*4.5.3 The region west of the lagoon: sediment migration around Ogudu Region*

The Ogudu inlet area shows a complicated bed pattern, which potentially endangers small boat movement because of its extremely shallow depth possibly due to the influx of industrial effluents and sediments that have been channelled through the place.

It was impossible to take measurements around the Ogudu Region of the lagoon during the research field data collection; possibly, it could be inferred from the result in Section 4.5.1 that a fast accretion of sediment takes place in the western zone of the lagoon where there is a large human population and industrial settlements are located. This region is where the Ogudu channel brings the largest quantity of sediment into the lagoon.

Generally, the mean difference of the depth value of 2008 and 2014 dataset was found to be extremely small. This was shown in the multiple range tests in **Table 4**

as approximately 0.251, the mean difference of profile C of 2014 and profile D of 2008. This implies that whatever the depths range from the area of this region of the lagoon in 2008 it has been reduced excessively in 2014. The decrease in the depth of the lagoon water bed could likely be increasing as a result of urbanisation that has exposed the majority of the lagoon ecosystem, which invariably causes increased

*Chart showing spatial variability of sediment accretion and erosion on Lagos Lagoon bed based on two repeated*

*Spatial variability of sediment accretion and erosion on Lagos Lagoon water based on 2008 and 2014 repeated*

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

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

*bathymetric data. Area = metres squared and volume = metres cubed.*

*bathymetric data of 2008 and 2014. Area = metres squared and volume = metres cubed.*

Four locations near to urban growth were chosen to take lagoon bed samples. Very significant to the sediments from each of the locations is that their grain sizes

erosion of sediment to the lagoon.

**Figure 19.**

**103**

**Figure 18.**

**4.6 Significance of the accretion spatial variability**


**Table 11.**

*Summary of erosion/accretion calculation on the lagoon water bed.*

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

**Figure 18.**

Volume = area height.

*4.5.2 Evidence base*

through the place.

**Table 11.**

**102**

quantity of sediment into the lagoon.

**Sediment status Volume (m3**

*Summary of erosion/accretion calculation on the lagoon water bed.*

137,429.161 = 858,932.254 height.

Hence sediment gained = 137,429.161**/**858,932.254. Average height of sediment gained = 0.16 m.

*Lagoon Environments Around the World - A Scientific Perspective*

sedimentation, and can easily transform or go into extinction.

there local dredging is going on within the system.

The spatial variability of erosion and accretion on the lagoon bed

*4.5.3 The region west of the lagoon: sediment migration around Ogudu Region*

The Ogudu inlet area shows a complicated bed pattern, which potentially endangers small boat movement because of its extremely shallow depth possibly due to the influx of industrial effluents and sediments that have been channelled

during the research field data collection; possibly, it could be inferred from the result in Section 4.5.1 that a fast accretion of sediment takes place in the western zone of the lagoon where there is a large human population and industrial settlements are located. This region is where the Ogudu channel brings the largest

It was impossible to take measurements around the Ogudu Region of the lagoon

Generally, the mean difference of the depth value of 2008 and 2014 dataset was found to be extremely small. This was shown in the multiple range tests in **Table 4**

Accretion 54,148,636 70,944,744 Erosion 54,011,207 70,085,812 Total accretion or erosion 137,429 858,932

**) Area (m<sup>2</sup>**

**)**

(**Figures 18** and **19**) shows that a large area of about 70,944,744 m2 was submerged into accretion with approximately 54,148,636 m<sup>3</sup> volume of sediment gained around the area. This large sediment deposition gives an indication that change in the lagoon bed is evident, that sediment is drifting constantly into the lagoon through erosion reducing the depth of the lagoon very fast despite the fact that

Between 6 years, the average height of 0.16 m was gained by the lagoon. Going

If the yearly average accretion (0.026 m/year) persists in the lagoon without any dredging/other removal, the study area of the lagoon will have gained a sediment height of 1.3 m in 50 years. Kjerfve and Magill [9] confirm that lagoons are net material sinks and that they are often subject to rapid sedimentation and will transform into other types of environments through sediment infilling and land-use activities. Hence, its time scale of transition since it is geologically rapid can occur within decades to centuries, and the Lagos Lagoon, as is the case with any other lagoon, is susceptible to disappearing after some decades. Kjerfve and Magill [9] use a systematic review approach and concluded that lagoons will quickly transform into other types of coastal environment without using any data to substantiate their inference. However, this aspect of the research has been able to confirm with scientific evidence that the Lagos Lagoon is a net material sinks, subject to rapid

by this rate, it means that in 1 year the height of accretion will be 0.026 m.

