SHORELINE PREDICTION USING A NUMERICAL MODEL ALONG RATHNAGIRI COAST, WEST COAST OF INDIA

Abstract

Thorough understanding of coastal processes which are controlled by coastal hydrodynamics and the resulting sediment transport is essential for development of coastal zone management plans. A small pocket beach around 2 km length at Bhatiya, Rathnagiri in the southwestern part of Maharashtra has taken for present investigation. The state of art numerical model 'LITPACK' implemented at study region for estimation of Longshore Sediment Transport Rate (LSTR) and shoreline evolution. Major inputs required
for modelling are nearshore wave climate, bathymetry, sediment characteristics and initial coastline. Nearshore wave climate at a depth of 15 m were collected using wave rider buoy during 2014-2016. Bathymetry, nearshore sediments and shoreline tracking were also collected during the same period. LSTR estimated for southern, central and northern sector of the study region. Annual LSTR is about 3 * 105 m3/year for northern & southern sector and it is about 2.4* 105 m3/year for the central sector. The model result compared with the available field observation and earlier studies. Further sediment transport table generated from calibrated model for estimating shoreline evolution along the study region. Shoreline evolution has been simulated initially for one year. Initial coastline digitized for every 10 m distance for better representation of coastline. Coastline grid starts from extreme southern boundary of the study region and extends up to northern boundary. Shoreline evolution at Bhatiya shows advancement of shoreline at extreme northern boundary of the study region and it may be due to net northerly sediment
transport along the region. One year wave climate extended to ten years using 'LITVONV' utility of LITPACK software for understanding the shoreline evolution for upcoming years. Result shows that more accretion in the northern sector & some erosion in southern sector.

Full text

6th National Conference on Coastal,
Harbour and Ocean Engineering
26th to 28th September, 2018
Central Water & Power Research Station, Pune
Indian Society for Hydraulics, Pune 625
SHORELINE PREDICTION USING A NUMERICAL MODEL ALONG
RATHNAGIRI COAST, WEST COAST OF INDIA
V. NOUJAS, R.S. KANKARA
National Centre for Coastal Research (NCCR), MoES, Chennai, India -600100
Email: noujasphy@gmail.com
ABSTRACT
Thorough understanding of coastal processes which are controlled by coastal hydrodynamics and the
resulting sediment transport is essential for development of coastal zone management plans. A small
pocket beach around 2 km length at Bhatiya, Rathnagiri in the southwestern part of Maharashtra has taken
for present investigation. The state of art numerical model 'LITPACK' implemented at study region for
estimation of Longshore Sediment Transport Rate (LSTR) and shoreline evolution. Major inputs required
for modelling are nearshore wave climate, bathymetry, sediment characteristics and initial coastline.
Nearshore wave climate at a depth of 15 m were collected using wave rider buoy during 2014-2016.
Bathymetry, nearshore sediments and shoreline tracking were also collected during the same period.
LSTR estimated for southern, central and northern sector of the study region. Annual LSTR is about 3 *
105 m3/year for northern & southern sector and it is about 2.4* 105 m3/year for the central sector. The
model result compared with the available field observation and earlier studies. Further sediment transport
table generated from calibrated model for estimating shoreline evolution along the study region. Shoreline
evolution has been simulated initially for one year. Initial coastline digitized for every 10 m distance for
better representation of coastline. Coastline grid starts from extreme southern boundary of the study
region and extends up to northern boundary. Shoreline evolution at Bhatiya shows advancement of
shoreline at extreme northern boundary of the study region and it may be due to net northerly sediment
transport along the region. One year wave climate extended to ten years using 'LITVONV' utility of
LITPACK software for understanding the shoreline evolution for upcoming years. Result shows that
more accretion in the northern sector & some erosion in southern sector.
Keywords: Longshore Sediment Transport Rate (LSTR), Bathymetry, Waves, Shoreline evolution
1. INTRODUCTION
Shoreline is highly undulating either due to natural or human activities. Natural causes that affect
shoreline include waves, tides, currents, longshore currents and morphology of the coast.
Construction of coastal protective measures, harbours, sand mining are examples of human
interference in the coast. It has been recognized that the presence of artificial protection measures
like seawalls, groins and breakwaters can also trigger shoreline changes (Bakker et al., 1970;
Komar, 1998; Cuadrado et al., 2005; Kim et al., 2009; Thomas et al., 2013). Detection of
shoreline change, the rate of positional change and future prediction play an important role in
any coastal zone management such as hazard zonation, island development studies, marine
transport, sediment budget and the modeling of coastal morphodynamics (Zuzek et al., 2003;
Maiti and Bhattacharya, 2009). There are many studies carried out for shoreline change studies
along the west coast of India (Murty, 1977; Sreekala, et al., 1998; Jayappa et al., 2003; Navrajan
et al., 2005; Kumar and Jayappa, 2009; Kumar et al., 2010; Loveson et al., 2014; Mukesh

