Does Urbanization can contribute to changes in flooding intensity and frequency

The relationship between the increasing urban runoff and flooding due to increased imperviousness is better perceived than that between the cyclic impact of urban growth and the urban rainfall via microclimatic changes. Human activity in urbanization and migration from rural areas (and leave cultivation) can increase the risks of flooding and increase the intensity of this event, for example by paving over areas which were previously covered by vegetation, reducing the capacity of the land to absorb rainfall and causing it to run off more quickly. Planning decisions - such whether to build houses in flood plains or position caravans next to the sea - can also affect society's vulnerability to floods.

Due to urbanization two important phenomena will happen, including population growth and industrial growth, so the microclimate of the area would be changed, for example urban heat islands and aerosols would be occur, therefore these changes arise other events such as changes in extreme rainfall patterns - due to urban heat island effect-, emission further CO2, and other events. Heat island can increase the rainfall in the vicinity of the cities. For urbanization and create buildings and other essential structure, peoples have to cut trees, so evapotranspiration amounts decrease, hence the latent heat flux will reduce. As well, the increase in artificial surfaces due to urbanization causes an increase in flooding frequency due to poor infiltration and reduction of flow resistance (due to faster concentration times). Urbanization invariably increases the flood risk as a result of heightened vulnerability, stemming from population concentration, wealth, and infrastructure to smaller areas (Taisuke et al., 2009; UN, 2006). Indeed, urban development increases flood risk in cities due to local changes in hydrological and hydro-meteorological conditions that increase flood hazard (WMO/GWP, 2008).

The hydrological changes of urbanization will increase the flood peaks and decrease the concentration times. The dramatic change in land use within and surrounding the city will impose impacts on the urban water cycle, resulting in more frequent and higher-magnitude floods. Drainage systems of urbanization are also important. If the drainage system in a city doesn't create by legal constructions or with the technical requirements, thus leading to limited capacity to collect waste-water or rainwater, also it causes sediments to remain, so when a heavy rainfall would have happened then the city seriously faces to intensive flood. As well, Canals crisscrossing the city play an important role, acting as outlets for drainage network (Nirupama and Simonovic, 2007). If they have not installed in proper and legal ways, in case of heavy rainfall, would cause flooding.

Another outcome of urbanization that affects the severity of the flood is: impervious paving, such as streets, highways, roads, railroad lines, parking lots, and waterways. An impervious area is considered as the main cause of urbanization which all rainfall falling on the surface is presumed to produce surface runoff, then the risk of flooding and its intensity and frequency after heavy rainfall can increase and intensify.

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