Extreme rainfall events and urban floods: Explained, pointwise

Introduction

Recently, heavy rainfall in parts of North India led to extensive flooding, impacting major cities such as Delhi, Gurugram, Patiala, Ludhiana and Jalandhar. Urban floods have now become a widespread occurrence across the nation, affecting numerous mega and metropolitan cities in India on an almost annual basis. While climate change and global warming are often cited as major causes of urban floods, several other factors also contribute to the problem. 

What caused the recent episode of heavy rainfall? 

Recent weather conditions which triggered heavy rains in Northwest India and the Himalayas were like those which caused 2013 Uttarakhand floods. 

First, an active monsoon with strong winds in the lower atmosphere, brought moisture into the region from the Bay of Bengal or the Arabian Sea.  

Second, a large-scale atmospheric forcing (a dynamic process that forces the air to rise) occurred. Outflows in the upper layers of the atmosphere moved eastward through mid-latitude troughs (Western Disturbance). These troughs can also direct the flow of moisture towards the Himalaya.  

Third, deep convection was triggered by orographic uplift due to the steep terrain of the Himalaya. 

Thus, an interaction between a western disturbance and active monsoon, combined with the topography of the Himalayas led to extremely heavy rainfall over Himachal Pradesh, Uttarakhand, Punjab and Haryana 

Excess rainfall over Northwest India is also related with the warm Arabian Sea (warmed by about 1.5 degrees Celsius since January) which has infused excess moisture over North-Northwest India 

Why are extreme rainfall events increasing? 

A study on the 2013 floods in Uttarakhand identified the Himalayan region is conducive to formation of deep, intense convective clouds. Instances of such deep convection have increased over time mainly due to climate change. Warmer temperatures lead to increased evaporation, which leads to more moisture in the atmosphere. This extra moisture leads to heavy rainfall when weather systems interact with the steep topography of the region.  

Human influence is causing the Arctic to warm and sea ice to shrink. There is increasing evidence that Arctic warming impacts monsoon climate through mid-latitude circulation. Observations and models suggest that due to Arctic warming, the frequency of occurrence of deep mid-latitude troughs is likely to increase. 

Due to climate change, hilly areas and surroundings like the Himalayan foothills or the Western Ghats are more vulnerable to heavy rains and landslides. Because of global warming, there’s extra moisture in the atmosphere which is lifted orographically and comes down as heavy rains.  

Changes in land use and land cover also appear to be reasons for the increase in extreme monsoon rainfall, especially its intensity.  

The IPCC’s scenarios for climate change indicate that these trends will continue. Multi-day flood events are projected to increase faster than single-day events in the future. 

What are the other causes of recurrent urban flooding? 

Urban planning: According to a NITI Aayog report titled ‘Reforms in Urban Planning Capability in India’, 65% of the urban settlements do not have any master plan. As a result, there are fragmented interventions, disorderly constructions and urban sprawl, all of which can worsen problems like urban flooding.  

Urban governance: On the governance front, municipal administrations have disregarded unauthorized construction. They also failed to prevent encroachments, particularly on water bodies and allowed construction in low-lying areas without adequate mitigation measures. Failure to stop indiscriminate disposal of solid waste and illegal dumping of construction debris also adds to the problem.  

Encroachment of water bodies: Water bodies such as wetlands, marshes, lakes, riverbeds and their floodplains act as natural rainwater sinks and buffers against floods. Their encroachment due to rapid and unplanned urbanisation has limited their ability to perform this vital function. 

Deforestation in the catchment area: The catchment regions of rivers and their tributaries have seen uncontrolled deforestation and degradation of vegetation. This has increased soil erosion and siltation which has increased the load of sediments in riverbeds and reduced their water-carrying capacity. The discharge of waste into the rivers has added to this problem. 

Storm water drains: In many urban localities, there is absence of storm water drains. At places where a storm water drainage network exists, it was planned several decades ago and thus inadequate to handle increasing instances of extreme rainfall. For example, Delhi’s storm water drainage network can carry a maximum of 50 mm of rainfall in 24 hours but recently, the city recently saw rainfall of 153 mm in a day. Poor maintenance of existing storm water drains due to incompetent municipal administration leads to their clogging with mud and material which further compounds the problem. 

What are the steps taken by the government? 

The National Disaster Management Authority (NDMA) has issued the National Guidelines on Urban Flood Disaster Management. The guidelines are comprehensive and identify the institutional framework required for improving flood resilience through better flood disaster management. 

A National Disaster Management Plan has been developed. The plan has a special section on urban flood disaster risk reduction.  

A standard operating procedure (SOP) for urban flooding was also brought out by the Ministry of Housing and Urban Affairs. The SOP identified the objectives, mitigation strategies for urban flooding, nodal agencies for early warning, city-level action plans including the establishment of the emergency operations center and other related measures including reporting formats.  

Also, the Ministry of Housing and Urban Affairs has released the first dedicated stormwater drainage manual. The manual has taken inspiration from established international codes of practice and has been modified to suit the local conditions. 

The government has also launched schemes like Atal Mission for Rejuvenation and Urban Transformation (AMRUT), National Heritage City Development and Augmentation Yojana (HRIDAY) and Smart Cities Mission for capacity building of urban local bodies. 

What should be done? 

Resilient design in urban planning: This should involve restoring traditional water bodies, revitalizing lakes, building artificial lakes, and developing green infrastructure such as rain gardens. These measures will enhance permeability, create open spaces, and effectively manage stormwater runoff. Rainwater harvesting in parks and open spaces will act as sponges and reduce flooding. 

Comprehensive stormwater drainage master plan: Cities should have separate stormwater and sewage drains. Master plans should be developed for all urban settlements keeping in mind the city’s land use patterns and drainage infrastructure. Encroachment of stormwater drains should be stopped, and they should be solely used carrying excess rainwater. 

Assess flood-carrying capacity of Indian cities and prepare urban flood risk database: This assessment would involve evaluating the ability of cities to manage flooding by examining the level of inundation during different intensities and durations of precipitation. This information would aid in fine-tuning forecasting models, allowing for timely issuance of early warnings and alerts based on specific regional thresholds.  

Governance measures: There should be regulated real-estate growth with proper implementation of land use planning and zoning regulations. For example, areas prone to flash floods can be designated as non-residential or restricted areas. 

Restoration of ecosystems: Ecosystems, such as forests and wetlands, should be protected and restored so that they can act as a natural buffer, absorbing rainfall and reducing runoff. 

NDMA guidelines: The recommendations made by the NDMA guidelines should be implemented. 

Sources: Indian Express, Times of India, Hindustan Times and Deccan Herald.