**5.2 Dynamics of the lagoon sea bed**

A lagoon system and its adjacent basins are dynamic on different spatial and temporal scales. As human activities increase with increased urbanisation, the volume of sediment accreting into the lagoon is assumed to be increasing on daily basis. This, in turn, influences the natural morphology of the lagoon coastline. Van Der Wal and Pye [79] investigated the morphological changes in estuaries with the use of historical bathymetric charts. Again, Hicks and Hume [80] determined sand volume and bathymetric changes on an ebb-tidal delta using repeated bathymetric surveys and they were able to detect net sand gains or losses over the ebb-tidal delta. The repeated bathymetric surveys were treated independently even though they were plotted together on the same ArcGIS interface. They exhibited that the accuracy of the surface-fitting and determinations of mean surface levels varied depending on the local sea bed topography [80]; hence, to avoid error and uncertainty, an interpolation method (kriging) that supported the local geographic spread of the data was adopted. A triangular irregular network (TIN) was chosen because it incorporates original height (Z) values not estimates; hence, the calculation of volumes at different spatial locations and differences in mean bed levels between the repeated surveys was performed.

The result shows that over a 6-year period that the repeated bathymetric data covered, the lagoon decreased in depth by an average of 0.16 m (0.026 m/year). Without any dredging or other removal, the study area of the lagoon will have gained 1.3 m of sediment in a 50-year period. Indeed, this result supports Kjerfve [32], Kjerfve [25] and Barnes [8] who said lagoons are short lived in geological time. This fact assisted to understand the choice of data type (temporal scale data) that is fit to detect short-term changes in any lagoon as it was in the research case study area. Hence, a proper monitoring measure must be taken to avert the sudden disappearance of the lagoon some decades from now.

The results in this section are also supported by Van Der Wal and Pye [79] that indicated repeated and sequential bathymetric mapping or bed surveys can be used to calculate erosion rates and sedimentation. Sources of error and uncertainty are due mainly as a result of the surveying techniques used [81], the density of depth sampling points [82], interpolation and averaging [83] during compilation. The error and uncertainty due to survey methods and density of depth sampling are cared for during the survey exercise, while the careful choice of the interpolation method helps to reduce the uncertainty that could result from interpolation. Documentation on the sea bed morphological development of a lagoon is often needed to support its management, such as navigation, flood defence and habitat preservation, and the effects of changes in natural forcing factors (sea level rise) on the lagoon ecosystem. The present rate of change in the lagoon sea floor must be made a baseline for assessing historic evolution in order to understand and predict its sea bed dynamic trend. However, this demands both reliable data and consistent effective survey methods.

major part of research objective that aims at assessing the dynamic nature of the

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

This chapter has been able to sum up its findings in this research that Lagos Lagoon is highly vulnerable to morphodynamic changes, and these changes include, as investigated in this research, interaction and the adjustment of its floor topography, and sequences of change involving the lagoon spatial sediment. Hence, it has been discovered from the research finding that the lagoon faces the challenge of

Mitigating the potential effects of morphological and hydrodynamic changes on

The use of a functional mechanism to build a model for detecting the coastline changes of the lagoon was made possible with the application of ArcGIS 10.1. The model derived has been useful to ascertain the degree of transgression and regression of the lagoon coastline. From literature, it was discovered in 2010 that the

disappearing. Likewise, in the lagoon seafloor, specifically in the region used as a case study, the depth has decreased by an average of 0.16 m (0.026 m/year). By implication, without any dredging, the study area will have gained 1.3 m of sedi-

For better management and sustainability of the lagoon, consistent measurement should go on henceforth especially measurement regarding bathymetry sur-

The authors are grateful to the Tertiary Education Fund (TETFUND) under the

Prof. Manning's contribution towards this research (book chapter) was made possible in part by a grant from The Gulf of Mexico Research Initiative (CSOMIO: Consortium for Simulation of Oil-Microbial Interactions in the Ocean) and in part by the US National Science Foundation under grant OCE-1736668 and HR Wallingford company research FineScale project (ACK3002\_62). Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org (DOI: 10.7266/n7-0sht-6s68). The authors wish to thank Dr Leiping Ye for his kind assistance with the data

Federal Government of Nigeria and Surveyor Registration Council of Nigeria (SURCON) that provided the funding for this research. Thanks to Professor Andrew Manning for his consistent encouragement and effort to ensure that this research is published. Dr Victor Abbott and Prof. Richard Whitehouse are commended for their kind assistance while the research was on-going.

. However, the results of the model revealed the

. Hence, the lagoon is gradually

sustainability and extinction due to poor planning across its ecosystem.

a lagoon is a controversial issue, with many unanswered questions and a great

lagoon and assesses what effect the changes induce.

**6. Summary and recommendations**

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

portion of uncertainty.

lagoon surface area was 208 km<sup>2</sup>

ment during a 50-year period.

**Acknowledgements**

archive uploading.

**109**

vey, flow and mixing in the lagoon.

present surface area to be approximately 204 km<sup>2</sup>

## **5.3 Sea-level rise and its impact on the coastal lagoon**

Numerous possible responses to sea level rise abound among which are inundation and flooding [55, 58, 84–86]. Prospective studies that focus on identifying the complex nature of the changes along the Nigeria coast should precede assessment of sea level; hence, the two combined can be evaluated to see the effect of sea level rise on the lagoon and other lagoons bounded along the Nigerian coastline. This is because the same rate of sea level rise scenario could bring different degrees of impact on different spatial locations on the coastline.
