Geoscience Reference
In-Depth Information
settlements and communication lines are sited unknow-
ingly in hazardous zones (Foody, Ghoneim and Arnell,
2004). Small and dry drainage channels are often ignored
by urban planners who do not appreciate the flashy na-
ture of their flooding dynamic, the long intervals between
floods and the serious hazard that they present (Laronne
and Shulker, 2002).
Given the unfeasibility of applying standard hydrolog-
ical modelling to dryland flood events (Greenbaum
et al.
,
1998) other geomorphological techniques can be applied
to determine flood hazard indirectly. High flood risks are
found on the piedmont areas of dryland mountains where
high-intensity floods disgorge into alluvial fans and lower
slope plains. Prediction of flood washout zones here is
complicated by the often unchannelised sheet floods and
the sudden changes in stream course that can occur within
the duration of a single flood (Rhoads, 1986; Grodek,
Lekach and Schick, 2000). Rhoads (1986) investigated
flood hazards in such an environment around Scottsdale
in Arizona, where the landscape was formed from an as-
semblage of pediments, alluvial fans, alluvial slopes and
alluvial plains. He used geomorphological evidence such
as variations in slope, depth of channel incision, local re-
lief, drainage texture and drainage density to identify five
distinct flood hazard zones. Rhoads found the most haz-
ardous zones to be floodplains and active alluvial fans.
On floodplains, while the flow was confined by a channel,
high risk was associated with overbank flows and high
flow velocities leading to scour and bank failure. On ac-
tive alluvial fans he found that where the channel was
unentrenched flooding was possible over the entire fan
surface. On such active fans the depth of flow decreases
downstream as the area of flow increases. In this way
the severity of flooding is greatest at the upstream end of
the fan where it intersects with the mountain front, but the
potential for more widespread and less severe flooding in-
creases towards the foot of the fan where flood depths and
velocities are reduced (Rhoads, 1986). Rhoads also notes
the planning considerations that should be applied on such
unstable, active fans, where a minor shift in the channel
position at the intersection point can have dramatic con-
sequences on the location of downfan flooding. Rhoads
(1986) identified the lowest flood risks as being associated
with abandoned alluvial fan segments that had undergone
dissection. Here, while major floods may occur in the en-
trenched fan portions, the dissected interfluves suffered
from only minor shallow sheetfloods.
There is general consensus that flood hazards in dry-
lands can be most efficiently reduced by maintaining nat-
ural drainage channels and infiltration processes as far
as possible (Rhoads, 1986; Schick, Grodek and Lekach,
ing flood peaks and distributing sediment. The zoning of
urban development to avoid surfaces with a high density
of such drainage channels is important (Schick, Grodek
and Lekach, 1997). However, where such development
cannot be avoided, Rhoads (1986) suggests that it should
be kept to low densities and should be permitted first near
the mountain front before proceeding downslope so that
its impact on downslope drainage can be assessed.
23.7
Conclusions
As humans make growing use of dryland resources, haz-
ards associated with aeolian and fluvial processes will
intensify. While some hazards are a result of the natural
processes that operate in dryland environments imping-
ing on human activity, it is clear that the most serious
issues develop where human activity has increased the
erodibility of stable surfaces. In this context, the impact
of aeolian hazards, such as agricultural wind erosion and
dust emissions from drained inland water bodies, far out-
weigh the problems associated with fluvial hazards. Such
wind erosion, while emanating from spatially distinct and
highly localised sources, can cause serious hazards over
huge downwind areas since aeolian dust is not constrained
by topography and can be carried hundreds of kilometres
in turbulent winds. In contrast, fluvial activity, while life-
threatening, causes hazards that are more site-specific,
topographically constrained and limited in downstream
impact as floods are attenuated by transmission losses to
groundwater.
Our appreciation of the impact of human activity on ae-
olian systems has grown enormously over the last 30 years
as research into the issue has flourished, and it is clear
that agricultural activity and water resource use in dry-
lands cannot be undertaken without consideration of the
potential acceleration in erosion that may result. However,
while research has offered some measures to mitigate the
hazard resulting from accelerated erosion, it has also high-
lighted the scientific, practical, financial and sometimes
political complexities involved in identifying and control-
ling hazardous areas.
Typically, human activity in drylands fails to appreci-
ate the fast-changing spatial and temporal dynamics of
aeolian and fluvial processes. Droughts are a natural part
of the system and can be a catalyst for accelerated wind
erosion on poorly managed land; a small, dry drainage
channel may be the conduit for a devastating flood. As
Houston (2006) points out, in deserts '
average condi-
tions do not exist'. Planning for the extremes is the only
way forward to minimise geomorphological hazards in
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