application of irs-p4 ocm data to study the impact of tidal...

Indian Journal of Marine Sciences Vol. 33(2), June 2004, pp. 129-137

Application of IRS-P4 OCM data to study the impact of tidal propagation on sediment dynamics in the Gulf of Kachchh

Yaswant Pradhan, A S Rajawat & Shailesh Nayak*

Marine and Water Resources Group, Space Applications Centre (ISRO), Ahmedabad - 38 0015, India *[E-mail: [email protected]]

Received 7 April 2003, revised 8 October 2003

This paper reports observations with respect to suspended sediment dispersal pattern at various stages of the tide cycles using the Indian Remote Sensing Satellite (IRS-P4) Ocean Colour Monitor (OCM) data, exploiting the high temporal resolution of two days and specific narrow spectral channels suitable to retrieve suspended sediment concentration. The shelf region of the Gulf, west of its mouth, plays a key role in sediment dispersal pattern within the Gulf of Kachchh as is evidenced by different linear shaped, clockwise and anticlockwise circulation patterns. The suspended sediments are transported as a V-shaped front into the Gulf during flooding conditions and as linear plumes fanning out of the Gulf during ebb conditions. Sediments are being transported into the Gulf along the northern shore from the Indus delta during flooding phase, whereas the southern shore gets sediments from the southern parts of the mouth. The study of sediment dispersal pattern led to dividing the Gulf of Kachchh into six zones, having characteristic sediment dispersal patterns. The information is useful for maintaining the approach channels of ports in addition to understanding dispersal of pollutants such as oil slicks under tidal influence.

[Key words: IRS-P4 OCM, suspended sediment concentration (SSC), tidal conditions, sediment dynamics]

Introduction It has been possible to utilise data from Landsat MSS, Landsat TM, IRS-1C & 1D for monitoring coastal environment of the country since last three decades1-5. These data sets have been extremely useful for understanding the impact on the coastal landforms in terms of net gain or loss. However, the poor temporal resolution of these remote sensing data has restricted the understanding of sediment transport in smaller time scales. Contemporary ocean colour sensors, onboard earth observing satellites, provide synoptic view with high temporal resolution, hence are excellent tools to map and monitor sediment patterns, to estimate relative changes in sediment concentrations and retrieve sea surface velocities using sediments as tracers in sequential images6-8. The Gulf of Kachchh being a tide-dominated region is ideal to be studied using sequential IRS-P4 OCM data providing information on suspended sediment dispersal in particular for identifying source to sink regions at various stages of tidal cycles. The Gulf of Kachchh [68° 20′ - 70° 40′ E and 22° 15′ - 23° 40′ N] is an east - west oriented water mass between the Saurashtra and the Kachchh Peninsulas (Fig. 1). It is about 170 km long and 75 km wide at the mouth and covers an area of around 7350 km2. It

is a shallow water body, with depth extending from about 60 m at the mouth to less than 20 m at the head of the Gulf. The Gulf of Kachchh is one of the three most prominent macro-tidal sites of the Indian coastline (other two sites are - the Gulf of Khambhat and the Sundarbans). The dominant motion in this part of the coast is the oscillatory flow driven by the tides of the adjacent Arabian Sea. The Gulf reportedly sucks the Indus sediments in its north-eastern sector and transports it out to the Arabian Sea near its south-eastern sector9. The tides in the Gulf of Kachchh are of mixed semi-diurnal type with a large diurnal inequality. The tidal range in the Gulf is about 4 m at the mouth and increases to around 7 m in the inner Gulf. This macro-tidal range is associated with strong water flows during ebb and flood tides. The Gulf waters behave as homogeneous one-layered structure due to high tidal range, low runoff from land, shallow depths and irregular bottom topography – all suitable for turbulent flow field; however the impacts of the complex physiographic features on the shelf circulation and shelf sediment dynamics remain unexplained. This behaviour of the Gulf is in sharp contrast to the estuaries on the west coast of India, which show seasonal stratification, sometimes with

