Nat Hazards (2009) 49:325–345 DOI 10.1007/s11069-008-9318-3 ORIGINAL PAPER
Wind waves and sediment transport regime off the south-central Kerala coast, India N. P. Kurian Æ K. Rajith Æ T. S. Shahul Hameed Æ L. Sheela Nair Æ M. V. Ramana Murthy Æ S. Arjun Æ V. R. Shamji
Received: 16 August 2008 / Accepted: 31 October 2008 / Published online: 25 November 2008 Ó Springer Science+Business Media B.V. 2008
Abstract Wind waves in the innershelf of the south-central Kerala coast, south-west India were measured at four locations during different seasons. Simultaneously, numerical models were developed to simulate the wave and sediment transport regime of the innershelf. Strong monsoonal influence is seen in the wave characteristics with greater amplitudes, lower periods and switch-over from SW to SWW–W direction. The net annual longshore sediment transport is southerly in the innershelf and northerly in the surf zone. These counter-directional transports are linked by seasonally reversing the cross-shore transports. In the locations where the transports in the longshore and cross-shore directions are balanced, stable beaches prevail. Erosion/accretion tendency prevails in locations where these transports are not balanced. The southern and northern parts of the coast where onshore transports are predominant could be accreting zones. The erosion/accretion pattern deduced from the sediment transport model corresponds well with the long-term erosion/ accretion trend for this coast. Keywords Wind waves Sediment transport Innershelf Kerala coast Numerical modelling Beach Erosion
1 Introduction Wind wave is the principal source of input energy into the coastal zone. It is particularly so for a microtidal coast like Kerala (Fig. 1) located in the south-west coast of India. Kerala is typical of a tropical, but monsoon-dominated coast with the south-west monsoon prevalent
N. P. Kurian (&) K. Rajith T. S. Shahul Hameed L. Sheela Nair S. Arjun V. R. Shamji Centre for Earth Science Studies, Thiruvananthapuram 695031, Kerala, India e-mail:
[email protected] M. V. Ramana Murthy ICMAM Project Directorate, MoES, Chennai 601302, Tamil Nadu, India
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from June till September. Thus, Kerala has the longest spell of south-west monsoon among all other parts of the Indian coast and offers an interesting case of monsoonal domination in the oceanographic and meteorological processes. Wind wave is one of the manifestations of monsoon along this coast. The high-steep monsoonal waves cause severe seasonal erosion all along the coast. Kerala has a high density of population along its coast. The monsoonal beach erosion results in loss of valuable property and dislocation of people in many locations. The erosion during monsoon and subsequent build-up of the beach during the postmonsoon period have been already studied for some locations of this coast (Kurian 1988; Thomas 1988; Shahul Hameed 1988, 2007; Harish 1988). The available studies conclusively show drastic spatial variations in the nearshore wave intensity and resultant coastal processes. The Trivandrum (southernmost) coast has the highest maximum wave height (Hmax) of 6 m, while Tellicherry in the north has the lowest Hmax of 2.58 m (Baba and Kurian 1988). Studies on innershelf sediment transport are lacking for this coast except for the work by Black et al. (2008). However, computations of wave-induced surf zone transport are available for a few locations (Chandramohan and Nayak 1991; Felix Jose et al. 1997; Black et al. 2008). The advent of numerical modelling techniques enables faster analysis of the hydrodynamic and sediment transport regime over a vast area. This study is aimed to understand the wave and sediment transport regime of the innershelf of a 110 km stretch of the south-central Kerala coast through field measurements and numerical modelling.
