The state-owned operator of two colossal Chinese hydro plants affected by 30% drops in generation over the April-to-June quarter has told The Energyst that reservoir filling upstream, and not seismic activity, caused their big, short-lived drops.
Without comment on 15 July, China Yangtze Power Corporation announced to the London Stock Exchange that water flows through the 120-mile long Xiluodu dam on the upper Yangtze fell by over 44% in the quarter.
That reservoir powers both Xiluodu’s own turbines and at a dam at Xiangjiaba, fifty miles downstream. Rated at 13.86GW and 6.45GW respectively, the two dams are behind only China’s Three Gorges complex in generation capacity. In world rankings, they are at numbers 3 and 9.
On 21 May, the US Geological Survey recorded a 6.1 magnitude quake near Dali City, 280 miles from the reservoir, and close to the Yangtze’s source in the geologically unstable foothills of the Himalayas.
Quoting the journal Nature and its citing of two named Chinese seismologists well versed in the region’s geology, our story speculated that the 21 May earthquake near Dali had caused the drops in waterflow and in power generation.
‘The Energyst’ immediately asked the China Yangtze Power Corporation for causes of the drop in water flows, and what its preparations were to combat earthquakes.
Two weeks after we’d asked requested their clarification, CYPC provided late last week a detailed reply including information not included in their announcement on 15 July to London’s capital markets.
For the Xiluodu dam’s near-halving in waterflow, CYPC gives two explanations:
“first, the reservoir water inflow was noticeably ample in the same period last year, so the comparison base is larger;
“second, several new reservoirs were built and were in the process of impoundment at the upper reaches of Xiluodu in Q2, 2021 which reduces the water inflow at (the) reservoir”.
The impoundment – or filling of reservoirs – around the unidentified new dams was a one-off process, said CYPC. Generation levels are expected to return quickly to normal, the statement implied.
The company provides no further details as to how many new dams on the upper Yangtze have had reservoirs filled, or the volumes and timescales needed to do so. It merely said;
“When the new reservoirs are put into normal operation, the water volume in the upstream reservoirs will increase, which will help improve the power generation of Xiluodu and Xiangjiaba..”.
“We operate and maintain these plants following the concept of accurate control, lean operation and meticulous maintenance”, the operator went on.
“Up to now, the generating capacity of Xiluodu and Xiangjiaba hydropower stations is always kept at normal level, and the specific power generation mainly depends on the water inflow”.
On earthquake activity in the river basin of the upper Yangtze – also known as the Jinsha – , the CYPC notes:
“Generally, (a) reservoir-induced earthquake is triggered by water weight and seepage. It always occurs within 5 to 10km (of) the reservoir. Any earthquake beyond this range cannot be regarded as induced by the reservoir”.
Hundreds of thousands of dams exist around the world, CYPC asserts, only about 100 of whom have experienced large, reservoir-induced earthquakes.
“In the past 12 months, we analyzed data from the reservoir-induced earthquake monitoring system set up by the company.
“(We) concluded that the water pressure in Xiangjiaba and Xiluodu reservoir areas at the Jinsha River Basin increased due to the impoundment has little effect on the underground crust several kilometres deep.
“No large earthquake is induced or triggered by reservoir impoundment”.
“Earthquakes (are) frequent and strong in the downstream of the Jinsha River”, CYPC concedes. For this reason, it says;
“the company has invested lots of funds (sic) to build a reservoir-induced earthquake monitoring system covering Xiangjiaba and Xiluodu reservoir areas…. This system includes (networks for) earthquake measurement, observation of active fault and strong motion, dynamic monitoring of groundwater, (and a) network for monitoring crustal deformation”.
“In recent years, several large earthquakes have occurred in southwest China”, CYPC agrees. “However, based on the analysis of monitored data, Xiluodu and Xiangjiaba hydropower projects operate reliably and safely”.
The CYPC statement cites the Wenchuan earthquake in May 2008, a 7.9 magnitude quake 200 miles from Xiluodu. It said a team of specialist seismologists who investigated from the International Committee on Large Dams (ICOLD)concluded:
“first, earthquakes don’t produce the failure of dams. No large dam in the world has ever collapsed due to an earthquake. ….second, dam filling applies pressure to the earth’s surface ….which may induce a small earthquake below magnitude 2 around the reservoir area, but no large ones”.
Dr Martin Wieland, the Swiss-based seismologist who chairs ICOLD’s committee on seismic aspects of dam design, endorsed the CYPC statement to ‘The Energyst’.
“A magnitude 6.1 earthquake occurring at distances of 40 km from a well designed, well constructed dam should not be a safety problem at all, “ wrote Wieland.
“However, transmission lines could be damaged by rockfalls etc., which would lead to a temporary stop of power generation”,
Dr. Wieland named Baihetan as a new dam recently commissioned 150 miles upstream of Xiluodu, with a reservoir of 18 cubic kilometres. Though not familiar with the new dam’s filling procedure, Dr Wieland said its impoundment could easily explain the drop in energy production at the two established dams.