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

a Estimation of sail loss from the Upper Rajang Sub-Catchments during the development of Lhe Bakun HEP b Soil loss estimation in relation to changing discharge in the watershed, 1.4, Si

Faculty of Resource Science and Technology

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)

2005

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 vauses

of soil erosion (Lopez and Alhaladejo, 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 uman aelivily over the last 45 yeurs (World Bank, 1992)

From the engineering perspective, soil crosion is defined as a general destruction of soil structure by the action of water and wind It is essentially the smoothing

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

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

away, Unuy leaving bebind ars allered bare earth surface (Wishchmveier et ai, 1978)

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

disperse the aggregate material Lighter agpregatle materials euch as very fine sand,

sill, clay and organic malier can be eusily removed by the raindrop yplash und runoff wuler: greater raindrop energy or runoff amounts might be required ty move

the larger sand and gravel particles Soll movement by ceinfall (raindrop splash) is usually greatest and most noticeable during short-duration, high-intensity Uhunderstoums, Although the erosion caused by longlasling and lesyinLense

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

Runoffcan occur whenever there is excers water on a slope that cannot be absorbed

inta the soil or trapped on the surface The amount of rimoff will increase if infiltration is reduced due tn soil compaction, crusting or freezing Runoff from the

agricultural land may be greatest during spring months when the soils are usually galuruted, snow is melting and veyelative cover ia muumal,

1n Malaysia, there are many soil eruvion prone zones eypecielly hilly areas al the newly eetublished oil palin plantation wall along the riverbanks Tn the case of slope,

an altered bare surface of the elope with sheet, rill and gully erosion features will cause instability of the slope ‘This situation will gradually cause slope failure or landslide as commonly Imow The soil erosion phenomenon is basically the function

af the erasivity of the oil (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 beet management practices A number of other impacts are expected to be residual Progressive construction and use of access roalls and camps in rugged and sleep topography intersected by many

watercourses would initiate unavoidable erosion and sedimentatian in the rezervoir

areu, Removal of biomass in this environment would increase the risk o[aeoelerated erosion and sedimentation over a larger area, Following biomass removal, the sediment yield in the calchment ulso 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 unpoliuted, contain suspended matter consisting of natural ailt, sand, etc, derived from the stream bed and banks There are several reasons why suspended solids are

objectionable in a stream, amoug which ace:

+ They interfere with selfpurification by diminishing photosynthesis and by smothering benthic organisms,

* Reduce resorvoir 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 scdimentation and

shallowing of river bed, etc

The coil erosion related problems should thus be identified to enhance understanding and to minimize effects Soil loss estimation in relation ta changing discharge in the watershed provides vital infarmation on this issue

1.8 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 Bukun caldlunent area iv lovated between latitudes 1.5°N aud 3.0°N and longitudes 118.5°E and 115.3°E The catchment upstream of the dam site covers an area of about 16 million hectares (ha) The watershed and river are respectively the largest (44,200 km9) and the longest (900 lan) in Malaysia and the Balui or Upper Rajang sub-walershed

represente 34% of the entire Rajang watershed

1.8, Objectives ofthe Study

A vel of research projects van be iniliated in relation to the developuient of the Balun HEP dam with the aim of producing data and information useful for an integrated approach to river basin and land use management ‘he present study

focuses on the following objectives!

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

development of Lhe 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 und stream channels, silling of reservoirs (reduce reservoir storage capacity), damages to fish spawning grounds and depiction of downstream water quality, Pesticides and fertilizers, frequently transported along

with the eroding soil can contaminate or pollute downslream water sources and recreational areas, Because of the potential seriousness of some impacts, the

estimation of soil loss is uevesvary, The estimation is useful, among olhers in

understanding the sources, predict the trend of.crosion and support further studies

