MATERIALS AND METHODS

Study area: The study area, Chandipur is situated in Balasore district, Orissa, eastern coast of India (Fig. 1).

Fig. 1:

Study area in Chandipur, marked by red square along the east coast, India

Fig. 2:

Study area along with sampling locations of coastal sediments, Chandipur

The coastal sediments belong to Quaternary fluviomarine origin developed during post-glacial Flandrian transgression (6.5 Ka onwards) during the Holocene^20-21. The Chandipur coastal area has broadly two geomorphologically distinct zones, land part (away from sea) is characterized by the fluvial processes from the Buribalam river, seaward part is a long beach, parallel to the coastline of the Bay of Bengal¹¹. Beach has distinctive dune field and a very low dipping (<3°) silty intertidal flat, along with a shore parallel, sandy longitudinal bar at Buribalam estuary mouth¹⁶. The intertidal flat is quite wide, approximately 1-4 km. The intertidal mud flat of Chandipur is an exclusive coastal ecosystem which hosts red crabs, endangered horseshoe crabs, sea turtles and varied intertidal benthic fauna. This ecosystem is under threat because of amplified tourism activities. The other part of this coastline is controlled by Indian Army as an integrated test range. Chandipur coastline belongs to one of the identified “Important Coastal and Marine Biodiversity Area” (ICMBA) for the better management of coastal and marine flora and fauna. The study area lies between 21°44’ N-21°47’ N and 87°02’E-87°07’E (Fig. 2), covering a stretch along NE trending Chandipur coastline, ending at Buribalam river estuary mouth. This study was conducted from December, 2016-January, 2019.

Sample processing and calculation: A detailed field work had been carried out along the long stretch of Chandipur coastline. Modern day beach sediment samples have been collected from eleven locations, presented in Fig. 2. Table 1 represents the coordinates of the sampling stations. Surface sediments were removed to avoid any kind of dilution in analyses and sediments were collected from 1 m×1 m grids and preserved in ziploc plastic bags. Collected sediments were dried in sunlight and 300 g sediment from each of the eleven locations was further dried on oven for 1 h at 50°C temperature. Once fully dried, samples were sieved using Table Sieve Shaker. Various mesh sizes have been used for grain size analyses, which are 425, 300, 150, 125, 90 and 75 μm.

Fig. 3:

Methodology of sample collection to preparation for grain size and sorting analysis

Figure 3 briefly demonstrates the method of sample collection at study area too sample preparation in laboratory. After kinetic sieving, grain size (ϕ) values of different weights were obtained and used for various statistical parameters. Calculation methods of various output parameters have been discussed below.

Krumbein²² provided the phi scale (ϕ) for grain size, which basically translates the Wentworth grain size scale²³ into a logarithmic scale. The equation is as follows:

(1)

where, d is the particle diameter (mm).

Phi scale eases the statistical analysis (mean, skewness etc.) performed on grain size data. Sieving is an established method in classical grains size analysis. Sieve analysis data are plotted as histogram distribution curve and frequency distribution curve to get definitive outlook of sedimentation pattern and spatial grain size distribution as well as depositional environment. From grain size graphical representation, a mean grain size (M) can be interpreted, which represents a typical grain size middle property or a central location value from the distribution⁵. Following is the equation to measure graphic mean (M):

(2)

where, ϕ₁₆, ϕ₅₀ and ϕ₈₄ are the grain sizes at 16, 50 and 84% of the sample by weight respectively (percentiles). The Φ₅₀ represents the graphical median value.

McCammon²⁴ suggested inclusion of the central section and extremes of data distribution^25-27 to characterize the sediment distribution, which is known as Sorting (S). Sorting is calculated by McCammon²⁴:

(3)

RESULTS

Statistical mean grain size (phi) and sorting (phi) from the recent coastal sediments have been studied. Data has been presented in Table 2. Result shows average coarse to medium grained sand distribution (L1, 4 and 7) closest to beach whereas the traverse along L-3, 6 and 9 reveals a finer mean grain size, being the furthest sampling points from beach (Fig. 4). Figure 4 evidently picks decrease in grain size from beach to tidal flat as well as towards estuary.

L-10 and L-11 records very fine to fine sands, being on active sedimentation zone of the estuary. A distinct grain size increase (decreasing phi values) away from estuary mouth can be found in Fig. 5a. Figure 5b reflects a gradual decrease in grain size from medium-coarse sand to fine sands (increasing phi values) from beach to tidal flat area.

Fig. 4:

Mean grain size variation from beach to estuary and towards estuary

Fig. 5(a-b):

Grain size variation away from (a) Estuary mouth and (b) Coastline Source: Folk et al.28

Fig. 6:

Grain size vs. sorting at various sampling stations

Both these interpretations have been supported by a strong correlation in data plots.

Mean grain size vs. sorting values at various sampling locations have presented in Fig. 6. Classification has been used while interpreting Fig. 5a and b. Three distinct sorting clusters have been identified. L-8, 9, 10 and L-11 reveals poor sorting (increasing sorting phi values). L-6 and L-5 indicate moderate to poor and moderate sorting characteristics respectively. L-1, 2, 3, 4and L-7 display very good degree sorting.

