EM 1110-2-1100 (Part V)
31 Jul 2003
(d) Kraus, Hanson, and Blomgren 1994 also discovered that the updrift shoreline rarely reached the
seaward end of the groin (Table V-3-8, No. 6). On-shore sediment transport processes appear to be necessary
for a groin to be filled naturally so that the shoreline reaches the tip, or the tip is buried. Longshore sand
transport direction reversals, sand bypassing under water at the end, and groin permeability all normally keep
the updrift shoreline well landward of the end of the groin.
(e) Properties No. 7, 8, and 9 in Table V-3-8 have long been accepted standard conditions for groin field
design and construction. Even so, as noted in No. 10, filling a groin field does not guarantee that 100 percent
of the original longshore transport will continue as sand bypassing. The entire, filled, groin field system will
impound sand updrift to cause some erosion downdrift of the system.
(f) Properties No. 11 and 12 are often stated reasons by opponents of groins but are questionable because
they cannot be supported by physical mechanisms nor principles of conservation of sand. No. 13 will be
considered under innovations discussed in the following paragraphs.
(g) A critical review of the literature also shows that little, if any, previous discussion exists on how to
judge success (or failure) of a groin design. As discussed further, success should be judged on two factors:
to maintain a minimum, dry beach width for specified storm conditions for protection beyond a reference
baseline; and to bypass an average, annual amount of sediment to minimize downdrift impacts.
(3) Physical processes.
(a) Normal morphological response. How do groins work? Waves breaking alongshore at an angle create
a time-averaged, longshore current and longshore sediment transport. The cross-shore distribution of
longshore sediment transport is discussed in Part III-2 (see Equation III-2-23 and Example Problem III-2-7).
A key variable is the surf zone width for the theory cited (Bodge and Dean 1987) which assumes sediment
mobilized in proportion to the local rate of wave energy dissipation and transported alongshore by the local,
wave-induced current. The groin simply blocks a part of this normal transport of sand alongshore and causes
it to accumulate in a fillet on the groin's updrift side (the side from which the sediment is coming). This
accumulation reorients the shoreline and reduces the angle between the shoreline and the prevailing incident
wave direction. The reorientation reduces the local rate of longshore sand transport to produce accumulation
and/or redistribution of sand updrift of the groin. The amount of sand transported past the groin is greatly
reduced (or eliminated) to significantly impact the downdrift area. The ratio of groin length to some
statistical measure of surf zone width (or water depth at the groin tip) is a key factor in sand bypassing, as
discussed further in the following paragraphs. Wave diffraction causes reduced wave energy in the lee of the
groin relative to the midcompartment, mean water-level setup gradients, and setup induced currents behind
the groin. These contribute to complex, current circulation patterns that move sediment alongshore and
offshore along the leeside of the groin (Dean 1978). The strength of these internal current patterns depends
on groin planform geometry, but also on groin cross-sectional elevation and permeability across the surf zone.
Waves diffract around the groin tip, propagate over the submerged section and reflect off the body of the
groin. These interactions vary with water depth changes during the tidal cycle. Consequently, sediment can
also move over the top of submerged groins (over-passing), through the permeable, groin structure (through-
passing) and behind the end of the structure (shore-passing). Impermeable, high crested groins created
internal and external current patterns that are far different than permeable, submerged structures. Fleming
(1990) discusses the results of physical model studies with current and sediment movements for both high
and low groin cross sections. Complex flow patterns were produced, and it was stated that strong local
currents may cause a net loss of sediment from the compartment by offshore movement during storm events.
(b) Storm response. Groins offer little or no reduction in wave energy to shore-normal waves during
storms. Consequently, cross-shore sediment transport processes as discussed in Part III-3-3 for natural
beaches are similar for groin field compartments. And, for near normal wave incidence, the groin system can
Shore Protection Projects
V-3-69
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