Some aspects of the sediment transit on the Mekong river in relation with hydropower development

Daniel Loudière Société Hydrotechnique de France

A presentation in three parts

- the situation when starting the project with FFEM support,

- some considerations proposed by CNR

- data base and data use

Strenghts and weaknesses of the project starting early 2010

•  A large interest in the French Hydropower sector: edf, cnr, sogreah, cemagref-irstea, cerege, JFB+HP, AFD + FFEM

•  Decision support for generating sustainable hydropower in the Mekong basin (Knowledge of sediment transport and discharges in relation to fluvial geomorphology for detecting the

impact of large-scale hydropower projects) •  Many studies, many data, many modelization

works,still large difficulties to develop an accepted method, reach a consensus and mitigate large discripancies

Many challenges

•  A key factor : the dynamism of hydropower developers on the upper Mekong in China, on tributaries, on the lower Mekong

•  The multiple and vital services brought by the river

•  The global changes, climate change

•  A relative weakness of MRCs in comparison with some HP developers

•  Time management between short, middle and long term

Three points to include or develop in the project

•  A participation of external experts in three domains: sediment metrology, fluvial geomorphology and environmental engineering

•  Inclusion of the project in the MRCs’ specific or cross-cutting programmes, adoption by national stakeholders, possible modifications to meet their expectations

•  Expansion of the project upstream (implying knowledge of Chinese practices) and downstream (including influences on marine resources and security in the delta), impacts and other policies (abstraction of aggregates or flood prevention) especially in Cambodia and Vietnam

ACNRcnSETS : Industrial and river sites

 19 development schemes

19 Hydro Power Plants 25% of French hydropower generation 3.000 MW of installed capacity

14 large-gauges locks on 300km of large-gauges navigable waterways Maintenance and Control of Flood capacities 400 km of maintained dikes Development of Portuary infrastructures 29 industrial and portuary sites including E.Herriot Port in Lyon

 Development and operation of Wind and PV Farms   And also 32 pumping stations, 27000 ha of area

CNR contribution

Hydraulic systems management

Lao, Vietnam, Cambodia, Thailand

Technical Assistance for the implementation of a reliable and efficient system of collection and transmission of hydro-meteorological data in the Lower Mekong basin. Strengthening national and regional capacities.

Client: MRC - 2007-2012

Mekong-Hycos project for MRC

Recommendations for : •  new locks to be implemented in association with the hydro generation plan on the Mekong Main stream. • Improvement of navigation safety through a set of preventive actions.

Client: MRC – 2008-2009

Navigation on the Mekong-River

Optimization study of hydropower plants on the Mekong river

Lao PDR

Feasibility study for an hydraulic and Energetic optimization of a chain of 5 hydro power plants on the Mekong river. Including social and Environmental assessments studies.

Client: Ministry of Energy and Mines - 2008-2009

Hydrology and Hydropower

Sustainable Development of Hydropower Resources in Siphandone

Assessment of the hydropower potential of the area in sorting out the most favorable sites for hydropower resources exploitation taking into account technical, economic, social and environmental aspects within an integrated approach. pre-feasibility studies for the selected sites in partnership with WWF.

Client: Ministry of Energy and Mines - 2009-2011

The overall impression is that it is necessary to specify the difference between a large dam and a run-of-river development. This run-of-river development does not have a large storage capacity regarding the input flow contrary to the huge dams and does not have the heavy impacts of large reservoirs.

Xayaburi development is planned on the Mekong River on the principle of run-of-river. The reservoir stretches from Xayaburi to Luang Prabang upstream. Purposes are mainly power generation and navigation improvement. The barrage will be operated using day to day water flows naturally available.

The Xayaburi power plant has an installed capacity of 1,260 MW with a 810 m long and 32 m high dam and a reservoir area of 49 km² and live storage of 225 Mm3.

XAYABURI Run-of-river infrastructure or large dam ??

General design of Run-of-River development

• Barrage with gate structure spillway with sufficient capacity for handling at least the 1,000 year return period flood,

• hydropower plant,

• lock for crossing the head when navigation is already existing,

• upper reaches upstream the dam whose length can exceed 100 km. If necessary, local dikes are added on both main sides and on tributaries banks to prevent flooding of alluvial plains.

When the river flow exceeds the design flow of the powerhouse, the spillway gates are opened progressively allowing excess water to flow downstream. Although the main aim of contemplated dams is hydroelectricity production, multipurpose comprehensive development is recommended for optimizing the project and benefiting local populations.

Run-of-River Concept

Run-of-River concept : • The storage capacity of impoundments is negligible compared to river flow volumes, especially flood volumes,

• There is neither possibility of inter annual regulation nor seasonal regulation,

• There is no significant active storage, with regards to the large storage of the entire reservoir. Use of the total storage capacity would lead to low water level i.e not enough head at the power station, limited draught jeopardizing navigation, high head for pumping stations…,

• Output flow sent downstream of the dam equals Input flow entering upstream the reservoir,

• The turbine flow equals the Mekong River flow for river flow less or equal to the design flow of the power station.

