The storage capacity in a reservoir is nationally divided into three or four parts distinguished by corresponding levels.
These specific levels and parts are generally defined as follows:
- 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 is 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 below
- 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.