Management of High Flood in Mahanadi & its Tributaries below Naraj
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Mahanadi is an important river in India. At its delta head at Mundali, it has branched into several rivers and drains its water to the Bay of Bengal. The deltaic region is affected by flood, drainage and salinity problems almost every year due to presence of low level escapes.. These escapes start functioning with a flood of 17000 cumecs of undivided flood in Mahanadi at Naraj . Some part of the deltaic region is safe up to a flood of 28300 cumecs due to presence of double continuos embankments. The flood at the delta head has crossed 28300 cumecs eight times between 1958-1998 and made serious problem to entire delta command area. The annual flood damage of the deltaic region is 37 crores of rupees (1992 base). Drainage problem also poses a major constraint to agricultural development of the area. Out of total CCA of 3.03 lakh hect. more than one third suffers from ill drainage. In the present context it is advocated to provide and improve the structural measures and to provide some special treatment to the affected area to check the major floods up to 35000 cumecs. Structural measures to high flood is not feasible and these are proposed to be mitigated by Non structural measures . INTRODUCTION The river Mahanadi branches off to several courses at its delta head, Naraj. The entire delta area is largely affected by flood and drainage almost every year. Floods play havoc at the deltaic region and causes untold miseries. It causes immense loss of life, disruption of human activities, damage to properties, agricultural crops and poses serious health hazard. The problem starts when Mahanadi flood exceeds the limit of 17000 cumecs. After the devastating flood of 1855 a spur was constructed at Naraj. Later it was modified to a weir which functioned for more than a century. Now a Gated barrage is constructed at the downstream of the old dying weir to control the flood and provide irrigation to its command area. The flood and drainage problems in the deltaic area are mainly due to following 1. Precipitation in the command area, 2. Floodwater entering the doabs through low level escapes in the river embankments, 3. Irrigation water through canals and over irrigation, 4. The other causes which enhance the problems inside the doabs like (a) The delta rivers are at higher level than the valleys inside the doabs. (b) Delta country is very flat, (c) Construction of cross bunds by local people, (d) Encroachment in bed and berms of drainage channels by local people, (e) Inadvertent leasing of berm lands of drains by the Government, (f) Construction of bridges and culverts and other cross drainage works without provision of sufficient water ways, (g) Lack of proper maintenance of the existing drainage channels, (h) Insufficient spacing of both the embankments of double embankments, (I) Absence of sluices and tidal structures at the outfalls . The problems become more acute when floodwater combines with rainwater .The observed maximum value of precipitation is shown in Table -1. River Mahanadi draws water from a catchment area of 1,32,197 sq. km at Naraj. Out of this catchment the uncontrolled catchment of 49250 sq. km, which are below Hirakud, experiences heavy rainfall during south west monsoon in view of its location just south of tracks of monsoon storms and these floods cannot be controlled by Hirakud. In 1980 such a flood, which came at a later part of monsoon (August & September) was routed by Hirakud reservoir partly to produce a flood of 34753 cumecs at Naraj. This also happened in 1982, when practically no contribution was there from Hirakud for the highest flood. In between (1926-1940) and in the post Hirakud period (1958-1998) twenty seven major floods occurred when the regulated flood peak discharges at delta head exceeded 28300 cumecs twenty times. The flood peaks and Return periods at the delta head are shown in Figs 3 and 4. Once the river water enters the doabs it doesn't get drained out easily. More and more lands are subjected to continuos poor drainage conditions and get water logged. The area affected by poor drainage is 2.695 lakh hectare out of C.C.A.of 3.19 lakh hectare. The poorly drainage areas can be divided into the following four catagories. 1. Area unsuitable for cultivation throughout the year: These areas of 0.132 lakh hectare are marshy and swampy lands, remain submerged throughout the year and are unsuitable for growth of any type of crops. Provision for surface drainage will retrieve the area. 2. Unsuitable for cultivation in kharif: These are lowlying lands with poor land grading and land shaping. They remain waterlogged for most part of kharif. Only Rabi cultivation is practised in these 0.421 lakh hectare area. The problem can be solved if appropriate drainage facilities are provided. 