Luận án Thạc sĩ Khoa học Môi trường: Estimation of soil loss from the Upper Rajang Sub-catchments in Sarawak, Malaysia during the development of the Bakun Hydroelectric Project

A field's sediment yield is thesum of soil losses from slope segments minus deposition, The deposition may occur in depressions, at the toes of slopes, along filed boundaries, and in ter

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

of soil erosion (Lopez and Albaladejo, 1990) A review of the impacts of soil degradation found that 1.2 billion ha (almost 11% of the vegetative area in the

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

last 45 years (World Bank, 1992).

From the engineering perspective, sil 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, rlled and washed down by the force

of gravity (Beasley, 1972) Rainfall isthe prime agent of sil erosion, whereby therain's runoff will scour away, loosen and break soil particles and then earry themaway, thus leaving behind an altered bare earth surface (Wishehmeier ot al, 1978)

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

disperse the aggrogate material Lightor aggregate materials such as very fine sand,

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

‘runoff water: greater raindrop energy or runoff amounts might be required to move

the largor sand and gravel particles Soil movement hy rainfall (raindrop splash) is

weually greatest and most noticeable during shortduration, high-intensity

thunderstorms Although the erosion caused by long-lasting and leseintense

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

the amount of sol 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 oF trapped on the surface, The amount of runoff will increase if

infiltration is reduced duc to soil compaction, crusting oF freezing Runolf from the

agricultural land may be greatest during spring months when the soils are usuallysaturated, snow is melting and vegetative cover is minimal

Tn Malaysia, there are many sol erosion prone zones especialy hilly areas at thenowly established ol palm plantation and along the riverbanks In the ease of slope,

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

‘cause instability of the slope Tis situation will gradually cause slope failure or

}andslide as commonly know The sil erosion phenomenon is basically the function

of the erosivity ofthe soil (Roslan, 1992).

watercourses would initiate unavoidable erosion and sedimentation in the reservoir

area, Removal of biomass in this environment would inerease the risk of acclorated

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

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

also unavoidably affect the terrestrial and aquatic resources within the reservoir

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

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

tunpolluted, contain suspended matter consisting of natural sll, sand, etc, derived

from the stream bed and banks, There ae several reasons why suspended solids areobjectionable in a stream, among which are

+ They interfere with self purification by diminishing photosynthesis and by

smothering benthie organisms,

+ Reduce reservoir storage capacity,

+ They can result in the reduction of fish and other aquatie 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 iver bed, ete

‘The soil erosion related problems should thus be identified to enhance

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

discharge in the watershed provides vital information on this issue

1.2, The Study Site

‘The proposed study area is located within the Balui sub-watershed of the upperRajang River Basin in the interior of Sarawak The Bakun catchment area is

located between latitudes 1.5°N and 8.0°N and longitudes 113.5°E and 116.9" 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

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

represents 34% of the entire Rajang watershed

1.8 Objectives of the Study

‘A sot of research projects ean be initiated in relation to the development of theBakun HEP dam with the aim of producing data and information useful for anintegrated approach to river basin and land use management, The present study

focuses on the following objectives

8) Estimation of soil loss from the Upper Rajang SubCatchments during the

development of the Bakun HEP

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

1.4 Significance of the Study

Sediment which reaches streams or watereourses can accelerate bank erosion,clogging of drainage ditches and stream channels, silting of reservoirs (reducereservoir storage capacity), damages to fish spawning grounds and depletion ofdownstream water quality Pesticides and fortilizers, frequently transported along

with the eroding soil can contaminate or pollute downstream water sourcos andrecreational areas, Because of the potential soriousness 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 loas 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’

+ Deseribe the total suspended solids (TSS) measurement methods, and to

Aevelop 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

catehment

+ Make recommendations on implementation of an integrated watershed

‘management approach with respect to management of soi 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 87 km

upstream of Belaga Town in the State of Sarawak, Malaysia,

‘The implementation of the hydro project was initially privatized to Bhran 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 he

shelved Later in 2000, the Government reinstated the project and vested all therights of Blran Berhad to Sarawak Hidro Sdn, Bhd (SHSB), In the meantime, the

river diversion works continued and were completed and handed over to SHSB atthe end of April 2001

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

awarded to Global Upline Sản Bhd and the work was completed in June 2002,Further construction of the dam and ancillary facilities (the main civil works) wasoffered to Malaysia-China Hydro Joint Venture on 8 October 2002, The main eivil

