ERDC/CHL CHETN-II-47
March 2004
with erosional hot spots. In the wave focusing related cases, it may be possible to remove the
geologic feature that produces the wave focusing. Hot spots that result from geologic controls can be
more difficult to resolve, in that there is often little that can be done to change those geologic
controls. In these cases, alternative approaches including limiting future development, buy-out of
existing infrastructure, and alternative land use should be considered.
Structure-induced erosional hot spots come about primarily due to interruption of the net longshore
transport along the beach by the structure or wave reflection from the structure. These hot spots have
also been treated in a number of ways. Sand bypassing around navigation channel structures can be
an effective way of eliminating the interruption longshore transport that creates some hot spots.
Notching of groins in the swash zone has proven effective in restoring longshore transport in New
Jersey. Periodically renourishing the beach at Surfside/Sunset Beach is the approach taken to deal
with wave reflection from the Anaheim jetties.
Hot spots at locations where development encroached on the active beach were also identified in a
number of the case studies. These often result because the design required a uniform berm width
along the project. Consequently, at the location of the encroaching structure there is a seaward bulge
in the design shoreline, which behaves as an extremely short nourishment project embedded within
the larger project. Based on experience and theory, the longevity of a beach nourishment project
goes with the square of the project length. In the absence of any other measures, it will be difficult if
not impossible to maintain the desired beach width. As such, when there are encroaching structures
within a given project reach, it will be difficult to maintain a uniform design berm width throughout
the project. Alternatively, a reduced level of protection at the encroaching structure should be
acknowledged or storm protection for the encroaching structure should be provided by other means
(structural or otherwise). If a structural approach is taken, it is important to minimize direct
interaction between the shore protection structure and the surf zone to minimize what little sand is in
the system from being scoured out and transported offshore and downdrift of the beach.
On some projects, the nonuniform size distribution of sand along the project also enhanced the
erosion potential in areas where finer borrow material was placed. Identification and use of multiple
borrow areas with similar sediment characteristics are recommended. If unanticipated changes in
sediment characteristics are found in designated borrow areas during construction, it is advisable to
investigate the possibility of using other borrow areas in order to limit the use of fine grain size fill
material. An unexpected change in borrow area sediment characteristics indicates insufficient
borrow sampling.
Hot spots at the ends of projects were identified in several of the case studies. The erosion and fill
losses at the nourishment project's terminal ends should be anticipated in all projects. These areas
probably should not be considered hot spots at all. That is, the design analysis should specifically
examine and quantify project end losses and expected beach widths in the vicinity of the terminal
ends of the project. The project shareholders and sponsors should be made aware of these
expectations in the design phase prior to project construction. Considerations should be given to add
a terminal structure at the end of a fill if high end losses are anticipated. If a project is to be
terminated adjacent to a terminal groin or inlet jetty, consideration should be given to sand
tightening the structure to prevent loss of fill material through the structure and unwanted deposition
in adjacent navigation channels. Sand tightening can be achieved through reconstruction of the
structure, use of grout, sheet piles, or the placement of sand filled geo-tubes.
28
Previous Page
