Question

The USBR recommendations for minimum freeboard of a control spillway over its maximum water level for earth dam having height less than 60m is:

• A. Between 2m to 3m
• B. 2.5m above top of gates
• C. 3m above top of gates
• D. Between 1.5m to 2.5m

Hint:

Unlike concrete dams, earthen dams cannot tolerate any overtopping.
Therefore, spillway capacity and freeboard safety margins are designed using conservative maximum probable flood (MPF) estimates.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Freeboard is the vertical distance between the maximum reservoir water level (when the spillway is operating at full design capacity) and the top of the dam.
Maintaining adequate freeboard on earthen dams is critical to prevent overtopping, which can quickly cause catastrophic erosion and dam failure.

Step 2: Detailed Explanation:
The United States Bureau of Reclamation (USBR) provides design standards for freeboard based on dam height, reservoir wind fetch, wave action, and spillway type.
For earthen dams with heights less than \( 60\text{ m} \) (which covers most small to medium agricultural and irrigation dams), USBR guidelines state:
The minimum freeboard during design flood conditions should be maintained between \( 1.5\text{ m} \) and \( 2.5\text{ m} \).
This safety margin accommodates wave run-up, wind setup, and potential settlement of the earth-fill embankment over time without risking overtopping.
For very high dams (greater than \( 60\text{ m} \)) or reservoirs with large wind fetches, higher freeboard margins are required.

Step 3: Final Answer:
The USBR recommended minimum freeboard for an earth dam less than \( 60\text{ m} \) high is between \( 1.5\text{ m} \) and \( 2.5\text{ m} \).