2 Area of study The area of study is the innershelf of the south-central Kerala coast. This is a 110-km long stretch of the Kerala coast extending from Thrikunnapuzha in the south to Cochin in the north (Fig. 1). The coast faces the Arabian Sea and the Indian Ocean and the coastline is more or less aligned in the NNW–SSE direction. Consequently, the coast receives moderate to large waves from as far away as the Southern Ocean and from the north-west across the Arabian Sea (Baba and Kurian 1988; Baba 2005). The winds are predominantly W and NW (Kurian et al. 2007a, b). The innershelf of this coast has a gentle slope with the 20 and 50 m isobaths at average distances of 10.75 and 33 km, respectively, from the shoreline. Within the area of study, the innershelf of the southern part is relatively steeper with the 20 and 50 m isobaths at distances of 9.2 and 28 km, respectively. The innershelf of Alleppey is the gentlest. There is a major tidal inlet at Cochin connecting the sea to the Vembanad estuary, the second largest in India, with an area of 205 km2. About 5 km south of Thrikunnapuzha, there is a minor inlet at Kayamkulam. A major part of the coast is protected by sea wall; while the seawalls in the sector on both the sides of the Cochin inlet are well built and well maintained, the seawalls in the rest of the coast are slumped or in the process of slumping (Kurian et al. 2007b). The beaches of the southern sector of the coast are enriched with heavy minerals and consequently, the sediments are fine sand (\ 0.25 mm). The beach sediments tend to be coarser further north and are confined mostly to the fine to medium sand (\ 0.5 mm) category. The innershelf (upto 20 m depth) surficial sediments are silty/clayey during pre- and post-monsoon, while it is very fine silty during monsoon. This coastal sector is well-known for the persistent occurrence of mud banks (Kurup 1969; Gopinathan and Qasim 1974), which is a unique phenomenon causing dampening of waves and generating very calm zones in the nearshore, at a few locations.
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Fig. 1 Location map: a area of study; b unstructured mesh created for the area of study
3 Offshore deployments for wave measurements Wave measurements were carried out by deploying wave gauges at depths of around 8 m off the four locations viz. Thrikunnapuzha, Mararikulam, Andhakaranazhi and Njarakkal (see Fig. 1). Due to lack of sufficient number of equipments, the measurements were carried out in the year 2004 at two locations in the south and in the year 2005 in the northern locations. However, simultaneous measurements at four locations spread over the study region were carried out during November–December 2005 (see Table 1). The seasons are described as ‘monsoon’ for the south-west monsoon period of June– September. The periods March–May and October–December are described as ‘premonsoon’ and ‘post-monsoon’, respectively. The measurements were carried out for durations of 21–30 days during pre-monsoon, monsoon and post-monsoon seasons. Since monsoon data are considered to be very precious, care was taken to ensure data collection continuously for a period of 1 month at each location. The equipments used are the ADCP with directional wave (RD Instruments Inc., USA), directional wave and tide gauges (Valeport Limited, UK) and Dobie wave gauge (NIWA, New Zealand). The details of the deployments of the equipments are given in Table 1. The ADCP and Valeport wave gauges were set up to record the waves at 2 Hz with 2,048 samples every 3 h. The Dobie wave gauge was programmed at 1 Hz only due to its memory limitations. In addition to the data measured for the study as described above, wind and water level data collected under the National Data Buoy Programme (NDBP) in the Arabian Sea at
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Table 1 Details of offshore deployment of equipment S. No.
Period
Location
Position Lat. °N
1 2 3 4 5 6
Long. °E 76°240 2300
Valeport, ADCP
Mararikulam
00
9°36 18
76°160 1300
Valeport
Monsoon (13/07/04–13/08/04)
Trikunnapuzha
9°130 2500
76°240 1600
Valeport
Mararikulam
9°360 0100
76°160 5100
Dobie
Post-monsoon (27/10/04–18/11/04)
Trikunnapuzha
9°130 0200
76°240 0200
ADCP
Mararikulam
9°360 0200
76°160 0200
Valeport
Pre-monsoon (10/03/05–01/04/05)
Anthakaranazhi
9°440 0600
76°160 2200
ADCP, Dobie
Njarakkal
10°020 1200
76°100 5700
Valeport
Monsoon (18/07/05–18/08/05)
Anthakaranazhi
9°440 0300
76°150 2100
ADCP, Dobie
Njarakkal
10°020 1100
76°100 5900
Valeport
Post-monsoon (03/11/05–03/12/05)
Anthakaranazhi
9°440 3500
76°160 0500
Dobie
Njarakkal
10°020 1100
76°100 4800
Valeport
Tharayilkadavu
9°100 4600
7600 250 3000
Valeport
Kayamkulam
9°070 5200
76°280 1500
Valeport
Pre-monsoon (27/02/04–20/03/04)
Trikunnapuzha
9°130 0200
Wave gauges deployed
0
8.3°N, 72.6°E (DS7) and 10.6°N, 72.5°E (DS2) for the year 2005 were used for providing the boundary values for the numerical model.
