Grain Size Analysis of Sediments From Okpoama – Brass Beach in the Niger Delta

Granulometric analysis was carried out on sixty – three sediment samples collected from the Okpoama – Brass Beach, in the Niger Delta, to determine the particle size distribution, the nature of the sediments and transport mode of the sediments. The sediment population is predominantly unimodal, with the 0.25 – 0.125 mm (fine sand) as the modal class, though, a significant population are polymodal. Sorting ranges from 0.1 (very well sorted for the unimodal samples) to 1.01 (poorly sorted for the polymodal samples). The average sorting value for the entire sampled population is 0.89 (Moderately sorted). The skewness of the population range from symmetrical to negatively skewed. The average values for the kurtosis is 1.2 (Leptokurtic). The average value for the Median (φ) is 1.92 which represents the medium sand size grade, while the average Mean value is 1.95 φ. The predominant mode of transport is by saltation for the modal class, and suspension and traction for the finer and coarser sediments respectively. The sediment load from Okpoama – Brass beach are dominantly fine sands, indicative of distant source rock and a gentle depositional slope.


3.Methodology
Sampling of the beach was done in transects. Each transect included the sampling of the sand dunes, backshore and beach face deposits where possible, otherwise only two samples were collected from sections of the beach that was reachable in that transect. A total of sixty -three (63) samples were collected into sample bags and properly labeled. They were taken to the laboratory for granulometric analysis. Mechanical sieving by Folks and Ward (1957) was employed. Samples were oven dried to remove moisture content before pouring the loose samples through a stack of wide sieves (set of sieves) with different apertures of known sizes with a minimum time of fifteen minutes used to shake the samples contained in the sieves. This is done to separate the grains into their various sizes. The results got after sieving were used to calculate grain size parameters and to plot frequency and cumulative curves graphs. The calculated parameters include median, mean, mode, sorting (standard deviation), skewness and kurtosis of the grain population.

Median Diameter (Md):
This is the average size of the sample grain which corresponds to the second quartile associated with the 50% percentile Md=φ50.

Mode (Mo):
This is the size class in which the greatest percentage of grains is represented. The mode class means the commonest grain size which is got from a size frequency histogram.

Kurtosis (K)
It is a measure of the peakedness of the grain distribution. It is the ratio of the spread of the falls and centre of the distribution. K = φ95 -φ5 …………………………………Equation (4) 2.44(φ75 -φ25)

4.Presentation And Discussion Of Results. 4.1 Presentation of Samples and Description
The sample points, brief textural description, elevations and their associated geo-references are presented in Table  2. A total of 30 transects were taken along the Okpoama -Brass beach.  Table 3 shows the various statistical parameters for the individual sample calculated from the grain size analysis carried out. Frequency curves and cumulative curves of some of the samples, showing the distribution of the sample population are also plotted for graphical presentation in Figures 3 -54. The grain size population of the sampled sediments shows variations in the modal presentation. Most of the samples are unimodal as can be seen from the frequency histograms, with the 0.25 -0.125 mm as the modal class (fine sand grade), but a significant number are polymodal (Figures 3-8, 15-22, 31-54). The unimodal samples show very good sorting while the polymodal sediments exhibit poor sorting. Sorting ranges from 0.1 (very well sorted) to 1.01 (poorly sorted). The average sorting value for the entire sampled population is 0.89 (Moderately sorted). The average value for kurtosis is 1.2, which indicates the sediments are leptokurtic. The kurtosis values range from 0.51 (very platykurtic) for the polymodal sediments to 2.25 for the unimodal sediments which are very leptokurtic. The skewness of the population range from symmetrical to negatively skewed. The average value for the Median (φ) is 1.92 which represents the medium sand size grade. This does not tie exactly with the modal class, which is the 2 -3 φ, (fine sand grade). The average Mean value is 1.95 φ.

Presentation of Grain Size Parameters and Discussions.
The size distribution consists of sizes from coarse sands to silt size class. According to Visher (1969), coarse grains larger than 0.5 φ are transported by traction method, bedload between 0.5φ and 3 φ are by saltation method, whereas, those finer than 3 φ are carried by suspension. All the three modes of transportation are inferred from the cumulative curves of the sediments (Figures 9-14, 23-30).
Waves and currents are responsible for sediment transport and reworking within the beach. In a prograding coast like the Niger Delta, rivers carry sediment load from the in-land into the ocean. These sediment loads are reworked by waves and current and deposited in the beaches. Coastal sediments are usually made up of sands, and are gravelly where source rock is nearby (Reineck and Singh, 1980). The sediment load from Okpoama -Brass beach are dominantly fine sands, indicative of distant source rock.
Weigel (1964) observed that a systematic relationship exist between grain size, beach slope and exposure to wave action. For a given grain size, a beach slope will increase with decreasing wave action. Alternatively, under similar wave conditions, coarser sand will always form a steeper slope than finer sand (Reineck and Singh, 1980). Conclusively, Okpoama -Brass beach is a low lying beach composed mainly of fine sand grade.

Conclusion
The Okpoama -Brass beach is an active beach that sits at the edge of the Atlantic Ocean parallel to the shore line. The beach is low lying and is composed of sand, predominantly the fine sand grade. Most of the samples are unimodal with a significant number polymodal. The modal class is the 0.25 -0.125 mm class (fine sand grade). The average value for the median (φ) is 1.92. Sorting ranges from 0.1 (very well sorted) to 1.01 (poorly sorted), the average value is 0.89 (moderately sorted). The sediments are symmetrical to negatively skew. The average value for kurtosis is 1.2, which indicates the sediments are leptokurtic.
The modal class of the sediment is transported by saltation, while, the finer and coarser sediments which occur in lower quantities are transported by suspension and traction respectively.