What is Hydrographic Survey

Hydrographic Survey

These are surveys that include all types of hydrological observations necessary for the design of hydraulic or marine structures. A Hydraulic or marine structure includes the design of dams, barrages & weirs, ports, docks, lighthouses, etc. This knowledge of the amount of rainfall in the area, the location of hills, sand bars under the sea, and tidal survey are critical.

Hydrographic survey includes the following:

Rain Gauging: The amount of rainfall in the catchment area of a basin or a river is generally recorded. This helps predict any probable flood in the downstream area in case of heavy rainfall, and necessary precautions may be taken to avert disaster.

River Gauging: River gauging involves the measurement of the discharge of a river and the establishment of a gauge post on one of its banks. This is done to directly read the HFL so that a warning may be given to surrounding areas for any precautionary measures to be taken if necessary.

In addition, river gauging is undertaken for the following reason:

To fix the height of the guide bank.
To fix the number of spans of road and railway bridges, unexpected high floods may not cause any damage to the structure.
To fix the spillway level and the height of the dam or barrage.
To design the cross drainage works when a canal crosses a river.

Marine Survey: Marine survey involves the following procedure

The shoreline at low and high tide is marked. Then an open traverse is conducted by compass or theodolite to obtain the configuration of the shoreline.
The water depth around the harbor is determined by echo-sounder, and obstructions like large rocks, sandbars, etc., are located.
Current and tidal observations are taken to identify the nature of the seawater and take necessary precautions for the safe movement of ships.
Positions, where lighthouses are to be set up for indicating a safe route for ships from one port to another, are established. Large rocks, submerged hills, sandbars, etc., are located by measuring the depth by echosounder and other appliances.
The profile of the bottom of the sea is prepared by depth recording instruments mounted on a motorboat or streamer along a pre-determined alignment fixed by a compass. Soundings are taken at regular intervals and recorded.
Maps are prepared to indicate the position of submerged hills, sandbars, or any other obstructions. The safe route is indicated on the map by showing the positions of lighthouses. The profile of the sea bed is prepared for navigational guidance.

Methods used for measurement:

Depth:-

Sounding rod: A wooden or bamboo pole of 5cm diameter suitable for measuring the depths of small rivers about 2 to 2.5.
Sounding Cable: For high velocity of water cable with pulley system is used.
Echo-Sounder: The echo-sounder is used for measuring depths of large rivers more than 10m.

The velocity of flow:-

Surface float Method: Surface floats are made of cork, painted red or white, and have a flag attached on top. The velocities of the surface floats are measured, which represents the surface velocity.
Sub-Surface float Method: Similar to the surface float method, a hollow cylinder is attached to the floats by a chord.
Velocity rod Method
Pilot tube Method
Current meter Method: The price current meter is a commonly used meter for measuring the velocity of the water.