4 Numerical modelling 4.1 Models used The MIKE21 Spectral Wave Model (MIKE21 SW) has been used for the simulation of wave climate in the nearshore area. MIKE21 SW is a new generation spectral wind–wave model based on unstructured meshes, which takes into account all the important phenomena like wave growth by influence of wind, non-linear wave–wave interaction, dissipations due to white-capping, bottom friction and depth-induced breaking, refraction and shoaling due to depth variations and wave–current interaction. The model simulates the growth, decay and transformation of wind-generated waves and swells in offshore and coastal areas. For the present study, the fully spectral formulation based on the wave action conservation equation (Komen et al. 1994; Young 1999) was used. Outputs from the model are the wave parameters like significant wave height, peak wave period, mean wave direction etc. The radiation stresses induced as a result of the wave action was also generated, as this data was required for further computation using the Mike Flow Model FM. The MIKE21 sediment transport (ST module) of MIKE Flow Model FM was used for sediment transport computations. MIKE 21 Flow Model FM is a comprehensive system developed by DHI for 2D flows based on an unstructured mesh with linear triangular elements. It uses a cell-centred finite volume solution technique. The ST module is dynamically linked to the hydrodynamic model. Surface elevation and current-related outputs from the hydrodynamic (HD) module of flow model as well as wave data from MIKE21 SW model and sea bed material characteristics (from Kurian et al. 2007b) were given as input for running the ST module.
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4.2 Mesh and model boundaries The bathymetry data for the model was obtained using MIKE C-MAP. The model domain selected is about 97 9 45 km with a maximum water depth of about 110 m. An unstructured mesh having 995 nodes and 1,804 elements with a local refinement in the nearshore region (Fig. 1) has been created. The mesh model has three open boundaries—north, south and west—and one land boundary on the east. For the western boundary which is also the offshore boundary, wave data from deepwater offshore buoy in the area was given as input. The northern and southern boundaries were defined as lateral boundaries. 4.3 Input data Wind and surface elevation or water level conditions were the other input data required for simulation. The data from deep water offshore buoy was used for water level variations and wind. Separate models were set up for the three seasons—namely pre-monsoon, monsoon and post-monsoon by giving appropriate input data (wave, wind, surface elevation, sediment grain size, etc.). The simulations for the pre-monsoon and post-monsoon seasons were carried out for duration of 1 month each with March and November 2005 being taken as the representative months. As the monsoon data from the offshore buoy was available only for a period of 2 weeks, the simulation was limited to a period of 2 weeks during the second half of July 2005. 4.4 Model calibration and validation Calibration of the model was done by adjusting the bottom friction to match the physical and nearshore characteristics of the domain. For calculating the bottom friction, the spatial variation of sediment grain size and distribution was taken into account. This was repeated for all the three seasons as these parameters are subjected to seasonal variation. The results were validated using the measured wave data at innershelf locations, as given in Sect. 3. Comparisons of time series of measured data and model outputs for all the three seasons are presented in Fig. 2. The statistical parameters quantifying the comparisons for Hs are given in Table 2.
5 Results 5.1 Innershelf waves The salient characteristics of innershelf waves in the study area are derived from the measured data as well as model simulations. The frequency distributions of wave heights, periods and directions at different locations are given in Figs. 3–9, while Fig. 10 gives the model outputs which give the spatial distribution in the whole innershelf of the study area. 5.1.1 Wave heights The Hs for the pre-monsoon period at Trikkunnapuzha ranges from 0.29 to 0.83 m with an average value of 0.51 m. Almost 50% of the Hs are in the range 0.25–0.5 m and 48% in the range 0.5–0.75 m, indicating very low wave activity during the pre-monsoon period.
