Debris flow behavior: Impacts of compositional variability and channel geometry at Mt. Ruapehu, New Zealand

Brian Kastl
University of Hawaii, SOEST/Geology and Geophysics
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Country: New Zealand
State/Province:North Island
UTM coordinates and datum: none


Climate Setting: Semi-Arid
Tectonic setting: Continental Collision Margin
Type: Process


Debris flows are violent geomorphological hazards that threaten lives and infrastructure near mountains around the globe. Areas surrounding volcanoes are particularly susceptible to these torrents of boulders, sediment, and water due to the instability of volcanic summits. The competence of steep, rocky terrain is often weakened by the flow of acidic water, heated at depth by magma. Such hydrothermal alteration turns solid rock into fragile clays. Entire flanks may collapse, rapidly evolving into mobile debris flows as they travel downslope. At New Zealand's largest volcano, Mt. Ruapehu, such a failure occurred 4.6 thousand years ago and its fluid-like behavior is recorded by deposits called the Mangaio Formation that cover an area of 37 km2 (Figure 1). So how can we predict debris flow inundation of future flank collapses to gauge their threat?

Clay content
For a given collapse volume, many factors will impact the coverage area. High clay content from sites of intense hydrothermal alteration improves cohesion. Particles within debris flows stick together, such that less material is deposited in channels near the source and more material is deposited further downstream where the flow can spread laterally. Inundation area to collapse volume (A/V) ratios are consequently higher for debris flows sourced from more heavily altered flanks (Figure 2). In fact, measurements from volcanic flank collapses around the world suggest that hydrothermally-altered sources generate flank collapses with A/V ratios many times higher than those from unaltered sources (Figure 3). The ratios may also scale with clay content, considering the Ruapehu flank had an anomalously high clay content of 12% and an anomalously high A/V ratio of 530:1.

Water content
Water content plays a key role in enhancing flow mobility as well. Water, both derived from the source and entrained during travel, decreases proximal depositional volumes. It concomitantly increased distal depositional volumes and inundation areas. Figure 4 is an aerial photograph that shows the Mangaio formation deposits as well as more recent 2007 water-rich debris flow deposits. The 2007 debris flow was sourced from a crater lake breakout and maintained a water content of 47-52%. The 2007 debris flow had a volume only 2% of the Mangaio debris flow, but it had a greater water content, allowing it to inundate comparable areas in the channel and on the alluvial fan. Mangaio water content was high enough for full alluvial fan inundation, yet low enough to maintain a sufficient thickness to cover all topographic expression in the alluvial fan.

Channel gradient heavily controls the kinetic energy of debris flows, allowing those on steeper slopes to travel greater distances. They will be more erosive where this energy is high, and more depositional where energy is low. Preservation potential is consequently low for debris flows in steep terrain. However, the Mangaio debris flow overflowed the erosional channel due to its momentum and high mobility in the steep upper slopes of Mt. Ruapehu. Like race cars elevating along a cambered turn, the Mangaio debris flow "super-elevated" along a bend in the channel, such that an instantaneous velocity could be calculated. It was traveling at least 16 m/s (the speed of a hyena in a full sprint) 5 km from source.

Channel geometry
Finally, channel geometry also influences depositional behavior. In combination with gradient, downstream changes in valley width and depth affect flow energy. Over a distance of less than 2 km the Mangaio debris flow drastically changed its emplacement behavior from topography-mantling to infilling. This dissipation of energy occurred in association with a 2.5° reduction in the slope and a 22% valley width expansion. (Figure 5).

Clay and water content, gradient, and channel geometry all impact the depositional behavior of volcanic and non-volcanic debris flows. Other studies quantify their relative controls and integrate this data into simulation-based predictive flow modeling. With these studies, more accurate inundation areas of debris flows can be predicted and hazard assessments can be made.

Associated References

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