Know your Waterbodies: A Practical Guide to Tank Networks

1. Introduction

Tamil Nadu has 41,123 tanks with capacity to irrigate 3.98 lakh hectare agricultural land. For more than two centuries, the Revenue Department of the state has governed theses tanks network systems and continues to hold most of the legal authority over them. The maintenance of small tanks is largely entrusted to the RD&PR through Panchayat Unions, while larger tanks are maintained by the WRD. Many channel networks are managed by local farmers, and in some cases by the WRD. Except for anicuts and headworks, design standards for most structures are already well established and documented through government circulars. The law in the state is clear that none of the standards of any tank and channel networks can be changed.

Apart from government, now many civil society groups and farmers groups have started restoration works of tanks in Tamil Nadu. The restoration work includes strengthening bunds and repairing sluices, weirs and channels. Such efforts rarely require new design work; rather, they depend on sound understanding of existing system and their context. PRADAN (Professional Assistance for Development Action) a leading developmental organization of India had put together information in context to educate various government and non-government actors involved in restoration of tank system in Tamil Nadu. A part of this document is presented in this article.

 2. Tank and Its Components

 2.1 What is a Tank

The word “tank” is believed to have originated from the Gujarati word tankha, which later became tanque in Portuguese and eventually “tank” in English. Tanks are found in many parts of the world, particularly in South and East Asia. In South India and Sri Lanka, highly evolved tank systems have existed and functioned continuously for more than a thousand years.

Tanks in Tamil are called Eri, Kanmai and Kulam. They are artificial, earthen-bunded reservoirs that receive, store, and convey water by gravity for irrigation to lands located downstream. They are formed by constructing a bund across a natural slope, allowing runoff to accumulate behind it and be utilised below. Thus, the bund defines the tank and gives it its functional identity.

2.2 Size and Shape

In rolling topography, most tanks tend to have a semi-circular configuration, characterised by relatively short bunds and a longer foreshore (water spread edge). In contrast, in plain areas, tanks typically have longer bunds, shallower storage depths, and comparatively narrower water fronts.

Most tanks are shallow, often storing water to depths of less than two metres. However, this does not imply that they are small in extent. There are tanks in South India that store water to depths of even 10 metres. The size and storage depend on local topography and catchment conditions.

 2.3 Classification of Tanks

Tanks may exist as independent systems or as part of interconnected networks. A tank that receives water only from its own drainage area is called an isolated tank (a tank dependent entirely on its own rainfall catchment). However, most tanks are interconnected because surplus water naturally flows downstream. Channels that carry this surplus water from one tank to another, forming chains of tanks.

Administratively, tanks are classified into two categories. Smaller tanks irrigating less than 100 acres and not part of any system are classified as Panchayat tanks, while larger tanks, including system tanks, are classified as WRD (Water Resources Department) tanks.

In South India, major rivers such as the Cauvery River and the Krishna River feed extensive tank systems through diversion structures known as anicuts (low dams built across rivers to divert water). The Vaigai River is a well-known example, feeding hundreds of tanks along its course from its origin to the sea.

From an engineering perspective, tanks that are connected to such river or reservoir systems are called system tanks (tanks that receive water through a regulated network such as canals or river diversions). These tanks have some assurance of water supply. In contrast, non-system tanks (tanks dependent only on local rainfall) rely entirely on their own catchment and may not fill every year.

Defining what constitutes a tank is complex; consequently, the scope of rehabilitation work also varies. A tank typically includes the bund, tank bed, and parts of the channels. In some cases, portions of the tank bed have been allocated for cultivation, and farmers hold patta; such situations are not uncommon. Ownership of tanks generally rests with the Government, while certain management and conservancy responsibilities are assigned to specific Government departments. Under normal circumstances, the Revenue Department, through the District Collector, retains ownership of tanks and related structures, rather than individual departments or Panchayats. Hence dealing with encroachments of any part of tanks are also the functions of Revenue Department.