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Fig. 2 Comparisons of time series of measured and simulated wave parameters: a Hs and b Tp
During monsoon period, as expected, wave heights have shifted to higher values, with minimum Hs of 0.86 m (more than the maximum pre-monsoon wave height) and a maximum of 2.06 m. Forty-two percent of the waves are having Hs in the range 1.25– 1.5 m, while the average value is 1.43 m. During post-monsoon, Hs vary between 0.34 and
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Nat Hazards (2009) 49:325–345 Table 2 Statistical parameters of the calibrated model for Hs
331
Parameter
Pre-monsoon
Monsoon
Post-monsoon
Mean
0.37
1.03
0.43
Bias
0.02
-0.08
0.05
RMS error
0.15
0.23
0.16
Corr. coeff.
0.73
0.85
0.69
1.5 m, with an average of 0.57 m. The frequency distribution is characterised by a prominent peak with 60% of occurrence in the range 0.5–0.75 m. The maximum values of Hmax for pre-monsoon, monsoon and post-monsoon are 1.30, 3.23 and 1.46 m, respectively. The peaks of Hmax distribution for pre-monsoon, monsoon and post-monsoon are in the ranges of 0.50–1.00, 2.0–2.50 and 0.75–1.0 m, respectively. Off Mararikulam, the wave intensity is relatively less when compared to Thrikunnapuzha. The Hs ranges from 0.26 to 0.69 m with an average of 0.46 m during premonsoon, 0.85–2.27 m with an average of 1.36 m during monsoon and 0.3–0.9 m with an average of 0.53 m during post-monsoon. The relatively calmer condition off Mararikulam is evident from the fact that almost 70% of the Hs are in the range 0.25–0.5 m during the pre-monsoon period and as against 42% off Thrikunnapuzha, the peak has only 37% in the height range of 1.25–1.5 m during monsoon. During post-monsoon, 49% of the waves are in the range 0.5–0.75 m and 43% are in the range 0.25–0.5 m. The maximum values of Hmax for pre-monsoon, monsoon and post-monsoon are 1.08, 3.67 and 1.41 m, respectively. The maximum occurrences of Hmax for pre-monsoon, monsoon and post-monsoon are in the range of 0.5–0.75, 2.0–2.25 and 0.5–0.75 m, respectively. Off Anthakaranazhi, the Hs for the pre-monsoon period ranges from 0.25 to 0.68 m with an average value of 0.42 m. Almost 80% of the waves have Hs in the range of 0.25–0.5 m and the rest 20% are in the range of 0.5–0.75 m. During monsoon period, Hs ranges from 0.84 to 2.3 m with an average of 1.47 m. Forty percent of the Hs are in the range of 1.25– 1.5 m and 20% are in the range of 1–1.25 m and 1.5–1.75 m. During the post-monsoon period, Hs ranges from 0.2 to 1.0 m with an average value of 0.48 m. Fifty-four percent of the waves are in the range of 0.25–0.50 m and 36% are in the range of 0.50–0.75. The wave intensity at Anthakaranazhi though measured in 2005 is more comparable to Mararikulam than Thrikunnapuzha. The Hs, off Njarakkal (diagram not presented), ranges from 0.29 to 0.93 m with an average of 0.5 m during the pre-monsoon and from 0.72 to 2.2 m with an average of 1.16 m during monsoon. During the pre-monsoon, about 55% of the Hs are in the range 0.25–0.50 m followed by about 40% in the range 0.50–0.75 m. The peak of the frequency distribution, during the monsoon, lies in the range 1.0–1.25 m with a percentage occurrence of 35%, closely followed by the range 0.75–1.0 m. A comparison of the Hs for the two locations Andhakaranazhi and Njarakkal shows that Andhakaranazhi has a higher energy regime than Njarakkal, though the pre-monsoon conditions show the reverse. The Hmax distribution during pre-monsoon has the peak in the range 0.50–0.75 m closely followed by 0.75–1.0 m while during the monsoon, the peak is not that well pronounced and the distribution spreads over a very wide range of higher values. 5.1.2 Wave periods The frequency distributions of wave periods (Tz and Tp) for different locations are given in Figs. 6–8. The ranges of Tz are 3–13, 6–10 and 3–10 s, respectively, for the pre-monsoon,
