Design of a sustainable flood carrier system in Cuddalore of Tamilnadu

Cuddalore district is located along the south west coast of Indian in the state of Tamilnadu bordering bay of Bengal.. The district is blessed with presence of wetlands, minor port and beach. The annual rainfall of the place is about 1300 mm. The district receives 70% of rainfall during north east monsoon from October to December. Agriculture is the main occupation. The rainfall along the coast is higher compared to upper regions. The high intensity rainfall occurrence is a common one. The district has anciently constructed hydraulic structures for conserving water called tanks. Perumal tank is the last source. The entire flood water during rainy season flows through a drain called Paravanar. This runs almost parallel to the coast for about 18km and finally merges with Bay of Bengal. Because of length and shallow slope, it takes more time for the water to recede. Hence a new canal was designed to drain some water directly in to the sea. HECRAS numerical model was adopted to study canal performance. The details of hydraulic studies and coastal sediment transport computations are highlighted.


Introduction
The district of Cuddalore is located along 78 o 45'E and 11 o 15'N coordinates along the south west coast of India in Tamilnadu state. The sea of Bay of Bengal is bordering the district on its east side. The river Pennar is draining in the sea along the north side and river Vellar is confluencing at the southside. Apart from this another drain called Paravanar river is also draining in the sea in between Pennar (Fig 1).Cuddalore is a low lying area and highly flood prone as described by Evany Nithya and Priyanka(2019).

Hydrology of Paravanar
The river is drain carrying the rainwater during floods. The drain commences from surplus portion of the existing Perumal tank. The river takes a crescent like plan shape at a place called Aruvamukku and then flows for a distance of about 18km and finally confluences with Bay of Bengal at Cuddalore port. The average slope of the river is 1 in 3000. The annual rainfall of the district is about 1300mm.Number of effective rainfall days are around 30 in a year. Nearly 60% of the rainfall occur during north east monsoon period from October to December. Out of this it is common to have high intensity rainfall based on studies of Sandeep and Jegankumar (2018) which results in drainage problem. Moreover, due to very shallow slope the drainage process is getting more delayed (Fig 2 to  4).   The district of Cuddalore is generally a low lying one .The slope of river Paravanar is very shallow. The place is highly flood prone especially during north east monsoon. So this drain because of its geometry takes a long time for complete drainage of flood water. This in turn results in heavy water logging as observed by Yovan Felix and Sasipraba (2020). Because of waterlogging the people living here are in high flood threat for more days even if the rainfall stops. It also results in unhygienic situation. Based on their representation the PWD officials have proposed a flood carrier directly connecting Paravanar river to sea by a flood carrier canal. (Fig 5). The existing river Paravanar can carry 1190 m 3 /s. It is proposed to diver 600 m 3 /s to new canal.

Desk studies
This includes study of shoreline changes and rainfall data. The satellite imageries will give the historical coastline changes over a period of time. In the present case the imageries during 2003, 2005, 2006, 2008.2011 and 2015 were downloaded from google earth and analysed. The pattern of changes that takes place along the estuary was studied. The river mouth is already trained by constructing a pair of breakwater. Hence it was not all creating any block. Accretion of beach was noticed on the south side and erosion on the north side of breakwater indicating that littoral drift is towards north. (Fig 6). Rainfall. The district has a hot tropical climate. The summer season, which is very oppressive, is from March to May. The southwest monsoon, which follows, lasts till September. October to December constitutes northeast monsoon season. January to February is the comparatively cooler period. The annual normal rainfall for the period (1901-2000) ranges from 1050 -1400 mm. The normal annual rainfall over the district varies from about 1050 mm. The contributions of individual seasons are as follows: NE-57%, SW-31%, Summer-7% and winter 5%. But the rainfall pattern in third decade shows high magnitudes of north east monsoon. (Fig 7 &8). . The study of rainfall pattern indicates that north east monsoon is dominating. The number of rainy days is in the range of about 25. But during high intensity rains the flooding process is the creating inundation Table1

Sediment transport
In order to understand the shoreline dynamics, it is very much essential to have an estimation of littoral drift that takes place along the coast. This will be useful to arrive any training works at river entrance. Here the littoral drift is estimated adopting Van Rijn (2001) approach (Table 2). It is to be mentioned that the net littoral drift along east coast during Jan to Sept is towards north, while during Oct to Dec it is directed toward south as pointed out by NarasimhaRao (1983).

Conclusions
The proposed carrier canal designed for 600cumecs or 12200 cusecs was modelled for different boundary conditions without any spill • The channel is hydraulically capable of carrying the designed discharges • The sea bed bathymetry and coast line changes using the imageries wereanalysed. The estimation of alongshore sediment transport is directed towards north direction • Even though it was initially felt that there is a need to construct training wall for about 100m length to keep the outlet of proposed channel open, the littoral drift pattern indicate that it will create high erosion on its north side which is populated. Hence it is recommended not to adopt any such hard measures • The present site is located at about 10km south side of existing Cuddalore minor port. It was assessed that present opening of new inlet to sea will not be permanently open as the net drift is towards north. Hence the confluence point is to be periodically dredged and the dredge material should be dumped on the coast on the north side so as to maintain the sediment transport equilibrium. Hence no artificial interferences were made at exit and it is designed as a sustainable one. • The present canal will definitely reduce the receding of flood water because the distance is reduced by the construction of flood carrier • The project in general will help the social population in clearing the flood.