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Orange County Coastal Regional Sediment Management Plan
Southern hemisphere swell is derived from extratropical cyclones from the South Pacific Ocean
with the majority occurring from spring through early fall. These swells approach from about
170 degrees to 215 degrees, decaying significantly as they traverse across the Pacific Ocean.
Local sea is the term applied to steep, short period waves which for this project are generated
by local winds and northwest winds in the outer coastal waters. The local winds can be further
separated into pre-frontal winds from the southeast, gradient winds during the passage of a
winter low pressure system from the west, and westerly sea breezes.
With the predominance of wave energy reaching the Orange County coast from the northern
hemisphere, wave driven currents typically run from northwest to southeast throughout the
winter and spring and cause the majority of longshore sediment transport. As this coast is also
significantly exposed to southern swell, seasonal reversals in longshore sediment transport
occur, typically in the summer. These seasonal longshore transport components tend to be
much greater than the net difference between the two. Thus the combined gross longshore
transport (south transport + north transport) is much greater than the net longshore transport
(south transport – north transport). Variable climatic cycles result in a range of conditions from
dominant southeastward sediment transport over certain periods, followed by periods of more
balanced sediment transport directions. The shoreline morphology has equilibrated over time to
follow predominant conditions and over the long-term is oriented to southeastward sediment
transport, with sediment inputs to the littoral cells typically from the northwest and outputs from
the littoral cells typically in the southeast.
2.3 Sediment Transport and Beaches
The Plan covers the entire 39 mile coastline of Orange County extending from the San Gabriel
River in the north to the San Mateo Creek in the south. This stretch of coastline can be divided
by many possible features, of which the physical process of sand transport seems most
appropriate for a RSM plan. These physical processes are most easily described by a sand
accounting system called the sediment budget and a geographical grouping method based on
the littoral cell.
The sediment budget approach was developed to understand the impact of coastal processes
on shoreline change. The sediment budget conceptually accounts for inflows (sources),
outflows (sinks), and storage of sediment within a littoral cell. A littoral cell is a coastal
compartment or physiographic unit that contains sediment sources, transport paths, and
sediment sinks (USACE, 2002b). A littoral cell is typically a portion of the coastline that does
not significantly transport to or receive littoral sediment from another cell in either the upcoast or
downcoast direction (USACE, 1991). Most cells, however, are not absolutely separated and do
have some leakage between them. Orange County has four major littoral cells as shown in
Figure 2.5. These littoral cells are the Seal Beach Littoral Cell, the Huntington Beach Littoral
Cell, the Laguna Beach Cells (USACE, 2002b), and the northern portion of the Oceanside
Littoral Cell (USACE, 1991).
Everest International Consultants, Inc. 2.10