Assessment of marine environment by study of textural characteristics of the surface Sediments of a tropical mangrove ecosystem Gulf of
Kachchh, Gujarat, India: A GIS approach
Goutam Kumar* & AL. Ramanathan
1 School of Environmental Science, Jawaharlal Nehru University, New Delh-110067, India.
*[ E.Mail: goutamses@gmail.com]
Received 24 August 2012 ; revised 08 November 2012
Mangroves in Gulf of Kachchh, Gujarat, India have been studied for grain size spectrum and textural parameters namely phi mean, standard deviation, skewness and kurtosis. In general, grain size spectrum shows a marked variation in the upstream riverine input stretch. Observation shows that variation of energy conditions is controlled by the fluvial profile in the channel. Fluctuation in the physico-chemical environment results seems to alter the textural pattern prevailing in the system.
Textural analysis of the very fine sand to fine sand shows comparatively low energy condition in the fluvial and mangrove creek zones. A GIS approach reveals one of the prevailing condition i.e. topography, which influences the textural variability in the surface sediment.
[Keywords: Grain size, Mangroves, Gulf of Kachchh, Gujarat].
Introduction
Mangroves ecosystem are one of the most productive ecosystems in the world, which contains the salt tolerant shrubs and trees as well as other flora and fauna. The strategic presences of these ecosystems at the land-sea interface and are highly productive and interactive. The mangrove ecosystem holds and stabilizes sedimentary deposits from erosion and also acts as a buffer between transitional near shore and estuarine or lagoonal environments with respect to fresh water discharge, salinity regime and with the adjacent aquatic system. However, a mangrove directly affects the sediment flux by influencing the hydrodynamic regime through their physical configuration1,2. Hence, it has been attempted to investigate to understand the textural characteristics of the sediments of Gulf of Kachchh riverine and creeks to have proper insight into particle size distribution3. Environmental status of the basin has been elucidated by studying the energy conditions and transportation pattern of the sediment. Fresh water discharge from any perennial river does not take place in this mangrove wetland and hence marine processes are dominant throughout the year4. The mangroves of the Gulf of Kachchh are the second largest after Sundarban in the mainland of India.
Various studies have been carried out in order to reveal the tidal circulation and source and dispersal of suspended sediment in Gulf of
Kachchh5&6. The surface sediments in the mangroves of Gulf of Kachchh are fine to very fine sand7. Hence an attempt has been made in this investigation to understand the influence of topographical feature of the surroundings on textural characteristics of the mangroves sediment of Gulf of Kachchh by using GIS application.
Material and Methods
The gulf of Kachchh lies approximately between latitudes 22° to 23°N and between Longitudes 68° to 70° 30° E with an area of approximately 7300Km2. It has a complex set up of a macro- tidal region. The depth ranges from 60 m at the mouth to less than 20m at the head of the gulf .the floor of the Gulf of Kachchh is highly irregular. Climate is semi- arid and the maximum rainfall is of the order of 50cm yr-
1.there is no major river flow into it and hence little runoff has been observed. Topography is very irregular at the mouth and the central part of the Gulf and consists of pinnacles and scarps ranging in height from 6 to 32m 8. Towards the head, the relief is subdued due to the covering fine-grained sediments. Major source of this sediment is considered to be the shore material and the load transported by the Indus River 9. It is now recognized that the damming of the Indus River has drastically reduced the sediment delivery to the Arabian Sea10, 11. Reduction in sediment supply of the Indus River results
changes in the Indus delta. Process of sediment supply to the Gulf of Kachchh via tidal erosion of the abandoned delta is still active. High turbidity is noticed throughout the year off the Indus delta especially the eastern part. The eroded material from this region can be transported into the Gulf of Kachchh due to tidal and long-shore currents.
Tidal processes, especially settling lag effects help in import of fine grained sediments into high tidal areas 12.
Collection and preservation of sediment samples.
