Environmental Engineering Reference
In-Depth Information
4.5 Monitoring Changes After Beach Renourishment
Changes will occur on beaches that have been renourished. These beaches will be
eroded by the same processes that depleted the preceding natural beaches (Riddell
and Young
1992
). It is often observed in practice that erosion, sediment volume
loss and coastline retreat, are greater on a renourished beach than the histori-
cal rates on a natural beach (Dean
2000
). As a result it is impossible to meet the
expectations of stakeholders who expect renourishment to last indefinitely. Most
renourished beaches begin to lose sediment as soon as they have been emplaced,
some of the beach material being washed or blown away alongshore, some being
swept to the backshore and beyond, and some withdrawn to the sea floor.
Monitoring of coastal processes and morphology following beach nourish-
ment is necessary to gain understanding of the underlying causes of beach erosion,
and improve subsequent project design (NRC
1995
), while providing guidance
and calibration for numerical models (Dean
2002
). Monitoring and mapping are
also necessary to determine the rates and patterns of sediment losses, indicating
when and how much supplementary beach material is required and where it should
be placed (Foxley and Shave
1983
). Changes are usually measured by making
repeated surveys along transverse profiles from the back of the renourished beach
out on to the nearshore sea floor, and linking these by alongshore surveys. The use
of remote sensing techniques is also common, such as LIDAR survey and satel-
lite imagery. Supporting evidence is usually available from sequential air photo-
graphs and video imagery has also been used (Elko et al.
2005
, Ojeda et al.
2008
).
In the absence of sufficient profile survey data or wide-coverage satellite imagery
Raman Murthy et al. (
2008
) used remote sensing to monitor coastline changes fol-
lowing a beach renourishment project on the southeast coast of India. Gares et al.
(
2006
) examined the use of LIDAR data for beach renourishment monitoring at
Wrightsville Beach, North Carolina, and demonstrated that this information pro-
vides data for use in both horizontal (shoreline) and volumetric (sediment budget)
analyses of changes following renourishment.
Multiple sources of data can be useful when monitoring changes following a
renourishment. Since the completion of the 'sand engine' project on the Dutch
coast (Sect.
4.3.2
, p. 62, 63) in 2011, the topographic evolution of the renourished
profile has been monitored monthly using a purpose-built jet-ski mounted with a
global positioning system and an echo- sounder, in addition to four-yearly coastal
profile measurements (Stive et al.
2013
). Furthermore, two high-resolution video
cameras have been installed overviewing the 'sand engine' and adjacent beaches,
and regular aerial photographs are collected.
The aim of monitoring is to understand the processes that erode and distribute
emplaced beach material. Such information can guide future beach management,
such as the insertion of groynes, the introduction of regular renourishment updrift
on beaches that are losing sediment alongshore, localised renourishment at places
of severe erosion, or the need to repeatedly restore the profile of a beach that is
losing sediment offshore.