6th National Conference on Coastal,
Harbour and Ocean Engineering
26th to 28th September, 2018
Central Water & Power Research Station, Pune
Indian Society for Hydraulics, Pune 626
Gupta, 2014; Noujas and Thomas, 2015) and limited studies only for shoreline prediction (Maiti
and Bhattacharya, 2009; Shamji et al., 2010, Noujas et al., 2014; Rajasree et al., , 2016; Prasad et
al., 2016; Noujas et al., 2017).
Shoreline can be predicted by using statistical or numerical models and for this study highly
established numerical model LITPACK were used. There are many studies about waves carried
out along Rathnagiri coast (Glejin et al., 2012; Kumar et al., 2014) and as of our best knowledge
there is no study regarding shoreline prediction and hence present investigation carried out.
2. STUDY REGION
Rathnagiri is a port city (Fig. 1) in the southwestern part of Maharashtra, India (16 57' 53.76" N,
7317' 36.86"E to 1658'51.05" N, 7317' 38.02"E) and Bhatiya is a small pocket beach around 2
km length. It is one of the minor port situated along the south west coast of India. Port is
maintaining the channel depth of 11 m. It has an average elevation of 11 meters from MSL. The
Sahyadri mountains border the Rathnagiri on the east. Heavy rainfall during monsoon results in
highly eroded landscape in the coastal region.
Figure 1: Study region
3. METHODOLOGY
Bhatiya

6th National Conference on Coastal,
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26th to 28th September, 2018
Central Water & Power Research Station, Pune
Indian Society for Hydraulics, Pune 627
3.1 Data Collection
The wave data collected in the year 2015 January to December using wave rider buoy has been
used for the present investigation. Waves were measured using the Datawell directional wave
rider buoy at a depth of 15 m. The vertical and horizontal (eastward and northward) displacement
data were obtained from the respective accelerations measured by the buoy. The data were
recorded continuously at 1.28 Hz, and the data for every 30 min were processed as one record.
The bathymetry survey carried out using echo sounder for the nearshore region up to 12 m depth
in a closed interval of 500 m transect along Vengurla region during January 2016. For getting
better accuracy in bathymetry while interpolating; alongshore transects were also collected at
depths of 3, 6 and 9. The echo sounder was integrated with GPS (Global Positioning System) for
accurate positions. The shoreline data was collected during Low tide time and beach profiling
collected using Trimble RTK-GPS prior to bathymetry data collection. Therefore bathymetric
data, shoreline and beach profiles were combined together and interpolated using MIKE 21 Mesh
Generator for obtaining the continuous bathymetry of the area (Fig. 2).
Figure 2: Bathymetry along Bhatiya, Rathnagiri region

6th National Conference on Coastal,
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26th to 28th September, 2018
Central Water & Power Research Station, Pune
Indian Society for Hydraulics, Pune 628
3.2 Numerical Modelling
Sediment transport and shoreline change were estimated using the model LITPACK (DHI,
2014). It is a numerical model in MIKE software package for simulating non-cohesive sediment
transport driven by waves and currents along quasi-uniform beaches. Littoral drift, coastline
evolution and profile development were also computed using various modules. The main
modules of LITPACK consist of LITDRIFT (Longshore current and littoral drift) and LITLINE
(Coastline evolution).
The LITDRIFT calculates the net/gross littoral transport over a specific design period. It
combines Sediment Transport Program (STP) with coastal hydrodynamic model to give a
deterministic description of the littoral drift. It simulates the cross-shore distribution of wave
height, setup and longshore current for an arbitrary coastal profile. It provides a detailed
deterministic description of the cross-shore distribution of the longshore sediment transport for
an arbitrary bathymetry for both regular and irregular sea states. The longshore and cross-shore
momentum balance equation is solved to give the cross-shore distribution of longshore current
and setup.
LITDRFIT consists mainly of two computation steps: longshore current calculation
(hydrodynamic model) and sediment transport computation (sediment transport model, STP).
The cross-shore distribution of longshore current, wave height and setup for an arbitrary coastal
profile, is found by solving the long and cross-shore momentum balance equations. The
hydrodynamic model includes a description for regular and irregular waves, influence of tidal
current, wind stress and non-uniform bottom friction as well as wave refraction, shoaling and
breaking.
Based on the results from LITDRIFT, the LITLINE simulates the coastal response to gradients in
the longshore sediment transport. The LITLINE calculates the coastline evolution by solving a
continuity equation for the sediment in the littoral zone. The influence of structures, sources and
sinks is included. With jetties and breakwaters, the influence of diffraction on the wave climate
was also included. It calculates the coastline position based on input of the wave climate as a
time series. The model, with minor modifications, is based on one-line theory, in which the
cross-shore profile is assumed to remain unchanged during erosion/accretion. Thus, the coastal
morphology is solely described by the coastline position (cross-shore direction) and the coastal
profile at a given longshore position. Through successive calls to LITDRIFT, the associated
program LINTABL calculates and tabulates transport rates as functions of the water level, the
surface slope due to regional currents, wave period, height and direction with respect to the
coastline normal.
The governing sand conservation equation is given by
()
 = 1
()()
 +()
  (1)

