Flood Control Challenges for Large Hydroelectric Reservoirs

FLOOD CONTROL CHALLENGES FOR LARGE HYDROELECTRIC RESERVOIRSEXAMPLE FROM NAM THEUN-NAM KADING BASIN IN LAO PDRPeter R.B. Ward, Timo A. Räsänen, Peter-John Meynell, Tarek Ketelsen, Khamfeuane Sioudom and Jeremy Carew-Reid

CHALLENGE PROGRAM ON WATER & FOOD IN THE MEKONG: Optimizing the management of hydropower cascades at the catchment level (MK3)

Flood Control for Large Reservoirs:Design and Operation

• Climate and Expected Large Flood Events

• Storage for Flood Pulse Modulation

• Spillway Flow Capacity and Operation

• Mechanical Reliability and Servicing


Libby Dam, Kootenai River Basin US/Canada


Thailand Examples

SRINAGARIND DAM

UBOL RATANA DAM

Srinagarind Dam, Thailand


• 1966 Dam commissioned, primarily for storage for agriculture • 1978 Disaster narrowly averted when a flood overtopped the core of the dam

by 24 cm. Average daily peak inflow more than triple the designed maximum spillway capacity.

• Original design flood estimate was insufficient.

• Outcome:

• EGAT began formal dam safety practices, such as comprehensive dam safety reviews, emergency preparedness

• 1986-7 Dam was raised by 3.1 m, and spillway capacity was increased by 40%. Additional work cost $15 million, an outlay of 70% on top of the original cost.

Ubol Ratana Dam, Thailand

• Difficult climate: periodic extremely heavy rain events associated with tropical cyclone weather: typhoons and tropical storms. Global climate change promises more extreme future weather.

• Short number of years for hydrological data set for the basin: design risk

• Three projects in operation, and others proposed

• Dams are not owned by one entity, communications question

• Nam Theun 2 flow scheduling and reservoir operation is done from another country

Nam Theun River Basin: overview

Nam Theun River Basin: hydropower development

Nam Theun River basin:hydropower development

Table 1. Main characteristics of Theun-Hinboun, Nam Theun 2 and Nam Gnouang hydropower projects. Data from Then-Hinboun are before the (2012) expansion project

Theun-Hinboun Nam Theun 2* Nam Gnouan

Dam height [m] 27 39 67 Active storage [mcm] 15 3530 2260 Flood buffer [mcm] 1410 470 Reservoir drawdown [m] 5 12.5 35 Surface area at FSL [km2] 6.3 450 107 Average discharge [m3/s] 220 (460 before NT2) 240 95 Catchment area [km2] 8937 4013 2942 Installed capacity [MW] 210 to be enlarged 1090 60 Head [m] 230 348 47 Turbine discharge [m3/s] 110 330 144 Annual production [GWh] 1356 (1645 after NG) 5936 294 Spillway type 2 radial gates

(1160m3/s/gate); 1 flap gate (50m3/s); 4 sand flushing gates (20m3/s/gate); fixed overflow weir

5 radial gates (1374 m3/s/gate ); 2 flap gates (192m3/s/gate)

5 radial gates (3144 m3/s/gate)

Spillway capacity [m3/s] 12500 6870 15700

* Data are for Nakai Dam on Nam Theun River. Reservoir is also contained by an (earth) saddle dam, on the south side.

Nam Theun: river flowPeak: 18,000 m3/s Peak: 9,000 m3/s

Two largest events in 10 year daily record: Theun Hinboun damsite

Nam Theun: peak flow events

Typhoon tracks 2002 (left) and 2011 (right) Sept and Oct

Nam Theun: typhoon tracks

• We used simple model for NT2 reservoir, with computed inflows based on flow data before and during the 2002 extreme flood event, to check on reservoir water surface heights. Peak inflow on 22nd Sept was 12,000 m3/s.

• We wanted to see what would happen during a “Normal” procedure, and a “Too little too late” procedure for operating the spillway gates.

• We assumed that at the start of the extreme runoff event the reservoir was full to the FSL, and that there was still available the (large) flood buffer storage above this level.

Nam Theun: Water balance

NT2 reservoir water surface during typhoon induced major runoff eventAll gates at Nakai dam working, and timely response

Nam Theun: simulated reservoir storage

Too-little-too-late operation. One spillway gate not functioning, and delayed maximum opening of other gates. Dangerously high water level in reservoir.

Nam Theun: simulated reservoir storage

• We concluded that a great deal of care and a very tight requirement for timely response is needed at NT2 dam, even for management of a flood event (10 year event?) that is not extreme.

• Not clear how future exceptional flood events will be managed successfully.

• Need for a variety of management/governance/co-ordination activities.

Nam Theun: conclusions

1. Adherence to a well conceived rule curve for reservoir surface levels in the flood season, with periodic updating of the rule curve to reflect improved knowledge of the basin, and long term shifts from climate change

2. Ready access to long term and short term weather predictions, particularly for heavy rain expected from typhoon events.

3. Responsiveness to daily and hourly developments during major floods

4. Periodic comprehensive dam safety reviews, by an independent team that should include engineering experts, with backgrounds in hydrology, geotechnology, mechanical engineering and electrical engineering.

Nam Theun: Recommendations

Suggestions are made for • the establishment of a strengthened capacity for the

NT-NK River Basin Committee Secretariat

• emergency communications capability between stakeholders, e.g. dam operators and downstream stakeholders

• periodic, long-term requirements for engineering assessment concerning safe operations at the dams.

Nam Theun: Recommendations

• Ongoing research work on climate periodicities in the Mekong region is encouraging, as it promises to offer ways of achieving advance notice of the likelihood of an extreme weather season.

• Links between the WNPM, ENSO and PDO events is being established, and this understanding will provide extremely helpful with advanced warning and preparedness.

Flood Control for Large Reservoirs: Design and Operation

Thank you for your attention

Thank you for your attention

• 11 years of discussions post 1948.

• Ratified in 1964 for 60 year period. 10 year prior notification before expiry

• Basic idea was to construct and operate four projects (3 in Canada) that would achieve hydropower generation and flood control.

• Several downstream projects in the US benefit from the upstream Treaty projects with the ongoing benefit of generating more HP energy

• Treaty covers Canadian entitlement “one half of the estimated increase in US downstream power benefits”

Columbia River Treaty

• Periodic extremely heavy rain events, determined by monsoon conditions, and weather associated with tropical cyclone activity

• Short data set available when design was undertaken (early 1970s).• Design flood selected, and spillway size with operating procedures

determined. Construction was delayed, and dam finally completed in 1980.

• 18 years later, review of hydrological conditions, spillway capacity shown to be less than desirable.

• Decision made to operate the reservoir with a different set of RULE CURVES, for dam safety reasons.



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Flood Control Challenges for Large Hydroelectric Reservoirs