EM 1110-2-1100 (Part V)
31 Jul 2003
Natural beaches coexist in front of the rocky cliffs and naturally-hardened shorelines at many
locations throughout the world. A major, comprehensive research effort is needed to quantify the
effect of sand trapping on frontal and downdrift beaches.
(3) Active volume in the cross-shore profile. Successive cross-shore surveys of the beach profile to
closure depth reveal spatial variations in vertical elevation at each location. In the absence of lateral transport,
the eroded sections balance the accreted areas, i.e., sediment volume is conserved. The active sediment
volume is defined as one-half of the total volume change between two successive surveys. The 12-year,
biweekly nearshore bathymetric data set surveyed at the Corps of Engineers Field Research Facility, Duck,
North Carolina, has been analyzed to quantify the total active sand volume, its spatial variation across the
profile, and its relation to long-term, fair-weather, and storm periods (Ozger 2000). An empirical relationship
between storm wave power and active sand volume has been developed for the Duck site. Prestorm
morphology and duration of storm surge are possible factors for the scatter in the power versus active sand
volume relationship. The maximum value of active sand volume was 140m3/m (350 cu yd/ft). Different tidal
conditions, wave climate and hard bottoms (or reefs) limit the cross-shore movement of sediment.
Determinations of the naturally active, sand volume should be made for other sites. Basco and Ozger (2001)
summarize the above results and discuss various applications in coastal engineering. The seawall trap ratio,
WTR can be defined as:
Wall Trap Volume
WTR '
(V-3-5)
Active Sediment Volume
to quantify the relative impact of the sand volume removed from the system. Dean (1987) failed to consider
the WTR. Weggel (1988) qualitatively addresses the importance of the numerator increasing with type
number but also did not consider the significance of the denominator. The spatial distribution of the WTR
is also important relative to wall location. At locations with seawalls where the WTR is small, annual
mitigation may be economic.
(4) Sand rights and mitigation. A few states have adopted sand mitigation polices and procedures to
permit seawall construction and maintain a healthy beach. The idea is to annually replace the beach materials
trapped behind the structure with a volume calculated by some formula. The methodology in Florida has both
on offshore and longshore transport components which require knowledge of the annual erosion rate and net,
longshore transport rate, respectively (Terchunain 1988). Sarb and Ewing (1996) present formulas for cliff
and bluff erosion impacted by seawall construction in California. These formulas attempt to deterministically
estimate the wall trap volume (numerator) but do not consider the active sediment volume (denominator) in
Equation V-3-5. The relative volume trapped is unknown. Field research efforts to date have yet to confirm
the trapping theory of Dean (1987). The reason may be that the trapped volume is only a small percentage
of the total, active sand volume in the profile. The WTR is near zero so that downdrift impacts are minimal
and lost in the data scatter. See also Part III-5-13 for discussion of measures to manage human influence on
sediment supply. Who owns the sediments on eroding shorelines, the local property owners or downdrift
interests, is a legal question facing society. The subject of sand rights (Magoon 2000) is an extremely
complicated issue that may require settlement by the nation's courts.
V-3-34
Shore Protection Projects
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