**4.2 West to east profiles**

Detailed analyses were performed on transects that were created by considering west to east direction to indicate changes along the north-south direction on the lagoon bed. The essence of creating the west to east direction profile lines is to ascertain the trend of changes on the lagoon bed moving southward from the freshwater inlets in the north where major sediments from upland intrude into the lagoon. Thus, this analysis determines if there is a significant variation on each of the profile lines on 2008 and 2014 data moving from the north to the south. Therefore, the hypothesis is set as follows to examine if there are significant changes in the lagoon water bed topography:


In testing the hypothesis, this study carried out t-test to test the significant variation of the depth variables of the two repeated bathymetric data that produced the result of the changes on the lagoon water bed between 2008 and 2014 for the section covered on the lagoon. The t-test compares the actual difference between the means of the two samples: depth of 2008 data and the depth of 2014 data. It constructs confidence intervals or bounds for each mean and for the difference between the means. Of particular interest is the confidence interval for the ratio of the variances that extend between particular ranges of value, and the results show

*Morphodynamics in a Tropical Shallow Lagoon: Observation and Inferences of Change DOI: http://dx.doi.org/10.5772/intechopen.90189*


*\*denotes a statistically significant difference.*

*Data for all the transect lines along west-east direction.*

#### **Table 2.**

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 bathy-

*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.*

*Lagoon Environments Around the World - A Scientific Perspective*

Detailed analyses were performed on transects that were created by considering west to east direction to indicate changes along the north-south direction on the lagoon bed. The essence of creating the west to east direction profile lines is to ascertain the trend of changes on the lagoon bed moving southward from the freshwater inlets in the north where major sediments from upland intrude into the lagoon. Thus, this analysis determines if there is a significant variation on each of the profile lines on 2008 and 2014 data moving from the north to the south. Therefore, the hypothesis is set as follows to examine if there are significant

1.*H*0: There is no significant difference between the 2008 bathymetric data sample and the 2014 bathymetric data sample in predicting changes in the

In testing the hypothesis, this study carried out t-test to test the significant variation of the depth variables of the two repeated bathymetric data that produced the result of the changes on the lagoon water bed between 2008 and 2014 for the section covered on the lagoon. The t-test compares the actual difference between the means of the two samples: depth of 2008 data and the depth of 2014 data. It constructs confidence intervals or bounds for each mean and for the difference between the means. Of particular interest is the confidence interval for the ratio of the variances that extend between particular ranges of value, and the results show

2.*H*1: There is significant difference in the 2008 bathymetric data sample and the

metric data of 2008 and 2014.

changes in the lagoon water bed topography:

**4.2 West to east profiles**

**Figure 3.**

lagoon bed.

**88**

2014 bathymetric data.

*t-test for 2008 bathymetric data against 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).

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 prominent at the end of profiles D-D' and F-F<sup>0</sup> , and this could mainly be because of 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 A-A<sup>0</sup> , B-B<sup>0</sup> , C-C<sup>0</sup> and E-E<sup>0</sup> (**Figures 4**–**6** and **8**).

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 around and along the transect C-C<sup>0</sup> . This implies that navigation could be very dangerous for boats with draft above 1.4 m along the corridor of this transect. However, along transect D-D<sup>0</sup> and E-E<sup>0</sup> , there was infill somewhere along the midway 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 Ogudu channel and ends near Five Cowrie channel.

It could be observed from **Figure 7** that channel lateral migration (the geomorphological process that involves the lateral migration of sediment across floodplain. This process is mainly driven by the combination of bank erosion and bank

**Figure 4.** *Profile section A-A*<sup>0</sup> *showing trend of variation in the repeated bathymetric data.*

**Figure 5.** *Profile section B-B*<sup>0</sup> *showing trend of variation in the repeated bathymetric data.*

**Figure 6.**

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

deposition over time. Hence, channel's change is driven by sediment transport) occurred at the end of transect D-D<sup>0</sup> toward Five Cowrie channel. Comparing this result with existing literature [73–75], it could be confirmed that lateral migration that occurred at this region that is as a result of the lagoon bank erosion and sedimentation depends upon the ecology of the watershed corridors of the lagoon ecosystem. Hence, the volume of sediment eroded from the watershed corridors is shown to be largely a function of the watershed size and grain size of sediment at the base of the outer bank. Consequently, it appears that bank erosion and channel migration are basically problems of sediment entrainment, which is dependent on

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

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

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

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

Transect F-F<sup>0</sup> was characterised by channel movement (which in this case is the up and down meandering of the lagoon bottom morphology), the channel migration by erosion on one side leads to deposition towards the Lagos Island side of the transect; however, toward the end of the transect, there was dredging. This was

total flow from the watershed and sediment size.

