24 Reservoir Regulation

Deeksha Katyal

Introduction

Reservoirs are the integral part of any scheme associated and intended with the purpose of management of water resources. Generally, most of the human needs are satisfied with the natural streams available to us but all streams are not perennial but these are season dependent. Therefore, it is necessary to regulate the natural stream flow in a certain manner to ensure the spatial and temporal availability of water according to human requirements. Reservoirs are meant to store water to ensure continuous supply of water during low flow periods. The stored water can be used for several purposes such as, irrigation, domestic and industrial needs, power generation etc. Reservoirs can be serves as the storehouse of excessive water flow at the time of flood and hence protect downstream areas from the smash up caused due to flood. Additional uses of reservoirs include its use as recreational sites, navigational pool and serves as habitat for aquatic flora and fauna.

Categorization of reservoirs

There are various ways to categorize reservoirs but from its operation point of view, the most common way is to classify it according to the purpose they serve.

A.     Single purpose reservoir: When a reservoir is constructed in a way that it can serve one particular purpose, it is called as single purpose reservoir. The purpose may vary from irrigation, domestic or industrial needs, recreation to many conservational purposes such as flood control and so on. Idduki dam built on Periyar river is the example of single purpose reservoir. It was built in1974for the purpose of hydroelectric generation. Other examples of single purpose reservoirs are Baglihar dam built on Chenab River in Jammu and Kashmir, Chamera dam built on Ravi River in Himanchal Pradesh etc.

B.     Multipurpose reservoir: When a reservoir is constructed with the aim to serve combination of two or more purposes, it is termed as multipurpose reservoir. There are several multipurpose reservoirs constructed in India. One such example is Indira Sagar dam built on the Narmada River located in Madhya Pradesh. It has the installed capacity of 1000 MW. Another example of multipurpose reservoir is Maithan dam in Jharkhand. It is built on Barakar River and one of the most successful multipurpose project in India. This dam was specially designed for flood control and generates high electric power. Other popular examples of multipurpose reservoirs are Tehri dam built over Bhagirathi River, Bhakra dam on Satluj river, Sardar sarovar dam on Narmada River and so on.

C.     Pondage reservoirs: These reservoirs are the ventures which give more consideration to the storage in comparison to the diversion projects with the help of pondage. Nevertheless, it is important to note that the storage would not be so big so that it can assure the season of rising or declining storage. These reservoirs may spill in low flow period when flows the comparatively good and not succeed in even high flow period when flows are slightly low. To evaluate the performance of such reservoirs, simulations must be carried out frequently (on monthly basis). Banasura sagar dam is the example of pondage reservoir which satisfied the demand for irrigation and drinking water in seasonal dry period.

D.    Within the year storage reservoirs: These reservoirs are constructed in a way that in ordinary situations, they totally filled up and even may dribble during the period of flood and in the low flow period, they are approximately exhausted. For such type of reservoirs it is easy to define storage accretion and exhaustion more competently and precisely.

E.     Carryover storage reservoirs: Carryover storage reservoirs are constructed in a way that they posses’ rather bigger active storage than usual inflows and outflows. By doing this spill can be avoided in any season. The objective of such reservoir is to neglect the effect of modification in the allocation system on its performance. The working sheets of these types of reservoirs can be made with high precision on yearly or bi seasonal basis.

F.     System of reservoirs: This term is used to denote the system where cluster of single or multiple reservoirs are operated in an integrated manner so as to make use of the natural resource (river) in an optimized way.

Purpose of reservoirs: The services provided by the reservoirs are diverse and can range from conservational uses to management of flood calamity etc. In monsoon season (high flow), water is conserved in the reservoirs to serve various purposes such as irrigation, domestic and industrial needs, recreational use, navigation, power generation, water quality management etc.

A reservoir must be well matched to serve particular purpose for which it is being constructed. The compatibility of the reservoir to meet a particular purpose is very important in its successful operation. If the purpose is not compatible then operation of reservoir becomes complicated. Therefore it is necessary to understand the purposes met by the reservoirs and their compatibility.

A.     Agriculture: In agriculture sector, stored water can be used as the source of irrigation. The crops grown in the agricultural fields are seasonal in nature and so is the requirement of water for irrigation. The demand of water for irrigation is consumptive in nature. Some portion of water used for the irrigation returns back as the backflow from the agricultural fields. The requirement of water for irrigation is directly proportional to the rainfall in that particular area i.e. high rainfall means low demand of water from reservoir. The principle of regulation of reservoir for irrigational demand is to store runoff water during monsoon and redistribute it during minimal flow.

B.     Hydroelectric power generation: The requirement of the hydroelectric power varies seasonally. The extent of variation depends upon the nature of loads being served viz; industrial, municipal or domestic. The requirement of hydroelectric power is non consumptive in nature as the water used in hydroelectric power generation could be consumed for other purposes downstream.

The reservoirs designed for hydropower generation can be categories in to two categories

1.      Storage reservoirs, which have sufficient storage space to regulate stream flow

2.      Run-off-the-river projects, which have small space compared to the volume of total flow.

The hydropower generation units are being operated mainly as “base load stations” or “peaking stations”. Base load stations follows to meet a definite pattern of demand whereas peaking stations works on random demands and does not follow pre defined pattern of demands.

