Luận văn estimation of soil loss from the upper rajang sub catchments in sarawak malaysia during the development of the bakun hydroelectric project

Keli 1985 and Ke 1986 pointed out that the total soil loss in Loess Plateau area is about 53 million hectares with population of 70 million located in middle reaches of the Yellow River

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ESTIMATION OF SOIL LOSS FROM THE UPPER RAJANG SUB-CATCHMENTS IN SARAWAK,

MALAYSIA DURING THE DEVELOPMENT OF THE

BAKUN HYDROELECTRIC PROJECT

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Chapter 1: Introduction

1.1 Background

1.1.1 Erosion

The degradation of soils is a serious problem in developing countries, especially in

highland, forest and river catchment areas Soil degradation is one of the greatest

challenges facing mankind and its extent and impact on human welfare and the

global environment are greater now than ever before (Lal and Stewart, 1990)

Water erosion is the main degradation process, while human activities, the

reduction of plant cover, and the nature of the parent material are the main causes

of soil erosion (Lopez and Albaladejo, 1990) A review of the impacts of soil

degradation found that 1.2 billion ha (almost 11% of the vegetative area in the

world) have undergone moderate or worse degradation by human activity over the

last 45 years (World Bank, 1992)

From the engineering perspective, soil erosion is defined as a general destruction of

soil structure by the action of water and wind It is essentially the smoothing

process with soil particles being carried away, rolled and washed down by the force

of gravity (Beasley, 1972) Rainfall is the prime agent of soil erosion, whereby the

rain’s runoff will scour away, loosen and break soil particles and then carry them

away, thus leaving behind an altered bare earth surface (Wishchmeier et al., 1978)

The impact of raindrops on the soil surface can break down soil aggregates and

disperse the aggregate material Lighter aggregate materials such as very fine sand,

silt, clay and organic matter can be easily removed by the raindrop splash and

dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai

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the larger sand and gravel particles Soil movement by rainfall (raindrop splash) is

usually greatest and most noticeable during short-duration, high-intensity

thunderstorms Although the erosion caused by long-lasting and less-intense

storms is not as spectacular or noticeable as that produced during thunderstorms,

the amount of soil loss can be significant, especially when compounded over time

Runoff can occur whenever there is excess water on a slope that cannot be absorbed

into the soil or trapped on the surface The amount of runoff will increase if

infiltration is reduced due to soil compaction, crusting or freezing Runoff from the

agricultural land may be greatest during spring months when the soils are usually

saturated, snow is melting and vegetative cover is minimal

In Malaysia, there are many soil erosion prone zones especially hilly areas at the

newly established oil palm plantation and along the riverbanks In the case of slope,

an altered bare surface of the slope with sheet, rill and gully erosion features will

cause instability of the slope This situation will gradually cause slope failure or

landslide as commonly know The soil erosion phenomenon is basically the function

of the erosivity of the soil (Roslan, 1992)

1.1.2 Sediment Yield

Several of the impacts stemming from the construction process and earthworks at

work sites are predictable and mitigable to a significant extent through careful site

planning, supervision and application of best management practices A number of

other impacts are expected to be residual Progressive construction and use of

access roads and camps in rugged and steep topography intersected by many

watercourses would initiate unavoidable erosion and sedimentation in the reservoir

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area Removal of biomass in this environment would increase the risk of accelerated

erosion and sedimentation over a larger area Following biomass removal, the

sediment yield in the catchment also increases rapidly Removal of biomass would

also unavoidably affect the terrestrial and aquatic resources within the reservoir

area

Insoluble matter in suspension is one of commonest forms of pollution, being recent

in river and reservoir All rivers and reservoir, even those which are relatively

unpolluted, contain suspended matter consisting of natural silt, sand, etc, derived

from the stream bed and banks There are several reasons why suspended solids are

objectionable in a stream, among which are:

• They interfere with self-purification by diminishing photosynthesis and by

smothering benthic organisms,

• Reduce reservoir storage capacity,

• They can result in the reduction of fish and other aquatic species,

• They are unsightly and are a nuisance aesthetically,

• They can also cause mechanical problem to installations such as pumps,

turbines,

• They can affect navigation in waterway through sedimentation and

shallowing of river bed, etc

The soil erosion related problems should thus be identified to enhance

understanding and to minimize effects Soil loss estimation in relation to changing

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1.2 The Study Site

The proposed study area is located within the Balui sub-watershed of the upper

Rajang River Basin in the interior of Sarawak The Bakun catchment area is

located between latitudes 1.5°N and 3.0°N and longitudes 113.5°E and 115.3°E The

catchment upstream of the dam site covers an area of about 1.5 million hectares

(ha) The watershed and river are respectively the largest (44,200 km2) and the

longest (>900 km) in Malaysia and the Balui or Upper Rajang sub-watershed

represents 34% of the entire Rajang watershed

1.3 Objectives of the Study

A set of research projects can be initiated in relation to the development of the

Bakun HEP dam with the aim of producing data and information useful for an

integrated approach to river basin and land use management The present study

focuses on the following objectives:

a) Estimation of soil loss from the Upper Rajang Sub-Catchments during the

development of the Bakun HEP

b) Soil loss estimation in relation to changing discharge in the watershed

1.4 Significance of the Study

Sediment which reaches streams or watercourses can accelerate bank erosion,

clogging of drainage ditches and stream channels, silting of reservoirs (reduce

reservoir storage capacity), damages to fish spawning grounds and depletion of

downstream water quality Pesticides and fertilizers, frequently transported along

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with the eroding soil can contaminate or pollute downstream water sources and

recreational areas Because of the potential seriousness of some impacts, the

estimation of soil loss is necessary The estimation is useful, among others in

understanding the sources, predict the trend of erosion and support further studies

Soil loss and transport in the upland watershed are difficult to measure, and may go

unnoticed until it is a severe problem Deposition is often easier to identify and

measure Water samples collected at downstream locations can be used for

sediment analysis for the assessment of cumulative sediment yield for all the

catchments in the watershed or river basin The research is intended to:

• Describe the total suspended solids (TSS) measurement methods, and to

develop a relationship between daily discharge (or water level) and daily

TSS From the daily TSS readings, the total yield of the TSS for the whole

year can be determined

• Discuss the chronological changes of sediment yield of the upper Rajang

catchment

• Make recommendations on implementation of an integrated watershed

management approach with respect to management of soil base on changing

of soil loss over different years

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Chapter 2: Literature Review