*Spatial variability of sediment accretion and erosion on Lagos Lagoon water based on 2008 and 2014 repeated bathymetric data. Area = metres squared and volume = metres cubed.*

#### **Figure 19.**

*Chart showing spatial variability of sediment accretion and erosion on Lagos Lagoon bed based on two repeated bathymetric data of 2008 and 2014. Area = metres squared and volume = metres cubed.*

as approximately 0.251, the mean difference of profile C of 2014 and profile D of 2008. This implies that whatever the depths range from the area of this region of the lagoon in 2008 it has been reduced excessively in 2014. The decrease in the depth of the lagoon water bed could likely be increasing as a result of urbanisation that has exposed the majority of the lagoon ecosystem, which invariably causes increased erosion of sediment to the lagoon.

#### **4.6 Significance of the accretion spatial variability**

Four locations near to urban growth were chosen to take lagoon bed samples. Very significant to the sediments from each of the locations is that their grain sizes


sediments in three of the locations, this could imply the effect of increased stress (through human activities) on the lagoon ecosystem where the habitats live, and hence, their displacement probably leads to their extinction as their habitation is depleted. The sediments around Ebute-Metta show large grain size than any other locations. This could likely be sediments of industrial refuse that are channelled into

**Source Sum of squares Df Mean square F-Ratio P-Value** Model 1.11515E15 1 1.11515E15 2446.67 **0.0000**

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

At Ijede, the grain with the largest percentage of sediment during sieve analysis was silt and sand; hence, the prevailing colours were mostly brown (silt sediment) and grey (sand sediment). The texture of the sample at this location was slightly cohesive and frictional. However, the sample at Ebute-Metta was slightly different from that of Ijede in that there is more cobble sediment at Ebute-Metta than the proportion present in Ijede. The sample at the inlet completely displays sediment that is largely cohesive clay, dark brownish in colour, but completely void of

Further analysis was carried out on quantitative verification of the sediment gain in some part of the lagoon bed using the initial four spatial locations. The volume of sediment accreted in the area was calculated with the coverage area. To establish the relationship that exists between the volume of sediment and area covered, an analysis of variance (ANOVA) was carried out to test whether there is significant difference between the volume of sediment and the area (with 95% confident interval). The result of the ANOVA test is summarised in **Table 13**, which shows that there is a significant difference in the volume of sediment accretion/erosion in

This section outlines the basic procedure that is used for calculating volumetric

errors provided the estimates of the vertical (Δd) are known. If Δd values are unavailable for the specific surveys, standard errors of 0.15, 0.3 or 0.45 m can be used based on the class of survey [66]. For every coastal survey (surveys on lagoons, estuaries, lakes and surveys close to the shore), it is assumed that errors in horizontal positioning (Δx and Δy) are random and have an insignificant effect on the volumes compared with possible errors in water depth measurements, tide

The volumetric error difference between different repeated bathymetric surveys was estimated by determining how much the average depth in each

the lagoon through Ebute-Metta channel where the sample was collected.

cobble-sized sediments from the remains of water snail shells.

Residual 2.27891E12 5 4.55782E11

*ANOVA test on change in sediment deposition in six spatial locations on the lagoon.*

Total (Corr.) 1.11743E15 6

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

**Table 13.**

the area subject to the test.

R-squared = **99.7961%**.

correction and data reduction.

**4.7 Error analysis**

**105**

*4.6.1 Summary of the analysis of variance*

Correlation coefficient = **0.99898.**

Standard error of est. = **675,116.** Mean absolute error = **477,716.**

R-squared (adjusted for d.f.) = **99.7553%**.

#### **Table 12.**

*Sieve analysis of sediment from four spatial locations around the lagoon.*

are very similar both in colour and texture, and **Table 12(i-iv)** shows the summary of the sieve analysis performed on the sediments collected from the four locations. The results show the composition of the whitish shell as a major boulder or cobble

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


**Table 13.**

*ANOVA test on change in sediment deposition in six spatial locations on the lagoon.*

sediments in three of the locations, this could imply the effect of increased stress (through human activities) on the lagoon ecosystem where the habitats live, and hence, their displacement probably leads to their extinction as their habitation is depleted. The sediments around Ebute-Metta show large grain size than any other locations. This could likely be sediments of industrial refuse that are channelled into the lagoon through Ebute-Metta channel where the sample was collected.

At Ijede, the grain with the largest percentage of sediment during sieve analysis was silt and sand; hence, the prevailing colours were mostly brown (silt sediment) and grey (sand sediment). The texture of the sample at this location was slightly cohesive and frictional. However, the sample at Ebute-Metta was slightly different from that of Ijede in that there is more cobble sediment at Ebute-Metta than the proportion present in Ijede. The sample at the inlet completely displays sediment that is largely cohesive clay, dark brownish in colour, but completely void of cobble-sized sediments from the remains of water snail shells.

