EM 1110-2-1100 (Part V)
31 Jul 2003
(c) Benefit/cost ratios. A useful indicator of economic performance of each alternative is the benefit to
cost ratio (BCR). As previously noted, the total, life-cycle costs do not depend on the other constraints,
therefore remain constant. However, the benefits included in the ratio can be limited by the funding authority.
Consequently, the BCR calculated can be significantly different depending upon whether all the potential
benefits or only limited benefits are considered. Because the Federal government limits the benefits allowable
to only storm damage reduction benefits, the total or true BCR is always greater than that specified for Corps
projects. In effect, two BCR's exist.
Federal government, (BCR)F
Storm damage reduction benefits
Total, life-cycle cost
Total, true (BCR)T
Total, life-cycle cost
Further details regarding the (BCR)F for Federal-sponsored projects and other methods to measure
economic performance are discussed in Part V-8. The total (BCR)T is never calculated for Corps
projects. Consequently, the local sponsors, general public, and media may not understand nor
appreciate the true value of shore protection projects to their community. These institutional,
political (social), and legal constraints are discussed further in the following paragraphs.
(d) Sea level rise. A detailed summary of present day knowledge of mean sea level change of the world's
oceans is given in Part IV-1-6. Over the last 100 years, average, relative sea level rise has been 30 cm (3
mm/year) on the East Coast and 11 cm (1.1 mm/year) along the West Coast (excluding Alaska). The Gulf
of Mexico coast is highly variable ranging from 100 cm (10 mm/year) in the Mississippi Delta plain to 20
cm (2 mm/year) along Florida's west coast (National Research Council 1987). Substantial local variability
exists. The question remains as to whether these average rates will increase (substantially), stay constant, or
decrease in the future. Three things remain clear, however. The existing rates of mean sea level rise at
specific sites have not been a severe economic constraint for the shore protection alternatives selected. At
many locations, anthropogenic effects (e.g., jettied tidal inlets) causing downdrift, beach erosion are clearly
much larger than those occurring due to sea level rise. And finally, long-term, relative changes in sea level
can be incorporated into storm surge analysis and the economic design of coastal structures.
(3) Environmental. A third major constraint of shore protection works is their impact on the
environment. The Eastern Scheldt, storm-surge barrier shown in Figure V-3-1 was the focus of much
discussion in the early 1970s. Environmental scientists favored raising the dikes around the periphery to
maintain the saltwater ecology of the tidal estuary. Agricultural and water boards favored a solid dam across
the mouth that would create an inland, freshwater lake. A compromise was reached: a storm-surge barrier
with movable gates which stay open under normal conditions but are closed at very high storm-surge events.
The final design, construction methods and equipment required much research and challenged the ingenuity
and technical process of Dutch coastal engineers. In the final analysis, the environmental constraint, to
maintain the saltwater ecology, dictated the final design. The additional engineering and construction cost
proved to not be the deciding factor.
(a) Types of environmental concerns. As in the preceding example, modification of upland habitat such
as land use, resting areas for turtles and shore birds, wetlands, flora and fauna beneficial to the ecosystem,
threatened and endangered species, etc. can take place. The aquatic habitat can also be important, for
example, water quality, aquatic species, benthic organisms, hazardous, toxic and radiological sediment in
Shore Protection Projects