Reservoir management,reservoir capacity,storage space of reservoir,reservoir sedimentation

A reservoir is a storage structure that stores water in periods of excess flow (over demand) in order to enable a regulation of the storage to best meet the specified demands. Modelling a reservoir system, in general, is specific to the system, and the assumptions one may use to make the model formulation simple enough for problem-solving through known techniques in systems analysis. This lecture illustrates how a reservoir may be modeled using deterministic inputs. Model formulations for two important aspects of reservoir modeling are discussed: reservoir sizing and reservoir operation.

section of reservoir

Capacity of the Reservoir

Once it is decided to build a reservoir on a river by constructing a dam across it, it is necessary to arrive at a suitable design capacity of the reservoir. The reservoir storage generally consists of there main parts

which may be broadly classified as:

1.Inactive storage including dead storage

2.Active or conservation storage, and

3.Flood and surcharge storage.

In general, these storage capacities have to be designed based on certain specified considerations,

which have been discussed separately in the following Bureau of Indian Standard codes:

IS: 5477 Fixing the capacities of reservoirs- Methods

(Part 1) : 1999 General requirements

(Part 2) : 1994 Dead storage

(Part 3) : 1969 Line storage

(Part 4) : 1911 Flood storage

The data and information required for fixing the various components of the design capacity of a reservoir are as follows:

a) Precipitation, run-off and silt records available in the region;

b) Erodibility of catchment upstream of reservoir for estimating sediment yield;

c) Area capacity curves at the proposed location;

d) Trap efficiency;

e) Losses in the reservoir;

f) Water demand from the reservoir for different uses;

g) Committed and future upstream uses;

h) Criteria for assessing the success of the project;

i) Density current aspects and location of outlets;

j) Data required for economic analysis; and

k) Data on engineering and geological aspects

Storage space of reservoir

The annual demand for water at a particular site may be less than the total inflow there, but the time distribution of the demand may not match the time distribution of inflow, resulting in surplus in some periods and deficit in some other periods of the year. A reservoir serves the purpose of temporarily storing water in periods of excess inflow and releasing it in periods of low flow so that the demands may be met in all periods. The problem of reservoir sizing involves determination of the required storage capacity of the reservoir when inflows and demands in a sequence of periods are given. The total storage can be divided into three components: dead storage (for accumulation of sediments), active storage (for conservation purpose such as water supply and hydro power production), and flood storage (for reducing flood peak). While each of these components may be determined by separate modelling studies,we confine ourselves in this section only to the determination of the active storage capacity of the The inflow to the reservoir is in fact a random variable. The problem gets complicated if the randomness of the inflow has to be taken into account.One common method, extensively used in practice, is to determine the active storage capacity using the Rippl diagram or the mass diagram by plotting cumulative inflow with time. The method involves finding the maximum positive cumulative difference between a sequence of reservoir releases (equal to demands) and historical inflows over a sequence of time periods in which the demand is constant. For details of this method, the reader may refer any standard textbook on hydrology.If the demand is constant in each time period, the method is quite simple to apply. When the demand varies across time periods, the procedure requires a plot of the cumulative deficits in time from the period in which a deficit sets in, for the duration of the deficit, and finding the maximum deficit among all such duration. The total deficit duration containing this maximum deficit is known as the critical period.

This method is still cumbersome compared to a simple analytical technique, known as sequent peak method, which is described next. If we ignore the evaporation losses in the reservoir, the sequent peak method is quite simple. We shall first assume this situation and look at the algorithm to determine the reservoir capacity by the sequent peak method. Later, we shall see how the reservoir losses can be incorporated into the method to determine the active storage capacity of the reservoir.

approx of reservoir

Reservoir sedimentation

Sedimentation of a reservoir is a natural phenomenon and is a matter of vital concern for storage projects in meeting various demands, like irrigation, hydroelectric power, flood control, etc. Since it affects the useful capacity of the reservoir based on which projects are expected to be productive for a design period.Further, the deposited sediment adds to the forces on structures in dams, spillways, etc. The rate of sedimentation will depend largely on the annual sediment load carried by the stream and the extent to which the same will be retained in the reservoir. This, in turn, depends upon a number of factors such as the area and nature of the catchment,level use pattern (cultivation practices, grazing, logging, construction activities and conservation practices), rainfall pattern,storage capacity, period of storage in relation to the sediment load of the stream, particle size distribution in the suspended sediment, channel hydraulics, location and size of sluices, outlet works, configuration of the reservoir,and the method and purpose of releases through the dam. Therefore, attention is required to each one of these factors for the efficient control of sedimentation of reservoirs with a view to enhancing their useful life and some of these methods are discussed in the Bureau of Indian Standard code IS: 6518-1992 “Code of practice for control of sediment in reservoirs”. In this section, these factors are briefly discussed. There are different techniques of controlling sedimentation in reservoirs which may broadly be classified as follows:

• Adequate design of reservoir

• Control of sediment inflow

• Control of sediment deposition

• Removal of deposited sediment