Seventeen surface sediment samples were collected from sites of the mangrove region during October 2008 to November 2008.
Extensive sampling was done at Mundra, Jodiya, Sachana, New Bedi Port, Old Bedi Port, Sikka, Narara and Salaya. Sampling sites (Fig.1) has been selected by keeping in consideration of different components like anthropogenic input, mixing zones, natural weathering input zones, etc.
Sediments samples (1 kg each) were collected in the polyethylene bags. In laboratory, the samples were air-dried, gently pounded and homogenized using coning and quartering procedure.
Granulometric separation was carried out by combining dry and wet sieving. Dry sieving was carried out by electromagnetic sieve shaker (Fritsch Analysette-3) into 250 µm (medium sands) , 125 µm (fine sands), 63 µm (very fine sands), 37 µm(very coarse silt) and < 37 µm (clay). The further separation of 63 µm was done by wet sieving by Attenburg sedimentation cylinders methods based on Stokes’ law. Textural parameters were computed using the formulae of Folk and Ward (1957).13-17 Mean (Mz), Standard deviation (σ1), Skewness (Ski) and Kurtosis (KG) are widely used to reconstruct the depositional environments of sediments and sedimentary rocks. Grain size distribution was also studied to determine sedimentary environment with the help of log-probability studies.
Application of GIS for assessment of topology of the study area
Geographical Information System(GIS) application can reveal the topological structure of the area viz. slope, land use pattern, flow of the sediments/dunes to the study area. Mangroves of Gulf of Kachchh is surrounded by cities, bare lands, sand dunes and other side by Arabian Sea.
Flow of the sediment is mainly from the river Indus, which is presently hindered due to dam construction at the upper head of the river Indus.
Sediments are contributed from the erosion of the
earlier landform i.e mud islands. On the other hand fine sand, very fine sand and medium sands are coming from the surrounding sand dunes, which is mainly carried out by wind. Seasonal rivers also contribute some very fine and fine sand in their mouth near the mangroves.
Results and Discussion
Textural attributes of sediments viz. mean (Mz), standard deviation (σ1), skewness (SkI) and kurtosis (KG) are widely used to reconstruct the depositional environments of sediments18. Correlation between size parameters and transport processes/depositional mechanisms of sediments has been established by exhaustive studies from many modern and ancient sedimentary environments19-26.
The mean size indicates that the fine sands were deposited at a moderately low energy conditions. The phi mean size of samples varies from 1.962 to 3.289 Ф with an average mean of 2.532 Ф (Fig.2a). Majority of the sample lies in fine sand category (58%), rest in very fine sand and medium sand category. Mean size of the sediments influenced by the source of supply, transporting medium and the energy conditions of the depositing environment. Mean size indicates that the fine sands were deposited at a moderately low energy conditions.
Standard deviation (1) measures the sorting of sediments and indicates the fluctuations in the kinetic energy or velocity conditions of the depositing agent27. Standard deviation values of the sediments range between 0.508 and 1.107 Ф with an average value of -0.827Ф (Fig.2b).
Sorting of sediments ranges from poorly sorted to moderately sorted character. About 76% of samples fall under moderately sorted nature.
Abundance of the particle dispersion is derived from the riverine and creeks sediments. In the riverine zones only four samples (location S8 &
S9, S12 and S15) which were collected from Salaya, Mundra and Jodiya exhibit moderately sorted to moderately well sorted particle dispersion.
Fig 1. Map showing the study area.
Skewness (Ski) measures the asymmetry of a frequency distribution. Skewness values range between -0.280 and 0.678 with an average value of 0.362(Fig.2c). Symmetry of the samples varies from very fine skewed to coarse skewed nature.
About 65% of the samples fall under very fine skewed category. The very fine skewed nature of
sediments indicates excessive riverine input.
Positive skewness (70%) of sediments indicates the unidirectional transport (channel) or the deposition of sediments in sheltered low energy environment28.
