The working methodology used to achieve our results is based on the use of several participatory tools from different methods. We made this choice to allow strong involvement of the population and the authorities in the diagnosis of problems and in the formulation of solution approaches.

The institutional anchoring phase was based on hearings with the competent authorities of the town hall, the prefecture, the ministry and delegates to the populations. It was necessary to explain in detail the goals, objectives and expected results of our research work in order to benefit from their agreements, their research authorizations and to facilitate the participation of the population in the overall success of the project.

The documentary research phase constituted the prelegomena of our project because it made it possible to collect the information essential to the development of our work problem. To achieve this, we first had to list our information needs, then prioritize them. We had to gather documents from other related sciences that could help us achieve this task. The approach consisted of an initial collection of field data from the appropriate administrations (Ministry, University, Prefecture, Town Hall). To carry out the second we used the archives of the WAPCO company.

Once the site was chosen, we carried out series of descents on the ground. They allowed us to better familiarize ourselves with our work site, to take ownership of it to know and understand its components and to derive an inventory. During this reconnaissance phase, surveys in public services and interviews with resource people took place. They helped justify the choice of site and theme.

Built between 2019 and 2022 by the Chinese company WAPCO, the Sèmè-Podji wharf built as part of the Niger – Benin pipeline project (export of oil from Niger to other countries) has at its end (sea side) a quay allowing the docking of the three launches available to the company, to carry out operations at sea. The structure with a total length of 666.96 meters is made up of:

- a maritime bridge with a length of 257.3 meters and a width of 25 meters comprising three rows of reinforced concrete piles with a diameter of 1.20 meters, each row being crowned by a reinforced concrete beam of prefabricated elements keyed by concreting on the piles;

- a protective dike with a length of 409.66 meters and a width of 18 meters;

- a quay with a length of 60 meters and a width of 25 meters, comprising three rows of reinforced concrete piles with a diameter of 1.20 meters, each row being crowned by a reinforced concrete beam of keyed prefabricated elements by concreting on the piles. Figure 3 and Figure 4 respectively present the plan view of the wharf and the cross section of the maritime bridge of the wharf of the Sèmè-Podji pipeline which is the subject of our study.

Built between 1994 and 1998 by the NOVAPONTE group (Campenon Bernard SGE pilot), the structure with a total length of 12,300 meters is made up as follows:

- a North viaduct, 488 m long (11 spans with an average of 45 m). The piles are built on piles 1 m in diameter.

- an Expo viaduct 672 m long (12 spans from 45 m to 62 m). The piles are built on piles 1.70 m in diameter.

- a main cable-stayed bridge 820 m long, with a central span of 420 m above the main navigable channel. The deck is held transversely by three double piers on each side of the main span. The foundations are made of 2.20 m piles.

- a central viaduct 6531 m long, made up of 09 viaducts of 09 spans of approximately 80 m. Each navigable channel has special spans. The piers are built on piles with a diameter of 1.70 m for standard spans, and 2.20 m for navigation spans.

- a South viaduct, 3825 m long, has a section similar to that of the North viaduct. The piles are built on piles of 1.80 m or 2 m in diameter.

Figure 5 and Figure 6 respectively present the plan views of the project and the general view of said project identified as a control project within the framework of this study.

The wharf of the Sèmè-Podji pipeline project being a recent work (carried out between 2019 and 2022), the diagnosis of the latter was made based on the structure because no disorder was observed in the materials. The program for this diagnosis is established from the analysis of available input data—execution plans—bathymetric data—discussions with the manager and a preliminary visit to the site.

The comparative analysis of the geometric measurements of the quay of the Sèmè-Podji pipeline project and the maritime piles of the Wasco da Gama bridge, both maritime works, revealed an under-dimensioning at the level of the spans of the quay bridge of the Sèmè-Podji pipeline project. Podji, especially since for the Sèmè-Podji quay on spans of 7 m in length, a value which is well below 45 m and 62 m considered as minimum and maximum values recorded at the Wasco de Gama bridge taken as project witnesses.

Bringing the piles closer together (small spans) reduces the current which promotes the accumulation of sediment.

The wharf of the Sèmè-Podji pipeline project has completely modified the natural hydrology. It no longer experiences a flow capable of setting in motion the sands accumulated since at least 2022. This element appears to be a fundamental characteristic explaining the erosion observed to the east towards Nigeria.

The main action is to promote the transport of sediments by removing all the sediments accumulated under the wharf bridge. Two other means of action can be cited: increasing the quantities of missing sediments and moving the quantities of excess sediments.

It seems necessary to define the state that is desired for the Sèmè-Podji coastline in the different sectors of the pipeline project wharf.

It appears essential in the short term to return to the initial state of the coastline ( Figure 7 ), that is to say, to remove all the sediments accumulated under the wharf bridge in order to once again promote the achievement of the state of balance.

We can increase the quantities of missing sediment and move the quantities of excess sediment, by using nautical means such as dredges ( Figure 8 ). This action was proposed by the wharf manager, the Chinese company WAPCO. The company has identified five sand sampling areas for a volume of 398,024 m³ ( Figure 9 ). The coordinates of the points delimiting these zones, their surfaces and the volumes of sand which can be dredged there are presented in Table 1 and Table 2 . These volumes of sand will only allow part of a beach of length 600 ml to be recharged on an average width of 67 ml ( Figure 9 ). The coordinates of this recharged part are presented in Table 3 . These five identified areas are easy to access and are in the vicinity of the areas to be recharged.

Source: WAPCO, 2023.

Source: WAPCO, 2023.

Source: WAPCO, 2023.

Depending on the direction of the incident swell, breakwater or groyne batteries can be used.

The breakwater is a tool for managing the longitudinal coastline to protect the coast from transverse swells ( Figure 10 ). Placed on the foreshore or further offshore, the breakwater aims to break swells perpendicular to the coast in order to attenuate their force.

Groynes are coastal structures designed to retain sediment by preventing it from moving along the shoreline and to help maintain wide beaches by minimizing or slowing erosion [5] . It is a transversal shoreline management tool ( Figure 11 ) relying on currents to modify the coastline. Placed on the foreshore, the groyne aims to break the dynamics of coastal drift. Cross-shore currents are not affected by this type of structure.

The solutions proposed by the manager of the Sèmè-Podji wharf, the company WAPCO, in order to combat the phenomenon of erosion to the east of the structure are not sustainable. In-depth studies will have to be carried out in order to define, depending on the direction of the incident swell, other solutions such as the use of breakwater batteries, groynes, etc.

A combination of all the solutions defined in this article will make it possible to effectively combat the phenomenon of growing erosion that we are currently observing in the commune of Sèmè-Podji towards Nigeria, with the aim of avoiding diplomatic incidents.