Keywords

1 Introduction

The littoral sediment transport pattern along a particular coast usually takes place due to the action of breaking waves [1,2,3,4]. The breaking waves result in the pile-up of water along the coast which in turn leads to the generation of littoral currents and moves the sediment either in alongshore or offshore/onshore direction. The onshore/offshore and alongshore movement of sediment is usually dependent on several factors such as beach slope, wave period, current direction and bottom friction [5, 6]. Both the strength and direction of sediment movement varies with season and location [3, 7,8,9,10,11,12,13]. Chandramohan et al. [14] revealed that the sediment transport pattern along the east coast of India takes place northward for 8 months (March–October) and southward for the remaining four months (November–February) of the year. According to Chandramohan and Nayak [15], the net annual sediment transport along the east coast is northward whereas along the west coast it is southward except for the south Gujarat Coast. In the present study, an attempt was made to study the nearshore wave climate and associated sediment transport pattern along the SE coast of India.

2 Study Area

The 580 km length of the study area (SE coast of India) starting from Kavali in Andhra Pradesh to Nagapattinam in Tamilnadu has three major ports (Ennore, Chennai, and Thoothukudi) and two medium ports (Cuddalore and Nagapattinam). Natural factors like cyclones, coastal flooding, coastal sediment deposition, tsunami, etc. and the anthropogenic factors such as domestic and industrial sewage due to urbanization and industrialization play a major role on the sediment dynamics along the coast. The material inputs from industries, aquaculture, tourism sector, petroleum products, fertilizer, pesticides, etc. enter the nearshore waters affecting the mangroves and related coastal environments [16].

3 Data and Methodology

In order to study the spatial as well as temporal variations of the sediment transport along the SE coast of India, five cross-shore transects (A–E) were fixed as shown in the Fig. 1. Each transect covers the distance from shoreline to 50 m depth contour offshore and the details of the physiographic characteristics of each transect are given in Table 1.

Fig. 1
figure 1

The study area showing the five transects (A, B, C, D and E) with cross-shore bathymetry profiles (values on each profile indicate the ‘bed slopes’ at corresponding depth contour)

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Table 1 Details of the five transects along the SE coast
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For each transect, the data on cross-shore bathymetry (from 0 to 50 m depth) was extracted by re-analysis of the bathymetry data from ICMAM database and by combining online bathymetry data, c-map and ICMAM field data. The directional wave data needed at the offshore depths were collected from the wave atlas (1998–2012) of the Indian Coast prepared by NIOT [17]. The offshore location at which the wave parameters of the NIOT wave atlas represents is around 24–26 m depending on the transect. The extracted data from the NIOT wave atlas was used to calculate the wave parameters at 0.8, 2, 5, 10, 30 and 50 m depth contours using Linear Wave Calculator (LWC) of DHI (2005). The calculated wave parameters at 0.8, 2, 5, 10, 20, 30 and 50 m depth contours for each of the five transects were later utilized as input for the LITSTP model of LITPACK [18].

The LITSTP is a one-dimensional model that includes the hydrodynamic processes and the intra-wave sediment transport mechanisms. The input data for the LITSTP module are the wave period (T), wave height (H), wave direction (θ), current speed and direction, shoreline angle, seabed slope and the mean sediment size. The wave direction alpha (α) is the angle between the wave orthogonal and the coast orthogonal (i.e. α is 0° if the wavefronts are parallel to the coast). The g value taken as 9.82 m2/s and the density as 1028 kg/m3. While simulating the model the depth and the bed slope values were specified at each calculated location. Both current speed and direction were specified at each of the simulated location and the sediment size (0.2 mm) was represented as the mean grain diameter d50. The uniformly graded sediment process was considered and the related fall velocity and the specific gravity were adopted as 0.022 m/s and 2.65 respectively. The model simulations to derive sediment transport rates along the SE coast were carried out for January and August months representing NE and SW monsoon wave climatic conditions respectively.

4 Results and Discussion

4.1 Wave Climate

January (NE Monsoon): The percentage distribution of wave height, wave period and wave direction at an offshore location derived from the NIOT wave atlas for each of the five transects (A–E) are shown in Fig. 2. In January the transects C, D and E showed higher wave strengths of around 2.0 m whereas transects A and B showed comparatively lower strengths of around 1.5 m. Overall, the occurrence of higher strength waves in the offshore area (NIOT buoy location) comprised only 10% of the total waves and were within the range of 1.5–2.2 m. However, the lower strength waves which is a regular phenomena comprised 90% of the waves and ranged from 0.5 to 0.8 m height.

