Flood Monitoring, Mitigation and Management
Floods wash away or destroy homes, pollute drinking water, wipe out croplands and give way to diseases like water-borne cholera, diarrhea, dysentery, malaria and hepatitis in their wake. In developing countries with limited infrastructure, locating flood waters in order to assess the risk it poses to people – and help decision-makers prioritize aid efforts – is one of the most important jobs of forecasters. This is not always easy. Even as climatologists project an increase in extreme weather events including floods, extensive building activity along riverbanks and flood plains brings more property under threat. In the pre-flood stage, satellites can help civil protection planners anticipate where a river would be most prone to burst its banks, and take action accordingly.
One of the biggest problems in coping with floods is obtaining a clear picture of the overall extent of the flood. During a flood event near-real-time images are a management tool for authorities coping with the disaster. Satellite images, in conjunction with radar instruments especially suited for differentiating between waterlogged and dry land can show an entire flood within a single picture. A time sequence of satellite images can show if the flood is growing or diminishing over time, and highlight further areas coming under threat of inundation. Satellite data can provide highly detailed digital elevation models of areas at risk that can serve as the basis of computerized flood simulations. Simply comparing the before and after images of the flooded region makes possible a rapid and authoritative damage estimate, factoring in different land cover types to quantify the cost of the flood.
Data from weather satellites and NASA's precipitation satellites – the Global Precipitation Measurement (GPM) mission, and the recently decommissioned Tropical Rainfall Measuring Mission (TRMM) – are very useful. The real value of these data is exploited when they are incorporated into flood monitoring and prediction tools like the Global Flood Monitoring System, produced by the University of Maryland. Sentinel-1A, a C-band SAR imaging satellite high resolution data on July 24, 2017 helped to compare inundated regions in parts of Gujarat.
SCATSAT-1 – continuity mission for Oceansat-2 – was launched by the Indian Space Research Organization (ISRO) on September 26, 2016 to detect and monitor extreme events such as flood with high temporal resolution (on a daily basis). The satellite carries Ku-band Scatterometer similar to the one flown onboard Oceansat-2. The data from SCATSAT-1 has been used for the detection of the flood situations over India. The observations have been analyzed for flood detection and monitoring over India with special emphasis in Gujarat and southern parts of Rajasthan during the monsoon season of 2017. Observations on flooding conditions made through Sentinel-1A data were well correlated with SCATSAT-1 data.
Monitoring the aftermath, future flood risk maps can be created by combining a trio of variables – the flood extent, the type of land inundated and the likely flood return period – to guide civil protection officials as well as insurers as to how much money should reasonably be spent to protect or insure areas identified as flood-prone.
Flood Mitigation and Management
Many things can be done before an impending flood to reduce damage to home and property. Rather than trying to prevent floods altogether, flood mitigation involves the management and control of flood water movement, such as redirecting flood run-off through the use of floodwalls and flood gates, and wet proofing properties. It also involves the management of people, through measures such as evacuation. The management and mitigation of flooding can be handled at three levels: on individual properties, small communities, and whole towns or cities. The costs of protection rise as more people and property are protected. It is estimated that for every Rupee spent on mitigation, four rupees are saved.
The following strategies may be used to reduce the impact of floods.
Strategies for Management
1. Introduce better flood warning systems
The flood warning systems must be continually improved in order to give people more time to take action during flooding, potentially saving lives. Advance warning and pre-planning can significantly reduce the impact from flooding.
2. Modify structures to better withstand floods
The focus should be on “flood resilience” rather than defense schemes. Residents should be advised to concrete floors and replace materials such as MDF and plasterboard with more robust alternatives. Flooding will definitely occur, and we should be prepared for it. Electric sockets should be moved higher up the walls to increase resilience.
3. Construct buildings above flood levels
All new buildings should be constructed at least one metre from the ground to prevent flood damage. Conventional defenses need to be supplemented with more innovative methods to lower the risk of future disasters.
4. Tackle climate change
Climate change has contributed to a rise in extreme weather events, scientists believe. Governments from 195 countries have pledged to “pursue efforts” to limit the increase in global average temperatures to 1.5°C above pre-industrial levels.
5. Increase spending on flood defenses
Many governments around the world are reducing their spending on flood defenses when they should actually be increasing the same.
6. Protect wetlands and introduce plant trees strategically
The creation of more wetlands – which can act as sponges, soaking up moisture – and wooded areas can slow down waters when rivers overflow. These areas are often destroyed to make room for agriculture and development. Halting deforestation and wetland drainage, reforesting upstream areas and restoring damaged wetlands could significantly reduce the impact of climate change on flooding.
7. Restore rivers to their natural courses
Many river channels have been historically straightened to improve navigability. Remeandering straightened rivers by introducing their bends once more increases their length and can delay the flood flow and reduce the impact of flooding downstream.
8. Introduce water storage areas
Many countries have established flood storage reservoirs as part of their flood alleviation schemes. For example, in Britain, following the severe flooding of 2009 a 76,000m³ flood storage reservoir – the equivalent of 30 Olympic sized swimming pools – was constructed in Thacka Beck Nature Reserve in the UK to hold back flood water. The risk of flooding from the beck was reduced from a 20 per cent chance in any given year to a one per cent chance.
9. Improve soil conditions
Inappropriate soil management, machinery and animal hooves can cause soil to become compacted so that instead of absorbing moisture, holding it and slowly letting it go, water runs off it immediately. Well drained soil can absorb huge quantities of rainwater, preventing it from running into rivers.
10. Put up more flood barriers
Flood management agencies use a range of temporary or “demountable” defenses in at-risk areas. These can be removed completely when waters recede. Temporary barriers can also be added to permanent flood defenses, such as raised embankments, increasing the level of protection. As the threat and frequency of flood risk increases, the use of passive flood defense may be the only realistic long term solution.
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Department of Geology
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