Environmental Engineering Reference
In-Depth Information
4.4 Design Considerations
4.4.1 Introduction
There are a number of significant specific design considerations when planning
beach renourishment. In many cases, design will be limited, but it can be complex,
assessing a range of criteria in detail. For example, beach renourishment design
can vary from basic forms, such as the placement of sediment on the subaerial
beach re-using dredged sediment, through to complex design, considering multi-
ple factors such as the local rate of erosion, the natural beach slope, native and
imported sediment grain sizes, wave climate and tidal levels, existing structures
and infrastructure, and past engineering activities in the area. In addition the tim-
ing and effects of multiple renourishments can be considered (Dean
1995
). Design
considerations concerning grain size and grading, profile shape and volume, and
planform shape (all strongly interlinked) are now summarised. Detailed discussion
of all design considerations is beyond the scope of this textbook, but further tech-
Where possible, beach renourishment design should take as much information
as possible from the existing site, including erosion history. Mention has been made
of the Dutch Method of design (Sect.
4.3.7
, p. 72), described by Verhargen (1992).
This is used in most beach renourishment projects in the Netherlands, and follows
five steps: (1) regular (at least annual) surveys of beach profiles over a period of at
least 10 years; (2) calculation of annual sand loss in each coastal sector; (3) addi-
tion of 40 % to offset losses as the profile adjusts to greater beach width extending
into deeper water; (4) multiplication of this quantity by an duration; and (5) placing
of sediment between the dune foot and a line 1 m below low tide.
4.4.2 Grain Size and Grading
It is necessary to consider the calibre of the sediment needed for a beach renour-
ishment (USACE
2002
) because the median grain size and sorting of sediment
influences the stability of the renourished beach (CIRIA
2010
). It has long been
known that beach renourishment is most effective when an appropriate grain-sized
sediment is used (Newman
1976
). Sediment used for renourishment should have
similar grain size characteristics to the natural beach, for excessively fine sediment
is soon lost offshore or alongshore and excessively coarse sediment may form too
steep a beach, promoting nearshore reflection scour (Krumbein
1957
) and devel-
oping a beach profile unsuitable for recreational use (Blott and Pye 2003). In gen-
eral, coarser sediment is likely to persist longer on a beach, and finer sediment
to be lost more quickly (Dean
1983
; Bird
1996
; Nordstrom
2000
; Komar
2007
;
Kumada et al.
2007
). This has led to a number of renourishment projects incor-
porating coarser sediment than the native sediment. Monitoring of renourished
beaches on the shores of Lake Michigan, in the United States, has shown that
