1. Synthetic rainfall events were generated corresponding to selected observed extreme rainfall totals of given return period (RP), for a range of durations (1-day to 5-day) • A Generalised Extreme Value distribution was fitted to observed annual maxima data using L-moments. 207 207 196 147 109 218 210 204 159 233 237 231 224 208 167 237 237 225 210 169 237 237 226 214 175 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Caine 1980 - Worldwide Innes 1983 - Worldwide Crosta & Frattini 2001 - Worldwide Crosta & Frattini 2001 - Southern Alps Literature values of landslide thresholds Number of simulated landslides Estimated curves for constant number of simulated landslides { Results Acknowledgements: This work forms part of the LESSLOSS FP6 project (www.lessloss.org) (GOCE-CT-2003-505448) and builds upon the DAMOCLES FP5 project (EVG1-CT-1999-00007). References: Caine N. 1980, The rainfall intensity-duration control of shallow landslides and debris flows. Geografiska Annaler, 62 A, 1-2,23-27 Crosta G. B., Frattini P., 2001. Rainfall thresholds for triggering soil slips and debris flow. In Mugnai A., Guzzetti F., Roth G (eds) Mediterranean Storms, 2 nd Plinius Conference, pp 463-487 Innes J. L., 1983. Debris Flows. Progress in Physical Geography, 7, 469-501 Focus Area & Input data The Pioverna (Valsassina) catchment (160 km 2 ) & Esino catchment (20 km 2 ) are located in the pre-Alps of the Lecco province in Northern Italy. The rivers discharge into Lake Como (Lario). The area receives a mean annual rainfall of 1542 mm. C. Isabella Bovolo and James C. Bathurst School of Civil Engineering and Geosciences, University of Newcastle upon Tyne, UK [email protected] , [email protected] 0 5 10 15 20 0 2 4 6 8 10 12 14 16 18 20 22 24 "Decreasing" Rainfall "Increasing" Rainfall 0 10 20 30 40 2-year RP 5-year RP 10-year RP 25-year RP 50-year RP "Peak" Rainfall "Constant" Rainfall 0 20000 40000 60000 80000 100000 120000 140000 0 20 40 60 80 100 120 140 160 Modelling the Timing and Location of Shallow Landslides and Debris Flows using the SHETRAN Model Introduction Rainfall is an important factor for triggering landslides but its effect is poorly understood. The aim of this work is to investigate the occurrence, and the associated sediment release, of shallow landslides and debris flows at the river basin scale as a function of rainfall return period. The timings and locations of landslides were simulated in response to rainfall patterns of different combinations of intensity and duration. The simulations were carried out using the SHETRAN landslide model. Method 1 2 3 NoData Forests Pastures and meadows Rocky outcrops Conclusions • The effects of different rainfall properties on simulated shallow landslide incidence and sediment yield were explored. • The relationship between the simulated number of landslides and rainfall event intensity and duration was found to be similar to that in the literature. The upper bound simulations were found to be too high but the lower bound simulations were found to be consistent with Innes (1983) and Crosta & Frattini (2001). • The timing and occurrence of landslides was shown to be influenced by rainfall distribution pattern as well as magnitude. The effects of antecedent conditions