1. There are a number of anicuts in India most of which are at least a few decades old. These are constructed of stone or brick masonry, with cement or surkhi mortar. Many of the structures are intact except for a few minor cracks. In line with the continued economic growth, expansion of infrastructure facilities is a necessity. In addition to roads, water related facilities will find increased requirements in the form of domestic, industrial and irrigation demands.

In addition to new structures, often attempt is made to improve and utilize the existing structures where ever available. This helps in the better utilization of existing resources. Also minimizing the usage of resources for an entirely new structure will help in diverting for some other purpose, as well as disturbances to the environment are minimized. In this paper, the Engineering issues connected with various types of reservoir improvement projects are discussed.

There are a variety of reservoirs from natural lakes to check dams across rivers or an entirely artificial reservoir confined by bunds on all the sides. They exist in a variety of topographic conditions, different degrees and purposes of usage, as well as in different structural conditions. Hence the basic approach to the improvement issue depends on the above factors. Some projects are critically influenced by some factors like flash floods, recent urban expansion, falling under reserve forest area etc. Thorough understanding of purpose of improvement and the existing conditions of the structures is a prerequisite for undertaking any reservoir improvement project.

2. Improvement Requirements.2.1. Any reservoir performs best under regular monitoring and maintenance. When too many

unattended issues accumulate or the treatment required is of a major nature, a reservoir improvement project is formulated.

2.2. Increased safety requirement.This can become a reason for a project under the following conditions.a. New regulations require a more stringent safety standards.b. New studies revealed that a higher flood is probable.c. Recent flood of a significant size in the same or neighboring area Urban development in the

downstream area increasing the risk of loss of life and property.d. Change in hydrometereological conditions and topographic and land use changes in the

catchment area resulting in a change in runoff.2.3. Storage augmentation

Increase in demand due to and for the economic development for various purposes like domestic supply, industrial use, irrigation etc.

2.4. Some times when a new scheme is formulated, existing structures are utilized with or without full or partial modification. Typical examples include,a. Multistage lift irrigation schemes utilizing existing tanks or reservoirs for detention at

intermediate stages.b. Existing reservoir used as emergency storage during maintenance period. New flood

diversion or interbasin water transfer schemes utilizing existing reservoirs for storing and distribution.

Any of these conditions or a combination of the above can lead to the formulation of a project proposal.

3. Dimensions of reservoir improvement.Reservoirs can be improved in different aspects which fall under main categories as below.3.1. Reservoir capacity conservation: Over the passage of time, the useful reservoir volume reduces

due to sedimentation. Increasing the capacity of the reservoir is then required. This can be attempted in one or more of the following ways.

3.1.1.Desilting measures: These include excavation and removal of sediments during lean period, dredging etc.

3.1.2.Construction of channels, intakes etc, to utilize deeper waters. This method actually does not increase storage capacity, but utilizes more portion of existing reservoir.

3.1.3.Reallocation of specific purposes. These include allocation of a portion of irrigation reservoir for domestic purpose by making a bund partition. Or the construction of a new intake to supply water for increased urban demand.

3.1.4.Watershed area treatment to reduce soil erosion and subsequent deposition.3.1.5.Bank stabilization treatments in the periphery of the reservoir area.

3.2. Reservoir capacity augmentation: The challenge of reservoir augmentation essentially lies in increasing the usable storage water level, i.e. FRL (Full reservoir level) while keeping the effects of increased level during surplussing the flood over the spillway under manageable limits.Reservoir volume is increased by change in water spread area as well as water level. The increase in water level results in encroachment of more area by the water spread. Submergence of any habitat or infrastructure facility like road may occur, if the reservoir area is bounded by natural ground. In stretches where it is bounded by bund, the bund may have to be raised and strengthened.The possibility of increasing suitable volume by excavation (of deposited and bed material) in between the MDDL (minimum drawdown level) and FRL ( or up to MWL for flood storage) should also be considered. However such excavation should be done in mild slopes and after considering the geotechnical studies so as not to induce any stability or seepage problem.

3.3. Spillway augmentation: Spillway is a safety mechanism to let off the surplus waters. Demand for an increase in spillway capacity can occur due to increased safety requirements as discussed earlier.