3. Area unsuitable for cultivation for both seasons but with low yield: The problem is due to high water table. Drainage facilities are required for these areas of 0.553 lakh hectare 4. Area which can not be economically retrieved: These area of 0.036 lakh hectare contributes to central and deep portion of swamps. The annual loss due to flood damage alone is 37 crores of rupees(1992 base), is shown in Table 3. The indirect loss suffered by inhabitants in the area is also very high and retards the economic growth of the entire area for several years. HISTORY In the original project report of Hirakud dam no proposal was given to delta irrigation, instead in the revised project report 1,3 (1952) it was proposed to bring under irrigation in stage II with total closure of Kuakhai at its off take. Later the Majumdar committee (set up by government of India) recommended to construct a new barrage across Kuakhai river. The then Chief Engineer of Orissa proposed to have double embankments with high level escapes instead of Barrage at Kuakhai. In practice none of the above steps had been taken up. A proposal of a storage dam at Naraj has subsequently been dropped and a high level dam at Tikarpara came as a next proposal. This is also shelved. Now another Dam at Manibhadra at the upstream of Naraj has been proposed, which is awaiting sanction at different level. The following alternative method of studies over last several decades for flood control in Mahanadi delta area has been suggested. Method I: Diversion of Mahanadi flood from upstream of Hirakud through Tikara river to Brahmani valley by providing additional spillway capacity at Hirakud and an escape channel. It is not desirable as Brahmani valley has its own acute flood problems every year. Method II: Diversion of flood to Chilika lake through a pilot cut. Sir M. Viswesray has suggested to construct a dam across d/s junction of Mahanadi and Rana nallah to reduce the depth of the cut and to have some flood stored in the reservoir. It may ecologically and hydrologically disturb the lake Chilika and also is quite costly. Method III: In 1945, Dr. A. N. Khosla proposed three storage reservoirs in Mahanadi, i.e., at Hirakud, at Tikrapara, and at Naraj. Out of these three only Hirakud has been constructed. Method IV: To remove drainage congestion is to improve the drainage channels inside the doabs. But it is not practicable as it amounts to excavation of very big rivers. Also drainage channels may not fulfil the purpose due to poor out fall condition. So flood control is proposed in rivers. Drains with some tidal structures and embankments are designed to take care of local rainfall flood. RIVER BASIN CHARACTERISTICS Kathajuri is a major branch that off takes from the river Mahanadi at its delta head. The details of Mahanadi river and its branches are shown in Figs. 1 and 2. The total length of river Mahanadi is 858 km and it has 437 numbers of tributaries. Kathajuri, which has branched off from Mahanadi was widening alarmingly by 1855 and drawing more water than Mahanadi. The branch Kuakhai takes off from Kathajuri just down stream of Naraj with poor carrying capacity and its riverbed at the off take is about 2.5 m higher than its parent river. The river Kuakhai bifurcates into three rivers, viz. the river Kushabhadra, Bhargavi and Daya. The river Bhargavi has branched into several small rivers before joining the lake Chilika. Kushabhadra has out fallen into the Bay of Bengal and the rivers Bhargavi and Daya have out fallen into the lake Chilika. Water levels of Chilika is about 1.5 m higher than the mean sea level resulting the flood discharge is not quick and causes inundation on the upstream reaches. The river flood in delta area in the river Kushabhadra is also blocked as sand bars tends to form across mouth due to littoral drift. These sand bars show a clear tendency to grow in a easterly direction up to the coast as a result of coastal process. The river Bhargavi, runs a considerable length almost parallel to sea due to high sand spit condition in the shore. The rivers are badly silted up and do not have important function of discharging Mahanadi flood into the sea. The rivers flowing in the command area are almost flat in character. The average ground profile and bed slope of the river varies from 1 in 5000 to 1 in 6000. The doabs in the delta areas are intersected by several drainage channels and are draining either to the main stream or to the sea. The doabs tend to slope down from the river to the interior and from the top near the delta apex to the bottom at the coast. Flatness at the ground helps the lowlevel flood escapes to function at different stages of Mahanadi flood. The bed of river is full of sand and at the upstream reaches followed by clayey deposit at the downstream reaches nearer to the sea. The side slope of river are very much steep and almost 1V:1/2H. Rivers are more straight in the upstream than nearer to the sea. They through up their branches to their sides again branches combine to form loops and the process continue with the branches almost flowing at a redial pattern creating the actue delta in Mahanadi. FLOOD CONTROL Embankments: In t