should now be explored. • These results should help to develop forecasting, mitigation and hazard zonation strategies. Combined Sediment Yield for Pioverna & Esino River outlets Total Sediment Yield (tonnes) Rainfall event intensity (mm/day) Simulated number of landslides compared to published zero-landslide thresholds for lower bound simulations Average rainfall event intensity (mm/hr) Rainfall duration (days) Number of landslides associated with different return periods for the upper bound simulations Precipitation, river discharge and number of landslides for a 1-day rainfall event with a 2 year return period Number of landslides Precipitation (mm) Hours Location of landslides for rainfall events of 1 day duration & 2 year return period Landslide location (lower bound) Landslide location (upper bound) River Evapotranspiration Canopy interception Root zone Landslides, erosion & sediment transport 3D Variably saturated subsurface flow model Snowmelt Overland & channel flow SHETRAN V4 The SHETRAN landslide model • Landslide occurrence is determined as a function of the time- and space-varying soil saturation conditions simulated by SHETRAN using infinite-slope, factor-of-safety analysis. • The SHETRAN landslide model is relevant to catchment scales of up to 500 km 2 . • SHETRAN can predictively examine the impacts of possible future changes in climate (including rainfall characteristics) and can therefore be used in hazard assessment and as a tool for developing mitigation strategies. The landslide component uses the topographic index to link the SHETRAN grid resolution (typically 100m – 2km), at which the basin hydrology and sediment yield are modelled, to the landslide component’s sub- grid resolution (typically 10 – 20m), at which landslide occurrence and erosion are modelled. • SHETRAN is a physically based, spatially distributed, integrated surface / subsurface modelling system for water flow and sediment transport in river catchments. • The landslide component models shallow landslide occurrence and the resulting sediment yield at the catchment scale. Elevation (m) Esino river Pioverna river • 1- 5 day rainfall events, corresponding to selected return periods, were disaggregated to hourly resolution. • Four rainfall disaggregation patterns were evaluated with equal total precipitation but variable hourly intensity, called: Peak, Constant, Increasing and Decreasing. 2.Uncertainty was allowed for by setting upper and lower bounds on key model parameters. 3. The rainfall events were applied to the SHETRAN model which simulated landslides and the resulting sediment yield. 4. The results were analysed to investigate relationships between rainfall properties and the characteristics of the modelled landslides and their sediment yield. Rainfall disaggregation pattern. 1-day example shown below. 