SECTION III.

GENERAL DIRECTIONS FOR TAKING SURVEYS OF RIVERS, CANALS, EMBANKMENTS, AND LANDS RECLAIMED FROM THE SEA.

The directions in this section will be given under the following six examples, viz. :

EXAMPLE I.-Directions for the survey of lands to be reclaimed from the sea.

EXAMPLE II.-Directions for the survey of a tidal river.

EXAMPLE III.-Directions for the horizontal survey of lands reclaimed under Ex. I. and II.

EXAMPLE IV.-Directions for the survey of a river in a large valley from high water level upwards.

EXAMPLE V.-Directions for the survey of mountain rivers. EXAMPLE VI.-Directions for the survey of canals, irrigationworks, and warp-lands.

EXAMPLE I.

DIRECTIONS FOR THE SURVEY OF LANDS TO BE RECLAIMED FROM

THE SEA.

Before land is reclaimed from the estuary of a tidal river on one or both banks, by means of embankments, a survey of it is usually made. The object of the survey is to enable the owner or owners to ascertain the area that can be recovered, the dimensions of the embankments required, and whether the land, when reclaimed, will return sufficient interest to justify the investment of capital necessary to complete the work, and lay out the land for agricultural purposes.

Such a survey necessarily includes a plan and section of the embankments and mouth of the river. The land to be reclaimed may be presumed to extend to both sides, but as the two areas are similar in character, directions for one will be sufficient.

The embankments are thus what are usually termed the sea embankments, in contradistinction from those of the next example that extend inland along both sides of the tidal part of the river.

Parallel examples are to be found at most of our large rivers, such as those that flow into the Wash, from which large areas of fen land have been reclaimed and added to the counties of Lincoln, Huntingdon, and Cambridge. Holland presents examples of still greater magnitude. In this country, in our colonies, and other parts of the world, a vast amount of land may yet be profitably reclaimed.

In some places surveys are easily taken both for plan and section. Thus, where the land is only under water at spring tides, it will be covered with herbage, so that a complete survey, horizontal and vertical, may be taken at first, as no difficulty stands in the way of determining the position of the embankments, fences, watercourses, roads, buildings, &c., required in laying out the land reclaimed for agricultural purposes. In others the work embraces an indefinite amount of sectional detail, owing to the nature of the subsoil, and the difficulty of determining the position and magnitude of the embankments. The latter will be the most instructive example to adopt. The more advisable course in examples where the position of the embankment cannot be determined at once, is to include two areas of land by means of two boundary lines, the one area greater than will probably be reclaimed, and the other less.

The boundary line of the largest area may be termed the maximumextreme, or the exterior line, or outer line; and the boundary line of the lesser area the minimum-extreme, or the interior line, or inner line.

A preliminary horizontal survey for the plan will thus be necessary. It will embrace the details of the area between the outer and inner boundary lines, but only a general outline of the lands inside and outside.

The survey for the sections should precede that for the plan, as the position of the two boundary lines cannot be determined until the former work is concluded. This arises from the treacherous nature of the subsoil, which, in the vast majority of cases, cannot be judged as to quality from mere superficial appearances. The boundary lines have consequently to be carefully examined before the several stations are finally fixed upon for the maximum and minimum extremes between which the position of the embankment is presumed to run. In doing so, to avoid, if possible, going over the line a second time, the distances of the ground line below the surface level of the sea at high water should be taken and entered into the field-book, which will form part of the details of the sectional survey.

Directions for ranging station-poles.

The first station-poles that require to be set are those at the mouth of the river, two on each side, one for each boundary-line.

The outfall or mouth of the river is often the most difficult point in the survey to determine. In some cases, for example, it is more advisable to make a new outfall than to continue the old one; consequently this is the first point that requires to be settled, and the following general rules may be given for that purpose:

1. When the channel of the river at low water contracts at its mouth, when the subsoil is of such a quality as to permit of an embankment being easily thrown across this narrow place from one side to the other, and when the old bed of the river above this expands into basins, it may then be advisable to change the outfall.

2. When the land to be reclaimed extends to a great distance on each side of the river, and when an embankment can be thrown across the channel, the river may be divided, and its waters diɛcharged into the sea by two outfalls, one on each side. This may be done partly to intercept the water from tributary streams and drains, and partly to throw it into the sea at a higher level.

3. It may be advisable to change the outfall in order to economise capital invested in the formation of the embankment.

4. The outfall may be changed to afford increased facilities for inland navigation.

5. The outfall may be changed to facilitate the accumulation of mud, and the formation of new lands for subsequent reclamation from the ocean.