Soil loss and transport in the upland watershed arc 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 Iccations can he used for

sediment analysis for the asresement of cumulative eediment yield for all the

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

+ Describe the totel suspended solids (TS) measurement methods, and to develop a relationship between daily discharge (or water level) and daily TSS, From the daily TSS readings, the tolal 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 warershed

management approach with respect to management of soil base on changing:

of soil loss over different years

Chapter2: Literature Review

2.1 History ofthe Balsam HEP Project

The Bukun Hydroelectric Project (Bukun HEP) in Sarawak, with a proposed goncration 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 initielly privatized to Elvan 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 nighte of Itkran Berhad to Sarawak Hidro Sdn Bhd (SHSB) In the meantime, the river diversion works continued and were completed and handed over to SLISL at

the end of April 2001

Ơn 1* 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 Gailities (the unain 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 rescrvair impoundment is planned to commence earlier i.e on 1 January 2007

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

be elongated and dendritic in shape, spanning over the Batang Bului, Sg Murus, Sungai Behau and Sungai Linu The reservoir will lie bebween the base elevation

of 34m asl at the dam site and maximum operating level of clevation 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:

* toensure that the quality of water of the reservoir will improve: and

* tomake 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 ssluble-dissolved and ineoluble-solid materials

Physical erosion involves detachment and transport of insehable-soil particles, eg.,

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

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

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

causalive agent in sediment deladunent and transport is dhe wind (Lal, 1990)

Water Glaclated erosion

Slides Debris Creep

flow

erosion srosiơn bang ®/9S10H

erosion

Figure 2.1 Types of crosion (Sourve’ Lal, 1990)

Different types of erosion on the basis of major agents involved are shown in figure 2.1, Water erosion is dagsified into splash, sheet, rill, and gully erosion on the basis principal provesse invelved Splash or inter-rill erosion is caused by raindrop impact Sheet erosion is the removal of a thin, relatively uniform layer of scil

particles Rill erosion iz erosion in small of a thin, channel only a few millimeters wide and deap Rilla are transformed to gullies when they carmot be cbliterated by normal tillage Stream channel erosion and coastal erosion are caused, respectively,

by slream fow aud ocean waves Svil movernent en masge is caused by gravity

2.2.8, Sediment

‘The soil masz removed from one place is often depasited at another location when the energy of the erosion causing agent is diminished or too dissipated to trangport soil particles The term sediment refers to solid material that is detached from the svi] mass Ly erosion agents aud transported from its original place by suspension

in water or air or by gravity

The term soil erosion therefore is distinct from snil lose 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 lass

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

sơil dislodged und vedunentation, Sediment yield, in comperison, is soil logs delivered tn the specific point under consideration A field's sediment yield is the sui of soil losses from slope segments minus deposilion The deposilion may occur

in depressions, at the tocs of slopes, along filed boundarics, and m terrace channels The combined terms erosion and sedimentation by wat

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 engineer

, consider sedimentation ta be a major

process of which ervsivn is an initial step Fleming (1981) advpts a broader approach by stating that “the sediment problem may be defined as the detrimental depletion by ervsion and wansport uf veil resources from land surfoces and

subsequent accrelion by deposition in reservoirs and coastal areas”

2.3 Soil Erosion in Asian Countries

Soil crosion 1s perhaps the most scricus mechanism of land degradation in the tropies in general and the humid tropics in particular (Mi-Swaily et af, 1982) In the tropics, erosion by water, rather than by wind, assumes the primary

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

(1982) pointed ont that available geclogic data on erosion of different continents

indicate that Asia leads the way with L66 lonnes/ha‘year, followed by South

America, North and Central America, Africa, Furope, and Australia with 0.93, 0.73, 0.47, 0.42, and 0.82 tonnes‘ha/year, respectively These data were derived directly from vediment loads iat major rivers No allempt was made to convert these data to field soil logzes This was corroborated by the fact that the heavily populated vegions of Asia poszess the highest global sediment loads in their major rivera For examples, presented as an average scdiment removal from respective drainage basins (using appropriate sediment delivery ratios), were 550, 480, 480, 270, 217,

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

10

(India), Damodar River (India), Ganges River (India, Bangladesh, Nepal, Tibet), Red River (China, Vietnam), and Irrawady River (Burma) (l-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 12 million hectares The annual erosion rate vangee from 50-70 toneshectares/year (Defu, 1985; Dexing, 1986) The total sail

