What is Hydrographic Surveying?
Hydrographic surveying, also known as bathymetric surveying, is the survey of physical features present underwater. It is a science used to measure all factors beneath the water’s surface that affect marine activities such as dredging, offshore drilling, and marine construction. This type of surveying is usually conducted by government authorities and involves the use of sensors, soundings, and electronic sensor systems for shallow waters.
How Does Hydrographic Surveying Help?
Hydrographic surveying provides necessary data for the creation of nautical charts, which contain information about the available depths, improved channels, breakwaters, and other aids to navigation and harbor facilities. This surveying also helps to determine the loss in capacity due to silt and other uncertainties that may occur during the construction of port facilities such as piers.
Why is Hydrographic Surveying Important?
Hydrographic surveying is an important part of marine activities, providing data to ensure the safety and accuracy of these activities. This data is used to construct nautical charts that provide mariners with the information they need to navigate safely through underwater areas. Without this data, it would be much more difficult for ships to safely traverse the seas. In addition, hydrographic surveying helps to reduce the risks of maritime accidents, as it can provide detailed information on the depths and other features of the ocean floor.
Applications of Hydrographic Surveying
Dock and Harbor Engineering
Hydrographic surveying is an important tool for dock and harbor engineering. It provides precise measurements of water depths and bottom topography, enabling engineers to determine the most effective locations for docks, harbors, and other types of marine infrastructure. It also helps to ensure the safety of these structures by providing data on water currents, sedimentation, and other factors that can affect the stability of the structure.
Irrigation
Hydrographic surveying is also essential for irrigation projects. By providing detailed data on water depths, water flow, and the composition of the soil, engineers can determine the most effective irrigation methods for a particular area. This data helps to ensure efficient water usage, as well as reduce the risk of flooding and water pollution.
River Works
Hydrographic surveying is essential for the design and construction of bridges, dams, and other types of river works. By providing data on the depth and composition of the riverbed, engineers can determine the best locations for these structures. This data can also be used to develop strategies for controlling water levels and protecting against flooding.
Land Reclamation
Hydrographic surveying is also used for land reclamation projects. By providing data on the composition and depth of the land, engineers can determine the best methods for reclaiming land for agricultural or residential use. This data can also be used to create strategies for preventing soil erosion and preserving the environment.
Water Power
Hydrographic surveying is an important tool for the development of water power. By providing data on the depth and flow of rivers and streams, engineers can determine the most effective locations for hydroelectric dams or other types of water power generation. This data can also be used to create strategies for controlling water levels and protecting against flooding.
Flood Control
Hydrographic surveying is also used for flood control projects. By providing data on the depth and composition of rivers and streams, engineers can create strategies for controlling water levels and preventing flooding. This data can also be used to determine the best methods for constructing levees and other types of flood control structures.
Sewage Disposal
Hydrographic surveying is also used for sewage disposal projects. By providing data on the depth and composition of rivers and streams, engineers can determine the most effective locations for sewage treatment plants and other types of infrastructure. This data can also be used to create strategies for controlling water levels and preventing water pollution.
Uses of Hydrographic Surveying
Depth Measurement:
Hydrographic surveying is used to measure the depth of the bed. This helps to determine the depths of the ocean and water bodies and helps to establish the shorelines. It is also useful in navigation chart preparation, as it helps to identify the depths of the water bodies.
Navigation Chart Preparation:
Hydrographic surveying is used to prepare navigational charts which help in safe navigation. It helps to identify the depths of the water bodies and helps to establish the shorelines. It also helps to identify any irregularities in the bed of the water bodies.
River and Stream Discharge Measurement:
Hydrographic surveying is used to measure the discharge of rivers and streams. This helps to identify the rate at which water is flowing through the river or stream and helps to understand the behavior of the river or stream.
Massive Structures:
Hydrographic surveying is used to plan and design massive structures such as bridges, dams, and harbors. It helps to identify the depths of the water bodies, the shorelines, and any irregularities in the bed of the water bodies. This helps to ensure that the structures are built to the right specifications and are safe for use.
Preliminary Steps in Hydrographic Surveying
Step 1: Reconnaissance
Before any shoreline sounding can take place, reconnaissance is essential. This involves surveying the shoreline to identify the most suitable locations for soundings. The survey should consider factors such as the location of existing structures, the presence of hazardous materials, and the natural environment.
