Definition of flash flood

 

A flood, which is caused by heavy or excessive rainfall in a short period of time over a relatively small area, is referred as flash flood. In flash flood, water level rises and falls quite rapidly with little or no advance warning. Typically, flash floods occurs in areas where the upstream basin topography is relatively steep and the concentration time of the basin is relatively short. In Bangladesh flash floods generally occurs in the north-east, south-east and Chittagong region. But devastating and extended flash flood is a recurrent phenomenon for the north-east region of Bangladesh. The extreme flashy character of the rivers and sudden excessive rainfall in the region causes frequent flash floods in the northeastern Haor areas. In the haor area, flash flood comes from the very steep uplands adjacent to the region in Asam and Meghalaya hills range in India causing immense damage to the standing boro crops, lives and properties every year.

Characteristics of flash flood in north–east region of Bangladesh

Northeastern part of Bangladesh especially Sunamganj, Sylhet and Netrokona districts are located in one of the depressed portion of the country. Most of the rivers in these areas are originated from nearby hilly area of India. These rivers are extremely flashy that is characterized by sudden and wide variation in flow as a result of excessive rainfall. When heavy rainfall is occurred in the hilly region of India, water quickly moves towards the Haor area of Bangladesh through a number of rivers and Khals. This floodwater not only carries the water but also carry a huge amount of sediment originated mainly from hill. Over the time this sediment has deposited on the rivers and canals bed and has reduced the conveyance capacity more or less all of the water resources system with in the Haor area. As a result, when flash flood due to sudden heavy rainfall creates pressure on the water resources system, water easily overtopped and creates breaching at several locations on the submersible embankment eventually water quickly enter into the haor. Most of the cases, flood water comes into the haor very early in the monsoon and farmers are not get sufficient time to harvest their standing boro crop.

 

Temporal changes of flash flood in the north-east region of Bangladesh

 

In North-east region of Bangladesh, normally first flash flood came between middle of April to end of April. But according to the local people it is learned that in the recent year first flash flood is coming much earlier. They informed that over the last 5 years, the situation has further deteriorated especially during 2003 and 2004. They also informed that in 2004, most of the Haor area have fully flooded within 20^th April. In this study it was try to establish relation between the local people opinion and water level data collected at Sunamganj station. From this analysis it is found that

 

 

Impacts of flash flood on

 

One of the functions of flash floodwater is to carry sediments, which are eroded from the hilly catchment area. During heavy rainfall in the hilly region, a massive erosion is taken place on the exposed surface of the hill. If the high intensity rainfall is continued for certain period then courser sediment such as big sized stone, boulder etc start to erode and move along the rivers. Finally these sediments are deposited on the river bed, khals, canals and agricultural land. During flash flood, sediment transport rates increase significantly of the rivers and hence major flood events make a disproportionate distribution of sediment and changes in channel size, shape and even location.

Over the years, sediment has deposited mainly on the river bed and depressed portion of the haor. According to the local people, in some places about 4 to 5 m sediment has deposited on the river bed, which not only created obstruction for water flow but also hampered the navigation system in the north east region. Internal khals and canals have lost their carrying capacity due deposition of sediment on the bed and in some cases created drainage problem for the haor area. Due to deposition of sediment, bed of beels have raised which ultimately hampered the fisheries and agricultural activities as a result of scarcity of water in dry season. Flash flood has another impact on the plan form of the rivers and canals in the north east haor area. Due to high magnitude flash flood, sometime a severe erosion is occurred along the river bank causes not only a huge amount of national loss but also creates immense sufferings to the local people.

Sand carpeting is another problem induced from flash flood. This problem is normally found in haor very adjacent to the hill such as Matian haor, Angurali Haor.Karchar haor and Kalner haor under Sunamganj district.