Can land, trees, structures, farms, pipes, poles, pumps etc, along a river bank several kilometers downstream collapse if water flow in the river is suddenly reduced or stopped? Yes, reduced/ stopped, not increased? This kind of river bank slump or collapse or failure is not easily understood, but it does happen. Let us see how.
It happened last week in California state (USA), downstream of Oroville Dam along the Feather river. But more about it later.
When water flow in a river is suddenly stopped or drastically reduced, it can lead to slump in the river bank, including river bed structures. This is a well known phenomena, called River Bank Slump or Failure[i].
According to Wikipedia, River bank failure can be caused when the gravitational forces acting on a bank exceed the forces which hold the sediment together. River Bank Failure can occur due to many different reasons, but here we try to understand when this happens due to sudden water flow changes in the river.
David Petley said in his blog[ii]: “Riverbank collapse during rapid draw down is a well-known phenomenon – indeed riverbank failures are most likely on the falling limb of the hydrograph. As the river level rapidly falls the banks are left in a saturated state, which means that they are weak. But more importantly, they also have a hydraulic imbalance, which drives flow of water through the soil towards the river. This provides an additional force that reduces stability… Drawing the river down more slowly means that the hydraulic instability is avoided, and the riverbank failures should not occur.”
Avoidable Damage along the feather river in California So on Monday, Feb 27, 2017, farmers along the Feather river experienced a new round of damages when water flow was suddenly stopped from damaged spillway on Sunday, Feb 26, 2017, from 50000 cusecs to zero.
San Francisco Chronicle reported[iii], “But they apparently did not anticipate a side effect of their decision to stop feeding the gushing Feather River — a rapid drop in river level that, according to downstream landowners, caused miles of embankment to come crashing down. With high water no longer propping up the shores, the still-wet soil crashed under its own weight, sometimes dragging in trees, rural roads and farmland, they said. “The damage is catastrophic,” said Brad Foster, who has waterfront property in Marysville (Yuba County), about 25 miles south of Lake Oroville. The farmer not only saw 25-foot bluffs collapse, but also lost irrigation lines to his almonds. “When the bank pulled in,” he said, “it pulled the pumps in with it. It busted the steel pipes.””
It further reported that the dam operated had departed from their own practice: “The farm communities around Yuba City, where much of the riverbank damage was done, saw the rapid drop in water the next day. “Most of the time, they’ll give us three to five days, maybe even a week to lower the river,” said Phillip Filter, who’s been growing prunes, walnuts and peaches on waterfront property outside the town of Live Oak (Sutter County) for decades. “But what they did to this Feather River now is just ridiculous.””
Yuba County’s emergency operations manager, Scott Bryan, who has surveyed the deterioration of the riverside, said he hasn’t seen anything like it. “The landowners understand that the water will come up and do damage to their trees, but this is different,” he said. “This is actual loss of their land due to sloughing. “I can’t tell you 100 percent it was because the Department of Water Resources allowed the water to drop too rapidly,” he said. “But it’s consistent with that.”
The quick draw-down of water also impacted the fish in the river. It left chinook salmon and steelhead struggling to survive in isolated pools and puddles on the river’s edge. The fish, which migrate to the Pacific Ocean, were trapped as the main stem of the river swiftly shrunk.
Sacramento Bee reported: For miles along the channel, huge chunks of the river’s banks collapsed into the water, toppling wild cottonwood, oak and black walnut trees. Filter said neighboring farmers lost irrigation pumps into the river. Roots from some of Filter’s orchards are now dangerously close to the gaping wounds in the river bank… Filter, the Live Oak farmer, looking out over the mangled river bank and the downed trees – some of them oaks nearly a century old – said he wishes the state would have shut down water flows from the main spillway more gradually. He worries bigger chunks of the riverbank will collapse in the next round of storms, bringing down some of his valuable trees with them.[iv] State officials were yet to accept the damaging impacts of their action of suddenly stopping flow or if they could have reduced the flow more gradually to avoid the damages.
A brief video[v] shows how the land along the feather river has slumped by upto 20 ft.
How some other countries deal with this issue Across several countries in the world, a drawdown rule is set for hydropower plants so that their ramping up and ramping down (discharging and stopping discharge) rates do not cause soil erosion. In addition, Mekong Hydropower Dams are recommended by the Mekong River Commission to have Up ramping or down-ramping rules to minimise downstream bank erosion.
For the Gordon River in New Zealand, an explicit draw-down rule has existed since 2006 which stated: “Gordon Power Station had been discharging above 180 cumecs for greater than 60 minutes and Hydro Tasmania intended to reduce discharges to below 150 cumecs, the reduction in flow should occur at a rate of less than 30 cumecs per hour.”[vi] This is now further refined.
Ramping rates depend on a number of factors, including objectives to be achieved, prevailing flow range, if the channel is in low flow or High Flow regime, if the season is flood season or lean season, among others. The objectives could include protection for fish species or other biodiversity, protection of the river bed and river bank, temperature control, among others.
Please note that this exists when the hydro-peaking of these projects is a fraction of what we are envisaging in a river basin which is already notorious for erosion and sediment related problems. (Ganga and Brahmaputra)
How it is relevant This kind of sudden fluctuation in river flows can happen when hydropower projects operate on peaking mode, thus releasing huge amount of water for a few hours, say 4 hours in the morning and 4 in the evening, when power demand is at peak and almost none or namesake environment flows in rest of the hours. Such massive fluctuations in river flows twice a day, almost all round the year can have massive impacts on the river bed, river banks and all those using the river bed and river banks, including the biodiversity and also even the stability of the structures on the river bed and river banks. So one would expect that the environment and social impact assessment (EIA) of such hydropower projects would assess such impacts of the projects on river beds and river banks, but we have yet to see any such assessment or even mention of such impacts.
Similarly, when there are multiple hydropower projects along the same river, cumulative impact assessments (CIA) are done at river basin level. Such assessments have been done for a number of rivers like Siang, Subansiri, Lohit, Dibang, Tawang, Teesta, Sutlej, Bhagirathi-Alaknanda, among others. Again in none of these river basin wide assessment have we seen how the river beds and banks would be impacted by the cumulative impact of the peaking power operation of hydropower projects in the upstream, leading to sudden ups and downs in the river flows on daily basis. All these EIAs and CIAs are appraised and approved by the Expert Appraisal Committee on River Valley and Hydropower projects of Government of India’s Union Ministry of Environment, Forests and Climate Change (MoEFCC) and officials of MoEFCC. In case of CIAs, they are also approved by the Central Water Commission of Government of India’s Union Ministry of Water Resources, River Development and Ganga Rejuvenation.
And yet we have not seen any of these organizations raising such fundamental inadequacy of the EIAs and CIAs.
Let us hope that our official bodies will now learn and ask the CIAs and future EIAs to look into this important missing issue in the assessments.
Himanshu Thakkar (firstname.lastname@example.org, with inputs from Parineeta Dandekar), SANDRP
[vi] https://www.hydro.com.au/system/files/basslink/140342_Hydro_Gordon_River_Basslink_Monitoring_Program_Book-lr3.pdf, http://www.mrcmekong.org/assets/Publications/policies/1st-Interim-Report-ISH0306-Volume-2-Final.pdf