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# Bathymetric Mapping of Sediment Volume and Riverbed Changes for Optimized Dredging: A case study of Seplat Energies, in Mayuku River, Sapele LGA, Delta State, Nigeria

Kingsley Idhoko, Akpomuode C Ogenerume

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Kingsley Idhoko, Akpomuode C Ogenerume. Bathymetric Mapping of Sediment Volume and Riverbed Changes for Optimized Dredging: A case study of Seplat Energies, in Mayuku River, Sapele LGA, Delta State, Nigeria. International Journal of Research, In press, 12 (04). hal-05085742

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Submitted on 26 May 2025

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# Bathymetric Mapping of Sediment Volume and Riverbed Changes for Optimized Dredging: A case study of Seplat Energies, in Mayuku River, Sapele LGA, Delta State, Nigeria

Akpomuode Oghenerume Clark.¹, Idhoko Kingsley. E.¹,

1. Department of Surveying and Geoinformatics, Nnamdi Azikiwe University, Awka, Anambra State, Nigeria.

## Abstract

This research investigates applying bathymetric survey techniques in evaluating seabed morphology and quantifying sediment volumes before and after a dredging operation. The primary objective was to assess navigational improvements and sediment displacement through hydrographic surveying and geospatial analysis. Using tide-corrected depth readings referenced to a standard datum, pre-dredging depths ranged from 2.0 to 2.7 meters, while post-dredging depths reached the target range of 4.0 to 4.5 meters. High-resolution bathymetric data were processed within a GIS environment, allowing accurate cut volumes and surface area computations, with pre- and post-dredging volumes estimated at 1,554,307.706 m³ and 1,702,694.232 m³, respectively. The study also examined post-dredging morphology to detect depth uniformity and areas of overcutting or undercutting. Findings highlight the critical role of geospatial tools in managing dredging operations and ensuring channel optimization for safe marine navigation. Challenges encountered included tide variations, environmental constraints, and sediment inconsistencies. It is recommended that similar methodologies be adopted for future maintenance dredging projects, integrating real-time monitoring and environmental impact assessments to enhance sustainability.

**Keywords: Bathymetric survey, dredging, sediment volume, seabed morphology, navigational safety, GIS.**
# 1. Introduction

Nigeria remains one of the largest oil-producing countries in Africa, with a significant portion of its production concentrated in the Niger Delta region. This region is characterized by a complex network of rivers, creeks, and estuaries, which play a critical role in enabling access to remote oil field locations (Nwilo et al., 2007). Because much of the infrastructure and logistics in these environments are dependent on river transport, maintaining navigable waterways is essential to the smooth operation of oil exploration and production activities. However, these waterways are highly dynamic and are constantly shaped by sedimentation, tidal flows, and seasonal changes in river discharge.

Sediment buildup in rivers can obstruct access routes, reduce water depth, and impair marine operations. For oil companies like Seplat Energies, operating within the OML 38 oil block located in the Mayuku River, Okpe LGA of Delta State, such challenges demand regular dredging interventions to clear pathways and restore operational depth. Yet, dredging without accurate data on the riverbed's condition can be inefficient and environmentally unsound. The absence of precise, location-specific sediment data often leads to either under- or over-dredging, which may result in incomplete work or unintended environmental disturbance (Ibe, 1991).

Bathymetric surveying addresses this challenge by providing detailed information about the underwater topography of rivers, lakes, and oceans. It involves the measurement of depth and the mapping of the riverbed surface using sonar equipment, GPS positioning, and geospatial
processing tools (IHO, 2020). Through pre- and post-dredging bathymetric surveys, it becomes
possible to accurately determine the volume of sediment removed and identify morphological
changes in the riverbed. This approach helps guide dredging efforts, ensuring that operations are
targeted and environmentally responsible.

The need for accurate bathymetric data is even more pronounced in sensitive and ecologically
complex environments like the Niger Delta. In many riverine communities, the environmental
impact of industrial activity, including dredging, is a major concern. Studies have indicated that
unregulated or poorly monitored dredging can lead to increased water turbidity, habitat
degradation, and erosion of riverbanks (Adelana & Olasehinde, 2015). Therefore, integrating
geospatial and hydrographic data into dredging operations is essential for balancing economic
development with environmental stewardship.

