3 2 Relief Physiography elevation of various regions

3. 2 Relief & Physiography (elevation of various regions of BD from the sea level, three major classification of physiographic features)

Relief & Physiography • Bangladesh is crisscrossed by a larger number of rivers maintaining an intricate network with their numerous tributaries & distributaries. • The elevation in the major part of BD is low since the most of the country’s land is originated through fluvial processes. • Nearly 50% of the country’s elevation is less than 10 m from sea level. • The southeastern hilly region exceeds an elevation of 300 meters. • Around 20% of the land consists of low-lying tidal plain having an average elevation of 3 m above the sea level.

Physiography

Physiographic Classification • Most of the deltaic southern part of the Bengal Basin is not more than 10, 000 years old (Rashid, 1991). • Based on the age & formation of landmasses, the physiography of BD can be divided into three categories • a) Hills (Tertiary period) b) terraces (Pleistocene age) & c) flood plains (relatively recent).

Physiographic Classification • Hills: developed in Tertiary period (35 -15 m yrs ago) occupying 12% of the land area -located mainly in the southeastern regions of Ctg & CHTs and northern part of great Sylhet District. -topography is largely determined by geologic structure (folded, faulted & uplifted) & rock type (sandstone, siltstone & shale of mid-Tertiary age).

Physiographic Classification • Terraces -major terraces are the Barind (located in Rajshahi, Dinajpur & Bogra districts with an area of around 8000 sq km) & Madhupur Tracts are located in the northwestern & central parts of BD respectively. -Lalmai (with an area of 33 sq km), west of Comilla Town is also a smaller terrace. -These terraces cover 8% of the land which are also recognized as OLD ALLUVIUM (flood plain deposits of the earlier Ganges & Brahmaputra rivers). -the Madhupur Tract extends from north of Dhaka in the south to Jamalpur & Mymensingh to the north with an area of around 4000 sq km.

Physiographic Classification • Flood plains -are also known as alluvial plains through fluviodeltaic processes. -flood plains are created due to the deposition (alluvium) of the GBM rivers covering 80% of the land in Bangladesh. -there as many as 9 types of flood plains in BD, of which the Ganges plains cover a major part.

BANGLADESH: Physiographic Unit Ref. (Rasheed, 2008)

Flood Plains 1 Old Himalayan Floodplain 2. Teesta Floodplain 3. Brahmaputra Jamuna Floodplain 4. Ganges Floodplain 5. Meghna Floodplain 6. Surma-Kushiara Floodplain 7. Ganges Tidal Floodplain 8. Meghna Estuarine Floodplain 9. Ctg coastal Floodplain

1 Old Himalayan Piedmont Plain

2 2 Teesta Floodplain 2 2

3 Meghna 3 3 Brahmaputra-Jamuna Floodplain 3

4 4 Ganges Floodplain 4 4 4

5 5 5 Meghna Floodplai

6 Surma Kushira Floodplain

7 Ganges Tidal Floodplain

8 Meghna Estuarine Floodplain 8 8 8

9 9 Ctg Coastal plain

Physiographic Classification • Subdivisions of Delta 1. Moribund delta 2. Immature Delta 3. Active Delta 4. Mature Delta

1 3 2 4