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

‘The total seriously eveded area in China under water erosicn would be at least, 150 million hectares The total soil loas in China was caleulated to be more than 4500

auillion tonnes, which accuunls fur an estimaled 20% of total workd soil loss

(Dazhong, 1994) About 40% of total soil evaded from the land, or about 2000

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

8500 million tonnes of sediments are deposited in lakes, rivers, and various water conservation fxilities (TMB, 1984; Zlengslwn, 1987),

‘The Yellow River is 5464 km long, watershed of 680,000 kin? and carries 40 billion cubic meters of total annual mmaff 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 88 kfm? During periods of flooding, sill content in the Yellow River cun rise lo more than 650 ky/n? (Gueliang, 1987)

‘The Yangtze, which is the lowest river in China, is about 6800 ku long with a trillion cubic meters of armual runoff and collecting 2400 million tannes of sail sedinent About 680 million tonnes of sediment are depusited at the mouth of the river The remaining deposits are in the river system, Takes, and reservoirs (Youugeng and Jinlin, 1986 Yansheng, et al., 1987) The large Dongling Luke inn the middle area of Yangtze River has an input of 180 million tonnes of silt About 70% of this silt is doposited on the lakebed and raiscs it about 3.5 cm annually From 1949 to 1977, the water arca, storage capacity, and navigable section of the Jake have beon reduced by 37%, 39%, and 31%, respectively (I'MB, 1984; Youngeng

and Jinlin, 1986) It is also eetimated 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 veservoirs 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 inflow has been about 200% more than the designed inflow Nizamsagar reservoir,

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 ASZALAND

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 aveas have been reduced to lesa than #0% These are the most critical areas undergoing environmental changes, especially through land degradation and soil erosion Predicted soil lose was estimated al 30-150 tonnes/ha’year, depending on parameters such ag goil characteristics, land slope, land cover, und farming

systems

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

on sleep-land areas 11 causes gevere ons and off'sile enviruomental, ecouamic, und social impacts On site, it reduces the chemival fertilily of the soil by autrient und organic matter deplelivn, und in same vases, exposes Che acid subsoil Erosion slvo damages Lhe physical fertility by removing surface soil, und reducing the suil depth and water holding capacity These soil changes will slowly reduce crop yiclds, farm incomes, and household mutrition ‘Ihe offsite effects of crosion on the quality and availability of water can also be very serious Major offsite effects include increased surface runoff, offen resulting in flooding which displaces people in low- lying areas and damages voad iniastructure; inereased sediment, nutrient and pollution loads in streams, whicb degrade the quality of household water supplies and increase the risk on human health; siltation of dame and irrigation canale, resulting in reduced water supply for irvigated crops and shorter life of reservoire: and sediment deposition in offshore fisheries, reducing the availability of aquatic

supplies and promotion of eco-lourism

The Mekong Basin

In astudy about soil crosion and sediment transport in the Mekong Basin, Al-Souti (2003) found that the crosion in the Mckong Basin is meunly rainfail based runoff’ erosion eubject to the effects of land cover Soil erosion patterns in the basin are

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 km* The Lower Mekong Basin

excludes Yunnan and Myanmar and thus the catchments area is estimated around

615,800 km° 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 kan), 21st with respect to catchment’s area and 8th with respect to average

annual runoff (475 x 10° m? per year or 15000 m/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 Harden

| 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

Harden 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 32472 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 vain 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 km? (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/halyear (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 occur (Bakun HEP EIA, 1995) For instance, in Peninsular Malaysia, Baharuddin

