**4. Results**

This section presents results from repeated bathymetric surveys to measure and monitor the changes in the lagoon water bed in terms of erosion and accretion. The results were based on the process of achieving bathymetric survey that produced the data, description of the results of vertical profiles in the area that was covered with the acquired data and then the computation of accretion and erosion geomorphologic units in the survey area during the study. Bathymetric survey was carried out on the lagoon to cover a section of the lagoon that was easily accessed based on the manpower and logistic available during the research data collection in wet and dry seasons. The survey covers the western part of the lagoon through to the near-central region.

#### **4.1 Analysis of the lagoon bed dynamics**

Profile analysis was carried out on the bathymetric data of 2008 and 2014 from the lagoon, which were plotted in ArcGIS software by creating 10 profile sections (**Figure 3**) at distance interval of 100 m along the coverage area on the lagoon (profile lines A-A<sup>0</sup> , B-B<sup>0</sup> , C-C<sup>0</sup> , D-D<sup>0</sup> , E-E<sup>0</sup> , F-F<sup>0</sup> , G-G<sup>0</sup> , H-H<sup>0</sup> , I-I<sup>0</sup> and J-J<sup>0</sup> ). This analysis was performed in order to reveal the variability in the lagoon bed elevation patterns and volume dynamic that occur along the profile lines. This method was used by [72] for analysis of beach fill profile, where the result reveals clearly regions of erosion and accretion.

The bathymetric charts (2008 and 2014) were used to depict the changes along each of the profile lines to quantify whether erosion or accretion occurs at a particular location on the lagoon bed. Over the 6-year period, the changes in the lagoon depth were examined and discussed in the subsequent sections. The detailed

#### **Figure 3.**

*Lagos Lagoon with profile lines on the study area. The area on the map with colour blue indicates area covered by the bathymetric survey of 2008, while the area with greenish yellow colour shows area surveyed in 2014.*

in **Table 3** the profile lines with a significant difference between the means of the two samples at the 95% confidence level not containing the value zero (0).

**Profile t-Statistic p-Value** Profile A-A<sup>0</sup> �3.62781 0.00061912\* Profile B-B<sup>0</sup> �1.08967 0.281534 Profile C-C<sup>0</sup> �0.0174967 0.986164 Profile D-D<sup>0</sup> 1.95931 0.060101 Profile E-E<sup>0</sup> �0.180016 0.857449 Profile F-F<sup>0</sup> 1.02115 0.314395

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

The first step in this analysis is to present (**Table 2**) extent of changes on the lagoon water bed that is represented by the change on the two repeated datasets on each of the profiles depth variables on the lagoon (**Figures 4**–**9**). Erosion was very

dredging (**Figures 7**–**9**). The proving evidence that dredging has taken place at the far end of profile D-D' is the huge sand fill area appearing white on the map in **Figure 3**. However, accretion was the common phenomenon at the end of profiles

Movement of shoals (submerged ridge of sand and unconsolidated materials rising from the bed of the lagoon to near water surface, **Figure 6**) was exhibited

dangerous for boats with draft above 1.4 m along the corridor of this transect.

way of each transects. The depth of the infill in each transects (approximately 2.3 m) implies fast sediment accretion inside the lagoon and fast erosion of sediment from the lagoon ecosystem basin. Transect D-D' begins from somewhere closer to

This process is mainly driven by the combination of bank erosion and bank

It could be observed from **Figure 7** that channel lateral migration (the geomorphological process that involves the lateral migration of sediment across floodplain.

, and this could mainly be because of

. This implies that navigation could be very

, there was infill somewhere along the mid-

prominent at the end of profiles D-D' and F-F<sup>0</sup>

around and along the transect C-C<sup>0</sup>

*\*denotes a statistically significant difference. Data for all the transect lines along west-east direction.*

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

*t-test for 2008 bathymetric data against 2014.*

However, along transect D-D<sup>0</sup> and E-E<sup>0</sup>

, C-C<sup>0</sup> and E-E<sup>0</sup> (**Figures 4**–**6** and **8**).

Ogudu channel and ends near Five Cowrie channel.

*Profile section A-A*<sup>0</sup> *showing trend of variation in the repeated bathymetric data.*

A-A<sup>0</sup>

**Figure 4.**

**89**

**Table 2.**

, B-B<sup>0</sup>

comprehensive results in this section are given in two different segments as comparative results of the profiles running through a west-east direction and a southnorth direction on the lagoon. This made use of the depth datasets for the bathymetric data of 2008 and 2014.
