A novel method for measuring the timing of bed erosion during debris flows and floods

We describe a novel channel bed erosion sensor and the first results of its usage. The sensor is based on a resistance chain built into a column of 5 cm tall cylindrical aluminum elements. The timing and change in resistance as elements are removed from the column are logged outside of the channel. Five sensor columns were placed in the Illgraben torrent bed prior to the start of the 2008 flow season. Before the first debris flow event, 0.1 m of sediment was deposited by small flood events. During the 16 June 2008 debris flow, this layer was eroded, and erosion was therefore first measured 44 min after front arrival and took place progressively over the next 12 min. The results at the erosion sensors are consistent with measurements over the entire reach using traditional surveying methods, and data from the adjacent debris flow monitoring station provide a basis for interpretation. The main limitation is that this sensor records only the rate and timing of the maximum bed erosion and cannot be used to determine deposition and subsequent erosion within a given event.