(1988) observed an increase of 70% (from 0.07 to 0.13 tonnes/haiyear) in suspended sediment yield after supervised logging of a small rain forest catchment on granite

rock (area 0.3 km’) 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 tonnesha/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/halyear for undisturbed (Sg Lawing, 5 km?) and partly logged (Sg Lui, 68 km2, 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 km?) and Sg Chongkak (13 km) 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 km?) 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/hayear 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 km? (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 (2330 km?) and Jeniang (1770 ku’) river basins Earlier measurement of

sediment yield at Jambatan Syed Omar toLalled 0.88 tonnesha/year (Wam Ruslan,

1989) and concluded that the observell increase could partly be attributed to changes in Jani use in the aren Wan Ruslan (1992)

Values presented in hydropower feasibility studies carried out in Sabah and Sarawak (Syed Mubumunal and Electrowalt Engineering Services Lid, 1994) range from 2,05 loones/ha!year Jr undisturbed Upper Padas calchiment (1790 kun}

to 12.50 tonmewhuvyear for the Butang Ai catchment (1200 km), 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 fow detailed studies, apart from a long running sct of plot cxporiments by the Research Branch of the Department of Agriculture Uniirtunately there has been Little work on forest hydrology in Sarawalc and no measurements of the impact of logging on erosion rates and stream sedimentation Comments by forestere 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

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

much as recent arguments have suggested Ifa clearad araa ts left to be ra"

colonized by secondary vegetation, peak stream flows and sediment yields

gradually return to near natural lavels The continuation of those affects in

logging areas is due to the mad systam which remaing after timber

extraction as 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 0.88 Lonnesfha/year, while those for imterracad pepper cultivation are &lto 90 tonnes/ha’year (Petch,

1985)

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

general belief that the slash-and-brrn 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 enrface, results suggested otherwize Only 0.45 tonnes‘ha were lost in the first year after clearing At Tehedu, Teck

(1992) recorded 0.46 lunnes‘ha soil loss in Lhe first year aller dlearance These field

data from plot studies (Table 2.1) clearly show that soil losy under vhifling cultivation is of the sume magnitude as thet under uutural forest, wherens once a

cultivation system Icaves bare ground between row crops, as in traditional popper,

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

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

(all values of soil loge in tonnes/year)

Land Use Location Blope | Period | Soilloss | Scillose

(degrees) | {years) | mean range

Hill Padi/ Shifting Cultivation

cover

¢) bush fallow Semonggok | 16-36 3 0933 | 006045

2.5 Soil Loss Estimation Methodologies

The measurement soil loss or soil erosion rates are arelatively 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

2.8.1 Universal Soil Loss Equation (USLE)

The universal goil 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 evasion

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 ata particular location The erosion vatues reflected by these factors can vary considerably due to varying weather conditions Therefore, the values abtained from the USLE more accurately represent long-term averages The USLE:

«Ris the rainfall and runoff factor hy geographic location The greater the

intensity and duration of the rain storm, the higher the erosion potential The R factor is calculated as u product of storm kinelic energy times the maximum 30 minutes storm depth and summed for all sloum in year The R factor represents the input that drives the sheet and rill erosion processes

‘Thus differences in R-valuos represent differences in crosivity of the climate

+ Kis the soil erodibility factor It is the average suil loss in tonnes!a

2 per unit urea for a particular soil in cultivated, continuous fullow with an arbilrarily selected slope length of 72.6 1 and slope steepness of 946 Kis a measure of the susceptibility of soil particles to detachment and transport

by rainfell ond runoff, Texture is the principal factor ullecting K, but structure, orgame matter and permeability alsa contribute

22

* LS is the slope length-gradient, factor The LS factar represents a ratio af goil loss umder 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

* Cis the cropivegetaton and management factor It is used to determine the velative effectiveness of soil and crop management systems in terms af preventing eoil loss The C! factor ia a ratin comparing the soil loes from land

umder a specific crop and management system ta the corresponding loss

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

aelecting the crop type and tillage method that corresponde to the field and

then multiplying these factors together

* P is the support practice factor It refiects the cffocts 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 vatia af soil loss by a eupport practice

ta that of atraight-row farming up and down the slope The most commonly used supporting cropland practices are cross slope cultivation, contour

farming and siripwopping

Table 2.2: Management strategies to reduce soil losses

The R Factor for a field cannot be

altered, The K Factor for a ficld cannot be

altered

Terraces may be constructed to additional investment and will

LS | reduee the slope length resulting cause some ineonvenience in

in lower svil losses farming Investigale other soi

congervation practices first

ỡ 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 loss soil crosion

will provide maximum protection for the geil) Use imininum tillage

systems where possible

The selection af 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 af sediment to occur close to the source