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

impoundment is planned to commence earlier 1ø 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, Muram,Sungai Bahau and Sungai Linau The reservoir will lie betwoen the base elevation

of 84 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 BIA report, only selective or partial biomass removal of the reservoir for all

trees down to Lem 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 improves and

+ to make sure that the future development and use of shoreline and reservoirmay 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 implios 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, includingsuch processes as gravitational ereep” (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 sessluble-soil particles, eg

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

surface or vertical within the soil profile through voids, eracks, and crevicesErosion 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 sol erosion on arable land in the tropics

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

principal processes involved Splash or interrill erosion is caused by raindrop

impact Sheet eroslon 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 bynormal tillage Stream channel erosion and coastal erosion are caused, respectively,

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

2.2.8, Sediment

‘The soil mass remaved 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 is orginal place by suspension

in water or air or by gravity

“The term soil erosion therefore is distinet 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, eo, or gravity Soil loss

is the net amount of soil moved off a particular field or area, the difference betweensoil dislodged and sedimentation, Sediment yield, in comparison, is soil lossdelivered to the specific point under consideration A field's sediment yield is thesum 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 impaet and runoff over the soil surface (ASCE, 195),

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

roach the ocean following repeated cycles of revdetachment and reentrainment 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 Sedimentationdecreases the capacity of reservoir, rivers, and chokes irrigation canals andtributaries, 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”

2.8, 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 (EISwaify e£ al, 1989), In

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

importance (EI-Swaify, 1999) Various authors, cited by El‘Swaify and Dangler

(1982) pointed out that available geologic data on erosion of different continentsindicate that Asia leads the way with 1.66 tonnes/hafyear, followed by South

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

0.47, 0.48, and 0.82 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 fuct 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), wore 550, 480, 480, 270, 217,

and 189 tonnes/halyear, respectively, from the Yellow River (China), Kosi River

lo

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

Red River (China, Vietnam), and Irrawady River (Burma) (EI-Swaify, 1991)

Soil erosion in China

According to Dazhong (1998), 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 cerious soil erosion The statistics from the early 1950s

‘quantified 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 totalcultivated land, are undergoing serious water and wind erosion (Fude, 1987)

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

is about 58 millon heetares with population of 70 millon located in middle reaches

of the Yellow River) is about 2200 million tonnes annually or 51 tonnesthalyear

‘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 byYang 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, 1884: Junfong, 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 tonesha/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 rateranges from 50-70 toneshectares/year (Dofu, 1985; Doxing, 1986) The total soil

loss in this region is about 150 million tonnes, 80% of whieh 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 ealeulated to be more than 5500

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

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

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

1500 million tonnes of sediments are deposited in lakes, rivers, and vatiows waterconservation facilities (MB, 1984: Zhengshan, 1987).

“The Yellow River is 5464 km long, watershed of 680,000 km? and carries 40 billioncubic meters of total annual runoff The highly concentrated sediments give theriver the highest silt content of any river in the world The average silt content inthe river water is 38 kg/m’, During periods of flooding, silt content in the Yellow

River can rise to more than 650 kg/m’ (Gueliang, 1987).

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

trilion cubie 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 inthe middle area of Yangtze River has an input of 190 million tonnes of silt About70% of this silt is deposited on the lakebed and raises it about 3.5 em annually

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

lake have beon reduced by 87%, 39%, and 816, 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

890 million eubie meters of waterstorage capacity are lost in the 20 largest

reservoirs in the upper area of Yangtze River annually hecause of sedimentdeposits, This reduces the total storage capacity about 1% per year (Youngeng andJinlin, 1986) ‘The waterway transportation distance of the Yangtze River systemhas been reduced about 40% because of sedimentation since the 19608 (Zhan andChuanguo, 1982)

‘Soil erosion in India

“The first gross national estimate made in 1950s reported that about 6000 milliontonnes of soll were eroded by water every year in India (Kanwar! vide Vohra, 198),

‘This was subsequently verified (Tojwani and Rambabu, 1981; Narayana and

‘Rambabu, 1983) by using the information on the land resources indifferent 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 (1988) concluded that, annually, 5834 million tonnes of soil was eroded.

