Morphology and sediment movement in a monsoon influenced open beach at Gangavali,near Gokarn (central west coast of India)

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1 Indian Journal of Marine Sciences Vol. 32(1), March 2003, pp Morphology and sediment movement in a monsoon influenced open beach at Gangavali,near Gokarn (central west coast of India) *M.S. Bhat, **V.C. Chavadi & *V.S. Hegde *Department of Civil Engineering, SDM college of Engineering and Technology, Dharwad ,Karnataka, India **Geology department, Karnatak University, Dharwad ,Karnataka,India [ E- mail : vshegde2001@yahoo.com ] Received 13 December 2001, revised 16 December 2002 Morphological and grain size studies have been carried out at monthly intervals (February 1993 to February 1994) to understand sedimentation pattern. Eight study sites were selected for morphological studies and five for grain size studies. It is observed that the morphological changes are cyclic i.e. erosion during monsoon and deposition during fair-weather season. The beach has experienced a loss of about 951 m 3 /m -1, a gain of about 1677 m 3 /m -1 and hence a net gain of about 725 m 3 /m -1 of sediments along the line of transect (all the 8 stations together) over a period of one year. A secondary cycle of erosion is observed between December and February. In general, the sediments of the study area are fine grained, well sorted, and symmetrical and platykurtic in nature. Mean grain size decreases, sorting improves and percentage of fine sand, positively skewed grains increases from north to south (G1 to G8). Beach experiences moderate turbulent energy conditions and the sediment movement is towards south in this beach.the prevailing conditions in this beach are ideal for recreational development. [ Key words: Beach morphology, sedimentation, grain size, longshore transport ] In micro tidal beaches, waves are the dominating forces in changing the morphology 1. In general, beaches with >0.5 km length are exposed to wide window (open to wave attack) wave approach 2, due to wave refraction, waves reach the shore at an angle resulting in alongshore current. Contrary to this, in the beaches with narrow window (sheltered) wave approach, waves reach almost normal to the shore and beach reflects most of its incident energy and thus behaves as reflective. Generally beaches with slope 6 to 12 have been defined as reflective and < 6 are designated as dissipative 3. Along the central west coast of India, beaches with wide window wave approach and with narrow window wave approach exist. Those with narrow window wave approach remain reflective. Contrary to this, the open beaches (present beach) owing to SW approach of monsoon (June to September) and NNW orientation of the coast, become reflective during monsoon and dissipative during other seasons. Therefore grain size grading is unstable 4 as against reflective beaches where size grading is stable over a period of time. In the present paper, the stability of the microtidal (tidal range < 2m), monsoon influenced open beach was studied through morphological and grain size trends to evaluate its potential to develop recreation center. Materials and Methods The study area is located on the central west coast of India between Gokarn and Gangavali (long , East. and lat , N). River Gangavali joins the sea in the north and the southern part is well protected by Umamaheshwari hillock with steep rocky cliffs. Eight stations/reference points along the beach (G1, G2, G3, G4, G5, G6, G7 and G8) have been selected (Fig. 1). The first location G1 has been fixed using Brunton compass with respect to Umamaheswari hillock and a temple in the Gangavali village and the subsequent positions by engineering survey with respect to G1. Beach profiles are monitored at monthly intervals 5. Sediment samples were collected using plastic coreliner of 8 cm diameter (from February 1993 to February1994), representing 5 cm depth from different micro-environments viz., backshore (berm), foreshore (high, mid and low tide) and at breaker levels (Brl) at selected locations (G1, G3, G4, G6 and G8). The sediment samples were washed, dried and treated 6 and subjected to size analysis to deduce textural parameters 7. Graphic measures were calculated to understand the depositional environment 8.

2 32 Indian J. Mar. Sci., Vol. 32, No. 1, March 2003 Fig. 1 Location map of the study area Results and Discussion The monthly beach profiles for all the locations (Fig. 2), show some salient and common features at each of these locations. During premonsoon (February to May), the beaches exhibit alternate erosion and accretion. The beach reaches its maximum growth by May with the development of wide berm and foreshore. During this season, prevailing long period waves with breaker height < 1.5 m favor deposition. During southwest monsoon (June to September), the beach experiences erosion. Maximum erosion was observed during July and August. The beach is characterized by narrow foreshore with concave slope, absence of berm and at places with sandy cliff. The high wave energy (H/T >0.25), short period waves with near normal approach are considered to be responsible for erosion of the beach. During intermonsoon (SW-NE) season (October to November), the beach experiences the replenishment of the materials that were lost during monsoon, however a secondary cycle of erosion was observed during NE monsoon (December-January). The concave features of the monsoon foreshore become complicated with development of berms and often with submerged bars few meter seaward of the low water line. However in the later stages with the progress in accretion, the submerged bar welded together to the Fig. 2 Monthly profiles changes along Gangavali-Gokarn beach foreshore. These features suggest more accretion in the lower foreshore (G8-Sept, G1- Oct, G3-Nov, G4- Oct and G5-Nov). It is interesting to note that the development of such submerged bar is not found uniformly in all the stations. They are found at different stations of the beach at different months and at different distances with respect to reference point. These observations suggest that they are migratory in nature and the lower foreshore is highly dynamic. During this season, the direction of wave approach changes from west to southwest with corresponding changes in wave characters. Prevailing long period waves with lower breaker height are responsible for accretion. The central part of the study area shows the

