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 x 10 (5) m(3) of fall deposits on the Mangatoetoenui Glacier and generated a lah ar event that flowed east-northeast of Ruapehu volcano, down the Mangatoeto enui Stream, and eventually entered the Tongariro River 19 km downstream Th e lahar left multiple stacked deposits of at least three major flow surges, or pulses, which traveled down slope at calculated velocities of as much a s 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 furrowe d lag of unsorted ash and lapilli margined by steep headwall scarps marking the edges of undisturbed fall deposits. Failure of fall deposits in the so urce area triggered debris flows down a north and south branch of the Manga toetoenui 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 de posits 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 sno wpack and the junction with perennial streamflow, the depositional record o f this lahar shows clear evidence for downstream transformation of individu al peak flows from debris flow to hyperconcentrated flow. Transformation be gan 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 flow's sediment load, The h yperconcentrated-flow deposits frequently may be subdivided into multiple u nits, which is evidence that the lahar flowed as a series of surges or wave s. We consider that these were caused by episodic, but contemporaneous, inp uts from a divergent flow from a single source area or by pending and episo dic release of debris within the channel. Estimated Froude numbers for the Mangatoetoenui lahar are greater than 3, implying supercritical flow and th e development of fluid mechanical instabilities that enhanced these flow su rges.