HYDROGRAPHIC SURVEYING
A hydrographic survey is one whose principal purpose is to
secure information concerning the physical features of water areas. Such
information is essential for the preparation of modern nautical charts on which
are shown available depths, improved channels, breakwaters, piers, aids to
navigation harbor facilities, shoals, menaces to navigation, magnetic
declinations, sailing courses, and other details of concern to mariners. Also,
a hydrographic survey may deal with various subaqueous investigations which are
conducted to secure information needed for the construction, development, and
improvement of port facilities to obtain data necessary for the design of piers
and subaqueous structures, to determine the loss in capacity of lakes or
reservoirs because of silting, and to ascertain the quantity of dredged
material.
Hydrographic
surveying has obvious application in the field of dock and harbor engineering
and coast protection works. There are many more applications in civil
engineering projects involving irrigation, water supply, water power, flood
control, river works, land reclamation, and sewage disposal. The measurement of
discharges of rivers and streams is also one of the activities of hydrographic
surveys.
The most
common uses of the hydrographic surveys can be listed as:
1. Measurement of tides
2. Determination of bed depths
3. Determination of scour, silting, and
irregularities of the bed
4. Determination of shore line
5. Establishment of the mean-sea level
6. Preparation of navigation charts
7. Measurement of discharge of rivers
and streams
8. Determination of direction of
currents to locate sewer fall
9. Providing help in the planning of
projects like bridges, dams, harbours, reservoirs.
METHOD OF HYDROGRAPHIC SURVEYS
To locate the subaqueous details on hydrographic charts, a
number of suitably special control points are established along the shore line.
The depth of water at various points forming a network is determined by making
soundings from stationary boats. The locations of the soundings can be
determined by making observations either from the boat to the control points or
from the control points to a target fixed in the boat.
Before
locating the details in hydrographic survey, the following preliminary steps
are required to be carried out:
1. Reconnaissance
2. Establishing horizontal control
3. Establishing vertical control.
Reconnaissance
Although
the principal operation of the hydrographic survey is obtaining the hydrograph
or making the soundings. This phase of the project cannot be performed until
certain preliminary steps are taken. The first of these is a careful
reconnaissance of the area to be surveyed in order to select the most-
expeditious manner of performing the survey, and to plan all operation so that
the survey is satisfactorily completed in accordance with the requirements and
specifications governing such work. The use of aerial photographs can be of
considerable help in this preliminary study.
Establishment of horizontal control:
The
horizontal control is a framework by which land and marine features are held in
their true relative positions. It usually consists of a series of connected
lines whose lengths and azimuths are determined accurately. Triangulation and,
to a lesser extent, traverse are most commonly executed to provide horizontal
control. For rough works a tacheometric traverse or plane table traverse may be
run. However, for more precise control, a theodolite and a tape traverse is
usually run. For extended surveys, where great precision is required,
horizontal control is provided by triangulation.
As the general
topography, vegetation, type and size of water body railways and highways of
the coast line are the factors which decide the character of control, no
definite rules can be given for providing horizontal control for hydrographic
surveys. However, the following guide lines may help in selection of the type
of control.
1. If the water body is more than one
kilometer wide, it is advisable to run two traverse along each shore,
connecting to each other by frequent tie lines.
2. If the water body is narrow, i.e.,
rivers, inlets, etc, and shore conditions are favorable it is advisable to run
a traverse line only along one of the banks.
3. If the shore lines are obscured by
vegetation it is advisable to adopt triangulation system.
4. For large lakes and ocean shore
lines, the horizontal control consists of a network of connected triangulation
system on shore.
These triangulation systems are usually, supplemented by
traverses run along the shore and connected to the primary triangulation
system.
Previously established control is very important asset in any
hydrographic survey. Every effort should be made to obtain and utilize data
from earlier surveys.
Establishment of vertical control
Before sounding
operations are begun it is essential to establish the vertical control in order
that the stage or elevation of the water surface can be known when the
soundings are obtained. Vertical control data are also needed for the limited topography
shown on all nautical charts.
A number of
bench marks are established near the shore line at close spacing to serve as
vertical control, the bench marks are used for setting and checking the levels
of gauges to which the sounding refer to.
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