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Fig. 3 Frequency distribution of Hs and Hmax off Trikunnapuzha during pre-monsoon, monsoon and postmonsoon
monsoon and post-monsoon respectively off Thrikunnapuzha. The corresponding ranges of Tp for the three seasons are 4–22, 6–18 and 4–16 s. During the pre-monsoon, about 28% of each of the occurrences of Tz is in the range 8–9 and 9–10 s, while in monsoon, the periods
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Fig. 4 Frequency distribution of Hs and Hmax off Mararikulam during pre-monsoon, monsoon and postmonsoon
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Fig. 5 Frequency distribution of Hs off Andhakaranazhi during pre-monsoon, monsoon and post-monsoon
are lower with 45% of the values occurring in the range 7–8 s, closely followed by the range 8–9 s. The post-monsoon distribution of Tz is characterized by a peak of 40% in the range 6–7 s, closely followed by the range 7–8 s. While the Tp is concentrated in the range 10–12 s, during monsoon, the distribution is spread more towards higher periods in the post-monsoon season. Tp distribution during pre-monsoon is notable for a prominent peak of 12–13 s with 82% of occurrence. Off Mararikulam, the Tz lies in the ranges of 4–15, 6–10 and 4–13 s in the pre-monsoon, monsoon and post-monsoon seasons, respectively. The peak during pre-monsoon is in the range of 7–9 s, which is slightly lower than Thrikunnapuzha. During monsoon, 62% of the
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Fig. 6 Frequency distribution of Tz/Tmean and Tp off Trikunnapuzha during pre-monsoon, monsoon and post-monsoon
waves have Tz 8–9 s. As in the case of pre-monsoon, the peak during post-monsoon is lower when compared to Thrikunnapuzha. For monsoon season, Tp has a prominent peak at 10–11 s which is lower than that for Thrikunnapuzha, while in the post-monsoon season,
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Fig. 7 Frequency distribution of Tz/Tp off Mararikulam during pre-monsoon, monsoon and post-monsoon
the occurrences are more in the higher periods and higher than that of Thrikunnapuzha. The average of Tp for pre-monsoon, monsoon and post-monsoon are 12.0, 10.5 and 11.9 s, respectively.
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Fig. 8 Frequency Distribution of Tz/Tp off Andhakaranazhi during pre-monsoon, monsoon and postmonsoon
Off Andhakaranazhi, the ranges of Tz are 4–11 and 5–13 s, respectively, for the premonsoon and post-monsoon. The ranges of Tp during pre-monsoon, monsoon and postmonsoon are 4–19, 7–19 and 6–19 s, respectively. While Tz in the ranges 7–8 and 8–9 s together constitute more than 50% of the distribution during the pre-monsoon, for Tp, a
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Fig. 9 Rose diagram of peak wave direction during pre-monsoon and monsoon
Fig. 10 Simulated Hs in the innershelf during different seasons
predominant peak in the range 12–13 s constitutes 43% for the same season. In the monsoon, the peak for Tp is evenly distributed in the 9–12 s range. In the post-monsoon, no prominent peaks are seen in the distributions of Tz and Tp.
