Flood Forecasting/ Hydrological Observation
Flood causes considerable damage to human lives and property almost every year. About 48% of total flood prone area (40 mha assessed by the Rashtriya Barh Ayog) of the country has been provided with reasonable protection against flood of a low to moderate magnitude due to technological and economic constraints. It is not possible to provide protection against all magnitude of flood. Flood Forecasting & warning has been recognized as the most important, reliable and cost effective non-structural measures for flood mitigation.
At present, CWC is entrusted with monitoring of flood situation in the country during monsoon by observing water levels/ discharges along the major rivers in the country and issuing flood forecasts to the local administration/ project authorities/ State Governments and the Home Ministry, G/o India; covering 148 low lying area/ cities and towns besides 28 reservoirs all over the country. The network is spread on ten major river systems viz. the Ganga & its tributaries, the Indus/ Jhelum, the Brahmaputra & its tributaries, the Barak, the Eastern Rivers, the Mahanadi, the Godavari, the Krishna and the West flowing rivers covering 72 river sub-basins, over 17 states viz. Andhra Pradesh, Assam, Bihar, Chhattisgarh, Gujarat, Haryana, Jammu & Kashmir, Jharkhand, Karnataka, Madhya Pradesh, Maharashtra, Orissa, Telangana, Tripura, Uttarakhand, Uttar Pradesh & West Bengal, one Union Territory of Dadra & Nagar Haveli and National Capital Territory of Delhi. The flood forecasting and advance warning for 148 low lying areas/ towns and 28 reservoirs help the user agencies in deciding mitigating measures like evacuation of people and shifting their movable property to safer locations. The Inflow Forecasts at 28 reservoirs are used by the dam authorities in timely operation of reservoir gates for safe flood discharges downstream as well as to ensure adequate storage in the reservoirs for meeting irrigation and hydropower generation demands during non-monsoon period. Such services are normally available during flood period from 1st May/1st June to 31st October/31st December every year. The forecast are disseminated using all types of communication means such as fax, wireless, phone, mob, SMS, email, electronic media, print media, social media, website, etc. Annually, over 6000 flood forecasts and advance warnings are issued by CWC regional offices across the country to the user agencies during floods. The overall accuracy of forecasts issued by CWC over the past years is around 97%.
The Central Water Commission has categorized various flood situations, for monitoring the floods in the country though its flood forecasting network, into four different categories namely low, moderate, high & unprecedented, depending upon the river water level with reference to warning level, danger level, and highest flood level.
- LOW FLOOD:
The river is said to be in “LOW FLOOD” situation at any flood forecasting sites when the water level of the river touches or crosses the warning level, but remains below the danger level of the forecasting site. Yellow colour has been assigned to this category.
- MODERATE FLOOD :
If the water level of the river touches or crosses its danger level, but remains 0.50 m below the Highest Flood Level of the site (commonly known as “HFL”) then the flood situation is called the “MODERATE FLOOD” situation. Yellow colour has been assigned to this category.
- HIGH FLOOD:
If the water level of the river at the forecasting site is below the Highest Flood Level of the forecasting site but still within 0.50m of the HFL then the flood situation is called “HIGH FLOOD” situation. Orange colour has been assigned to this category. In “High Flood Situations” a special “Orange Bulletin” is being issued by the Central Water Commission to the users agencies which contains the “special flood message” related to the high flood.
- UNPRECEDENTED FLOOD :
The flood situation is said to be “UNPRECEDENTED” when the water level of the river touches or crosses the “HIGHEST FLOOD LEVEL” recorded at any forecasting site so far. Red colour has been assigned to this category. In “Unprecedented Flood Situations” a special “Red Bulletin” is being issued by the Central Water Commission to the users agencies which contains the “special flood message” related to the unprecedented flood.