Location map was concerned to understand the effect of relative distance from estuary mouth on spatial variation of grain size and sorting. L-1, 4 and L-7 reveals coarse to medium mean grain size (1.35-1.66 phi) and very well sorting (0.25-0.40 phi). Being located on the beach and far away from estuarine mouth, high energy waves are dominant force acting on these locations responsible for such uniform sediment characteristics in these three sampling stations. L-10 and L-11 are located on estuary bank and estuary mouth respectively, which is the active sedimentation zone.

Fig. 7:

Active estuarine sedimentation zone, marked by yellow, arrow indicates shore parallel sediment transfer from bar area to away, zone marked by red indicates active sediment mixing zone resulting poor sorting

And hence very fine to fine mean grain size (2.70-2.95 phi) with poor sorting (>1.00 phi) have been identified in these two locations. L-9 and L-11 are located in either flanks of the estuary mouth bar, although L-9 is away from mouth when compared to L-11. Both these station are characterized by poor sediment sorting and close mean grain size values (2.79-2.95 phi). Moderate to poor sorting at L-6, 8 and L-9 points to the effect sediment mixing resulted from shore parallel current moving sediment from estuary mouth towards these locations. Active estuarine sedimentation and potential mixing zones have been mapped and demarcated in map (Fig. 7).

Result shows that beach sands are relatively coarser grained (coarse-medium sands) and well sorted, resulting from high energy wave action. This uniform sediment characteristics have been pretty consistent along the beach, parallel to the coastline. Traverse from beach to tidal flat to sea (foreshore-backshore-inshore direction) display a gradual decrease in grain size because of diminishing wave energy effect. Traverse along L1-2-3, L4-5-6 and L7-8-9 provides the decreasing grain size pattern from beach to tidal flat. Fine grained sands are dominant towards estuary revealing very poor sorting, as can be seen in L-10 and L-11. An intermediate sorting and grain size distribution has been found in three sampling stations (L6, 8 and L-9). This is a result of sediment mixing phenomenon by shore parallel currents responsible for sediment transportation from estuary mouth to these locations. Figure 7 demonstrates the active mixing zone in study area map.

DISCUSSION

This study interprets the lateral variation of grain size and sorting in the coastal sediments of Chandipur. Sampling points closest to the beach (L1, 4 and 7) indicates coarse to medium mean grain size (1.35-1.66 phi) and very well sorting (0.25-0.40 phi). Very fine to fine mean grain size (2.70-2.95 phi) with poor sorting (>1.00 phi) have been recorded in the locations close to estuary bank and estuary mouth (L-10 and L-11) in the upper tidal flat. Sediment mixing phenomenon by shore parallel currents resulted in intermediate sorting and grain size distribution for the in-between sampling stations (L6, 8 and L-9).

Chakrabarti^17,18 interpreted mostly of fine sand (2-3 phi) in the upper tidal flat along with a seasonal influx of muddy sediments (fine grained) through the tidal creeks draining the upland areas. Saha et al.¹⁹ suggested that the sand transportation by tidal currents might have resulted in sediment mixing, which resulted in fine to very fine sand size sediment distribution therefore. They advocated the reduction of sorting by the mixing of two size population-sand and silt²⁰. Bardhan et al.¹⁶ identifies that the contact between beach and tidal flat is marked by abrupt gradient change along with variations in grain size. Folk and Ward²⁸ classified sorting <0.35 as very well sorted, 0.35-0.50 as well sorted, 0.50-0.71 as moderately well sorted, 0.71-1.00 as moderately sorted and >1.00 as poorly sorted.

It is suggested to deploy the coastal sediment sample collection and statistical analysis in Chandipur area in different months of the year to decipher the temporal variation of sediment distribution and identify potential seasonal sediment flux, if in place. With more number of closely placed sampling points, a minute spatial sediment variation can be modeled.

CONCLUSION

This work employs graphical method to estimate the statistical distribution of mean grain size and sorting behavior of coastal sediments from Chandipur, eastern India. Variation of these statistical parameters has been documented and a spatial correlation between the sampling points has been presented. The sediment mixing pattern near estuary and mouth bar has been established in this study and that might be a potential path for active pollutant contamination towards the tidal flat zone, a rich habitat for microorganisms.

SIGNIFICANCE STATEMENT

Characterization of sediment grain size and sorting on a beach are key input parameter to morphological models. The analysis of these parameters helps to identify the spatial sediment distribution in active sedimentation environment. As the redistribution of different grain sizes takes place, the local morphology changes owing to subtle changes in erodibility and transportability. This study captures the spatial distribution of modern day coastal sediments and deciphers the active sediment mixing zone. The Buribalam River flows through a populated human settlement and present urban drainage and local small scale river side industries are prominent sources of pollutants which are directly or indirectly being mixed up in river water and reaches the sea/estuarine without any barrier. The study area, although being a designated ecologically sensitive zone, is therefore very vulnerable to potential pollutant affected.

ACKNOWLEDGMENT

This work is a part of SD’s thesis work during his Masters in Jadavpur University. SD thanks Jadavpur University for the infrastructure for this work. All those who supported and helped during the fieldwork, sample collection, processing and preparation are thanked. The ideas presented here in, are solely the authors’ interpretation.