At the OML 38 oil block in the Mayuku River, sedimentation has been identified as a key
operational issue, particularly in maintaining access to oil wellheads and associated infrastructure.
This study proposes a focused investigation into the changes in riverbed elevation and sediment
volume before and after dredging in this area. By utilizing bathymetric survey methods, the study
aims to provide an evidence-based assessment of dredging efficiency and riverbed alteration. Such
data will not only support operational planning but also contribute to regulatory compliance and
long-term sustainability efforts in the oil and gas sector.

Furthermore, this research aligns with broader efforts in hydrographic science to apply data-driven
approaches to coastal and inland water management. Modern bathymetric techniques, when
integrated with Geographic Information Systems (GIS), allow for high-resolution mapping and volumetric calculations that enhance our understanding of sediment dynamics (Ukponmwan & Alakinde, 2019). By applying these tools to a real-world oil field scenario, this study adds practical value to the field of hydrography and serves as a reference model for similar projects in the region.

This study's background is rooted in the operational needs of oil exploration in sediment-prone river systems and the importance of accurate bathymetric data in guiding and evaluating dredging activities. It reflects the intersection of engineering, environmental management, and geospatial science in addressing one of the common but complex challenges in riverine oil field operations.

## 2. Materials and Methods

The materials/Equipment used for this study are listed below

1. TERSUS GNSS RECEIVER (BASE AND ROVER)

2. HI-TARGET GNSS RECEIVER (BASE AND ROVER)

3. LEVEL INSTRUMENT AND STAFF

4. HI-TARGET DIGITAL ECHO SOUNDER AND ACCESSORIES

5. SOUTH DIGITAL ECHO SOUNDER

6. METAL DETECTOR

7. TIDE GUAGE
<figure><img src="image_2.png" /><figcaption>Fig 1. Methodology Flowchart</figcaption></figure>

Survey Control Establishment

Two geodetic control points, designated as OVHR-GPS4 and OVHR-GPS4A, were identified near the main site to serve as reference points for all surveying activities. In-situ integrity checks were performed to assess their stability and suitability for use. Upon successful validation, these points were adopted as the primary control references for the project (Trimble, 2023).
Satellite Constellation Availability

To evaluate satellite visibility and accuracy, the Trimble GNSS Planning tool was used. Parameters included a 10° elevation mask, 45 m antenna height, and coordinates at N 5° 43' 31.1212", E 5° 23' 14.7144". The tool provided:

a. Sky plots for GNSS constellations

b. 24-hour satellite elevation charts

c. 3D Dilution of Precision (DOP)

d. Ionospheric activity forecasts

e. Satellite visibility predictions

<figure><img src="image_3.png" /><figcaption>Fig 2. Sky plot of satellite availability (https://www.gnssplanning.com/#/settings)</figcaption></figure>

These metrics were essential for identifying optimal observation periods and minimizing positioning errors (GNSS Planning Online, 2023; Teunissen & Montenbruck, 2017).
# Tidal Gauge Establishment and Monitoring

A tide gauge was installed and referenced to OVHR-GPS4 using precise leveling methods. A closed leveling loop was conducted through the water line and benchmark using an automatic level. The established vertical control was used to calibrate the tide gauge, which provided data for converting observed depths to mean sea level during both pre- and post-dredging surveys (IHO, 2020).

Tide readings were logged at 15-minute intervals during bathymetric data acquisition and at 10-minute intervals during dredging. These observations were corrected to the Lagos Vertical Datum using tidal reduction techniques consistent with hydrographic standards (Harari et al., 2020).

# Bathymetric Survey and Instrument Calibration

A dual-frequency echo sounder interfaced with a dual-frequency GNSS receiver was employed to capture bathymetric data. Calibration was done via a bar check method at standardized depths to ensure measurement accuracy. Pre-dredge surveys were conducted across the Mayuku Creek slot entrance and Well Slots 8/9 and 13, using systematic sailing lines and 10-meter interval soundings (Garcia et al., 2021).

Post-dredge surveys mirrored the pre-dredge methodology, with real-time RTK GNSS positioning validated using a digital layout of the project area (DFFW 01). Field calibration ensured consistent positional integrity throughout data collection.

# Dredging Operations

Dredging was conducted in two phases:

a. **Dredging to Flotation**: Six swamp excavators and a GNSS-RTK-guided grab dredger were deployed. Long-arm excavators transferred spoil material to a designated dump site. This phase enabled sufficient draft for service craft operations.
b. **Full-Depth Dredging**: Conducted in five designated zones (Area 1, A1, 2, 3, and 4) beginning at Mayuku Creek, this phase targeted deeper sediment removal to design depths (Cheng et al., 2019).

