AN ASSESSMENT OF THE ACCURACY AND REPEATABILITY OF DEBRIS FLOW VELOCITY CALCULATIONS

For analysis of debris flows, estimates of flow velocity are important for sizing mitigation channels, berms, walls and basins. Velocity estimates influence calculations of run-out distance and impact force and are, thus, reflective of a level of potential damage. Equations for predicting and measuring debris flow velocities are numerous, but many rely on parameters that cannot be measured with precision and/or parameters with an unknown consistency of measurement. In addition, due to the vast number of equations, it is difficult to know which to use and under what conditions. Therefore, a preliminary study has been done to assess the accuracy, repeatability, and applicability of such calculations.

The methods of calculating debris flow velocities were divided into three groups by similarity of calculation and parameters. Each group was evaluated using published measurements and recent measurements taken in the field. An analysis of relative accuracy was completed from these evaluations. Two separate sets of data were run through each method of velocity calculation and results were compared to each other using a 1:1 line and R2 values. From these evaluations, key sources of error were identified in each of the groups. The main source of error was the difficulty collecting reliable values for the radius of curvature along the debris path. Next, a sensitivity analysis was completed by changing input parameters incrementally and recording the effect on the velocity calculation. From this information, recommendations are made for field measurements and further investigation into the methods of debris flow velocity calculation, including general guidelines for measurement of input parameters and the possible exploration of a consistent measurement method of the ‘radius of curvature' parameter.