ERDC/CHL CETN-IV-29
September 2000
Circulation Patterns at Tidal Inlets
with Jetties
by Adele Militello and Steven A. Hughes
PURPOSE: This Coastal Engineering Technical Note (CETN) provides guidance on
interpreting horizontal circulation patterns at inlets.
BACKGROUND: Tidal inlets provide a conduit for water exchange between the ocean and
coastal bays, lagoons, and estuaries. They also serve as navigation routes for commercial and
recreational vessels. The U.S. Army Corps of Engineers maintains Federal inlets on all coasts of
the United States and its territories. The Corps manages inlets primarily by construction of
jetties and by dredging. Jetties stabilize the entrance and the entrance channel, and they also
protect vessels from waves as they travel between the surf zone and deep water.
Tools for inlet management have become more sophisticated in recent years with advances in
field instrumentation and computing capabilities. These improvements have yielded high-quality
data and detailed calculations of inlet currents. Engineering problems that can benefit from
interpretation and understanding of circulation patterns at inlets are prediction and prevention of
scour, channel migration, and navigation safety.
Circulation patterns are specific to each inlet, but certain properties are common to many inlets.
Common properties include ebb or flood dominance, preferred channels on ebb and flood tide,
eddy formation and migration, and jetty control on flow patterns. This CETN describes the
circulation patterns and related processes common to many inlets with focus on those with dual
jetties. Circulation patterns described herein assume that the tide is the sole or dominant forcing.
geometry, presence and configuration of structures, bottom topography, and nontidal forcing,
such as wind and river inflow. The rise and fall of the ocean tide is the primary forcing. Inlet
currents are strong with typical maximum speed in mainland U.S. Atlantic inlets being about 1 to
2 m/s. Representative circulation patterns and morphological features at inlets are shown in
Figure 1. Flood currents form channels on both sides of the inlet entrance. As water traverses
the inlet and enters the bay, the current is primarily aligned with the inlet and flows over the
flood shoal where the velocity is reduced and material is deposited. This process forms a flood
ramp, which is the sloped front face of the flood shoal. During ebb tide, the primary conduits of
water are channels located between the flood shoal and the barrier island. These ebb channels
merge at the inlet forming a main ebb channel. Strong ebb currents exiting the inlet form a jet
(Joshi 1982; Joshi and Taylor 1983; Mehta and Joshi 1988). As the jet exits the inlet, it expands
and loses velocity, depositing material onto the ebb shoal.
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