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Containment of Coastal Erosion

Forestalling Coastal Erosion at Sagar Island


In case of Sagar Island, recent observations by John Pethick (2008) suggest that four major areas of erosion are present: in the north east (Kuchuberia); the south east centre; Dublat on the south coast, and Beguakhali on the south coast. In contrast to these erosional areas, the remainder of the Island appears to be accreting. Large areas of recently developed mangrove and salt marsh are present on both east and west coasts and a substantial area of accretion was observed in the centre of the south coast east of Sagar Mela ground. Moreover both the south and west coasts of the Island have wide intertidal areas with sediment consisting mainly of fine sand. Sand accretion on the north-east coast is particularly rapid although, further north, the island of Ghoramara, that was formerly accreting, is reported to have eroded significantly over the past few years. In order to plan strengthening of any existing remedial measures and to bring in new remedial measures in both the areas, a thorough understanding of the coastal processes is required.

The erosion-accretion pattern in the Sundarban coastal zone has probably been made over-simplified because of the scale of investigation and reliance on only archived map and imagery data. Detailed picture may be exemplified by observations made in 2008 in Sagar Island.

Figure 12: Major embankment erosion sites on Sagar Island. Ghoramara Island is an accretional site included for identification purposes (Pethick, 2008).

All inhabited islands of Sundarban including the Sagar Island have embankments along the periphery of the islands to prevent over topping due to high flood/tide and erosion by meandering creeks bordering them. In addition to the low standard of defence provided by the embankment, a further hazard is presented by the erosion of the largely earth embankment. Recent observations suggest that four major areas of erosion are present: in the north east (Kuchuberia); the south east centre; Dublat on the south coast, and Beguakhali on the south coast (see Figure 12). In addition to these localised areas of high erosion, it appears from a preliminary scrutiny of the 1967 topographic maps and the 2006 satellite image that the entire south coast of the island has been eroding at least for the past 50 years. This general erosion appears to be at a much slower rate than that presently experienced at either Dublat or Beguakhali. In contrast to these erosional areas, the remainder of the Island appears, from field observations supported by map/satellite analysis, to be accreting. Large areas of recently developed mangrove and salt marsh are present on both east and west coasts and a substantial area of accretion was observed in the centre of the south coast east of Sagar Mela ground. Moreover both the south and west coasts of the Island have wide intertidal areas with sediment consisting mainly of fine sand. Sand accretion on the north-east coast is particularly rapid although, further north, the island of Ghoramara, that was formerly accreting, is reported to have eroded significantly over the past few years (see Figure 12). These sand beaches appear to play a central role in protecting the embankment from erosion and encouraging mangrove and salty marsh colonisation.

Estuarine meanders

According to John Pethick (2008) the pattern of erosion and accretion on the east coast of Sagar Island, where it is bordered by the large estuarine channel of the Muriganga, appears to follow a meandering pattern of the tidal channel. Meander wave length and amplitude are positively correlated with tidal discharge and, as discharge increases towards the estuary mouth, so the meander pattern is one of gradual increase seaward.
East bank

If the satellite image for the Island is inspected (Figure 13), it will be seen that the pattern of erosion at the present time could be explained by the meandering pattern of the Muriganga channel.  The outside bend of the meander forms characteristic scrolls or bays in the east coast of Sagar Island and similar ones on the east bank of the Muriganga. These scrolls develop by erosion of the banks of the channel forming the bays seen both in the field and in the satellite image. A possible configuration of the meander responsible for these scrolls is shown in Figure 13. Further inspection shows that there are several other such meander scrolls, now abandoned, located along both banks of the Muri Ganga. These scrolls or bays have been subsequently infilled by sediment deposition and colonised by mangrove (the dark red colour in the satellite image: Figure 13) A possible former meander pattern that might explain these abandoned meander scrolls is shown in Figure 13. It is possible that these two distinct meander patterns are caused by deposition in the Hugli estuary forcing the channel in the Muri Ganga to avulse (i.e. switch from one pathway to another) possibly as a response to the alternate accretion and erosion of the estuarine bed surrounding the island of Ghoramara.