2.6.2 Measuring Sediment Yield from River Basin

According 1o Walling (1994), information on Ube vediment yield at the oullel 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 trom the channel TTawever, it is essential ta realize that, there are a number of conetraints

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

erosion studies

Scdiment yicld measurement possess the advantage of providing a spatially

integrated ascesement of erosion rates in the upstream catchment area and

thereby avoid many of the sampling problems associated with direct measurements

‘Thus, in principle, maasurement of sediment yield at a single pomt at hasin 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

erosiun wl soil loss within drainage basis

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

method was reported by Murtedza et af (1987) for the 9180 kin? Padas 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 a/ (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)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 =¢.ss.flow where ¢ 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’

i a

Suspended solids discharge = sỉ (flow)>"

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

Log (discharge) = (b+ 1)Log (low) + Log (al) The suspended solids discharge can thus be related to flow by a linear relationship The values for the consiunis a and b + 1 depend on couditions im the watershed Once thie equation is determined for a particular watershed and as leng az

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

Using flow dala fom the year 1969 — 1980 tơ calculate, they found that annual solids discharge at Tenom increased fron 768,300 touney or 0.84 tounes/hwyear in

1969 to 2,698,800 tonnes or 2.94 tonnes/hatyear in 1977

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

ie that no solids

suspended solids are a conservative parame! ttle out of the

waler belween the upstream sites and outlel of catchment This assumption is of

course not accurate! much of the suspended golids carried by he water under high flow conditions will settle out if flow rates and turbulence in the river decreaze However, the above assumption didnot affect the finding based on the calculations First, solids that settle out under flow conditions will be re-suspended when flow inereases again, 30 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 cutict of the catchment came during a fw high flow days It was found that the solids discharge on the top 12 flow days (ar 3% of the total year) was

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

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2.6.3, Measuring Sediment Yield by Using Tracers

In the second edition of the book “Soil Exosion and Conservation’, Morgan (1998)

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

radioactive isotope, caesium-187 Caesium-127 wus produced in Uhe fall-out of atmospheric testing of nuclcar woapons from 1950s to 1970s It was distributed globally in the stratosphere and deposited on the carth’s surface by the ranfall Regionally, the amount deposited varics with the amount ofrain 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 10x 10 m to ‘lx 20m,

the spatial pattern of isotope loading is established

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

thus method cau be used to estumale erosion rales This ca be done be taking

samples on crosion 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

cacsium-137,

2.6, Previous Estimations of Soil Loss in the Baku Catchment

26.1, The Study of SAMA in Bakun Catchment

In 1983, SAMA came up with the first cstimate of sediment yield in Bakun catchment The sediment rating curve was cstablished by mcans of computer program KYFTT Their fitted sediment rating curve has following equation:

S — 0.0103 x (Q — 199)" 86:

where?

2?

* S: Suspended Sediment Transport (ka's)

* @ Water discharge (ms)

They used suzpended sediment data measurement by Drainage and Trrigation Department in 1982 and 24 data taken by them in the month of March, and the revl in November 1982 at Stalion 7002 - 42 kim downstream of the Bukun Dom Site The uverage unnual suypended sediment wausporl wus computed as 7.5 million tonnes cr 5.08 tonnes/heyear They assumed that bed load transport amgunts to 20% of the suspended sediment transport, so that the total average annual scdiment inflow into the Bakun rescrvoir was computed as @ million tonnes

‘per annum

26.2 Estimated TSS Yield in Bakun HEP EIA report

To 1995, as acomponent of the ELA for the proposed Bakun HEP project (Appendix 8B, Bukun HEP EIA, 1995), The Center for Water Research: (CWR) at the

