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Urban floods are floods that happen in a relatively short period of time and can inundate an area with several feet of water. As areas become 'urbanized' or go through the process of urbanization, there are increased flood risks that result. The main problem with urban flooding is the fact that they occur in highly populated areas. Although not as severe as a flash flood, the property damages and death toll can be significant as surface water runoff is controlled and managed by humans in a concrete world. hy Does Urban Flooding Occur? As more and more farmlands and wooded areas are converted to urban and suburban areas, the amount of surface area available for water infiltration into the soils decreases. Home sites, parking lots, buildings, and roadways all decrease the surface area of soil on Earth's surface. According to the National Weather Service, about 10% of the land in the United States is paved roads. Because of all the pavement, water is redirected into sewage and storm drain systems. In addition to the increase in urban structures, there is a resultant decrease in vegetation. Trees, shrubs, and grasses cannot take away some of the water that enters into an area as a result of the water cycle. Part of the devastating game of playing with Mother Nature is when humans interfere with the natural flow of water in the Earth system. In nature, water will flow as rivers and streams change course. In urban areas, water must follow the prescribed pathways set forth by large water systems that direct water where to flow. Simply put...humans usually will not suffer the inconvenience of water flowing in an area deemed 'unfit' for water. According to FEMA, the basic philosophy of urban drainage systems, or "stormwater management", for re-directing water flow has been to seek maximum convenience at an individual site by the most rapid possible elimination of excess surface water after a rainfall and the containment and disposal of that water as quickly as possible through a closed conveyance system. In other words, "get that water out of here NOW" has been the overlying philosophy of creating drainage systems in urban areas. If the chance of rainfall is high, people do not want that water around for long. How Urbanization Fails To image how all these efforts have actually increased the risks of significant flooding in urban areas, imagine a severe thunderstorm that dumps a significant amount of precipitation in an area over a relatively short period of time. Without urbanization, the waters that flood the area would be more likely to infiltrate into the ground, move to nearby streams, collect in natural ditches and low-lying areas, or move into the groundwater supply. In other words, the water cycle would work fairly undisturbed and the water would either evaporate, infiltrate, or move to rivers and then to the ocean. With storm water collection systems, the water will actually reach the river system faster than Mother Nature would have moved it. Therefore, flooding will happen more quickly and with a greater intensity than before urbanization occurred. As noted by FEMA, about 20-25% of all economic losses resulting from flooding occur in areas not designated as being in a “floodplain,� but as a consequence of urban drainage. Clearly there is a price for economic development and the rise of the concrete world. For more discussion of this topic, check the links below: floods west auckland
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