Alluvial Fans

An alluvial fan is a build up of river or stream sediments which form a sloping landform, shaped like an open fan or a segment of a cone. Flooding on alluvial fans can be damaging as the fans have steeper gradients than river floodplains. 

Alluvial fan activity near Flaxmill Creek, Makarora East

Alluvial fan activity near Flaxmill Creek, Makarora East

They typically occur near the boundary between hillslopes and valleys and owe their origins to changes in the slope of natural drainage systems, for example where a steep gully merges onto a flatter valley floor.

The gradient decrease and widening of the flood path where a gully meets the valley floor encourages the deposition of sediment, which accumulates to form a fan-shaped landform. More than 2,000 alluvial fan areas, or 6% of the total land area, have been mapped in Otago.

The fans' elevated profile, with good drainage, make them attractive places for people to live. However, flooding on alluvial fans can be more damaging than other types of flooding because the fans have steeper gradients than river floodplains.


Types of fans

There are two types of alluvial fans; debris dominated and floodwater dominated. 

Debris dominated: these fans involve flows of dense viscous mixtures of water, mud, sand, and gravel, mixed with boulders and commonly woody debris. 

Floodwater dominated: During a flood, water will spill across the fan surface. Floodwater flows contain finer sediment than debris flows and cannot transport large boulders. Often water flows down these fans in a thin continuous sheet. These fans are usually not as steep as debris fans, but can still be just as destructive.

The main hazards affecting alluvial fans include inundation by flood water, debris flow and debris flood deposits, channel migration, deposition, and erosion. Considerable buildup of sedimentation may result from alluvial fan floods. However, debris and flood flows are only intermittent and usually occur over decades or centuries.

Alluvial Fans in Otago

Alluvial fans are shown in the Otago Natural Hazards Database. These are shown in three stages: 

Stage 1 - Regional Areas: This first stage was mapped through a regional desktop assessment at a 1:50,000 scale, with hazard areas identified largely from aerial photography and scientific knowledge. The mapped edges of these alluvial fan features are indicative only, and in some places are only accurate to +/- 200 metres. In addition, the Regional Areas only show alluvial fans greater than 0.5km2.

Stage 2 - Selected Areas: The second part of this staged approach was to map alluvial fans at about a 1:10,000 scale, with field checking of all areas identified as active alluvial fans. This provides a more in-depth picture of the nature and characteristics of selected fans. 

Stage 3 - High Hazard Fans: This stage is a subset of 11 fans where existing or future community areas also have a high level of hazard exposure. This sTate summarises the existing catchment and characteristics of each alluvial fan and described the hazard associated with each.

Other information and documents providing relevant information on alluvial fan hazards are available in our technical reports section. In particular, a 2008 scientific paper entitled ‘Principles of Sustainable Development on Fans’ by Davies and McSaveney looks at the need to understand the hazards associated with alluvial fans so these features can be developed sustainably. 

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