Flood risk measurment [i.e. measurement] on alluvial fans : a laboratory study : A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering in the University of Canterbury [Lincoln College], New Zealand

Abstract

In spite of the application of active protection measures against floods, snow avalanches, and torrents, increasing damage has been observed in several countries. It has been realized that passive measures should be included in a protection scheme to decrease the damage potential. An important basis for formulating passive and designing active protection measures is the risk or hazard map. The methods available for formulating hazard maps on torrent fans or on alluvial fans are based either on observations or on calculations. In laboratory conditions three alluvial fans with differing input conditions were redeveloped. With the help of video film the frequency distrlbutions of active channels at grid squares on the alluvial fans were determined. The frequency distribution over a long observation period (equivalent to the long term risk distribution) was found to be related to the 3-dimensional shape of an alluvial fan. The long term risk at a point is a functlon of the absolute location of that point on the alluvial fan. The shorter term risk a is function of the absolute location of the point on the alluvial fan, aswell as of its location relative to the channel position, and of the shape of the alluvial fan and its slope condition. Two major processes of fan formation were observed which influenced the short term risk distribution: The entrenchment-aggradation process, and the channel migrating process which occurred as a result of concave bank erosion. The applicability of the available methods is discussed in general terms, for alluvial fan with low land value, and in areas where neither dlscharge nor previous damage events are known to have occurred formulating risk maps on such alluvial fans is suggested.