EM 1110-2-1100 (Part V)
31 Jul 2003
1986; Pope and Dean 1986; Kraft and Herbich 1989; Pope 1989; Rosati 1990; Rosati and Truitt 1990;
EM 1110-2-1617; and Chasten et al. 1993). One key aspect of all these efforts has been to determine under
what conditions salients or tombolos will naturally form behind the breakwater.
(a) Salients or tombolos. A salient is the preferred shoreline response for a detached breakwater system
designed for the Corps as stated in EM 1110-2-1617 and in Chasten et al. (1993). This is to allow longshore
sediment transport to continue to move through the project area to downdrift beaches. Salients are likely to
predominate when the breakwaters are sufficiently far from shore, short relative to incident wavelength, and
relatively transmissible (low crested or large gaps with low sediment input). Wave action and longshore
currents tend to keep the salient from connecting to the structure.
Sand will more likely accumulate in the structure lee and form a tombolo when the breakwater is
close to shore, is long relative to incident wavelength, and is relatively impermeable (high crest and
small gaps and with large sediment input). A tombolo-detached breakwater functions like a tee-
shaped groin by blocking longshore transport and promoting sediment movements offshore in rip
currents through the gaps. Although some longshore transport can occur seaward of the breakwater,
the interruption in the littoral system may starve downdrift beaches of their normal sediment supply,
causing erosion. Variable wave energy regimes may produce periodic tombolos to temporarily store
and then release sediment to the downdrift region.
Salient formation provides a recreational swimming environment and limits access for maintenance
and to the public. Tombolo formation provides a recreational beach environment and allows direct
access to the structure for maintenance, but public access may not be desirable.
Figure V-3-21 presents a definition sketch for the variables that have been employed to develop
empirical relationships for detached, breakwater design by many research efforts listed in Table V-3-
5 dating back to the 1960s. For salient or tombolo formation, the key variables are:
Y Distance of breakwater from nourished shoreline
Ls Length of breakwater structure
Lg Gap distance between adjacent breakwater segments
ds Depth (average) at breakwater structure below mean water level
Three dimensionless ratios, Y/ds, Ls/Lg and Ls/Y have emerged to separate salient and tombolo response.
As qualitatively discussed, when the breakwater is long and/or located close to shore, conditions
favor tombolo formation. As shown in Table V-3-6, "Conditions for the Formation of Tombolos,"
many references say Ls/Y > 1-2 for tombolo formation (except Gourlay 1981). Dally and Pope
(1986) recommend
Ls
single breakwater
(V-3-6)
' 1.5 to 2
Y
Ls
L # Lg # Ls segmented breakwater
' 1.5
(V-3-7)
Y
where L is the wavelength at the structure.
V-3-50
Shore Protection Projects
Previous Page