As per the Board Standing Orders (BSO), tanks are assigned to Panchayat Unions or the Water Resources Department (WRD) for maintenance and management. Tanks with a command area of less than 40 hectares fall under Panchayat Unions, while those with a command area of more than 40 hectares fall under the WRD. In addition, all tanks connected to reservoirs or channel systems controlled by the WRD are managed by the WRD. Ideally, every tank is expected to be maintained once in five years under a cyclical maintenance system. However such practices are not followed in all districts.

The law also requires villagers to undertake kudimaramathu (community-based repair works), and revenue officials are empowered to mobilise local communities during emergencies such as floods, as well as for routine maintenance.

2.4 Revenue Classification of Tanks

As per the BSO, irrigation sources (including tanks) are classified into five classes based on the period of water availability:

1. First Class: Water available throughout the year (12 months)
2. Second Class: Water available for 8 to 10 months
3. Third Class: Water available for 5 to 8 months
4. Fourth Class: Water available for 3 to 5 months
5. Fifth Class: Water available for less than 3 months

As the revenue classification is based on the productivity of a tank, determined by the duration and reliability of its water storage, higher classes indicate greater irrigation potential and administrative importance. However, all tanks are important. For instance, fifth-class tanks play a critical role in dry areas, where even a short duration of water availability is of significant value.

2.5 Types of Catchment

Understanding how much water a tank receives requires understanding its catchment. The area that directly drains into a tank is called its free catchment (the immediate drainage area contributing runoff directly to the tank). When upstream tanks release surplus water that flows into downstream tanks, the receiving tank effectively has a larger contributing area called the combined catchment (total area contributing water including upstream flows). The difference between these is referred to as intercepted catchment (the portion of upstream catchment whose flows are intercepted before reaching the tank).

For estimating water inflow, engineers traditionally consider a fraction (commonly one-fifth) of the upstream catchment in addition to the free catchment. These estimation methods were standardised during the nineteenth century in the Madras Presidency.

Such technical details are documented in Tank Restoration Survey (TRS) Memoirs (official records containing technical data on tank capacity, catchment, and water inflow estimates). These were prepared for most tanks before the 1930s and updated until the 1980s, and are available with the Water Resources Department.

2.6 Channels

Channels are watercourses that collect, convey, and distribute water. A supply channel (a channel that brings water into a tank) feeds the tank, while a surplus channel (a channel that carries excess water away from the tank) drains it. The same channel may function differently depending on its position in the system.

Channels are usually marked in revenue maps. Their width varies widely depending on the catchment, ranging from about 1 metre to as much as 100 metres. In most southern districts, channel widths are typically between 3 and 20 metres.

The area behind the bund where water is stored is called the waterspread area or tank bed (the portion of land that gets submerged when the tank fills). Tank bunds are generally constructed using earth and become compacted and stable over time. Some are strengthened with stone pitching to prevent erosion.

2.7 Irrigation from Tanks

The area irrigated by the tank is called the command area or ayacut (the agricultural land receiving water from the tank). Water is delivered through sluices and distributed through field channels, which are often marked in revenue maps.

In larger ayacuts, drainage channels may exist to remove excess water. In many cases, the same channels serve both irrigation and drainage functions.

2.8 Weirs and Sluices

Weirs and sluices are key structural components of a tank system, next in importance after the bund and channels.

Weir: A masonry or concrete structure provided in the bund, usually at one end, which allows surplus water to safely overflow into the surplus channel. It protects the bund from damage.

Sluices are structures used to release water from the tank into irrigation channels. Traditionally, they were made using hollowed palmyra trunks buried within the bund. Today, they are constructed in masonry with pipes and control mechanisms such as shutters. Depending on the design and depth of storage, sluices may vary from simple openings to more complex multi-level outlet systems.

