Hydrographic Surveying

The branch of surveying that deals with the measurement and description of the physical features of bodies of water and their marginal land areas. This includes bathymetry (depth measurement), tidal analysis, and locating navigational hazards, which are vital for maritime navigation and coastal engineering. Key aspects include measuring tides, currents, and shoreline mapping.

Measurement of Depth (Sounding)

The process of measuring the depth of a body of water at a specific point.

Methods of Sounding

Methods of Sounding

  • Lead Line: A weighted line marked with depth graduations. Used for shallow waters and check measurements.
  • Sounding Pole: A graduated pole (rod). Used for depths up to 6 m in calm water.
  • Echo Sounder (Fathometer): Uses sound pulses (sonar) to measure depth. It measures the time for a sound wave to travel to the bottom and return.

Echo Sounding Formula

D=v×t2 D = \frac{v \times t}{2}
Where:

Checklist

Position Fixing (Location of Soundings)

Three-Point Resection Simulator

Determine the position of an unknown station (O) by observing three known points (A, B, C).

Resection Principle

By setting up an instrument at unknown Station O and measuring the horizontal angles to three known control points (A, B, C), the coordinates of Station O can be calculated.

*Note: This simulation provides a schematic visual representation of the concept. Real coordinate computation involves complex trigonometric formulas (e.g., Collins Point Method or Tienstra's Method).*

To map the depth, the horizontal position (x,yx, y) of each sounding must be known.

Methods of Fixing Position

  • Intersection: From two shore stations (using theodolites).
  • Resection (Three-Point Problem): Measuring two horizontal angles simultaneously from the boat to three known shore stations using a Sextant. Solved graphically (Three-arm protractor) or analytically.
  • GPS/GNSS: Modern method using RTK GPS on the survey vessel.

Capacity of Reservoirs

Hydrographic surveying is often used to compute the volume or capacity of a reservoir, lake, or proposed dam site by evaluating the areas enclosed by successive contour lines.

Methods for Computing Capacity

  • End-Area Method: The volume between two successive contours is estimated using the average of their enclosed areas (A1A_1 and A2A_2) multiplied by the contour interval (hh).
  • Prismoidal Method: Used for higher accuracy, requiring an intermediate contour area. It follows the standard prismoidal formula: V=h6(Abottom+4Amid+Atop)V = \frac{h}{6}(A_{bottom} + 4A_{mid} + A_{top}) where hh is the total vertical distance between the top and bottom contours.

Discharge Measurement

Discharge (QQ) is the volume of water flowing past a section per unit time (m³/s or cfs). It is critical for hydrology and water resource planning.

Area-Velocity Method

Discharge Formula

Q=A×V Q = A \times V
Where:

Checklist

Measurement of Velocity

Measurement of Velocity

  • Current Meter: Mechanical (propeller) or acoustic (Doppler) device lowered into the water to measure velocity at specific depths.
    • One-Point Method: Measure at 0.6d0.6d (0.6 of depth from surface). Used for shallow streams.
    • Two-Point Method: Measure at 0.2d0.2d and 0.8d0.8d. Average them. The standard for deep streams.
  • Floats: Surface floats used to estimate surface velocity (VsV_s). A rough estimation technique.

Two-Point Method Formula

Vmean=V0.2+V0.82 V_{mean} = \frac{V_{0.2} + V_{0.8}}{2}

Float Method Formula

Vmean≈0.85Vs V_{mean} \approx 0.85 V_s

Stream Gauging Stations

To continuously monitor discharge over time, permanent stream gauging stations are established to log continuous discharge monitoring data. They measure the water level (stage) continuously, which is then converted to discharge using a calibrated rating curve.

Gauging Equipment

  • Staff Gauge: A simple graduated scale fixed to a pier or post in the stream. Read manually. This is the simplest setup of a stream gauging station.
  • Recording Gauge (Stilling Well): A float inside a protected well that continuously records water levels on a chart or digital logger. Essential for continuous discharge monitoring without manual observation.

Tides

Tides are the periodic rise and fall of the ocean surface caused by the gravitational pull of the Moon and the Sun on the Earth. Understanding tides is essential for establishing vertical datums for hydrographic surveys.

Types of Tides

  • Spring Tides: Occur during full and new moons when the Earth, Moon, and Sun are aligned. Gravitational forces combine to produce the highest high tides and lowest low tides.
  • Neap Tides: Occur during quarter moons when the Sun and Moon are at right angles to each other. Gravitational forces counteract, producing the smallest tidal range.

Datum Planes

  • Mean Sea Level (MSL): The average height of the sea for all stages of the tide over a 19-year period (Metonic cycle). The primary reference for terrestrial elevations.
  • Mean Low Water (MLW): The average height of all low waters. Reference for soundings (Chart Datum).
  • Mean High Water (MHW): The average height of all high waters. Reference for bridge clearances.
  • Mean Lower Low Water (MLLW): The average of the lower of the two daily low tides. Often used as the standard Chart Datum for navigational charts.
Key Takeaways
  • Sounding: Measuring depth (D=vt/2D = vt/2).
  • Chart Datum: Reference level for soundings (usually MLLW).
  • Three-Point Problem: Used for position fixing by resection (Sextant).
  • Discharge (QQ): Volume flow rate (A×VA \times V).
  • Velocity Methods: One-point (0.6d) or Two-point (0.2d & 0.8d).
  • Tides: Spring tides (maximum range, aligned), Neap tides (minimum range, right angles).
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