Geoscience Reference
In-Depth Information
Case study 7.2 The impact of dams on deltas and estuaries: the Nile delta and the Aswan High Dam
BACKGROUND
The Aswan Dam was built in order to provide irrigation and drinking water, via the construction
of a large reservoir (Lake Nassar), to the Nile catchment, and also to facilitate the development
of hydroelectric power to the growing population of Egypt (Rashad & Ismail 2000). Such a
supply was considered vital for a country whose population (
c
. 48 million) had complete depend-
ence on the Nile. Although its completion in 1964 facilitated this, environmental impacts were
also common, not least in the delta area downstream of the dam. This was not the first structure
to be used to regulate water along the Nile, however, with various schemes, such as construction
of the Aswan Low Dam in 1902, having gradual accumulatory impacts on the delta.
As with any coastal landform, deltas undergo alternate constructive and destructive episodes
(see Fig. 7.11 for the example of the Mississippi). Cores taken in the area have shown that there
have been a series of deltas at the mouth of the Nile, which have accreted and eroded throughout
Pliocene and Pleistocene times (i.e. over the past 5 million years) (Stanley & Warne 1998), with
the present delta starting to form
c
. 8000 to 6500 years ago. In the early stages of the formation
of the present delta, seven major channels drained across the delta surface, each with its own
lobe at the coast (Badr & Lofty 1999). Significantly, by 2000 years ago, two of these had dis-
appeared, and by 1000 years ago, a further three had gone (McManus 2002), leaving the two
we now recognize as the Rosetta and Damietta. This loss of channels was partly the result of
natural evolution of the delta, but also early attempts at irrigation and management.
Over the past 150 years, the delta has been in an erosional phase (Stanley & Warne 1998)
with the majority of the 260 km long delta face undergoing erosion and landward retreat (Frihy
1996). Only small areas of localized accretion now occur, such as in association with jetty
construction at Damietta Harbour (El-Asmar & White 2002) (Case Fig. 7.2). The reason for
the widespread erosion has, to a great extent, been more organized water regulation, which
has affected the balance between sediment input, coastal sediment drift and subsidence in the
delta area. Despite earlier management efforts, the Aswan High Dam has, arguably, had the
greatest impact on delta decline. Pre-closure in 1964, the Nile supplied
c
. 124 million tonnes of
sediment a year to the delta area (Carter 1988). Such a flow of both water and sediment was
sufficient to maintain the two major channels, but also a series of minor distributary channels,
which provided an annual deposit of fertile silt on farmland. Following closure, the amount of
sediment delivered per year dropped to
c
. 50 million tonnes (Carter 1988). This sediment was
notably finer grained than pre-closure (Stanley & Wingerath 1996), and largely derived from
the river downstream of the dam, and also from the delta hinterland itself. More significantly,
however, the volume proved insufficient to compensate for that lost to erosion. As a result, the
delta front became net erosional.
As well as reduced sediment load, the reduced river flow led to the silting up of many of the
distributary channels, leading to major changes in delta morphology and ecology (Case Fig. 7.2).
For example, reduced discharge and the easterly drift of sediment at the coast blocked many
of the smaller estuaries (Frihy 1996). Chesworth (1990) shows that of the
c
. 55.5 billion m
3
of
water released from the dam, only 17.5 billion m
3
reaches the delta coast, the rest being either
extracted or lost to evaporation. Of the two main channels, the Damietta and Rosetta, the
former is generally dry for much of the year, and the latter carries the bulk of what flow remains
(Bird 1985).
