**2.2 The response of coastal lagoon to sea level rise (SLR)**

The fourth Intergovernmental Plan on Climate Change (IPCC) report (AR4) projected the estimate of sea level rise for this century that it could likely range from 18 to 59 cm [52]. However, the estimation of IPCC's AR4 did not include the contributions from Greenland and Antarctica [53]. Basically, the actual rise may be higher or lower than the projection of IPCC. Hence, there is uncertainty in the estimation of sea level rise; this dilemma in the rise projection could be as a result of variation in the greenhouse gas both now and in the future. Climate model of IPCC 2001 report indicates spontaneous rise in the annual global mean temperatures [54, 55].

Sea level is raised by warmer temperature that melts the glacier ice sheets, the melted ice sheet is discharged into the ocean and this in turn increases and expands the volume of the ocean water, which splits into the enclosed water bodies like the lagoons and the estuaries and increases the water level in the systems [53]. The effect of increasing sea level brings negative hazards for coastal areas, including increased erosion, increased flooding/submergence, increased salinisation and threats to coastal cities in terms of storm surges, and all these could create direct negative impact on the urban coastal communities, wetlands, coastal ecosystem and the various infrastructural development around the coast [56–59]. Due to the negative effect of sea level rise, scientists and coastal policy makers face the challenge of

a semi-diurnal tidal regime; as urbanisation and human activities around the lagoon increase, the volume of sediment that is entering into the basin is believed to be increasing on a daily basis. Changes in the Lagos Lagoon water bed over 6 years' time scale using repeated bathymetric data (2008 and 2014) are presented in this section. Bathymetric surveys were carried out on the Lagos Lagoon to cover some section of the lagoon that was easily accessed based on the manpower and logistic available during the research data collection in the wet seasons. The surveys primarily focus on the western part of the lagoon through to the near-central region. The survey vessel (length—5.84 m, width—1.69 m) was equipped with a single beam echo sounder (frequency—200 kHz, model—SDE-285 Single Frequency Digital Echo sounder, type—South) for collection of bathymetric data on Lagos Lagoon. Initially, an overview of the process of acquiring the bathymetric data that was used in the research is outlined. The procedure of the bathymetry and data reduction is followed by analysis of the lagoon water bed dynamics using abstracted profile lines from the bathymetric data. The results of the significant accretion and erosion inside the lagoon were analysed spatially to quantify the volume of sediment gain or loss on the lagoon water floor; this enhanced the possibility of verifying if the lagoon is gradually disappearing. This aspect of the research, to the best of the authors' knowledge, reveals for the first time the various kinds of evolutionary changes (channel movement, accretion, erosion, infill and movement of shoal) on

*Morphodynamics in a Tropical Shallow Lagoon: Observation and Inferences of Change*

*DOI: http://dx.doi.org/10.5772/intechopen.90189*

This section presents the procedures utilised for gathering bathymetric data used in the analysis of the lagoon bed geomorphology. Hydrographic charting has always been of critical concern for navigation; however, bathymetric survey charts are often out of date due to geomorphic changes in many submarine areas, which most of the time occur rapidly [66], and also lack the detailed resolution required for scientific research level studies. On some navigation charts, it is highly possible that 10 years old bathymetry and the marked depths might have all changed considerably during the period since the chart was first published. This is especially relevant in the areas of strong current activity, of a mass movement, and where there is strong storm activity, as fast changes could be highly likely. Water depths are measured by both direct contact procedures and acoustic methods, and this research made use of a bathymetric chart that was obtained directly with the use of single beam echo sounder. Acoustic depth sounders measure the elapse time an acoustic pulse takes to travel from a generating transducer to the seafloor and back, and with the velocity of sound in water known, the travel time of the reflected wave can be measured and converted into distance. With the use of the single beam echo sounder, the section of the lagoon covered in this study was sounded in October 2014 taking note of the reference datum used in the bathymetric survey of the

The depth data acquired were referenced to the local chart datum used in Nigeria (Lagos 1955 height). However, tidal height readings were not measured during the course of the bathymetry survey relative to chart datum at a tidal station (because of security challenge and lack of personnel). Hence, predicted tidal values were used to reduce the measured depth to chart datum. The tidal heights are a variation in the sea level that is associated with the gravitational forces maintaining the sun, moon and the earth in their orbits [67, 68]. The reduction of soundings

the lagoon water bed.

lagoon in 2008.

**85**

**3.3 Reduction of soundings to chart datum**

**3.2 Bathymetric survey**

**Figure 1.**

*Map showing Lagos Lagoon as situated in Lagos within Nigeria. The lagoon is surrounded by settlements (local government areas).*

understanding how the sea level rise will affect the coastal area and the best management plan that can enhance sustainability [60]. If the sea level rise proceeds at the present rate, it may lead to submergence of most of the coastal lagoons turning it to part of the ocean.
