Coastal and nearshore sediment data along the eastern coastal zone of Bangladesh of the northern Bay of Bengal

A seafloor sediment dataset of 42 samples has been collected from the coastal and nearshore areas of the northern Bay of Bengal, which is situated in the southern part of the eastern coastal zone of Bangladesh. From February 2018 to March 2019, Van Veen grab samplers were employed to gather samples at a depth of 4 m and 23 m. The unconsolidated bottom surface has been sampled, taking bathymetry and fishing boat accessibility into account. Using the dry sieving technique, grain size distribution analysis was performed on the obtained sediments. The retrieved dataset was measured to identify seafloor sediment grain size distribution and determine the sediment dynamics. This data can be used to delineate sediment characteristics, depositional processes, and the environment.


Specifications
Earth-Surface Processes Specific subject area Sedimentology, Marine Geology, Coastal Geomorphology Type of data Table  Image Graph Figure  Map How the data were acquired A total of 42 sediment samples were collected from the seafloor. The samples were collected using a Van Veen grab sampler. Sediment sampling was conducted with the goal of delineating the coastal and nearshore processes, considering depth and distance from the shoreline. The survey was conducted using local fishing trawlers/boats, so the depth and distance were based on safety considerations and trying to maintain a grid pattern of data collection using spot location with GPS. Collected sediment samples were taken to the laboratory, where different measurements of sieve shaking were conducted. ASTM sieve mesh sizes 18, 35, 60, 120, and 230 were used to separate the sediment into various fractions such as very coarse, coarse, medium, fine, and very fine sand, as well as mud (which passed through the 230 mesh sizes). The separated samples were then measured using a laboratory analytical balance. The statistical parameters, including histograms and cumulative curves, were calculated using MS Excel software. The cumulative curve was used to extract the phi size values of different percentages, which were then used to calculate the statistical parameters of grain size, including mean size, sorting, skewness, and kurtosis, using MS Excel software. Data

Value of the Data
• Surficial unconsolidated sediment of the seafloor of the northern Bay of Bengal, Bangladesh was poorly or never studied prior to the collection and analysis of this shallow marine dataset. • The coastal and nearshore sediment data can be used for sediment dynamics, depositional processes, depositional environment, and engineering studies for the benefit of policymakers, researchers, consultants, and stakeholders to make their decisions. • There is the longest uninterrupted sandy sea beach, named Cox's Bazar sea beach, situated along the sampling zone. Nowadays, the beach area is undergoing erosion in the popular tourist spot. The dataset can be used for the reconstruction and nourishment of the beach. • The dataset can help with the potential delineation of sediment transportation, distribution, erosion, and deposition in the area.

Objective
The dataset was created to study grain size characteristics, sediment depositional processes, sediment transportation, and the identification of the depositional environment of Bangladesh's eastern coastal zone, which is part of the northern Bay of Bengal. This data article will be used to determine the proper results by utilizing the insights output of the data for the research article. Also, the dataset will be an important tool for understanding the physical, chemical, and biological processes that shape coastal and nearshore ecosystems and for developing effective strategies for coastal management and conservation. This dataset can be used to study the impact of sedimentation on water quality in coastal areas, design and implementation of coastal habitat restoration, understand the processes of coastal erosion and sediment transport, identification of suitable sediment sources of beach nourishment and, source and level of pollution monitoring (such as distribution of heavy metals, pesticides, and microplastics). Besides this dataset can also be used for seafloor mapping and exploration, geohazards assessment, and, engineering applications.