3.3.1.Ungated spillway: The crest of an ungated spillway can be raised to the required level by adding concrete on the crest with suitable arrangement for ensuring monolithic action. The consequent raise in MWL is to be dealt with by raising the peripheral bund. Sometimes the increase in MWL is small sufficient free board is provided in previous design, it can be allowed to be accommodated in the free board portion.

3.3.1.1. Labyrinth: Labyrinth can pass a given discharge with lesser head. The applicability is possible in a limited range of head.

3.3.1.2. Fuse plugs/ Fuse gates: These arrangements typically block the water up to FRL and are washed away or opens when the water reaches at some critical level beyond the crest, thereby passing a higher discharge.

3.3.1.3. Combination of Labyrinth and fuse plug can be used for effectively keeping the MWL at a lower level still enjoying the benefits of a higher FRL

3.3.2.Gated spillway: A gated spillway can be made to store water by increasing the top level of the gate. This is often done by welding plates on top of the gate. The incremental water depth is accommodated in the free board. If the free board is already critical parapet wall height is to be increased. Lowering the crest and adding plates at the bottom can also be checked for its suitability.

3.3.3.Design checks: Before carrying out any of these methods, the length and stability of the structure is to be confirmed for the changed conditions. The structural stability is enhanced by the following measures.

Adding mass (typically concrete) on the upstream. Adding mass (typically concrete) on the downstream. Adding mass (typically concrete) on the piers.

The added concrete has to be stitched to the existing structure to ensure monolithic action. For this holes are drilled in the existing structure and anchor rods are inserted and grouted. The projecting end of the anchor rod is embedded in the new concrete, as well as tied to the new skin reinforcement if required. Sufficient bond is to be ensured in both new and old concrete.The mass of concrete added on the upstream (close to the crest )has the effect of reducing the spillway coefficient. The concrete on the downstream may change it from the original ogee profile. These effects should be considered and suitable profile modification adopted. If the weir extends below the apron and concrete is added in this portion, hydraulics is not affected. Seepage studies to find the exit gradient and uplift have to be carried out. In case the depth of cutoff appears to be near critical, and the existing cutoff depth and condition should be ascertained. Suitable method to increase the depth of cutoff should be studied and in worst case the existing cut off may have to be removed and new one inserted. The floor length and thickness required shall be checked. Upstream floor thickness can be increased if required.Energy dissipation: Design flood to be passed may be changed due to hydrometerological or catchment area changes. Or less stringent criteria for design would have been used for design during construction. The downstream river cross section may be changed resulting in modification of tail water rating curve. Either of the reason may require a change in floor level or length or both.Strength: Low grade of material or workmanship, non-maintenance or specific problems like alkali aggregate reaction can result in deterioration of the structure. In such cases, cement grouting or injection of specific chemicals can improve material properties. Providing synthetic geo-membranes on upstream face can reduce seepage through the body, thereby limiting deterioration. This can be applied on

both concrete dams and rockfill or earth dams. Extending the structure may be done by suitable strengthening methods.

4. Typical issues4.1. General: The topography of the reservoir area and details of the existing structure are to be

collected. The collected data is to be verified for it’s reliability and authenticity. For instance there may be situations where no drawing is available or number of alternate proposals were made and identification of the final drawing which was executed at site is not easily known. Or there may be changes made in the founding levels etc., made at site to suit the local conditions, which were not properly recorded.

4.2. The topographic data of reservoir area and above ground data of structures like pier thickness etc., should be verified. After conducting the design study using these parameters for the new FRL and MWL conditions, near critical or critical aspects are to be identified. These factors shall be confirmed at site. For example, if exit gradient or scour depth is suspected to be critical, the depth of cutoff may have to be ascertained by geophysical exploration, open excavation or bore hole data or both. Having discussed above the general requirement of any project, typical constraints faced during the implementation of a project are discussed.Topography of reservoir area: Topographic survey of reservoir area is to be reasonably accurate if reservoir capacity is to be conserved or augmented by a definite amount. Any minor increase in water level is required to be accommodated within the free board during initial planning and later existing capacity turns out to be much less. In such a case the increase in FRL/MWL has to be accommodated by raising the bund and strengthening requiring additional time, effort, money and materials.Existing condition of structures: Strength and damage of existing structures is to be reliably estimated. Any suspected damage visible or not visible on the surface should be investigated and extent of damage if any ascertained. Also the structures should be checked whether they satisfy current design and construction standards or not. A clear decision on whether to upgrade it to current standard is required or not and if so to what extent and method is to be made.