**5.1 Overview**

chart changes from one survey to another. Maximum likely error (MLE) was

*Δzave* is the average of depth changes over the entire survey area.

*Lagoon Environments Around the World - A Scientific Perspective*

*MLE* <sup>¼</sup> <sup>2</sup> � *<sup>Δ</sup><sup>z</sup>*

**4.8 Accounting for uncertainty in the lagoon bed dynamics**

*Δzave* <sup>¼</sup> <sup>2</sup> � <sup>0</sup>*:*<sup>27</sup> 1*:*211

This means that the maximum likely possible error from the two repeated bathymetric data is 45%. The lesser the percentage, the better the surveys and the better the specifications used in the surveys [66]. The computed percentage is allowable for engineers'survey in the coastal area [66]. Hence, for monitoring purpose, the maximum likely error MLE is suitable to detect changes on the

Depth plays a significant role in the monitoring of the lagoon bed dynamics because depth measurement is a key parameter that influences many processes in lagoon water bed dynamics as is the case in coastal changes [77]. This section of the study has produced maps and statistical summaries of the potential risk of losing the lagoon to sediment accretion and that it could be filled up with sand in a few decades. Limitations of the monitoring assessment using repeated hydrographic surveys to serve as the uncertainties, which include the disturbances produced by small vessels and the uncontrolled human activities on the water, cannot easily be accounted for. For this study, the uncertainty in the monitoring assessment was not accounted for because of the short time that was allotted for data gathering and

From the four spatial locations selected for comparative analysis of erosion and accretion variability on the lagoon bed floor (**Figure 18**), three of the locations (Ibeshe, Inlet and Ogudu) show that the areas are prone to accretion more than erosion. Ibeshe area (north eastern) of the lagoon recorded the highest rate of sediment accretion. In

contrast, the lagoon outlet area exhibits more erosion than accretion.

¼ 0*:*446, approximately 45%

*MLE* <sup>¼</sup> <sup>2</sup> � *<sup>Δ</sup><sup>z</sup> Δzave*

where Δz is the change in depth between the different surveys at a point and

Three points were sampled at approximately mid-region on the area where bathymetric data were collected on the lagoon, and depth difference between the two repeated bathymetric data was determined, averaged and recorded as Δz. *Δzave* was determined by taking difference in the depth between the two bathy data at different parts of the study area and ensure these was distributed almost equally over the data coverage, and the mean was taken and recorded as average of depth changes over the entire survey area. The values of the two variables were

> Δz ¼ 0*:*27 m (2) *Δzave* ¼ 1*:*211 m (3)

(1)

(4)

computed as:

computed as:

Therefore,

lagoon bed.

**106**

unavailability of personnel.

This study explores comparative analysis between available two repeated bathymetric data of 2008 and 2014. The findings indicate that overall the Ibeshe region of the lagoon experienced the largest volume of accretion and it has the widest area covered by accretion. Generally speaking, the total accretion was found higher than the erosion that takes place in the lagoon. This gives a signal that the depth of the lagoon is reducing. Joining this finding with the result of Taiwo and Areola [78] that shows loss in the lagoon ecosystem and a gradual reduction in the surface area of the lagoon due to encroachment on its coastline, it can be concluded that as a result of increasing urbanisation, the lagoon is moving toward extinction despite its large area of coverage.