Figure 13: Conjectural meander pathways in the Muriganga channel.  The present situation is indicated by the thick black line.  A possible former meander pathway is shown by the dotted line. Note that erosional areas are predicted at the outside of each meander bend (Pethick, 2008).

Since the amplitude of a tidal meander is related to the tidal discharge and since the tidal discharge of the Muriganga is unlikely to change significantly, even allowing for sea level rise, it may be proposed that the size of these meander scrolls and thus the erosion associated with them, will not increase.  Thus a finite erosional zone could be defined along the east coast of Sagar Island. It is equally apparent that the location of these erosion scrolls may switch from place to place along the coast of Sagar Island depending on the pattern of change in the larger Hugli system.

Figure 14. Conjectural meander pathways in the Hooghly Estuary. Present situation is shown by the thick white line. A possible former pathway is indicated by the dotted line. The location of a training wall that may have triggered avulsion is indicated (Pethick, 2008).

West bank

The western or Hugli bank of Sagar Island is almost wholly accretional at the present time. It appears from anecdotal evidence that this is a relatively recent development and that erosion formerly characterized much of this coast. There may be several explanations of this reversal from erosion to accretion. One possibility proposed here is again connected to the tidal meander, this time of the Hugli itself.  The mangrove area located immediately south of the arrow shown in Figure 14 represents a former meander bend. The meander pathway that may have caused this erosion is shown as a dotted white line in Figure 14. Avulsion of this meander to its present pathway, shown as a thick white line in Figure 14, may be a result of the construction of a training wall running north east from the extreme northerly tip of Nayachar Island. This training wall could have intercepted the dominant ebb tidal flow that previously was directed into the Haldia channel, forcing flow eastward into the main channel and causing avulsion of the meander to its present location.  This in turn  has led to the west coast of Sagar becoming located at the extreme southerly tip of the  inside of the meander bend and thus receiving sediment from the flow rather than losing it to erosion as formerly was the case. Majority of sediment, both fine sand and silts are derived from the land and carried down to Sagar Island by the Hugli. Most of this sediment transport will be during monsoon periods when fresh water discharge is high.

The sediment pathways diverge to the north of Sagar Island at Ghoramara Island (Figure15). Most sediment carries on in the Hugli estuary but some is diverted into the Muriganga. Before the building of the embankment, the fine grained silts carried by these flows would have been carried into the interior  of Sagar island and deposited from the low velocity flows that would have resulted there both due to the shallow water depths and the presence of salt marsh and mangrove vegetation. The embankment has prevented this sediment deposition save for those areas where mangrove lies seaward of the embankment. Instead the silts are unable to be deposited in the high inter-tidal flows and are carried seaward where they are lost to coastal systems.

The critical deposition velocity for the coarser grained fine sand content of the tidal flows is much higher than that of the silts so that these sands are deposited seaward of the embankment in areas not covered by salt marsh or mangrove.  It is these sands that form the extensive inter-tidal beaches on both south and west coast of the island and to a lesser extent the inter-tidal area of the east coast.

After field investigation, coupled with detailed examination of satellite imagery, of the coastal landforms along the coast of the island, John Pethick suggests the pattern of sediment movement as shown in Figure 15. Sediment moves south along the shore of the Muriganga driven by tidal currents.  As it moves into the south shore, the transport process becomes wave dominated with waves approaching from the south to south west, and sediment is moved westward towards Beguakhali. Here it enters the Hugli system and continues northwards until it meets the southerly moving sediment brought down stream by the Hugli ebb tide.  The meeting of these two opposing flows results in a sediment convergence and extensive and rapid deposition. This deposition has in turn resulted in the formation of  a wide inter-tidal area in the centre of the west coast of Sagar with significant areas of mangrove forming inshore of this.

 Figure 15: Possible sediment transport pathways around Sagar Island based on configuration of coastal landforms. A sediment convergence zone is proposed on the west coast (Pethick, 2008).

