It is essential to determine the changes in the hydraulic characteristics of mud flows in order to better understand the flow mechanism and to develop efficient measures. We successfully observed the behavior of large]scale mud flows triggered by the bursting of a crater lake on Mount Ruapehu, New Zealand on March 18, 2007. We collected various images of the mud flow, including serial static pictures taken by our automated camera systems and video footage taken by New Zealand authorities and local television companies. Hydraulic calculations, including image]analysis, were performed to reveal mud flow flowing characteristics and to create mud flow hydrographs. Then these calculations were used to reveal travel distance.
The peak discharge of the mud flow exponentially decreased with travel distance. The remarkable peak discharge decrease at Rangipo Desert can be explained by a gradient decrease, mud flow separation, and underground water infiltration. The mud flow behaved as a turbulent debris flow in the lower reach of the Tranzrail Gauge Station. The continuous drift currents of the mud flow between Strachans Bridge and Kauangaroa Station may suggest that one factor contributing to the decrease in mud flow energy was the friction of the bending river banks.

Key wordsFcrater lake burst]induced mud flow, observation, hydraulic characteristics changes, hydraulic calculation