‘The country’s rivers carry an approximate quantity of 2052 million tonnes of soil(6.26 tonneshhalyear) of this, nearly 480 million tonnes are deposited in variousreservoirs resulting in a lose 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 eatchment has beon40 to 2166% morethan was assumed at the time of reservoir project design (it has been lower in the

cease of only one reservoir) Using the average of 21 reservoirs, the actual sedimentinflow has boon about 200% more than the designed inflow Nizamsagar reservoir,

‘Which is the oldest in India (193D, had loss 52.1% capacity hy 1967 (CBIP, 1981),

‘Most of existing reservoirs wore planned with provision of dead storage designed to

store the incoming silt with a trap efficieney determined separately for cach

reservoir It was assumed that the entire sedimentation would take place below thedead 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 thedead storage (CBIP, 1981; Sinha, 1981)

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 countey’s total landarea of 30 million ha, The sloping topography and the high rainfall would subjectthe cultivated sloping lands to various degrees of erosion and other forms of landdegradation, Field experiments conducted in the TBSRAM ASTALANDManagement of Sloping Lands network sites in the Philippines showed that up-

and-down slope cultivation resulted in annual erosion rates averaging about 08.4

tonnesha, depending on the rainfall and type of soil Tt was estimated by theBureau of Soils and Water Management that about 623 million tonnes of soi is lostannually 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 soilcrosion, Predicted soil loss was estimated at 30-150 tonnesiha/yeaz, depending onparameters such as soil characteristies, land slope, land cover, and farming

systems,

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

‘on steeprland areas It causes severe on- and off-site environmental, economic, and

social impacts On site it reduces the chemical fertility of he soil by mutriont 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 yield, farmincomes, and household nutrition, The offsite effects of erosion on the quality andavailability of water can also be very serious, Major offsite effects includeinereased surface runof, often resulting in Mooding which displaces people in lo”

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 irigation eanals,

resulting in reduced water supply for iigated 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

Ấn a study about soil erosion and sediment transport in the Mekong Basin, Al'8oufi(2003) found that the erosion in the Mekong Basin is mainly rainfall based runofTerosion subject to the effects of land cover Soil erosion patterns in the basin are

| heterogencous The river basin lying across six countries has-enusedmade the

aystem analyses a significantly complex task, He used the Modified Universal SoilLoss Equation within the Soil and Water Assessment ‘Tool (SWAT) model to

| determine soil erosion and sediments transport loading patterns SWAT model isdeveloped 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 eatchment’s 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(1880 km), 21st with respect to eatchment’s area and sth with respect to average

‘annual runoff (475 x 10° m* per year or 15000 mi) 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 mnvyear (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 werewashed from the LancingJiang 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 65.98 million tonneslyear at Chiang Saen, 107.26 milliontonneslyear at Vientiane and 129,89 million tonnes iyear at Khone Falls Harden

| ‘and Sundborg (1992) conducted a study in Laos and North:ast 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 Pakso, the minimum value presented was 79.7 million tonnes in 1967 tothe maximum value of 224.72 million tonnes in 1978, The variation might be

attributed to the variation in river diseharge 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 ofMalaysia have predominantly been concentrated on the effect of land use changes

on hillslope plot (Morgan e¢ al, 1982: Hatch, 1988, Malmer, 1998: Brooks et al,

1998) or on relatively small catchments up to 140 kmẺ (Shallow, 1956: Douglas,

1967, 1968: Leigh and Low, 1978: Baharuddin, 1988: Greer et al., 1989 Malmer,

1980, Zulkifi ot al., 1991: Douglas et al., 1992: Lai, 1999) In Malaysia, measuredsediment yields from field plots or relatively small catchments covered byundisturbed rain forest range from less than 1 tonneshalyear (ef Douglas, 1968

Leigh and Low, 1973: Baharuddin, 1988: Malmer, 1993) to just over 3 tonneshha

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 tooccur (Bakun HEP EIA, 1995), For instance, in Peninsular Malaysia, Baharuddin

(1988) observed an inerease of 70% (from 0.07 to 0.12 tonnes/ha/year) in suspendedsediment yield after supervised logging of a small rain forest catchment on graniterock (area 0.3 km) and of 97% after unsupervised logging (irom 0.14 to 0.27

tonnesfbajyear) Shallow (1956) observed sediment yield of 0.56 tonnesfhalyoar and

