Precision and Accuracy in Debris-Flow Volume Measurement

Measurements of debris-flow volumes are important for design and maintenance of retention basins and conveyance structures such as bridges and culverts, for analysis of inundation and run-out, and for input into predictive models. While these uses require accurate volume measurements, reported values in the technical literature do not consistently include estimates of accuracy, precision, and error. The purposes of this study are to systematically quantify these parameters so that they can be optimized, and to identify the most trustworthy types of measurements at different expected volume ranges. The most accurate and inexpensive measurement method uses capacity curves for debris retention basins. Precision of capacity curves is dramatically improved by installing finely graduated height scales in the basin to carefully estimate the contained volume after an event. For small-volume events (a few thousand cubic meters), measurement of channel cross sections to estimate debris-flow scour for runoff-generated flows is an accurate, but labor-intensive, method of estimating debris-flow volume. Map measurement of debris deposit extent typically results in underestimation of volume, with large error ranges resulting primarily from difficulty in accurately estimating the thickness of the deposit. Truck counts typically result in overestimation of volume, but accuracy could be considerably improved by accounting for percentage under-or over-loading rather than rounding the count up or down. Analyses or designs that use measured debris volumes should recognize that there are inherent errors: Even in the best case, error can be expected to be at least +/- 4 percent. In most cases, regardless of the method of volume measurement, error can be expected to be much higher, typically at least +/- 10-20 percent.