Flow transformations and transitions

Lahars are a common feature in volcanic terrains. Their formation requires an adequate water source, abundant unconsolidated debris, substantial relief at the source and a triggering event (Vallance 2000). Lahars may be of primary (syn-eruptive) or secondary (post-eruptive or unrelated to eruptive activity) origin and can be initiated by a variety of mechanisms (Neall 1976; Waitt et al. 1983; Cronin et al. 1997; Mothes et al. 1998; Hodgson & Manville 1999; Lavigne et al. 2000; Vallance 2000; van Westen & Daag 2005; Németh & Martin 2007). Lahars can be triggered by pyroclastic flows entering streams, phreatomagmatic explosions or directed blasts (Neall 1976b, Vallance 2000). Furthermore, lahars can form after sudden melting of a glacier or snow and ice by hot pyroclastics, lava flows, or growing lava domes, as well as an elevated heat gradient due to hydrothermal and magmatic fluids in the upper edifice (Janda et al. 1981, Pierson et al. 1990; Pierson & Janda 1994, Branney & Gilbert 1995, Cronin et al. 1996a, Thouret et al. 1998, Manville et al. 2000, Stern 2004; Major et al. 2005). Some flows are associated with existing crater lakes and are produced by an eruption through the crater lake or from a non-volcanic crater lake rim failure (Cronin et al. 1996a; 1999, Lecointre et al. 1998, Manville et al. 1998). Rain-triggered lahars often occur after large explosive eruptions, e.g. Mt. Pinatubo in the Philippines (Newhall & Punongbayan 1996; Chorowicz et al. 1997, van Westen & Daag 2005, Carranza & Castro 2006) or Mt. Merapi in Indonesia (Lavigne et al. 2000, Lavigne & Thouret 2002), and represent a major hazard especially in equatorial latitudes and temperate zones of high rainfall.

Debris flows can form by a variety of transformations from different kinds of flows, i.e. by water incorporation from pyroclastic flows and debris avalanches as well as from floods through sediment erosion and entrainment (Neall 1976a; Janda et al. 1981; Waitt et al. 1983; Scott 1988a; Pierson et al. 1990; Pierson & Janda 1994; Branney & Gilbert 1995; Newhall & Punongbayan 1996; Mothes et al. 1998; Thouret et al. 1998; Hodgson & Manville 1999; Lavigne et al. 2000; Vallance 2000; Lavigne & Thouret 2002; Major et al. 2005; van Westen & Daag 2005; Carranza & Castro 2006; Németh & Martin 2007). The sediment-water ratio within a flow can change longitudinally due to bulking or depositional processes as well as laterally, due to flow depth and velocity variations.

Confined lahars with low clay contents typically transform from debris flow to hyperconcentrated flow at some point along their path due to progressive incorporation of water

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and continuous sedimentation (Janda et al. 1981; Pierson & Scott 1985, Lowe et al. 1986, Smith 1986, Major & Scott 1988; Scott 1988a, 1988b; Alloway 1989; Major & Newhall 1989; Smith & Lowe 1991; Scott et al. 1995; Major et al. 1996; Pringle & Cameron 1997; Thouret et al. 1998; Cronin et al. 1999, 2000; Lavigne et al. 2000; Lavigne & Suva 2004; Major et al. 2005). With distance and further dilution, more sediment is transported as bedload, deposits are more distinctly stratified and high-angle cross-bedding can be formed (Cronin et al 2000), which reflects gradation into normal streamflow (Pierson & Scott 1985; Scott 1988a; Scott et al. 1995). This gradual dilution and transformation process only affects lahars that are small compared to the volume of available water along the river catchment; it does not significantly influence the rheology and flow behaviour of large-volume lahars (Cronin 2000, Lavigne & Thouret 2000).

Pierson & Scott (1985) suggested that the progressive head to tail dilution of the lahar with stream water leads to the development of a hyperconcentrated flow phase preceding the main debris flow body. The fluid component of the flow separates from and outruns the sediment-rich component, producing a lag between the two flow components (Scott 1988a). A later study by Cronin et al. (1999) showed that the initial portion of a lahar wave is stream water pushed along the channel in front of the lahar rather than fluid separated from the bulk flow. The lahar is essentially an invading solution that contrasts with the resident stream water which is pushed ahead. The studied non-cohesive lahars showed four phases: resident stream water pushed ahead of the lahar, a downstream-lengthening mixing zone between stream water and the lahar, the relatively undiluted original lahar, and the tail of the lahar surge. The resident stream water portion indicates that lahars do not entrain and mix perfectly with water sources in their paths. The increasing lag between peak stage and peak sediment concentration described by Scott (1988a) is hence probably due to lengthening of the mixing interval between streamflow and lahar with distance from source.

A distinct change in slope or widening of the channel can cause a more abrupt transformation from debris flow to hyperconcentrated flow (Cronin et al. 2000). For example, the 25/09/95 lahar from Mt. Ruapehu, New Zealand, spread out over a broad area when it exited a confined gorge onto the Whangaehu fan, which led to a loss of flow competence and flow depth encouraging sediment deposition. In contrast to dilution by addition of water, this type of flow became water-rich through a sudden loss of velocity and a consequential loss of sediment load (Cronin et al. 2000). Under certain circumstances, i.e. strong erosive energy of the flow in combination with a high supply of loose sediment along the flow path, a hyperconcentrated flow can bulk up and transform into a debris flow (Costa 1984; Pierson & Scott 1985; Scott 1988a, 1988b; Smith & Lowe 1991; Scott et al. 1995; Major et al. 2005). A transition from

Chapter 3. Sedimentology 115

hyperconcentrated flow to debris flow can also occur due to rapid loss of water through infiltration into the surface of the underlying volcaniclastic and fluvial sequences (Smith 1986).

The Taranaki deposits appear to have been produced by a range of all the different flow types within the sediment-water spectrum described above. Their characteristics, transitions and lateral changes exposed in cross-section in the coastal cliffs of the Taranaki peninsula were documented in order to construct a spatial-temporal model of ring plain evolution, and to interpret the flow and sedimentation process operating over various time and spatial scales.

3.4.

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