Step 2: Locate Horizontal Control
Once the reconnaissance is complete, horizontal control points must be identified. These points will provide reference points for the sounding operation, and should be placed at regular intervals along the shoreline.
Step 3: Locate Vertical Control
The next step is to establish vertical control points. These will be used to determine the depth of the water at each sounding location. Vertical control points should be established at regular intervals along the shoreline, and their depth should be recorded.
Step 4: Sounding
The final step is to take soundings at each of the control points. This is done by either fixing a point in the boat and taking soundings from the control point, or by sounding from the boat to the control points. The depths recorded at each point will provide an accurate indication of the depths along the shoreline.
Reconnaissance
Introduction to survey requirement
Before starting any survey project, it is essential to undergo a reconnaissance of the whole survey area. This would help in selecting the most suitable approach and tools to perform the survey in a cost-effective and efficient manner in accordance with the requirements and specifications. Aerial photography can be used as a great tool to facilitate the study.
Benefits of reconnaissance
Reconnaissance helps in selecting the most effective way of performing a survey as per the given requirements. It also helps to identify any potential risks or challenges that may arise during the survey. Aerial photography can be used to gain a comprehensive overview of the area which would be helpful in the successful completion of the survey.
Use of aerial photography
Aerial photography is a great tool for surveying large areas. It can provide a comprehensive overview of the area and the various features present in it. This helps in identifying the most suitable approach for performing the survey. It also helps to identify any potential risks or challenges that may arise during the survey.
Locating Horizontal Control
What is Horizontal Control?
Horizontal control is a series of lines whose lengths and azimuths are determined by means of either triangulation or other methods, used to accurately locate all features of the land and marine in true relative positions. It is essential for many surveying, mapping and engineering projects.
Methods for Establishing Horizontal Control
Tachometric and plane table survey can be used to conduct rough works. However, no standard rules are kept for establishing horizontal control as topography, vegetation, type, size of topography all affect the rules. Generally, it is advisable to:
- Run traverses along each shore and connect them with frequent tie lines if the water body is wider than 1 km.
- Run transverse lines only along one of the banks if the water body is narrow.
- Use triangulation system if shorelines are filled with vegetation.
- Use a large network of triangulation system for large lakes and ocean shore lines.
Example of Combined Triangulation and Traversing
A combined triangulation and traversing system is shown in Figure 1. This method combines the principles of triangulation and traversing to accurately measure the coordinates of landforms and other features.
Locating Vertical Control
Establishment of Vertical Control
Before any construction or surveying project can be undertaken, vertical control must be established. This involves placing several benchmarks at strategic locations within the project area which will act as a reference point for the surveyors. The levels of each benchmark must then be checked in order to ensure accuracy. This process of setting and checking the levels of the benchmarks is referred to as the establishment of vertical control.
Sounding in Hydrographic Survey
Sounding: The Process of Determining Depth Below Water Surface
Sounding is the process of determining the depth below the water surface. It is done by subtracting the sounding from the mean sea level, which is the step before undergoing sounding. This is analogous to levelling and can help to determine the reduced level of any point of a water body.
The specific need for sounding are
- Preparation of Navigation Charts for Future Use
Creating and maintaining navigation charts is an important part of maritime navigation, as these provide necessary information for navigating safely through waterways. Navigation charts are created using data obtained from hydrographic surveys and include information such as the depth of water, soundings, dangers and obstructions, and the location of aids to navigation. These charts are regularly updated and provide an all-time source of information for sailors. - Determining the Material to be Dredged
In order for a successful dredging operation to be carried out, it is important that the material to be dredged is determined early on. This allows for the dredging to be done more efficiently, as there won’t be any confusion as to what material needs to be dredged. The dredged material should also be accompanied by any necessary filling operations, as well as the dumping of any excess material. - Design of Backwaters and Sea Wells
Designing backwaters and sea wells requires detailed information that is obtained from sounding. Sounding is the process of measuring the depth of water in a body of water. This information is used to create accurate and detailed designs for backwaters and sea wells, allowing for safe navigation and usage.
Equipment for Sounding
Shore Signals and Buoys for Sounding
Shore signals and buoys are essential tools for undergoing sounding. These tools help to identify the depths and characteristics of a particular body of water. Shore signals are typically used to mark a specific area and buoys are used to indicate the location and depth of a particular area. By using a combination of these two tools, it is possible to accurately measure the depths and characteristics of a body of water.