3 Bhat et al.: Morphology and sediment movement 33 Table 1 Net change in the volume of sediments in m 3 /m Stations G1 G2 G3 G4 G5 G6 G7 G8 Erosion Accretion Sum % 18.78% 11.76% 17.42% 1.18% 16.85% 7.82% 14.25% 12% Total loss= m 3 /m, Total gain= m 3 /m. Net gain = m 3 /m development of parallel berms pointing to more supply of materials in this region and movement of berm crest onshore-offshore particularly during May. Such onshore-offshore transfer of material is the characteristic feature of reflective beaches. The volume computations (Table 1) show that the beach at all the locations experienced accretion through episodic as well as continuous cycles of erosion/accretion giving rise to a net gain of about 725 m 3 /m of sediments along the line of transect in all the 8 stations together during the study period. Annual changes during the study period indicated that there is more or less uniform accretion pattern in different parts of the beach. Except station 4, in all the station(g1,g2,g3,g5,g7 and G8) significant gain was observed. In general accretion in the southern half of the beaches(stationsg5 to G8) is almost equal to the accretion in the northern half i.e. from station G3 to G1.These observations reflect the stability of the beach stretch between Gangavali and Gokarn. The longshore sediment transport largely depends on the wave approach and also on the nearshore topography. In general, along the west coast, breaker angles are generally 5-10 during premonsoon (Feb.-May) and postmonsoon (Oct.-Jan.) seasons and they changed to during monsoon (June Sept.) thus indicating a reversal in the direction of longshore current 9. The observed longshore currents (Table 2A) show that the southern currents are dominant. However, both northern and southern currents prevail along the beach. The velocity of the northern currents ranges from 0.05 to 1.0 (m/sec) and southern currents ranges from 0.06 to 0.75 m/sec. It was observed that the waves of 1 m to 1.75 m height prevail during monsoon and 0.25 m to 1 m during fair-weather season (Table 2B). The wave period varies from 5-8 sec during monsoon and sec during fair-weather season (Table 2C). It is observed that there is lack of correlation between the observed longshore currents and erosion/accretion events. Granulometric studies It is a well known fact that the grain size character of sediments is controlled by the wave energy input and it changes seasonally. The seasonal variation in grain size shows that there are only two modal classes i.e. medium and fine sand. Fine sand was dominant during all the seasons. The cross shore and alongshore grain size variations and percentage of samples are presented in Tables 3 and 4..Mean size decreases (Table 3A) and percentage of fine sand increases (Table 4A) from G1 to G8 i.e. from north to south during all the seasons. During postmonsoon, station G4 showed little coarser size compared to G3 and G6 and percentage of fines was also less compared to G3 and G6, indicating a zone of divergence. Sorting improved from G1 to G8 during premonsoon and monsoon and in general during postmonsoon. Positively skewed grains increased towards south. From north to south (G1 to G8), mean size decreases, sorting improves and percentage of positively skewed grains increases, indicating southerly movement of sand 10. The grain sorting process in the foreshore is initiated by the breaking of waves. The suspended materials derived from breaking waves along with the sediment load derived partly from the lower foreshore are transported by rolling in the form of turbulent sheet flows, first flowing up the sloping beach face, then slowing down to a halt and the water that has not percolated into the sediment flows straight back down the slope 11. During this movement, coarser material is left behind and finer material carried up the slope. This is the reason for increase in grain size at low tide level (LTL) samples compared to mid tide level (MTL) samples during pre and postmonsoon (Table 3B). The MTL sample is ascribed to the entrapment of fines during the retreat of backwash. Hence the percentage of fines is more at the MTL samples (Table 4B). The degree of sorting is less at breaker level (BRL) samples than others and is negative to symmetrically skewed. The negative values reflect high energy of

4 34 Indian J. Mar. Sci., Vol. 32, No. 1, March 2003 Table 2 Observed longshore currents wave heights, and wave periods [N-North (Up coast), S-South (Down coast)] Stations G1 G2 G3 G4 G5 G6 G7 G8 A. Longshore currents (m/sec) in different months March 93.35N.33N.35N.34N.37S.35S.37N.35S April.17N.33S.33S.33S.5S.4S.5S.4N May.17S.27S.4S..67S.38S.5S.33S.67S June.83N.2N.33N.33N.4N 0n-off.33N.5N July.64N.75S.63S.23N.53N.15S.41S.5S August.43N.5N.5S.64S.37S.64S.43S.43S Sept.47N.15S.7N.32S.49S.41S.38S.5N Oct.25S.41N.37N.34S.51S.46S.40N.23S Nov 1.0N.3N.66N.28S.2N.2N.26N.3S Dec.25N.3N.2N.27N.27N.42N.3N.13N Jan 94.23N.08N.13N.18S.1S on-off.05n.13n Feb 94.06S.63S.12S.42S.07S.13S.07S.1S B. Wave heights (meter) March April May June July August Sept Oct Nov Dec Jan Feb C. Wave period (in sec) March April May June July August Sept Oct Nov Dec Jan Feb