2.1 History of the Bakun HEP Project

The Bakun Hydroelectric Project (Bakun HEP) in Sarawak, with a proposed

generation capacity of 2,400 MW, is located on the Balui River about 37 km

upstream of Belaga Town in the State of Sarawak, Malaysia

The implementation of the hydro project was initially privatized to Ekran Berhad

in 1994 and the preliminary works and river diversion works commenced in 1995

However, the economic slowdown beginning in 1997 had forced the project to be

shelved Later in 2000, the Government reinstated the project and vested all the

rights of Ekran Berhad to Sarawak Hidro Sdn Bhd (SHSB) In the meantime, the

river diversion works continued and were completed and handed over to SHSB at

the end of April 2001

On 1P st P June 2001, the construction of the upstream auxiliary cofferdam was

awarded to Global Upline Sdn Bhd and the work was completed in June 2002

Further construction of the dam and ancillary facilities (the main civil works) was

offered to Malaysia-China Hydro Joint Venture on 8 October 2002 The main civil

works is scheduled to be completed by 22 September 2007 while the reservoir

impoundment is planned to commence earlier i.e on 1 January 2007

The reservoir of the Bakun Hydro Dam by virtue of the topography and relief will

be elongated and dendritic in shape, spanning over the Batang Balui, Sg Murum,

Sungai Bahau and Sungai Linau The reservoir will lie between the base elevation

of 34 m asl at the dam site and maximum operating level of elevation of 228 m asl,

encompassing an area of 69,640 ha, with a corresponding perimeter of about 2,000

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km This Reservoir preparation (RP) comprise inventory, perimeter survey and

marking, biomass removal planning, partial biomass removal over the entire

reservoir and complete biomass removal of a 100 km reservoir rim between

elevation 180 m asl and 228 m asl identified for future use

Biomass removal forms the main activity of the reservoir preparation Complete

biomass removal of the entire Bakun Dam reservoir is not practical or feasible due

to its immense size As such, as recommended by the environmental consultants in

the EIA report, only selective or partial biomass removal of the reservoir for all

trees down to 15cm dbh will be carried out The complete biomass removal at

certain zone of the shorelines is to be implemented for the following reasons:

• to ensure that the quality of water of the reservoir will improve; and

• to make sure that the future development and use of shoreline and reservoir

may not be hindered

2.2 Definitions

2.2.1 Soil Erosion

The word erosion is derived from the Latin word erosio, meaning “to gnaw away”

In general terms, soil erosion implies the physical removal of topsoil by various

agents, including falling raindrops, water flowing over and through the soil profile,

wind velocity, and gravitational pull Erosion is defined as “the wearing away of

the land surface by running water, wind, ice or other geological agents, including

such processes as gravitational creep” (SCSA, 1982) The process of wearing away

by water involves the removal of soluble dissolved and insoluble solid materials

Physical erosion involves detachment and transport of insoluble soil particles, e.g.,

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sand, silt, clay, and organic matter The transport may be lateral on the soil

surface or vertical within the soil profile through voids, cracks, and crevices

Erosion by wind involves processes similar to those by water except that the

causative agent in sediment detachment and transport is the wind (Lal, 1990)

2.2.2 Types of Erosion

Different types of soil erosion can be classified on the basis of major erosion agents

Fluids or gravity is the principal agent of erosion Wind, rainfall, and running

water are the principal agents of soil erosion on arable land in the tropics

Rain

er osi on

Ri l l

er osi on Spl ash

Gul ly

er osi on

fl ow Sur face

St r eam bank

Flowi ng wat er Wat er

Figure 2.1 Types of erosion (Source: Lal, 1990) Different types of erosion on the basis of major agents involved are shown in figure

2.1 Water erosion is classified into splash, sheet, rill, and gully erosion on the basis

principal processes involved Splash or inter-rill erosion is caused by raindrop

impact Sheet erosion is the removal of a thin, relatively uniform layer of soil

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particles Rill erosion is erosion in small of a thin, channel only a few millimeters

wide and deep Rills are transformed to gullies when they cannot be obliterated by

normal tillage Stream channel erosion and coastal erosion are caused, respectively,

by stream flow and ocean waves Soil movement en masse is caused by gravity

2.2.3 Sediment

The soil mass removed from one place is often deposited at another location when

the energy of the erosion causing agent is diminished or too dissipated to transport

soil particles The term sediment refers to solid material that is detached from the

soil mass by erosion agents and transported from its original place by suspension

in water or air or by gravity

The term soil erosion therefore is distinct from soil loss and sediment yield

(Wischmeier, 1976; Mitchell and Bubenzer, 1980) Soil erosion refers to the gross

amount of soil dislodged by raindrops, overland flow, wind, ice, or gravity Soil loss

is the net amount of soil moved off a particular field or area, the difference between

soil dislodged and sedimentation Sediment yield, in comparison, is soil loss

delivered to the specific point under consideration A field’s sediment yield is the

sum of soil losses from slope segments minus deposition The deposition may occur

in depressions, at the toes of slopes, along filed boundaries, and in terrace channels

The combined terms erosion and sedimentation by water embody the process of

detachment, transportation, and deposition of sediment by erosive and transport

agents including raindrop impact and runoff over the soil surface (ASCE, 1975)

Sediments from one location may be deposited at another site and may eventually

reach the ocean following repeated cycles of re-detachment and re-entrainment in

rills, channels, streams, river valleys, flood plains, and delta The process begins

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with sediment detachment from uplands and ends with an eventual transport to

the ocean

Sedimentation has serious environmental and economic implication Sedimentation

decreases the capacity of reservoir, rivers, and chokes irrigation canals and

tributaries Researchers, especially engineers, consider sedimentation to be a major

process of which erosion is an initial step Fleming (1981) adopts a broader

approach by stating that “the sediment problem may be defined as the detrimental

depletion by erosion and transport of soil resources from land surfaces and

subsequent accretion by deposition in reservoirs and coastal areas”

2.3 Soil Erosion in Asian Countries

Soil erosion is perhaps the most serious mechanism of land degradation in the

tropics in general and the humid tropics in particular (El-Swaify et al., 1982) In

the tropics, erosion by water, rather than by wind, assumes the primary

importance (El-Swaify, 1993) Various authors, cited by El-Swaify and Dangler

(1982) pointed out that available geologic data on erosion of different continents

indicate that Asia leads the way with 1.66 tonnes/ha/year, followed by South

America, North and Central America, Africa, Europe, and Australia with 0.93, 0.73,

