Cinder movement experiments on scoria cones slopes : Rates and direction of transport

As part of a field experiment to examine slope processes, four experimental grids with painted and numbered cinders were placed on the outer crater rims of two scoria cones in the San Francisco volcanic field, Arizona. Each grid contained 50 cinders placed in five rows of ten each. Rows were placed parallel to local slope contours. The mean diameter for each cinder was 1.7±0.2 cm (n = 200) and the average grid slope was 20.2°. Grids were set in July 1992. They were revisited one month later in August 1992 and again two years later in August 1994. Although several cinders failed to show any movement in the August 1992 survey, the average length of movement was 11.2 cm (n = 118). If the total movement is averaged over the 197 cinders that were relocated, the average length of movement then becomes 6.7 cm (n = 197). All cinders showed movement in the August 1994 survey and the average distance of movement was 32.8 cm (n= 141 with 59 missing cinders). The mean annual rate of movement after 25 months was 15.8cm/yr(n= 141). Using the convention that the 180° azimuth direction is downslope and perpendicular to local slope contours, the direction of cinder movement more closely approaches 180° with an increase in time. The azimuth directions calculated after just one month of emplacement display greater scatter and variability than the more correlated results measured after 25 months. The mean azimuth value after the 1992 survey was 163.6±54.2° (n= 118), while the mean azimuth after the 1994 survey was 177.9±20.7° (n = 141). Several painted cinders displayed upslope movement when the grids were first visited after one month. However, after two years the cumulative movement for every cinder was downslope from its original position. Non-channel overland flow is interpreted to be the primary erosional agent responsible for moving the cinders in the downhill direction. Rainsplash is interpreted to be responsible for moving the cinders in the upslope direction and is believed to be the major contributor to the variability in the azimuth measurements.