Data Description
A total of 42 samples have been collected from the eastern coastal zone of Bangladesh, where 17 samples are from the Cox's Bazar coast, 10 samples from the Teknaf coast, and 15 samples from the Saint Martin's Island coast ( Fig. 1 ). The survey covered approximately 1300 sq. km of the coastal and nearshore area of the eastern coastal zone of Bangladesh.
The sample collected from the nearshore area shows a high amount of sand (average 92.99%) and a low amount of mud (silt and clay) (average 7.01%). On the Cox's Bazar coast, all of the samples (except sample C15) have a higher percentage of sand than all of the samples (more than 98%). On the Cox's Bazar coast, only one sample (C15) contains 13.34% of mud and the rest of the samples show less than 5% of mud, except for C12 ( Fig. 2 b). The average sand is 97.06%, and mud is 2.94% on the Cox's Bazar coast. On the Teknaf coast, sample T16 shows more than 99% sand, and samples T24 (60%) and T25 (79%) show less sand. The range of mud of the sample T17-T23 is about 3%-8%, and sand is about 91%-96%. The mud content is higher in Samples T24 and T25, with a maximum of 39.67%. The average sand is 89.61% and mud is 10.93% along the Teknaf coast. Here, sand (2 mm to 0.063 mm in size) particles are typically larger in size than mud ( < 0.063 mm in size) particles, and sieve analysis used to distinguish between them based on their size differences [3] .
The maximum sand found in the sample T7 (99.99%), where T6, T8, T9, and T13 show more than 98% of sand in the Saint Martin's Island coast samples. Samples T1, T2, T5, T12, and T14 show a lower percentage of sand (70-90%) and a higher amount of mud (10-30%). Sample T14 contains 72% sand and 27% mud. On Saint Martin's Island coast, the average sand is 90.61%, and the average mud is 9.36%. The average value shows that the Cox's Bazar coast contains a higher amount of sand and a lower amount of mud than the Teknaf coast, which shows a higher amount of mud and a lower amount of sand. But as per distribution, Saint Martin's Island coast shows a higher percentage of mud and a lower percentage of sand than the Teknaf coast ( Fig. 2 b).
The frequency curve is used to determine the modal nature through histogram construction [4] . On the Cox's Bazar coast, the frequency curve shows the dominant unimodal nature of the sediment. The sediment in samples C11 and C14 exhibits a wide range of bimodal characteristics. Samples C6 and C16 show a sharp peak of frequency at the 2 mm phi size in the graph, and samples C3, C2, and C10 show a standard peak at 3 mm phi size, where C1, C4, C9, C12, C13, and C17 show a sharp peak in the frequency curve at 4 mm of phi size ( Fig. 3 a).
The frequency curve of the Teknaf Zone shows the dominant bimodal nature of the sediment. Sample T16 shows unimodal with a 2 mm phi size, and samples T19 and T20 show unimodal with a 4 mm phi size of sediment. T22 shows a unimodal but flat peak in the frequency curve ( Fig. 3 b). Samples T3, T4, T5, T11, and T13 from Saint Martin's Island coast are bimodal, with the dominant peak at 4 mm phi size. T9 has a flat peak, T6 has a wide unimodal peak, and T7 has a unimodal with 2 mm of phi size sediment. The T8 and T13 show a sharp peak with a 3 mm phi size due to the unimodal nature of the sediment. T2 and T15 show unimodal sediment with a phi size of 4 mm ( Fig. 3 c).
Grain size (phi scale) plotted against cumulative weight percentage generates a grain size cumulative curve, which is the most useful curve for grain size analysis [4] . Generally, the grain size cumulative curve generates an S-shaped curve. On the Cox's Bazar coast, the cumulative curve shows a steep slope. But C16 shows a very steep slope with medium-grain sand and C6 shows a steep slope with medium to fine-grain sand. Samples C4, C7, C9, C15, and C17 show a steep slope curve with a fine sand size distribution ( Fig. 4 c).
The majority of the sediment in the Teknaf Zone cumulative curve has a very gentle slope, with only sample T16 showing a very steep curve with medium sand size. Sample T13 shows a steep curve with fine sand size, whereas samples T19 and T20 show steep slopes with very  fine sediment ( Fig. 4 b). The sediment of Saint Martin's Island coast shows a gentle slope in the cumulative curve where T7, T8, and T13 show steep slopes of medium to fine-grain sand. T9 shows a gentle slope with medium to fine-grained sand. Other samples of Saint Martin's Island coast show a gentle slope with fine to very fine sediment ( Fig. 4 a).
Mean grain size is a descriptive parameter of grain size that measures the arithmetic average size of all the particles in a sample. It indicates the central tendency of a grain size distribution. The mean grain size of the samples mostly shows the fine sand size (2 to 3 mm). Of the 42 samples, 23 contain fine sand, 15 contain very fine sand, and 4 contain medium sand. The distributions of mean grain size are almost similar for all three coasts. The Teknaf coast sediment shows a dominant fine grain size. On the Teknaf and Saint Martin's Island coasts, samples show dominant finer sand than on the Cox's Bazar coast, where Cox's Bazar coast sediments are distributed with medium, fine, and very fine sand ( Fig. 5 a).
Grain-size sorting is a measure of the range of grain sizes present and the degree of spread or scatter of these sizes around the average or mean size. The inclusive graphic standard deviation is the mathematical expression for sorting. The sorting of the samples shows mostly poorly sorted sediment (14 samples), 10 samples are in moderately sorted sediment, 13 are in moderately well-sorted sediment, 4 are in well sorted, and 1 is in very well-sorted sediment. Most of the samples from the Teknaf coast and Saint Martin's Island are poorly sorted, whereas sediments from the Cox's Bazar coast are dominantly sorted. Also, the Cox's Bazar coast contains less poorly sorted sediments. The distribution of poor sorting of sediment is higher in the middle of the sampling area (C11, C12, T22 to T25) ( Fig. 5 b).
Skewness is a measure of symmetrical distribution, i.e., the proportion of coarse or fine fractions. A symmetrical curve with excess fine material shows a positive value, whereas one with excess coarse material shows a negative value; a zero value is indicated by a symmetrical curve. Positive skewness indicates a fine fraction abundant in the sediment [5] . The skewness shows dominants of near symmetrical value on all three coasts ( n = 15). 12 samples show coarse skew, 12 are strongly coarse skewed, and 3 are fine skewed. The distribution of skewness values shows similarity along the three coasts ( Fig. 5 c).
The kurtosis expresses the packedness of the grain size distribution. The majority of the survey area's samples ( n = 15) show platykurtic nature sediment, 11 show mesokurtic, 7 show leptokurtic, and 8 show very leptokurtic. Mesokurtic nature is dominant on the Saint Martin's Island coast. In terms of kurtosis, the majority of the samples show extreme values on the three coasts (platykurtic, very leptokurtic) ( Fig. 5 d). The extreme value of kurtosis can be used for the identification of the depositional environment [6] .

