Beach stability in relation to the nearshore wave energy distribution along the Quilon coast, SW coast of India

Indian Journal of Marine Sciences Vol. 28, September 1999, pp. 233-239

Beach stability in relation to the nearshore wave energy distribution along the Quilon coast, SW coast of India

T.N.Prakash, N.P.Kurian & Felix Jose

Centre for Earth Science Studies, Trivandrum -695 031, India Received 7 Jail/tal)'. 1998. revised 21 Jlllle 1999

The study covers the short- and long-term beach volume changes, shoreline changes, wave refraction pattern and nearshore wave energy distribution. In general there is a decrease of wave energy towards north. The distribution of wave energy for monsoon shows that the highest value of 5215 J/m² is found in the southern most station and lowest value of 4043 J/m² is found in the northern most station. The computation of short- and long-term beach volume changes indicate that the beaches of the southern sector are nearly stable while the northern sector beaches are eroding. In addition to the wave and wave-induced processes, the anthropogenic factors such as sand mining by IRE, presence of breakwaters at Ashtamudi inlet and lack of sediment supply from the rivers contribute to the continuous erosion in the northern sector. The erosionlaccretion pattern closely follows the longshore energy gradient except in ioeations where anthropogenic factors dominate.

Along monsoon dominated coasts such as SW coast of India, though significant erosion occurs every year due to monsoonal waves lashing on the coast, the beach builds up subsequently during the fair-weather period^t•5. However, certain stretches of the coast undergo continuous erosion thus affecting the stability of the coas{ The wave and wave-induced processes are the predominant factors responsible for the beach erosion/accretion. The wave transformation processes in shallow waters bring about spatial variation in wave energy leading to erosion/accretion along the coast^7.8.

The anthropogenic factors may also contribute significantly to the beach stability. The coast of Quilon is noted for a high density of population combined with intensive fishing activity. The rich placer deposit occurring in the northern part of this coast makes it economically important. There has been a gl'eat concern in the recent years about the erosion occurring in some parts of this coast which is reported to be due to the placer mining by IRE. Hence, a study on the stability of the beaches of this coast has been undertaken in relation to nearshore wave energy distribution.

Materials and Methods

The area of investigation is confined to the coastal terrain of Quilon (Fig. I) extending over a length of 41 km. The shoreline changes its orientation from 290⁰ to 350⁰N at Thangassery headland. This

demarcates the coast south of Thangassery as southern sector and north of it as northern sector. The Kallada river is a major river system debouching sediments into the Asthamudi estuary which opens into the sea at Neendakara. The major rock types in the hinterland regIOn are Khondalite suite of rocks

,

\

,

\ , ,

,e' ,

\

\

\ ,

\

~

;

\

~

..

"V '-

." ,

"V '

<S> '

-

"t- ... ',;"\,

,

Itm 15 0 6 Itm

... =="""'== ... , , ,

Fig. I- Location map

234 ^{INDIAN 1.} MAR SCI., VOL. 28, SEPTEMBER 1999

with a narrow strip of Tertiary sediments on the coast.

This rich beach placers in the northern sector are being mined since a couple of decades. The beach sediments are coarser in the southern sector and progressively becomes finer towards the northern sector. The southern inner-shel f is steeper than the northern shelf. No measured data on waves IS

available for this coast. According to Kurian? this coast falls under the moderate wave energy regime.

The present study IS based on beach profiles, satellite imageries, wave refraction diagrams and nearshore wave energy distribution for the area represented by stations CES-77 to 116 (Fig. I). The beach profiles for the period 19806, 1981⁶ and 1995 were processed for long- and short-term volume changes, and cIassi fied into erosionallaccretional beaches. The beach volume IS represented as m'llinear metre of beach. The long-term shoreline changes for the period 1968-1990 were estimated by

comparing the IRS-lA, LISS II geo-coded imagery with the toposheet of 1968. Sufficient number of ground control points were taken while estimating the shoreline changes.

The wave refraction diagrams and wave energy distribution for the area of study are obtained from deep water wave data by making use of a wave transformation model? This model computes nearshore wave parameters from deep water data by making use of the equation H=K,K,K,Ho, where H is the nearshore wave heights, K, ^IS the refraction coefficient, Ks is the shoaling coefficient, K, is the friction coefficient and Ho is the deep water wave height. The bathymetric grids. with an element of 1.8 km were prepared for the shelf area utilising the hydrographic charts. The deep water wave statistics (Table I) for the study was obtained from the raw IDWR data compiled and supplied by National Institute of Oceanography. Utilising the bathymetric

Table I- Deep water wave statistics Direction

South

South-south-west

South-west

West-south-west

West

Period (s)

