Flash Floods in the Arid and Semi-Arid Regions

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Blog Post: S. Fazal Daoud Firdausi 

Most of us have grown-up reading geography school textbooks stating arid region of India i.e. Thar Desert is a rainfall deficit region and it hardly receives 12-15 cm annual rainfall. If we analyse the rainfall record of last five years of the region, it may compel us to change our notion about the same. In 2015 there was an early onset of monsoon in the state of Rajasthan. It ended up with 78 % excess rainfall in Western part and 54 % excess rainfall in East Rajasthan. In 2017, districts like Barmer recorded 49 mm, Jaiselmer 28 mm and Bikaner 7 mm in a span of 24 hours. The given map above developed by Indian Meteorological Department (IMD) for the period between 01st June, 2018 to 19th July, 2018 shows that there has been 20 per cent to 59 per cent excess rainfall in Jaiselmer district of Rajasthan. As of 17 July, 2018 East Rajasthan is rain surplus by 16 per cent, while west Rajasthan is 15 per cent in excess. In 2018, heavy downpours had created flood like situation in several cities of Rajasthan.

Reasons behind Flash Flood havoc in Arid Parts of Rajasthan

One third of the World’s land surface can be classified as arid or semi-arid. In those areas, on the one hand precipitation is rare and serious drought occurs frequently. On the other hand flash floods may cause serious disasters. To understand the flash floods it is very important to know the precipitation pattern in these areas.

Nature of Precipitation in arid and semi-arid areas

Smaller Frequency of occurrence of rainstorms: In these areas rainstorms are random events with a small frequency. Storms are formed over a much smaller area. Short Duration and High Intensity Storms: In arid regions of tropical zones rainstorms results from short duration convective storms which usually lasts from 15 minutes to two hours with an intensity of 100-150 mm/hour.
Major characteristics of Flash Floods in arid and semi-arid areas

Suddenness of Occurrence: Since flash floods are generated by high intensity storms of limited areal extent or glacier-dammed lake outburst, conventional forecasting methods cannot provide proper warning and people have insufficient time to react about the situation.

Randomness of Distribution: In any given catchment the frequency of occurrence of flash floods is very small, perhaps, once in many years. However, due to climate change such occurrences are becoming more regular (Mishra, Anupam, DTE).

Complex Formations: Flash floods in these regions occur from storms or cloudburst of limited areal extent, or snow ice-melt dam-break, or combination of all these factors.

Impact of Flash Floods in Arid and Semi-Arid Regions
  • Loss of life and destruction of infrastructure
  • Loss of livelihood through damages in agriculture and death of livestock
  • Disturbance of the drainage path of the rivers
  • Difficulty in draining out water due to the presence of Gypsum belt in many regions.
  • Destruction of traditional water harvesting structures such as ‘Tankas’ and ‘Johars’.
  • Soil Erosion and Land Degradation
  • Loss of local habitat and ecosystem
Recommendations and Adaptive Strategies
  • Engineering Measures: It varies because of different natural geographical conditions and properties of specific floods.
  • Building Storage Reservoirs: For controlled release and protection of the downstream floodplain.
  • River Basin Management: It is carried out to reduce surface flow. For example:
  • Shallow ditches may be created where the river emerges from the hill for flood interception and diversion.
  • Terraces and check dams on slope may be developed to weaken the power of flood and reduce the flood peak.
  • Clearing Obstacles in water course: It improves discharge condition and enlarges flood carrying capacity of the river.
  • Building flood diversion storage and flood retardation areas.
Non-Engineering Measures
  • Establishment of Unified Command for Disaster Reduction.
  • Establishment of advance monitoring systems for forecasting and warning.
  • Afforestation measures – plantation forestry can improve catchment conditions, prevents floods and erosion efficiently.
  • Flood Insurance.
  • Rapid Action for Disaster Alleviation.
  • Arid Region Risk Identification which includes hazard mapping.
  • Specific Management Strategies for Arid Regions.
Major Obstacles
  • Engineering Structures requires huge capital investment.
  • Inimical threat of damage and breakage due to flash floods.
  • Accumulation of sediments in reservoirs.
  • Due to less frequent occurrence of storms in arid areas flood diversion storage may remain unused.
  • Lack of awareness among the local population.
  • Lack of coordination among various agencies.
Remarks:

The most essential and significant measure to manage flash floods in arid region is real-time monitoring of flash floods, development of real-time and precise warning system and hazard mapping possible through the induction of remote-sensing and GIS techniques. It is also important to integrate the new technology with traditional knowledge base, keeping in mind the ground realities. 

Location: Jaisalmer, Rajasthan 345001, India