South coast erosion

The overall erosion of the south coast of Sagar Island appeared to John Pethick(2008)  to be a product of rising sea levels and is to be explained by the estuarine roll-over model. This general background erosion is however at a much slower rate (approximately 1m per year) than the erosion rate at Dublat and Beguakhali where rates of 10m per year are experienced at present. It is noticeable that both these erosional areas are located at the ‘corners’ of the island: that is where sediment transport pathways must change direction abruptly.  A possible explanation for these high erosion rates is therefore that sediment carried along the coast is temporarily moved offshore at these ‘corners’ causing sediment starvation and erosion as they do so. This process may be exacerbated by the specific landforms at each site.

Figure 16 shows that the hard coastal defences at Beguakhali erected to protect the former lighthouse and pilot station, although they have failed to arrest the erosion here, have succeeded in slowing the erosion rate down relative to that of the earth embankments on either side.  The result is that the defended stretch now forms a headland along which increased current velocities prevent the deposition of fine sand. Figure 12 shows that at Dublat the formation of a spit, probably aided by the planting of casuarinas trees, has forced the sediment transport pathway offshore and caused a sediment starvation zone immediately to the west.

Figure 16: Sketch map showing the coastal headland at Beguakhali. Proposed embankment realignment shown as dashed line (Pethick, 2008).

Figure 16: Sketch map showing the impact of a spit on the sediment pathway at Dublat. Proposed embankment realignment shown as dashed line (Pethick, 2008).

In both cases erosion may be reduced, if not eliminated, if a gentler shoreline curve was to develop, rather than the existing abrupt change in orientation as shown in Figure 15 and Figure 16. It seems probable that such pathways will eventually emerge as a result of the differential erosion and accretion on both sections of coast.

Preventive Measures at Sagar Island under ICZM Project

The initial study has indicated that there will be two prong solutions, namely short-term and long-term. The long-term solution will be to devise and design measures like construction of hydraulic groins through opening up of the mouth of estuaries, removal of naturally created barrier to supply sediment at the starved regions, etc. and will be based on sediment flow analysis and outcome of more specific research on coastal processes around Sagar Island as per Component N. Such designing will be completed concurrently with coastal process monitoring scheduled for 3 years.

Short-term solution will be taken up at Dublat where heavy waves breaking continuously at the foot of the embankment erode the embankment during high tide period. The embankment as a result fails very often resulting in tidal inundation and consequence loss of crops and public properties. The planned short-term solution will include strengthening and armouring of the embankments. It is proposed to armour the embankment for 1500 m length by 2.0 m x 1.0 m x 0.60 m thick M-15 concrete block pitching works over filter beds in addition to construction of cross walls. The design parameters are as follows:

Existing ground level (average)

(+) 2.00 m GTS

Highest and average high tide level

(+) 5.40 m GTS and (+) 4.50 m GTS

Average and lowest low tide level

(-) 1.20 m GTS and (-) 2.00 m GTS

Sea side berm level and height of wave

1.00 – 2.00 m and 1.50 – 2.00 m

Formation level of proposed sea dyke

(+) 8.20 m GTS

Top level of proposed block pitching

(+) 7.90 m GTS

Sea- and country side slope of proposed sea dyke

1:6 and 1:2

Height of proposed sea dyke above beach level

6.70 m

Sea dyke encased in sea side below berm level

1.50 m

Conventional size of C.C. (1:2:4) block

2.00 m x 1.00 m x 0.60 m

Size of sausage at the toe of block pitching

1.00 m x 0.60 m

Top of sea side portion at +8.20 m level

Brick wall with corbelling in R/S Top

Length of the proposed sea dyke

1500.00 m

Total sea side slope length of proposed sea dyke

48.05 m

Free board of the proposed dyke

3.70 m

Thickness of geofabric filter

2.0 mm

Spacing of intermediate cross wall

15.00 m apart

Thickness of cross wall and depth

400 cm thick, 1050 mm depth


The implementing agency for the short-term construction work will be Sundarban Infrastructure Development Corporation Ltd (SIDCL). The cost has been estimated at Rs 27.33 crores having a works cost component of 25.36 crores. The short term work will be completed in 3 years but at present no budgetary fund allocation has been made.