University of Western Al

alia 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 canditians during both construction and operatian of project

2.6.2.1 Eroston and Sediment Yield Modeling

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

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

Mast of the madel use regular grids for the calculation of water and sediment, transport between grid cells Such models are impractical for use in large

calchment 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 pluses inay be dislinguished in the erosion-sediment delivery

proces, which determines the amount vf sediment leaving a calchment (Bennet,

1974), The first phuye is the upland phase, where Glory yuch ox rainfall uniount, intensily and ducution, evil type, soil condition and veil moisture content, slupe und slope length, vegetation and litter cover govern the crosion from hill-slopes and its transport to drainage network ‘he second phase is the in-channel phaso, 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 distributien ef the bed-load, the amount and nature

af 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:

$Bc + BLo — 85: - BL¡ + SDO + D8o where:

* SSo and Blo are the suspended sediment and bed load yield at the

catchment cutlct,

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* SS, and Bl ave inputs of suspended sediment and bed load into the

catchment from mpatream areas,

* SDCie the sediment delivery to the drainage network, and

« DSc represents changes in the sediment storage within the drainage

network,

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

catchment to which the models quotes above apply because DSe may be considered negligible The predicted soil loss from hill-slopes is therefore similar to the

sediment vield at Uhe oullet of the caichunent area

‘The sediment delivery ratio is known to decrease with the size of the catchment due ta increased sediment deposition opportunities within the drainage network

(Trune, 1948: Wileon, 1973) Sediment: delivery ie a runoff transport process and

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

(Poster, 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 (eg land use) cannot easily be incorporated into the model and new coefficients need Lherefore be determined aller euch change

26.22 Reserwir Preparation and Operational Options

Five Possible catchment and reservoir operational conditions were modelled by CWR These conditions encompaseed:

» Scenario $1 — ‘Worst case‘no build’ scenario: Selective timber harvesting

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

extraction methods (ie 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 impoumdment area selectively logged and then submerged

Scenario $2 Selective timber harvesting continues in the catchment using

+ (1995) mechsnized timber extraction methods until 1996 From

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

(a, Tlelicopter lagging) No logging takes place in avea for which logging licenses have not yet been issued The remaining forest in the impoundment area selectively logged and then submerged

Scenario $3 ‘Most likely’ scenario: Sclcctive timber harvosting 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 {7.e, Helienpter logging) No logging takes place

in area for which logging licenses have not yet heen issued The remaining forest in the impormdment area is selectively logged A portion of residual biomass in the impoundment, area (1.6 between 10% and 40% af the total

residual biomass) is cleared and burned prior te inundation The remaining impoundinent acea is submerged without deuring and burning

Scenario $4: Selective timber harvesting continues in the catchment using

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

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

Ge, Helicopter logging) No logging lakes place in avea for wihich logging licenves have not yet been issued The remaining forest in the impoundment

area is selectively logged and 100% residuel biowass is cleared and bummed

prior W inundation

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* Scenario S5 — Test case’ scenario: Selective timber harvesting continues in

the eatchment 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 biomasa is cleared and burned prior to imumdation

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

catchment commenced, was alan modeled to assesa the “total” effect of lagging on the water and sediment yield from the Rakun catchment and the likely impacts on

walter quality

2.6.2.3 Sediment Yield Modeling Hesult

Predicted suspended sediment yield over the period 1983 until 1998

From the modeling exercise, the CWR team fond that the cumulative predict suspended sediment yield over the period 198% until 1998 for the baseline scenario amounted to 107 million tonnes Selective logging of the forest increased the predicted cumulative suspended sediment yield more Uuan three-fold to between

840 and 345 million tonnes for scenariv $1 Lo $3 respectively, as compared lo the

baseline scenario The predicted annual meximum values of suspended sediment yicld tor scenarios $1 to S& increased even more, to about 4.3 times that for the

baseline scenario The total sodimont yield for scenario $1 to $5 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 activitics on the catchment Annual mean, minimum

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

different scenarios over the period 198-1998, 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 2.5 below

