Dirtwork term
Freeboard
also called: safety storage
The vertical distance between the design water surface in a pond or basin and the top of the bank, safety margin for big storms.
Freeboard is the extra height built into a pond, detention basin, or levee above the calculated maximum water surface. If the engineer's calculation says the pond fills to elevation 102.0 in the design storm, the bank gets built to 103.0 or 103.5, the extra foot or 18 inches is freeboard.
The point is to keep the next bigger storm (the one above the design event), or wind-driven waves, or accumulated sediment that's reduced the basin's effective volume, from overtopping the bank. Overtopping is how earthen ponds and basins fail, water erodes the back of the bank, the bank breaches, and the whole impoundment lets go at once.
Freeboard is cheap insurance at construction time. Adding it later, after a near-miss, costs an order of magnitude more.
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Freeboard comes up on excavation jobs all over East Texas.
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When you need it
When freeboard comes up on a job
Freeboard is the margin you build in when a pond or a detention basin is sized, holding the dam or bank above the design water level. It is what lets a bigger storm than planned spill over the spillway instead of over the dam.
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Related terms
Other words that come up alongside this one
Retention Pond
Excavated pond that holds water permanently, with storage capacity above the normal pool for storm events.
Detention Basin
Excavated basin sized to temporarily hold stormwater and release it slowly to a downstream system.
Watershed
The area of land that drains to a single low point, every drop of rain that falls inside it ends up at the same place.
Slope Ratio
How steep a slope is, expressed as horizontal:vertical (e.g. 3:1 means 3 ft over for every 1 ft up).
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