1. Parvathy K.G. National Institute of Technology Karnataka, Surathkal, India A numerical modeling approach for study of mudbank impact on coastline Introduction Southwest coast of India bearing the brunt of the S. W. monsoon waters are highly productive and sustain a large pelagic fishery. With the onset of monsoon a very special phenomenon occurs along specific parts of the southwest coast, particularly along Kerala coast, known as Mudbanks or in local terms called ‘Chakara’. Mudbanks are defined as patches of calm, turbid water with high load of suspended sediment, appearing close to the shore with a clay substratum during the rough monsoon season. Globally mudbanks reported along the southwest coast of India, northeast coast of South America, Lousiana coast and the northeast coast of China. Mudbanks of southwest coast of India make it unique from the mudbanks occurring along other parts of the world.. Mudbanks, the formation of which is a curious phenomenon reported hitherto from southwest coast of India, appear over restricted areas on the Kerala-south Karnataka coast during the inclement S.W. monsoon as bodies of calm water, where the wave action the coastal water abounds with is almost absent due to a fine mud in a state of suspension. On a squally, surf beaten coast, these areas facilitate easy fishing operations of country crafts, which otherwise idle away during this harsh weather. For this reason it gained much popularity, particularly of late among the coastal people. This phenomenon, taking place in the coastal zone, has been known from time immemorial. The phenomenon is all the more important as it also enables a thrilling fishery to develop in the coastal area where the mudbank appears during the height of the monsoon period. The waters of the mudbank remain practically an undisturbed sheet, while turbulent conditions prevail along its outer fringes, a situation brought about by the colloidal suspension of fine silt or clay particles in the water column. This calmness facilitates easy operation of fishing canoes, which are transported from fishing villages several kilometres north and south of the mudbank. Objectives  Socioeconomic Significance : rich fishing ground particularly during monsoon season when the rest of the sea area hostile for fishing.  Coastal Protection :protects a section of the coast when the major portion of the southwest coast of India faces severe erosion due to monsoon waves.  Natural Breakwater : unique phenomenon which has tremendous ability to damp the wave.  Shore Equilibrium : direct influence on the equilibrium conditions of the shore in its vicinity. Schematic profile of a mudbank region ( after Nair1988). Mudbanks are an inquisitive coastal phenomenon which occurs only at a few locations in the nearshore waters of the world ocean. Mudbanks, its occurrence, nature, properties and characteristics are interesting subjects from engineering point of view Considering the socio economic importance of this coastal phenomenon the present work is an initiative to study the impact of mudbanks on coastal dynamics along Southwest coast of India details a numerical approach using a software package called LITPACK Motivation Study the influence of mudbank on nearshore wave attenuation To provide an insight of mudbank impact on coastal morphology through a numerical modeling approach. Objectives Regional Setting Straight sandy coast extending for about 39 km from Munambam inlet to Chettuwa inlet in in Thrissur district of Kerala, India. Mudbank locations Methodology Working of Model Impact of Mudbank on Nearshore Wave Characteristics  Analyses of measured field data of offshore and nearshore waves during mudbank period for wave attenuation studies  Wave transmission is quantified by the use of the wave transmission coefficient, Kt = Ht/Hi Kt is the wave transmission coefficient, Ht is the height of the transmitted wave on the landward side of the structure Hi is the height of the incident wave on the seaward side of the structure (U.S. Army Corps of Engineers, 1984). Shoreline Evolution Model, LITPACK  Modelling of shoreline evolution characteristics has been done using LITLINE module of LITPACK in MIKE software package.  LITPACK is a numerical model in MIKE software package for simulating non cohesive sediment transport driven by waves and currents, littoral drift, coastline evolution and profile development along quasi uniform beaches Munambam Chettuwa Sector Model Algorithm