INDIAN J. MAR. SCI., VOL. 33, No. 2, JUNE 2004

130

S

Fig. 1—Location map of the study area (bathymetry contours are in meters). Tidal data used in the present study are for Kandla, Navlakhi, Sikka and Okha ports. Different locations shown in solid squares are: (1) Jakhau, (2) Mandvi, (3) Mundra, (4) Kandla, (5) Navlakhi, (6) Jodiya, (7) Sikka, (8) Kalubhar, and (9) Okha

salt-wedge formation, associated with the height of the two monsoons10. Sediment dispersal pattern studies within the Gulf are essential in view of the maintenance of the existing/proposed ports/harbours and power plants, navigation, understand the dispersal of industrial pollutants (brine, oil, etc.) and protection of coastal habitats like coral reefs. Hence, it forms a major input towards achieving the goals of Integrated Coastal Zone Management (ICZM). Materials and Methods Sequential IRS-P4 OCM (spectral channel central wavelength [bandwidth]: 412[20], 443[20], 490[20], 510[20], 555[20], 670[20], 780[40], 860[40] nm; Spatial resolution: 0.36 km; Payload altitude: 760 km; Recurrent period: 2-days; Satellite overpass: 1200±0020 hrs LST) data for path-09 row-13 covering January 2000 (12 cloud-free scenes) were used for this observation. Retrieval and mapping of suspended sediment concentration (hereafter SSC) from the OCM data were performed using the in-house OCM-processing software11 and SeaDAS (SeaWiFS Data Analyses Software), respectively, on UNIX (SGI - IRIX 6.3) operating system. The algorithm initially proposed by Tassan 12 was used to compute suspended sediments from OCM data. The SSC maps were derived using water-leaving radiance in band 490, 555 and 670 nm. The algorithm used for retrieval of SSC is:

( ) ( )log 1.83 1.26log 0.0 40.0SS X= + ∀ ≤ ≤

where is suspended sediment concentration (in mg l

S-1) and

( ) ( ) ( )( )

0.5555

555 670490S

RrsX Rrs Rrs

Rrs⎡ ⎤

= ⎡ + ⎤ × ⎢ ⎥⎣ ⎦ ⎢ ⎥⎣ ⎦

Rrs(λ)is remote sensing reflectance in respective wavelengths (λ). The retrieval accuracy13 of SSC from OCM data is within 15%. Tidal information for three major ports in the Gulf - Okha, Navlakhi and Kandla, during January 2000, was taken from the Indian Tide Table14. To observe the west-east propagation of tide in the Gulf, the tide conditions at Sikka were computed (a minor port in the south central gulf) using the conventional method:

[ ] mint t t th H r H H majδ χ δ= × + ∀ = − χ where min min and maj majMSL SCF MSL SCFχ χ= + = +

th = tidal height at the minor port (here Sikka) at time t in meters

tH = tidal height at the major port (here Kandla) at time t in meters

minχ = corrected mean level for the minor port (here Sikka) in meters

majχ = corrected mean level for the major port (here Kandla) in meters

r = average ratio for the minor port SCF = seasonal correction factor MSL = mean sea level

PRADHAN et al.: APPLICATION OF IRS-P4 OCM DATA

131

Corresponding times at the minor port were calculated as tmin=tmaj+δ t where δ t is the time difference in hours. Figure 2 shows tide curves at Okha, Sikka, Kandla and Navlakhi during January 2000. Also depicted are the tidal conditions at the time of OCM pass. The SSC maps of consecutive dates were observed to be at different stages of tidal cycle and do not provide unequivocal information about the sediment dispersal

alone. Therefore, the tidal information14 was used to arrange SSC maps following sequential flooding and ebbing conditions. QuikSCAT wind vectors (data available for few days over the study area) were compiled to verify the possible contribution to the transport processes over the shelf region. The SeaWinds on QuikSCAT Level-3 data (25 km gridded values of scalar wind speed, meridional and zonal components of wind velocity)

Fig. 2—Tidal conditions at Kandla, Navlakhi, Sikka and Okha during OCM pass time in January 2000


132

were obtained from the NASA15. Daily wind vectors were generated by averaging data for morning (0600 hrs LST) and evening (1800 hrs LST) overpasses. The sediment dispersal patterns were studied along with corresponding wind and bathymetry data.