5 Bhat et al.: Morphology and sediment movement 35 waves 12. The near symmetrical nature result from the presence of fine sand entrapped in the coarse mode derived from the vertical sorting action of waves and partly by the material carried by the backwash. The better sorted nature of HTL and MTL samples could be attributed to the effect of turbulent sheet flows in Table 3 Alongshore and cross shore variation in mean size (in phi) Station Premonsoon Mean size(φ) Mean size(φ) Mean size(φ) A. Alongshore variation G ± ± ± G ± ± G ± ± ± G ± ± ± G ± ± ± B. Cross shore variation Premonsoon Mean (φ) Mean (φ) Mean (φ) HTL 1.98 ± ± ± MTL LTL 2.26 ± ± ± ± ± ± BRL ± ± ± Table 4 Percentage of samples along the shore. and across the shore A. Alongshore G1 G3 G4 G6 G8 Premonsoon Medium Sand Fine Sand Medium Sand Fine Sand Medium Sand Fine Sand Premonsoon Positively Skewed Symmetrical Negatively Skewed Positively Skewed Symmetrical Negatively Skewed Positively Skewed Symmetrical Negatively Skewed B. Across the shore Premonsoon HTL MTL LTL BRL Medium Sand Fine Sand Medium Sand Fine Sand Medium Sand Fine Sand Premonsoon Positively Skewed Symmetrical Negatively Skewed Positively Skewed Symmetrical Negatively Skewed 37.5 Fig. 3 Coarsest first percentile v/s median plot for the sediments of the study area Positively Skewed Symmetrical Negatively Skewed

6 36 Indian J. Mar. Sci., Vol. 32, No. 1, March 2003 the foreshore zone and prolonged winnowing action of waves. Coarsest first percentile versus median diameter plot for the sediments (Fig.3) was drawn to understand the mode of transport and the depositional agencies 8. The sediments fall in the region IV and V, predominantly in the region V. Sediments falling in these regions are suspension sediments that may contain rolled grains smaller than 1 mm. Type IV and V are generally graded suspension sediments, type IV being high and type V being moderate turbulent deposit. From the foregoing account, the following conclusions can be drawn. (i) Morphological changes are cyclic i.e. erosion during monsoon and deposition during fair weather season resulting in net accretion. (ii) A secondary cycle of erosion was observed during December/February due to the effect of northeast monsoon. (iii) In general, the sediments are fine grained, well sorted, and symmetrical and platykurtic in nature. (iv) Grain size decreases, degree of sorting improves and percentage of fine sand, positively skewed grains increases towards south indicating southern movement of sediments. (v) Transport mode by graded suspension of sediments reflect moderate turbulent energy conditions during deposition and the beach experiences low to moderate energy conditions. The present study suggests that the beaches to the north of Gokarn are stable and suitable for recreational development. Acknowledgment Authors are thankful to the Department of Science and Technology, Govt. of India for providing financial assistance and Dept. of Civil Engineering; SDM college of Engineering and Technology, Dharwad for the facility extended to carry out the present work. References 1 Haynes M O, Barrier island morphology as a function of tidal and wave regime, in Barrier Island, edited by S P Leatherman, (Academic Press, New York) 1979, pp Bryant E, Behavior of grain size characteristics on reflective and dissipative foreshores, Broken Bay, Australia, J Sed Petrol, 52 (1982) Wright L D, Chapell J, Thom B G, Bradshaw M P & Cowell P J, Morphodynamic of reflective and dissipative beach and inshore systems, Southern Australia, Mar Geol, 32 (1979) Pickrill R A, Beach changes on low energy lake shorelines Lakes Manapouri and Te Anau, New Zealand, J Coast Res, 14 (1985) Emery K O, A simple method of measuring beach profiles, Limnol Oceanogr, 6 (1961) Ingram R L, Sieve analyses procedures, in Sedimentary petrology, edited by Carver R E, (Wiley, New York) 1970, pp Folk R L & Ward W C, Brazos River bar, a study in the significance of grain size parameters, J Sed Petrol, 27 (1957) Passega R, Grain size representation of CM pattern as a geological tool, J Sed Petrol, 34 (1964) Narayanaswamy G, Udayavarma P & Abhraham Pylee, Wave climate of Trivendrum, Kerala, Mahasagr Bull Natn. Inst. Oceanogr, 12 (1979) Mc Laren P &Bowles D, The effect of sediment transport on grain size distribution, J Sed Petrol, 55 (1985) Friedman G M & Sanders J E, Principles of sedimentology, (John Wiley and Sons, New York) 1978, pp Chaudhri R S, Khan H M M & Kaur S, Sedimentology of beach sediments of the west coast of India, J Sed.Geol,. 30 (1981)