River Survey - Surveying a River & Sounding
The survey of the shore line of a river is made by running a theologize and tape traverse on a shore at a convenient distance form the edge of the water. If the river is narrow, a single theologize and tape traverse is on one bank and both banks. Located by staid or plane table methods.
If the river is wide, it is necessary to run traverses on both banks and locate each shore line by staid or plane tabling form its traverse. For checking purposes, the two traverses should be tide to each other at intervals by cross-bearing or angles.
When the shore lines of rivers and lakes are obstructed by woods, it is not economical to locate it by traversing. It is required to use a sys of triangulation. As a check upon the survey a base line is measured at the end of the survey and also additional check bases are measured at intervals of 10-15miles.
AB = Base line C, D, E, F, --------- are trigonometric stations
The measurements of depths below the water surface is called Sounding.
2. Objective of sounding:
The object of making soundings is to determine the configuration of the bottom of the body of water. This is done by measuring form the boat, the depth of water at various points.
3. Uses and Applications of Soundings
The gages may be divided in to two classes.
An observer is req to read oneself registering gage while the self registering gauge is automatic and is generally used when accurate and continuous record of fluctuations the water surface is required.
The gauge should be established at a convenient place where it is unaffected by the action of waves and is protected for storms.
5. Equipments for making Sounding:
It should be sufficiently roomy and stable. Flat bottomed boat is suitable in quite water is round bottomed boat is convenient in rough water. A power boat (steam or motor aunch) is most suitable when wind is blowing and the water currents are strong.
Sounding rods or poles are convenient in shallow and smooth water up to depths of about 4 to 6 m (15-20ft). they are made of well-seasoned timber and are auricular in section of abut 5cm(2//) diameter and 3 to 7.5m long (12-25) graduated in meter or centimeter (ft or inches) with a metal shoe at the bottom.
Direct depth measurements are taken by lowering it vertically into the H2O until it hits the bottom and reading the graduation at the surface.
The lead lines also called sounding lines are used for depth over 6m (20/). It consist of a suitable length of stretch-resistant cord or other material to which a heavy lead weight (5 to 10) is attached. The cord is make with feet or meter graduations and these should be checked frequently against an steel tape fro their accuracy.
Use: In use the weigh is lowered into the water being careful to keep the cord vertical. The graduation at the surface is read when the weight hits the bottom.
For regular sounding a brass such-chin is most satisfactory since its length is practically const i.e, the links are welded. The brass tags are attached at 0.2m (1') interval but leather or cloth tags are preferred as the brass tags can injure the hands of the surveyor. The chain should be tested periodically.
The attached to a lead line is conical in shape and very strong, (2.5-12.5) kg (5-25lbs) depending upon the depth of water current
The wt is circular in cross-section and length equal to 3 to 4 times the diameter and slightly tapers towards the top end.
For ocean sounding an insert. Known as fathometer is used. It is electric device and measure the time required for the sound (impulses) travel to the bottom of water and back. The travel time is converted into depth displayed in either digital or graphic for fathometer is also called echo sourer.
It is very use full much sounding is to be done. The type commonly used is hand driven and consists
The theologize and other instrument used in land surveys are not used in a boat where the support is unstable. The sextant is well suited to hydro graphic work and has the added advantage of measuring angles in any plane. It is the most precise hand instrument yet device for measuring angles. There are two versions of the instruments
Shore signals are required to mark the ranges i.e, lines along which sounding are to taken and the reference points to which angular observations are to be taken from the boat. They should be clearly visible for considerable distances. If the water is shallow, ordinary pole signal may be used but if water deep buoys are used as signals.
The lines on which sounding are taken are called ranges or range lines. They are laid on the shore parallel to each other and at right angles to the shore line or radiating form a prom nay natural object when the shore line is very irregular.
Each range line should be marked by means of signals erected at 2 points it, at considerable distance apart. The spacing of range lines vary form 6m 30m (20 to 100ft) depending upon the object of survey and the nature of the bottom.
6. Making the Soundings:
7. Methods of Locating Soundings:
Soundings may be located by the following methods which are commonly used
In this method the positions of sounding are located by measuring two angles simultaneously with a sextant, from the boat (P) to three shore signals or any points (A, B, and C) whose positions have been previously known.
The points sighted should be well defined such as chimneys, light houses etc. In order to minimize the error in measuring the angles and plotting them, the nearer object should be proffered to distant one. This method is commonly used where to range are employed.
In this method a wire or rope is stretched b/w fixed points on opposite banks and is marked by means of cloth or metal tage, at equal intervals along the rope or wire. The boat is rowed to these points and sounding are taken. This is most accurate but most expensive method. It is used when sounding are to be tsken along the cross-section of a canal or narrow river.
It is also used when it is required to determine the quality of material removed by dredging. The soundings are taken b/t and after dredging work is done.