Bathymetry, Sediment Profiling & Secchi Disk in SEZAD Port

Overview

The project involved providing comprehensive services for Bathymetric Survey, Secchi Disk measurement, and Sediment Sample collection for the Coastal Erosion Assessment and design of coastal protection at the Special Economic Zone at Ad Duqm, Sultanate of Oman. The aim of the marine survey was to perform bathymetric survey, Secchi disk measurement, and sediment profiling.

Bathymetry Survey

A pre-survey investigation was carried out at the survey site to ensure a safe working area and to meet the client's survey objectives within the predefined areas. For the survey, a Valeport Tide Gauge was installed for tide correction. The GPS antenna was positioned at the top of and aligned with the echo-sounder transducer pole to achieve accurate horizontal positioning. An Odom EchoTrack MKIII Dual frequency Echo Sounder was utilized, logging 210 kHz and 33 kHz digital outputs via the navigation computer. CTDS dips and bar checks were performed daily before starting the survey. The Hemisphere Sokkia RTK system, equipped with HydroPro software, was used to convert the antenna position into local UTM projection coordinates.

Sediment Collection

A grab sampler was employed to collect samples from the topmost layers of the seabed by bringing two steel clamshells together and cutting a sample from the soil. This method is used to examine surface sediment (about 10-15 cm deep) while minimizing disturbance from the pressure wave as the sampler descends.

Secchi Disk Measurement

A Secchi disk, a circular metal disk painted with black and white quadrants, was lowered into the water by hand. The depth at which the disk vanished from sight was recorded. The distance of disappearance and reappearance indicates water clarity. The Secchi Disk Depth reflects the absorption and scattering of light by particles and dissolved substances in the water, including algae, sediments, detritus, and organic acids from plant and algae breakdown.

Results

Contour maps at 0.5m and 2.5m intervals of the surveyed area were presented to the client, with drawings prepared at a scale of 1:2500. Sediment grain-size analysis was conducted using the sieve method. Samples were washed through sieves to separate sand and gravel from the silt-clay fraction. Coarse fractions were oven-dried, weighed, and then passed through a 2-mm sieve to collect the gravel fraction. The sand fraction was treated with hydrogen peroxide to remove organic matter, washed, dried, and weighed. A sieve analysis was performed on the silt-clay fraction. Water transparency (Secchi disk depth) was greater in deeper areas and decreased towards shallower depths, indicating a more turbid near-shore environment.