Experimental Design, Materials, and Methods
Samples have been collected from the seafloor of the coastal and nearshore areas of the northern Bay of Bengal, located in the eastern coastal zone of Bangladesh. Samples were designed to be collected at 4-5 km intervals using fishing trawlers or boats according to a grid system. Samples are collected during the dry season (November-January) and pre-monsoon season (February-April) because sampling during the monsoon and post-monsoon is difficult using fishing trawlers or boats in rough sea conditions in the Bay of Bengal. A total of 42 samples were collected, and the sampling location was level. 'T' indicates the sample is near the Teknaf coast and Saint Martin's Island coast, and 'C' indicates the sample is near the Cox's Bazar coast. The sampling looks like there are some samples missing between samples T25 and C17 (in the middle of the map) ( Fig. 1 ). It could be due to the hard or compact nature of the seafloor's sediment. Five times an attempt was made to capture samples from the gap area using the Van Veen grab sampler, but it failed. It is possible to assume that humans manually ran the sampler while sampling from the small floating fishing boat. Nonetheless, the other sample stations were successful using the same method. The 5 km grid interval sampling design was attempted to be maintained, but due to current and tidal influences, the fishing boat could not remain stable in the anchoring location while using a grab sampler.   Table 1 ). The shallowest sample derived from the Cox's Bazar coast is 4 m in depth, and the deepest sample from the Teknaf coast is 23 m in depth. On the Teknaf coast, the shallowest depth (5 m) sample is T20, and the deepest sample (C13) of the Cox's Bazar coast is 18.30 m. The Ekman grab sampler has been attempted several times during sampling, but the sample was not retrieved with it.
The sediment has been placed in a geological bag and transported to the lab after sample collection. The sample was then subsampled to 100 g for dry sieve analysis after being dried in the open air for 30 days. Between sand and mud grain size 1 mm, 0.5 mm, 0.25 mm, 0.125 mm, 0.063 mm, and pan mesh sizes were used in a sieve machine (Advantech Dura Tape type) with 100 g dry samples that were shaken for 20 min. Using mesh size falls between sand and mud particle size. Then the measured sieved data is calculated for grain size analysis using Folk and Ward's [6] formula.
With the use of MS Excel analysis, the attribute table data values for the map were created using ArcGIS 10.5 [7] software

Ethics Statements
This work did not involve human subjects, animal experiments or data collected from social media platforms.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data Availability
Coastal and nearshore sediment samples of the eastern coastal zone of Bangladesh