**7. Landslides susceptibility zoning and early warning**

The hilly part of Chittagong City and its surrounding areas have been classified into four zones on the basis of locations of past landslides, rock types and their geotechnical properties (**Figure 6**). These potential zones are very closely correlated with the geology of the area. Based on these parameters, Zone-I is most dangerous and Zone-IV is relatively less dangerous for landslides hazard. In Zone-I, the rock type is mostly loose, highly weathered sandstone and Zone-IV constitutes mostly of compacted shale. Considering the age of the rock types, Zone-I is younger than Zone-IV. Zone-I in the map represents Dupi Tila and Upper Tipam Formations, zone-II corresponds to Middle Tipam, zone-III falls into Lower Tipam and finally zone-IV to Boka Bil Formation.

Rainfall-induced landslides are common phenomenon in Chittagong and in adjoining areas where scores of people are being killed each year due to landslides. Under present situation, it

with landslide events. Hence, rainfall threshold values for landslides were set for even shorter

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After rainfall data analysis, the threshold values for Chittagong City are set to.

periods such as 3 h, 1 day and 3 days.

**Figure 8.** Return periods analysis of extreme rainfall events.

**Figure 7.** Statistical analyses of rainfall data.

3 h to 100 mm.

24 h (1 day) to 200 mm. 72 h (3 Days) to 350 mm.

**Figure 6.** Landslide hazard-zoning map of Chittagong City and its surroundings.

is most important to save lives and properties from the potential hazard of landslides. To serve this purpose, the Geological Survey of Bangladesh (GSB) and the Norwegian Geotechnical Institute (NGI) are working together to lessen the landslides hazard level in Chittagong City. The co-operating partners have developed an early-warning system to forecast rainfall-induced landslides in Chittagong so that people can be aware of the impending landslides beforehand.

GSB has carried out a study to rectify empirical data from years 2003 to 2010 and still studying various landslides to rectify the rainfall threshold value. Event-based rainfall data are statistically analysed (**Figures 7** and **8**) to determine the rainfall threshold values for landslides in Chittagong City (**Table 2**). For analysis on experimental basis, the total rainfall duration was split into three separate durations such as 1 day, 3 days and 7 days. For all these periods, respective landslide events were correlated with the total amount of rainfall. During analysis, it was found that rainfall within a very short period of time (such as 3 h) is also associated

**Figure 7.** Statistical analyses of rainfall data.

with landslide events. Hence, rainfall threshold values for landslides were set for even shorter periods such as 3 h, 1 day and 3 days.

After rainfall data analysis, the threshold values for Chittagong City are set to.

3 h to 100 mm.

is most important to save lives and properties from the potential hazard of landslides. To serve this purpose, the Geological Survey of Bangladesh (GSB) and the Norwegian Geotechnical Institute (NGI) are working together to lessen the landslides hazard level in Chittagong City. The co-operating partners have developed an early-warning system to forecast rainfall-induced landslides in Chittagong so that people can be aware of the impending landslides beforehand. GSB has carried out a study to rectify empirical data from years 2003 to 2010 and still studying various landslides to rectify the rainfall threshold value. Event-based rainfall data are statistically analysed (**Figures 7** and **8**) to determine the rainfall threshold values for landslides in Chittagong City (**Table 2**). For analysis on experimental basis, the total rainfall duration was split into three separate durations such as 1 day, 3 days and 7 days. For all these periods, respective landslide events were correlated with the total amount of rainfall. During analysis, it was found that rainfall within a very short period of time (such as 3 h) is also associated

**Figure 6.** Landslide hazard-zoning map of Chittagong City and its surroundings.

142 Engineering and Mathematical Topics in Rainfall

24 h (1 day) to 200 mm.

72 h (3 Days) to 350 mm.

**Figure 8.** Return periods analysis of extreme rainfall events.


charge the batteries and require only mobile telephone coverage at the installed location. The stations work with standard mobile telephone SIM cards and do not require any special infrastructure from the local mobile telephone operator. Installation is simple as the station is a compact unit that can be placed on the ground or on a roof, or mounted on a

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The main cause of landslides in and around Chittagong City is heavy rainfall in a short span of time. At the same time, geotechnical properties of the slope materials (loose, well-graded sand, high permeability and porosity, low internal friction angle and weathering characteristics) are also responsible. In several cases, hill cutting accelerates the process of landslides but

Depending on the locations of landslides, geology and geotechnical properties of rocks, Chittagong City and its surrounding areas have been classified into four landslide hazard zones. Of these, Zone-I is very much susceptible to landslides and Zones-II, -III and -IV are

Rainfall becomes the primary triggering factor of landslides, and it is statistically calculated that 100 mm of rainfall in 3 h, 200 mm in 24 h (1 day) and 350 mm in 72 h (3 days) can be the threshold values of rainfall to initiate landslide occurrence. Event-based statistical analysis of

Accordingly, two automated rain gauges have been installed in and around Chittagong City. If threshold values exceed, the system automatically sends landslide early warning to 10 selected mobiles of local administration and the first responding organizations so that they can alert people in advance before landslides. GSB is also working to increase the number of such systems especially in other landslide-prone areas of the city. The ultimate goal is to save the people from landslide disaster and build a landslide resilience

The authors are indebted to Ms. Moonira Akhter Chowdhury, Director General of GSB, for her kind permission to publish a chapter titled 'Landslides Susceptibility of Chittagong City, Bangladesh and Development of Landslides Early Warning System' in the book Landslide Science for a Safer Geoenvironment by Springer International Publishing (2014). It is to state that with kind permission from the authors to re-use it, a considerable part of the present work was taken from this previously published chapter. Thanks to the Government of Bangladesh and the Norwegian Ministry of Foreign Affairs to provide fund to establish and maintain the landslides early-warning system in Bangladesh. Thanks to Ms. Shahtaj Karim, Assistant

rainfall data is still taking place to rectify these threshold values for the city.

Director (geology) of GSB, for her assistance to prepare the chapter.

mast.

**8. Conclusions**

community.

**Acknowledgements**

was not the main cause of landslides.

progressively rather less vulnerable.

**Table 2.** Results of statistical analysis of rainfall data.

Automatic rain gauges for landslides early warning have been installed in Chittagong City Corporation and Chittagong University Campus which record rainfall data every 15 min and send it to a server through an online system. Historical rainfall data are stored in the server for further analysis. In addition, when the rainfall threshold values, which have been set in the system, are exceeded, the rain gauges are programmed to send SMS messages to registered mobile phones of 10 different organizations/personnel (**Figure 9**).

These rain gauges send real-time early warning to the registered mobile phones so that the first responding organizations can take action straight away before landslides happen. This is intended to facilitate the timely evacuation of the people residing close to landslidesprone areas. The automatic rain gauges installed are receiving power from a solar panel to

**Figure 9.** Schematic diagram for landslide early-warning system.

charge the batteries and require only mobile telephone coverage at the installed location. The stations work with standard mobile telephone SIM cards and do not require any special infrastructure from the local mobile telephone operator. Installation is simple as the station is a compact unit that can be placed on the ground or on a roof, or mounted on a mast.
