Conservation and Restoration of Lakes (Irrigation Lakes in Tamilnadu)

Transcription

1 Sengupta, M. and Dalwani, R. (Editors) Proceedings of Taal2007: The 12 th World Lake Conference: Conservation and Restoration of Lakes (Irrigation Lakes in Tamilnadu) K. Thachanamoorthy Assistant Executive Engineer, Water Resources Organisation, Public Works Department, Tanks (Lakes) Modernisation, Gingee, Villupuram District,Tamilnadu ABSTRACT In Tamil Nadu, about 5,250 rain-fed PWD (Public Woks Department) lakes are available, irrigating about 5.12 lakh hectares (30% of the total irrigated land). To emphasize the conservation and restoration of irrigation lakes in Tamil Nadu as a multidisciplinary system and to analyze the system with the combination of conventional and modern methods, a survey, planning and modernization work was undertaken during the implementation of irrigation lakes modernization project in Tamil Nadu. The study included field investigation and survey, reservoir storage analysis and simulation, engineering designs of lake components (physical), repairs and restoration, agricultural and socio-economical aspects and so on. The water yield from catchments and supply channels and the evaporation loss of lakes were simulated. The crop water requirements of the ayacut area were estimated and the surplus / deficit were calculated. Based on the results, plans were drawn for the scientific and rationalized method of maximum flood discharge, physical renovation of the lake and restoration of lake components based on actual site and economic conditions. Effective Participatory Irrigation Management (PIM) with modern agricultural practices involving all stake holders with legal and social bindings are implemented. Conservation and restoration of irrigation lakes by the combination of both conventional and modern techniques giving due recognition for practical and scientific approach with social background was found to be an optimal solution. Keywords: Irrigation lakes - hydrology and maximum flood discharge consumptive water use - reservoir Simulation - engineering design of lakes (physical components) - Physical restoration of lake components - multidisciplinary system approach. INTRODUCTION Lake is a formation of small earthen embankment in valleys and local depressions to store runoff from its catchment or diverted from nearby rivers / streams In Tamilnadu about 5,250 rainfed PWD ( Public Woks Department ) lakes are available irrigating about 5.12 lakhs hectares. It is about 30% of the total irrigated land in Tamilnadu. Everyone believes that if there is a world war again, it may be for water only. So it is needless to say the importance of conservation and restoration of lakes. As in Tamilnadu more than 75% of people depend mainly on agriculture, it is very essential to conserve and restore the irrigation lakes in a technological way with the much needed social approach. Figure 1. Shows the three functions of the lake with its physical components. Important Functions of Lakes Collection / Harvesting of water from the upstream side component of the lake ( ie - river, check dam, feeder canal, catchments, etc ). Storage of water into the lake. Components involved: water spread area, bund, sluices, weir & regulating arrangements. Proper utilisation of water for irrigation. Components involved: Main field channels, on farm distribution network, cultivable area, surplus and cross drainage arrangements. Figure 1. Schematic Diagram of lake functions.

2 Collection / Harvesting Investigation Thorough investigation is to be done for upstream side river / stream, check dam supply channel, catchment area, longest main stream, shape of catchment group of lakes etc. Yield Calculation With available daily rainfall data for particular period (minimum of 12 years) the run -off is arrived. The yield thus obtained in mm is to be converted into volume. The average depth of flow over a period is to be ascertained for the supply channel to the lake. (For yield from supply channel ) Chart (1) Design of Supply Channel Data Required Supply Channels, Catchment Drains and Group Network: Standardisation by desilting and encroachment evictions to the designed size. In let and outlet points across the supply channels & catchment drains are to be provided with revetments, masonry etc. Identifying the group of tanks and the network is to be fully renovated without any omission. It leads to effective functioning of the entire group. Storage of Water The main physical components of the lake related to storage are tank bund, sluices and sills, weir, water spread area and encroachment. Contour leveling is to be done for calculating water spread, depth of storage and capacity calculation. A sample calculation with contour leveling and capacity calculation using the trapezoidal formula is given in Figure 3 & Table 1 respectively Levels are to be taken from the off take point to end reach. Fix up the gradient reach wise Q = AV Where V = 1/n R 2/3 S ½ (Manning s formula) for earthen channel n = Adopting a trapezoidal section with side slopes 1:1, section is designed for required discharge with the available bed fall. Restoration of Physical Components Check Dam: Repairs for body wall, grouting aprons and other parts like wing walls, return walls, sandvent, head regulator etc. Figure 2. shows a Checkdam under repair Figure 3. Tank Water Spread Map Q = h/3 [ A1 + A2 + A1 A2 ] where A1 & A2 Lower & Upper contour Area in m2 h = Contour interval in m. Q = discharge in cumec Table1 shows yield & related capacity. Figure 2. Check Dam 1776

