Why Himachal Pradesh and Uttarakhand need to step up flood management

Two of India’s Himalayan states, Himachal Pradesh and Uttarakhand have seen multiple instances of flash floods this monsoon season. Is this a result of abnormal rainfall, or something else? An HT analysis of various data sets shows that multiple factors, including growth in habitations, need to be taken into account to understand the growing risks of floods in these regions and that mechanical interpretation of rainfall data can be misleading.

Two of India’s Himalayan states, Himachal Pradesh and Uttarakhand have seen multiple instances of flash floods this monsoon season. Is this a result of abnormal rainfall, or something else? An HT analysis of various data sets shows that multiple factors, including growth in habitations, need to be taken into account to understand the growing risks of floods in these regions and that mechanical interpretation of rainfall data can be misleading. Here are five charts which explain this argument in detail.

A general view of the section of a railway bridge washed away by the deluge at the Chakki River after flash floods in Kangra district, in India’s Himachal Pradesh state on August 20, 2022. - At least 15 people were killed in India after heavy monsoon rains triggered flash floods and landslides near the Himalayan foothills, authorities said August 20, 2022. (Photo by AFP) (AFP)

While the overall monsoon is normal, the hills saw more intense rainfall events

So far, the 2022 monsoon is far from being rainiest monsoon in Himachal Pradesh and Uttarakhand. In the June 1 – August 29 interval, the 10.5% and 5.2% surplus they have compared to the 1961-2010 average is ranked only 58th and 61st since 1901. This headline number, however, does not tell us about extreme rainfall events, which are the biggest cause of flash floods in the hills.

There were floods in both states in the week ending August 20. This week in 2022 was ranked 13th and 53rd since 1901 for Himachal Pradesh and Uttarakhand in terms of overall rainfall, but 8th and 17th for heavy and extreme intensity rain. The India Meteorological Department (IMD) classifies more than 35.5 mm rain within 24 hours as heavy and extreme rainfall events. To make matters worse, such intense rain was more than 2-3 times the 1961-2010 average in most places where it fell, which included some of the most mountainous regions of Himachal Pradesh and almost all of Uttarakhand. This is best seen by looking at the rainfall and altitude map of the two states together.

See Map 1 and 2

Rainfall became more intense in the hills in the last decade

An analysis of high intensity rainfall data from IMD shows that large parts of Himachal Pradesh and Uttarakhand have seen an increase in this in the decade ending 2020 over the period from 1961-2010. But this piece of statistical information is not enough to understand the risks of flooding in these states. For example, Map 3 shows that overall high intensity rain did not increase in a large part of Lahaul and Spiti district in the 2011-2020 decade, where a flash flood took place in early August. It has also decreased 5.3% if one looks at the Himachal Pradesh as a whole.

See Map 3

So, this data does adequately capture flooding risks. What these numbers hide, however, are other ways in which rainfall can lead to disaster. For example, if a place received two days of 50 mm rain each in the 1961-2010 decade (giving a total of 100 mm of heavy intensity rain) and just one day of 80 mm rain in 2011-2020, it will show a decrease in overall amount of high intensity rain. A significantly high intensity rainfall in a day than what the threshold for such classification is (80 mm versus 50 mm, for example) will significantly increase the risks of flooding in the hills. One way to capture this it to look at how much high intensity rain averaged per day of such rain. This number has increased even in some regions where overall high intensity rain has not, such as the Lahaul and Spiti district. The two numbers taken together show that there are hardly any regions of the two hilly states where rainfall has not become more intense.

See Chart 4

Receding glaciers, dams, and urbanisation increase risk of floods

More intense rain is not the only thing that has increased the risk of disasters in the hill states. As HT reported earlier (https://bit.ly/3ApbW5y), temperatures have also increased in these states, and faster than in the rest of the country. This has increased the speed at which glaciers are receding in the Himalayas, leaving rock and sediment deposits that act like natural dams. When they breach (due to high intensity rains) they cause floods and landslides. Excess water and collapsing earth would still not be a big problem if the water was absorbed by land or did not affect human lives and property. However, this mitigating factor has decreased fast in the past three decades, land cover data from the European Space Agency’s (ESA) Climate Change Initiative (CCI) shows. This data tracks changes in majority land cover for grids of around one square kilometre area, and shows that urban areas have increased almost ten times between 1992 and 2015 in the two states. In 12 of 25 districts in the two states, all the urban area has been built only after 1992, the earliest year for which the ESA has this data. While some of this dramatic growth shown by the data can be the result of the low resolution of the dataset, it can also miss smaller changes like roads built to connect the urban areas. Whatever the exact rate of urbanisation may be, the data shows that it has been fast, increasing the risk of disaster. That risk can be lowered only if urban areas are adapted for floods and landslides.