0.47, 0.43, and 0.32 tonnes/ha/year, respectively These data were derived directly

from sediment loads in major rivers No attempt was made to convert these data to

field soil losses This was corroborated by the fact that the heavily populated

regions of Asia possess the highest global sediment loads in their major rivers For

examples, presented as an average sediment removal from respective drainage

basins (using appropriate sediment delivery ratios), were 550, 480, 430, 270, 217,

and 139 tonnes/ha/year, respectively, from the Yellow River (China), Kosi River

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(India), Damodar River (India), Ganges River (India, Bangladesh, Nepal, Tibet),

Red River (China, Vietnam), and Irrawady River (Burma) (El-Swaify, 1993)

Soil erosion in China

According to Dazhong (1993), China has a vast territory, a large population, and

abundant natural resources The total land area of China is 960 million hectares,

which accounts for 1/15 of the total world land area China’s vast mountain-land

areas plateaus are suffering serious soil erosion The statistics from the early 1950s

quantlified that one-sixth of soil surface in China was prone to erosion (TMB, 1984)

About 42 million hectares of China’s cultivated land, or one-third of the total

cultivated land, are undergoing serious water and wind erosion (Fude, 1987)

Keli (1985) and Ke (1986) pointed out that the total soil loss in Loess Plateau (area

is about 53 million hectares with population of 70 million located in middle reaches

of the Yellow River) is about 2200 million tonnes annually or 51 tonnes/ha/year

Three-quarters of loss soil is transported to the lower reaches of the Yellow River

Southern of China is located in tropical and subtropical zones The total area is

about 160 million hectares with population of 200 million The soil loss study by

Yang et al (1987) indicated about 35.2 million hectares area was being eroded with

a total annual soil loss of 1600 million tonnes

The northern region of China is located in warm temperate zones Several sources

(NADC, 1981; HCH, 1984; Junfeng, 1985) estimate that soil erosion in this region

covers about 23 million hectares, the soil erosion is about 20 tonnes/ha/year, but

may reach as high as 50 tones/ha/year (IFS, 1985; Defu, 1985) Total soil loss for

the region is about 500 million tonnes annually

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The northeastern region covers about 13 million hectares The annual erosion rate

ranges from 50-70 tones/hectares/year (Defu, 1985; Dexing, 1986) The total soil

loss in this region is about 150 million tonnes, 80% of which is from cultivated land

The total seriously eroded area in China under water erosion would be at least 150

million hectares The total soil loss in China was calculated to be more than 5500

million tonnes, which accounts for an estimated 20% of total world soil loss

(Dazhong, 1993) About 40% of total soil eroded from the land, or about 2000

million tonnes of soil, is carried to the mouths of the river in China The remaining

3500 million tonnes of sediments are deposited in lakes, rivers, and various water

conservation facilities (TMB, 1984; Zhengshan, 1987)

The Yellow River is 5464 km long, watershed of 680,000 kmP 2 P and carries 40 billion

cubic meters of total annual runoff The highly concentrated sediments give the

river the highest silt content of any river in the world The average silt content in

the river water is 38 kg/mP 3

P During periods of flooding, silt content in the Yellow River can rise to more than 650 kg/mP 3

P(Gueliang, 1987)

The Yangtze, which is the longest river in China, is about 6300 km long with a

trillion cubic meters of annual runoff and collecting 2400 million tonnes of soil

sediment About 680 million tonnes of sediment are deposited at the mouth of the

river The remaining deposits are in the river system, lakes, and reservoirs

(Youngeng and Jinlin, 1986; Yansheng, et al., 1987) The large Dongting Lake in

the middle area of Yangtze River has an input of 130 million tonnes of silt About

70% of this silt is deposited on the lakebed and raises it about 3.5 cm annually

From 1949 to 1977, the water area, storage capacity, and navigable section of the

lake have been reduced by 37%, 39%, and 31%, respectively (TMB, 1984; Youngeng

and Jinlin, 1986) It is also estimated that about a thousand million tones of silt

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are deposited in the reservoirs on the Yangtze River system annually, and about

390 million cubic meters of water-storage capacity are lost in the 20 largest

reservoirs in the upper area of Yangtze River annually because of sediment

deposits This reduces the total storage capacity about 1% per year (Youngeng and

Jinlin, 1986) The waterway transportation distance of the Yangtze River system

has been reduced about 40% because of sedimentation since the 1960s (Zhan and

Chuanguo, 1982)

Soil erosion in India

The first gross national estimate made in 1950s reported that about 6000 million

tonnes of soil were eroded by water every year in India (Kanwar: vide Vohra, 1981)

This was subsequently verified (Tejwani and Rambabu, 1981; Narayana and

Rambabu, 1983) by using the information on the land resources in different regions

of India (Gupta et al., 1970), the average values and iso-erodent map of India, and

sediment data for 21 rivers of Himalayan region and 15 rivers of the

non-Himalayan region (Gupta, 1975; Rao, 1975; Chaturvedi, 1978) Narayana and

Rambabu (1983) concluded that, annually, 5334 million tonnes of soil was eroded

The country’s rivers carry an approximate quantity of 2052 million tonnes of soil

(6.26 tonnes/ha/year); of this, nearly 480 million tonnes are deposited in various

reservoirs resulting in a loss of 1-2% storage capacity per year and 1572 million

tonnes are carried out to the sea

Sedimentation studies of 21 major reservoirs in India (Gupta, 1980) have shown

that the annual rate of siltation from a unit catchment has been 40 to 2166% more

than was assumed at the time of reservoir project design (it has been lower in the

case of only one reservoir) Using the average of 21 reservoirs, the actual sediment

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which is the oldest in India (1931), had loss 52.1% capacity by 1967 (CBIP, 1981)

Most of existing reservoirs were planned with provision of dead storage designed to

store the incoming silt with a trap efficiency determined separately for each

reservoir It was assumed that the entire sedimentation would take place below the

dead storage level and the designed live storage would be available for utilization

throughout the projected life of the reservoir These assumptions have not realized,

since observations have show that the siltation is not confined to dead storage only,

and the quantum of siltation in the live storage is equal to or more than that in the

dead storage (CBIP, 1981; Sinha, 1984)

Soil erosion in the Philippines is a major threat to sustainable production on

sloping lands where mainly subsistence farmers carry out food and fibre production

