Advance Surveying

Unit-II

Hydrographic Surveying

Ashu S. Kedia

Lecturer, Dept. of Civil Engineering,

School of Technology,

PDPU, Raisan, Gandhinagar

Introduction: Hydrographic Surveying

It is that branch of surveying which deals with the measurement of water bodies.

It is the art of delineating the contours and features of seas, gulfs, rivers and lakes.

Applications: Making nautical charts for navigation and determination of rocks

To secure information needed for the construction, development and improvement of port facilities (Eg. Chek lap kok airport)

Measurement of areas subject to scour or silting and to ascertain the quantities of dredged material

Controlling and planning of engineering projects like bridges, tunnels, dams, reservoirs, docks and harbours

Establishing mean sea level and observation of tides

Determination of shore lines

Measurement of discharge of rivers

Offshore engineering and the shipping industry requirements.

Drilling rigs (extracting oil, gas etc. from deep sea) locating up to 125 miles offshore, search for resources particularly oil and gas.

To construct offshore islands from dredged material (to bring material form some where and dump there) to support marine structure.

Hydrographic surveys are made to acquire and present data on oceans, lakes and harbors. It comprises of all surveys made for determination of soundings (measurement of depth below the water level), depth available for navigation and velocity as well

as characteristics of flow of water.

Saadiyat Island in Abu Dhabi Incheon Bridge, Korea

Chek lap kok, Hong Kong Airport Palm Jumeriah Island, Dubai

Need of Hydrographic Surveys

In the past, many fishing ports were built at convenient locations, with no

particular attention paid to such environmental factors as wave heights, sudden

changes in water depths, water currents, tidal streams, seaweed and sand drift.

Many of the structures around the world, suffered from problems that were

considered minor earlier but have now developed into major ones, for example,

fouling up with seaweed or silting up (shelter mouth facing the wrong direction) or

just being inaccessible in rough weather.

A hydrographic survey, also known as a bathymetric survey, is therefore essential

if the correct design decisions are to be made right from the project inception stage

to ensure that the landing is easy to use and free of major maintenance problems

under all conditions.

Steps of Hydrographic Survey

Reconnaissance

Although principal function of the hydrographic survey is obtaining the hydrography or making the soundings, it is must

to perform certain preliminary steps.

Careful reconnaissance of the area to be surveyed, to select most-expeditious manner of performing the survey.

To plan all operations such that survey can be completed in accordance with requirements and specifications governing

such work.

Aerial photographs can be of considerable help in this preliminary study

Steps of Hydrographic Survey

Establishing Horizontal and Vertical Control

The main operation in hydrographic surveying is to determine the depth of water at certain points

The measurement of depth below the water surface is called sounding.

To take the sounding, a vertical control is necessary and to locate the sounding (i.e., the point where the sounding is taken), a

horizontal control is necessary.

Horizontal control is a framework by which land and marine features are held in their true relative positions.

For rough works, a Tacheometric traverse or a plane-table traverse may be run.

However, for precise control, a theodolite and tape traverse is usually run.

Steps of Hydrographic Survey

When the soundings are recorded, it is essential to know the gauge reading. i.e. the level of water which continuously goes on

changing.

Tide gauges are kept in operation to establish the common datum and to give the height of water for which each sounding is taken.

Number of benchmarks are established near the shore line at close spacing to serve as vertical control.

Mean Sea Level

The elevation of the points generally refer to the mean sea-level (M.S.L) which is considered as datum for all types of surveys.

It is determined by making observations extended over a period of 19 years.

Mean Sea Level as datum:

A gauge is set at a location where it is protected from rough waves and where the water level is not affected by local conditions

Gauge should be located at a low level in sufficient depth of water so that the gauge reading can be taken even at low tide

The zero of the gauge is referred to a bench mark located on shore

The readings of the elevation of water surface are taken continuously for 19 years and the mean of all readings is the mean sea-level (M.S.L).

The gauge reading corresponding to the mean sea-level is transferred to a permanent point on the shore which serves as a bench mark for determining the elevations of other points with reference to the datum, i.e., M.S.L.

