478
Climatic Geomorphology
through the system. This is a critical energy threshold of the stream flow (Bull, 1979).
Temporal variations can cause modifications to the sedimentation velocity and even the
aggradation or dissection of the fan surface.
Alluvial fans present serious difficulties to human activity (Beaty, 1974; French, 1987).
Flows are rare and unpredictable, and there can be wide variation within and between
successive flows. Further, they can be dominated by water or by detritus which can even
block channels. They may also undergo important migration and avulsions are common
(Cooke, 1984).
The risk of flooding is evident, but not all parts of fans are subject to the same degree.
The risk is extreme along the active channel, moderate at the head and distal areas of the
fan, and low in the mid sector (Kesseli and Beaty, 1959). A common planning and
engineering solution is to design and maintain a single channel for the flow (Graf, 1988).
But the best defence against flooding is to identify the most dangerous areas and make
flood risk maps such as those produced for the city of Suez and its environs (Cooke et al.,
1982). These maps help to determine construction densities and to situate projects away
from the channels (Fig. 19.15). Sediment deposition can also be fought using check dams
and by dredging the channels in the catchment area. Flood alert systems can also be
installed (Porath and Shick, 1974).
The channels running across fans are unstable and frequently overflow because of
boulder sized sediment on their bottoms. This creates new channels which on occasion
form part of abandoned channels. Important floods occurred in the south of Jordan in 1966
that partially destroyed the town of Maan, killing 70 people. The cause was mainly the reuse of an abandoned channel by the rising water (Graf, 1988).
Debris flow is one of the most common types of mass movement in semi-arid areas. It
can be caused by intense rainfall moving abundant colluvial material through channels and
over alluvial fans (Johnson and Rodine, 1984) and depositing material in areas with gentle
slopes - commonly at the head of the fan. The distinction between the deposits made by
debris flows and fluvial processes is not always easy to make because there is a continuous
gradient between them (Hooke, 1987). Both are generated by concentrated flows of large
transport and erosion capacities, but with different viscosities.
In their movement, debris flows can uproot vegetation, cover roads, channels and railways,
drag along cars, destroy constructions and cover fields (Hooke, 1987). In semi-arid areas with
clay soils, dangerous mud flows can occur that become channelled down fills and slopes.
Debris flows can be started by landslides, along with materials supplied from other
sources and by a sudden saturation in detritus (the fire-hose effect) (Johnson and Rodine,
1984). Debris flows are very common in the semi-arid regions of the world and have been
frequently studied, especially in California. In the La Canada Valley, Los Angeles County,
debris flows occurred in 1934, 1938 and again between 1962 and 1971, causing the deaths
of 63 people and important material losses (see Johnson and Rodine, 1984).
The area around Gissar in Tajikistan, close to the Afghanistan border, suffered an
earthquake in 1988 that caused landslides on loess hills. The deposits were saturated by
irrigation water and in the initial moments reached a liquid state producing a debris flow
that moved some 20 million m 3 of material a distance of 2 km. Houses on the outskirts of
the town were buried (Ishihara, 1989).
In some areas, urban development reaches up to the apex of alluvial fans, allowing
detritus and mud flows to extend beyond their source area. Urbanized substrates facilitate
Climatic Geomorphology
through the system. This is a critical energy threshold of the stream flow (Bull, 1979).
Temporal variations can cause modifications to the sedimentation velocity and even the
aggradation or dissection of the fan surface.
Alluvial fans present serious difficulties to human activity (Beaty, 1974; French, 1987).
Flows are rare and unpredictable, and there can be wide variation within and between
successive flows. Further, they can be dominated by water or by detritus which can even
block channels. They may also undergo important migration and avulsions are common
(Cooke, 1984).
The risk of flooding is evident, but not all parts of fans are subject to the same degree.
The risk is extreme along the active channel, moderate at the head and distal areas of the
fan, and low in the mid sector (Kesseli and Beaty, 1959). A common planning and
engineering solution is to design and maintain a single channel for the flow (Graf, 1988).
But the best defence against flooding is to identify the most dangerous areas and make
flood risk maps such as those produced for the city of Suez and its environs (Cooke et al.,
1982). These maps help to determine construction densities and to situate projects away
from the channels (Fig. 19.15). Sediment deposition can also be fought using check dams
and by dredging the channels in the catchment area. Flood alert systems can also be
installed (Porath and Shick, 1974).
The channels running across fans are unstable and frequently overflow because of
boulder sized sediment on their bottoms. This creates new channels which on occasion
form part of abandoned channels. Important floods occurred in the south of Jordan in 1966
that partially destroyed the town of Maan, killing 70 people. The cause was mainly the reuse of an abandoned channel by the rising water (Graf, 1988).
Debris flow is one of the most common types of mass movement in semi-arid areas. It
can be caused by intense rainfall moving abundant colluvial material through channels and
over alluvial fans (Johnson and Rodine, 1984) and depositing material in areas with gentle
slopes - commonly at the head of the fan. The distinction between the deposits made by
debris flows and fluvial processes is not always easy to make because there is a continuous
gradient between them (Hooke, 1987). Both are generated by concentrated flows of large
transport and erosion capacities, but with different viscosities.
In their movement, debris flows can uproot vegetation, cover roads, channels and railways,
drag along cars, destroy constructions and cover fields (Hooke, 1987). In semi-arid areas with
clay soils, dangerous mud flows can occur that become channelled down fills and slopes.
Debris flows can be started by landslides, along with materials supplied from other
sources and by a sudden saturation in detritus (the fire-hose effect) (Johnson and Rodine,
1984). Debris flows are very common in the semi-arid regions of the world and have been
frequently studied, especially in California. In the La Canada Valley, Los Angeles County,
debris flows occurred in 1934, 1938 and again between 1962 and 1971, causing the deaths
of 63 people and important material losses (see Johnson and Rodine, 1984).
The area around Gissar in Tajikistan, close to the Afghanistan border, suffered an
earthquake in 1988 that caused landslides on loess hills. The deposits were saturated by
irrigation water and in the initial moments reached a liquid state producing a debris flow
that moved some 20 million m 3 of material a distance of 2 km. Houses on the outskirts of
the town were buried (Ishihara, 1989).
In some areas, urban development reaches up to the apex of alluvial fans, allowing
detritus and mud flows to extend beyond their source area. Urbanized substrates facilitate