Sloping lands occupy about 9.4 million ha or one-third of the country’s total land

area of 30 million ha The sloping topography and the high rainfall would subject

the cultivated sloping lands to various degrees of erosion and other forms of land

degradation Field experiments conducted in the IBSRAM ASIALAND

Management of Sloping Lands network sites in the Philippines showed that

up-and-down slope cultivation resulted in annual erosion rates averaging about 98.4

tonnes/ha, depending on the rainfall and type of soil It was estimated by the

Bureau of Soils and Water Management that about 623 million tonnes of soil is lost

annually from 28 million ha of land in the country

Soil erosion in Laos

Natural resources in Laos have been depleted gradually by mostly human activities,

the most common being deforestation through slash-and-burn agriculture Forest

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encroachment in the northern and central regions has accelerated rapidly and the

forest areas have been reduced to less than 30% These are the most critical areas

undergoing environmental changes, especially through land degradation and soil

erosion Predicted soil loss was estimated at 30–150 tonnes/ha/year, depending on

parameters such as soil characteristics, land slope, land cover, and farming

systems

Soil erosion has been identified as the major problem for sustainable agriculture

on steep-land areas It causes severe on- and off-site environmental, economic, and

social impacts On site, it reduces the chemical fertility of the soil by nutrient and

organic matter depletion, and in some cases, exposes the acid subsoil Erosion also

damages the physical fertility by removing surface soil, and reducing the soil depth

and water holding capacity These soil changes will slowly reduce crop yields, farm

incomes, and household nutrition The off-site effects of erosion on the quality and

availability of water can also be very serious Major off-site effects include

increased surface runoff, often resulting in flooding which displaces people in

low-lying areas and damages road infrastructure; increased sediment, nutrient and

pollution loads in streams, which degrade the quality of household water supplies

and increase the risk on human health; siltation of dams and irrigation canals,

resulting in reduced water supply for irrigated crops and shorter life of reservoirs;

and sediment deposition in offshore fisheries, reducing the availability of aquatic

supplies and promotion of eco-tourism

The Mekong Basin

In a study about soil erosion and sediment transport in the Mekong Basin, Al-Soufi

(2003) found that the erosion in the Mekong Basin is mainly rainfall based runoff

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heterogeneous The river basin lying across six countries has causedmade the

system analyses a significantly complex task He used the Modified Universal Soil

Loss Equation within the Soil and Water Assessment Tool (SWAT) model to

determine soil erosion and sediments transport loading patterns SWAT model is

developed to evaluate surface runoff from different agricultural and hydrologic

management practices

The Basin covers an area of approximately 795,000 kmP 2

P The Lower Mekong Basin excludes Yunnan and Myanmar and thus the catchment’s area is estimated around

615,800 kmP 2 P The basin consists of approximately 33 percent forests Compared to

other major rivers of the world, the Mekong ranks 12th with respect to length

(4880 km), 21st with respect to catchment’s area and 8th with respect to average

annual runoff (475 x 10P 9 P mP 3 P per year or 15000 mP 3 P/s) The Mekong river flow within

the territory of China forms about 51% of the flow at Vientiane (Lao) and 16% of

the flow at Kratie which is the beginning of the lower flood plain (Al-Soufi and

Richey, 2003) The wet season lasts from May to October where the average

rainfall around 80-90% of the annual total The Dry season period starts from

November and lasts until April The minimum annual rainfall is 1000 mm/year

(NE of Thailand) and the Maximum is 4000 mm/year (West of Vietnam) The

Mekong River itself deposits a considerable amount of fertile silt each year during

the flood season on lower forests and flood plain in Cambodia and Vietnam

Published records have shown that in 1997, 83.25 million tonnes of soil were

washed from the Lancing-Jiang to the lower Mekong (Kelin & Chun, 1999)

Pantulu (1986) pointed out in his study that the annual sediment load of the Basin

was estimated around 65.93 million tonnes/year at Chiang Saen, 107.26 million

tonnes/year at Vientiane and 129.89 million tonnes /year at Khone Falls Hården

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and Sundborg (1992) conducted a study in Laos and North-East of Thailand on the

suspended sediment transport in the Mekong River network They found that

sediments vary very regularly with water discharge At Pakse, their published data

indicated an increase in the sediment load of about 50% between the 60s and 1992

This was attributed to the sediment inflow from tributaries in Laos The report of

Hården and Sundborg (1992) presented a wide range of load values at Luang

Prabang from a minimum of 62 million tonnes in 1987 to 361 million tonnes in

1966 At Pakse, the minimum value presented was 79.7 million tonnes in 1967 to

the maximum value of 324.72 million tonnes in 1978 The variation might be

attributed to the variation in river discharge particularly the year 1978 when the

flood was the highest ever recorded

Soil erosion in Malaysia

Erosion and sediment yield studies in the tropical rain forest environmental of

Malaysia have predominantly been concentrated on the effect of land use changes

on hill-slope plot (Morgan et al., 1982; Hatch, 1983, Malmer, 1993; Brooks et al.,

1993) or on relatively small catchments up to 140 kmP 2

P (Shallow, 1956; Douglas,

1967, 1968; Leigh and Low, 1973; Baharuddin, 1988; Greer et al., 1989; Malmer,

1990, Zulkifli et al., 1991; Douglas et al., 1992; Lai, 1993) In Malaysia, measured

sediment yields from field plots or relatively small catchments covered by

undisturbed rain forest range from less than 1 tonnes/ha/year (cf Douglas, 1968;

Leigh and Low, 1973; Baharuddin, 1988; Malmer, 1993) to just over 3 tonnes/ha

per year (Douglas et al., 1992)

Unless logging of such areas under rain forest is carried out very carefully, large

increases in sediment production, and therefore also in sediment yield, are likely to

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(1988) observed an increase of 70% (from 0.07 to 0.12 tonnes/ha/year) in suspended

sediment yield after supervised logging of a small rain forest catchment on granite

rock (area 0.3 kmP 2

P) and of 97% after unsupervised logging (from 0.14 to 0.27 tonnes/ha/year) Shallow (1956) observed sediment yield of 0.56 tonnes/ha/year and

1.03 tonnes/ha/year in the Cameron highland in Peninsular Malaysia with forest

covers of 94% and 64%, respectively Chong (1985) found 8-17 times increase in the

sediment load of peak flows shortly after clear felling In a study of five steeps

catchments on granitic rock along the Sungai Langat, Lai (1993) observed

sediment yield of 0.54 and 0.90 tonnes/ha/year for undisturbed (Sg Lawing, 5 kmP 2 P)

and partly logged (Sg Lui, 68 kmP 2 P, 20% logged in 1978) catchments, respectively

These low values contrast sharply with the suspended sediment yield of 28.26 and

24.58 tonnes/ha/year observed in the first year after logging (mechanised) of the Sg

Batangsi (20 kmP 2

P) and Sg Chongkak (13 kmP 2

P) catchments, respectively The suspended sediment yield of the Sg Chongkak decreased to 13.35 tonnes/ha/year in

the second year after logging

In Sabah, east Malaysia, Malmer (1990) observed increased in suspended sediment

yield from small catchments (0.03 – 0.18 kmP 2 P) and unbounded runoff plots from