## Data Processing and Visualization

Depth readings were exported in CSV format and corrected using the corresponding tidal gauge records. These corrected depths were then processed in AutoCAD to superimpose them onto the study layout for both pre- and post-dredge visualization (Esri, 2022).

## Interpolation and Surface Modeling

Surface modeling employed the Triangulation with Linear Interpolation method, using Delaunay triangulation to construct triangular meshes connecting survey points. This technique preserved measurement fidelity and ensured smooth surface rendering. It was particularly effective in this study due to the relatively uniform point distribution (Zhou et al., 2021).

## 3. Study Area

The project area is located at Ovhor Field Oil Mining Lease (OML) 38 situated in Sapele LGA, Delta State of Nigeria, and lies between (5.2310E, 6.2120N and 6.3370E, 5.3360N). The area is a typical rainforest with a tidal swamp. The geological formation of the area is classified as possessing peat and soft mud.

The Mayuku River flows through a landscape characterized by tropical vegetation and rich biodiversity. The river supports a variety of fish species, crustaceans, and riparian vegetation, contributing to the ecological health and stability of the region. The surrounding lands are used for agriculture, settlements, and small-scale industries, while mangroves and palms provide natural erosion control along the riverbanks. Human activities have created some environmental pressures
on the river ecosystem, underscoring the need for sustainable management practices to maintain
this
resource.

<figure><img src="image_4.png" /><figcaption>Fig 3. Map showing Study area</figcaption></figure>
## 4. Results and Presentations

<figure><img src="image_5.png" /><figcaption>Fig 4. Map showing 3d depth surface level during the pre-bathymetry survey.</figcaption></figure>
<figure><img src="image_6.png" /></figure>
<caption>Fig 5. Map Showing 3d Depth Surface Level During Post-Bathymetry Survey</caption>

Dredging activities significantly influenced the morphometric characteristics of the study area, as revealed through the comparative analysis of pre-dredging and post-dredging datasets. The processed results were presented using a suite of visual tools, including tables, thematic maps, 3D surface plots, and charts, which collectively illustrated spatial and volumetric transformations. These geospatial outputs were generated using GIS software, enabling accurate assessment and interpretation of terrain modifications.

A key finding from the analysis of the oil well slot area indicates a measurable lateral expansion post-dredging. This morphometric change supports the enhancement of navigational pathways for marine and service vessels within the dredged corridor, aligning with project objectives aimed at improving accessibility to oil well slots.

## Depth Variation and Bathymetric Interpretation

The findings from this bathymetric survey provide a comprehensive assessment of sediment volume, riverbed morphology, and depth variation before and after dredging at the OML 38 oil block site in the Mayuku River. By integrating pre- and post-dredging bathymetric data tied to tidal datum, precise measurements of channel depth and sediment displacement were achieved, vital for evaluating the effectiveness of dredging and its implications for navigability.

The pre-dredging survey indicated uneven bed elevations, with depths averaging 2.0 m to 2.7 m, revealing sediment build-up that limited vessel access and posed navigational hazards. These findings align with observations by Lorenz (2021), who emphasized the risks of sediment accumulation in operational waterways. In some locations, depths fell below 2.0 m, underscoring the need for immediate dredging intervention to support oilfield access and service vessel operations.
Post-dredging bathymetric measurements showed a notable improvement, with depths ranging from 4.0 m to 4.5 m, aligning with the targeted operational depth.

## Volume and Area Analysis

### Volumes

Z Scale Factor: 1

**Total Volumes by:**

Trapezoidal Rule: 1554307.7060392
Simpson's Rule: 1554372.6231285
Simpson's 3/8 Rule: 1553772.8436691

**Cut & Fill Volumes**

Positive Volume [Cut]: 1554307.70604
Negative Volume [Fill]: 0
Net Volume [Cut-Fill]: 1554307.70604

### Areas

**Planar Areas**

Positive Planar Area [Cut]: 2100826
Negative Planar Area [Fill]: 0
Blanked Planar Area: 0
Total Planar Area: 2100826