Results and Discussion The high tidal influx covers the low-lying areas of over 1500 km2 comprising intricate network of creeks, marshy tidal flats and rock outcrops. The associated

tidal currents are fairly strong and bimodal in nature having two dominant directions - upstream during flood and downstream during ebb in all encompassing oscillatory motions. Surface currents vary from 1.5 - 2.5 knots at the mouth to 3 - 5 knots in the central portion of the Gulf 16. Strong currents normally occur during mid-tide, i.e., 2 - 3 hrs before and after low and high tides. Spring currents are about 60 - 65% stronger than neap currents.

Fig. 3—Sediment dispersal patterns as per ebbing tide conditions for Okha, sequenced using OCM data. (Refer Fig. 2 for tide conditions

at Okha, Sikka, Kandla and Navlakhi during time of OCM imaging). Rectangles on 18th and 14th has reference to Fig. 5


133

The study of suspended sediment dispersal pattern retrieved using OCM data (Figs 3 & 4) along with corresponding wind and bathymetry data (Fig. 5) led to dividing the Gulf of Kachchh into six zones (Fig. 6). The identified zones are: 1) NE of Jodiya - Kandla sector; 2) Region between Kalubhar - Mundra sector and Jodiya- Kandla sector;

3) Region between Okha - Mandvi sector and Kalubhar - Mundra sector; 4) Northern shore; 5) Southern shore; 6) West of Okha - Mandvi sector (shelf region). The role of tidal stream currents in sediment dispersal in and around the Gulf is observed to be dominant. The major pathways of suspended sediments during flooding and ebbing conditions are derived and schematically shown in Fig. 7. Relative

Fig. 4—Sediment dispersal patterns as per flooding tide conditions for Okha - sequenced using OCM data. (Refer Fig. 2 for tide conditions at Okha, Sikka, Kandla and Navlakhi during time of OCM imaging). Rectangles on 26th and 6th has reference to Fig. 5


134

turbidity conditions during a tidal cycle for these zones as observed on OCM derived suspended sediment concentration maps (Figs 3,4) are shown in Table 1. Observations of these zones are discussed in the following paragraphs: Region west of Okha forming the shelf region at the mouth of the Gulf behaves as a highly dynamic zone (Fig. 6, zone 6) and was observed to play a key role in sediment dispersal pattern within the Gulf of Kachchh as is evidenced by different linear shaped, clockwise and anticlockwise circulation patterns observed on OCM derived suspended sediment maps (Figs 3,4). The different patterns indicate different surface current directions, primarily, East-West, NW,

SE and SW are observed. The sediment concentrations are relatively less (Table 1), and the dispersal patterns are controlled by depth, tidal current and wind conditions. The patterns of the suspended sediments in this region are broadly of three types i.e. i) moderate to high suspended sediments seen as green patches (Fig. 3, panel 2 and 4; Fig. 4, panel 1, 4 and 5), indicating that area gets sediment flux from the mouth of the Indus ii) low suspended sediments seen as dark blue to blue patches (Fig. 3, panel 1 and 6; Fig. 4, panel 2), indicating sink regions and iii) moderate suspended sediments with linear, curvilinear patches seen as green patches within light blue regions (Fig. 3, panel 3 and 5; Fig. 4,

Fig. 5—Wind speed and direction over the shelf region off the Gulf of Kachchh during OCM overpass dates used in the present study

Fig. 6—Turbidity zonation in and around the Gulf interpreted using Figs 3 and 4. 1-NE of Jodiya - Kandla sector; 2-Region between Kalubhar - Mundra sector and Jodiya- Kandla sector; 3-Region between Okha - Mandvi sector and Kalubhar - Mundra sector; 4-Northern shore; 5-Southern shore; 6-West of Okha - Mandvi sector


135

Fig. 7—Schematic diagram of the suspended sediment dispersal pathways over a complete tidal cycle in and around the Gulf of Kachchh;

Note: The eastward arrows represent predominant flooding condition and vice versa

Table 1—OCM derived turbidity conditions in the Gulf of Kachchh during tidal cycle (Sequenced as per flooding and ebbing conditions at Okha)