0.04 tonnes/ha/year for undisturbed forest to 0.7 tonnes/ha/year after burning of

secondary forest, 1.5 tonnes/ha/year after manual extraction and 2.1

tonnes/ha/year after tractor extraction

The only sediment yield data available for catchment in Malaysia with area larger

than 1000 kmP 2

P(size can comparable to Balui River drainage basin) are those presented by Wan Ruslan (1992) He presented sediment yield for two sub-

catchments of the Muda River basin in Peninsular Malaysia, which were under

padi cultivation and partly under rubber plantations Annually sediment yield was

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calculated using a single sediment-rating curve for both catchments and annual

sediment yield of 1.12 and 0.42 tonnes/ha/year obtained for the Jambatan Syed

Omar (3330 kmP 2

P) and Jeniang (1770 kmP 2

P) river basins Earlier measurement of sediment yield at Jambatan Syed Omar totalled 0.83 tonnes/ha/year (Wan Ruslan,

1989) and concluded that the observed increase could partly be attributed to

changes in land use in the area Wan Ruslan (1992)

Values presented in hydropower feasibility studies carried out in Sabah and

Sarawak (Syed Muhammad and Electrowatt Engineering Services Ltd., 1994)

range from 2.05 tonnes/ha/year for undisturbed Upper Padas catchment (1790 kmP 2 P)

to 12.50 tonnes/ha/year for the Batang Ai catchment (1200 kmP 2

P), the latter was affected by logging

Soil erosion in Sarawak has been the subject of many comments by observers, but

few detailed studies, apart from a long running set of plot experiments by the

Research Branch of the Department of Agriculture Unfortunately there has been

little work on forest hydrology in Sarawak and no measurements of the impact of

logging on erosion rates and stream sedimentation Comments by foresters include

the following:

"While floods in several basins in Sarawak have been attributed to

extensive forest clearing, it is impossible to be sure of the exact role that

clearing has played However, in areas where the bush fallow period is not

too short, shifting cultivation may not disrupt the hydrologic regime as

much as recent arguments have suggested If a cleared area is left to be

re-dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai loi dai

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colonized by secondary vegetation, peak stream flows and sediment yields

gradually return to near natural levels The continuation of those effects in

logging areas is due to the road system which remains after timber

extraction has finished" (Butt, 1983)

Plot experiments, covering small areas of slope indicate that mean values of

erosion under natural forests in Sarawak range from 0.1 to 0.23 tonnes/ha/year,

while those for unterraced pepper cultivation are 81to 90 tonnes/ha/year (Petch,

1985)

A study on Semonggok Series soils (Ng and Tek, 1992) noted that contrary to the

general belief that the slash-and-burn system of growing hill padi and maize as a

companion crop on hillslopes will incur severe soil and nutrient losses due to

greater surface runoff and the very "open" soil surface, results suggested otherwise

Only 0.45 tonnes/ha were lost in the first year after clearing At Tebedu, Teck

(1992) recorded 0.46 tonnes/ha soil loss in the first year after clearance These field

data from plot studies (Table 2.1) clearly show that soil loss under shifting

cultivation is of the same magnitude as that under natural forest, whereas once a

cultivation system leaves bare ground between row crops, as in traditional pepper,

erosion rates rise to 100 times that under natural forest (Murtedza, 2004)

Table 2.1: Data on erosion rates under forest and shifting cultivation for Sarawak

(all values of soil loss in tonnes/ha/year)

Land Use Location (degrees) Slope (years) Period Soil loss mean Soil loss range

Primary Forest Niah F.R 25-30 4 0.19 0.083-0.31

Semonggok 25-30 11 0.24 0.07-0.77 Secondary Forest

a) logged 10 years

b) with hill padi Semonggok 25-30 11 0.10 0.02-0.17

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c) 2 month old

Hill Padi/ Shifting Cultivation

b) terraced with

c) bush fallow Semonggok 16-26 3 0.233 0.06-0.45

Traditional

Pepper Semonggok 25-30 11 89.44 139.12

51.18-2.5 Soil Loss Estimation Methodologies

The measurement soil loss or soil erosion rates are a relatively young science Some

of the earlier reported data are based on measurements initiated in the first and

second decades of the twentieth century Consequently, most of the techniques

used still require standardization Further more, new methods are rapidly being

developed (Lal, 1990)

The technique used to evaluate the soil loss depends on the types of erosion to be

monitored, the scale of measurement, and the objectives The following sections

highlight some of popular methods used in the estimation of soil loss

2.5.1 Universal Soil Loss Equation (USLE)

The universal soil loss equation (USLE) developed by Wischmeier and Smith (1958)

has been the most widely used as forecasting tool for two decades ending in

mid-1980 Although developed mainly as a forecasting cum planning tool for

agricultural land, USLE has been modified and adapted to predict the erosion

potential from watershed and non-agricultural sites (Lal, 1990)

The Universal Soil Loss Equation predicts the long-term average annual rate of

erosion on a field slope based on rainfall pattern, soil type, topography, and crop

system and management practices USLE only predicts the amount of soil loss that

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results from sheet or rill erosion on a single slope and does not account for

additional soil losses that might occur from gully, wind or tillage erosion

Five major factors are used to calculate the soil loss for a given site Each factor is

the numerical estimate of a specific condition that affects the severity of soil

erosion at a particular location The erosion values reflected by these factors can

vary considerably due to varying weather conditions Therefore, the values

obtained from the USLE more accurately represent long-term averages The USLE

is given as:

A =R x K x LS x C x P

• A represents the potential long term average annual soil loss in tonnes per

acre per year This is the amount, which is compared to the "tolerable soil

loss" limits

• R is the rainfall and runoff factor by geographic location The greater the

intensity and duration of the rain storm, the higher the erosion potential

The R factor is calculated as a product of storm kinetic energy times the

maximum 30 minutes storm depth and summed for all storm in year The R

factor represents the input that drives the sheet and rill erosion processes

Thus differences in R-values represent differences in erosivity of the climate

• K is the soil erodibility factor It is the average soil loss in tonnes/acre per

unit area for a particular soil in cultivated, continuous fallow with an

arbitrarily selected slope length of 72.6 ft and slope steepness of 9% K is a

measure of the susceptibility of soil particles to detachment and transport

by rainfall and runoff Texture is the principal factor affecting K, but

structure, organic matter and permeability also contribute

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• LS is the slope length-gradient factor The LS factor represents a ratio of

soil loss under given conditions to that at a site with the "standard" slope

steepness of 9% and slope length of 72.6 feet The steeper and longer the

slope, the higher is the risk for erosion

• C is the crop/vegetation and management factor It is used to determine the

relative effectiveness of soil and crop management systems in terms of

preventing soil loss The C factor is a ratio comparing the soil loss from land

under a specific crop and management system to the corresponding loss

from continuously fallow and tilled land The C Factor can be determined by

selecting the crop type and tillage method that corresponds to the field and

then multiplying these factors together

• P is the support practice factor It reflects the effects of practices that will

reduce the amount and rate of the water runoff and thus reduce the amount

of erosion The P factor represents the ratio of soil loss by a support practice

to that of straight-row farming up and down the slope The most commonly

used supporting cropland practices are cross slope cultivation, contour

farming and strip-cropping

Table 2.2: Management strategies to reduce soil losses

0B

R The R Factor for a field cannot be altered -

K The K Factor for a field cannot be altered -

LS Terraces may be constructed to reduce the slope length resulting

in lower soil losses

Terracing requires additional investment and will

cause some inconvenience in farming Investigate other soil conservation practices first

C The selection of crop types and Consider cropping systems that

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tillage methods that result in the

lowest possible C factor will result

in less soil erosion

will provide maximum protection for the soil Use minimum tillage systems where possible

P

The selection of a support practice

that has the lowest possible factor

associated with it will result in

lower soil losses

Use support practices such as cross slope farming that will cause deposition of sediment to occur close to the source

2.5.2 Measuring Sediment Yield from River Basin

According to Walling (1994), information on the sediment yield at the outlet of a

basin can provide a useful perspective on the rates of erosion and soil loss in the

watershed upstream He contends that in most rivers the suspended sediment

component will account for the majority of the total load This is most relevant in

soil erosion investigations, since most of the bed load will be eroded from the

channel However, it is essential to realize that there are a number of constraints

that must be recognized in attempting to use sediment yield measurements in soil

erosion studies

Sediment yield measurement possess the advantage of providing a spatially

integrated assessment of erosion rates in the upstream catchment area and

thereby avoid many of the sampling problems associated with direct measurements

Thus, in principle, measurement of sediment yield at a single point at basin outlet

can provide information on average rates of erosion within the basin, whereas a

large number of plot or similar measurements might be required in order to derive

an equivalent average However, there are several major problems that need to be

recognized in any attempt to provide meaningful information about on-site rates of

erosion and soil loss within drainage basin

A typical example of sediment yield determination using basin and sub-basin outlet

method was reported by Murtedza et al (1987) for the 9180 kmP 2 PPadas River basin

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in Sabah, Malaysia The basic requisite for determination of the source and solids

loading at any point of a river stretch is sufficient data on flow and solids

concentration at various upstream locations Murtedza et al (1987) used daily flow

rates and limited suspended solids concentrations at different flow data collected

from the Drainage and Irrigation Department of Sabah The Padas watershed was

divided into four smaller areas based on the location of gauging-station to identify

the general area from which most of the solids at output of catchment

To determine the output of solids from each of four areas, daily solidss loading at

each gauging station wereas calculated based on daily flow data Since all of the

stations have some missing daily flow data, a method was developed for calculating

the missing flow data from the flow data at other stations

When complete daily flow data was available, daily and yearly solids loading from

each station were estimated using an exponential relationship between suspended

solids concentration and flow:

ss = a(flow)P b

where: ss is suspended solids concentration,

a, b are constant

Suspended solids discharge, i.e the total amount of suspended solids carried by the

river in some time period, is:

Suspended solids discharge = c.ss.flow where c is a conversion factor If the suspended solids are in mg/L and the flow in

cubic meters per second, the conversion factor to tonne per day is 0.0864

Combining the equation for suspended solids concentration and discharge gives:

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Suspended solids discharge = a’.(flow)P b+1

Where a’ is a times c taking the log of both sides of the equation gives:

Log (discharge) = (b + 1).Log (flow) + Log (a’) The suspended solids discharge can thus be related to flow by a linear relationship

The values for the constants a’ and b + 1 depend on conditions in the watershed

Once this equation is determined for a particular watershed and as long as

conditions do not change, it can be used for calculating daily solids discharge from

daily flow data

Using flow data from the year 1969 – 1980 to calculate, they found that annual

solids discharge at Tenom increased from 768,300 tonnes or 0.84 tonnes/ha/year in

1969 to 2,698,300 tonnes or 2.94 tonnes/ha/year in 1977

They also point out that implicit in the calculations is the assumption that

suspended solids are a conservative parameter, i.e that no solids settle out of the

water between the upstream sites and outlet of catchment This assumption is of

course not accurate; much of the suspended solids carried by the water under high

flow conditions will settle out if flow rates and turbulence in the river decrease

However, the above assumption did not affect the finding based on the calculations

First, solids that settle out under flow conditions will be re-suspended when flow

increases again, so on an annual basis the assumption is more valid than it is on

daily basis Another interesting finding is that a large fraction of the total annual

solids loading at outlet of the catchment came during a few high flow days It was

found that the solids discharge on the top 12 flow days (or 3% of the total year) was

29.1 (in 1978), 20.9 (in 1979) and 30.6% (1980), respectively, of the total annual

solids discharge

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2.5.3 Measuring Sediment Yield by Using Tracers

In the second edition of the book “Soil Erosion and Conservation”, Morgan (1995)

wrote that the most commonly used tracer in soil erosion measurement is the

radioactive isotope, caesium-137 Caesium-137 was produced in the fall-out of

atmospheric testing of nuclear weapons from 1950s to 1970s It was distributed

globally in the stratosphere and deposited on the earth’s surface by the rainfall

Regionally, the amount deposited varies with the amount of rain but within a small

area, the deposition is reasonably uniform By analysing the isotope content of soil

cores collected on the grid system varying in density from 10 x 10 m to 20 x 20 m,

the spatial pattern of isotope loading is established

The changes in isotope loading can be correlated with measured sediment yield;

thus method can be used to estimate erosion rates This can be done be taking

samples on erosion plots and comparing the isotope loss, expressed as a percentage

of the reference level, to the measured erosion rate or by applying a simple model

which assumes that net soil loss is directly proportional to the percentage loss of

caesium-137

2.6 Previous Estimations of Soil Loss in the Bakun Catchment

2.6.1 The Study of SAMA in Bakun Catchment

In 1983, SAMA came up with the first estimate of sediment yield in Bakun

catchment The sediment rating curve was established by means of computer

program XYFIT Their fitted sediment rating curve has following equation:

