If the boom is held at its ends, upstream from the oil spill, it will assume a catenary shape and capture the oil that drifts against it. The oil will form a stagnant pool with its maximum depth near the boom. If critical depth is exceeded at the boom, there may be a rapid drainage failure, during which most of the captured oil is lost. In a 1-kt current, with oils of 0.8, 0.9 and 0.98 specific gravity, tests indicate that boom depth must exceed 3, 10, and 55 in, respectively, to prevent drainage. In a 2-kt current the minimum depth almost triples. From near the boom to near the upstream edge, oil thickness decreases parabolically. At the leading edge, there is a rapid thickening due to a gravity or head wave.
When the current exceeds ]/3kt, oil can be torn off the bottom of the head wave and entrained in the current flowing under the captured pool. If the distance from the barrier, the barrier's draft, and the oil's buoyancy are insufficient, the entrained oil will sweep under the boom and will not resurface to coalesce with the captured pool. For a 2-ft draft boom and for oil specific gravities of 0.9 and 0.98, the respective critical current speeds are 2 and 0.9 kts. If the current is higher and there is sufficient sea room, the boom must drift with the current to keep the relative speed below critical. In a tidal channel or river, the boom can be streamed at an angle such that the current velocity component normal to the boom is below the critical speed. In this way, oil can be diverted without major loss from midchannel to a lower current area or to the river bank.
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