Geoscience Reference
In-Depth Information
extraction and land surface stabilization under
agriculture (Coltorti 1997). Similarly, dams on
rivers have been responsible for reduction of
sediment supply through direct entrapment and
through reduction of transport capacity by redu-
cing water flows (Case Study 8.3), and large-scale
urbanization may reduce sediment discharge
from paved areas. Mining and quarrying may also
locally alter sediment supply. The disposal of
mine waste in Corsica introduced vast quantities
of sediment to the beach systems and caused
shoreline accretion of over 400 m (Bernier et al.
1996). More widespread sediment inputs to the
coastal zone arise from waste disposal - few
beaches do not contain occasional bricks, glass
fragments or other human debris.
Small-scale sediment removal from beaches and
dunes has taken place for a variety of reasons,
including aggregate, construction, fertilizer and
animal bedding. Carter et al. (1991) reported
5000 - 6000 t of sand removal over a 10-year
period from eleven Northern Ireland beaches and
Case Study 8.3 Impacts of reduced fluvial sediment supply to California beaches, USA
The sandy beaches of southern California are an important economic resource. Over the past
few decades many beaches have experienced severe erosion that has reduced their amenity
value and placed human infrastructure at risk. Griggs & Savoy (1985) estimated that > 80% of
the California coast was eroding and
c
. 30% was in the high risk category. The reasons for the
rapid coastal erosion relate largely to reduction in fluvial sediment supply, which was estimated
to yield 70 - 85% of all beach sand (Sherman et al. 2002) (Case Fig. 8.3a). Additional reductions
in sediment supply relate to interruptions of alongshore sediment movement by engineering
structures (mainly jetties) and protection of eroding bluffs by seawalls (Liedersdorf et al. 1994).
The fluvial sediment supply reduction was related to the construction of dams on sediment-yielding
rivers in the steep, arid California hinterland (Case Fig. 8.3b). More than 500 dams impound
more than 42,000 km
2
(38% of the surface area) of California (Willis & Griggs 2003). Sherman
et al. (2002) calculated the sediment retention in 28 dams and 150 debris basins in southern
California and concluded that these structures impounded
4 million m
3
of sediment per year,
equivalent to 3 m
3
of sediment per metre of shoreline in the five southern coastal counties in
the State. Willis & Griggs (2003) determined that 70 dams (13%) were responsible for 90% of
sediment reductions to the coast (Case Fig. 8.3c). Half the dams are old and have lost significant
water retention capacity as a result of sedimentation and are also in need of maintenance. In
view of their reduced functionality, a number of dams have been identified for removal.
In the face of rapid beach erosion, a study on the management options was undertaken
(Coyne & Sterrett 2002). This study identified an annual tax revenue to the State of $4.6 billion
based on beach tourism and recreation. Potential losses in revenue through beach erosion were
calculated at $1 billion in taxes. There was thus a strong economic argument for beach restora-
tion. The study recommended a twofold approach to beach restoration involving opportunistic
beach nourishment and dam removal. Opportunistic beach nourishment involves the emplace-
ment of 'sand of opportunity' that becomes available from construction or excavation. Previous
experience had shown such emplacements to be successful in improving beach longevity. The
more far-reaching recommendation is the removal of dams that are no longer serving a useful
function, in order to increase the natural sediment supply to beaches. The economics of beaches
versus the reduced value of dams was probably an important factor in enabling this manage-
ment strategy to be adopted. A number of studies are presently underway in advance of dam
removal on several sediment yielding rivers.
>
