Theory

• Precipitation falls in greater amounts and with greater intensity in towns and cities than in the surrounding rural area. Natural landscapes like forests, wetlands and greenlands trap precipitation and then allow it to infiltrate slowly into the ground, contrasting impermeable urban surfaces such as roads and car parks.
• Urban areas are designed to shed water quickly. Sloping roofs, smooth, rounded guttering and cambered roads all contribute to the rapid movement of water away from the surface. Water is then gathered in smooth storm sewer systems which act like a high-density drainage system. It gathers speed and erosional power as it travels underground. When the water leaves the storm drains and empties into streams, they fill rapidly.
• Because much of the precipitation is unable to infiltrate impermeable urban surfaces, groundwater and soil water storage levels are reduced. Since this is the water that feeds streams during dry periods, base level flows are reduced. The resultant storm hydrograph for an urban river shows a river with a flashy discharge but low base flow.
• According to the Asian Development Bank, the Asian population that is vulnerable to inland flooding is expected to reach 350 million by 2025.