S = 0.0103 x (Q – 139)P 1.3806

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• S: Suspended Sediment Transport (kg/s)

• Q: Water discharge (mP 3

P/s) They used suspended sediment data measurement by Drainage and Irrigation

Department in 1982 and 24 data taken by them in the month of March, and the

rest in November 1982 at Station 7002 – 4.2 km downstream of the Bakun Dam

Site The average annual suspended sediment transport was computed as 7.5

million tonnes or 5.08 tonnes/ha/year They assumed that bed load transport

amounts to 20% of the suspended sediment transport, so that the total average

annual sediment inflow into the Bakun reservoir was computed as 9 million tonnes

per annum

2.6.2 Estimated TSS Yield in Bakun HEP EIA report

In 1995, as a component of the EIA for the proposed Bakun HEP project (Appendix

3B, Bakun HEP EIA, 1995), The Center for Water Research (CWR) at the

University of Western Australia carried out an environmental assessment of the

potential impact of the development on the hydrological features of the catchment

upstream of the proposed Bakun HEP dam and on the future quality of water to be

stored within, and released from, the resulting impoundment The assessment was

based upon computer model simulation of: (1) Catchment area water yield and

sediment yield, and (2) water quality in the reservoir (specifically temperature,

suspended solids, nutrient, etc.) under a range of catchment and reservoir

operational conditions during both construction and operation of project

2.6.2.1 Erosion and Sediment Yield Modeling

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According to CWR, erosion models have primarily been developed to predict soil

loss for hill-slopes under agriculture, for field sized areas or for small catchment

Most of the model use regular grids for the calculation of water and sediment

transport between grid cells Such models are impractical for use in large

catchment modeling studies due to large amounts of cells that would be necessary

to perform the calculations In addition, it may be difficult to collect the necessary

input data when dealing with such large catchments

In general, two phases may be distinguished in the erosion-sediment delivery

process, which determines the amount of sediment leaving a catchment (Bennet,

1974) The first phase is the upland phase, where factors such as rainfall amount,

intensity and duration, soil type, soil condition and soil moisture content, slope and

slope length, vegetation and litter cover govern the erosion from hill-slopes and its

transport to drainage network The second phase is the in-channel phase, which

determines the transport of sediment over larger distances through the drainage

network The amount of sediment transported by a stream depends mainly on the

channel slope and particle size distribution of the bed-load, the amount and nature

of sediment delivered by the upland phase, and velocity and depth of flow in the

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• SSR i R and BLR i R are inputs of suspended sediment and bed load into the

catchment from upstream areas,

• SDC is the sediment delivery to the drainage network, and

• DSR C Rrepresents changes in the sediment storage within the drainage

network

The sediment delivery ratio may be assumed to be close to unity for the small

catchment to which the models quotes above apply because DSR C R may be considered

negligible The predicted soil loss from hill-slopes is therefore similar to the

sediment yield at the outlet of the catchment area

The sediment delivery ratio is known to decrease with the size of the catchment

due to increased sediment deposition opportunities within the drainage network

(Brune, 1948; Wilson, 1973) Sediment delivery is a runoff transport process and

this makes it highly correlated with the volume of runoff and peak runoff rate

(Foster, 1988) Empirical models (e.g sediment rating curve) have therefore been

commonly used to predict sediment for larger catchments The disadvantage of

empirical models is that changes in one of the parameters affecting sediment yield

(e.g land use) cannot easily be incorporated into the model and new coefficients

need therefore be determined after each change

2.6.2.2 Reservoir Preparation and Operational Options

Five Possible catchment and reservoir operational conditions were modelled by

CWR These conditions encompassed:

• Scenario S1 – ‘Worst case/no build’ scenario: Selective timber harvesting

continues in the catchment using the present (1995) mechanized timber

extraction methods (i.e., tractors, high-lead yarding) No logging takes place

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in area for which logging licenses have not yet been issued The remaining

forest in the impoundment area selectively logged and then submerged

• Scenario S2: Selective timber harvesting continues in the catchment using

the present (1995) mechanized timber extraction methods until 1996 From

1996, timber extraction is carried out by least impact logging techniques

licenses have not yet been issued The remaining forest in the impoundment

area selectively logged and then submerged

• Scenario S3 – ‘Most likely’ scenario: Selective timber harvesting continues

in the catchment using the present (1995) mechanized timber extraction

methods until 1996 From 1996, timber extraction is carried out by least

impact logging techniques (i.e., Helicopter logging) No logging takes place

in area for which logging licenses have not yet been issued The remaining

forest in the impoundment area is selectively logged A portion of residual

biomass in the impoundment area (i.e between 10% and 40% of the total

residual biomass) is cleared and burned prior to inundation The remaining

impoundment area is submerged without clearing and burning

• Scenario S4: Selective timber harvesting continues in the catchment using

the present (1995) mechanized timber extraction methods until 1996 From

1996, timber extraction is carried out by least impact logging techniques

licenses have not yet been issued The remaining forest in the impoundment

area is selectively logged and 100% residual biomass is cleared and burned

prior to inundation

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• Scenario S5 – ‘Best case’ scenario: Selective timber harvesting continues in

the catchment using the present (1995) mechanized timber extraction

methods until 1996 From 1996, no further timber harvesting takes place in

the catchment The remaining forest in the impoundment area is selectively

logged and 100% residual biomass is cleared and burned prior to inundation

A baseline scenario (S6), representing pre-1983 conditions before logging of the

catchment commenced, was also modeled to assess the “total” effect of logging on

the water and sediment yield from the Bakun catchment and the likely impacts on

water quality

2.6.2.3 Sediment Yield Modeling Result

Predicted suspended sediment yield over the period 1983 until 1998

From the modeling exercise, the CWR team found that the cumulative predict

suspended sediment yield over the period 1983 until 1998 for the baseline scenario

amounted to 107 million tonnes Selective logging of the forest increased the

predicted cumulative suspended sediment yield more than three-fold to between

340 and 345 million tonnes for scenario S1 to S5 respectively, as compared to the

baseline scenario The predicted annual maximum values of suspended sediment

yield for scenarios S1 to S5 increased even more, to about 4.3 times that for the

baseline scenario The total sediment yield for scenario S1 to S5 was therefore 2.1

times that of the baseline scenario whilst the annual maximum increased by factor

of 2.7 as a result of logging activities on the catchment Annual mean, minimum

and maximum values of predicted suspended sediment loads and bed-loads for the

different scenarios over the period 1983-1998, and the corresponding values of total