3 Table 1. Storage Calculation Table INCREASING THE STORAGE CAPACITY To increase the depth of storage or to restore the water spread area desilting the foreshore can be done. It can be also increased by raising the height of bund and weir etc based on submergence of foreshore land. If there is no submergence of foreshore lands the capacity of the lake can be increased economically by raising the height of the bund. Estimation Desilting of 50,000 m 3 = 50,000 x Rs. 50 / m 3 = Rs. 25 Lakhs Increase in volume = 50,000 m 3 If depth of storage increased to 0.10m with the water spread area of 50ha, Total increase in volume = 5,00,000 x 0.10 = 50,000 m 3 To increase the same volume with the tank bund of 1000 m length. The quantity of earth work involved = 1000 x (3.53 ) x 0.10 = 353 m 3 Add for sloping portion extra = 397 m 3 Add for weir improvement etc = Rs. 12,500 /- Total Cost = Rs. 50,000 /- For the same quantity of increase in capacity by desilting we need Rs 25 Lakhs. But by increasing the height of bund we need 0.5 lakhs (ie) about 1/50th of the desilting cost. (only 2% of desiling cost ) Lake Bund Standardisation of lake bund is to be done with required standard side slope, top width & revetments. Initial levels are to be taken Longitudinal and Cross section are to be drawn based on levels. The quantity of earthwork to be arrived at and the standards to be maintained as per requirements. Typical sections are shown in Figure 4. Total = 750 m 3 Rate of 750 m 3 = 750 x 50 = Rs. 37,500 /- 1777

4 Figure 4. LS & CS of Bund Sluices and Sill Levels Sill level of sluices are fixed on the existing ayacut & commandability. Hence sill level should not be changed during the rehabilitation ( or ) reconstruction of sluices. Tower head type sluice is provided if depth of storage exceeds 3.00m. If it is less than 3m, wing wall type can be provided. Plug rod with screw gearing arrangement are to be provided for regulating water from sluices. Gauge plate also to be fixed for every sluice Figure 5a. Isopluvial Maps Surplus Weir Design aspects Maximum Flood Discharge (MFD) is calculated based on hydrometerlogical method which is evolved by Central Water Commission (CWC) India, in collaboration with Ministry of Railways, India. For 50 years return period. (As per simplified approach) Q 50 = A0.831 S0.181 (R50) 1.242/(L)0.196(LC)0.556 Where Q = Discharge in cumec A = Area in catchment in sq. km S = Equivalent stope of catchment in m/km. R50 = To be findout from isopluvial map for return period of 50 years Figure 5a&b. Figure 5b. Isopluvial Maps TD = Storm duration in hours Where TD = 1.1 (0.376 LLC / S )

5 Table 2. Equation Slope and Td Ratio Td ratio Sl. No Duration Sub zone Sub Zone Sl. No Duration Sub zone Sub Zone In hours 3(1) 4(a,b & c) In hours 3(1) 4(a& b) Designed Flood Discharge (DFD) of the surplus arrangement; Q = 2/3 cd lh (2gh)0.5 Where cd = co.eff of Discharge L = length of weir in m H = depth of flow over crest of weir in m g = 9.81 m /sec2 Cd = 0.62, 0.562, or based on type If MFD < DFD, The Design is sufficient otherwise surplusing capacity of the lake is to be increased to avoid breaching of lake. Based on the discharging capacity of the weir the surplus course is to be designed & desilted to the designed section & falls. PHYSICAL RESTORATION OF WEIR Two types of weirs 1. wing wall type 2. Core wall type Crest of weir, abutments, wings, aprons, cut off walls are to be repaired. Eviction of encroachment In the foreshore and bund, the encroachment is to be evicted and the water spread area are to be restored to its original area as per documents. While desilting of lake, it is better to do it from the outer boundary towards the deep bed of the water spread area. 1779