2.9 Key Terms related to Tanks

To understand tank storage and functioning, certain key levels must be clearly understood:

• Full Tank Level (FTL): The maximum level up to which water is normally stored in the tank. It is usually determined by the crest level of the weir.
• Maximum Water Level (MWL): The highest level water may reach during heavy inflow conditions, slightly above FTL, typically by about 0.45 m to 0.90 m.
• Tank Bund Level (TBL): The top level of the bund. This is maintained above MWL.
• Freeboard (the height between MWL and the top of the bund): This provides safety against overtopping and bund failure during floods.
• Sill Level of Sluice: The lowest level at which water can be drawn from the tank through the sluice. Water below this level cannot be released by gravity.
• Dead Storage (water below sill level that cannot be drawn out) and Live Storage (water above sill level that can be used for irrigation) are important concepts in understanding usable water.
• Berm (a horizontal strip between the bund toe and borrow area): Over time, this becomes stable and strengthens the bund.
• Breaching Section: A designated or locally known portion of the bund where controlled breaching may occur during extreme floods to protect the main structure.

Tanks also play an important role in groundwater recharge. Water stored in tanks enters the ground through infiltration (downward entry of water into soil), seepage (lateral movement through soil), and percolation (deeper downward movement of water into soil), thereby replenishing wells and borewells.

3. Rehabilitation of Tanks and Waterbodies:  Selection and Implementation of Works

3.1 Selection of Tanks

In most districts, the number of tanks is large, and the demand for rehabilitation is consistently high. It is therefore necessary to prioritise tanks based on clear and practical criteria. The following considerations may be applied, depending on local conditions and community expectations:

• Presence of a larger number of small and marginal farmers, rather than concentration of land in the hands of a few large holders within the ayacut.
• Clear scope for performance improvement through rehabilitation works.
  • For example, where the hydrology is adequate but supply channels are silted and unable to convey sufficient water.
• Willingness of farmers to work collectively, beyond local or parochial differences.
• Readiness of the community to participate, contribute, and take responsibility for implementation.

3.2 Inspection of Tanks for Identifying Problems

A systematic field inspection is essential before planning any intervention.

Assessment of Supply Channels

• Walk along the entire length of the supply channel.
• Compare the dimensions on the ground with the FMB and decide the condition of the channel.
• Identify:
  • Extent of siltation
  • Growth of bushes and shrubs
  • Encroachments and obstructions
  • Any other factors affecting flow

Assessment of the Bund

• Walk along the full length of the bund and observe:
  • Sections where the bund height may need to be raised
  • Breaches, weak portions, and erosion-prone areas
  • Presence of encroachments or obstructions
• Alongside technical assessment:
  • Organise farmers into Tank Associations (TAs)
    • Form a local committee to:
    • Supervise works
    • Support procurement of materials
    • Liaise with government officials

Assessment of Sluices

• Examine the physical condition of the sluice: whether intact, damaged, or collapsed.
• Assess whether the sluice is:
  • Fully functional
  • Partially functional
  • Non-functional
• In case of leakage:
  • Identify the source of leakage
  • If leakage is in masonry (cistern or walls), repairs may be feasible
  • If leakage is within pipes or barrels, assess the need for partial or full dismantling

Assessment of the Weir

• Inspect the structural condition of the weir:
  • Whether it is intact or collapsed
  • Whether the crest and core wall can withstand repair
• If the structure is not stable, it is preferable to dismantle and reconstruct rather than undertake partial repairs.

3.3 Pre-Measurement and Fixing of Levels

• The bund level (TBL) should be fixed with reference to the Full Tank Level (FTL) or the crest level of the weir.
• The sill level of sluices must be strictly maintained.
• While dismantling, mark the sill level clearly on a nearby permanent structure.
• The crest level of the weir must be marked and preserved before any dismantling.

3.4 Sequencing of Field Execution

• Involve the Executive Committee of the Tank Association at every stage.
• Sequence the work based on:
  • Current condition of the tank
  • Availability of water
  • Status of standing crops
  • Any other factors affecting farmers
• General sequence to be followed:
  • Bund strengthening should commence from the flank, preferably near the weir, using this point to establish TBL.
  • Sluice dismantling should be carried out carefully, ensuring that sill levels are not disturbed.
  • Weir dismantling should also be done carefully, ensuring that crest levels are preserved.

