The water surface immediately downstream of the culvert is known as the Tailwater, often synonymous with the water surface of the outlet pool. The tailwater elevation is controlled by a geomorphic feature downstream of the outlet pool.
Often this feature is the pool tailout or riffle crest, where gravel that is scoured from the outlet pool gets deposited. In other cases the tailwater control might be a weir or grade control structure designed to backwater the culvert inlet. The tailwater control elevation also determines the residual pool depth.
In some cases there will be no outlet pool downstream and flow will be unimpeded. In this situation the channel roughness, slope and cross section shape will govern the water surface elevation downstream of the culvert.
Tailwater Control Photos
See also: Tailwater Rating Curve, Cross Section Method