Fig. 2
figure 2

Deepwater wave parameters—a wave height (m) b wave period (s) and c wave direction (deg) derived from NIOT wave atlas for the offshore region (around 22.8 and 30.4 m) for the five different transects (A–E) along the SE coast for January representing NE monsoon conditions

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The wave period of 90% of the waves (Fig. 2b) along the SE coast usually falls within the range of 4–5 s whereas the remaining 10% fall within 5–7 s. The wave direction in January (NE monsoon) at the different transects (Fig. 2c) indicates that, in general, the waves approach the coast between 75 (ENE) and 155 (SSE) direction. However, 90% of the waves approach the shore between 75 and 90 (ENE and E) and the remaining 10% of the waves from 125 and 155 (ESE and SSE). Thus, during the NE monsoon season about 10% of the waves having higher strengths approach the coast from the SE direction while 90% of the waves approach the coast from ENE direction.

August (SW Monsoon): In August, the transects C and D (Fig. 3a) showed higher wave strengths of around 1.5 m whereas the transects A, B and E showed lower strengths of around 1.0 m. About 90% of the waves with strengths of 0.4–0.7 m and periods of 4–5 s (Fig. 3b) approach the coast from SE and SW direction (Fig. 3c).

Fig. 3
figure 3

Deep water wave parameters—a wave height (m), b wave period (s) and b wave direction (deg) derived from the NIOT wave atlas for the offshore region (around 22.8 and 30.4 m) for the five different transects (A–E) along the SE coast for August representing SW monsoon conditions

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Overall, it is observed that there is a clear demarcation in the approach direction between higher and lower strength waves along the SE coast. The higher strength waves (1.0–1.5 m) even though they are of less percentage approach predominantly from the S and SE directions during both the seasons. The lower strength waves approach the coast from ENE in NE monsoon and S and SSE in SW monsoon. Further, the NE monsoon is characterized by higher strength waves as compared to that of during the SW monsoon season. The lower strength of waves in the SW monsoon season may be due to the shadowiness of Sri Lanka peninsula for S wave propagation originating from the equatorial Indian Ocean region.

4.2 Wave Transformation in Shallow Water

January (NE Monsoon): The calculated wave height (H), wave period (T) and wave direction (θ) at the different depth contours (0.8, 2, 5, 10, 20, 30, and 50 m) based on LWC for each of the five transects (A–E) for January representing NE monsoon conditions are shown in Table 2. At transect A, 90% of the wave heights varied from 0.6 m (in deep water areas) to 0.5 m (in shallow waters near shoreline), whereas the wave direction vary from 96° in deep water areas to 88° in shallow waters areas. However, 10% of the waves with heights vary between 1.6 m (in deep water areas) to 0.7 m (in shallow water areas close to shoreline) and the directions vary from 125° to 123° as the waves approach from deep water areas to shallow water areas. More or less similar trend of wave transformation was noticed along transect B.

Table 2 Transformed wave parameters at different depth contours covering deep water areas (50 m) to shallow water areas (0.8 m) across transects A–E in January (NE monsoon)
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Along transects C–E the deep water wave heights at these three transects varied between 0.7 and 2.0 m whereas the shallow water wave heights varied between 0.7 and 0.8 m. Similarly, the directions varied between 135 and 81 in deep water areas and between 105 and 78 in shallow water areas (close to the coast). On the whole, the waves transformed from greater heights (around 0.7–2.0 m) to lower heights (0.7–0.8 m) as the waves approach the shore. This kind of wave transformation in January representing NE monsoon conditions may lead to southward sediment transport along the coast.

August (SW Monsoon): The calculated wave height (H), Wave period (T) and wave direction (θ) at different depth contours (0.8, 2, 5, 10, 20, 30, and 50 m) utilizing NIOT wave atlas data based on LWC along the five transects (A–E) for the month August representing SW monsoon conditions are shown in Table 3. At transect A, 90% of wave heights varied from 0.5 m (in deep water areas) to 0.6 m (in shallow waters near shoreline), whereas the wave direction varies from 135° in deep water areas to 121° in shallow waters areas. However, 10% of waves with heights vary between 1.0 m (in deep water areas) to 0.64 m (in shallow water areas close to shoreline) and the directions vary from 158° to 121° as the waves approach from deep water areas to shallow water areas. More or less similar trend of wave transformation was noticed along transect B.