6th National Conference on Coastal,
Harbour and Ocean Engineering
26th to 28th September, 2018
Central Water & Power Research Station, Pune
Indian Society for Hydraulics, Pune 629
Where,  () =distance from the baseline to the coastline; t =time;  = height of active
cross-shore profile; =longshore transport of sediment expressed in volumes; x=longshore
position;  =Longshore discretization step; = source/sink term expressed in volume.
  and  are calculated based on user specifications, while the longshore transport
rate () is determined from tables relating the transport rate to the hydrodynamic conditions at
breaking.  is user specified, while  is determined from stability criteria. From an initial
coastline position () (x), the evolution in time is determined by solving the above equation,
using an implicit Crank-Nicholson scheme.
4. RESULTS AND DISCUSSIONS
4.1 Nearshore Wave Climate
Nearshore wave data collected at a depth of 15 m in 2015 (Fig. 3). During January-May more
than 90% of root mean square wave height (Hrms) is less than 1 m and it starts to increase in first
week of June due to onset of monsoon. Maximum Hrms about 3.9 m observed on 23rd June and
second highest Hrms observed during the 18th July and it is around 2.5 m. Hrms starts to decrease
during the second week of October and it is less than 1 m in remaining period. The zero
crossing wave period (Tz) is comes in the range of 2.6 to 10.3 s and mean value of 5 s. Mean
wave direction shows mainly in the range of 184 - 234 with a mean value of 254.
Figure 3: Wave climate off Rathnagiri during the year 2015
4.2 Longshore Sediment Transport Rate (LSTR)
Longshore sediment transport along Bhatiya coast estimated for southern, central and northern

6th National Conference on Coastal,
Harbour and Ocean Engineering
26th to 28th September, 2018
Central Water & Power Research Station, Pune
Indian Society for Hydraulics, Pune 630
sector of the study region with measured wave data of the year 2015, surveyed bathymetric data
and measured sediment characteristics. Annual longshore sediment transport is about 3 * 105
m3/year for northern & southern sector and it is about 2.4 * 105 m3/year for the central sector.
Sediment transport is towards north except December, Jan- Feb month and rate is also less
during those months (Fig. 4). The coastline normal is 265 in Bhatiya coast and this is the reason
for northerly transport in most of the months. Northerly sediment transport is much higher during
the month of June and followed by July. The model result is comparable with the study
conducted by Chandramohan and Nayak (1992).
Figure 4: Monthly variation of LSTR along Bhatiya coast
4.3 Shoreline Evolution for One Year
Shoreline evolutions at Rathnagiri have been simulated initially for one year. Initial coastline
digitized for every 10 m distance for better representation of coastline. Coastline grid starts from
extreme southern boundary extends up to northern boundary. Shoreline evolution at Rathnagiri
shows advancement of shoreline at extreme northern boundary of the study region due to net
northerly sediment transport (Fig. 5).

6th National Conference on Coastal,
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26th to 28th September, 2018
Central Water & Power Research Station, Pune
Indian Society for Hydraulics, Pune 631
Figure 5: Shoreline evolution for one year along Bhatiya coast
4.4 Shoreline Evolution for Next Ten Years
One year wave climate extended to ten years using 'LITVONV' utility of LITPACK software for
understanding the shoreline evolution for upcoming 10 years. All other input conditions are
same as one year model run. The Result showing more accretion in the northern sector & some
erosion in southern sector (Fig. 6). Accretion in northern sector is due to net northerly transport
at Bhatiya. However, the cumulative advancement of beach in the northern boundary may not
happen in real time scenario due to loss of sediments as a cross-shore transport during the
monsoon season.
Figure 5: Shoreline evolution for ten years along Bhatiya coast

6th National Conference on Coastal,
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26th to 28th September, 2018
Central Water & Power Research Station, Pune
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5. CONCLUSION
Longshore Sediment Transport Rate (LSTR) and Shoreline evolution computed along Bhatiya
coast using state of art LITPACK model. Initial coastline, nearshore wave climate, sediment
characteristics and bathymetry collected during 2015-16. LSTR is about 3 * 105 m3/year for
northern & southern sector and it is about 2.4* 105 m3/year for the central sector. Further
sediment transport results were used to run shoreline evolution model. Shoreline evolution for
one year shows very small accretion in northern boundary of the study region. Further model run
continued for ten years and results shows advancement of shoreline extreme northern boundary
of the study region due to net northerly sediment transport. However, the cumulative
advancement of beach in the northern boundary may not happen in real time scenario due to loss
of sediments as a cross-shore transport during the monsoon season. In addition some minor
erosion observed southern side of the study region. The result of this study can be used by
coastal managers for suggesting suitable management plans along the sector
ACKNOWLEDGEMENT
We express our sincere thanks to Director, National Centre for Coastal Research (NCCR), MoES,
Chennai for providing the facility for carrying out this work and his keen interest. We are grateful to all
team members of NCCR and CSIR-NIO, Goa for helping us in data collection.
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6th National Conference on Coastal,
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6th National Conference on Coastal,
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