Date of OCM pass

for which SSC maps were generated

Region NE of Jodiya-Kandla sector (mg l-1)

Region between Kalubhar-Mundra and Jodiya-Kandla

sector (mg l-1)

Region between Okha-Mandvi and Kalubhar-Mundra

sector (mg l-1)

West of Okha-Mandvi sector

(mg l-1)

Northern shore (mgl

Southern shore (mg l-1) -1)

Jan.26, 2000 >40 8-10 10-40 <1-15 15->40 15->40 Jan.10, 2000 >40 3-10 7->40 8-12 15->40 15->40 Jan.08, 2000 >40 3-10 7->40 <1-15 15->40 7->40 Jan.24, 2000 >40 8-10 7->40 <1-15 15->40 15->40 Jan.22, 2000 >40 8-10 15->40 15->40 15->40 15->40 Jan.06, 2000 (Flooding)

>40 3-10 7->40 8-12 15->40 7->40

Jan.04, 2000 >40 2-5 8-10 8-12 8-10 15->40 Jan.20, 2000 >40 8-10 <1-15 8-12 8-12 15->40 Jan.02, 2000 >40 2-5 8-12 8-10 <1-15 15->40 Jan.18, 2000 >40 8-10 <1-15 8-12 15->40 15->40 Jan.16, 2000 >40 5->40 15->40 <1-15 15->40 15->40 Jan.14, 2000

(Ebbing) >40 5->40 15->40 <1-15 15->40 15->40

panel 3 and 6), indicating transport direction of sediments. During high to near high tide condition at Okha (Fig. 3, panel 1; Fig. 4, panel 6; see with reference to Fig. 2), it is observed that sediments are being sucked into the Gulf of Kachchh from the shelf region. This takes place even during unfavourable wind conditions for example even in the presence of strong north-easterly winds (Fig. 4, panel 6; see with reference to Fig. 5, lower right panel). During low to near low tide condition at Okha (Fig. 3, panel 6; Fig. 4, panel 1; see with reference to Fig. 2), it is observed

that sediments are fanning out of the Gulf of Kachchh towards the shelf region. These sediments transport to larger distance under weak wind conditions (Fig. 4, panel 1; see with reference to Fig. 5, lower left panel) or otherwise form linear to curvilinear clockwise and anti clockwise patterns in the shelf region under influence of strong opposing wind conditions (Fig. 3, panel 6; see with reference to Fig. 5, upper right panel). It is observed that during mid-tide condition at Okha and under influnce of strong northeasterly wind large transport of sediments takes place in the shelf


136

region (Fig. 3, panel 4; see with reference to Fig. 5, upper left panel). The interplay of high magnitude east-west currents with other prevalent currents in this region determines the nature of sediments being transported into and out of the Gulf of Kachchh. Extensive work on the shelf sedimentation based on analysis of sediment samples collected from the macro-tidal Gulf of Kachchh has reported17 two distinct sources of sediments – (i) the Indus sediments (typically chlorite, illite and fluvial mica) in the northern regions and (ii) sediments derived from the Saurashtra peninsula (usually smectite and bioclastics) in the southern region. Within the Gulf of Kachchh, distribution of suspended sediments is highly dynamic for the region between Okha - Mandvi sector and Kalubhar - Mundra sector (Fig. 6, zone 3). It is observed that suspended sediments are transported as a V-shaped front into the Gulf during flooding conditions (Fig. 4, panel 3-5, seen as red patches) and as linear plumes fanning out of the Gulf during ebb conditions (Fig. 3, panel 4-6, seen as green and yellow patches). The shape of the bottom sediment map of Geological Survey of India (GSI, 1975; not shown here) supports this phenomenon and is also partly in agreement with the surface flow patterns as observed in the suspended sediment maps (Figs 3,4). It suggests that the source of suspended sediments is from both northern as well southern parts of the mouth of the Gulf of Kachchh. Sediments are being transported into the Gulf along the northern shore from the Indus delta during flooding phase. The southern shore gets sediments from the southern parts (Saurashtra peninsula) of the Mouth. This region shows that during flooding phase SSC varies from 3 - 10 mg l-1 and during ebbing phase 2 - >40 mg l-1 (Table 1). Presence of suspended sediments, varying from moderately turbid to highly turbid, and exposure of several shoals during the tide cycle were also noticed. The orientations of these shoals indicate predominantly East-West currents. These shoals also act as barrier to smooth flow of tidal waters, which reduces the current speed considerably. The northern shore (Fig. 6, zone 4) was under higher suspended sediment load during flooding phase (Fig. 4, panel 4 and 5) with an eastward current direction (see Fig. 7). Sediments were being transported in the Gulf along northern shore from the Indus delta during flooding phase as evidenced by sediment plumes oriented towards SE (Figs. 4, panel 6). River Indus debouches about 50 × 106 tons of

