A combined influence of surface temperature and daily rainfall to the historical landslides occurred on 7th September 2007 over sub-Himalayan region, India

Occurrences of substantial landslides in connection with immoderate daily and cumulative rainfalls in the sub-Himalayan region are a well-established fact. In this work, we incorporate strong influences of the regional climate parameters, such as daily maximum surface temperature ( T_max ) and daily minimum surface temperature ( T_min ) to the failure condition of wet soil state. Variations of these climate factors are coupled with the variation of daily rainfall R as to explain the typical landslide events. To find a possible correlation, we, for our current analysis, consider the prominent landslides reported to have occurred on 7th September 2007 in Sikkim and greater Darjeeling and Kalimpong regions, West Bengal (India). Within the vicinity of locations of these reported landslides, we studied numerically T_max , T_min and R across the geographical region for a specific day-window ( DoY ) from 23rd August to 12th September in each year ( Y ) starting from 2001 to 2010. We systematically analyzed the possible conjugal effects of variations in T_max (Y, DoY) , T_min (Y, DoY) and R (Y, DoY) during the occurrence or non-occurrence of these landslide phenomena. We define a normalized integrating parameter called as ‘La-coupling factor’ L_cf (Y, DoY) and computed the same for the aforementioned DoY and Y period. The noticeable value of L_cf (Y, DoY) is numerically obtained to be of about 0.19 and 0.42 respectively, on 6th and 7th September 2007 while the same is obtained to be extremely small for all other non-landslide days. Consequently, we find an acceptable correlation between the computed L_cf (Y, DoY) and the possibility of occurrence or non-occurrence of any such landslides. Explanation of our unique results is supported by the theory of wet granulates in presence of those key climatological parameters.