• Discharge through turbine is adjusted in real time to fit Mekong River flow fluctuations.

Run-of-River Concept

Normal flow conditions In normal conditions i.e. without floods, the surface of the pond area can be considered as horizontal along the river upstream each dam, since the backwater slope is very low. All the flow crosses the powerhouse turbines. The spillway and bottom gates are closed until the input flow reaches the installed flow capacity of the powerhouse.

Run-of-River Concept

Example of a cascade of 3 dams in normal hydrological conditions (out of flood period).

Flood management During high flows and floods conditions (for example flood 30 to 100 year return period flood), the slope of the backwater curve increases along the river upstream of each dam.

Run-of-River Concept

Cascade of 3 dams during an intermediate flood

Flood management (continued) Keeping the same regulated water level upstream dam would lead to too high backwater curves and inundation hazard. Correct management implies to decrease operated level during flood as follows:

Run-of-River Concept

Cascade of 3 dams during an intermediate flood

Design Flood For largest floods as the 1000 year or 10’000 year design flood for example, all spillway gates are completely opened, water level in the reservoir is close to the natural one, preventing extra inundation

Run-of-River Concept

Cascade of 3 dams during the design flood : all gates of the spillway are opened.

Hydrology, water flows in Mekong mainstream

•  Comment on hydrological data of project: Hydrological data used in the feasibility study are based on the most relevant

hydro meteorological information available on the mainstream, MRC data. The 45 000 m3/s design flood assessment is consistent.

•  Yunnan Cascade impact after completion and filling period of reservoirs: Impacts are expected on flow regarding the yearly flow regulation. During the

dry season the low flow are increased by roughly 50%. During the wet season, flood peaks are reduced for floods lower than the 20 year flood. For greater floods, the impact should be weaker.

•  Impact of planned hydroelectric projects on tributaries in Laos, upstream Xayaburi:

These dams could, if operated accordingly, lead to significant contribution to flood mitigation and to flow increase in dry season. These Lao dams on Mekong tributaries will increase hydrological impact of the Yunnan Cascade.

Hydrology 1/5

Hydrology, water flows in Mekong mainstream (cont’d)

•  Cumulative impact of Yunnan cascade dams and Lao tributaries dams: •  Such impacts will add substantially to the economic viability of the schemes

on the Mekong River in Lao PDR particularly during the low flow season. •  This impact could be mitigated by the evolution of water use in the Mekong

Basin (irrigation and water supply) and Climate Change.

Hydrology 2/5

Hydrology, water flows in Mekong mainstream (cont’d)

•  Document planned operation pattern of Xayaburi and its effect on reservoir water levels and water release

Hydrology 3/5

Operating waterlevel at the Xayaburi dam site

Z1 Z2 Z3 Z4 Z5

275 MSL

Flushing 268 MSL

0 F1 5000

F2 ≈ 10000

F3 ≈ 18000

F4 Discharge

240 MSL

Waterlevel (MSL)

Fine materials: Friendly flushing

Sediment Transport 9/14

Coarse material shuttle - for mainstream dams with lock

dredger barges

Transportation across the reservoir

Dumping downstream the barrage

lock

Sediment Transport 10/14

Data base in relation with engineering needs

Dealing with water discharges

•  The report presented recently by Poyrie shows that data are to-day available to describe the actual situation, its saisonnal variation, its yearly variation and the role of the main tributaries.

•  Simulation works have been carried out to study the effects on the flow pattern in relation with existing reservoirs and different projects on the main stream or on tributaries

Data base in relation with sediment transport

•  It starts with a basic watershed knowledge •  It goes on with the erosion process •  It appears according to different size: clay and

silt, sand (fine, middle, coarse), gravel pebbles and blocks

•  It is stored and may be mobilised again •  Transportation takes place mainly during

floods and may concern various material (bedload, suspension) and suspended matters may vary deeply in fine sand content in such a deep river as the Mekong

On site more accurate sediment transport measurement is needed

•  A campaign foreseen next september

•  New devices should be tested specially ADCP, they may bring new information if properly calibrated and handled

•  The proposed campaign is necessary to develop human resources and obtain the expected data related to sediment transport in the Mekong river

Some comments as a conclusion

The links between flow measurement, sediment transport and fluvial geomorphology are about to be established for a better understanding ot the nature of displaced materials, their transport and their deposit.

Modelisation opportunities will exist for the engineers in charge of the study of possible new hydropower schemes.

Technical solutions do exist with acceptable cost/benefit relation to mitigate most problems. Some aspects still need to be checked and improved at the level of specific studies during the pre-design stage.

Again, the project proposed to the FFEM for a financial support seems to respond to our expectations; but my feeling is that basic works have still to go on to reach an operationnal understanding of the sediment transport in the Mekong basin

Thank you for your attention

Conclusion