Hydrographic Survey at GRSE: Pre- and Post-Dredging Survey for Two Jetties

Accurate hydrographic surveys form the backbone of every successful dredging and marine infrastructure project. Recently, our team at Topo India completed a comprehensive Pre-Dredging and Post-Dredging Hydrographic Survey for Garden Reach Shipbuilders & Engineers (GRSE), covering two strategic jetties within their operational waterway. The objective was to map existing underwater conditions, guide dredging operations, and finally determine the exact volume of earth removed from beneath the water body.

The survey was executed safely, efficiently, and with high-precision instrumentation, ensuring GRSE received reliable geospatial intelligence to support marine operations, draft maintenance, and navigation planning.

Project Overview

The scope of work involved:

• Detailed Pre-Dredging Bathymetric Survey along the two jetty stretches

• Depth data acquisition at 1-metre interval cross-sections

• Post-Dredging Bathymetric Survey using identical transects for perfect comparability

• Generation of TIN (Triangulated Irregular Network) surfaces for both pre- and post-dredging datasets

• Detailed contour generation, underwater terrain visualisation, and quantitative analysis

• Computation of accurate dredged volume based on differential bed profiles

Carrying out the survey in a working shipyard environment, surrounded by large vessels and ongoing marine activities, required strict safety protocols, precise planning, and coordinated execution.

 Equipment Used

1. Trimble GNSS System for Precision Positioning

We deployed a Trimble multi-frequency GNSS system that ensured centimetre-level coordinate accuracy, even in challenging satellite visibility conditions near large docked ships and infrastructure. This accuracy was critical for depth–location synchronisation.

2. Hydrographic Echo Sounder for Depth Measurement

Our professional hydrographic echo sounder delivered stable and reliable depth data with:

• Depth accuracy: ±5 cm

• High-frequency bottom detection

• Continuous real-time draft measurement

• Consistent performance even in turbid or low-visibility waters

Integration of GNSS + Echo Sounder enabled recording of 3D bathymetric data points with exceptional precision.

Continuous depth readings are being captured at every 1m interval.

Survey Methodology

1. Pre-Dredging Bathymetric Survey

Before dredging, the team navigated the inflatable survey boat along planned lines spaced at 1-metre intervals.At each interval, synchronized GNSS coordinates and echo sounder depth readings were recorded.

The output was a high-density point cloud capturing the exact underwater bed profile.

2. TIN Surface Generation & Contouring

Using advanced hydrographic processing software, we generated TIN surfaces for both the pre- and post-dredging datasets.

TIN modeling allowed:

• Accurate interpolation of irregular underwater terrain

• Smooth representation of bed undulations

• Generation of detailed depth contours, underwater slope patterns, and navigational depth maps

• Reliable surface-to-surface comparison for precise volume estimation

The contour maps gave GRSE clear visibility of depth variations and dredging effectiveness.

3. Post-Dredging Survey

After completion of dredging, the same transects were surveyed again using identical methodology and equipment.This ensured perfect alignment between pre- and post-dredging surfaces, eliminating discrepancies.

Setting up the instruments and checking sample drafts.

4. Volume Computation

The dredged volume was calculated using:

• Pre- and post-dredging TIN surfaces

• Cut–fill algorithms

• Surface differencing techniques

This allowed us to quantify the exact cubic volume of earth removed, offering an audit-proof verification for project stakeholders.

Safety & Field Execution

Working near operational jetties, docked ships, and industrial marine movements required strict safety adherence:

• Constant communication with jetty control

• Working from a stable inflatable boat

• Controlled navigation around large hull structures

The team executed the entire project smoothly, without any safety deviations.

Outcome for GRSE

The hydrographic survey delivered:

• High-resolution pre and post dredging bathymetric maps

• Accurate depth measurements with ±5 cm precision

• Detailed TIN-based contour maps

• Surface difference models for analysis

• Verified dredging volume computation

• Reliable documentation for engineering and marine operations

This data now supports GRSE in ensuring safe depth availability, improving navigational efficiency, and validating dredging performance.

Conclusion

This hydrographic survey project at GRSE highlights the critical role of accurate bathymetry in maritime engineering. With advanced GNSS positioning, precise echo sounding, and high-fidelity TIN modeling, Topo India continues to deliver dependable, engineering-grade geospatial solutions for ports, shipyards, inland waterways, and coastal infrastructure.