The top water level refers to the maximum water level a reservoir can hold, considering its design and operational features.
Understanding Top Water Level
The definition of "top water level" depends on the type of reservoir and how its overflow is managed. According to the provided reference:
- For reservoirs with a fixed overflow sill: The top water level is the height of the lowest point on the crest of that sill. This means water will begin to overflow once it reaches this level.
- For reservoirs with controlled overflow: (using gates, syphons, or other mechanisms) The top water level is the maximum level the water can reach without accounting for any additional capacity reserved for flood control. In other words, it's the highest level for normal storage.
Key Considerations
- Fixed Overflow Sill: Imagine a bathtub with an overflow drain. The height of that drain is analogous to the crest of the sill.
- Controlled Overflow: Think of a dam with gates that can be opened to release water. The top water level would be the highest water level allowed before the gates are opened, excluding any extra space kept empty to accommodate potential floods.
- Flood Control: Reservoirs often have extra capacity to handle flood events. The top water level excludes this flood storage capacity; it represents the maximum normal operating level.
Example
Consider a reservoir with gates used to control water release. The reservoir is designed to normally hold water up to 100 meters. However, it also has an additional 5 meters of capacity specifically for flood control. In this case, the top water level is 100 meters. When heavy rains are predicted, the operators might lower the water level to, say, 95 meters, creating 5 meters of empty space to absorb floodwaters. This lowering of water level does not change the top water level designation.