Further analysis was carried out on quantitative verification of the sediment gain in some part of the lagoon bed using the initial four spatial locations. The volume of sediment accreted in the area was calculated with the coverage area. To establish the relationship that exists between the volume of sediment and area covered, an analysis of variance (ANOVA) was carried out to test whether there is significant difference between the volume of sediment and the area (with 95% confident interval). The result of the ANOVA test is summarised in **Table 13**, which shows that there is a significant difference in the volume of sediment accretion/erosion in the area subject to the test.

#### *4.6.1 Summary of the analysis of variance*

Correlation coefficient = **0.99898.** R-squared = **99.7961%**. R-squared (adjusted for d.f.) = **99.7553%**. Standard error of est. = **675,116.** Mean absolute error = **477,716.**

## **4.7 Error analysis**

This section outlines the basic procedure that is used for calculating volumetric errors provided the estimates of the vertical (Δd) are known. If Δd values are unavailable for the specific surveys, standard errors of 0.15, 0.3 or 0.45 m can be used based on the class of survey [66]. For every coastal survey (surveys on lagoons, estuaries, lakes and surveys close to the shore), it is assumed that errors in horizontal positioning (Δx and Δy) are random and have an insignificant effect on the volumes compared with possible errors in water depth measurements, tide correction and data reduction.

The volumetric error difference between different repeated bathymetric surveys was estimated by determining how much the average depth in each

are very similar both in colour and texture, and **Table 12(i-iv)** shows the summary of the sieve analysis performed on the sediments collected from the four locations. The results show the composition of the whitish shell as a major boulder or cobble

**Size of mesh (mm) Sediment weight (g) Remark**

*Lagoon Environments Around the World - A Scientific Perspective*

2.36 2.5 100% whitish brown shell 1.18 1.8 Whitish brown 600 μm 3.1 Whitish brown

2.36 8.22 90% whitish shell

600 μm 12.05 Brownish grains

300 μm 5.64 Dark and brownish grains 212 μm 10.79 Dark and brownish grains 150 μm 12.95 Dark and brownish grains

2.36 1.7 100% whitish shell 1.18 1.63 60% brown pebbles

 μm 14.27 Grains with black patches μm 16.44 Grains with black patches μm 21.85 Brown with dark grains μm 13.26 Brown with dark grains μm 7.66 Darkish brown grains

1.18 0.12 Blackish grains 600 μm 1.28 Blackish grains 425 μm 0.63 Blackish grains

*Sieve analysis of sediment from four spatial locations around the lagoon.*

 μm 0.78 Blackish grains + whitish patches μm 1.00 Blackish grains + whitish patches μm 1.19 Blackish grains with shinning whitish grains μm 26.26 Dark brownish with whitish grains

600 μm 20.46 99% brown pebbles with traces of whitish grains

1.18 6.07 White shell and 60% brownish grains

**(i) Five Cowries**

(ii) Ebute-Meta

(iii) Ijede

(iv) Inlet

**Table 12.**

**104**

425 μm 3.6 300 μm 10.4 212 μm 29.0 150 μm 24.0 75 μm 23.7

425 μm 4.97

75 μm 21.08

2.36 0

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

$$\text{MLE} = \frac{\text{2} \times \Delta z}{\Delta z\_{\text{ave}}} \tag{1}$$

Considering the degree of accretion on the lagoon water bed and the impact it

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

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

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

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

will have on the lagoon and its ecosystem, it is clear that consistent repeated bathymetric data will be suitable to monitor the dynamics of the lagoon bed. In further investigation, there is need for a multi-beam hydrographic data with a high

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

accuracy of depth values.

**5.1 Overview**

area of coverage.

**107**

**5. Discussion and conclusions**

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

**5.2 Dynamics of the lagoon sea bed**

between the repeated surveys was performed.

disappearance of the lagoon some decades from now.

where Δz is the change in depth between the different surveys at a point and *Δzave* is the average of depth changes over the entire survey area.

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 computed as:

$$
\Delta \mathbf{z} = \mathbf{0}.27 \,\mathrm{m} \tag{2}
$$

$$
\Delta z\_{\text{ave}} = \mathbf{1.211m} \tag{3}
$$

Therefore,

$$\begin{aligned} \text{MLE} &= \frac{2 \times \Delta z}{\Delta z\_{\text{ave}}} \\ &= \frac{2 \times 0.27}{1.211} \\ &= 0.446, \text{ approximately } 45\% \end{aligned} \tag{4}$$

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 lagoon bed.

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

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 unavailability of personnel.

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.

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

Considering the degree of accretion on the lagoon water bed and the impact it will have on the lagoon and its ecosystem, it is clear that consistent repeated bathymetric data will be suitable to monitor the dynamics of the lagoon bed. In further investigation, there is need for a multi-beam hydrographic data with a high accuracy of depth values.