6 8 10 12 14 6 8 10 12 14 6 8 10 12 14 6 8 10 12 14

6 8 10 12 14

0.5

I)

4 4 7 7 5 5

o

6 J

7 2 4 2 3 II

I 3 9 5

10 7 9 8

1.0

10 8 7 8 7

7 6 5 4 6

I)

9 3 3 6 10

8 3 5 7

II 9 5 8

I)

Freq. of occurrence of wave height (Ill) 1.5

8 8 9 6 4 5 6 5 2 6 9 8 5 4 7

9 8 5 4 6 II

l)

9 3 9

2.0

II 10 10 5 5 10 8 5 3 6 9 8 6 2 4

8 9 4 2 3

o

I)

4 6

I)

2.5 6 8 7 3 2 5 6 2

o o

7 6 5

9 8 5 2

9 7 10

4 3

3.0 3

5 2 3

4 4 2

()

o

5 6 4 2 2

8

(,

4 2

o

7 6 :\

3

35

()

:\

I 2

()

2 5 J

() ()

I 4 2

()

o

3 4 :\

I ()

4

(,

2 2 3

;:::4.0 5

(,

6 3 2 :1 :\

5

() ()

3 3 3

()

7

l)

5 :1 4 10 II

2 5

,..

PRAKASH e/ al.: BEACH STABILITY ALONG QUILON COAST 235

grids, the computations were made for each height- period-direction using the model. Refraction diagrams were prepared and zones of convergence and divergence were identified. From the wave height computed at 10m depth for each component, the energy is calculated using the equation^lO

E=1/8pgH²

where E is the wave energy, p is the density of water, g is the acceleration due to gravity and H is the mean wave height. The average energy at each station is obtained by taking the weighted average according to the frequency distribution (Table I).

Results

The refraction diagrams were prepared for all the directions and periods, and those for the predominant directions and periods are presented here. From the deep water wave climate (Table I) it can be inferred that the westerlies, i.e direction 270°, constitutes the predominant ones. This is followed by the WSW, i.e. 24rN. The refraction diagrams for the westerly and WSW direction and typical periods 10 and 12 s are shown in Fig. 2. Though the lower periods are dominant the higher periods are the ones which are important as far as the energy distribution is concerned. The westerly deep water waves of period las show convergence in the southern sector at stations-78 to 81, and in the northern sector at stations

CES-107 to \09. A predominant divergence is seen around the Thangasseri headland. The refraction diagram for 12 s period shows that the convergence points have shifted slightly towards north. As expected the convergences and divergences have become more pronounced. The WSW waves also create prominent zones of convergence and divergence with some shifts in their position when compared to the westerlies. There are strong convergences to the south of Chavara and near the headland for WSW waves of both the periods.

The distribution of nearshore wave energy along the coast is shown in Fig. 3. The distribution for monsoon shows that the highest value of 5215 Jim"

was found in the southern most station CES-77 and lowest value of 4043 Jim" is found in the northern most station CES-116. From the highest value at CES-77 it decreases gradually to a lower value of 4238 J/m² at the headland. On either side of the Neendakara inlet high values around 5000 J/m² are noticed. In general the southern sector has higher wave energy compared to the northern sector. Un like the southern sector the energy varies with intermittent peab at CES-97, lOa, 107 and 114 in the northern sector (Fig. 3). The distribution of the whole year average also shows the same trend as in the monsoon period. It varies from the highest value of 3382 J/m² at CES-77 to the lowest value of 260 I J/m² at CES-

116.

Fig. 2-Wave refraction diagrams for (a) westerly period I 0 ~lI1d 12 s (b) west-south-west period (10 ,mel 12 s)

236 ^{INDIAN J.} MAR SCI., VOL. 28, SEPTEMBER 1999

The volume change for monsoon and net yea.rly change during 1980-81 are shown in Fig. 4. The southern most zone (CES-77 to 79) shows eroding trend during monsoon period. The beaches from station CES-80 to headland are showing a prominent accretionary trend contrary to the usual beach behaviour during monsoon. A cumulative accretion of 32 m'/m was observed for this sector. On either side of the Neendakara inlet an average deposition of 28 m³/m of beach was noticed. The northern sector beaches starting from CES-98 show eroding tendency except at CES-106. The erosion was vigorous in the northern most zone. During this period a cumulative erosion Of 159 m³/m has been observed for the northern sector.

The net yearly change for the entire coast showed more or less same pattern as that of the monsoon. A

5500 5000

"""

~

cumulative accretion of 124 mJ/m for the s'outhern sector and erosion of 335m'/m for the northern sector were observed. For the entire coast a cumuiative erosion of 211 m'/m was computed.