6 UTILISATION present land use cropping pattern Crop water requirement calculation [ Table 3 ] Localisation and grouping of ayacut [ Fig 6] Irrigation channels & Distribution network. [ Design 1] Optimal Design of main channels & distributaries, based on gradient and commandability. (Typical Design is explained with commandability and back water effect. The design data required: Existing field channel & ayacut levels ). Lake storage Analysis & simulation (relevant table enclosed) table 4. Physical restoration of irrigation network Lining of field channels (photo enclosed) Regulating cisterns with shutters Cross drainage works such as syphon, aqueduct, inlet and outlet etc. The above three functions are to be carried out preciously to conserve & restore the irrigation lakes of Tamilnadu. OTHER ENGINEERING ASPECTS Research studies conducted for seepage losses and about 33% of the water is saved thro lined canals Stabilisation of the existing cultivable command area have been achieved If there is excess water available, additional new ayacut can also be brought under irrigation due to conservation of water based on accurate calculations and proper Micro level irrigation network using pipelines. ie.drip, sprinkler etc. Optimisation of reinforced concrete elements/structures involved in physical restoration so as to minimize the cost of the project. OTHER DISCIPLINES Agriculture: Suitability of Soil: Soil survey to the ayacut area is to be conducted and the suitable crop recommendation for the available water is to be recommended by the Agricultural department. It should be based on the crop budget & net benefit. Uniformity in cropping pattern and time for each seasonal crop. Supply of inputs, modern agricultural extension services, & marketing facilities (ex. System Rice Intensification (SRI) method ). Remunerative price to the farmers. Table 3. Crop water requirement 1780

7 Table 4. Working Table Figure 6. Command Area Map with Grouping for Design. 1781

8 Figure 7. Lining of Field Channel Design - Design of Field Channel 1782

9 Design (1.2): Design of Field Channel 1783

10 Chart - Yield and Storage Capacity Chart (1) Tank Capacity Yield in Mcft. Social Factors: Involvement of WUA for physical rehabilitation and restoration works in the Water Users Association (WUA) is to be planning and execution stage. formed for each lake. Case study : In Kedar lake of villupuram district, WUA have been involved in the construction of lined canals, the quality is very good and they maintain them also in good condition. Creating awareness for water users, Panchayat Raj Institution (PRI) members & other stakeholders by educating, training, demonstration & by field visits etc. The social forestry in the water spread area is to be avoided. It reduces the capacity & also leads to more evapo transpiration losses. Selecting & involvement of Non Government Organisations (NGO) for formulation and functioning of Water Users Association. Encroachment eviction, avoiding further encroachments by maintaining the lake through Water Users Association ( WUA ) For further maintenance as they are facing resource constraints, they have been permitted to the usufructs rights. Role of women, small and marginal farmers should be more. ECONOMY Creating credit facilities to the farmer either by money, machineries or equipments Crop budget and the benefit cost ratio of particular crop is to be studied then and 1784

11 there and the crop rotation is to be recommended based on economy. LEGAL ASPECTS Legal bindings: Memorandum of understanding is to be evolved with the farmers for further maintenance, irrigation scheduling, crop rotation and other credit facilities. Legal binding between stake holders like Engineers, Agronomists, PRI members and other Administrators Elections are to be conducted for WUA & the new legislation on Tamilnadu Farmers Management of Irrigation Systems Act ( TNFMIS Act ) may have to be enforced at the earliest. Documents related to lake like field measurement sketches, survey numbers, list of ownership, maps are to be maintained by WUA. ENVIRONMENTAL ASPECTS Sewage water should not be allowed into the lake If so, there should be proper treatment before allowing the water into the lake. Urbanization of Lake Area: to be incorporated suitably. ACKNOWLEDGEMENT The author is grateful to the Government of Tamilnadu, Secretary to Government Public Works Department, Engineer in Chief, Water Resources Organisation and Chief Engineer (Design Research and Construction Support) for sponsoring me to present this paper in the 12th World Lake Conference held at Jaipur, India from 28th October to 2nd November He also acknowledges his parents, family members, friends and other fellow workers for their constant encouragement and support. REFERENCES Ellis, W.M (1950) College of Engineering manual on Irrigation Europeon Economic Community (EEC) Consultant (1990) Feasibility study for EEC assisted lake modernisation project in Tamil Nadu Garg, S.K. (2003) Irrigation Engineering and hydraulic structures Hydrology study organization publication by central water commission (1990) -workshop on Design flood estimation for small and medium catchments in Tamil Nadu Karmegam, M (2003) Director Centre for water recourses, Anna University Chennai Role of Women, small and marginal farmers as stakeholders Karmegam, M (2003) Director Centre for Water Resources, Anna University Chennai Water losses in tank field channels under different soil and channel condition Krishnamoorthy, S and K. Thachanamoorthy (1996) Optimization of Reinforced Concrete Elements using geometric programming technique Lynn, E.J. (1992), Associate Professor University of Coloroda U.S.A - Reservoir storage analysis Lynn, E.J. (1992), Associate Professor University of Coloroda U.S.A - Deterministic Reservoir Simulation Ministry of Water Resources, Government of India (2007) workshop on Restoration of water bodies that are directly linked to Agriculture at New Delhi. 1785