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In hydraulic design, this is a height that is added to an allowable water level for design purposes. For example, in determining the height of a flood wall, or a channel that is required to contain flows of a given frequency (ARI or AEP), a level higher than the calculated water surface level is selected to allow for wave action and debris (and possible model inaccuracies). Another example is where allowable water levels within inlet pits might be set a certain distance below street level, to ensure easy inflow of stormwater.

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