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

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

BAKUN HYDROELECTRIC PROJECT

Vu Ngoc Chau

Master of Environmental Science (Land Use and Water Resource Management) Faculty of Resource Science and Technology

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

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

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

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

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

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

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.,

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

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

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

(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

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

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

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

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

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

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

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

(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 steepscatchments 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

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

2.4 Studies on Rates of Soil Erosion in Sarawak

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-

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

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

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

• 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

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

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:

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

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

• 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

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 channel

The catchment mass balance for sediment may thus be represented by the following equation:

SSR O R + BLR O R= SSR i R + BLR i R + SDC + DSR O

where:

• SSR O R and BLR O R are the suspended sediment and bed load yield at the catchment outlet,

• 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

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

• 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

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

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)

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

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

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