Why are Patna & Bhagalpur facing unprecedented Ganges floods

Floodwaters entered colonies in Patna near Ganges (Photo India Today)

From SANDRP
As India celebrates 75th Independence day on August 15, 2021, large parts of Bihar along the Ganga river, including Patna (flood water entered colonies near Ganga in Patna) and Bhagalpur are preparing to face unprecedented floods. In fact, Central Water Commission’s (CWC) flood monitoring site at Hathidah in Patna district crossed the HIGHEST FLOOD LEVEL (HFL) of 43.17 m at 0300 hours in the early morning on Aug 13, 2021. The water level is already at 43.33 m at 1300 hrs on Aug 14. It is forecast to reach 43.45 m by 0800 on independence day still with a rising trend. This is apparent from the CWC hydrograph of this site shown above.
CWC FF tweet about severe inundation around Hathidah in Patna dist in Bihar along Ganga on Aug 13, 2021:
https://twitter.com/CWCOfficial_FF/status/1426126406136655879
At upstream Gandhighat in Patna, the water level is already at 50.38 m, within 14 cm of its HFL (50.52 m), the CWC forecast says it will be within 11 cms of HFL in the morning of independence day, still showing a rising trend.
At downstream Bhagalpur, the Ganga river water level is forecast to reach 34.7. m, just 2 centimetres within its HFL of 34.72 m by 1600 hrs on Independence Day. It is already at 34.53 m at 1100 hrs on Aug 14.
A little further upstream at Ballia in Uttar Pradesh, the water level is at 60.25 m still with a rising trend, just 14 cms lower than its HFL of 60.39 m. CWC has forecast that this will now go down to 60.15 m at 0800 hrs on Aug 15, but let us see how correct that forecast is.
Why these unprecedented Ganga Flood levels? This could be largely seen as the effect of huge rainfall and floods in upstream Ganga basin areas of Madhya Pradesh and Rajasthan during Aug 3-6, which became a flood peak along the Yamuna that is now travelling downstream, releases from a number of large dams in those areas also contributing to the flood peak.
There is no doubt that flood peak is playing a role in Ganga crossing HFL at Hathidah now. But is that explanation good enough? The fact is the water level did not reach the HFL levels at almost any of the numerous sites along the Yamuna (with the exception of Auraiya, possibly because of its close proximity with the point where Chambal brought the flood waters to the Yamuna) or Ganga upstream of Hathidah and the sites mentioned above.
The Farakka factor: Farakka Influence Zone There is another factor that is playing a role here. The name of that factor is FARAKKA dam on the Ganga downstream from Bihar. Farakka dam, as Bihar Chief Minister Nitish Kumar, has said many times earlier, is acting as a bottleneck, leading to the accumulation of hundreds of millions of cubic meters of debris (including silt, sand, boulders and other materials that the river brings) on the river beds each year. This has led to huge drainage congestion along the river, reducing the carrying capacity of the river massively. It is this then which does not allow the quick drainage of the flood peak, a problem that the river upstream from what we can call Farakka Influence Zone (FIZ) does not face, but the river flow starts slowing down in FIZ.
the Ganga water level at Farakka went above the danger level of 22.25 m in the evening hours of Aug 5, 2021, and has shown a rising trend since then. This level is likely to remain above danger level till close to the end of the monsoon season, as is normally the case.
2017 Union Govt Committee on Role of Farakka: CWC had no data What is this impact of FIZ on the river and how far this zone is extended is a subject of investigation. Following the unprecedented Bihar floods of Aug 2016 (see below for details) the issue of the role of Farakka had arisen. On request of Bihar CM Nitish Kumar, the Union Government had set up a committee in Sept 2017. However the Central Water Commission, which is supposed to be the repository of all the Ganga silt and flood-related information, did not have ANY pre-Farakka information (which is key to understand the role of Farakka) and not even most of the post-Farakka information, so there was no way to convincingly arrive at the answers to questions related Farakka Influence Zone. This author was a member of the committee. The committee was wound up with a split report: Bihar Govt appointed 5 members of the committee submitting a detailed report about the key issues, information non-availability and so on; the CWC appointed members submitting a report that could not answer any of the issues raised. Because of CWC’s inability to provide the basic information required for the work of the committee, the committee could not come to any useful consensus conclusions.
Aug 2016: Unprecedented Bihar floods The unprecedented flood levels along Ganga in Bihar this year is a repeat of what happened almost exactly five years ago, in Aug 2016[i]. That time, as SANDRP wrote then, there was a role of upstream (water release from Ban Sagar Dam in Madhya Pradesh in upstream Ganga Basin) and a downstream (Farakka) factor. That time too the Ganga water levels along Ballia, Gandhighat, Hathidah and Bhagalpur had breached the HFL levels, as it is happening this year.
This seems to indicate that huge, widespread areas along Ganga upstream of Farakka, possibly right up to Ballia will continue to face such unprecedented floods every few years, possibly with increasing frequency and intensity under climate change-induced rainfall pattern, as long as we are unable to convincingly diagnose or solve the impact of Farakka. In fact, Farakka increases the floods along the Ganga even in normal monsoon years. We hope Bihar CM Nitish Kumar, Bihar Water Resources Department and others concerned will raise this issue again as Bihar seems to be the biggest sufferers of FIZ.
Himanshu Thakkar (ht.sandrp@gmail.com)
Source: https://sandrp.in/2021/08/14/why-are-patna-bhagalpur-facing-unprecedented-floods-on-indias-75th-independence-day/