2. Acknowledgement I am thankful to Director, Centre for Earth Science Studies, Thiruvananthapuram for facilitating the research.. I acknowledge DHI India for their encouragement and support.. I also acknowledge OCEAN’14 MTS/IEEE team for providing the opportunity to present my work. I also thank ONR for its grant Noujas V Senior Research Fellow Centre for Earth Science Studies Thomas K.V. Scientist G Centre for Earth Science Studies Ramesh H. Assistant Professor National Institute of Technology Karnataka Co-Authors 1.P.G.Kurup, “Studies on Physical Aspects of Mudbank along the Kerala coast with Special Reference to the Purakkad Mudbank,” Bull. Dept. of Mar. Sci.,Univ. Cochin, 1977, pp. 57. 2.A.S.K. Nair, “Mudbanks (Chakara) of Kerala- a marine environment to be protected,” In. Proc. of the National Seminar on Environmental Issues University of Kerala Golden Jubilee Seminar, Trivandrum, India, pp 76- 93. 3.J. Mathew, M. Baba, and N. P. Kurian, “Mudbanks of the southwest coast of India. I: Wave characteristics,” Jour. Coast. Res., 11 (1), 1995, pp.168-178. 4.Centre for Earth Science Studies, Thiruvananthapuram,“Shoreline management plan for selected locations along Kerala coast”. Final project report submitted to ICMAM Project Directorate, Ministry of Earth Science, 2013. 5.Danish Hydraulic Institute (DHI), “LITLINE Coastline Evolution User Guide,” DHI Software, 2014. 6.Nguyen Ngoc Thach1, Nguyen Ngoc Truc1, Luong Phuong Hau2, “Studying shoreline change by using LITPACK mathematical model (case study in Cat Hai Island, Hai Phong City, Vietnam),” VNU Journal of Science, Earth Sciences, 23, 2007, pp. 244 ‐ 252. 7.V. Noujas, K.V. Thomas and K.O. Badarees, “Shoreline management plan for a mudbank influenced coast along Munambam-Chettuwa in central Kerala,”Proc. of HYDRO 2013 International, IIT Madras, India, 2013, pp. 118-126. References Nearshore wave characteristics during mudbank formation Wave ParametersRange of valuesRemarks Significant Wave Height (Hs) (m) 0.05-0.589 Very low value in the nearshore regions of Kara Attributed to presence of mudbanks during monsoon months. Wave Period6.9-19.69 (-) Wave Direction117⁰-261⁰ (-) Plot showing the relation between the wave transmission coefficient and wave height attenuation Wave transmission coefficient calculated for the mudbank region found to be less Wave energy dissipation is more in mudbank region. Present mudbank area reported wave attenuation about 50-95 % Similar studies conducted by Mathew et al (1995) along Alleppy coast observed wave height was attenuated by about 75-80 % in the developing and dissipating stages of the mudbank Results & Discussion  Mudbanks, a region of calm and highly turbid waters with copious fish catch is a miracle of nature and a boon to coastal community.  Formation and functions of mudbank remain unique and enigmatic along littoral waters of southwest coast of India.  Detailed study of Munambam-Chettuwa sector gives a better view of mud banks impacts on coastline along with shift and migration of mudbank locations.  Developing solid knowledge of this local phenomenon would benefit in positioning fishing harbours and implementing coastal protection measures effectively along a mudbank dominated coast.  A better understanding of mudbank impact on coastal dynamics finds an important place in the national context of coastal zone management of the country.  It is found that LITLINE module of LITPACK has no mechanism to account for cohesive sediment grain size and wave damping effect of mudbanks and hence the model as such is not capable of producing accurate results in mudbank areas even though it gives satisfactory results in this study.  Being a 1D model, the shoreline evolution module, LITLINE of LITPACK accommodate alongshore transport alone. Lack of crossshore transport especially during the monsoon and subsequent beach building period may underestimate or overestimate the sediment transport computations Stable and accreting mudbank sector and further north requires no interventions and seawalls In the eroding sector immediately south of mudbank seawalls may be strengthened. Providing the eroding coast with appropriate selection and design of coastal protective measures will facilitate beach accretion along the coast. Coastal Management Strategies Model Studies Shoreline Evolution due to mudbank  Predicting this local phenomenon with the aid of numerical models, would help in redesigning the strategy in positioning fishing harbours and implementing costal protection measures. Conclusion  Shoreline evolution during mudbank formation along the selected coast modeled using LITLINEi  Impact of reappearance/migration of mudbank analyzed Beach sediments accumulate in the mudbank region and its north Location of the structure is altered from grid point 25 to grid point 10 for this purpose.  Eroded area by the old mudbank is accreting as the new mudbank has formed in the eroded area  Accretion observed to the north of new mudbank.  Erosion is accelerated on the southern side of new mudbank.  This is because during monsoon season the net current direction is southerly. Severe erosion found along the coastal stretch south of the mudbank. 1 2 1 2 1 2 Migration of Mudbank 3 3 Model Constraints Scope of Future Work