1.08 tonneshalyear in the Cameron highland in Peninsular Malaysia with forestcovers of 94% and 64%, respectively Chong (1985) found 8:17 times inerease in thesediment load of peak flows shortly after clear felling In a study of five stoops

catchments on granitic rock along the Sungai Langat, Lai (1999) observedsediment yield of 0.54 and 0.90 tonnos/ha/year for undisturbed (Sg Lawing, 5 km?)

and partly logged (Sg Lui, 68 km’, 20% logged in 1978) eatchments, respectively

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

24.58 tonneshalyear observed in the first year after logging (mechanised) ofthe SeBatangsi @20 km2) and Sq Chongkak (13 kam!) catchments, respectively The

suspended sediment yield ofthe Sy Chongkak decreased to 13.35 tonnewha/year in

the second year after logging

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

yield from small catchments (0,08 ~ 0.18 km?) and unbounded runoff plots from

0.04 tonnesfhalyear for undisturbed forest to 0.7 tonnes/halyear after burning of

secondary forest, Lỗ tonnes/halyear after manual extraction and 2

tonnoshhalyear 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 underpadi 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/halyear obtained for the Jambatan SyedOmar (3830 km) and Jeniang (1770 km*) 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 tochanges in land uso in the area Wan Ruslan (1992)

Values presented in hydropower feasibility studies carried out in Sabah andSarawak (Syed Muhammad and Blectrowatt Engineering Serviees Ltd., 1994)

‘range from 2.05 tonnes/halyear for undisturbed Upper Padas catchment (1790 km’)

to 12.50 tomnes/haiyear for the Batang Ai catchment (1200 km), the latter wasaffected by logging,

24, Studies on Rates of Soil Erosion in Sarawak

Soil erosion in Sarawak has been the subject of many comments by observers, butfew 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 toextensive forest clearing, itis impossible to be sure of the exact role thatclearing has played However, in areas whet the bush fallow period is nottao short, shifting cultivation may not disrupt the hydrologic regime asmuch as recent arguments have suggested Ifa cleared area is left to be re

colonized by secondary vegetation, peak stream flows and sediment yields

sradually roturn to near natural levels The continuation of those effects inJogging areas is due to the road system which remains after timber

extraction has fished" (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 0.28 tonnes/halyear,while those for unterraced pepper cultivation are Sito 90 tonnewhalvear (Petch,

1988)

A study on Semonggok Series soils (Ng and Tek, 1992) noted that contrary to thegeneral 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 tonnestha wore lost in the first year after clearing, At Tebedu, Teck

(2992) recorded 0.46 tonnesfha soil loss in the first year after clearance, These field data from plot studies (Fable 2.1) clearly show that soil loss under shifting

cultivation is of the same magnitude as that under natural forest, whereas once acultivation system leaves bare ground between row crops, as in traditional pepper,

‘erosion rates rise lo 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 tonneshalyeat)

> - Slope] Period [ Soilloss [ SolllosTạng Use Lecation | (degrees) | (years) | mean | range:PrimayForst | NhhER | 3530 | 4 0.19 | 0053031

Semonggok | 2330 | —11 031 | 007041Seeondary Forest

') with hill padi | Semonsgok | 2590 | 11 | 040 | 0020.17

20

Jallang and sorub_|_ Ninh FR 88 3 ul 4518Hill Padi Shifting Cultivation

2) normal Rg, BemE | 5550 | 1 | 0

Breraced with Í semonggok | a0 | HO | 120 | 08346

© hush flow | Wmmommk| 1638 | 1 | 058.) 008945a) bash fallow | Thu | 25 | 3 | 034 | 032046

Traditional DU Pra Semongeok | 2540 | 11 | S944 | SIS)

2.5, Soil Loss Estimation Methodologies

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

of the canlier reported data are based on measurements initiated in the first andsecond decades of the twentioth century Consequently, most of the techniquesused still roquire 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 sil loss,

2.5.1, Universal Soil Loss Equation (USLE)

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

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

mid-1980 Although developed mainly as a forecasting cum planning tool foragricultural land, USLE has heen 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 prediets the amount of soil loss that

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

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

Five major factors are used to calculate the soil loss for a given sie, Each factor isthe numerical estimate of a specific condition that affects the severity of soicrosion at a particular location, The evosion values reflected by these factors eanvary considerably due to varying weather conditions, Therefore, the values

obtained from the USLE more accurately represent longterm averages The USLE

is given ag

ASRxKxLSxCxP

‘+ Arrepresents the potential long term average annual soil loss in tonnes per

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

Toss" Limits,

‘+ Rs 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 themaximum 30 minutes storm depth and summed forall 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,

