- GeoRef, Copyright 2008, American Geological Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
Tidal inlets penetrating narrow barriers and with or without tidal deltas typically have a central trough with terminal channel fans. The trough is centrally narrowly branched at each end with multiple, lesser, converging lateral channels. Deltaic sand, where present, tends to form peripheral shoals and shoals separating the two sets of channels. The trough is a narrow horizontal slot, ratio 1/24 to 1/75 in typical examples. Deep holes occur in axial and other narrows. Detailed studies by others show that the individual channels of an inlet can usually be identified as either entering or issuing channels, subject to being shifted by longshore currents as the tidal phases change. Without further direct current studies, the gross flow characteristics of the channel systems may be interpreted from the described conditions. In both flood and ebb (rising and falling) phases, the water level is higher on one side of the barrier than the other. For the high sheet of water to get through the inlet, it must drain centripetally into it along all unobstructed radii, a tidal drain. The wide distribution of the minor lateral channels gives them most of the drainage. The inlet flow issues with appreciable velocity, scouring the trough and hole and forming a local maximum of current energy in its flow direction. The trough typically has 2 major branches at each end separated by shoals but narrowly and sub- centrally confined. If unobstructed and unbranched, the issuing channel flares like a horizontal jet, which ideally flares at 20 degrees . Whether single or branched, the issuing current is a tidal jet. Thus, at the ends of the inlet both the flood and ebb tides are drains on the high side and jets on the low side of the barrier, in both cases being there local maxima of hydraulic energy. The mouths of shallow estuaries, but not those of deep straits, show the type of channel pattern characteristic of the tidal inlet.