sediments16, and part of this discharge moves along the coast through shelf circulation. The SSC varies between 15 - > 40 mg l-1 during flooding phase and <1 - > 40 mg l-1 during ebbing phase (Table 1). The region west of Mundra showed sediments with higher suspended sediment load during flooding (Fig. 4, panel 4 and 5, seen as red patches) and moderate SSC during ebbing conditions. The region east of Mundra upto Kandla shows higher SSC during flooding and lower SSC during ebbing (Table 1). The southern shore (Fig. 6, zone 5) was perceived with higher sediment concentrations during ebbing phase (Fig. 3, panel 6). Although the tidal stream currents are weaker along both the shores, we speculate the higher concentrations down the southern shore could be because of the re-suspension of the bottom sediments along its wider intertidal zone compared to the counterpart in the northern shore. It is around 1 km at Okha and around 5 km at Jodiya. Many of the wave cut, eroded islands and shallow banks on the southern shores of the Gulf have coral reefs on their northern and western sides extending from intertidal to subtidal zones. These are mostly fringing reefs, which protect the coast, increase its stability and help establish sheltered harbours. The sediments from the Saurashtra peninsula are transported in NW direction as observed by sediment plume (Fig. 3, panel 4-6) and deposit on southern part of the shelf and southern shores. The region between Kalubhar-Mundra and Jodiya-Kandla sector is comparatively calm and shows clear to low turbid water compared to its surroundings for most parts during flooding as well ebbing (Fig. 3 and 4, seen as blue and green colour). This region is identified with low sediment concentrations at all tide conditions (Table-1), which provides feasible environmental condition for coral reefs to develop in this region. Irrespective of tidal conditions, the region NE of Jodiya-Kandla sector (Fig. 6, zone 1) show high SSC (Fig. 3 and 4 seen as red patches in all panels; Table 1) as the tidal range of more than 6 m floods vast areas up to Little Rann and the fine sediments remain in suspension for most of the time. Physiographically, this region is divided into two major creeks i.e., Kandla creek and Hansthal creek separated by Sathsaida Bet (Island). As the tide propagates into these creek systems, it amplifies during flood tide and there is also increase in the magnitude of currents. These currents erode the adjoining region of fine sediments18, which are brought into suspension and are transported outwards


137

during ebbing phase. They form shoals at the outer part of this region oriented in east-west direction, which are exposed during ebb conditions. This study demonstrated that because of its synoptic view (1420 km2), narrow spectral channels specifically chosen to retrieve suspended sediment concentration, high spatial resolution (among presently available ocean colour sensors) and temporal frequency of two days, IRS-P4 OCM data is extremely useful in understanding regional sediment dynamics. It is possible to generate suspended sediment maps at different stages of a tidal cycle in the macro-tidal regime to understand sediment transport. The information is useful for maintaining the approach channels of ports in addition to understanding dispersal of pollutants such as oil slicks under tidal influence. It has been planned to improve the algorithms for retrieving suspended sediment concentrations and utilise the information in mathematical models related to sediment transport. Acknowledgement Dr. A.K.S. Gopalan, former Director SAC, Dr. K.N. Sankara, Director SAC, Dr. R.R. Navalgund, Director, NRSA (and former Deputy Director, Remote Sensing Applications Area, SAC) and Mr. B.N. Krishnamurthy, former Adviser, DOD are thanked for their support as well technical guidance from time to time while reviewing the project on Coastal Processes. Y P thanks Mr. Benidhar Deshmukh for useful discussions. References

1 Nayak S R, Monitoring the coastal environment of India using satellite data, Science, Technology and Development (Frank Cass & Co Ltd Essex, U K), 14 (2), (1996) 100-120.