3.5 Documentation and Coordination

• Maintain clear photographic records at every stage of work.
• Keep farmers regularly informed and record minutes of meetings.
• Inform and coordinate with relevant authorities, particularly:
  • Engineers of the Panchayat Union
  • Section officers of the Water Resources Department (WRD)

 4. Desilting Tank Beds for Improving Soils and Agricultural Productivity

4.1 Tank Silt and Its Agricultural Value

The use of tank silt in Tamil Nadu is a long-established and sustainable practice for improving soil fertility, strengthening water management, and revitalising dryland agriculture. Over time, irrigation tanks accumulate fine sediments carried by runoff from upper catchments. These sediments consist of nutrient-rich particles of sand, clay, and organic matter derived from eroded topsoil.

Tank silt is therefore rich in organic carbon, nitrogen (N), phosphorus (P), potassium (K), and beneficial soil quality, while simultaneously restoring the storage capacity of tanks.

The major benefits of tank silt include:

• Soil fertilisation: Supplies N, P, K, and micronutrients
• Soil enrichment: Improves overall soil health and biological activity
• Improved soil structure: Enhances aggregation and reduces compaction
• Better water retention: Increases the soil’s ability to hold moisture
• Erosion control: Stabilises soils and reduces runoff losses
• Higher crop productivity: Leads to improved yields and crop performance

4.2 Government Policy and Regulations

The Government of Tamil Nadu recognises the customary practice of farmers removing silt from tank beds for agricultural use. To regulate this activity, G.O. No. 50 (2015), Industries Department, issued under the Minor Minerals Act, 1952, permits farmers to remove tank silt free of cost for agricultural purposes.

Applications for desilting are processed through a centralised online system. Farmers are permitted to remove silt only from tanks officially notified by the district administration.

The permitted quantities are:

• Wetlands: 75 cubic metres per acre (approximately 38 tractor loads)
• Drylands: 90 cubic metres per acre (approximately 45 tractor loads)

Typically, the district administration identifies eligible tanks between April and June, and the list is published through the district gazette. Farmers may apply through Common Service Centres (CSCs) after identifying nearby notified tanks.

Applicants are required to provide:

• Aadhaar card and proof of land ownership
• Details of landholding and soil requirement
• Information on tractors and excavators to be used

Excavation and transport are supervised by designated government officials. Violations—such as unauthorised removal or commercial sale—are treated seriously and may attract penalties for farmers, machine operators, and supervising officials.

4.3 Preconditions for Desilting

Desilting operations require careful preparation to ensure safe excavation and identification of suitable silt.

Key preparatory actions include:

• Clearing vegetation from the tank bed to identify workable areas
• Marking excavation zones based on trial pits to assess silt depth and quality
• Mobilising farmers and facilitating applications for silt removal

Farmer participation is typically organised through:

• Announcing the scheme within the revenue village and surrounding areas
• Collecting applications as per official procedures
• Estimating demand based on applications received
• Prioritising farmers based on intended agricultural use

4.4 Major Conditions for Desilting

Permits issued by revenue authorities include specific safeguards to protect tank structures:

1. Excavation must be carried out at least 10 metres away from the toe of the bund.
2. Bunds must not be cut or damaged to create access routes.

• Excavation depth must not exceed 1 metre, and excavated silt must not be stockpiled within the tank bed.

1. Only certified tank silt may be removed; other types of earth are not permitted.
2. Excavation should be undertaken only when the tank bed is dry, and excavation areas must be marked in the Field Measurement Book (FMB).
3. Excavation below the sill level of the tank is not permitted.

• Electric poles and transmission structures must not be disturbed.
• Trees, stone pitching, and other structures must not be damaged.

1. Excavation near bunds and hydraulic structures is strictly prohibited.
2. Violations are punishable under applicable regulations.

4.5 Risks and Safeguards

Desilting programmes carry certain risks, particularly the misuse or commercial sale of tank silt. These risks can be mitigated through strong local oversight. Farmer associations and village committees play a key role in:

• Monitoring excavation activities
• Ensuring that silt is used only for agricultural purposes
• Maintaining transparency in allocation and usage

Regular monitoring of both tank beds and farmers’ fields is essential. Coordination with district authorities—including the Revenue, Police, and Mining Departments—further strengthens compliance and enforcement.

