Debris Flow Run-Out and Landslide Sediment Delivery Model Tests

Five models are investigated for determining the percentage delivery of sediment to streams for shallow landslides that evolve into debris flows. Models I–III are based on debris flow run-out distance, Model IV on snow avalanche run-out distance. For these it is assumed that percentage delivery is 100% for slopes above 10° and zero for slopes below 4°. For intermediate slopes, delivery is based on the run-out distance estimate. Model V, derived from field data, estimates percentage delivery directly without the need for slope conditions. The model performances are investigated using 100 hypothetical hillslopes and a limited database on landslide sediment delivery in Idaho. Model IV performs best but Model I is a close second and its design (run-out distance varying with elevation drop) may be preferable for modeling purposes. Model II (run-out distance varying with debris flow slope and volume) provides a conservative upper limit on percentage delivery. Parallel use of models to provide bounds on the percentage delivery estimate is recommended. The assumption of no deposition at slopes above 10° is found to be not entirely correct and requires further research.