+ Kis the soil erodibility factor It isthe average soil loss in tonneslacre per

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

arbitrarily selected slope longth of 72.6 f and slope steepness of 9% Kis @

measure of the susceptibility of soil particles to detachment and transport

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

structure, organie matter and permeability also contribute

2

‘+ LS is the slope length-gradient factor The LS factor represents a ratio ofsoil 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 isthe risk for erosion

‘+ Cis the cropivegetation and management factor It is used to determine therelative 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 ean be determined by.selecting the erop type and tillage method that corresponds to the field andthen 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-eropping

‘Table 2.2: Management strategies to reduce sol losses

Factor ‘Management Strategic Example

R | The R Factor for a field cannot be

altered

K _| The K Factor for a field cannot be

altered

Torracing requires

‘Terraces may be constructed to | additional investment and will

LS - | reduce the slope length resulting | cause some inconvenience in

in lower soil losses forming Investigate other soil

‘conservation practices firstC | The selection of erop types and | Consider cropping systems that

tillage methods that result in the [will provide maximum protectionlowest possible C factor will result | for the soil Use minimum tillage

in loss soil erosion, systems where possible

“The solection of a support practice | Use support practicos such a

that has the lowest possible factor | eross slope farming that will causeassociated with it will result in| deposition of sediment to occurlower soil losses close to the source,

P

2.5.2, Measuring Sediment Yield from River Basin

According to Walling (1994), information on the sediment yield at the outlet of abasin can provide a useful perspective on the rates of erosion and soil oss in the

watershed upstream He contends that in most sivers the suspended sediment

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

sail exosion investigations, since most of the bed load will be eroded from thechannel, 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 outletcan provide information on average rates of erosion within the basin, whereas alarge 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 infarmation about on-site rates of

‘erosion and soil loss within drainage basin,

Atypical example of sediment yield determination using basin and sub-basin outlet

‘method was reported by Murtedza o£ al (1987) for the 9180 km# Padas 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 solidsconcentration at various upstream locations, Murtedza e¢ af (1987) used daily flowrates and limited suspended solids concentrations at different low data collectedfrom the Drainage and Irrigation Department of Sabah, The Padas watershed was

divided into our smaller areas based on the location of gauging-station to identity

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 suspendedsolids concentration and flow!

se= allow?

where 8 is suspended solids concentration,

bare constant,

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

river in some time period, is

Suspended solids discharge = e.ssflow

where c is a conversion factor If the suspended solids are in mg/L and the flow incubic meters per second, the conversion factor to tonne per day is 0.0864

Combining the equation for suspended solids concentration and discharge gives!

Suspended solids discharge = a’ (flow)*!

Where ais a times e taking the log of both sides of the equation gives:

Log (ischarge) = (b + 1):Log (ow) + 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,Onve this equation is determined for a particular watershed and as long asconditions do not change, it can be used for ealeulating daily solids discharge from

daily flow data

Using flow data from the year 1969 — 1980 to caleulate, 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 tonnosfha/yoar in 1977

‘They also point out that implicit in the calculations is the assumption thatsuspended solids are a conservative parameter, Le hat no solids settle out of thewater between the upstream sites and outlet of catehment This assumption is ofcourse not aertratoi much of the suspended solids carried by the water under high

‘Now conditions will settle out if flow rates and turbulence in the river decreaseHowever, the above assumption did not affect the finding based on the calculations

First, solids that settle out under flow conditions will be resuspended when flow

increases again, sơ on an annual basis the assumption is more valid than it is ondaily basis: Anothor interesting finding is that a large fraction ofthe total annualsolids loading at outlet of the catchment came during a few high flow days It wasfound that the solids discharge on the top 12 lw days or 8% ofthe total year) was

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

solids discharge,

26

2.8.3 Measuring Sediment Yield by Using Tracers

In the second edition of the book “ il Erosion and Conservation’, Morgan (1995)

‘wrote that the most commonly used tracer in soil erosion measurement is theradioactive isotope, eaesium-187 Caesiunr137 was produced in the fallout of

atmospheric testing of muclear 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 vaties with the amount of rain but within a small

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

‘thus method can be used to estimate erosion rates, This can be done be takingsamples 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 modelwhich assumes that net sol loss is directly proportional to the percentage loss ofcaesium "137

