidal rivers vary so on the coast As the crest of the tidal wave passes the mouth of the river a branch wave is sent up the river This wave has first to overco in opposition to it, and in so doing causes a banking up of the water to such a height that the inclination of the surface is reversed to an extent sufficient to cause a tidal current to run up the river Theup-streaher level towards the head of the river than at thedown the river gains sufficient impetus to scoop out the water at theOwing to a flow of upland water down a river the ebb lasts longer than the flood tide by a period, increasing in length as the distance from the mouth of the river increases; and, similarly to the sea, the current may continue to run down a river after the tide has turned and the level of the water is rising Theup the centre of the river is in excess of that along the banks, so that the current changes near the shore before it does in the ravity than the fresh, weighing 64 lb per cubic foot against 62-1/2 lb, it flows up the bed of the river at the commencement of the tide, while the fresh water on the surface is running in the opposite direction After a time the salt water becomes diffused in the fresh, so that the density of the water in a river decreases as the distance froed into a river is due pri place; inasent rarely flows more rapidly than from two to four miles per hour, or, say, twelve to fifteen miles per tide The extent to which the suspended ain up stream when the current turns depends upon the quantity of upland water which has flowed into the upper tidal part of the river during the ebb tide, as this water occupies a certain a to the depth and width of the river, and thus prevents the sea water flowing back to the position it occupied on the previous tide, and carrying with it the e discharged into the Tha when there is only a small quantity of upland water is at the rate of about onethirty days to travel the thirty-one miles to the sea, while at the mouth of the river the rate does not exceed one- third of a mile per day
CHAPTER IV
SELECTION OF SITE FOR OUTFALL SEWER
The selection of the site for the sea outfall sewer is aaon the point, and the pere disposal depends primarily upon the skill shown in this eneral idea of the tidal conditions, and to examine the Adeneral set of the et as quickly as possible The main currents may be at some considerable distance from the shore, especially if the town is situated in a bay, when theacross the mouth of it from headland to headland The sea outfall should not be in the vicinity of the bathing grounds, the pier, or parts of the shore where visitors ate; it should not be near oyster beds or lobster grounds The prosperity--in fact, the very existence--ofvisitors, whose susceptibilitiesquestions, and there are always sentined and conducted they may be
It is desirable that the sea outfall should be buried in the shore for the greater part of its length, not only on account of these sentis, but as a protection from the force of the waves, and so that it should not interfere with boating; and, further, where any part of the outfall between high and loaterof the beach will inevitably take place on each side of it The extreme end of the outfall should be beloater mark of equinoctial tides, as it is very objectionable to have sewage running across the beach from the pipe to the water, and if the foul e of the water it will probably be brought inland by the rising tide Several possible positions may present themselves for the sea outfall, and a few trial current observations should be made in these localities at various states of the tides and plotted on to a 1:2500 ordnance map The results of these observations will probably reduce the choice of sites very considerably
Levels should be taken of the existing subsidiary sewers in the town, or, if there are none, the proposed arrangement of internal sewers should be sketched out with a view to their discharging their contents at one or other of the points under consideration It may be that the levels of the sewers are such that by the time they reach the shore they are below the level of loater, when, obviously, pue must be resorted to; if they are above loater, but below high water, the sewage could be stored during high water and run off at or near loater; or, if they are above high water, the sewage could run off continuously, or at any particular tiht be decided
Observations of the currents should now bespecial attention to those periods during which it is possible to discharge the sewage having regard to the levels of the sewers These should be reatest care and accuracy, as the final selection of the type of scheely on the results obtained and the proper interpretation of the influences, such as wind, etc Care ht of the tide and the relative positions of the sun,the observations, and in esti from such information the extent to which the tides and currents may vary at other times when those bodies are differently situated
It is obvious that if the levels of the sewers and other circued at loater, and the works are to be constructed accordingly, it is most important to have accurate inforhest loater which may occur in any ordinary circuiven by a casual observation, is adopted without consideration of the governing conditions, it may easily be that the tide in question is a low one, that may not be repeated for several years, and the result would be that, instead of having a free outlet at loater, the pipe would generally be subreatly reduced
The run of the currents will probably differ at each of the points under consideration, so that if one point were selected the best result would be obtained by discharging the sewage at high water and at another point at loater, whereas at a third point the results would show that to discharge there would not be satisfactory at any stage of the tide unless the seere first partially or even wholly purified If these results are considered in conjunction with the levels of the sewers definite alternative schemes, each of which would work satisfactory h outline, comparative approximate estimates should be prepared, when a final sche the most efficient result at the reater extent than is inherent to all sche thus selected the exact position of the outfall, the current observations froineer may be in a position to state definitely the course which would be taken by sewage if discharged under any conditions of time or tide This inforineer, but the scheme will have to receive the sanction of the Local Government Board or of Parliament, and probably considerable opposition will be raised by interested parties, which must be met at all points and overcome In addition to this, it may be possible, and necessary, when heavy rain occurs, to allow the diluted sewage to escape into the sea at any stage of the tide; and, while it is easy to contend that it will not then be ed into inland strea heavy rainfall, the aforesaid sentimentalists may conjure up many possibilities of serious results As far as possible the records should indicate the course taken by floats starting froular hour afterwards on the ebb tide, as well as at loater and every hour on the flood tide It is not, however, by any means necessary that they should be taken in this or any particular order, because as the height of the tide varies each day an observation taken at high water one day is not directly coh water the next day, and while perhaps relatively the greatest aleaned from a series of observations taken at the sahts, still, every observation tells its own story and serves a useful purpose