Sounding Equipment for Sounding
Sounding equipment is necessary for accurately measuring the depths and characteristics of a body of water. This equipment typically includes an echo sounder, sonar, and other types of equipment that can measure depths and characteristics of a particular body of water. The echo sounder is used to measure the depths of a body of water and the sonar is used to measure the characteristics such as temperature, salinity, and other characteristics of a body of water.
Instruments for Measuring Angles
In addition to shore signals, buoys and sounding equipment, instruments for measuring angles are also essential for undergoing sounding. These instruments are typically used to measure the slope of the bottom of a body of water and the angles of a shoreline. By using these angles, it is possible to accurately measure the depths and characteristics of a particular body of water.
1. Shore signal and buoys
Range Lines
Range lines are used to mark the boundaries of an area in the sea. They are created by joining two or three signals in a straight line, which is perpendicular to the shore line. This allows for angular observations to be made from sounding boats. To make the range lines visible from a distance, they are made highly conspicuous by using buoys. These are floats made of light wood or air tight vessels, which are weighted at the bottom and kept vertical by anchoring with guywires. A hole may be drilled into the buoy to accommodate a flag.
Shore Signals and Buoys
In order to mark range lines underwater, shore signals and buoys are used. Buoys are usually made of light wood or air tight vessels, which are weighted at the bottom and kept vertical by anchoring with guywires. A hole may be drilled into the buoy to accommodate a flag. This makes the range lines visible from a distance. Shore signals are used to provide direction and orientation to the buoys, allowing them to be placed in the correct position.
2. Sounding Equipment
Sounding Boat: An Overview
A sounding boat is a type of flat-bottom boat with a low draft designed for the purpose of carrying out sounding operations. Typically, larger boats equipped with a motor are used for sounding in the sea. The soundings are taken through wells provided in the boat. An illustration of a sounding boat can be seen in Figure 2.
Sounding Pole or Rod
A sounding pole or rod is a device used to measure the depth of water. It is typically a rod made of seasoned timber, with a diameter of 5-10 cm and a length of 5-8 m. A lead shoe of sufficient weight is connected to the bottom of the pole to keep it vertical. Measurements are taken from the bottom of the pole to the water surface, with graduations marked from the bottom upwards. Hence, the readings on the rod corresponding to the water surface can be used to calculate the water depth.
Lead Line
A lead line is a device used to measure the depth of water. It is typically a graduated rope made of chain, connected to a lead or sinker of 5-10 kg, depending on the current strength and water depth. It is important to note that due to deep and swift flowing water, the true depth may vary. As such, a correction may be necessary when using a lead line.
Sounding Equipment
Sounding is a method used to measure the depth of the ocean or sea. Various types of sounding equipment are used to measure the depth of the ocean. The most commonly used sounding equipment are lead lines, sounding poles, and sounding weights. Lead lines are weighted loops of rope that are dropped into the water to measure the depth. Sounding poles are long poles with weights attached to them, which are lowered into the water to measure the depth. Sounding weights are large weights that are dropped into the water to measure the depth.
Weddell’s Sounding Machine
Weddell’s sounding machine is another type of sounding equipment used for large-scale sounding work. This machine is designed to measure depths of up to 30-40m and is bolted onto the well of the sounding boat.
Fathometer
Fathometer is an echo-sounding instrument used to measure ocean depth. This instrument records the time of travel from the surface of the water to the bottom of the ocean and back. Knowing the velocity of sound waves, the depth can be calculated.
Calculating Depth D with a Lead Line
A lead line is a tool used to measure the depth of the water. It is usually made of rope or chain with a weight attached to the bottom. By throwing the lead line into the water, the user can measure the depth by counting the number of fathoms (1 fathom = 6 feet). This method is relatively easy to use, but it is not as accurate as other methods.
Calculating Depth D with a Plumb Bob
A plumb bob is a tool used to measure vertical angles and depths. It consists of a weight attached to a string and suspended from a fixed point. By dropping the plumb bob from the fixed point, the user can measure the depth by the angle formed between the string and the ground. From this angle, the depth D can be calculated if AB can be found. This method gives truly vertical and accurate results and is found to be more sensitive than a lead line.