*4.5.1 Calculation of volume gained*

The single beam hydrographic data of 2008 and 2014 were used to determine the degree of changes that took place over a period of 6 years. Hence, the amount of sediment eroded or gained was calculated using the depth range from the datasets created on triangular irregular network (TIN). The TIN morphological surface was converted to raster data and was used in the CUTFILL tool to determine the volume gain or loss. To analyse the change in the sediment volume between 2008 and 2014, a statistical summary from the ArcGIS model was used. A summary of the gain/loss analysis is depicted in **Table 11**. The amount of accretion was found to be higher than that of erosion/dredging on the lagoon water bed despite all the local sand extraction going on consistently in the lagoon.

It can be inferred from **Table 11** that 858,932 m2 on the lagoon gained 137,429 m<sup>3</sup> volume of sediment between 2008 and 2014. Hence, the depth of accreted sediment over the area was computed as:

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

Between 6 years, the average height of 0.16 m was gained by the lagoon. Going by this rate, it means that in 1 year the height of accretion will be 0.026 m.
