A LOW-COST METHOD TO MEASURE THE TIMING OF POST-FIRE DEBRIS FLOWS AND FLASH FLOODS

Data on the specific timing of post-fire debris-flows and flash floods is very limited. This lack of information has made it difficult to quantify precisely the hydrologic triggering conditions of these destructive events. We have developed an approach for recording the timing of debris flows and floods using relatively inexpensive pressure transducers, which are typically used to monitor groundwater levels and streamflow. Although the sensors cannot sample at the sub-second rates used at more advanced debris-flow monitoring sites, they can sample at a rate sufficient to detect the passage of most debris flow surges and floods. Post-event examination of flow deposits is used to distinguish whether the measured event was a debris flow or flood. The pressure transducers are installed into holes drilled vertically in exposed bedrock sections of the channel bed. The limited exposure to the flow increases the chances of survival during large debris flows. When paired with standard hydrologic measurements (e.g. rainfall, soil moisture) and post-event field measurements (e.g. peak flow depth), the combined data set documents the hydrologic conditions of an event at a level of detail only surpassed by more advanced (and expensive) monitoring sites.

In this work, we evaluate our method by comparing data with higher resolution measurements made at our more advanced monitoring stations in southern California burned areas. Comparisons show that a 30-second sampling rate can detect the timing of a variety of debris flows ranging in height from 20 cm to 4 m. This rate was also found to be sufficient to detect the timing of floods with both non-hydrostatic and hydrostatic pressure fields. Finally, we describe how this approach was applied to the 2009 Station Fire to measure the timing of debris flows and floods in eight watersheds, and to reconstruct the debris-flow events of 6 February 2010, which damaged or destroyed over 30 homes downstream of our pressure transducers.