Bathymetric and Hydrographic Surveys in Kenya and East Africa
Hydrographic surveying or bathymetric surveying is the survey of physical features present underwater. It is the science of measuring all factors beneath water that affect all the marine activities like dredging, marine constructions, offshore drilling etc.
Bathymetry is performed to map the under-water bottom with a high level of accuracy. It will help correlation and interpretation of the data obtained from other methods, which yield sub-bottom information and allow a quality check of the results. Since the other methods measure thickness from the bottom, this accuracy is essential.
The principle of the method is to send an acoustic signal and measure the travel time to derive a depth. This depth conversion process is done by first measuring the velocity of sound in the water at different depths. This calibration is done twice a day to ensure a good accuracy.
The water depth measurements can be expected to be accurate to within ± 10 cm.The bathymetry equipment is a small equipment which is mounted on a boat and survey conducted along with other geo-physical methods.
Survey is conducted in a grid pattern. The line spacing is decided based on the resolution required. The accurate positioning is achieved using a Differential Global Positioning System.
- Multi-beam surveying: A multibeam echo sounder attached to a boat sends out a wide array of beams across a "swath" of the waterbody floor. As the beams are bounced back from the waterbody floor, the data is collected and processed. The processed data can be viewed in real time on the boat during the survey. Multi-beam surveying is generally done in larger water bodies.
- Single-beam surveying: Rather than sending out a wide set of beams, single-beam bathymetry measures the water depth directly under the boat. Single-beam surveys are generally used for smaller water bodies.
- Acoustic Doppler Current Profiler (ADCP): ADCPs are used throughout USGS to measure streamflow. ADCPs measure water velocity by transmitting sound waves which are reflected off sediment and other materials in the water. Data collected from ADCPs can then be used to for bathymetric mapping.
- Sub-bottom profilers: Sub-bottom profilers are most commonly used to view the layers of sediment and rocks under the water body floor. A transducer sends a sound wave to the water body floor. This sound wave can penetrate the water body floor. The data returned from the sound waves can be mapped to show the layers beneath the water body floor.
- Ecomapper Autonomous Underwater Vehicle: The Ecomapper can collect detailed bathymetric data, down to one-foot contours, in places that are difficult to reach with boats. The Ecomapper uses side-scan sonar and a Doppler velocity log.
- Flood inundation
- Contour of streams and reservoirs
- Scour and stabilization
- Water-quality studies
- Dam removal
- Biological and spill
- Storage and fill in reservoirs and ponds
Is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, oil exploration/drilling, pipeline damage assessment and related disciplines. Strong emphasis is placed on water depth soundings, shorelines, tides, currents, sea floor and submerged obstructions that relate to the previously mentioned activities. Hydrographic surveys uses the raw data collected through the hydrographic survey process into information usable by the end user.
SECO has been conducting hydrographic surveys over the last decades; however, by today's standards, the older methods of conducting these surveys could be considered antiquated. Before the advent of electronic surveying equipment including electronic distance meters, GPS and echosounders (water depth recorders), shallow water depths were measured with sounding poles or lengths of ropes. The approximate locations of the soundings were put on maps to the best ability of the survey party chief. As one can imagine, this method was not nearly as accurate as it could be. Echosounders made determination of water depths a lot more accurate; however, the horizontal accuracy of the soundings was still based on the judgment of the individual surveyor. Adding electronic distance measuring techniques to the process meant that the horizontal and vertical positions of the soundings could be determined more accurately with minimal judgment calls on the part of the surveyor.
With the introduction of GPS into the hydrographic surveying process, it became possible to conduct hydrographic surveys in a real-time fashion and allowed surveys to be done without the use of standard surveying equipment and allowed the surveys to be done out of sight of land.
SECO has gone through all of these processes and is now capable of conducting just about any level of hydrographic/side scan sonar surveys to any required degree of accuracy.
Following are the applications of hydrographic surveying:
- Dock and Harbor Engineering
- River Works
- Land reclamation
- Water Power
- Flood Control
- Sewage Disposal
Uses of hydrographic surveying are given below:
- Depth of the bed can be determined
- Shore lines can be determined
- Navigation Chart Preparation
- Locate sewer fall by measuring direct currents
- Locating mean sea level
- Scouring, silting and irregularities of the bed can be identified
- Tide measurement
- River and stream discharge measurement
- Massive structures like bridges, dams harbors are planned