Table 3 Transformed wave parameters at different depth contours covering deep water areas (50 m) to shallow water areas (0.8 m) across transects A–E in August (SW monsoon)
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Along transects C–E the deep water wave heights at these three transects varied between 0.4 and 1.7 m whereas the shallow water wave heights varied between 0.4 and 0.7 m. Similarly, the directions varied between 179 and 189 in deep water areas whereas in between 116 and 133 correspondingly in shallow water areas close to the coast. On the whole, the waves transformed from greater heights (around 0.5–1.5 m) to lower heights (0.4–0.7 m) as the waves approach the shore. Such wave transformation in August representing SW monsoon conditions may lead to northward sediment transport especially along the southern part of SE coast.

4.3 Nearshore Sediment Transport––LITSTP

After obtaining the transformed wave parameters at the required depth from LWC, the sediment transport rate at that depth was simulated using LITSTP model for each of the five transects as described in Sect. 3. To study the nearshore sediment transport pattern along the SE coast simulations for the months of January and August representing the NE and SW monsoon conditions respectively were carried out and the results were plotted as shown in Fig. 4.

Fig. 4
figure 4

Simulated sediment transport rate at different depth (0.8, 2.0, 5.0, 10.0 and 20.0 m) contours along each of the five transects (A–E) under the wave conditions of January (NE monsoon). Values on the figure represent the magnitude in m3/month/m while the arrows indicate the direction of sediment transport

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In general, it is observed that the rate of sediment transport in January (NE Monsoon) is higher close to the coast and gradually decreased towards the offshore area up to 20 m depth contour. Sediment transport was negligible beyond 20 m depth at almost all the five transects studied. The sediment transport rate due to 90% of the waves is low (21–105 m3/month/m) (Fig. 4a). High sediment transport rate of 192–798 m3/month/m (Fig. 4b) occurs at 2 m depth contour due to 10% of the waves.

More or less similar features were observed in August (SW monsoon) as that of January (NE monsoon) but the direction of sediment transport was predominantly northward with lower sediment transport rates. The sediment transport rate during the SW monsoon is relatively lower than that of during the NE monsoon (Fig. 5). The highest sediment transport rate of 1.3–47 m3/month/m (Fig. 5a) was seen due to 90% of the waves while 8–22 m3/month/m (Fig. 5b) due to 10% of the waves representing SW monsoon conditions.

Fig. 5
figure 5

Simulated sediment transport rate at different depth (0.8, 2.0, 5.0, 10.0 and 20.0 m) contours along each of the five transects (A–E) under the wave conditions of August (SW monsoon). Values on the figure represent the magnitude in m3/month/m while the arrows indicate the direction of sediment transport

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From the simulated sediment transport rates as mentioned under Figs. 4 and 5, the computed gross sediment transport rate along each of the five transects due to 90% wave occurrences for the months January (NE monsoon) and August (SW monsoon) is given in Table 4. It is clearly noticed that the sediment transport rate during the SW monsoon is low (975–42,251 m3/month) when compared to that of during the NE monsoon (13,319–73,967 m3/month) at almost all the transects. The highest sediment transport rate of 73,967 m3/month was noticed during the NE monsoon season while the lowest sediment transport rate of 975 m3/month was noticed during the SW monsoon along the SE coast.

Table 4 Sediment transport (ST) rate (m3/month) due to 90% of wave occurrences along the SE coast
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The gross sediment transport rate along each of the five transects due to 10% wave occurrences for the month of January (NE monsoon) and August (SW monsoon) is given in Table 5. It is noticed that the sediment transport rate during the SW monsoon is low compared to that of during the NE monsoon season at almost all the transects.

Table 5 Sediment transport (ST) rate (m3/month) due to 10% of wave occurrences along the SE coast
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For validation, the simulated sediment transport rates were compared with published studies for this region [15, 19–21]. The simulated sediment transport rates reasonably matched well with the values obtained from the past studies as shown in Fig. 6.

Fig. 6
figure 6

Comparison between the simulated sediment transport rates and studies reported for the study area

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5 Conclusion

Along the SE coast of India, 90% of the waves have wave heights ranging from 0.5 to 0.8 m whereas 10% of the waves have heights of 1.5–2.0 m. The wave period usually varies between 4 and 6 s. During the NE monsoon season (January) the wave direction is predominantly ESE whereas during the SW monsoon it is SSE. A remarkable feature identified in the present study is that most of the sediment transport is confined to the nearshore waters within 10 m depth contour i.e. within 5 km from the shoreline. The simulated results indicated that the sediment transport rate usually varied between a minimum value of 975 m3/month and a maximum value of 73,967 m3/month. The sediment transport rates along the coast are relatively higher during the NE monsoon season as compared to that of the SW monsoon. The direction of the sediment transport is southward in January representing the NE monsoon wave climatic conditions whereas northward in August representing the SW monsoon wave climatic conditions.