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predicted sediment yields (suspended sediment plus bed-load) for 5 scenarios and

the baseline scenario are given in table 2.3 below

The different management options proposed for the impoundment area (i.e., within

Scenarios S2, S3, and S4) had little effect on the suspended sediment yield as the

period during which they were applied was relatively short and because the

impoundment area cover less than 5% of the total catchment area

Table 2.3: Predicted and annual suspended sediment yields, sediment yield and

bed-load from the Balui River catchment at the Bakun Dam site over period

1983-1998 for different catchment operational scenarios (all values in million tonne,

standard deviations in brackets)

Mean annual bed-load

Mean annual sediment yield

Min annual sediment yield

Max annual sediment yield S1 21.6 (7.9) 7.4 (0.6) 29.0 (8.1) 19.0 43.1

It is clear that the difference in sediment yields between Scenarios S1 to S5 and

the baseline scenario increased significantly as logging progressed The difference

in total sediment yield was mainly caused by variations in the suspended sediment

yield, as bed-load predictions were almost identical for all scenarios (refer Table

2.3) Although bed-load are likely to increase as a result of logging (Lai, 1993),

changes in bed-load were effected only indirectly by changes in water yield in the

current model Since the differences in water yield were small between the

different scenarios, no large differences were predicted for bed-load component of

the total sediment yield between the various scenarios Total predicted bed-load

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over the period 1983-1998 ranged from 116 million tonnes for the baseline scenario

to 119 million tonnes for the other scenario As such, the bed-load amounted to 52%

of the total sediment yield predicted for the baseline scenario and to 26% of those

predicted for the other scenarios

Predicted suspended sediment yield over the period 1999 until 2043

Annual mean, minimum and maximum values of predicted suspended sediment

yield and bed load, the corresponding values of total predicted sediment yield

(suspended sediment plus bed load) over the period 1999 – 2043 for the three

relevant catchment scenarios and baseline scenario are given in table 2.4

From the modeled result, they point out that the patterns indicate that the

different in annual sediment yield between the baseline scenario and the other

scenarios was highest in the period during and shortly after logging (1999-2015)

and decreased significantly between 2015 and 2043 as a result of re-growth of the

secondary vegetation in the selectively logged areas

The average suspended sediment yield for the baseline scenario over two periods

(1983 – 1998 and 1999 - 2043) was predicted to be 6.4 million tonnes/year or 4.32

tonnes/ha/year The predicted average suspended sediment yield over two periods

modeled (1983 – 1998 and 1999 - 2043) was 20.22, 16.35 and 12.75 tonnes/ha/year

for scenarios S1, S3, S5

The predicted average bed load over two periods modeled (1983 – 1998 and 1999 -

2043) was 7.5, 7.4, 7.3 and 7.2 million tones/year for scenarios S1, S3, S5, and the

baseline scenario, respectively Such proportions of bed load to total sediment load

are not uncommon and similar ratios have been measured in Peninsular Malaysia

by Lai (1993, refer Section 2.1.2)

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Table 2.4: Predicted and annual suspended sediment yields, sediment yield and bed-load from the Balui River catchment at the

Bakun Dam site over period 1999-2043 for different catchment operational scenarios (all values in million tonne, standard

Min annual suspended sediment yield

Max annual suspended sediment yield

Mean annual bed-load

Mean annual sediment yield

Min annual sediment yield

Max annual sediment yield

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2.6.3 Using GIS to Study Soil Erosion and Hydrology in Bakun HEP

Roslinah Samad and Norizan Abdul Patah (1997) of the Malaysian Centre for

remote Sensing (MACRES) had reported soil erosion and hydrological study of the

Bakun Dam Catchment Area using remote sensing and geographic information

system (GIS) The landsat TM data (1988 and 1994) with false color composites

band 4, 5, 3 were used in their study Rainfall data, soils map and tophographic

maps at scale 1:25,000 also were used as an ancillary data The methodology

adopted in the generation of the R, K, LS and C digital raster layers for soil erosion

modeling and hydrological studies was done in MICSIS (Micro-computer Spatial

Information Special system for soil erosion modeling based on the parameters of

the USLE was incorporated in MICSIS The Universal Soil Loss Equation (USLE)

(Wichmeier and Smit, 1978) is an erosion model designed to predict average soil

loss from specific tracks tracks of land under different land use management

systems The USLE was adopted in this study with minor modifications in

estimating the R and K parameters to suit the Malaysian conditions

In the study, they found that rainfall erosivity of the Bakun catchment area ranges

from 880-1400 US units In the southern part of the cathment area, the erosivity is

very high whilst in the vicinity of the dam area is high Bakun is predominantly

characterized by soils of the Skeletal and Red-Yellow Podzolic Group They are well

to excessively drained soils with shallow to moderate depth (25-50 cm of the

surface) Their erodibility value of 0.18 is moderate attributed mainly to the high

very fine sand and silt content (49%) Soils of high erodibility (>.3) such as the

podzols, gely soils, skeletal & podzols, skeletal & gley soils and podzols & gley soils

groups occur in very limited extent

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Bakun has a rugged topography with sharp crest and steep slopes Most of the area

is above 500 m a.s.l with the highest elevation being 2040 m slope Length varies

from 3-10 m for the gentler slopes (2-12) and 10 ³ 20 m for the steeper slope (>12)

Except for the logging and shifting cultivation activities in the immediate

surroundings of the proposed dam site and also along Balui River towards its

headwaters upstream, the catchment area is basically under densed forest cover

Abandoned areas of shifting cultivation have been transitioned into natural bush

and grassland over short periods The extent of inundation at the three proposed

flood levels - (i) probable maximum operational flood level 233 m produced 632.44

kmP 2 P inundation extent of water and 36.93 kmP 2 P volume storage of water' (ii)

maximum operational flood level 228 m produced 593.96 kmP 2 P) inundation extent of

water and 33.84 kmP 3

P volume storage of water; (iii) minimum operational flood level

195 m produced 388.68 km2 inundation extent of water and 18.42 kmP 3

P Soil loss in tonnes/ha/year was estimated based on 6 classes in table 2.5

Attention should be focused on the logged over forest (including logging tracks) and

shifting cultivation areas where no or minimal conservation practice has been

employed Soil loss here ranges from moderate to severe and is estimated to be 6.6

million tonnes/year Given the rainfall erosivity, topographical and soil factors the

area, the worst-case scenario would present a soil loss of some 221 million tonnes,

should the area be completely depleted of vegetation

Table 2.5: Soil erosion in Bakun catchment estimated by using GIS

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