The main problem with urban flooding is the fact that they occur in highly populated areas. 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. 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. The water rises relatively slow and the water level usually does not reach life endangering heights. The water slowly rises on the city streets. Because of all the pavement, water is redirected into sewage and storm drain systems. High intensity rainfall can cause flooding when the city sewage system and draining canals do not have the necessary capacity to drain away the amounts of rain that are falling. When the city is on flat terrain the flow speed is low and you can still see people driving through it.
In nature, water will flow as rivers and streams change course. Simply put…humans usually will not suffer the inconvenience of water flowing in an area deemed ‘unfit’ for water. Sometimes you see dancing drain covers. The economic damages are high but the number of casualties is usually very limited, because of the nature of the flood. If the chance of rainfall is high, people do not want that water around for long. Urban floods are a great disturbance of daily life in the city. 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. 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.
Water may even enter the sewage system in one place and then get deposited somewhere else in the city on the streets. 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. As there is little open soil that can be used for water storage nearly all the precipitation needs to be transport to surface water or the sewage system. In addition to the increase in urban structures, there is a resultant decrease in vegetation. Therefore, flooding will happen more quickly and with a greater intensity than before urbanization occurred. Urban flooding is specific in the fact that the cause is a lack of drainage in an urban area. In urban areas, water must follow the prescribed pathways set forth by large water systems that direct water where to flow. As areas become ‘urbanized’ or go through the process of urbanization, there are increased flood risks that result.