ERDC/CHL CETN-IV-29
September 2000
FLOOD AND EBB CHANNELS: Inlets often possess two channels, with one channel
favoring flood flow and the other favoring ebb. These channels are formed by strong currents
that occupy particular locations as dictated by the local hydrodynamics. Controls on channel
location are inlet orientation, structures, and channels leading into the inlet from the bay.
Figure 1 shows flood and ebb channels for a typical inlet. In this figure, the main ebb channel is
located in the center of the inlet and the flood channels are along the inlet margins. At Ponce de
Leon Inlet, FL, the ebb current is strongest along the side of the channel adjacent to the north
jetty, as shown in Figure 4. Scour near the north jetty at Ponce de Leon Inlet may be the result of
the strong ebb current (Militello and Zarillo 2000).
Jetty
Flood Shoal
Barrier Island
Flood Shoal
Figure 4. Ebb current at Ponce de Leon Inlet, FL
Scour can occur by an ebb current that impinges on a jetty (Hughes 1997), as illustrated in
Figure 5. As the ebb current is deflected by the jetty, the directional change in momentum
narrows the flow streamlines, accelerating flow along the jetty. The increased current speed
scours the bottom adjacent to the jetty toe, and sediment deposition occurs in the center of the
entrance channel where the current has decreased magnitude. Over time the deeper, more
navigable portion of the channel migrates closer to the jetty. Ebb currents can also be modified
by any type of flow constriction created by jetty or shoreline revetment alignment.
During flood tide, some inlets may experience flow separation at the seaward ends of the jetties
that tends to direct flow along a preferred path through the inlet throat. Flow separation often
generates vortex-like rotational flows that may be strong enough to disrupt small-craft
navigation. Discussion of jetty geometry control on flow patterns is given next.
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