The different management options proposed for the impoundment area (i.e, within Scenarios $2, $3, and $4) had little effect on the suspended eediment yield as the

period during which they were applicd was relatively short and because the impoundient area cover less than 5% of the total catchment area

Table 2.9: Predicted and annual suspended sediment yields, sediment yield and

bed-lowd from the Balui River cattlunent at the Bakun Dam site over period 1983-

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

standard deviations in brackets)

2 Mean annual | voan Mean | Min annual | Max annual

Il is clear thal the difference in sediment yields between Scenarios 81 le $5 and the baseline scenario increased significantly as logging progressed The difference

in total sediment yield was mainly cuused by variations in Use suspended sediment yield, as bed-load predictions were almost identical for alll scenarios (refer ‘Table 2.9), Although bed:load are likely to increase as a result of logging (Lai, 1993), changes in bed:-load were otfected only indiroctly by changes in water yield in the current model Since the diffevences in water yield were small between the different ecenarios, no large differences were predicted for bed-load component of

the total eediment yield between the various scenarios Total predicted bed load

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over the period 1963-1998 ranged from 116 million tonnes for the baseline scenario

to 119 million tonnes for the other scenario As such, the hed-load amaumted to 52%

%4 of those

of the total sediment yield predicted for the baseline seenario and +o 3t

predicted for the other scenarios

Predicted suspended sediment yield over the period 1999 until 2043

Annual mean, minimum and maximum valnes of predicted suspended sediment yield and bed load, the corresponding values of total predicted eediment yield

— 2043 for the three

(suspended sediment phis bed load) over the periad 19

relevant catclunent scenarios and baseline scenario are given in table 2.4

rom the modeled result, they point out that the patterns mdicate 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 - 20-13) was predicted to be 6.4 million tonnesyear or 4.32

tonnes/hatyear The preficted average suspended sediment, yield over two periods modeled (1983 — 1998 and 1999 - 2043) was 20.22, 16.85 and 12.75 tonnes/ha/year

for scenarivs 51, 83, Sẽ

‘The predicted average bed load over two periods modeled (1983 — 1998 and 1999 - 2043) was 7.5, 7.4, 7.1 and 7.2 million tones’year for scenarios $1, 93, 85, and the baseline scenaria, respectively Such proportions of bed Inad to total sediment, load are not uncommon and similar vatins have bean measured in Peninsular Malaysia

by Lai (1993, refer Section 2 1.2)

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TTaHe 34: Predicted and smnual suspended sediment yields, sediment yield and bed-luad from the Balui River catchment at the Bakun Dam site over period 1999-2043 for different calchment operutional svenarivs (ll velues in unillion tonne, viandard

deviatious in brackets)

ậ Mean annual = Min annual | Max annual | Mean | Mean annual | Min annual | Max annual

B sediment yield sediment yield | sediment yield | bed-load yield yield yield

2.6.3 Using GIS to Study Soil Hrosion 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

36

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

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

from 3-10 m for the gentler slopes (2-12) and 10 * 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

km? inundation extent of water and 36.93 km® volume storage of water’ (ii)

maximum operational flood level 228 m produced 593,96 km) inundation extent of

water and 33.84 km? volume storage of water; (iii) minimum operational flood level

195 m produced 388.68 km2 inundation extent of water and 18.42 km® 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

Chapter 3: Material and Methods

3.1 Description of Study Area

The following description of the study area hay been based primarily from the Detail Environmental Impact Asscssment of Bakun Hydroelectric Project, 1995

reservoir of the Bakun Dam encompasses a arca of about 69.640 ha with a total storage volume of 43.8x10%°m3

3.1.2 Topography

The topography of the Upper Rajang drainage basin is steeply dissected, with an average hull slope of 18.50 as determine from 185 slope measurements taken within the basin arca using 1:50,000 scale topographic maps ‘The clovations range from less than 40 m asl at the dam site up to more than 1500-m asl in the catchment | lowever,

the predominant elevations range from 100 m asi to 300m asl in the reservoir and up

to 1000 m asl in the catchment

40