This category includes any weir device that uses gravity to drain oil off the water surface. Typically, the top edge of the weir is positioned just below the upper surface of the slick, allowing oil to flow over the weir into a collection sump, where it is pumped to storage. With some devices in this category, the top edge of the weir is above the slick and some means is used to move oil up and over the weir. With some units, the weir can be adjusted to limit the amount of free water collected along with the oil. Devices that have a fixed weir may recover significant volumes of water along with the oil: oil/water separators should be considered with such skimmers to maximize the use of available storage.