Approach:

Introduction - Explain Flash Floods and it's impact in brief.

Body - Discuss the factors and points relating to urban planning helping in the mitigation of flash floods.

Conclusion - Give a way forward along with strong implementational measures to mitigate flash floods.

Answer:

Flash floods refer to sudden and rapid flooding events that occur within a short period, typically within a few hours or even minutes. They are characterized by a swift and significant rise in water levels in rivers, streams, or urban areas, often resulting from intense rainfall, dam failures, or the sudden release of water from reservoirs. They are a serious threat to urban areas and can occur suddenly and without warning, causing widespread damage and loss of life. Flash floods in urban areas can occur due to a combination of natural and human factors.

Some key factors that contribute to the occurrence of flash floods in urban areas:

1. Impervious Surfaces: Urban areas are characterized by extensive impervious surfaces such as roads, parking lots, and buildings, which prevent rainwater from infiltrating into the ground. Instead, rainfall quickly runs off these surfaces and overwhelms the drainage systems, leading to rapid accumulation of water and flash flooding.

For example: Mumbai Floods 2005: Heavy rainfall overwhelmed the drainage systems in Mumbai, causing widespread flooding. The city's extensive impervious surfaces contributed to the rapid accumulation of water. Over 500 mm (20 inches) of rain fell in a 24-hour period, resulting in the loss of over 1,000 lives and affecting millions of people.

1. Stormwater Infrastructure: Inadequate or poorly designed stormwater infrastructure is a significant factor contributing to flash floods. Insufficient capacity or blockages in storm drains, culverts, and sewer systems can cause water to back up and overflow onto streets and low-lying areas, exacerbating flood risks.

For example: Chennai Floods 2015: Insufficient stormwater infrastructure in Chennai, combined with heavy rainfall, led to severe flooding. The city's storm drains and channels were unable to handle the excess water, resulting in inundated streets and neighborhoods. The floods affected over 4.6 million people and caused economic losses of around $3 billion.

1. Urbanization and Land Use Changes: Urban development often involves modifying the natural landscape, including clearing vegetation, altering natural drainage patterns, and filling wetlands. These changes reduce the land's ability to absorb and retain water, increasing runoff and flood potential.

For example: Uttarakhand Floods 2013: Rapid urbanization and unregulated construction activities in the hilly regions of Uttarakhand contributed to flash floods and landslides. The excessive land use changes, including deforestation and encroachment on river banks, amplified the impact of heavy rainfall. The floods caused the loss of thousands of lives and extensive damage to infrastructure.

1. Climate Change: Climate change plays a role in increasing the frequency and intensity of extreme weather events, including heavy rainfall. Urban areas are particularly vulnerable due to their concentration of impervious surfaces and inadequate drainage systems. As climate change progresses, the risk of flash floods in urban areas is likely to escalate.

For example: Kerala Floods 2018: Intense rainfall events linked to climate change resulted in devastating floods in Kerala. The region experienced unprecedented rainfall, leading to overflowing rivers and landslides. The floods affected over 5.4 million people and resulted in the loss of over 480 lives. The economic damages were estimated to be around $2.8 billion.

To mitigate the risk of flash floods, urban planning should incorporate strategies that address these factors. Here are some measures that can be implemented:

1. Green Infrastructure: Urban planning should emphasize the integration of green infrastructure, such as green roofs, permeable pavements, rain gardens, and bioswales. These features help to absorb and retain rainfall, reducing runoff and alleviating stress on drainage systems.

1. Floodplain Management: Identifying and preserving natural floodplains can help accommodate excess water during heavy rainfall events. Restricting development in flood-prone areas and implementing zoning regulations can help prevent construction in high-risk zones.

1. Improved Drainage Systems: Upgrading and maintaining stormwater drainage systems is essential. This includes regular cleaning of drains and pipes, increasing the capacity of existing infrastructure, and incorporating innovative designs to improve water flow and prevent blockages.

1. Sustainable Land Use Planning: Urban planning should prioritize sustainable land use practices. Preserving green spaces, promoting compact development, and encouraging mixed land use can reduce the extent of impervious surfaces and promote natural drainage.

1. Early Warning Systems: Implementing effective early warning systems can provide residents with timely alerts about impending flash floods. This allows them to take necessary precautions and evacuate if required.

1. Public Awareness and Education: Educating the public about flood risks, emergency response protocols, and the importance of responsible urban practices is crucial. Promoting community engagement and awareness can foster a culture of preparedness and resilience.

To address these flood events and mitigate the risk of future flash floods, India has taken various measures, including the implementation of the National Urban Flood Management Program (NUFMP) and the integration of green infrastructure in urban planning. However, there is a need for continuous efforts to improve stormwater infrastructure, land use planning, and climate change adaptation strategies to minimize the impact of flash floods in urban areas.