5.
Alluvial-fan hazard
An alluvial fan is an accumulation of river or stream (alluvial) sediments that form a sloping
landform, shaped like an open fan or segment of a cone (Figure 5.1). They form where rivers
or streams exit a valley, allowing sediment-laden flows to spread over a broad area. As these
flows exit the confines of the valley, they lose energy, and their ability to carry sediment
decreases. This results in the deposition of layer upon layer of sediment along the boundary
of the hill slopes and valley floors. Figure 5.2 shows an example of an alluvial-fan feature on
the Taieri Plains.
CATCHMENT
ALLUVIAL FAN
CONFINED
VALLEY
FORMER CHANNELS
Channel migration
(avulsion) between
successive events.
CURRENT
CHANNEL
ALLUVIUM
BEDROCK
Figure 5.1
Stylised image of an alluvial fan.
Alluvial fans are composed of sediments derived from the rock and soils within the wider
alluvial-fan catchment. While not part of the alluvial fan itself, the catchment plays an
important role in initiating and sustaining alluvial-fan activity as it is the source of material
for development of the fan. Water from the catchment flows downslope across the fan
surface. Depending on the characteristics of the source catchment and the available sediment
supply, these flows may also transport and deposit sediment down and across the alluvial fan.
Alluvial fans can experience long periods of inactivity, often from decades to centuries, only
to re-activate due to extreme or unprecedented events, or due to environmental change (such
as tectonic uplift or changes in the catchment sediment supply, climatic conditions and/or
river/stream incision). Their often benign appearance between events does not give a full
appreciation of their potential hazardous nature. On the Taieri Plains, fan streams are often
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Ch 5. Alluvial-fan hazard
ephemeral or inconspicuous, creating the impression that little or no hazard exists. In fact,
these areas are often considered attractive places for development, with their elevated profile
providing striking views and good on-site drainage. A limited understanding of alluvial-fan
hazard may, therefore, lead to problems if these areas are preferred locations for development
rather than areas with more obvious hazards (such as flooding across the surface of the
plains).
Figure 5.2
Looking east across the lower reaches of an active floodwater-dominant
alluvial fan that drains from the Maungatua Range (location of photograph is shown in
Figure 5.3).
The main hazards associated with alluvial fans can include debris flow, debris flood,
inundation by floodwater flow, sedimentation, erosion and/or channel migration across the
fan (Opus, 2009).15 These processes can impact on the long-term shape, elevation and
characteristics of a fan surface and hence on how the surface of the fan is used. Furthermore,
flood and debris flows generally occur suddenly without warning and can be unpredictable
and destructive.
Alluvial-fan landforms throughout Otago, with a surface area greater than 0.5km2, including
the Taieri Plains (Figure 5.3), have been mapped (Opus, 2009). Alluvial fans on the Taieri
Plains are most common along the margins of the basin where sediments have been deposited
by streams draining the Maungatua, North Taieri and Titri fault scarps (Section 7) (Bishop
and Turnbull, 1996). The fans located along the boundaries of the Titri and North Taieri
faults are generally smaller and have experienced greater deformation than the Maungatua
fans. This is probably a reflection of the higher elevation, the widely developed landslides,
and the higher rainfall and erosion rates experienced along the Maungatua fault scarp (see
Figure 7.1 for fault locations) (Barrell et al., 1998).
Fans have been classified based on the activity and type of depositional processes that have
formed the fan surface, as these criteria reflect the scale and significance of the hazard.
15
See the Glossary for further explanation of these and other terms used within this report.
Ch 5. Alluvial-fan hazard
69
Alluvial fans on the Taieri Plains are mainly floodwater-dominant alluvial fans, experiencing
sheet and channel floods. No debris-dominant alluvial fans have been identified on the
margins of the Taieri Plains.
Floodwater-dominated alluvial fans, like those on the Taieri Plains are characterised by
water-dominated processes. During heavy rainfall, sediment-laden water from the alluvial-fan
catchment is concentrated into channels flowing downslope, eventually crossing the alluvialfan surface. Figure 3.6 shows the location of ephemeral swales on the north-eastern section of
the Taieri Plain that act in this manner.16 Larger watercourses and features, such as open
drains, may also capture and re-direct this sheet-flow across the Taieri Plain. Where flow is
unable to be contained within these channels (due to limited channel incision or high
magnitude ‘super-design’ flow events), floodwater spreads laterally across the surface of the
fan as sheet flow. As the fan’s gradient reduces and the flows begin to lose velocity, fine
sediment suspended in the flows is deposited across its surface.
Figure 5.3
Alluvial-fan deposits greater than 0.5km2 on the Taieri Plains (Opus
2009). The red circle indicates the approximate location of Figure 5.2.
The active floodwater-dominated alluvial fans on the east of the plains are those of Mill
Creek, Silver Stream, Owhiro Stream, Quarry Creek, Gracie Stream and several smaller
creeks that drain the hill catchments. They extend across areas of high-density, residential
land use, including much of Mosgiel (Figure 5.3). Despite being identified as ‘active’ (Opus,
2009), there is little evidence of recent active sediment deposition across the fans, which may
be due to a lack of recent storm events of sufficient magnitude to initiate alluvial fan activity,
16
Note that these features also exist elsewhere across the Taieri Plain (including the Mosgiel urban area), but
they have not been mapped.
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Ch 5. Alluvial-fan hazard
or to significant changes on the fan surface caused by human activity. Channels and open
drains are now generally well incised into the fan surface, and the lower reaches of these fans
have been modified due to changes in land use, stormwater and other works, such as the West
Taieri Contour Channel (Figure 4.15).
Alluvial fans always present an element of hazard due to their unpredictability. They may
continue to evolve in response to modifications to their upstream catchment (e.g. where
changes in land use, landslide activity or earthquakes affect the availability of sediment) and
changes in climate. Should long-term climate change bring about a change in rainfall
intensity or storm frequency, this may affect alluvial-fan activity on the Taieri Plains.
Ch 5. Alluvial-fan hazard
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