| By :
Adrianna Noton
Storm water management has been an issue for builders and engineers ever since the Bronze Age (2220-750 BC), when housing densities increased and impervious surfaces like roads were being put down. It is produced as a product of rainwater and snowmelt. When stormwater does not evaporate back into the atmosphere or soak into the ground, it becomes stormwater runoff. This is when it becomes an issue for civil engineers. Runoff is important for three reasons. If it is produced in a higher volume than can be assimilated into natural waterways or storm sewers, the excess can cause flooding. The degree of flooding is heavily influenced by the amount of surface area that is covered by impervious surfaces, like roads and buildings, etc. In a natural setting, say a woodland, 40 per cent of the runoff either evaporates or is transpired by plant life. This is called, 'evapotranspiration'. Roughly 25 per cent disappears into the ground as surface infiltration and another 25 per cent infiltrates deeply. In an urban environment, with 75 to 100 per cent coverage by impervious surfaces, not only does less water penetrate into the ground (only 15 per cent), there is also less evapotranspiration because there is less plant cover. This can mean runoff as high as 55 per cent. In the ordinary course of human occupation, pollutants are deposited on a daily basis on roofs, lawns, roads, and so on. When it rains, stormwater collects these pollutants and transfers them to natural water systems (rivers, lakes, oceans). This is called 'polluted runoff' and is the second reason why runoff is important. The third reason why runoff is so important is as a resource. The world's population is increasing at an alarming rate and vast populations of people do not have access to clean water. Stormwater harvesting and purification are essential to enable urban environments to become self sustaining as regards this increasingly precious natural resource. One of the earliest examples of a runoff management system was discovered in the ruins of the Minoan Palace of Phaistos on the island of Crete off the coast of Greece. This significant archaeological find is visited each year between the months of April and October by thousands of tourists. In the United States, managing stormwater runoff is overseen by the Environmental Protection Agency (EPA) Since the late 1960s and early 1970s, they have been developing and using a computerized model to assist them with this task. It provides a graphical interface for entering data and editing runoff watershed data and running simulations of hydraulic control as well as tracking real time rates and depths of water flow. Storm water management is a critical aspect of any building or urban regeneration project. It is important in the design, construction and maintenance of highways, tunnels and bridges. As the world's population continues to grow, flood management is becoming more and more important to enable the construction of residential projects in areas where this is a risk. The phenomenon of global warming, contributing as it does to rising sea levels, is making this a priority all over the world.
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