Initial assessment of landslide prone area is important in designing landslide mitigation measures. This study, a part 5 of our study on developing landslide spatial model, presents initial signal of landslide prone area. In here, we use soil depth to hardpan to assess landslide prone area in Western Central Java, a relatively small region where 23% of Indonesian landslide occurs. To this end, we interpolated soil depth to hardpan in a regular grid from irregularly distributed data. To do this, we employed three different methods: Inverse Distance Weighting (IDW), Ordinary Kriging (OK) and Co-Kriging (CK). For the latter, we experimented with several potential covariates. To determine the best fitting model, several tests on model 10 performance and its corresponding errors were done. Error measures used in this study are Mean Square Error (MSE), Root Mean Square Error (RMSE), Mean Absolute Error (MAE) and Mean Absolute Percentage Error (MAPE), while statistical measures employed are Standard Deviation, Variance, Interquartile Range (IQR), Mean Absolute Deviation and MedianAbsolute Deviation. The result shows that CK with covariate of slope and soil cohesion is the best fitting model and exhibits clear pattern related to recorded landslide disaster sites. We found that 64% of landslide disaster events occur in the area having 15 soil depth to hardpan of 5 -10 m. Moreover, 84% of landslide occurrences happen in regions where soil depth to hardpan ranges from 5 to 15 m. Hence, we suggest that landslide prone area is an area possessing soil depth to hardpan of 5-15 m. This finding is advantageous for policy makers in planning and designing efforts for landslide mitigation.
IntroductionIndonesia has a large number of landslide disaster occurrences. In last decade, 3,924 landslide disaster events were recorded 20 with 1,404 events (~36%) of them occurred in the Central Java province (BNPB 2018). Out of those numbers, 908 landslide events occurred in western part of the area. In this region, landslide disaster events have caused 211 casualties, nearly 20,000 people suffered and more than 7,000 house damaged. Moreover, the number of natural disaster in Indonesia shows increasing trend from 924 events in 2008 to 2,862 events in 2017 which accounts for more than threefold during the last decade (BNPB 2018). Considering the impact of global climate change, the number could be larger and the impact could be worsening 25 (Banholzer et al. 2014). Hence, appropriate mitigation measures in particular locations are critical. Accordingly, understanding landslide prone area is helpful in designing policy for appropriate mitigation efforts.Landslide occurs when safety factor (SF), defined as a ratio of shear strength and acting force (gravitational force, seepage pressure, etc) of soil layer disrupted (Das 1998). The higher the slope stability (SF > 1) the lower the possibility of landslide