THE LOWER MEKONG BASIN

THE « SEDIMENT BUDGET » IN THE PERSPECTIVE OF ECONOMIC DEVELOPMENT

Pr Dr Jean-Paul BRAVARD

University of Lyon, France

Training Workshop “Towards Sustainable Planning of Hydropower in the Mekong Region:

Environmental and Socioeconomic Baseline Study Approaches”. Bangkok, 10-12/12/2012

WHAT IS A SEDIMENT BUDGET?

• The concept was coined in Norway (Rapp, 1960) and in the USA in the 1970-80’s (Trimble, Schumm, Meade), UK (Walling), to account the sources, sinks and redistribution pathways of sediments, solutes and nutrients in a unit region over unit time (Slaymaker, 2003). For instance:

The sediment budget quantifies different upstream/downstream components of

sediment production/transit/deposition,

The delivery ratio is the % of sediment input which is able to reach the sea

(considered as the final sink)

The sediment budget may vary at different time scales: Holocene, historical, recent

(depending on land use and climate control)

* the amounts and the types of storage : colluvium, channels (conveyance

losses) and floodplains, lakes and wetlands (as natural sinks),

* the duration of particle storage (residence time),

* accelerated erosion (by humans), reservoirs and mining

change the nature of fluxes and the depositional places.

At the global scale, despite increased soil erosion sediment yield to the seas has been drastically reduced since 30-40 years due to trapping by dams

1950-1970 1.1 x 109 t year-1

1980’s 0.8 x 109 t year-1

1990’s ca 0.4 x 109 t year-1

2000-2004 ca 0.15 x 109 t year-1

Recent changes in the sediment load of the

Yellow river. Source: D. Walling

This global trend impacts deltas which are of prime importance

for human beings and economy

Beach retreat, Mekong delta

METHODOLOGY OF

SEDIMENT BUDGET STUDIES

Alluvial plains

• Topographic and bathymetric surveys,

• Coring alluvial plains along cross profiles (radiocarbone age),

• Measuring recent deposition using 137Cs

• Measuring fresh sediment deposition on pads (accumulation rate in mm + bulk density (1 ton m-3),

>> then calculating gross sediment storage by alluvial accretion

River channels and reservoirs • Sampling suspended sediment at gauging stations and add 10% bedload • Sampling the amount of fines deposited on the channel bed and infiltrated • Estimating bank erosion and lateral deposition • Surveying reservoir (change in capacity) >> virtual rate of sediment production in m-3 km2 year-1 (drainage area), * Modelling deposition

Production zone

• Measurement of soil erosion at the plot scale

• Estimation using average erosion rates depending on soil cover types

Understanding sediment deficit at river mouths requires the elaboration of individual sediment budgets at the basin scale.

This type of study has been implemented since 40 years, notably in small basins

After Beach, 1994

Bed load transit along the rivers of the Rhone basin(1860-2000)

A marked reduction of bed load transit due to reafforestion of the

watershed, gravel mining and damming (IIRS, SOGREAH)

Transit of suspended sediment in rivers of the Rhone basin (1860-2000)

A moderate reduction of suspended sediment (IIRS, SOGREAH)

Average sediment budget of Europe

After Owens & Batalla, 2003, modified

Turner & Rabalais, 2004

RECENT RESULTS ON SEDIMENT TRANSFER AND SEDIMENT BUDGET

IN THE MEKONG CHANNEL

- Suspended sediment : 145-160 millions tons per year at Pakse - sand bed load: 20 +/- 10 millions tons ?

- Bed load transport is discontinuous from upstream to the delta - Coarse sand transport is continuous. It depends on energy and discharge regime - Medium and fine size sand may be transported as graded or as uniform suspension depending on energy (function of slope, discharge, width) - Wash load transport is continuous from usptream to the delta

The sediment budget of the Mekong

basin in natural conditions (before

1993)

The Lancang provided 60% of the

sediment load,

The 3S area was one of the most

productive.