Shore Line Survey

The shore line surveys are conducted for purposes such as: delineation of shore lines,

locating prominent features on the shore to which horizontal positions of soundings may be connected,

determination of low and high water lines

Normal methods of chain survey and plane-table survey are adopted to survey irregularities in the shore line

Clearly visible points from the water surface, are selected as reference points such as church, temple, light houses, wind-mills, transmission pylons, etc. and their locations plotted easily

Determination of high-water line may be easily judged roughly from the marks on permanent rocks

To locate the line accurately, method of contouring may be adopted in which a number of points are established at the time of high tide. The line connecting these points so obtained, is the required high water line.

The determination of low-water line is usually done by interpolation from sounding since the low-water line does not remain exposed for a longer time

Tides and Tide Gauges

Tides Tides are periodical variations in the level of the surface of a large

water body like a sea or an ocean.

These variations are mainly due to the attraction between earth and celestial bodies (mainly the Sun and the moon). Tides have a bearing on a number of aspects of hydrographic surveys.

Tide Gauges Gauges used to measure the height of tidal water over the areas to

be surveyed are called tide gauges.

These instruments measure vertical movements of tides. It is desirable that the site for a gauge must have deep water and shelter from storms. Most popular tide gauges are as follows:

1. Non-registering type of tide gauges (requires attention of the observer)

2. Self-registering type of tide gauges (records the reading automatically)

Contd.

Non-registering gauges can be of three types:

Staff gauge

Staff gauge is the simplest type of tide gauge, consists of a graduated board, 150 to 250 mm wide and 100 mm thick, fixed in a vertical

position.

It is marked in metres and decimetres from bottom upward, board is long enough to accommodate highest and lowest tides.

The zero graduation is generally below the lowest water level so that all readings are positive.

The reduced level of the zero mark is obtained by levelling, staff gauge is fixed vertically at the site of observation.

Staff gauge is read by noting down the readings of crests and troughs of several waves, two values are recorded and the average value is

taken at the water level, difficult to read when the intensity of tides is

high

Staff gauge

Float gauge

Designed to overcome the difficulty in reading a staff gauge when the intensity of tides is high, and the variations of water

level is more.

Consists of a float to which a graduated vertical staff is attached

Float and staff are enclosed in a stilling well which is generally made of pipe or wooden boards having cross-

section of 300 mm X 300 mm with some orifices (holes) in

the bottom and the sides of the well to permit entry of the

water into the well

The reading is taken against an index mark through a slit window

Contd.

Float Gauge

Weight gauge

It consists of a weight attached to brass chain or wire, chain passes over a pulley and is laid horizontal along the side of a

graduated scale

The weight is lowered to touch the water surface, and the reading is taken on the graduated scale against an index attached

to the chain. Before making use of the weight gauge, it is

calibrated.

The R.L. of water surface corresponding to the zero reading of the gauge is determined by differential levelling.

Although the length of the chain has to match the range of water levels, the graduated scale need not be unduly long, as a second

or third index can be attached to the chain at suitable interval.

Weight Gauge

The self-registering tide gauges automatically record the variation of water level with time. It consists of a float protected from wind and waves.

The vertical movement of the weight is transferred through a float wheel and gearing to a stylus (or a pen) which traces a curve on a sheet of paper (graph paper) wound round a drum attached to clockwork machine.

The clockwork machine rotates the drum at a constant speed. The time is indicated on the horizontal axis of the graph, and the water level is indicated on the vertical axis by the stylus.

The gauge gives a graphical record of the movement of the float with time. The stylus wire is kept under constant tension by two counterweights.

Sometimes the self-registering tide gauges stop working. They require frequent visual checking by an attendant. They are usually housed in a well, constructed under a building in order to minimize the effect of wind and other disturbances

Self-registering Gauges

One of the main objectives of hydrographic surveys is determination

of general topography of the bottom of water bodies. The process of

determining depths below the water surface is called sounding.