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0 5 10 15 20 25 30 35 40 45 50 1 day events 2 day events 3 day events 4 day events 5 day events Rainfall Return Period Simulations without use of the landslide component are the same as for the Lower Bound simulations Upper Bound Lower Bound 1 day events 2 day events 3 day events 4 day events 25 year RP 50 year RP 5 day events 2 year RP 5 year RP 10 year RP Discharge (mm/s) Number of landslides (/10) Time (hours) 1 0 0 2 0 0 1 5 0 SHETRAN grid resolution = 500 m. Landslide grid resolution = 20 m. Soil class and sand content (%) (32 – 44) (48 – 54) (60 – 74) Stream – aquifer interactions ! ! ! ! ! ! !! ! ! ! !! !! ! ! ! !! !!! !! !! ! ! ! !! !! ! !!! !! !!!! !!! !!! !!! !!!!!! !!!! !!!!! ! !! ! !! ! !! ! ! ! !! !! ! !! ! ! !!! !! ! !!! !!! ! ! !!! !! ! !!! !!! !! !!! ! ! !! !! ! !! !!! !!!! !! ! !!!! ! !!!!! ! ! ! ! !! ! !! !!! !!! ! !! !!! ! !!! !!!!!! ! ! !!!!!!! !!! !! !!!!! ! !!! !! !! !!!! ! ! ! !!!!!!! ! !!!!!! !!!!! !!!!!!! ! ! !!!!! !!!! !!!!!!!!!! ! !!!!!! ! !!!!!! !!! ! !! ! !!!!!!!! ! !!!!! !! !!!!! !! !!!!!!! !!!! ! !! !!!!! ! ! !!! !! ! ! ! !!!!!! !!! ! !! ! !!! !! !!! !! ! ! ! ! ! !! ! !! !! ! !! ! !!!! !! ! !! !! !! !! !!! !! !!!!! ! !! !!!! !!! !! !! !! ! ! !!!! !! !!!! ! !!!!!! ! !! !!!!!! !! !!!!!! !! !!!!! !!! ! !! !!! !!!!!! !! ! !!! !!!! !! !! !!!! !! !! !!!!!!!! !!!!!! !!!!!!!!!! !!!!!!! !!! !!!!! !!!!! !!!! !!!! !!!! !!! ! ! ! ! ! ! !! !! ! !!!!! !! !!!!! !! ! !! ! ! ! ! !! ! ! ! ! !! ! ! !! ! !!! ! !!! ! ! !! ! !! ! !! !! ! !! ! ! ! ! ! !!! !!!! !! !! !!!!! !!!! !! !!! !!!! !!!! !!!!!! !!! !!! !!! !!!!!! !!! !!! !!! ! ! ! ! ! ! ! !!! ! ! !! ! ! !! ! ! ! ! ! ! ! ! !! ! ! ! ! !!! ! !!! ! !! !!! !!!! ! !! !!!!!! !! ! !!!! !!! ! ! ! !! !!!! ! !!!!! !!!!!!!!! !! !!!! !!!!!!! !!!!!!!!!!! ! !!! !!!! !!! !! !!!! !! ! !!! ! ! !! ! ! !!!! !!!!! ! !! ! !! ! !!!! ! !!!!!!! ! !! !!!! !! !!!! !!!! ! !! !! !!! ! !!! !! !!!! !! ! !! ! !!! !! !!!!!! !!! ! !!! !! ! ! !!!! !!! ! !! !!! ! !! !! !!! ! !! !!! !!!! ! !! ! ! ! ! !! ! !! ! ! ! !! ! !! !! ! ! !! ! ! ! !!! ! ! ! !! !!!! ! !!!! !! !!!! ! !!!!! ! !!! ! ! ! ! ! !! ! ! ! !! ! !!! !! !! ! !!! ! ! ! !!! ! ! !! !!!!! ! ! !!! ! ! ! !! !!! ! ! !! ! ! ! !! ! ! ! !! ! !! ! ! ! !! !!!!!! !!! !!!!!!! ! !! !!! !!!! !! ! !! !! ! !!!! !!!! !!! !! !! !! !!!! !!!!!! !!! ! !! !! ! ! ! ! !! !!!! !!! ! ! ! ! !! !! ! ! ! !!! ! !! !!!! ! ! !!! !! ! ! !! !!!!! ! !!!! !!! ! ! ! !!!!! ! ! !! !!!!!!! !! !! !!! ! !!!!!! !! ! !!!!!!! !!! !!! ! !! !!!! !!! !! !! ! !!!! ! ! !! ! !!! ! !!!! !!! !! !! !! !!! !! ! !! ! ! ! ! ! ! !! ! ! ! ! ! !! !! ! !! !!! !!! !!!! !!!! !!! ! !!! ! !!! ! ! ! ! ! ! ! !! !! !! !! !!! !!! ! ! !!! ! ! ! ! !!!! !! ! ! !!! !! ! !! ! ! !!!!!! ! ! !!!!!!!!! ! !!! !! ! ! ! !! !!!!!! ! ! !!!!! !! ! !! ! !!!! !! ! !!!! !! ! ! ! ! !! !! ! !! !!!! ! ! ! !!! ! !! ! ! ! !!!!! !!!!! !!! ! ! !!!! ! !!!! !!!! ! !!!!!! !!!!!! !!!!! ! !!!!!! !!!!!! !!!!! ! !!!!! ! !!! !! ! ! !!!! ! !!! !!! !! !! !! !!!! !!!!! !!!! !!!!!!! !! !! !!!!!!!!! !!!!! !!! !!!!!!! !! ! !! !!!!!! !! !!!!!! ! !! !!!!! !! !! !!! ! !! !! ! !! ! !! ! !! ! ! !!!! ! !!! !!!! !! !!!!!! !! !!!! !!!!! ! !! !! !! ! !! ! ! !! !! !! !!! ! !! ! !! !! ! !!! !! ! ! ! !! ! !! !!! !!! ! !! !! !! !! !! ! ! !!!!!! !! ! !! !!! ! !! !!! !! !! !!! !! ! ! !!! !! ! ! ! !!!!! ! !! ! !!! ! ! ! ! ! ! ! ! !! ! !! !! !! !!! !!! !!!! !!!!!! !! !!!! !!!!!!! ! !!!!!!! !!! ! !!! !!!! !!!! ! !!! !!!! ! !!!!! !!!! !!!!!! !!! ! !!!!!!! !!!!! !!!! ! ! ! !! ! ! ! !!! !! !! ! ! !!! !! !! ! !! !! ! ! ! ! ! ! ! ! !! !! !!! ! ! ! ! ! !!! ! !!! !!! !! ! ! ! ! ! !!! ! ! ! ! ! ! ! ! ! !! !! ! !! !!! ! !! ! ! !! ! !! ! !!! ! !!! !! ! ! ! ! !! ! !! ! ! ! !!! ! ! !! ! !! ! !! ! ! ! !!!! !! ! ! ! ! ! ! ! ! ! !