In order to get a comprehensive picture of the long- term beach volume changes along t is coast, the profiles of selected stations for the years 1980 and 1995 have been compared and the volume change obtained is shown Table 2. The station CES-87 in the southern sector shows a net accretion of 28 mJ/m. In the northern sector all the stations show erosion with a maximum of 60 m'/m at CES-I 00. Since the long- term volume changes are available for some stations only, the trend has been studied from the satellite imagery also which has some limitations. The net shoreline changes for the period 1968- 1990 is shown in Fig. 5. Since the' resolution of IRS-I A LISS II is

@

...

~

,

'" ..

---

,-

- - .

- ^-

C1)

~ 3000

~ «3 2500 2000

~

3³⁰

.t; ::

~ 10

-<

-10 5-30

f'>-

~ .~-50 1ol-70

e

-90

"-

- .. ... , _{, ,.} ^,.

... ,

.. _..

'.-.,.- ....

77 80 82 84 90 92 94 96 98 100102104106108110112 114 116 Station numbers

Fig. 3-Nearshore wave energy distribution along the coast.

,.____---Yearlyaverage

Fig. 4-Beach volume changes along the coast.

PRAKASH el a/.: BEACH STABILITY ALONG QUILON COAST 237

only 36 m, any change within 36 m is noted as 'no change'. From the Fig.5 it can be seen that there is no shoreline change in the southern sector. In the northern sector, in general a retreat of shoreline is observed. At CES-93 a shoreline retreat of more than 100 m is noticed. No change is observed between CES- 102 and 103 where the IRE factory is located close to the beach and which is well protected by seawall. The beaches of stations CES-106 to 108 where active mining for beach placers is being carried out by IRE, recorded a maximum retreat of 250 m. A retreat of around 100 to 150 m was observed further north up to CES-I 15.

Discussion

The major factors responsible for the beach erosion are wave and wave-induced nearshore processes though other factors like storm surges and anthropogenic activities may also contribute considerabl/o. Sea level rise (SLR) also can influence the long term changes. This coast does not have a pronounced SLR and hence it might not have affected the beach stability".

The wave energy distribution along the coast can be attributed to the refraction and other transfor- mation processes in the shallow waters⁸ The southern coast is having high energy because of the convergence it gives for the predominant wave direction and period components. The steeper innershelf which leads to lesser energy dissipation due to bottom friction could be another reason for higher energy in the southern sector. The energy level decreases towards the headland because of the divergence, which is due to the presence of bay. The convex shaped bottom contours off the headland and

Station numbers

immediate north of Neendakara inlet lead to convergence of wave rays resulting in a high energy regime between stations CES- 90 and 97. Between station CES- 101 and 103 where the IRE factory is located there is a divergence of rays resulting in lower energy. Further north from station CES-I06 to 109 where sand mining by IRE is .being carried out there is a convergence of rays resulting in high energy. In general there is a decrease of energy towards the northern side which may be due to the gradual slope of the shelf of this sector.

In order to discuss the observed erosion/accretion pattern with reference to energy distribution, the diagram (Fig. 6) showing the longshore energy gradient is used. In general it is observed that the longshore energy gradient ^IS lllaXllnUIll off Neendakara inlet followed by the Chavara coast between stations CES-97 and 109. The erosion observed in the southern most stations from CES-77 to 79 may be due to the longshore gradient towards north, which makes the materials move towards north.

The region from CES-80 to the headland experiences accretion possibly due to supply of material from

Table 2-Long-lerm volumc changes

St no. Volumc changc (1ll¹/m)

87 +2X

98 -54

100 -60 :

101 -17

III -5

115 -II

116 -33

+ Accretion -Erosion

79 81 83 Il5 87 ·93 97 1l9 . 101 ·103 1(J) 107 lCil 111 113 115

~ Nochonge

i

c:

i!!""50

I>

., c:

Em _o

.r: (/)

·150

-2DJ

Fig. 5-Long-term shorelinc changes along the coast from IRS-lA, LlSS-1I imagery (Resolution 36 m only)

238 ^{INDIAN J. MAR} SCI., VOL. 28, SEPTEMBER 1999

\

\ I

\

\

\

\

\

\

\ "0 _~

I- LEGEND

~Ymb<ll Energy Grod.{Jm2 }

0 <50_

I-I!!> 50 -1 ~o

e 150-250

~ > 250

.-

Fig. 6-Nearshore energy gradient: (A) monsoon (B) yearly average

both south as well as north. Though a very strong energy gradient was observed off Neendakara inlet, it is not reflected on the coast due to the presence of breakwater on either side of the inlet. There is no change in beach volume at stations CES-99, 104 and 111 though the energy gradient at these stations does indicate accretion. It can be inferred that the accretion predicted by energy gradient does not materialise possibly due to sand mining in the adjoining area.