Sounding is analogous to levelling on land.

Application of Soundings:

Preparation of accurate charts for navigation

Determination of the quantities of the material to be dredged

Location of the areas from where material to be dredged, and where to be dumped

Obtaining information for the design of breakwaters, wharves, sea-wells, etc.

Sounding

1.) Shore signals and buoys

2.) Sounding equipment

3.) Angle measuring instruments

Equipment for Sounding

1. Shore signals and buoys

Shore signals are required to mark the range lines. The line joining

two or three signals in a straight line, usually perpendicular to shore

line, constitutes the range line along which sounding are taken.

Signals are also used for making angular observations from the

sounding boats.

Signals are made sufficiently conspicuous to make them visible from

considerable distances in the sea.

10 cm X 10 cm masts painted white, and firmly braced at the bottom.

Different range lines are distinguished by flags of different colours,

attached to the mast or pole of the signal.

A buoy is a float made of light wood or hollow air tight vessel properly

weighted at the bottom, and is anchored in a vertical position by means

of guy wires. A hole is bored through the vertical axis of the buoy to

accommodate a flag. In deep waters, the range lines are marked by a

signal at shore and buoys in water.

Shore signals and buoys

Sounding Boat: The sounding operation is generally carried out from a

flat-bottom boat of low draft. The boats of large size equipped with a

motor, are used for sounding in sea. These boats are generally provided

with opening, called wells through which soundings are taken.

In smaller boats, sounding platforms extended far enough over the

sides, are provided so that the sounding line or sounding poles to the

bottom of the water body, does not strike the boat.

Sounding Equipment

These are made of strong timber usually 5 to 10 cm in diameter and 5 to

8 m in length. The sounding rods consist of two or three lengths

screwed together so that unnecessary length may be removed when not

required for soundings in shallow water.

A lead shoe of sufficient weight is fitted at the bottom to keep the rod

vertical in flowing water, and to avoid sinking in mud or sand. The

graduations on the rod are marked from bottom upwards.

Thus the readings on the rod corresponding to the water surface, is

directly the depth of the water.

Sounding Pole or Rod

A graduated rope made of hemp or chain attached to the lead or sinker.

Chains of brass are better than hemp lead lines because they maintain

their length.

To minimize elongation of hemp lead lines due to wetting, the line is

first stretched when wet and then dried.

The process of wetting and drying is repeated till the stretch becomes

negligible. The line is then soaked in water and graduated by tags of

cloth or leather at every metre intervals.

Mass of the lead is generally between 5 to 10 kg, depending upon the

strength of current and the depth of water. Leads are sometimes

provided with recesses (bucket) in their bottom for lifting the soil

sample.

Correction to measured length is applied to get true length.

Lead Line

A fathometer is an echo-sounding instrument used to determine the

depth of oceans indirectly. It works on the principle of recording time of

travel by sound waves.

Knowing the time of travel from a point on the surface of the water to

the bottom of the ocean and back, and the velocity of sound waves, the

depth can be calculated.

The fathometer gives truly vertical and accurate depths. It is more

sensitive than a lead line. It gives a continuous profile by recording the

measurements on a drum.

Fathometer (Echo-sounding)

The soundings can be made with greater speed. Since the velocity of

sound waves varies with the density of water, adjustments can be made

to read the depth of water of any type.

Angle Measuring Instruments

Most common instruments for measuring angles are theodolite,

prismatic compass, and sextant.

Prismatic Compass and Theodolite are not suitable for angle

measurements from sounding boats due to unstable rowing boats.

Sextant is suitable to measure angles from sounding boat.

Contd.

Locating the Soundings

Soundings Points made in the water body are required to be located for plotting on the drawing sheet.

Depending upon the places of observation points from where the locations are made, the methods can

be:

Observations from the shore

Location by transit and stadia or tacheometer

Location by range and one angle from the shore

Location by two angles from the shore line

Observations from the sounding boat

Range and one angle from the boat

Two angles from the boat

Observations from both the shore and the sounding boat