EM 1110-2-1100 (Part V)
31 Jul 2003
the project section. The effect of varying taper length can be estimated with the aid of analytical or numerical
shoreline planform response models (Walton 1994; Hanson and Kraus 1989).
(6) The GENESIS model was used in a series of simplified applications to investigate the effect of beach-
fill transitions. Figure V-4-27 shows plots of an initial rectangular 6-km-long fill section, and the calculated
shoreline at 1-year intervals, for two projects identical in every way except that the project in the top panel
has a 400-m-long fill transition and the project in the bottom panel has a 2,000-m-fill transition. The total
fill volume is the same in both projects, to allow assessment of how best to utilize a finite volume (or
alternatively a fixed cost). The duration for both simulation is 5 years. A single, normally-incident effective
wave is considered. The design objective is to maintain a beach width of 20 m throughout the 5-year interval,
everywhere within the project reach.
(7) At the end of the 5-year simulation, both projects have beach widths greater than the 20-m design
width; and therefore, both projects have satisfied the design objective. It is noted that the average annual
volumetric rate of sand loss from the 400-m fill transition project was 148,000 cu m/year; whereas for the
2,000-m fill transition project, the average end losses were 89,000 cu m/year (about a 40 percent reduction
in the rate of end losses). However, because approximately 22 percent of the fill volume was placed outside
the project reach at the time of construction in the 2,000-m fill transition sections, the percent of total placed
fill volume remaining within the 6-km project area is greater for the 400-m fill transition project (58 percent
remaining) than for the 2,000-m fill transition project (55 percent remaining). For this case, the example
suggests that if there are few or no economic benefits to claim in the transition section, then all the fill volume
should be placed within the limits of the project reach (i.e., no use of transition sections). The same
conclusion would be reached in this case if maximizing fill volume retention was the primary design goal.
However, recall from the previous section that project longevity varies with the square of the project length.
Generally, this rule of thumb concerning use of transitions is true for shorter fills. Also, for short fills, it may
not be desirable to place sand volume in transition sections which may require a significant percent of the
total volume placed within the project.
(8) For longer projects, the design requirement of maintaining the design berm width at the project
boundaries throughout the renourishment interval may favor the placement of beach-fill transition sections.
Figure V-4-28 provides plots of the initial fill and the calculated shoreline position after 5 years, for 6- and
22-km-long fill projects, with both 400- and 2,000-m fill transition sections. Again, the design berm width
is 20 m. In Figure V-4-28a, it is seen that for the 6-km fill project, the 400-m transition project shoreline is
seaward of the 2,000-m transition project shoreline after 5 years everywhere within the project domain.
However, for the 22-km fill project shown in Figure V-4-28b it is seen that at the project boundaries, the
400-m transition project is landward of the 2,000-m transition project, and more importantly, landward of the
design shoreline. This result illustrates the potential value of beach-fill transition sections for long beach-fill
(9) However, even for long projects, the benefit of placing a fill transition section is only realized during
the first few years after construction (within the first renourishment cycle, as a rough rule of thumb).
(10) Within only a few years the volume of fill material that has moved from within the project limits
onto the adjacent beaches, as a result of alongshore spreading, is likely to be much greater than the volume
that was initially placed in the transitions, even with 2,000-m-long transition sections. This conclusion is
further illustrated by the results shown in Figure V-4-28, where it is seen that the calculated shoreline position
adjacent to the main project after 5 years is for all practical purposes the same for both the project with the
400-m fill transition and the project with the 2,000-m fill transition. The work of Walton (1994) suggests that
transition sections have a more lasting significant impact on reducing rates of sand loss from the project only
when their length exceeds a value that is 0.25 times the project reach length. In past practice,
Beach Fill Design