Coastal erosion in response to wave dynamics operative in Sagar Island, Sundarban delta, India

doi:10.1007/s11707-009-0001-0

Front Earth Sci Chin

0, Vol.

Issue () : 21-33 https://doi.org/10.1007/s11707-009-0001-0

Research paper

Coastal erosion in response to wave dynamics operative in Sagar Island, Sundarban delta, India

Barendra PURKAIT(

)

Geological Survey of India, Map and Publication Division, 29, J. L. Nehru Road, Kolkata 700016, India

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Abstract

Coastal erosion at Sagar Island of Sunderban delta, India, has been critically studied. The area is in the subtropical humid region. There are mainly three seasons viz: winter, summer and the monsoon. Different wave dynamic parameters were measured from theodolite observations with leveling staff and measuring gauges during lunar days at two sections of the western and eastern parts of the coastal zone during post-and pre-monsoons. A comparative study was made on the erosion/depositional pattern between the two sections in relation to different hydrodynamic parameters prevailing in these two sections. Plane table mapping was carried out to demarcate the different geomorphic units. The marine coastal landforms show dune ridges with intervening flats bordered by gently sloping beach on one side and a flat beach on the other side. The western part of the beach is mainly sandy; whereas the eastern part is silty and clayey with mud bank remnants. Actual field measurements indicate that the coastal dune belt has retreated to the order by about 20 m since 1985. The eastern part of the beach has lowered by about 2 m since 1985 and the western part was raised almost to the same tune. It is observed that accretion in the western and central parts of the beach took place; whereas severe erosion in the eastern part made the beach very narrow with remnants of mud banks and tree roots. Frequent embankment failures, submergence and flooding, beach erosion and siltation at jetties and navigational channels, cyclones and storm surges made this area increasingly vulnerable.

Keywords Bay of Bengal coastal erosion Sagar Island Sundarban delta wave dynamics

Corresponding Author(s): PURKAIT Barendra,Email:baren_purkait@gmail.com

Issue Date: 05 March 2009

Cite this article:

Barendra PURKAIT. Coastal erosion in response to wave dynamics operative in Sagar Island, Sundarban delta, India[J]. Front Earth Sci Chin, 0, (): 21-33.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-009-0001-0
https://academic.hep.com.cn/fesci/EN/Y0/V/I/21

Fig.1 Location Map

Tab.1 Climate and hydrography at the mouth of the Ganga-Brahmaputra Rivers

Fig.2 Geomorphological map of the coastal area of Sagar Island at Central part (Topoban area, sector - B) and eastern part (Dublat area, sector - C), Bay of Bengal, District-24 Parganas (South), West Bengal

Fig.3 Field photographs showing beach erosion at Sagar Island: 3A- Beach erosion, 3B- Mud embankment with bamboo poles to prevent beach erosion. 3C- Beach erosion showing remnants of tree roots, 3D- Remnant of mud bank exposed on the beach

Sl. No.	parameters	new moon(NM)	NM+2	NM+8	NM+10	FM-1	average	NM+1	NM+3	NM+9	NM+11	full moon(FM)	average
1	date	20.2.04	22.2.04	28.2.04	2.3.04	6.3.04		21.2.04	23.2.04	29.2.04	3.3.04	7.3.04
2	inter-tidal width/(m	362	328	194	138	302	264.8	132	142	51	64	131	104
3	R.L difference between HWL & LWL	3.765	4.185	2.19	1.135	3.765	3.008	4.585	4.580	1.930	2.475	4.36	3.586
4	beach face angle (β) (degree)	0.596	0.731	0.647	0.471	0.714	0.651	1.991	1.848	2.169	2.216	1.907	1.976
5	Swash line bearing (degree)	295-115	105-285	106-286	108-288	108-288		91-271	86-266	95-275	92-272	89-269
6	distance of gauges on profile line fromfixed point /m	95, 115,152	278, 285,293	341, 349,358	365, 371,379	304, 312,320		372, 364,354	74, 79.6,87.6	334, 327,319	342, 335,329	359, 355,348.8
7	mean water depth/(m	0.143	0.250	0.354	0.250	0.417	0.283	0.496	0.550	0.571	0.417	0.546	0.516
8	mean wave height (H) in m	0.392	0.578	0.629	0.422	0.298	0.464	0.882	0.762	0.842	0.706	0.436	0.726
9	significant wave height (H_1/3)) in m	0.731	0.787	0.831	0.533	0.443	0.665	1.154	0..978	1.017	1.002	0.615	0.953
10	mean wave period /s	9.817	9.694	5.804	10.218	6.432	8.393	13.792	12.856	6.274	6.432	9.771	9.825
11	mean wave approach angle (α) in degree	9.25	3.75	1.25	1	12.75	5.6	8.25	5.75	13.25	8	3.75	7.8
12	velocity of longshore current/ (m·s^-1)	0.318	0.158	0.028	0.029	0.339	0.174	1.331	0.063	1.039	0.673	0.417	0.704
13	wave form velocity (C) in m/sec	2.290	2.849	3.106	2.567	2.648	2.692	3.676	3.588	3.723	3.318	3.103	3.482
14	wave length (L) in m	11.852	15.211	10.818	16.003	13.003	13.377	29.978	29.862	14.846	13.004	22.610	22.060
15	wave Steepness/ (H·L^-1)	0.033	0.038	0.058	0.026	0.023	0.036	0.030	0.026	0.057	0.054	0.037	0.037
16	max. & min. wave height (m)	0.90,0.10	1.00,0.10	0.90,0.40	0.70,0.30	0.70,0.10	0.84,0.2	1.8,0.30	1.0,0.50	1.30,0.50	1.30,0.30	0.80,0.10	1.3,0.34
17	surf scaling factor /∈	236.471	234.451	544.954	389.08	148.468	310.685	33.912	36.480	59.999	46.879	25.731	40.600
18	breakers type	collapsing	collapsing	collapsing	collapsing	collapsing		collapsing	collapsing	collapsing	collapsing	collapsing
19	rate of sediment transport/ (m·s^-1)	0.058	0.063	0.027	0.008	0.053	0.042	0.418	0.213	0.617	0.235	0.039	0.304
20	wave energy (E) in // (J·ms^-2)	191.31	415.55	492.98	221.83	110.687	286.471	969.455	724.386	885.149	621.347	236.55	687.377
21	transect	A-0						transect	C-0