The significant erosion occurring in zones CES - 107 to 109 and 113 to I 14 corroborates with the high longshore energy gradient at these stations. In addition to this, sand mllllllg might also be contributing to the erosion. The net yearly beach volume changes along the coast is also explained by the yearly longshore energy gradient and anthro- pogenic factors.

The long-term beach volume data (Table 2) and shoreline change diagram (Fig. 5) indicate there was a stable beach to the south of the headland and continuous erosion in the northern sector which is in conformity with the short-term changes obtained. The observed significant erosion at station CES-98 is very well reflected in the long-term volume changes. However, though the short-term change at CES-

roo

indicates a near stable condition the long-term change showed a high erosion of 60 m³/m. The area north of CES-IOO is the IRE factory which is well protected by seawall with no frontal beach. Hence there is a constant loss of material from the zone north of

Neendakara inlet due to northerly longshore current prevailing throughout the yeal.!'. There may not be any significant replenishment from the south due to lack of sediment supply from the river and partly due to the hindrance to longshore transport by breakwaters.

A study on short- and long-term beach volume changes, shoreline changes, wave refraction pattern and nearshore wave energy distribution along the Quilon coast indicate that the wave energy is higher in the southern sector station and decreases towards the northern sector station. Fro IT) the short-and long- term beach volume change data it is found that the beaches of the southern sector is nearly stable while the northern sector is eroding. The erosion/accretion pattern closely corroborates with the longshore energy gradient except in areas where anthropogenic factors dominate. In addition to the wave and wave- induced processes, the anthropogenic factors like sand mining, breakwaters and lack of sediment supply from the rivers are some of the reasons for continuous erosion in the northern sector. The shorel ine change data from the satellite imagery need to be checked with high resolution remote sensing data products.

Notwithstanding the above limitations. the shoreline change data nearly corroborates the beach volume change data.

Acknowledgement

Authors are grateful to Dr. M. Baba, Director and Head, Marine Sciences Division. for the encouragement and suggestions. We greatly appreciate the financial support by Dept. of Science and Technology, Govt. of lndia under the project sanction ESS No. 23/159/92. One of the authors FJ is thankful to CSIR for granting a JRF.

References

1 Udayavarma P. Rama Raiu V S. Abraham Pylcc &

Narayanaswamy, Mahll.\'{/gor- li/ll/. NOIIi. !lIsl.

OcemlOJ(raphy, 5. 1981. 85.

2 Samsuddin M & Suchindan G K . .1 COOSI Rcs. 1. IlJX7. 55.

3 Mallik T K. Samsuddin M. Prakash T f\. V~lsudcv<ln V &

Machado T. EIlI'iroll. Ceo!. W{/Ier Sci. 10. ilJX7. 105.

4 Thomas K V. in: Oce{/I/ 1I'(II'e.\· (llId heaeh ^!)I'()( esses. cdltcd by M Baba & N P Kurian (Celllrc for Earth SClcnce SllIdies.

Trivandrum) 1988.47.

5 Shahul Hameed T S. in: Occal/ 1I'{Ii'('.\' Ollt! /Jl'({ch processes.

edited by M Baba & N P Kurian (Centrc lor Earth SClencc

Studie~. Trivandrull1) 1988.67.

6 Prakash T N & Aby Verghesc P. J Ceo!. SOl'. IIll/ia. 2lJ.

1987,390.

7 Kurian N P & Baba M. in II/dio's c/II'irol/l//l'/II!)rohll'/l/s, alld per.l'pecliI'e. edited by B P Radhakrishn<l & K K Ramachandran (Geol. Soc. of India. I3ang,llorc) 5. i lJ86. 251.

...

PRAKASH et al.: BEACH STABILITY ALONG QUILON COAST 239

8 Kurian N P, in Oceall wave studies and applications, edited by M Baba & T S Shahul Hameed (Centre for Earth Science Studies, Trivandrum) 1988, 15.

9 Kurian N P, Wave height and spectral transformation in the shallow waters of Kerala coast and their prediction, Ph.D thesis, Cochin University of Science & Technology, India, 1987 .

10 US Army, Coastal Engineering Rl'search Cmlre (U S Gov!.

Printing Oftice, Washington D C) 19R4.

II Harish C M, Impact of sea level rise dill' /() green hOllse effect-statistical analysis of tide gallge £law along the entire west coast of India (Centre for Enrth Science Studies, Trivandrum) 1993, pp. 53.