Fixation of Storage capacity

 Fixation of Storage capacity

Instructional objectives

On completion of this lesson, the student shall learn:

1. The usual classification of the zones of a reservoir

2. The primary types of reservoirs and their functions

3. The steps for planning reservoirs

4. Effect of sedimentation in reservoirs

5.  What are the geological explorations required to be carried out for reservoirs

6. How to determine the capacities of reservoirs

7. How to determine the dead, live and flood storages of reservoirs

8. How to reduce the loss of water from reservoirs

9. How to control sedimentation of reservoirs

10. The principles to be followed for reservoir operations

Introduction

Water storage reservoirs may be created by constructing a dam across a river, along with suitable appurtenant structures. How ever, in that lesson not much was discussed about fixing the size of reservoir based on the demand for which it is being constructed.

Further, reservoirs are also meant to absorb a part of flood water and the excess is discharged through a spillway.

It is also essential to study the relation between flood discharge, reservoirs capacity and spillway size in order to propose an economic solution to the whole project.

These and topics on reservoir sedimentation have been discussed in this lesson which shall give an idea as to how a reservoir should be built and optimally operated.

Fundamentally, a reservoir serves to store water and the size of the reservoir is governed by the volume of the water that must be stored, which in turn is affected by the variability of the inflow available for the reservoir.

Reservoirs are of two main categories: (a) Impounding reservoirs into which a river flows naturally, and (b) Service or balancing reservoirs receiving supplies that are pumped or channeled into them artificially.

In general, service or balancing reservoirs are required to balance supply with demand. Reservoirs of the second type are relatively small in volume because the storage required by them is to balance flows for a few hours or a few days at the most.

Impounding or storage reservoirs are intended to accumulate a part of the flood flow of the river for use during the non-flood months.

In this lesson, our discussions would be centered on these types of reservoirs

Reservoir storage zone and uses of reservoir

The storage capacity in a reservoir is nationally divided into three or four parts (Figure 1) distinguished by corresponding levels.

Full Reservoir Level (FRL): It is the level corresponding to the storage which includes both inactive and active storages and also the flood storage, if provided for. In fact, this is the highest reservoir level that can be maintained without spillway discharge or without passing water downstream through sluice ways.

Minimum Drawdown Level (MDDL): It is the level below which the reservoir will not be drawn down so as to maintain a minimum head required in power projects.

Dead Storage Level (DSL): Below the level, there are no outlets to drain the water in the reservoir by gravity.

Maximum Water Level (MWL): This id the water level that is ever likely to be attained during the passage of the design flood. It depends upon the specified initial reservoir level and the spillway gate operation rule. This level is also called sometimes as the Highest Reservoir Level or the Highest Flood Level.

Live storage: This is the storage available for the intended purpose between Full Supply Level and the Invert Level of the lowest discharge outlet. The Full Supply Level is normally that level above which over spill to waste would take place. The minimum operating level must be sufficiently above the lowest discharge outlet to avoid vortex formation and air entrainment. This may also be termed as the volume of water actually available at any time between the Dead Storage Level and the lower of the actual water level and Full Reservoir Level.

Dead storage: It is the total storage below the invert level of the lowest discharge outlet from the reservoir. It may be available to contain sedimentation, provided the sediment does not adversely affect the lowest discharge.

Outlet Surcharge or Flood storage: This is required as a reserve between Full Reservoir Level and the Maximum Water level to contain the peaks of floods that might occur when there is insufficient storage capacity for them below Full Reservoir Level.

Some other terms related to reservoirs are defined as follows:

Buffer Storage: This is the space located just above the Dead Storage Level up to Minimum Drawdown Level. As the name implies, this zone is a buffer between the active and dead storage zones and releases from this zone are made in dry situations to cater for essential requirements only. Dead Storage and Buffer Storage together is called Interactive Storage.

Within-the-Year Storage: This term is used to denote the storage of a reservoir meant for meeting the demands of a specific hydrologic year used for planning the project.

Carry-Over Storage: When the entire water stored in a reservoir is not used up in a year, the unused water is stored as carry-over storage for use in subsequent years.

Silt / Sedimentation zones: The space occupied by the sediment in the reservoir can be divided into separate zones. A schematic diagram showing these zones is illustrated in Figure 2 (as defined in IS: 5477).

Freeboard: It is the margin kept for safety between the level at which the dam would be overtopped and the maximum still water level. This is required to allow for settlement of the dam, for wave run up above still water level and for unforeseen rises in water level, because of surges resulting from landslides into the reservoir from the peripheral hills, earthquakes or unforeseen floods or operational deficiencies.