4.6 Multiple Benefits of Tank Desilting

Tank desilting provides multiple benefits through a single intervention:

• Nutrient-rich silt improves soil fertility and crop productivity.
• Removal of silt increases the storage capacity of the tank.
• Improved storage enhances groundwater recharge, benefiting wells within a radius of approximately 1.5 to 2 km.

Farmers often spend two to three times the excavation cost on transporting silt to their fields. This willingness to invest reflects strong demand and highlights the potential for scaling up desilting programmes across dryland regions.

4.7 Conditions issued to farmers for Taking Silt from Tank Beds for Agricultural Uses

(Under G.O. Ms. No. 50/2017, Industries Department)

All permit holders for silt removal shall comply with the following conditions:

1. Excavation shall be carried out only beyond a distance equal to twice the height of the bund from its toe.
2. Silt shall be removed only from locations identified by the Rural Development Department or WRD.
3. Transport of silt shall be arranged by the permit holder at their own cost.
4. Loading shall be carried out under the supervision of authorised officials.
5. Silt removal shall take place only when the tank is dry.
6. Excavation shall not go below the bottom level of the sluice.
7. The bund shall not be cut or damaged; only designated access routes shall be used.
8. Excavation shall not be carried out near sluices or surplus weirs.
9. Excavation shall not go below sill levels.
10. Agricultural activities shall not be disrupted.
11. The natural condition of the tank shall not be disturbed.
12. Trees and plantations shall not be damaged.
13. Excavated silt shall not be stored within the tank area or on the bund.
14. Only authorised vehicles shall be used for transport.
15. Violations shall attract action as per applicable rules.
16. Misuse of permits shall result in cancellation and legal action.
17. Permit periods shall not be extended or renewed.

5. Government Policies, Rules and Guidelines for Rehabilitation of Tanks

Tamil Nadu government has announced as a policy to involve various stakeholders to work for the development of waterbodies in the state. In recent years, the government has specifically recognised the need to involve:

• Farmers’ organisations
• Local community groups
• Voluntary organisations
• Corporate entities

Although farmers and well-wishers of tanks and water bodies are encouraged to participate in rehabilitation works, they are not permitted to undertake such activities independently and must obtain prior approval from the appropriate departments. The government grants permission to interested parties subject to strict conditions, including the submission of Detailed Project Reports (DPRs) prepared by qualified engineering personnel.

The Government has issued various guidelines and regulatory conditions to ensure that tank rehabilitation and desilting activities are carried out in a scientific, transparent, and environmentally responsible manner. These guidelines aim to protect the structural integrity of tanks, prevent misuse of natural resources, and ensure that the benefits of tank silt are primarily utilised for agricultural improvement. Similarly, the Government has also introduced a permit system for the removal of silt, requiring applications to be submitted to the District Administration, in accordance with the provisions of the Minor Minerals Act, 1957, and G.O. Ms. No. 50/2017.

The following sections present the guidelines for tank rehabilitation and desilting operations to be followed during the implementation of restoration projects.

5.1 Concerned Authorities

Applications for tank rehabilitation and silt removal shall be submitted to different authorities, depending on the procedures followed in each district. At present, there is no uniform system across the State.

• Rehabilitation of Panchayat Tanks: Applications are generally submitted to the District Collector.
• Rehabilitation of WRD Tanks and Channels: Applications are typically submitted to the Executive Engineer of the concerned basin. In some districts, applications received by the District Collector are forwarded to the appropriate authority.
• Silt Removal for Agricultural Use: Applications by individual farmers shall be submitted through Common Service Centres (CSCs). Farmers must verify whether the concerned tank has been notified by the District Collector (through Gazette notification) for silt removal.

