Another great alluvial river, the Nile, when studied using the same geomorphic techniques
(Schumm and Galay 1994) is found to be much less variable and dynamic (Figure 5.8), but
because of the great population density along its course even minor changes are of
importance. There appears to have been little change of the pattern of the River Nile since
th
mapping in the 18 century, except in the reach between El Saff and Cairo.
A plot of the Nile valley slope (Figure 5.9) shows differences similar to that of the Mississippi
(Figure 5.7), but they are much less. Sinuosity does increase with increased valley slope,
and valley slope actually increases downvalley (Figure 5.10). The change of valley slope
and sinuosity below Qena is undoubtedly the result of the large Wadi Quena, which in the
past has introduced coarse sediments into the Nile Valley. The other changes of slope are
probably related to tectonics, such as faulting at Assiut.
The anticipated impact of the High Aswam Dam on hydrology and sediment loads was a
matter of great concern for engineers concerned with bank stability and potential channel
degradation. Perhaps of greatest concern was the potential for major degradation of the
Nile following construction of the dam. Because degradation has been controversial, it was
studied by many researchers prior to, and after, construction of the dam (Fathy 1956,
Mostafa 1957, Shalash 1980, Shalash 1983, Richardson and Clyma 1980, Gasser et al.
1978, and El-Moatassem and El-Mottaleb 1979). The construction of the High Aswam Dam
commenced in 1963 and proceeded to 1968. Pre-dam estimates of degradation ranged
from 2.0 to 8.5 m (6.5 to 28 ft), but 18 years after the dam was in operation, maximum
degradation was 0.70 m (2.3 ft).
Several factors account for the fact that degradation has been minimal after closure of the
dam. For example, it is important to recognize that during past humid periods in Egypt,
wadis delivered coarse sediments to the valley, which could act as a control of degradation
depending upon their location in the valley and the depth at which they are encountered. At
present, many wadis appear to contribute only relatively fine sediment that can be readily
transported downstream, and the bed of the Nile is sand. However, during wetter periods of
the past, the wadis probably contributed abundant coarse sediment to the river, and even
today wadi flooding must introduce coarse sediments into the river. For example, there is an
abundant supply of sand, gravel, and cobbles in Wadi Qena, which drains a large area to the
north of Qena. A deep trench was excavated in a small wadi that enters the Nile valley at
Khuzam about 30 km (18.5 mi) downstream from Armant. Boulders and cobbles are
abundant in the trench, and such sediments undoubtedly were moved into the Nile during
wetter periods. All of the wadis contain much stored sediment of gravel, cobble, and boulder
size.
Based upon an analysis of borings in the Nile valley, Attia (1954) concluded that within the
valley "coarse deposits composed of coarse sand, sand and gravel, or gravel lie beneath the
fine alluvial deposits." It is well known that only a small percentage of coarse bed material
can armor a bed and it appears that the minimal degradation by the River Nile in response to
the High Aswam Dam is the result of coarse sediment beneath a veneer of sand.
5.10
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