**1. Introduction**

In the southeast of the country, Chittagong City has developed partly in the hilly area and the rest in the coastal plain. The area is bounded by longitudes 91° 46′–91° 50′ E and latitudes 22° 22′–22° 29′ N (**Figure 1**) that cover the Survey of Bangladesh topographic sheet no. 79 N/15, scale 1:50,000. Hills are composed mostly of weathered and loose sedimentary rocks of Tertiary (65–1.8 Ma) age which are prone to landslides. More than 200 people were killed in and around Chittagong City due to landslides between the years 2006 and 2013. Rainfallinduced landslides are common in this part of country like in other tropical mountainous regions of Southeast Asia [1–3].

The investigated area falls under tropical monsoon climate. The mean monthly maximum and minimum temperature ranges between 78.76–90.44°F and 55.88–77.38°F. The monthly average minimum and maximum rainfall is 0.66 mm in the month of January and 74.70 mm in the month of July, respectively. The yearly average rainfall is about 2794 mm [4]. The northwester and monsoon clouds are primarily responsible for the rainfall in the area. Almost 90% of the total yearly precipitation takes place between the months of June and

Landslide Susceptibility of Chittagong City, Bangladesh, and Development of Landslides…

http://dx.doi.org/10.5772/intechopen.74743

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A topographic sheet on scale 1:50,000 has been used for landslide investigation. Locations of the landslide areas have been taken by global positioning system (GPS) and correspondingly marked on a topographic sheet. The geological and geotechnical properties of the landslide materials and the nature of failures have been studied in detail. Landslide spots have been mapped on the geological map of the area and correlated with the geological structure and regional geology of the area. For preparation of landslide hazard-zoning map of the area, rock types, geological structures, nature of weathering and groundwater condition have also been studied. The dimensions of each landslide (lengths, width, depth, etc.) have been measured in the field. Geotechnical properties of the rocks, specially unconfined compressive strength, have also been measured in the field. Samples from each landslide site have been collected to determine the geotechnical properties in the GSB's

In the study area, most of the landslides are triggered by rainfall. Event-based rainfall data of the area have been collected and statistically analysed to resolve the threshold values of rainfall which trigger landslides. Accordingly, an automated rain gauge network has been installed in the area to anticipate potential landslide hazard. This system notifies about an impending landslide occurrence to local administration and the first responding organizations via mobile text messages to take necessary steps to save lives

Chittagong area lies along the western margin of tectonically active Chittagong-Tripurafolded belt [5, 6] (**Figure 2**). The folds are mostly tight, plunging and faulted in nature with NNW-SSE alignment. On the basis of landforms, its genesis, evolution and morphodynamics, Chittagong City can be divided into three broad distinct geomorphological divisions: (1) hilly area, (2) fluvio-tidal plain and (3) tidal plain (**Figure 3**). The hilly part of the city is characterized by different types of erosional processes, and therefore landforms have distinctive erosional features, whereas Fluvio-tidal and tidal landforms are depositional landforms that has distinctive accretion features. Each type of landform is again divided into the number of

Each geomorphic unit on the map has been identified from the oldest available (years 1953–1954) aerial photograph interpretation and field checking of those units. Every small feature on the photograph was considered, and interpretation was made with the help of

October.

laboratory, Dhaka.

and properties.

geomorphic units.

**2. Geomorphology and geology of the area**

The scope of investigation is to realize the actual causes of landslides in order to recommend prevention and controlling measures against the catastrophe, zoning for landslides hazard and to establish an early-warning system for landslides to save lives and properties. A landslide-zoning map of Chittagong City has been prepared, and the investigation also includes establishment of a network of automatic rain gauges for landslides early warning. Moreover, the purpose of the study is to help the engineers, town planners, policy makers and geologists to build a reasonably safer hilly city.

**Figure 1.** Location map of Chittagong area.

The investigated area falls under tropical monsoon climate. The mean monthly maximum and minimum temperature ranges between 78.76–90.44°F and 55.88–77.38°F. The monthly average minimum and maximum rainfall is 0.66 mm in the month of January and 74.70 mm in the month of July, respectively. The yearly average rainfall is about 2794 mm [4]. The northwester and monsoon clouds are primarily responsible for the rainfall in the area. Almost 90% of the total yearly precipitation takes place between the months of June and October.

A topographic sheet on scale 1:50,000 has been used for landslide investigation. Locations of the landslide areas have been taken by global positioning system (GPS) and correspondingly marked on a topographic sheet. The geological and geotechnical properties of the landslide materials and the nature of failures have been studied in detail. Landslide spots have been mapped on the geological map of the area and correlated with the geological structure and regional geology of the area. For preparation of landslide hazard-zoning map of the area, rock types, geological structures, nature of weathering and groundwater condition have also been studied. The dimensions of each landslide (lengths, width, depth, etc.) have been measured in the field. Geotechnical properties of the rocks, specially unconfined compressive strength, have also been measured in the field. Samples from each landslide site have been collected to determine the geotechnical properties in the GSB's laboratory, Dhaka.

In the study area, most of the landslides are triggered by rainfall. Event-based rainfall data of the area have been collected and statistically analysed to resolve the threshold values of rainfall which trigger landslides. Accordingly, an automated rain gauge network has been installed in the area to anticipate potential landslide hazard. This system notifies about an impending landslide occurrence to local administration and the first responding organizations via mobile text messages to take necessary steps to save lives and properties.