C. Municipal  and  industrial:  The  demands  for  industrial  and  municipal  do  not  vary significantly throughout the year in contrast to irrigation and hydropower demands. However, it has been observed that demands may vary as population grows and high demands are recorded in summer seasons.

D.    Water quality control: The water quality can be defined as the physical, chemical and biological characteristics of the water. In the process of reservoir construction (particularly dam), the quality of water and aquatic life adversely affected. Therefore, to maintain the adequate water flows downstream is also one of the purpose being served by the reservoirs.

E.     Recreation: The sufficient water level at the downstream of dams can be used for many recreational activities as swimming, fishing and many others water games. The recreational activities can be attained at its best by keeping the water levels suitable for those activities throughout the season.

F.    Flood moderation: In addition to abovementioned purposes, the most important function of reservoir is to moderate flood flows. However, generally the reservoir is not constructed solely for flood moderation purpose but is a joint venture with conservational uses. Either the space above the reservoir is reserved for storage of excess flow in flood conditions or the space below is used for the purpose. The prime objective is to store excess flow during flood so that flows can be moderated and any disaster could be prevented. When sharing of storage is contemplated, flood storage zone capacity varies with time in year, instead of being fixed.

Conflicts in reservoir operation: When multipurpose reservoirs are operated, various absurdities happen due to varying and contending objectives of the uses of water. Some of them are listed below:

A.   Conflict in space: When the purposes being served are diverse in nature like water conservation and flood moderation then these types of conflicts occurs. If topographic and physiographic features of any area and sufficient finances are available then the dam of sufficient height can be constructed and separate space can be owed for each use. Though, this is not a reasonable option. Conservational demands are best satisfied when reservoir is full at the end of filling period and on the other hand empty reservoir is needed to store the excess flow during flood conditions.

B. Conflict in time: Temporal conflicts occur, when the prototype of water uses varies. For example if a reservoir is constructed to serve irrigation and hydroelectric power then the water required for irrigation depends upon the season, type of crops, type of soils, rainfall etc whereas the variations in the water requirement for hydropower generation depends upon various other factors. In these types of circumstances, the optimal policy for the operation of reservoir needs to be designed.

C. Conflict in discharge: The multipurpose reservoirs experience conflict of discharge if they are intended to serve purposes where daily discharge varies considerably.

Hydrologic forecast: Hydrological forecast plays an important function in the operation of reservoirs. Forecast can be categorized into short term (upto 2 days), medium term (2-10 days), long term (beyond 10 days) and seasonal (several months). The short term forecasts are highest in reliability and long term forecasts are lowest in reliability, therefore not extensively used in the operation of reservoirs.

Principles of reservoir operation

Operation of reservoirs depends upon set of rules and principles for releasing and storing of water according to the purpose being served. It is pretty difficult to develop a set of rules which can cover all complex situations but the objective can be achieved by following some rules in case of single purpose, multipurpose and system of reservoirs.

1.   Single purpose reservoir

a)      Flood control: Operation of reservoir for flood control depends upon the area available for storage, discharge capacity of the outlets, their locations, flood characteristics, flood forecasts etc. It is generally difficult to consider all complex situations but by following some principles the optimal guidelines could be outlined.

I. Effective use of available flood control storage: According to this principle, the operation of reservoir should be aimed at the minimizing the damages of the flood affected areas to the utmost degree by efficiently utilizing the available storage.

II.    Control of Reservoir design flood: Under this principle, assumption has been made that full storage capacity would be utilized only when flood turns into reservoir  design  flood.  As  Design  flood  is  usually  an  extreme  episode therefore regulation of minor and major flood is less agreeable when this method is applied.

III.    Amalgamation of principles I and II: This principle involves the combine use of both  abovementioned  principles.  The first  principle is  applied for the lower segment of the flood reserve to attain greatest benefits by directing the former part of the flood and subsequently the water is discharges as planned for the reservoir design flood. This principle gives best results in the best overall regulation.

IV. Flood moderations in emergencies: In order to overcome emergency conditions, it is desirable to prepare an emergency release schedule in which all information about the reservoir is available to be used by the operator.

b)     Conservation: Reservoirs meant for the extension of supplies during low period should usually be operated to fill as early as possible during filling period, while meeting the requirement.

2.    Multipurpose reservoirs: The guidelines or regulating principles for the reservoirs used for many purposes should be designed in the manner that takes into account the various purposes to be served according to the priorities. Generally the reservoirs are divided into five zones and allocated for the respective uses.

a)       Spill Zone: This zone is the space occupied mostly during high flood and releases from this space is substitution between structural safety and downstream flood damages.

b)      Flood control zone: This is the space utilized to store the excess flow during any flood event so as to avoid downstream damages. This space should vacant as early as possible in order to cope up with next flood event.

c)       Conservation zone: This is the space allocated to store the water for various future conservational demands.

d)      Buffer zone: This is the space made to meet very little demands in case of extreme situations. This zone lies above dead storage zone.

e)       Dead storage zone: This is commonly known as inactive zone and present at the lowest part of reservoir. The storage in this zone is meant to absorb some of the sediments entering into the reservoirs.