**Figure 8.**

**Figure 9.**

**91**

**Figure 7.** *Profile section D-D*<sup>0</sup> *showing trend of variation in the repeated bathymetric data.*

*Morphodynamics in a Tropical Shallow Lagoon: Observation and Inferences of Change DOI: http://dx.doi.org/10.5772/intechopen.90189*

**Figure 8.** *Profile section E-E*<sup>0</sup> *showing trend of variation in the repeated bathymetric data.*

**Figure 9.** *Profile section F-F*<sup>0</sup> *showing trend of variation in the repeated bathymetric data.*

deposition over time. Hence, channel's change is driven by sediment transport) occurred at the end of transect D-D<sup>0</sup> toward Five Cowrie channel. Comparing this result with existing literature [73–75], it could be confirmed that lateral migration that occurred at this region that is as a result of the lagoon bank erosion and sedimentation depends upon the ecology of the watershed corridors of the lagoon ecosystem. Hence, the volume of sediment eroded from the watershed corridors is shown to be largely a function of the watershed size and grain size of sediment at the base of the outer bank. Consequently, it appears that bank erosion and channel migration are basically problems of sediment entrainment, which is dependent on total flow from the watershed and sediment size.

Transect F-F<sup>0</sup> was characterised by channel movement (which in this case is the up and down meandering of the lagoon bottom morphology), the channel migration by erosion on one side leads to deposition towards the Lagos Island side of the transect; however, toward the end of the transect, there was dredging. This was

**Figure 7.**

**90**

**Figure 6.**

**Figure 5.**

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

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

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

*Lagoon Environments Around the World - A Scientific Perspective*

confirmed by visual observation during data collection, as serious local dredging was going on in the area by those who are constructing near the lagoon bank.

**Contrast Sig Difference Limits** A 2008-A 2014 0.425517 0.686908 A 2008-B 2008 0.262744 0.721798 A 2008-B 2014 0.0548276 0.721798 A 2008-C 2008 \* **0.890828** 0.831887 A 2008-C 2014 \* **0.885494** 0.831887 A 2008-D 2008 0.634828 0.831887 A 2008-D 2014 \* **1.37749** 0.831887 A 2008-E 2008 \* **1.33233** 0.591565 A 2008-E 2014 \* **1.28283** 0.591565 A 2008-F 2008 \* **2.01205** 0.784867 A 2008-F 2014 \* **2.83594** 0.784867 A 2014-B 2008 0.688261 0.721798 A 2014-B 2014 0.480345 0.721798 A 2014-C 2008 \* **1.31634** 0.831887 A 2014-C 2014 \* **1.31101** 0.831887 A 2014-D 2008 \* **1.06034** 0.831887 A 2014-D 2014 \* **1.80301** 0.831887 A 2014-E 2008 \* **1.75784** 0.591565 A 2014-E 2014 \* **1.70834** 0.591565 A 2014-F 2008 \* **2.43757** 0.784867 A 2014-F 2014 \* **3.26146** 0.784867 B 2008-B 2014 0.207917 0.755078 B 2008-C 2008 0.628083 0.860921 B 2008-C 2014 0.62275 0.860921 B 2008-D 2008 0.372083 0.860921 B 2008-D 2014 \* **1.11475** 0.860921 B 2008-E 2008 \* **1.06958** 0.631744 B 2008-E 2014 \* **1.02008** 0.631744 B 2008-F 2008 \* **1.74931** 0.815577 B 2008-F 2014 \* **2.57319** 0.815577 B 2014-C 2008 0.836 0.860921 B 2014-C 2014 0.830667 0.860921 B 2014-D 2008 0.58 0.860921 B 2014-D 2014 \* **1.32267** 0.860921 B 2014-E 2008 \* **1.2775** 0.631744 B 2014-E 2014 \* **1.228** 0.631744 B 2014-F 2008 \* **1.95722** 0.815577 B 2014-F 2014 \* **2.78111** 0.815577 C 2008-C 2014 0.005333 0.955106 C 2008-D 2008 0.256 0.955106

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

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

**93**