2 Nayak, SR & Sahai B, Coastal morphology: a case study of the Gulf of Khambhat (Cambay), Int J Remote Sensing, 6 (1985) 559-567.

3 Desai, P S, Narain, A, Nayak, S R, Manikiam, B, Adiga, S & Nath, A N, IRS 1A applications for coastal and marine resources, Curr Sci, 61 (1991) 204-208.

4 Nayak, S, Chauhan, P, Chauhan, HB, Bahuguna, A & Narendra Nath, A, IRS-1C applications for coastal zone management, Curr Sci, 70 (1996) 614-618.

5 Shaikh, M G, Nayak, S R, Shah, P N & Jambusaria, B B, Coastal landform mapping around the Gulf of Khambhat using Landsat TM data, J Indian Soc Rem Sen, 17 (1989) 41-48.

6 SAC Report, Coastal processes (IRS-P4 OCM Utilisation/SATCORE Project Report), Vol 3, Scientific Report No IRS-P4/SATCORE/SAC/RESIPA/MWRG/ SR/03/03, (Space Applications Centre, Ahmedabad, India), (2003) pp. 150.

7 Prasad, J S, Rajawat, A S, Pradhan Y, Chauhan, O S & Nayak, S R, Retrieval of sea surface velocities using sequential ocean colour monitor (OCM) data, Proc Indian Acad Sci (Earth Planet Sci), 111 (2002) 189-195.

8 IOCCG Report, Remote sensing of ocean colour in coastal, and other optically-complex waters, in Reports of the International Ocean Colour Coordinating Group, No 3, edited by Sathyendranath, S, (IOCCG, Dartmouth, Canada) 2000, pp. 140.

9 Gupta, H V S & Hashimi N H, Fluctuation in glacial and interglacial sediment discharge in the Gulf of Kutch, Indian J Mar Sci, 14 (1985) 66-70.

10 Bauer, S, Hitchcock, G L & Olson, D B, Influence of monsoonally-forced Ekman dynamics upon surface layer depth and plankton biomass distribution in the Arabian Sea, Deep Sea Res Part I, 38 (1991) 531-555.

11 Anonymous, Atmospheric corrections, bio-optical algorithm development and validation of IRS-P4 OCM data (IRS-P4 OCM Utilisation/SATCORE Project Report), Vol 1, Scientific Report No IRS-P4/SATCORE/SAC/RESIPA/ MWRG/SR/1/2003, (Space Applications Centre, Ahmedabad, India) (2003) pp 116.

12 Tassan, S, Local algorithm using SeaWiFS data for retrieval of phytoplankton pigment, suspended sediments and yellow substance in coastal waters, Appl Opt, 12 (1994) 2369-2378.

13 Chauhan, P, 2003; (Personal Communication). 14 Anonymous, Indian Tide Tables-2000, (Survey of India,

Dehra Dun, India), 1999. 15 NASA/PODAAC, JPL California Institute of Technology, Web

Resource ftp://podaacjplnasagov/pub/ocean_wind/quikscat/L3/ Accessed on March 21, 2001.

16 Nair, R R, Hashimi, N H & Rao P C, On the possibilities of high velocity tidal stream as dynamic barrier to the longshore transport: evidence from the continental shelf of the Gulf of Kutch-India, Mar Geol, 47 (1982) 77-86.

17 Chauhan, O S, Influence of macrotidal environment on shelf sedimentation, Gulf of Kachchh, India Cont Shelf Res, 14 (1994) 1477-1493.

18 Pattanshetti, S S, Chauhan O S, & Sivakholundu K M, Quantification of changes in seabed topography with special reference to Hansthal Creek, Gulf of Kachchh, India, J Coastal Res, 9 (1993) 934-943.

ftp://podaac.jpl.nasa.gov/pub/ocean_wind/quikscat/L3/

application of irs-p4 ocm data to study the impact of tidal...

Documents