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Tz off Njarakkal (diagram not presented) for pre-monsoon and monsoon ranges from 3 to 13 s and from 5 to 10 s, respectively. The corresponding ranges for the Tp are 3–20 and 5–18 s. During the pre-monsoon period, Tz in the range 4–5 s constitutes 25% of the distribution, followed by the ranges 7–8 and 8–9 s together constituting nearly 40%. During this period, Tp has the prominent peak in the range 13–14 s constituting 31% of the total occurrence. During the monsoon period, Tz in the ranges 7–8 and 8–9 s constitutes about 85% of the occurrence, while Tp has the peak in the range 11–12 s with 28% of the occurrence. 5.1.3 Wave directions Due to limitations in the number of directional wave gauges and its occasional malfunctioning, directional wave data is not available for all stations. Off Thrikunnapuzha, the directional wave data is available for all the three seasons (Fig. 9). During pre-monsoon, the average value of peak direction is 226°N i.e. 15° south of shore normal, the shore normal being 241°N. The average peak direction during monsoon is 246°N i.e. 5° north of shore normal. The average peak direction for post monsoon is 204°N i.e. 37° south of shore normal. Pre-monsoon record of wave direction at Mararikulam shows that the peak wave direction is almost same as that of Trikkunnapuzha 224.4°N, but it is 31° south of shore normal since the shoreline is more closer to the north-south direction at Mararikulam. Frequency distribution of wave directions shows that during the pre-monsoon, the major wave direction is SW followed by WSW, the two directions together constituting almost the total occurrence. Pre-monsoon data off Mararikulam shows the same trend as Thrikunnapuzha. In the monsoon, there is a predominant shift in the direction to W and WSW. Off Thrikunnapuzha, the W and SW directions together contribute about 35% of the distribution, while the westerlies alone constitute more than 50% off Mararikulam. Thus, it can be deduced that the waves approach from south of the shore normal during pre-monsoon while it is close to the shore normal (both north and south) during monsoon. The wave height (Hs) simulated using the SW model of MIKE21 for the pre-monsoon, monsoon and post-monsoon seasons (Fig. 10) gives an insight into the spatial variation of waves. In the cross-shore direction, the wave intensity increases towards offshore irrespective of the location and season. In the alongshore direction, the southern sector has the highest intensity during all the three seasons. Higher values of Hs in the range of 3.5– 3.75 m are seen at an offshore distance of only 22 km off Thrikunnapuzha while, it is at a distance of 36 km off Cochin. Lowest wave heights are seen in the Cochin area and to a smaller extent off Alleppey–Mararikulam area. 5.2 Innershelf sediment transport The longshore and cross-shore components of the accumulated bed load were analysed separately for a detailed understanding of the innershelf sediment transport pattern. The results are shown in the Fig. 11. During pre-monsoon, the onshore and offshore transports are more or less balanced areawise. Excepting an 18-km sector south of Alleppey, rest of the inshore region has onshore transport. The tongue of offshore transport south of Alleppey, which is strong extends further offshore, to the north and south. Relatively, the onshore component off Cochin is quite strong showing an accumulated transport of up to 58 m3/m for the 1-month period. The offshore component south of Alleppey is moderately strong with an accumulated transport of up to 26 m3/m. The areal extend of southerly longshore transport is more than northerly transport during pre-monsoon season. However,
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a relatively strong northerly transport with a maximum value of 23 m3/m is noticed off the Alleppey–Thottappally sector. This meets with the southerly transport, which is relatively weak prevalent in the northern region off Mararikkulam, north of Alleppey. Sediment transport, both cross-shore and longshore, is surprisingly weak during the monsoon. The cross-shore sediment transport during monsoon indicates a reversal of the direction for a major part of the coast barring the northern and southern region of the model domain where onshore transport is maintained. Off Mararikulam where onshore transport prevailed during pre-monsoon, offshore transport is seen, though with reduced intensity. Offshore transport observed in the inshore regions north of Alleppey, extended offshore as well as towards north as a tongue. The longshore transport is southerly all over the study area during this season. During the post-monsoon season, the cross-shore and longshore components of the transport are very weak, which is typical of the comparatively calm oceanographic conditions prevailing during this season. The cross-shore component is positive all over the region, which implies that onshore transport is prevalent during the post-monsoon season. Incidentally, this is the season when the beach building process starts after the monsoonal erosion. The longshore transport too is very weak during this season, though it shows northerly direction for a major part of the study area. Thus, the model brings out distinct spatial and temporal variations in sediment transport in the innershelf of the area of study. The model results show a predominance of onshore transport in the northern and southern parts during all the three seasons. In the rest of the coast, the cross-shore transport directions reverse with seasons. As regards longshore transport, it shows a predominance of southerly drift. Between the seasons, the pre-monsoon is characterized by stronger longshore and cross-shore transports. 5.3 Surf zone sediment transport The wave breaking induces longshore currents and sediment transport in the surf zone. For the coast under study, the waves approach from south of the shore normal during pre- and post-monsoon, while it is north of the shore normal during monsoon. This in turn could induce predominant northerly longshore currents during pre- and post-monsoon and southerly longshore current during monsoon. The northerly currents being more predominant annually, the net transport could be northerly. Computed results using the LITPACK model gives the anticipated results as can be seen from Table 3. It shows a predominance of northerly transport with a quantum of 2,07,379 m3/year towards north, while the southerly transport is 77,170 m3/year. Thus, there is a net northerly drift of 1,20,000 m3/ year. The computed results are comparable to the values obtained by Black et al. (2008) for the adjoining Chavara coast in the south.