An efficient conveyor belt from the

upland to Pakse

Extensive alluvial storage of silt-clay

In the alluvial reach downstream of

Pakse, in the Tonle Sap and over the

delta (how much?)

Protecting sand is a major

issue for the Lower

Mekong and its delta

Syke, 2011

Planned dams on the Lower Mekong

Dams in the Mekong basin (MRC)

Existing

Under construction

Planned

The modified sediment budget of the Lower Mekong

15

The contribution of the Lancang decreased from 80 to 11% due to reservoir trapping,

The Lower Mekong will be deprived

of 80% of its load due to the Lancang dams (probably ca 100% of sand),

The 3S area has been impacted by

dams,

Courtesy:

Lois Koehnken

SAND-GRAVEL HARVESTING DEEPLY CONTRIBUTES THE ALTERATION OF

THE SEDIMENT BUDGET IN THE MEKONG RIVER CHANNEL

System already under stress due to sand and

gravel in channel mining

Unstable/ weakened system (need to be restored_

90% of harvested sediment is sand. Mining exceeds 50 millions tons y-1

Sediment harvesting along the Mekong channel affects downstream

transit and delta shoreline since pits trap sand bed load,

Sand harvesting and dams explain recent and ongoing retreat of the

delta coastline,

The impacts of extraction are reversible. Dams prefentially trap sand

and in most of cases this impact is not reversible without flushes

which severely affect the environment.

ECOLOGICAL AND ECONOMIC VALUE OF SEDIMENT BUDGETS

• Anticipating geomorphic adjustment of rivers to the alteration of the sediment balance – Bed degradation and threats on river banks, stability of public infrastructures and water table

– Exchanges between the Mekong and the Tonlé Sap (increased floods downstream, reduction of TS productivity)

– bed aggradation & degradation and fish habitat; increase of fine-grained sediment within gravel-bed rivers and salmonid spawning)

• Understanding the loss of fertile soils and agricultural resources in erodible areas, in order to contain its cost and better manage watersheds,

• Anticipating N and P budgets of the future in high productivity flooded areas,

• Understanding/modelling sediment-associated pollutants and contaminants

(heavy metals, radionuclides, synthetic organic compounds). They are elements of conveyance losses to floodplains.

• Anticipating the future of sand and gravel resources (construction),

• Anticipating deposition vs erosion of delta coastlines, and behavior of deltas and estuaries in the context of climate change and sea level rise

Sediment budgets for the sake of sediment budgetting but for the sake

of a particular and well defined practical problems:

Practical consequences of damming the Mekong

• The processes involved in sand transport are much sensitive to the energy of floods,

• Dams and reservoirs break stream energy. They act as artificial sinks for sand,

• Since sand transport has been underestimated so far, the potential impact of dams has been underevaluated,

Dams and reservoirs should not be raised in the Mekong channel at least as long as sediment budget is so poorly documented. Food security and resources for construction are key elements for development. To be balanced with energy.

THE MEKONG BASIN NEEDS A GLOBAL SEDIMENT BUDGET

• It is not possible to obtain a sound evaluation of sediment sources and inputs into the Mekong River system, except in some sub-watersheds. It is a long term target,

• Estimate of sediment trapping in all the existing reservoirs (artificial upland sinks). This type of data may be correlated with the main characteristics of sub-watersheds and be a substitute to the lack of data concerning erosion. Extrapolation will be the further step,

• Since 50 years, significant efforts have been devoted to suspended sediment concentration and yield on the Mekong. However sand transport has been underestimated and longitudinal variations of suspended load are still difficult to explain. Tributaries are poorly documented. Bed load studies are still poor despite some localized investigations,

• Few data deal with sediment deposited in natural sinks (except in the Tonlé Sap and in limited areas of the delta). This type of study has to be developed,

• Sand and gravel extractions are part of the sediment budget at the reach scale

CONCLUSION

It is extremely important to base on better scientific knowledge, decisions that will surely alter the Mekong sediment budget. Further studies are required,

Dams should be planned in rivers where the issues are the less sensitive in terms of sediment supply and transfer

Thank you for your attention