2.6, Previous Estimations of Soil Loss in the Bakun Catehment

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 e lished by means of computerprogram XYFIT Their fitted sediment rating curve has fallowing equation:

00103 x (Q= 139)

where

‘+S: Suspended Sediment Transport (kg)

+ Q Water discharge (mis)

‘They used suspended sediment data measurement by Drainage and LivigationDepartment in 1982 and 24 data taken by them in the month of March, and the

rost 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 18 tonnesfha/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 TS Yield in Bakun HEP BIA report

[In 1995, as a component of the EIA for the proposed Bakun HEP project (Appendix4B, Bakun HEP EIA, 1995), The Center for Water Research (CWR) at theUniversity 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 assossment wasbased 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, ete) under a range of catchment and reservoir

‘operational conditions during both construction and operation of project

2.6.2.1, Brosion and Sediment Yield Modeling

28

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

loss for hillslopes under agriculture, for field sized areas oF far small eatchment

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

transport between grid cells, Such models sre impractical for use in largecatchment modeling studies due to large amounts of cells that would be necessary

to perform the caleulations, In addition, it may be dificult to collect the necessary

input data when dealing with such large catchments,

In general, two phases may be distinguished in the erosionrsediment deliveryprocess, 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, sil condition and soil moisture content, slope and

slope length, vegetation and litter cover govern the erosion from hill-slopes and itstransport to drainage network, The second phase is the in-channel phase, whichdetermines the transport of sediment over larger distances through the drainagenetwork 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

+ SS and BL: are inputs of suspended sediment and bed load into the

catchment from upstream areas,

‘+ §DC is the sediment delivery to the drainage network, and

+ DSc represents changes in the sediment storage within the drainagenetwork,

‘The sediment delivery ratio may be assumed to be close to unity for the smallcatchment to which the models quotes above apply because DSc may be considerednegligible The predicted soil loss from hillslopes is therefore similar to the

sediment yield at the outlet ofthe catchment arva

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

‘due to increased sediment deposition opportunities within the drainage network(Grune, 194% Wilson, 1973), Sediment dalivery is a runoff transport process andthis makes it highly correlated with the volume of runoff and peak runoff rate(Poster, 1988) Empirical models (e.g sediment rating curve) have therefore beencommonly used to predict sediment for larger catchments, The disadvantage of

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

(e.g land use) cannot easily be incorporated into the model and new coefficientsneed therefore be determined after each change

2.6.22, Reservoir Preparation and Operational Options

Five Possible catchment and reservoir operational conditions were modelled by

CWR These conditions encompassed:

‘+ Scenario Sl ~ ‘Worst ease/no build’ scenario: Selective timber harvestingcontinues in the catchment using the present (1995) mechanized timber

extraction methods (ie tractors, highlead yarding) No logging takes place

30

{in avea for which logging licenses have not yet been issued The remaining{forest in the impoundment area selectively logged and then submerged.

Scenario $2: Selective timber harvesting continues in the catchment usingthe prosent (1995) mechanized timber extraction methods until 1896, From

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

ie, Helicopter logging) No logging takes place in area for which logginglicenses have not yet been istted The remaining forest in the impoundment

‘area selectively logged and then submerged

Scenario S8 ~ ‘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 (e., Helicopter logging) No logging takes place

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

forest in the impoundment aroa is selectively logged A portion of residualbiomass in the impoundment area (ie between 10% and 40% of the totalresidual 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 usingthe present (1995) mechanized timber extraction methods until 1996 From

1996, timber extraction is carried out by least impact logging techniques(ie, Helicopter logging) No logging takes place in area for which logginglicenses 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 ease’ seenari Selective timber harvesting continues inthe catchment using the present (1995) mechanized timber extractionmethods 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 eleared and burned prior to inundation,

A baseline sconario (S6), representing pre-1983 conditions before logging of thecatchment 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 onwater quality

2.6.23, Sediment Yield Modeling Result

Predicted suspended sediment yield over the period 1988 until 1998

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

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 Số respectively, as compared to the

baseline seonario, The predicted annual maximum values of suspended sediment

ield for scenarios S1 to $5 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 bod-loads for the

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

3

predicted sediment yields (suspended sediment plus bod-load) for 5 scenarios andthe baseline scenario are given in table 2.8 below.

