**3.1 Overview**

In general, the lagoon system and its adjacent tidal basins exhibit dynamics that are significantly different on both spatial and temporal scales. This is expected from *Morphodynamics in a Tropical Shallow Lagoon: Observation and Inferences of Change DOI: http://dx.doi.org/10.5772/intechopen.90189*

a semi-diurnal tidal regime; as urbanisation and human activities around the lagoon increase, the volume of sediment that is entering into the basin is believed to be increasing on a daily basis. Changes in the Lagos Lagoon water bed over 6 years' time scale using repeated bathymetric data (2008 and 2014) are presented in this section. Bathymetric surveys were carried out on the Lagos Lagoon to cover some section of the lagoon that was easily accessed based on the manpower and logistic available during the research data collection in the wet seasons. The surveys primarily focus on the western part of the lagoon through to the near-central region. The survey vessel (length—5.84 m, width—1.69 m) was equipped with a single beam echo sounder (frequency—200 kHz, model—SDE-285 Single Frequency Digital Echo sounder, type—South) for collection of bathymetric data on Lagos Lagoon. Initially, an overview of the process of acquiring the bathymetric data that was used in the research is outlined. The procedure of the bathymetry and data reduction is followed by analysis of the lagoon water bed dynamics using abstracted profile lines from the bathymetric data. The results of the significant accretion and erosion inside the lagoon were analysed spatially to quantify the volume of sediment gain or loss on the lagoon water floor; this enhanced the possibility of verifying if the lagoon is gradually disappearing. This aspect of the research, to the best of the authors' knowledge, reveals for the first time the various kinds of evolutionary changes (channel movement, accretion, erosion, infill and movement of shoal) on the lagoon water bed.

### **3.2 Bathymetric survey**

understanding how the sea level rise will affect the coastal area and the best management plan that can enhance sustainability [60]. If the sea level rise proceeds at the present rate, it may lead to submergence of most of the coastal lagoons turning

*Map showing Lagos Lagoon as situated in Lagos within Nigeria. The lagoon is surrounded by settlements (local*

The Lagos Lagoon (**Figure 1**) is the largest of the four lagoon systems of the Gulf of Guinea [61, 62]. The lagoon complex stretches from Cotonu in the Republic of Benin and extends to the borders of the Niger Delta in Nigeria along its 257 km course [63], longitude 3° 3″ and 3° 53″ E and latitude 6° 26″ and 6° 37″ N. It is a shallow region of water with constrained movement in a micro-tidal environment. Fresh water from upland is fed into the lagoon from the northern part of the system by Ogun River, with a host of other smaller rivers as well as tidal creeks [17]. It discharges in the south into the South Atlantic Ocean through the Lagos Harbour. The vastness of the lagoon may easily hide the many shallow places present within the system [64]. The lagoon system is the final basin of a number of industrial discharges/effluents from the surrounding industries and run-offs at the Lagos

In general, the lagoon system and its adjacent tidal basins exhibit dynamics that are significantly different on both spatial and temporal scales. This is expected from

Metropolis [65] and there is high urbanisation along the coastline.

it to part of the ocean.

**Figure 1.**

*government areas).*

**3. Methodology**

**3.1 Overview**

**84**

**2.3 Overview of Lagos Lagoon (Nigeria)**

*Lagoon Environments Around the World - A Scientific Perspective*

This section presents the procedures utilised for gathering bathymetric data used in the analysis of the lagoon bed geomorphology. Hydrographic charting has always been of critical concern for navigation; however, bathymetric survey charts are often out of date due to geomorphic changes in many submarine areas, which most of the time occur rapidly [66], and also lack the detailed resolution required for scientific research level studies. On some navigation charts, it is highly possible that 10 years old bathymetry and the marked depths might have all changed considerably during the period since the chart was first published. This is especially relevant in the areas of strong current activity, of a mass movement, and where there is strong storm activity, as fast changes could be highly likely. Water depths are measured by both direct contact procedures and acoustic methods, and this research made use of a bathymetric chart that was obtained directly with the use of single beam echo sounder. Acoustic depth sounders measure the elapse time an acoustic pulse takes to travel from a generating transducer to the seafloor and back, and with the velocity of sound in water known, the travel time of the reflected wave can be measured and converted into distance. With the use of the single beam echo sounder, the section of the lagoon covered in this study was sounded in October 2014 taking note of the reference datum used in the bathymetric survey of the lagoon in 2008.

## **3.3 Reduction of soundings to chart datum**

The depth data acquired were referenced to the local chart datum used in Nigeria (Lagos 1955 height). However, tidal height readings were not measured during the course of the bathymetry survey relative to chart datum at a tidal station (because of security challenge and lack of personnel). Hence, predicted tidal values were used to reduce the measured depth to chart datum. The tidal heights are a variation in the sea level that is associated with the gravitational forces maintaining the sun, moon and the earth in their orbits [67, 68]. The reduction of soundings

from floating platforms is traditionally based on the observed tidal time and height at one or more tidal stations and some interpolating techniques together with the associated assumptions to obtain tidal height relative to chart datum at other places.

