EM 1110-2-1100 (Part V)
31 Jul 2003
Table V-3-9
Process-Based Factors Controlling Groins
Process
Parameter
Description
1
Bypassing
Dg/Hb
depth at groin tip/breaking wave height
2
Permeability
Over-passing
Zg(y)
groin elevation across profile, tidal range
Through-passing
P(y)
grain permeability across shore
Shore-passing
Zb/R
berm elevation/runup elevation
3
Longshore Transport
Qn/Qg
net rate/gross rate
Kraus, Hanson, and Blomgren (1994) exercised the GENESIS model for a single groin and studied
the bypassing formulation. Rapid filling was followed by a gradual buildup over time that meant an
increased amount of material must bypass the groin as the shoreline grows out towards the tip.
Filling to capacity was only possible for Qn/Qg =1 for the unidirectional case. As Qg>Qn, growth of
the shoreline seaward decreased. These tests mean groins seldom fill to capacity by longshore
transport processes alone. Cross-shore sediment transport processes must be added to understand
how beach elevation and width can build beyond that capable of representation on one-line, shoreline
change models. Simulations with multiple-groins and the Westhampton Beach, New York, groin field
are also discussed in this paper. An aerial view looking east of the Westhampton Beach, New York,
groin field with beach nourishment in 1998 is shown in Figure V-3-30.
Realistic distributions of longshore current and sediment transport across the surf zone, beach profile
shapes with bars and troughs, and other sediment transport mechanisms (wind, tide) are further
complicating factors that have yet to be addressed in numerical models.
(b) Groin profile. A typical groin profile with inshore (berm) section, sloping middle section, and
horizontal seaward section is shown in Figure V-3-31. In general the landward end is set at the elevation of
the natural, existing beach berm, the sloping section is set at the slope of the beach face in the swash zone and
the outer, seaward section at a lower elevation such as mean low water (mlw) or lower. The landward and
sloping sections function as a beach template for sand to accumulate on the updrift side. The groin profile
is shaped to approximately match the postproject beach profile, after nourishment is complete. The seaward
end (depth) and seaward elevation are set to the planned bypassing and overpassing in the surf zone. A lower
seaward section permits longshore currents to carry sediment over the structure and reduces wave reflections
from the groin. A significant amount of sand is transported on the beach face in the swash zone (Weggel and
Vitale 1985) and therefore overpassing also takes place in the sloped section when the groin has been filled
in this area. Prevention of flanking is the main concern to locate the shoreward end. As seen in Figure V-3-
29, calculations of the storm erosion distance, e, together with maximum shoreline recession are needed to
establish this position. Seaward limit of the shore section is set relative to the desire, nourished beach width,
W, or even further seaward to help retain the nourished beach. Seaward limit of the outer section is the groin
length, Yg, and depends on the amount of longshore sediment transport to be bypassed, as discussed.
(c) Permeability. In general, sheet-pile groins of all types are impermeable whereas rubble-mound groins
permit some material to wash through the structure. Some rubble-mound design contain impermeable cores
and/or are treated with sealant materials to ensure sand tightness (see EM 1110-2-1617). There are no
quantitative guidelines for determining the permeability of sand for a given groin geometry of the rubble-
mound type. Some patented, precast concrete groin systems are permeable, as will be discussed later.
V-3-72
Shore Protection Projects
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