6 Discussion The wave characteristics of the study region are typical of a monsoon dominated regime. As categorised by Kurian (1989), the coast falls under a moderate energy regime. Within the area of study, the alongshore variation in wave height is relatively less. However, the southern part around Thrikunnapuzha has a slightly higher wave intensity which is attributed to the relatively steeper slope of innershelf that causes lesser wave energy dissipation (Kurian and Baba 1987). The relative steepness of the southern part is indicated by the fact that the 20 and 50 m isobaths are at distances of
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Fig. 11 Simulations of accumulated bed load during different seasons: a cross-shore and b longshore components
9.2 and 28 km respectively while the average values for the area of study are 10.75 and 33 km, respectively. The wave intensification during monsoon with reduction in wave periods and shift of the peak wave direction from SW to WSW-W is in conformity with the earlier studies for this coast (Kurian 1988; Thomas 1988; Shahul Hameed 1988; Shahul Hameed et al. 2007;
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Table 3 Computed longshore sediment transport in the surf zone off Thrikkunnapuzha Northward (m3/year)
Southward (m3/year)
Net transport (m3/year)
2,07,379
77,170
1,30,209 (towards north)
Harish 1988). With the onset of monsoon wind speed picks up and the Arabian Sea becomes the generating area for waves during this season, resulting in the change in the wave characteristics. The deep water buoy data for the study period show wind speed as high as 33 knots. During the pre- and post-monsoon seasons, the wind speeds are much lower and the locally generated waves with smaller amplitudes and periods become insignificant in the wave spectrum. During these seasons the observed periods are higher with SSW–SW directions which show the predominance of swells of longer periods from the south Indian Ocean. The sediment transport pattern in the innershelf is complex. It shows an overall predominance of southerly transport which is in conformity with the results of Black et al. (2008) for Chavara which is further south of this coast. In the cross-shore direction, offshore transport prevails during monsoon and onshore transport during the pre- and postmonsoons barring the northern and southern parts where onshore transport prevails for most part of the year. The net annual wave-induced longshore sediment transport in the surf zone is northerly which is well corroborated with field observations (Kurian et al. 2007b) and deductions from the measured wave data. One interesting aspect of the results of sediment transport computations is the relatively smaller quantum of longshore sediment transport that is not commensurate with the wave intensity, be it in the innershelf or the surf zone, during monsoon. It is well known that the obliqueness of wave approach with respect to the shore normal is a more sensitive parameter than amplitude in sediment transport. During the peak monsoon, the direction of wave approach is close to shore normal while during the rest of the period it is far south of the shore normal (as high as 37°, as seen in Sect. 5.1). Thus, the quantum of longshore sediment transport commensurate with the high amplitude of the waves is not seen during the monsoon. The sediment transport pattern as a whole in the area of study is one with northerly transport close to the shore and southerly transport outside the surf zone. Linking these opposing transports is the cross-shore transport which again varies seasonally with offshore transport predominant during monsoon and onshore transport predominant during the postmonsoon beach building period. Black et al. (2008) introduces the step-ladder sediment transport hypothesis to explain this sort of sediment transport patterns. According to that hypothesis, the sediment transport pattern in the innershelf is in the form of a step-ladder. The main supports of the ladder are the two longshore transports in the opposite directions, one in the innershelf and the other in the surf zone. The rungs of the ladder are onshore/offshore sediment transport between the beach and the innershelf. Barring the southern and northern parts of the study area where onshore transport prevails for most part of the year, sediment transport off the coast appears to belong to this step-ladder pattern. The stability of a coast depends upon the balance between the constituents of the ladder. Where there is perfect balance, the beach will be stable. Where the opposing forces are not well balanced the beach may show tendency for erosion or accretion depending on the specific case. Viewed in the above context, the sector from Thottapally to Chellanum (south of Cochin inlet) can be in a state of dynamic equilibrium. Though inshore locations of Chellanum also have onshore transports during all the three seasons, the rates are not