**Surface Areas**

Positive Surface Area [Cut]: 2121850.2356141
Negative Surface Area [Fill]: 0

<caption>Fig 6. VOLUME COMPUTATION FROM PRE-BATHYMETRIC SURVEY</caption>

### Volumes

Z Scale Factor: 1

**Total Volumes by:**

Trapezoidal Rule: 1702694.2324314
Simpson's Rule: 1703111.1864401
Simpson's 3/8 Rule: 1702862.9364724

**Cut & Fill Volumes**

Positive Volume [Cut]: 1702694.232425
Negative Volume [Fill]: 0
Net Volume [Cut-Fill]: 1702694.232425

### Areas

**Planar Areas**

Positive Planar Area [Cut]: 2100826
Negative Planar Area [Fill]: 0
Blanked Planar Area: 0
Total Planar Area: 2100826

**Surface Areas**

Positive Surface Area [Cut]: 2131826.7566597
Negative Surface Area [Fill]: 0
<caption>Fig 7. VOLUME COMPUTATION FROM POST-BATHYMETRIC SURVEY</caption>

The pre-dredging grid-based survey calculated a cut volume of approximately 1,554,307.706 m³ and a coverage area of 2,121,850.235 m². In comparison, the post-dredging survey revealed a volume of 1,702,694.232 m³ and an area of 2,131,826.757 m². This indicates that approximately 148,386.526 m³ of sediment was removed and 9,976.52 m² of area was added. These figures substantiate the project's success in achieving depth and areal expansion, enhancing the operational capabilities of the dredged waterway.

## Surface Interpolation and Spatial Modeling

Triangulation with Linear Interpolation, employing optimal Delaunay triangulation, was utilized to model the bathymetric surfaces. This method, which connects data points into non-overlapping triangular meshes, was found to be robust for the grid density and point distribution observed in this study. It produced smooth and accurate representations of the riverbed elevation, thereby supporting precise volume estimation and contour generation (Zhou et al., 2021). Further research could investigate alternative interpolation methods—such as Kriging or Inverse Distance Weighting (IDW)—to evaluate comparative performance in similar hydrographic applications.

## 5. Conclusion and Recommendations

Dredging is a widely used practice in shallow-water environments that involves the removal of sediment and debris from the bottom of the waterbody. In order to ensure proper vessel navigation and oil exploration in the proposed area, a review process is necessary. This review process involves data acquisition, processing analysis, and documentation.
The study has shown that with the availability of advanced data processing equipment and software, it is possible to measure and monitor changes in morphometrics with greater accuracy and spatial detail. The data can then be output in a visually appealing and useful format, and easily manipulated. This allows for more informed decision-making and policy formulation related to shallow-water management.

It is important to note that this research should not be seen as an endorsement of the findings presented. Rather, it is a call for more rigorous research into the occurrences presented. This will allow for a better understanding of the impact of dredging on shallow-water environments and will help inform future policy decisions related to this practice.

Furthermore, this work has demonstrated the effectiveness of GNSS-RTK (Global Navigation Satellite System - Real-Time Kinematic) in positioning oil wells and preparing survey locations. Accurate positioning is critical for all activities related to oil well location, surveys, and equipment optimization. Differential GNSS RTK makes use of a base station and a rover that can be mounted on any equipment or platform to provide accurate positioning. This technology has proven to be a valuable tool in the oil exploration process, allowing for more efficient and effective results delivery and optimization.

## Recommendations

### 1. Adoption of Real-Time Monitoring Tools

Future dredging projects should employ real-time depth monitoring tools and RTK-GPS integration to improve the accuracy and efficiency of data acquisition and dredge tracking.
**2. Post-Dredging Environmental Monitoring**

Establish a long-term environmental monitoring plan to observe the rate of sedimentation and ecological impacts following dredging, ensuring sustainable outcomes.

**3. Training and Capacity Building**

Technical personnel involved in hydrographic surveys should be regularly trained on modern data processing tools, geodetic referencing, and hydrodynamic analysis to improve overall data quality.

**4. Integration of Advanced Technologies**

The inclusion of Artificial Intelligence (AI) and Machine Learning (ML) in future studies can support predictive sediment modeling and enhance proactive dredging operations.

**5. Policy Development and Data Sharing**

Stakeholders, including oil companies and regulatory agencies, should collaborate to develop policies that promote the standardization of bathymetric practices and data sharing for national marine and inland water management.

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Nicholas P. Lorenz. (2015) Determining a dredge production constant through Gis
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