Deep floats and surface floats should be used concurrently to show the effect of the wind, the direction and force of which should be noted If it appears that with an on-shore wind floating particles would drift to the shore, screening will be necessary before the sewage is discharged The floats should be followed as long as possible, but at least until the turn of the current--that is to say, a float put in at or near high water should be followed until the current has turned at or near loater, and one put in at loater should be followed until after high water In all references to loater the height of the tide given is that of the preceding high water
The tih and loater at any place will be found to vary with the height of the tide, and all the information obtained on this point should be plotted on squared paper as shown on Fig 10, which represents the result of observations taken near the estuary of a large river where the conditions would be so in the open sea The vertical lines represent the tih or loater at which the current turned, and the horizontal lines the height of the tide, but the data will, of course, vary in different localities
[Illustration: Hours before turn of tide FIG 10]
It will be noticed that certain of the points thus obtained can be joined up by a regular curve which can be utilised for ascertaining the probable tiht intermediate to those at which observations were actually taken For instance, froiven it can be seen that on a 20 ft tide the current will turn thirty minutes before the tide, or on a 15 ft tide the current will turn one hour before the tide Soular curve, showing that the currents on those occasions were affected by so influence which the observer will probably be able to explain by a reference to his notes, and therefore those particular observations must be used with caution
The rate of travel of the currents varies in accordance with the ti Directly after the turn there is scarcely any movement, but the speed increases until it reaches a maximum about three hours later and then it decreases until the next turn, when dead water occurs again
Those observations which were started at the turn of the current and continued through the whole tide should be plotted as shown in Fig 11, which gives the curves relating to three different tides, but, provided a sufficiently large scale is adopted, there is no reason why curves relating to the whole range of the tides should not be plotted on one diagram This chart shows the total distance that would be covered by a float according to the height of the tide; it also indicates the velocity of the current from time to time It can be used in several ways, but as this necessitates the assuht the flow of the currents is absolutely identical along the coast in the vicinity of the outfall, the diagram should be checked as far as possible by any observations that hts Suppose we require to kno far a float will travel if started at two hours after high water on a 12 ft tide Froht the current turns two hours and a quarter before the tide; therefore two hours after high water will be four hours and a quarter after the turn of the current If the float were started with the current, we see fro 11 that it would have travelled threethis from four miles, which is its full travel on a whole tide, we see that it will only cover onebefore the current turns to run back again
Although sewage discharged into the sea rapidly becomes so diffused as to lose its identity, still occasionally the extraneous substances in it, such as wooden matches, banana skins, etc, may be traced for a considerable distance; so that, as the sewage continues to be discharged into the seapast the outfall, there is for possibilities of sewage contaed is limited to two hours, and starts, say, at the turn of the current on a 12 ft tide, we see fro 11 that the front of this body of water will have reached a point five-eighths of a e ceases; so that there will be a virtual coluhths of a mile, in which is contained all that re the course of the current We see, further, that at a distance of three miles away this coluiven point The extent of this colu to the tide and the tie; for instance, on a 22 ft tide, if the discharge starts one hour after the turn of the current and continues for two hours, as in the previous exa, whereas if it started two hours after the current, and continued for the sath of ti, but the percentage of sewage in the water would be infinitesimal
[Illustration: Hours after turn of current FIG 11]
In soe should be borne past a certain point before the current turns in order to ensure that it shall not be brought back on the return tide to the shore near the starting point In other words, the sewage travelling along the line of a branch current must reach the junction on the line of the main current by a certain ti the period of discharge will be two hours, and that the point which it is necessary to clear is situated three miles and a half froe according to the height of the tide can be obtained fro the 22 ft tide first, it will be seen that if the float started with the current it would travel twelve ht and a half miles A vertical line dropped froht ives the time two hours and a half before the end, or four hours after the start of the current at which the discharge of the sewage must cease at the outfall in order that the rear part of the column can reach the required point before the current turns As on this tide high water is about fifteen minutes after the current, the latest tie must be from one hour and three-quarters to three hours and three-quarters after high water Si a total travel of four miles: three and a half from four leaves half a ives one hour and three-quarters after the start of the current as the tih water is two hours and a quarter after the current; therefore the latest tie would be froh water, but, as during the first quarter of an hour the ht, would be in the opposite direction, it would be advisable to curtail the tie, and say that it should be limited to between two hours and a quarter and half an hour before high water It is obvious that if sewage is discharged about two hours after high water the current will be nearing its maximum speed, but it will only have about three hours to run before it turns; so that, although the sewage may be removed with the maximum rapidity from the vicinity of the sea outfall, it will not be carried to any very great distance, and, of course, the greater the distance it is carried the oing data are only applicable to the locality they relate to, although after obtaining the necessary inforrah has been said to show that when it is necessary to utilise the full effect of the currents the sewage should be discharged at a varying ti to the height of the tide
CHAPTER V
VOLUME OF SEWAGE
The total quantity of sewage to be dealt with per day can be ascertained by gauging the flow in those cases where the sewers are already constructed, but where the scheme is an entirely new one the quantity must be estimated If there is a water supply syste due allowance for the quantity used for trade purposes and street watering, will be a useful guide The average amount of water used per head per day for domestic purposes only may be taken as follows:--