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Fig. 12 Long-term shoreline change along the south-central Kerala coast (after Kurian et al. 2007a)
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sufficient enough to cause accretion. The sector from Purakkad till Andhakaranazhi (on both the sides of Alleppey) is notable for a net offshore transport and thus could be prone for erosion. However, this sector is known for the occurrence of mud bank at a few locations like Mararikulam, Alleppey and Ambalapuzha where accretion does occur. Since mud bank is not incorporated in the model, such locations where mud banks occur may be aberrations in the pattern of erosion deduced from the model outputs. The Cochin and Thrikunnapuzha (north of Arattupuzha) coasts where onshore transport prevails for major part of the study period could be locations where beach accretion occurs. Among the two, the Cochin coast with an exceptionally high pre-monsoon onshore transport is ideally set for high accretion. Kurian et al. (2007a) has carried out an analysis of the long-term shoreline change trend along this coast during 1983–2001 which is reproduced in Fig. 12. As can be seen, there is a phenomenal accretion just north of Cochin inlet with an advance of shoreline by 1.8 km during the period 1985–2000. It also shows significant accretion in the sector north of Thrikunnapuzha. A major part of the shore in between these northern and southern sectors is either near stable or under erosion. Within the constraints of modelling, the correspondence between the deductions from the model outputs and the observed erosion/ accretion pattern is phenomenal. The accretion seen at Ambalapuzha and Mararikulam could be linked to the occurrence of mud bank, which is not incorporated in the model. Apart from natural processes, anthropogenic factors play an important role in the erosion/ accretion processes of this coast, as concluded by Kurian et al. (2007a). The erosion seen south of the Cochin inlet and exceptionally high accretion north of the Cochin inlet could be linked to anthropogenic factors relating to dredging and other activities of the Cochin Port.
7 Conclusions A study of the wave and sediment transport regime in the innershelf off the south-central Kerala coast, southwest India was undertaken through field measurements combined with numerical modelling techniques. The study showed that the net annual longshore transports are southerly in the innershelf and northerly in the surf zone. These counter-directional sediment pathways were linked by cross-shore transports. The northern and southern portions of the study area show distinct signs of accretion by way of predominant onshore transport. The correspondence between the erosion/accretion pattern deduced from the model outputs and the physical scenario from the field is excellent. Acknowledgements The Ministry of Earth Sciences (previously Department of Ocean Development) is thanked for funding the study. The authors are thankful to Dr BR Subramanian, Project Director, ICMAM Project Directorate, Ministry of Earth Sciences and Dr M Baba, Director, CESS for the support and encouragement extended for the work. The deep water data for model boundary was provided by NIOT. M/s Abilash P Pillai, P Kalaiarasan, BT Muralikrishna and KP Indulekha have contributed immensely to the success of the wave measurement programme which extended for 2 years. Thanks are due to Dr KV Thomas for useful discussions during various stages of the work. M/s SS Praveen, AK Reshmi, Kalarani and Gopika R Nair have provided valuable assistance in preparation of the manuscript.
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