A low‐cost method to measure the timing of postfire flash floods and debris flows relative to rainfall

Abstract

[1] Data on the specific timing of postfire flash floods and debris flows are very limited. We describe a method to measure the response times of small burned watersheds to rainfall using a low-cost pressure transducer, which can be installed quickly after a fire. Although the pressure transducer is not designed for sustained sampling at the fast rates ( 2 s) used at more advanced debris flow monitoring sites, comparisons with high-frequency stage data show that measured spikes in pressure sampled at 1 min intervals are sufficient to detect the passage of most debris flows and floods. Postevent site visits are used to measure the peak stage and identify flow type on the basis of deposit characteristics. The basin response time scale (t b ) to generate flow at each site was determined from an analysis of the cross correlation between time series of flow pressure and 5 min rainfall intensity. This time scale was found to be less than 30 min for 40 postfire floods and 11 postfire debris flows recorded in 15 southern California watersheds ( 1.4 km 2 ). Including data from 24 other debris flows recorded at 5 more instrumentally advanced monitoring stations, we find there is not a substantial difference in the median t b for floods and debris flows (11 and 9 min, respectively); however, there are slight, statistically significant differences in the trends of flood and debris flow t b with basin area, which are presumably related to differences in flow speed between floods and debris flows.

Developing and Testing Physically Based Triggering Thresholds for Runoff‐Generated Debris Flows

“…The choice to average values of dimensionless discharge and Shields stress over a 10‐min time window is based on past observations in the Transverse Ranges of Southern California that debris flows typically reach the basin outlet of small watersheds (0.01–1 km 2 ) within 5–10 min of intense rainfall (Kean et al, ). Simulations indicate that there is only a small amount of separation between values of τ *,1 ( S ) (i.e., 1‐min averages) associated with debris flow events and values of τ *,1 ( S ) associated with water‐dominated floods at Las Lomas (see supporting information Figure S4).…”

Testing seismic amplitude source location for fast debris-flow detection at Illgraben, Switzerland

“…Such alarms are useful to raise the general level of alert but are often not accurate enough to serve as a basis for rescue deployment, road closure or building evacuation. Furthermore, empirical relationships between rainfall and debris-flow initiation are not necessarily transferable to other regions because the hydrological response of a catchment depends on the amount of precipitation (Gregoretti et al, 2016) and may react to sudden environmental changes such as wildfires (Cannon et al, 2008;Kean et al, 2012;Rengers et al, 2016).…”

Amplification of postwildfire peak flow by debris

“…These constraints allow the model to be applied in a fully predictive mode without any calibration. The 28 test events are from eight burned watersheds in Southern California [ Kean et al ., , ]. The events reported in Kean et al .…”