The principles on which multipurpose reservoirs should be operated are listed as:

a)      Separate allocation of capacities: When storage is owed separately for each purpose then the operation of reservoirs should follow the principles formulated for respective conservational purpose along with flood control. Though, allotment of separate space for every use is not an economical affair.

b)     Joint use of storage space: In case of multipurpose reservoir where joint storage space is available, it is quite difficult to operate the reservoir as the demands are purpose specific For example, In order to moderate flood flows, the storage space should be of nearly empty while on the other hand high level storage is needed to fulfill the conservational demands. Consequently, the objectives of these functions are not attuned and a negotiation will have to be achieved in the operation of reservoir by sacrificing the requirements of these functions. In a number of cases, the space available for the conservational purposes is utilized for flood moderation during monsoon season and is used up later on for conservational purposes when flows are inadequate to meet these demands. This will obviously include some sacrifice of the flood control interests towards the end of the monsoon.

There should be cautious approach while designing the reservoirs with the idea of combined operation of storage space to make the most of the complementary effects and minimizing the competing effects. Better services can be extracted from such ventures if they are designed properly.

3.    System of reservoirs: In this case, an integrated operation policy should be implemented in order to attain the best possible use of water resources accessible to us.

In the process of formulation of the regulations for the operation of integrated system of reservoirs, firstly the principles designed for the individual reservoirs were applied separately and after that modifications should be made by checking a number of alternative plans. With the introduction of computers in the water resource development, optimization and simulation methods are broadly used in these systems.

The principles which are generally considered for the integrated operation of reservoirs are as follows:

a)      Flood control regulation: In system of reservoirs, flood conditions are considered basin wise not on the sub basin wise. The engagement of flood reserves in every storage unit, distribution of outflows among reservoirs and bank full phase at critical site must be considered concurrently. For example, reservoirs having least capacity is considered, if a decline in outflows is needed and if an increase in outflows is needed then in this case, the reservoir having highest percentage occupancy is considered.

b)      Conservation regulations: In order to develop the co-ordinated plan so as to achieve optimal profit and diminishing the evaporation losses, following points should be considered:

1.      The demands at a time for a variety of uses

2.        On hand conservation storage in each reservoirs

3.    Allotment of outflows among the reservoirs.

System engineering techniques

System engineered techniques have emerged as the most influential tool in designing various strategies to obtain certain objectives. Usually the problems associated with water resources are multidimensional and any solution can proved to be beneficial to one problem but at the same time can affect other. Therefore, the approach should be planned in a way so that it can maximally facilitate to attain desired objectives with least discordance. To come up with the best possible solution and best decision, system engineered techniques are the best. Some of the approaches are discussed below:

1.      Simulation: For assessing the alternatives, simulation is possibly the most extensively worn method because of its numerical simplicity and flexibility. The technique of simulation does not recognize the optimal design or optimal policy and it facilitates the assessment of predictable performance that is resultant from any operating policy and strategy. It may be deterministic or stochastic in nature. If events are the random input and generated internally then it is called as stochastic and if no random inputs are involved then it termed as deterministic model. In the study of reservoir operations, stochastic models are very helpful. For instance, in reservoir operation study, the demands and rainfall patterns are random and reservoir may be vacant, half filled and full at the beginning, stream flows are random therefore it is recommended to use stochastic models for reservoir operation problems.

2.      Optimisation: It is method of selecting the finest answer from a number of likely options. Optimisation involves the application of a suitable optimisation model in combination with the optimization algorithm. Linear programming, dynamic programming, non linear programming and goal programming are some optimisation techniques which are discussed below:

a)      Linear programming: It is the most readily available software packages of algorithm and hence is most widely accepted technique in field of water resource systems. This technique can only study the linear relationships, which is its foremost constraint. Most of the problems associated with the water resource management are non linear in nature hence, they are estimated by piecewise linear method in order to get optimal solution.

b)     Dynamic programming: It is the commanding analytical optimisation technique and widely accepted technique for designing the water resource systems, its operation and allocation. It is more accepted technique than linear methods as it involves the solution of non linear problems also and our most of the problems in the field of water resource management are non linear in nature. In addition, it has the advantage of simplifying the complex problems into its sub sets which are easier to solve.

c)      Non linear programming: This technique is not as popular as linear and dynamic models because this optimisation process is slow and occupies more system storage and time. The mathematics involved is also very complicated than the linear programming. The chief disadvantage of this method is that the result attained by this model does not confirm a comprehensive optimum solution.

d)     Goal programming: It is the popular method and is competent of considering manifold goals in the objective function. In contrast to the linear programming models, hierarchy of goals characterizes the operation objectives of the goal programming. In this model, all objectives are allotted with target levels for achievement and a relative precedence on attaining these levels. Goal programming treats these targets as goal, but not as absolute constraints mandatory and attempts to find a optimal solution as closely as possible. This method uses the simplex algorithm and is based on the minimization of weighted absolute deviations from target of each objective.

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