During the hydrographic survey, the single beam echo sounder on the boat simply measures the depth of the water as the boat moves over the water column. However, the boat as a platform moves vertically depending on the water tide. The lagoon being in tidal waters, meaning the elevation of the water surface in the absence of waves (still water), was measured relative to chart datum. Soundings, relative to chart datum, are simply the surveyed depth less than the height of the vessel relative to chart datum. Water depths that were a reference to known datum were obtained by reducing the sounding depth using predicted tidal values by referencing the water surface to a known on-shore reference benchmark (Unilag 01). Depth was estimated to the best efforts at equipment calibration and data processing, the practicably achievable accuracy for coastal surveys when using echo sounders as 0.15 m [69]. The bathymetric data from the field were processed in the office using HYPACK software; this is a package that contains programs for single beam survey design and data collection. A sample of the final data X, Y and Z (depth) coordinates as plotted on the lagoon is displayed in **Figure 2** and the sample data are displaced in **Table 1**. The number in the chart is the reduced depth value in metres plotted against its corresponding X and Y coordinates.

## **3.4 Error in bathymetric survey (sounding)**

Errors in depth determination using acoustic instruments are caused by physical and mechanical factors, and such factors could include the velocity of sound in water and waves. The velocity of sound (V) in near-surface water ranges from 1400 to 1525 m/s but varies with water density, which is a function of temperature, salinity and suspended sediments [70, 71]. Hence, change in salinity can change the velocity of the water, and due to this, the echo sounder was calibrated onsite frequently using bar check. This check was also necessary for boat specific corrections because as the survey progressed, the vessel's draft changes as loads are exchanged (reduced). Wave error occurs as a result of the survey vessel pitching up and down, in order to obtain true water floor depth, and the transducer was

installed on the heave-compensated mount. This allows the boat to move while the

**X (m) Y (m) Z or depth (m)** 544,673.4 711,969.8 4.3 544,771.2 711,991 3.93 544,847.7 712,109.9 5.81 544,868.9 712,012.2 4.22 544,890.1 711,914.5 3.42 544,945.4 712,131.1 5.66 544,966.6 712,033.4 4.54 544,987.8 711,935.7 3.85 545,043.2 712,152.3 6.49 545,064.3 712,054.6 4.83 545,085.5 711,956.8 4.25 545,106.7 711,859.1 3.3 545,140.9 712,173.5 7.37

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

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

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

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

, F-F<sup>0</sup>

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

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

, G-G<sup>0</sup>

, H-H<sup>0</sup>

, I-I<sup>0</sup> and J-J<sup>0</sup>

). This

with the acquired data and then the computation of accretion and erosion

instruments remain fixed.

*Sample of sounding data after reduction and applied correction.*

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

the near-central region.

(profile lines A-A<sup>0</sup>

**87**

of erosion and accretion.

**4.1 Analysis of the lagoon bed dynamics**

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

, C-C<sup>0</sup>

, D-D<sup>0</sup>

, E-E<sup>0</sup>

**4. Results**

**Table 1.**


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

#### **Table 1.**

from floating platforms is traditionally based on the observed tidal time and height at one or more tidal stations and some interpolating techniques together with the associated assumptions to obtain tidal height relative to chart datum at other places. During the hydrographic survey, the single beam echo sounder on the boat simply measures the depth of the water as the boat moves over the water column. However, the boat as a platform moves vertically depending on the water tide. The lagoon being in tidal waters, meaning the elevation of the water surface in the absence of waves (still water), was measured relative to chart datum. Soundings, relative to chart datum, are simply the surveyed depth less than the height of the vessel relative to chart datum. Water depths that were a reference to known datum were obtained by reducing the sounding depth using predicted tidal values by referencing the water surface to a known on-shore reference benchmark (Unilag 01). Depth was estimated to the best efforts at equipment calibration and data processing, the practicably achievable accuracy for coastal surveys when using echo sounders as 0.15 m [69]. The bathymetric data from the field were processed in the office using HYPACK software; this is a package that contains programs for single beam survey design and data collection. A sample of the final data X, Y and Z (depth) coordinates as plotted on the lagoon is displayed in **Figure 2** and the sample data are displaced in **Table 1**. The number in the chart is the reduced depth value in

Errors in depth determination using acoustic instruments are caused by physical

and mechanical factors, and such factors could include the velocity of sound in water and waves. The velocity of sound (V) in near-surface water ranges from 1400 to 1525 m/s but varies with water density, which is a function of temperature, salinity and suspended sediments [70, 71]. Hence, change in salinity can change the velocity of the water, and due to this, the echo sounder was calibrated onsite frequently using bar check. This check was also necessary for boat specific corrections because as the survey progressed, the vessel's draft changes as loads are exchanged (reduced). Wave error occurs as a result of the survey vessel pitching up and down, in order to obtain true water floor depth, and the transducer was

metres plotted against its corresponding X and Y coordinates.

*Lagoon Environments Around the World - A Scientific Perspective*

*Sample of charted bathymetric data of 2014 dry season, plotted in decimal number.*

**3.4 Error in bathymetric survey (sounding)**

**Figure 2.**

**86**

*Sample of sounding data after reduction and applied correction.*

installed on the heave-compensated mount. This allows the boat to move while the instruments remain fixed.