‘The different management options proposed for the impoundment area (2e, within

Scenarios $2, 88, and $4) had little effect on the suspended sediment yield as theperiod during which they were applied was relatively short and because the

‘impoundment area cover less than 5% of the total eatchment area

‘Table 2.8: Predicted and annual suspended sediment yields, sediment yield and

bed-load from the Balui River eatchment at the Bakun Dam site over period 1983+

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

standard deviations in brackets).

g | Mean annual Mean

Ệ Suspended | Am | ,mạụm, | Min, annual | Max, annual

E suspended | annual | anual | sediment | sediment

g NHAC | bedload | gine vield yield

the total sediment yield between the various scenarios, Total predicted bediload

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

4o 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 vield 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(suspendod sediment plus hed load) over the period 1999 ~ 2043 for the threerelevant catchment scenarios and baseline scenario are given in table 24

From the modeled result, they point out that the patterns indicate that thedifferent in annual sediment yield between the baseline seenario 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 thesecondary vegetation in the selectively logged areas,

‘Tho 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 4.92tonnesha/year The predicted average suspended sediment yield over two periods

‘modeled (1983 — 1998 and 1999 - 2043) was 20.22, 16.85 and 12.75 tannesfha/yenrfor scenarios $1, $3, $5

‘The predicted average bed load over two periods modeled (1933 ~ 1998 and 1999 +

2048) was 7.5, 7.4, 78 and 7.2 million tonesyear for scenarios S1, $8, S5, and thebaseline 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 (1998, refer Section 2.1.2)

”

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

Bakun Dam site over period 1999-2048 for different catchment operational scenarios (all values in million tonne, standard

deviations in brackets)

£ | Meansnmual | Minannual | Max annual | Mean | Meanannual| Min, annual | Mas, annual

2 | “sumended | ‘suspended | “ruspended | annual | sediment” | sediment | sediment

E | sediment yieta | sediment yield | sediment yield | ted load | "yield wield viel

Sỉ |waee aa 554 15604) 40484) — '381 ws

83 25.2 (6.9) 146 42.3 7.4 (0.4) | 32.5 (7.2) 23.1 50.3

$5_[isoG.) 129 287 780.4) 25.316.) 19.6 365

Baseline | 6.3 (0.5) 5.1 T4 7.2 (0.4) 13,5 (0.9) 114 15.3

2.6.8, Using GIS to Studv 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 ensing and geographic informationsystem (GIS), The landsat TM data (1988 and 1994) with false color compositesband 4, 5, 8 were used in their study Rainfall data, soils map and tophographie

‘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 studios was done in MICSIS (Micro-eomputer SpatialInformation Special system for soil erosion modeling based on the parameters ofthe 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 managementsystems, The USLE was adopted in this study with minor modifications inestimating the R and K parameters to suit the Malaysian conditions,

In the study, they found that rainfall orosivity 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 Podzolie Group, They are well

to excessively drained soils with shallow to moderate depth (25-50 em 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 C8) such as thepodzols, goly soils, skeletal & podzols, skeletal & gloy soils and podzols & gley soils

‘groups occur in very limited extent

36

akun has a rugged topography with sharp erest and stoop slopes Most of the area

ix above 500 m a.sl, with the highest elevation being 2040 m slope Length vasies

from 8110 m for the gentler slopes (212) and 10 * 20 m for the steepor slope (12)

Except for the logging and shifting cultivation activities in the immediatesurroundings of the proposed dam site and also along Balai River towards its

headwaters upstream, the eatchment 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

‘ood levels + 6) probable maximum operational flood level 233 m produced 632.44

km inundation extent of water and 36.93 km! volume storage of water’ Gi)

maximum operational flood level 228 m produced 584.96 kn!) inundation extent of

‘water and 83.84 km? volume storage of water! (i minimum operational flood level

195 m produced $88.68 km2 inundation extent of water and 18.42 kam Soil loss intonnestha/year was estimated based on 6 classes in table 25

Attention should be focused on the logged over forest Gncluding logging tracks) andshifting cultivation areas where no or minimal conservation practice has been

‘employed Soil loss here ranges from moderate to severe and is estimated to be 66

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 deploted of vogetation

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

No Class Em ‘Area,

lonnefha/yoan) had

3 |Slisht <200 1,312,000

2 [Acceptable — |z003500 pm

3 [Madmae 00:50.00 mm+ |Hh 5.00-80.00 mm

5 [Severe '50.00-150.00 2.700,

6 [Very Severe [>150.00 35,000

38