Fig.2(a-d): Showing horizontal variation of grain size parameters of surface sediments.
Values of the fourth moment kurtosis (KG) ranging between 0.624 (S10) and 2.418 (S6) with an average of 1.032(Fig.2d). Majority (65%) of the samples falls under platykurtic to very platykurtic nature of distributions. Friedman (1962) suggested that extreme high or low values of kurtosis imply that part of the sediment achieved its sorting elsewhere in a high energy environment.
The variation in the kurtosis values is a reflection of the flow Characteristics of the depositing medium4, 29. Finer in size and dominant platykurtic nature of sediments reflect maturity of the sand and variation in the sorting values are likely due to continuous addition of finer/ coarser materials in varying proportions30.
According to Sahu (1964), the variations in the energy and fluidity factors seem to have excellent correlation with the different processes and the environment of deposition. The process and environment of deposition were deciphered by Sahu’s linear discriminate functions of Y1 (Aeolian, beach), Y2 (Beach, shallow agitated water), Y3 (shallow marine, fluvial) and Y4 (turbidity, fluvial).With reference to the Y1 and Y2 values, most of the samples (88%) fall in beach process (Y1) and all (100%) samples fall in shallow marine waters (Y2) respectively. Further the samples fall in Y3 indicate they were of shallow marine environment (66%) and fluvial environment (34%). The Y4 values show that about 59% of the samples were deposited by fluvial action and 41% by turbidity action (Table 1).
Fig.3.Showing the slope of the study area in the ASTER data(2009).
1.962 2.162 2.362 2.562 2.762 2.962 3.162 3.362
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17
Mean ø
Location Average Mean
= 2.532
a.
).
-0.28 -0.08 0.12 0.32 0.52 0.72
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17
Skewness
Location Av. Skewness
=0.362
c.
0.624 1.124 1.624 2.124 2.624
S1 S2 S3 S4 S5 S6 S7 S8 S9 S… S… S… S… S… S… S… S…
Kurtosis
Location
Av. Kurtosis
= 1.032
d.
.
0.508 0.608 0.708 0.808 0.908 1.008 1.108
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17
Standarad dev.ø
Location Av. Standard dev.
=0.827
b.
GIS Applications
Surface sediment of the estuaries depends upon the supply of the sediments from the rivers. In the study area, only Indus rivers flow is perennial, which is the major source of sediments9 and others are seasonal rivers. The reduction in sediment supply of the Indus river results changes in the Indus delta12. As the study of the geographical area of the Gulf of Kachchh, the slope of the study area is towards the gulf (Fig.3). Majority of the high altitude area is covered with the very fine sand, fine and moderately fine sand, which reaches the study area by the Aeolian activity. Application of GIS reveals the slope pattern of the surrounding area of the mangroves of Gulf of Kachchh while interpreting the ASTER data.
Conclusion
Most of the sediments were deposited under shallow marine environment by beach and fluvial processes by a near shore whirlpool agitating turbidity action of water. Textural analysis indicates the dominance of fine and very fine sand in the Shallow River and creek due to low energy condition. The very fine grained sediments in the suspended load of the Gulf of Kachchh creeks are dominant as compared to other main channels. Fine-grained sediments in the suspended load of the Gulf of Kachchh creeks are dominant as compared to other main channels. Slope pattern revealed by analysis of ASTER data, shows the inclination of the slope towards the mangroves of Gulf of Kachchh. The slope towards the Gulf of Kachchh influences the sediment characteristics by adding sediments through seasonal flowing
rivers and Aeolian deposits.
Acknowledgement
Goutam Kumar is thankful to University Grant Commission (UGC), Government of India for the award of Junior Research Fellowship (CSIR-UGC Fellowship Programme) to carry out this work. Authors are also thankful to Department of Forest, Gujarat for granting permission to carry out our designed studies in the Marine National Park and Sanctuary.
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