tidal currents and waves. It was closed the following year by filling with material dredged from
the adjacent channel. CIRP laboratory work indicated the erosion area is created by wave action,
and an effective diffraction mound termination design was developed (Seabergh 2002).
Scour at Inlet Structures
Scour holes that form adjacent to navigation entrance jetties and breakwaters can jeopardize
the structure toe and possibly lead to partial failure of the protective structure slope. CIRP
research identified jet-like tidal flows at structured inlets as a major factor influencing the
location and severity of scour at structures. Inlet jetties are solid boundaries that direct tidal
flows. Depending on the inlet planform geometry, the jet velocity can increase substantially
adjacent to the structure, as illustrated in Fig. 15. The dashed lines show the approximate jet
boundary, and the arrow lengths represent the relative increase in water velocity. Scour holes at
numerous inlets exist at locations consist with jet-flow analysis.
Fig. 15. Examples of jet-like flows at structured inlets
Flow maps were created from an analytical jet theory for frictionless flow, from which initial
estimates of the flow discharge distribution for given inlet geometries can be obtained. Fig. 16
plots flow maps for ebb and flood tides at an inlet protected by an arrowhead jetty system. The
maps show streamlines aligned with the flow, and the other lines are contours of constant
discharge per unit width. Flow maps for other inlet structure geometries can be generated using
an online application available on the CIRP web site.
Tools for estimating scour at inlet structures are being developed based on field observation
and laboratory measurements. Modeling scour at laboratory scale has been a difficult
proposition because the sand cannot be scaled to model size with traditional geometric scaling.
New scaling guidance was developed based on the concept that an equilibrium state exists
between the depth and maximum discharge at every location across the inlet throat (Hughes
2000b, 2002), and this concept has opened new possibilities for applying laboratory movable-bed
models to inlets. For example, a joint effort between CHL and the Los Angeles District of the
Corps of Engineers successfully predicted scour that had occurred at Ventura Harbor, and then
demonstrated that additional toe scour would occur unless a protective scour blanket was
installed (Hughes and Schwichtenberg 1998). Preventative maintenance was completed at the
Ventura Harbor breakwater just months before a major storm arrived. The District estimated the
scour blanket prevented about a half-million dollars in damage to the breakwater.