Tab.2 Post-monsoon data of wave parameter measurements at Sagar coastal zone
L u n a r d a y s L u n a r d a y s

Sl. No.	parameters	new moon(NM)	NM+2	NM+8	NM+10	FM-1	average	NM+1	NM+3	NM+9	NM+11	full moon(FM)	average
1	date	20.4.04	22.4.04	28.4.04	30.4.04	4.5.04		21.4.04	23.4.04	29.4.04	1.5.04	5.5.04
2	inter-tidal width/(m	409	360	199	240.40	388	319.28	150	158	52	128	180	133.6
3	R.L difference between HWL & LWL	4.495	4.065	1.805	2.285	4.485	3.427	4.36	4.385	1.525	3.205	5.595	3.814
4	beach face angle (b) (degree)	0.630	0.647	0.520	0.545	0.662	0.615	1.666	1.590	1.681	1.435	1.781	1.636
5	Swash line bearing (degree)	104-284	109.5-289.5	100-280	95-275	105-285		93.5-273.5	99-279	95-275	86-266	92-272
6	distance of gauges on profile line fromfixed point /m	226, 233,242	257, 259.6282.6	335, 344,357	331.4, 340.8,348.8	263, 269,277		363, 358,351	351, 341,336	302, 297,290	327, 321,314	358, 350,342
7	mean water depth/(m	0.575	0.675	0.475	0.688	0.988	0.68	0.808	0.558	0.983	0.708	1.142	0.840
8	mean wave height (H) in m	0.436	0.531	0.479	0.547	0.606	0.520	0.556	0.621	0.851	0.787	0.891	0.741
9	significant wave height (H_1/3)) in m	0.664	0.737	0.611	0.731	1.095	0.768	0.777	0.856	1.108	1.146	1.208	1.019
10	mean wave period /s	9.385	8.585	8.375	7.866	7.841	8.410	9.667	8.403	10.143	6.888	9.959	9.012
11	mean wave approach angle (a) in degree	7.5	1.5	7.25	2.25	3.75	4.45	3.75	5.25	4.5	1.5	2.250	3.45
12	velocity of longshore current/ (m·s^-1)	0.262	0.050	0.187	0.058	0.116	0.135	0.360	0.417	0.457	0.002	0.238	0.295
13	wave form velocity (C) in m/sec	3.149	3.44	3.059	3.480	3.954	3.416	3.658	3.401	4.241	3.830	4.466	3.919
14	wave length (L) in m	22.290	22.092	18.079	20.428	24.405	21.459	27.222	19.666	31.503	18.157	33.329	25.975
15	wave Steepness/ (H·L^-1)	0.020	0.024	0.026	0.027	0.025	0.024	0.020	0.032	0.027	0.043	0.027	0.030
16	max. & min. wave height (m)	1.00,0.10	1.10,0.10	0.90,0.20	1.10,0.20	1.20,0.10		1.00,0.10	1.20,0.10	1.60,0.20	1.40,0.20	1.60,0.20
17	surf scaling factor /?	246.384	312.314	457.902	494.908	370.608	376.423	43.545	61.463	62.476	122.026	59.116	69.725
18	breakers type	collapsing	collapsing	collapsing	collapsing	collapsing		collapsing	collapsing	collapsing	collapsing	collapsing
19	rate of sediment transport/ (m·s^-1)	0.080	0.026	0.090	0.010	0.009	0.043	0.075	0.122	0.056	0.014	0.033	0.060
20	wave energy (E) in // (J·ms^-2)	237.782	353.482	286.941	374.230	459.49	342.385	386.938	482.524	906.154	775.960	994.996	709.314
21	transect	A-0						transect	C-0

Tab.3 Pre-monsoon data of wave parameter measurements at Sagar coastal zone
L u n a r d a y s L u n a r d a y s

Fig.4 Grain size distribution patterns at different positions on the beach profiles along transects A-1, A-0, and A-4, showing the development of secondary mode with gradual increase of weight frequency % from east (A-4) to central (A-0) to west (A-1). Increasing of weight frequency of secondary modes indicates the presence of strong long shore current from east to west of the beach at this sector

Fig.5 Day to day change of beach profiles along transect A-0 during lunar days. a) Post-monsoon period; b) Pre-monsoon period

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