Disaster Management · TEE · papers
Q1A · Discuss in detail types of Floods on basis of duration of onset of floods with releva…
2023-05-04 · Semester VI final
Actual older TEE paper · 100 marks · Q1 compulsory; any four of Q2–Q7
Q1A. Discuss in detail types of Floods on basis of duration of onset of floods with relevant examples. [5 marks]
Answer
Definition. A flood is the inundation of normally dry land by water. It may result from river overflow, intense rainfall, inadequate drainage, storm surge or sudden release of stored water.
Two separate ways of classifying floods
| By onset | Meaning | Planning implication |
|---|---|---|
| Slow-onset | Water rises gradually, often after prolonged rainfall. | There may be time for planned evacuation, but long duration disrupts livelihoods. |
| Rapid-onset | Water rises over a comparatively short period. | Warnings and prompt action become critical. |
| Flash flood | Very rapid rise, often within minutes to a few hours of the trigger. | Little lead time; fast flow and debris can be highly destructive. |
| By source | How it develops |
|---|---|
| River / fluvial | River discharge exceeds channel capacity and water spreads over its floodplain. |
| Urban / pluvial | Rainfall runoff exceeds drainage or infiltration capacity; a river need not overflow. |
| Dam-breach | Dam failure releases reservoir water suddenly downstream. |
| Coastal surge | Wind and low pressure raise sea level; high tide can compound flooding. |
| GLOF | Sudden release from a glacial lake, for example after its retaining barrier fails. |
“Flash” describes speed, whereas “dam-breach” describes the source. A dam-breach flood may therefore also be a flash flood. Onset speed and total flood duration are different; a rapidly starting event may leave some areas inundated for much longer. The PPT uses “rapid” and “flash” inconsistently, so describe the actual time-scale instead of forcing a rigid universal duration.
Indian examples: monsoon river floods in Assam illustrate river flooding; Mumbai in July 2005 illustrates severe urban flooding; Panshet in 1961 illustrates dam-failure flooding; coastal inundation during a cyclone illustrates coastal flooding. Uttarakhand in June 2013 included rapid mountain floods and debris flows. Source and speed of onset are different classification axes.
Keep source and onset classifications separate
| Classification axis | Type | Indian example and explanation |
|---|---|---|
| Source | River flood | Assam monsoon river flooding: rivers overflow or inundate adjacent low-lying land. |
| Source | Urban / surface-water flood | Mumbai, July 2005: exceptional rainfall combined with drainage and urban vulnerabilities. |
| Source | Dam-failure flood | Panshet, 1961: failure released stored water downstream. |
| Source | Coastal flood | The 1999 Odisha super cyclone produced destructive storm-surge inundation along the coast. |
| Onset / duration | Slow-onset or sustained flood | Seasonal Assam/Brahmaputra flood episodes can build and persist over days. The actual timing varies by event and location. |
| Onset / duration | Rapid-onset / flash flood | Uttarakhand, June 2013 included rapidly developing mountain floods and debris flows. A flash flood describes rapid onset, not a separate water source. |
One event can occupy both classifications: a sudden dam-break flood is a source-defined dam-failure flood and a rapid-onset flood. Do not treat “river” and “slow” as mutually exclusive categories or assume every river flood develops slowly.
Coastal-case source: NDMA cyclone-management guidelines, hosted by NIDM, discussion of storm surge and inland inundation during the October 1999 Odisha cyclone.