Dynamic interactions between lahars and stream flow: A case study from Ruapehu volcano, New Zealand

Three lahars were sampled in the Whangaehu River on the eastern flank of Ru apehu volcano, New Zealand, at 23.5 and 42 km from the source as they were flowing on September 27 and 29 and October 6, 1995, The lahars were generat ed by water explosively ejected from the highly mineralized Crater Lake. We used the chemical contrast between the lahars and resident stream water in their paths to describe a four-phase model of a noncohesive lahar wave in a river channel: (1) ambient stream mater pushed ahead of the lahar in a pr ocess of miscible displacement due to hydrodynamic dispersion; (2) a zone o f mixing between the stream water and the lahar that increases in length wi th distance from source; (3) a remnant of the original lahar, least diluted by stream water that decreases in length and dilutes downstream; (4) the t ail of the lahar surge. Peak discharge occurs at the end of phase 1 as wate r is pushed ahead of the lahar, Peak sediment concentration occurs at the e nd of phase 2, where debris entrainment by the flow is at its greatest (i.e ., in front of the lahar proper). Deposits record only phases 2 and 3 of th e lahar wave, phase 1 flow left only a tide line of organic debris, and pha se 4 deposits were rapidly eroded by later streamflow Downstream dilution b y stream water eventually caused transformation of phases 2 and 3 of the la hars from debris flow to hyper-concentrated streamflow and then to normal s tream-flow as the flows became progressively finer grained and more turbule nt.