Hydrographic Surveying

Hydrographic Surveying

Hydrographic surveying deals with the measurement and description of the physical features of bodies of water. It includes the shape and nature of the bed (bathymetry), tides, and currents.

Measurement of Depth (Sounding)

The vertical distance from the water surface to the bed is called sounding.

Methods of Sounding

1. Lead Line: A weighted line marked with depth graduations. Used for shallow waters.
2. Sounding Pole: A graduated pole. Used for depths up to 6 m.
3. Echo Sounder (Fathometer): Uses sound pulses (sonar) to measure depth. $D = \frac{v \times t}{2}$ Where:
   • $v$: Velocity of sound in water (approx. 1500 m/s).
   • $t$: Time interval between transmission and reception.

Discharge Measurement

Discharge ($Q$) is the volume of water flowing past a section per unit time (m³/s or cfs).

Area-Velocity Method

$Q = A \times V$ Where:

• $A$: Cross-sectional area of the stream.
• $V$: Mean velocity of flow.

Measurement of Velocity

1. Current Meter: Measures velocity at specific points (typically 0.2 and 0.8 depth). $V_{mean} = \frac{V_{0.2} + V_{0.8}}{2}$
2. Floats: Surface floats used to estimate surface velocity ($V_s$). $V_{mean} \approx 0.85 V_s$

Tide Levels

• Mean Sea Level (MSL): The average height of the sea for all stages of the tide.
• Mean Low Water (MLW): The average height of all low waters.
• Mean High Water (MHW): The average height of all high waters.
• Datum: The reference plane for soundings (usually MLW or MLLW - Mean Lower Low Water).

Solved Problems