CHARACTERIZATION AND VOLUME ESTIMATION FOR THE 12 SEPTEMBER 2022 DEBRIS FLOW EVENT AT BIRCH CREEK, OAK GLEN, SOUTHERN CALIFORNIA, SOURCED FROM THE 2020 EL DORADO FIRE

On September 12, 2022, heavy rainfall generated by Tropical Storm Kay impacted the Yucaipa Ridge area of southern California and generated multiple debris flows within the 2020 El Dorado Fire. The largest of these flows occurred at Birch Creek and inundated downstream floodplain areas at Oak Glen causing millions of dollars of damage to multiple structures and water facilities. A San Bernardino County camera captured images of the flow characteristics upstream of Oak Glen Road illustrating the occurrence of multiple flow pulses that progressively aggraded in the trunk channel until the channel capacity was exceeded. Water-dominated recessional flows then eroded the in-channel deposits back to the approximate original channel grade. No rain gages were present within the watershed at the time, but the nearest gage about 4 km to the east recorded a peak depth of 0.59 in (15 mm) in 15 minutes and 1.1 in (28 mm) in 60 minutes. However, multi-radar, multi-sensor (MRMS) precipitation estimates suggest a much heavier peak 60-minute accumulation of 1.5-3 in (38-76 mm) occurred within the Birch Creek basin. High-resolution digital elevation models (DEMs) derived from lidar or structure from motion were not available following the event to assess the total debris volume. However, CGS staff made field observations of the inundation area at 160 stations on September 13 and 14 including documentation of the spatial extent and depth of the flow deposits at representative stations along the flow path extending about four kilometers downstream of the mountain front, where the flows re-entered existing channels and were conveyed to the Oak Glen debris basin. The data from these field observations and flow limits refined based on Google Earth imagery (acquired on September 15, 2022) were used to define polygons with similar average debris depths. The data and analysis show that an area of about 0.4 sq km was inundated to depths of 0.1-1.5 m; summation of the volumes collected for each defined polygon resulted in a total minimum debris volume of about 75,000 cubic meters, not counting debris that reached Oak Glen basin. This sediment volume will add to the sparse existing database on post-fire debris-flow sediment yield volumes used in future modeling and emergency planning efforts.