The U.S. Army Corps of Engineers dredges and maintains federal channels in U.S. waters. Depths in these channels are of critical importance to deep draft commercial vessels. The Corps periodically surveys channel depths using sonar and reports the data to NOAA. Minimum depths are reported for different portions of a channel depending on its width. Minimum depths within a channel are also called "controlling depths," because they limit or "control" the size of ships that may safely enter that portion of a channel.
Usually, channels 400 feet wide and greater are divided into four quarters. Channels 100 to 399 feet wide are divided into the left outside quarter, mid-channel for half the channel width, and the right outside quarter. Channels less than 100 feet wide have the minimum depth reported for the middle 80 percent of the channel.
The method of dividing or "quartering" the channels of different widths is illustrated in the figure below. Note that the designation of the left and right side is from the perspective of a ship heading to port.
On a NOAA ENC®, channels are divided into separate dredged area polygons, according to the method used by the Corps of Engineers to "quarter" them, as described in the figure above. The controlling depth in each dredged area – the "depth range 1" or "DRVAL1" attribute – may be obtained by a "cursor pick" inquiry of the ENC. In the figure below, a cursor pick report shows the Brewerton Channel left outside quarter’s controlling depth as 14.9 meters (49 feet).
On NOAA’s raster chart products, channel depths are commonly presented in a "channel tabulation," a table listing the controlling depths found across the width of channels. Longer channels are often broken into shorter sections, called "reaches." The extent of reaches are marked with a magenta line and labeled with a name that matches the name appearing in the tabulation.
The channel tabulation from Chart 12281 is shown below. The columns of numbers on the left show controlling depths for each quarter of different channelsand channel reaches. For example, the top left value in the first row shows that the controlling depth found along the entire length of the left outside quarter of the Brewerton Channel is 49 feet (the same section noted in the ENC image above). Tabulations for channels that are between 100 ft and 399 ft wide will show three columns of controlling depths (for the left and right quarters and middle half), those for channels under 100 ft wide will show only one column (for the middle 80% of the channel width).
The dimensions in the far right columns of the tabulation show the intended "project dimensions" that are used when each channel is dredged, not the actual last surveyed depths, which are often shallower due to the accumulation of silt and other factors.
Note, in the image below, the chart shows the limits of the quarters in the Brewerton Angle, as irregularly shaped portions of channels often are. The center of straight portions of channels are sometimes shown on the chart with a line extending from a pair of range lights or leading light, if they are present (see U.S. Chart No. 1 symbol P20). In the Brewerton Channel, shown below, the centerline is marked by an extension of the range lights that are beyond the left edge of the chart image.
However, as the image of the West and East Channels in the chart image below shows, the centerline and quarters of channels are not normally marked on NOAA charts.
Page last edited: August 11, 2016