EM 1110-2-1100 (Part II)
30 Apr 02
atmospheric pressure, and rainfall can be the primary components of water level climate (Part II-5-5). In
lakes, seasonal fluctuations in water level can be dominant, as in the Great Lakes (Part II-5-4.b.(2)).
(3) Long-term changes. Long-term changes in relative water level can be caused by climatological
effects and secular fluctuations (such as melting of the polar ice caps, large-scale isostatic adjustments of the
earth's crust, and local subsidence). These long-term changes, which operate on time scales ranging from
semiannual to decades, may sufficiently shift the water level datum relative to project features to merit
consideration in design.
h. Currents. Surf zone currents are discussed in Part II-4-6. Currents at inlets and harbors are treated
in Part II-6 and 7, respectively.
(1) Design importance.
(a) Nearshore shelf. Currents over the continental shelf are important relative to rate and direction of
transport of fluids and solids, such as river discharge into the ocean, sewer outfall discharge, movement of
sediment from offshore dredge disposal sites, and movement of civic waste material from ocean dump sites.
They may also be important for their effect on navigation into harbors, particularly for large vessels subjected
to currents moving across the entrance channel. Currents are driven mainly by tides and winds, but
temperature and salinity gradients, Coriolis effect, river discharges, and organized current systems (such as
the Gulf Stream) can also be important. Currents can vary greatly between the surface and bottom.
(b) Surf zone. Surf zone currents are the driving force transporting sediments in both the longshore and
cross-shore directions. As such, they are the key factor in beach erosion and accretion. They may also be
important relative to scour and stability of breakwaters and revetments. Surf zone currents are driven by
breaking waves and nearshore winds. Currents are very sensitive to wave direction. The magnitude of
longshore transport can vary greatly over a time period of days, months, and even from year to year in
response to natural variations in wind and wave climate (Figure II-8-17). At many sites, even the dominant
direction during a single year (upcoast or downcoast) can deviate from the normal pattern. Thus an adequate
sample of years is necessary for stable design estimates. Surf zone currents are discussed in detail in
Part II-4-6. Nearshore sediment transport is covered in Part III.
(c) Inlets. Currents through inlets are the primary process affecting exchange of water and sediments
between the bay and ocean. They impact water quality, bay ecology, and erosion and shoaling patterns. They
can impede navigation by creating steepened, breaking waves when a strong ebb current opposes energetic
ocean waves. They may cause scour along jetties and other inlet structures and affect structure stability.
Tides, wind, and density differences are typical driving forces. Inlet currents are a necessary consideration
in design of projects at inlets.
(d) Harbors. Currents through harbor entrances are generally important in terms of circulation and
flushing of the harbor to maintain water quality and, in some cases, to reduce maintenance dredging. They
can be driven by tides, winds, and river discharge within the harbor. Additional detail is given in Part II-7-6.
(2) Estimation procedures.
(a) Currents are estimated in terms of time-averaged mean speed and direction and, often, some measure
of maximum current speed. For tidal currents, it is helpful to distinguish between ebb and flood tide maxima.
Hydrodynamic Analysis and Design Conditions
II-8-33
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