Having fixed upon the place for the station-poles at the mouth of the river, which we shall suppose is not to be changed, two on each side, a transverse section of the channel and embankments between them should be taken and entered into the field-book. The ranging of the other station-poles of the survey may then be proceeded with, at the different angles or points of the two boundary lines, together with those of the seaward and landward statinos.

If the ground cannot be conveniently driven with a chain, which is often the case, the poles should be ranged purposely for the three areas of land, being ascertained by trigonometry; the first comprising all included by the survey beyond the outer boundary line; the second that within the inner line; and the third area, the land lying between the two boundary lines.

One reason for ranging two boundary lines, is to furnish the necessary data for obtaining an estimate of any line which may be determined upon between them for the embankment. So far as the work of the surveyor is concerned, no more difficulty is experienced in giving a section of the embankment at the exterior boundary than at the interior one. In point of fact the two lines. are two sections, either of which may be adopted, or any third line between them. But were only one longitudinal section of the embankment given, it might be otherwise with those who have to carry out into practice the proposition of reclaiming the land from

the ocean. But when they have the alternative left of turning to the right hand or left, between two extremes sufficiently far apart to meet all the exigencies of the case, the practical solution of the problem as to the position of the embankment can then be determined satisfactorily.

When the boundary lines are covered with a sufficient depth of water to float a flat-bottomed boat or coble, the station-poles may often be more conveniently ranged by such means, during the flow and ebb of the tide, than at low water. And when thus ranged the levels of both boundary lines may be taken at high water by the same means, and the ground, longitudinally and transversely, between them also examined, by boring into the subsoil under the water over the edge of the boat. Between the seaward and landward stations the surveying staff can thus employ their time without interruption until the station-poles are ranged, the lines examined, and the levels are taken.

The station-poles in the two boundary lines are ranged purposely to obtain the levels of the transverse and longitudinal sections, as well as the boundary lines themselves, and care should be taken that the line of the intended embankment lies between them, so that when it is made, the exterior line shall fall without, upon the sea side, and the interior one shall fall within, upon the opposite or land side.

The area between the boundary lines should be carefully examined, so as to determine if a sound foundation for the embankment exists between them.

The station-poles in each boundary line should be numbered, and the two lines distinguished from each other by different letters of the alphabet, and this requires to be done both upon the plan and sections. Thus, the station-poles of the exterior line may be numbered la, 2a, 3a, &c., and those of the interior line 1b, 2b, 3b, &c. And as the survey commences at the mouth of the river, it will be more convenient for the consecutive numbers to run from that point, the two stations on each side; there being la and 16 on the left bank, and 1c, 1d on the right bank.

The number of station-poles in each boundary line require to be equal, and the corresponding numbers should be opposite each other. Thus la should be opposite 1b, 5a opposite 5b, and so on for the other numbers in each line.

The object of this order is to get the data for the transverse sections. Thus between 1a and 16 there is a vertical plane that must be surveyed and shown upon the plan by a section, the ground line being represented by the bottom line, and the surface or high

water level at the top of the station-poles being shown by the upper line.

The number of transverse sections will depend upon the nature of the ground. If the surface is a regular plane, one for each straight line will be sufficient, but if undulating, one may be required for every concave and convex part.

The transverse section has a twofold purpose to serve. First, It shows the slope or angle of depression seawards which is required both for the embankment aud the drainage of the land. Second, The levels being thus given at the two extremes (the outer and inner boundary lines), and shown upon the transverse section, the height of the surface level at high water above any point in the ground line between them can be easily ascertained from the scale upon which it is drawn.

Two longitudinal sections are required, one for each boundary line. They show the top and bottom levels at each station-pole along both lines, and the numbers and letters of the alphabet that distinguish them from each other should be carefully attended to, so as to obviate error in copying from the working plan and section.

The levels are taken at high water by placing a mark upon the station-poles at the highest point to which the tide rises for the surface level, and by measuring its distance from the ground to obtain the bottom level; and the marks thus placed upon the station-poles should be carefully examined a second time at high water, in order to ascertain if the levels are correctly taken, before the base line is ranged and the theodolite placed for taking the bearings. This is usually termed "checking the levels." It should. be done between flow and ebb, when the weather is calm and the surface of the water "smooth" and free from ripple, and when right they will all lie in one plane.

Directions for the Horizontal Survey.

As the horizontal plane intersects the vertical ones at the several pairs of station-poles, the points of intersection and lines which they form are consequently common to both surveys. In all cases, therefore, when the vertical lines cannot be measured with the chain, this part of the vertical survey may be profitably supplied from data taken under the horizontal survey. Such, according to hypothesis, is the case in the present example.

The base line should range parallel to the general direction of the boundary lines, or as nearly so as practicable. Level ground and easily gone over should be chosen, in order to secure the greatest accuracy in measuring with the chain. This is necessary, as the true