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Monday, May 29, 2017
By Eng. Mahinda Panapitiya-May 28, 2017, 8:28 pm
Flooding during heavy rains and water pollution in natural streams are common problems all over the world when human settlements are located near flood prone areas. For example about 7-10% land area in the US under human settlements are subjected to flooding
In ancient cultures flooding was perceived as a blessing in disguise because it was the main transportation method of fertilizers free of charge for agriculture activities in temporary submergence areas called flood plains. After moving people into flood plains because of shortage of space for settlement, flood phenomena transformed to a curse for humans. With time, because of so called development, the environmental features of flood plains have been changed vastly causing floods during heavy rains while malfunctioning natural water cleaning process by wetlands especially during droughts. Wetlands of Flood plain are the interface between aquatic and terrestrial areas. Plants in those wetlands play very vital role in cleaning water biologically before it falls into the main streams. Wetlands are in fact the kidneys of our nature.
Hidden Reasons for Frequent Flooding
=Unplanned urbanization by clearing tree cover resulting in impervious areas (Roofs, Roads etc) that increase the runoff rate
=Obstruction in natural stream and flow ways due to poor maintenance (very common along Kelani River basin)
= Encroachment into flood zones (filling of wetlands for development Ex; Gampaha, Jaela, Wattala)
=Deposition of sediments from erosion from upland areas due to erosion of stream banks because of encroaching reservations of stream banks either for agriculture in rural areas and for settlement in urban areas
= Inadequate flow capacity in streams due to invasive type weed growth associated with polluted water. (Wattala, Kalmunai)
How to get ready to face above challenges
People who are already settled and people who are planning to settle in flood prone areas subjected to above issues could be kept aware of the different risk levels inherent to the place where they are currently settled or intend to settle.For that there is a need to do an exercise called Flood Hazard Zoning, This approach is very common in developed world.
There are technologies available to simulate different flood conditions thatarise due to heavy rains before it occurs using satellite and survey data. This is called "modeling" in engineering. It is same as modeling in clothing industry where the user becomes aware of how he or she looks without actually wearing it. Similarly any area subject to floods can be modeled and divided in to zones so that land user will know how his lands get submerged or perform for different levels of flood situations.This type of performance based methods evaluates how an existing or newly introduced flood mitigation effort performs under different flood conditions.
A sketch below extracted from a technical guideline adapted in US shows a typical flood zoning map which could be used by a community to decide whether they should or should not build their houses in particular location. For example in that map, people who are in Zone A are in high risk area subjected to flooding. Zone C is a low risk area. Deciding to settle down in a flood prone area is a gamble. For example a person who wants to build a house in Zone A which is designated as "100Year Flood Zone", will have 26% chance of submerging his house once in 30 Years which is the normal Bank Lending period of a housing loan. For the next 70 Years, which is the normal lifetime of a building, chance is 50%. For a person who wants to build a house in Zone B which is designated as "500-Year Flood Zone" will have 18% chance of submerging his residence or building once in 70 Years. By knowing in advance through these flood zoning maps, people themselves become aware of flood danger before it occurs and therefore they also become responsible for the challenges to be faced during flood situation. When there is no such initial warning, government will automatically have to bear the whole responsibility.
This type of mapping would also be useful guide for Land Valuation as well as for Insurance against flood risks. With flood zoning, flood insurance becomes an option that adds a financial component in designing buildings to address those future risks. For example people can build their houses at elevated levels on columns to suit predicted flood levels. Also the sewerage systems can be introduced to suit the wetland environments.
Ground water mapping during droughts
Similar to mapping of surface water flows during heavy rains, during severe drought ground water behaviors can be studied by recording water level elevations of open wells. Those data could be used to suggest methods to face drought situations by identifying areas suitable for temporary water storages within flood plains. Modern technologies could be used to identify naturally available water cleaning wetlands to be preserved.
During previous drought situations before the current flood, there had been a water shortage Attanagalu Oya Basin though annually it dumps water into the sea equivalent to a volume of Parakrama Samudraya. Also note that impact of severe drought situations is worse than that of flood,especially in view of the current pollution levels of our natural streams.
King Parakramabahu's famous quote on water conservation and utilization "Do not release even a drop of rain water to the sea without using "Applies not only to Dry Zone but for the Wet Zone too