Upon submission, certain districts may require details such as the estimated financial value of the work, a Detailed Project Report (DPR), and periodic photographic documentation to monitor progress. All permissions are granted subject to specific conditions to be followed during the execution of the works. These conditions are summarised below.

5.2 Conditions issued by the Rural Development & Panchayat Raj Department

1. Government Approval and MoU: Tank rehabilitation and desilting works shall be carried out in accordance with relevant Government Orders. A Memorandum of Understanding (MoU) shall be executed between the implementing agency and the concerned authorities prior to commencement.
2. Consultation with Local Stakeholders: The implementing agency shall consult the village administration and local farmers and carry out the work with their cooperation and consent.
3. Authorised Implementation: The project shall be implemented only by authorised representatives, including Community-Based Organisations (CBOs), NGOs, farmer groups, educational institutions, or other approved organisations.
4. Identification of Excavation Areas: Earth required for strengthening the tank bund shall be excavated only from locations 10–15 metres (berm distance) away from the toe of the bund, ensuring its stability.
5. Protection of Tank Structures: Care shall be taken to prevent damage to tank bunds, sluices, surplus weirs, and other appurtenant structures.
6. Use of Excavated Earth: Excavated earth shall be used solely for strengthening and consolidating the tank bund and shall not be diverted for other purposes.
7. Desilting of Channels: Inlet, outlet, and link channels shall be desilted and improved to ensure proper water flow between tanks.
8. Technical Supervision: All works shall be carried out under the technical guidance of the concerned Government department.
9. Working Hours: Working hours for excavation activities shall be fixed and strictly adhered to.
10. Safety Measures: Adequate safety precautions shall be taken during excavation and transport of silt.
11. Qualified Personnel: The implementing agency shall engage qualified personnel and appropriate technology.
12. Plantation along Bunds: Palm trees shall be planted along the downstream side of the bund to enhance stability and environmental sustainability.
13. Documentation and Monitoring: Proper documentation, including photographic records (before, during, and after completion), shall be maintained.
14. GPS Monitoring: Excavation locations and transport routes shall be recorded using GPS coordinates.
15. Protection of Tank Capacity: The work shall not reduce the original size, extent, or storage capacity of the tank.
16. Compliance with Laws: All applicable laws, rules, and regulations shall be strictly followed.
17. Completion Timeline: Works shall be completed within the stipulated timeframe.
18. Handover: Upon completion, the tank shall be handed over to the concerned department without damage to structures.

5.3 Conditions Issued by the Water Resources Department (WRD)

1. Channel excavation shall follow approved cross-sections and shall not alter hydraulic properties.
2. Sluices shall be constructed at prescribed sill levels without alteration.
3. Pipes shall conform to NP standards and require departmental approval.
4. Road damage arising from works shall be repaired by the implementing organisation.
5. Works shall conform to Full Tank Level (FTL) and Maximum Water Level (MWL).
6. Appropriate shutters shall be installed in all sluices.
7. Channel bed levels and gradients shall not be altered.
8. Bund structures and trees shall be protected.
9. Site disputes shall be resolved by the implementing organisation.
10. Legal liabilities shall rest with the implementing organisation.
11. Prior intimation to the Section Officer is mandatory.
12. Poor-quality work may be stopped by the Department.
13. No structures shall be constructed across channels.
14. Channel discharge shall not be altered.
15. Natural flow shall not be obstructed.
16. Post-completion concurrence shall be obtained.
17. Accidents and damages shall be the responsibility of the implementing organisation.
18. Desilted material shall be placed along banks; sand shall not be extracted from the riverbed.
19. No ownership claims shall be made by the implementing organisation.

5.4 General Conditions

All project works shall be carried out subject to the above conditions. The Department reserves the right to cancel permission or orders in case of violations, without prior notice. No compensation shall be payable for any loss incurred by the implementing organisation.

The conditions listed above are from the districts where PRADAN teams are functioning. As there are no uniformity among the districts it is better to consult the conditions laid down by the respective districts.

Tanks in Tamil Nadu

Source: Season and Crop Report (2025).[ppl-ai-file-upload.s3.amazonaws]