The Contribution of Post-Wildfire Rilling to Generation of the 2018 Montecito, CA Debris Flows: Quantification and Interpretation of Rill Geometries and Patterns.Paul Alessio, Thomas Dunne, Kristin Morell
Abstract
The debris flows that occurred in Montecito, CA, on January 9th, 2018, significantly impacted infrastructure, homes, people, and the landscape. Initial field surveys and GIS mapping revealed dense networks of rills on bare, burned hillslopes of the shale formations. Despite similar rainfall and wildfire conditions, rill occurrence was virtually absent from sandstone formations and the most extensive ground disturbances were gullies and shallow colluvial failures. Field surveys, Structure from Motion photogrammetry, DEM analyses, and grain size distributions were used to quantify rill geometries, patterns, and their contribution to the rate of mud generation. Rills, together with intervening sheetwash erosion, exhibited anastomosing patterns at the top of the shale hillslopes before incising rapidly on gradients of 0.5-0.9 into dense networks with low convergence or divergence angles. The burned topsoil into which the rills incised consisted of a loose, cohesionless layer, 3-5 cm thick. The humic layer was converted to charcoal coatings and ash from roots (>1 mm) and the soil surface had been incorporated into the topsoil by infiltration and burial. Beneath the burned layer, the soil retained its fine-root reinforcement, but was progressively incised in the downhill direction. Debris levees on the rill margins reflected viscous behaviour of flows within short distances of initial incision, indicating that the water and sediment were intimately mixed upon entering stream channels and initiated scour and rafting of boulders and woody debris. Downslope changes in rill spacing, width, and depth were measured across various hill slope geometries in order to estimate the volume of material removed and to quantify the rate at which the granular fluid discharge increased along stream channel networks.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H21F..03P
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGYDE: 1824 Geomorphology: general;
- HYDROLOGYDE: 1838 Infiltration;
- HYDROLOGYDE: 1879 Watershed;
- HYDROLOGY