Sedimentology and flow behavior of a rain-triggered lahar, Mangatoetoenui Stream, Ruapehu volcano, New Zealand

On October 28, 1995, heavy rain triggered failure of approximately 3.3 × 10 5 m 3 of fall deposits on the Mangatoetoenui Glacier and generated a lahar event that flowed east-northeast of Ruapehu volcano, down the Mangatoetoenui Stream, and eventually entered the Tongariro River 19 km downstream. The lahar left multiple stacked deposits of at least three major flow surges, or pulses, which traveled down slope at calculated velocities of as much as 27 m s −1 and calculated maximum peak discharges of as much as 2900 m 3 s −1 . The source area of the Mangatoetoenui lahar is delineated by a thin furrowed lag of unsorted ash and lapilli margined by steep headwall scarps marking the edges of undisturbed fall deposits. Failure of fall deposits in the source area triggered debris flows down a north and south branch of the Mangatoetoenui Stream. These two debris flows coalesced and ponded, then flowed across seasonal snowpack covering the stream. In the proximal zone, to 5 km downstream of the source area, the deposits of the individual constituent debris flows cannot be distinguished. However, in the medial zone, between 5 and 16 km downstream of the source area, and beyond the limit of the snowpack and the junction with perennial streamflow, the depositional record of this lahar shows clear evidence for downstream transformation of individual peak flows from debris flow to hyperconcentrated flow. Transformation began at 5 km from source due to dilution (1) by each peak flow incorporating normal streamflow in the perennial Mangatoetoenui Stream and tributaries, and (2) through deposition of part of each peak flow9s sediment load. The hyperconcentrated-flow deposits frequently may be subdivided into multiple units, which is evidence that the lahar flowed as a series of surges or waves. We consider that these were caused by episodic, but contemporaneous, inputs from a divergent flow from a single source area or by ponding and episodic release of debris within the channel. Estimated Froude numbers for the Mangatoetoenui lahar are greater than 3, implying supercritical flow and the development of fluid mechanical instabilities that enhanced these flow surges.