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 Chapter 1: Introduction 1.1, Brosion ‘The degradation of soils is a serious problem in developing countries, especially in highland, forest and river eatchment areas, Soil degradation is one of the greatest challenges facing mankind and its extent and impact on human welfare and the global environment are greater now than ever before (Lal and Stewart, 1990) Water erosion is the main degradation process, while human activities, the reduction ofplant 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. 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 the rain's runoff will scour away, loosen and break soil particles and then earry them away, 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 usually saturated, snow is melting and vegetative cover is minimal. Tn Malaysia, there are many sol erosion prone zones especialy hilly areas at the nowly established ol palm plantation and along the riverbanks.
In the ease ofslope, 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).2, Sediment Yield Several ofthe impacts stemming from the construction process and earthworks at work sites are predictable and mitigable to significant extent through careful site planning, supervision and application of best management practices. A number of other impacts are expocted to be residual.
Progressive construction and use of access roads and camps in rugged and steep topogeaphy intersected by many watercourses would initiate unavoidable erosion and sedimentation in the reservoir area, Removal of biomass in this environment would inerease the risk ofacclorated 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 are objectionable 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 offish 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 ofiver 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.2, The Study Site ‘The proposed study area is located within the Balui sub-watershed of the upper Rajang River Basin in the interior of Sarawak.
The Bakun catchment area is located between latitudes 1.9" The catchment upstream of the dam site covers an area of about 1. 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. Objectives of the Study ‘A sot of research projects ean be initiated in relation to the development of the Bakun HEP dam with the aim of producing data and information useful for an integrated approach to river basin and land use management, The present study focuses on the following objectives 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. Significance of the Study Sediment which reaches streams or watereourses can accelerate bank erosion, clogging of drainage ditches and stream channels, silting of reservoirs (reduce reservoir storage capacity), damages to fish spawning grounds and depletion of downstream water quality.
Pesticides and fortilizers, frequently transported along with the eroding soil can contaminate or pollute downstream water sourcos and recreational 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 tomeasure, 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. 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 the rights 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 at the end of April 2001. On 1% June 2001, the construction of the upstream auxiliary cofferdam was awarded to Global Upline Sản. and the work was completed in June 2002, Further construction of the dam and ancillary facilities (the main civil works) was offered to Malaysia-China Hydro Joint Venture on 8 October 2002, The main 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 ofelevation 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 reservoir may not be hindered.
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, including such 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 crevices Erosion by wind involves processes similar to those by water except that the ‘causative agent in sediment detachment and transport is the wind (Lal, 1990).2, 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. vss [ | covet cosy cust| set "- wesnseset Mễ asain [ |e shee gaye Co sin ouput On % | | se, xen _ xẻ. vợ i nh cat, eam te te ae Figure 2.1 Types of erosion (Souree! Lal, 1990) Different types of erosion on the basis of major agents involved are shown in figure 2.
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 by normal tillage.
Stream channel erosion and coastal erosion are caused, respectively, by stream flow and ocean waves: Soil movement en masse is eaused by gravity.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).