Pertanika 8(1), 115-122 (1985)

Suspended and Dissolved Sediment Concentrations of TwoDisturbed Lowland Forested Watersheds in

Air Hitam Forest Reserve, Selangor

F.S. LAI andM.SAMSUDDINFaculty of Forestry,

Universiti Pertanian Malaysia,Serdang, Selangor, Malaysia.

Key words: Disturbed forested watersheds; suspended sediment; dissolved sediment.

ABSTRAK

Kepekatan endapan ampaian dan larutan dari dua kawasan tadahan hutan sudahdibalak yang terletak di Hutan Simpan Air Hitam, Selangor dikaji selama enam bulan.Sampel-sampel air diambil semasa paras air sungai itu rendah dan tinggi. Keputusan-keputusan yang diperolehi menunjukkan bahawa perbezaan kepekatan endapan ampaian diantara kedua-dua tadahan adalah bererti. Kepekatan endapan ampaian berkisar di antara2-1305 mg/l bagi tadahan A, kawasan yang lebih dibalak manakala bagi tadahan B, yangkurang dibalak kepekatan adalah di antara 1 - 292 mg/l. Kepekatan endapan larutan pulaberkisar di antara 4 -136 mg/l untuk tadahan A dan 3-90 mg/l bagi tadahan B, iaitu satuperbezaan yang tidak bererti secara statistik. Keputusan-keputusan yang diperolehi dalamkajian ini selanjutnya menunjukkan kesan pembalakan ke atas kepekatan endapan yang ber-variasi dengan masa walaupun operasi pembalakan di sesebuah tadahan telah tamat satutahun lebih awal dari tadahan yang lain.

ABSTRACT

Observations on suspended and dissolved sediment concentrations were made from'two disturbed watersheds situated in Air Hitam Forest Reserve, Selangor over a period of sixmonths. Samples were taken during low flows and high flows. Results obtained showed thatthe difference in suspended sediment concentrations from the watersheds is significant.Suspended sediment concentrations ranged from 2-1305 mg/l for watershed A, a relativelymore disturbed watershed and 1-292 mg/l for watershed B, a less disturbed catchment. Fordissolved sediment concentration, the range was 4-136 mg/l for watershed A and 3-90 mg/lfor watershed B, of which the difference is not statistically significant. The findings in thisstudy demonstrate rather clearly the effect of logging on sediment concentrations andvariance over time in the two watersheds with logging operations ceasing about a year apart.

INTRODUCTION

The vital role of forest cover in maintaininggood water quality has long been recognised.The usefulness of water supply for instance,depends in part on the quality of water, which isoften dependent on the sediment concentration(Kunkle, 1974; Anderson, 1981). Man's use of

forest land, for timber in particular, has oftenbeen the cause for concern not only over thequality of water but also on stream behaviour,water losses from catchment, floods anddroughts. Logging operations have always beendescribed as one of the major causes for alteringthe often near-complicated hydrological pro-cesses.

F.S. LAI ANDM. SAMSUDDIN

Although there is a growing uncomfortableawareness on the adverse hydrological impactresulting from increased exploitation of hillforests on the principal sources of water supply,extraction of logs in logged-over forests largely inlowland areas is still being carried out. Theintensity is however relatively lower. Hydrolo-gical information resulting from the latter willremain useful especially so since these forests arestill equally important source areas of watersupply for domestic, agricultural and industrialuse.

This paper reports a study on suspendedand dissolved sediment concentrations carriedout in two recently logged disturbed lowlandforested watersheds. The main aim is to assessthe impact of logging on sediment concentra-tions over time.

MATERIALS AND METHODS

Study Area

Two watersheds demarcated, WA and WB,are tributaries of Sg. Rasau, the main riverdraining Air Hitam Forest Reserve (Fig. 1). Theforest reserve comprises 22 compartments ofwhich WA drains compartments 6, 7, part of 9and a small portion of a mature oil palm planta-tion. Compartments 10, 11 and parts of 8, 9 and16 are located in WB. WA is larger in areacovering 7.3 km2 compared to 4.7 km2 of WB.Some physiographic characteristics of bothwatersheds are presented in Table 1.

BSSSS

I KCOTLT I

MMIMK mil

Fig. 1. Location of study area.

Soil and Vegetation

Soil type for both watersheds are similar,comprising the soil series of Serdang-KedahDurian association (Zainuddin, 1976). Serdangseries occur along places downslope while that ofKedah and Durian are found on the ridges andupper slope region. The local Alluvium Colluvium association occurs in the valley and foothillregion.

TABLE 1Some physiographic variables of the watersheds

Physiographic properties WA WB

Area (km2)

Length of main channel (km)

Drainage density (km/km2)

Maximum height difference (m)

Mean slope of main channel

Form factor

Circulatory ratio

7.3

5.2

4.11

106

0 004

0.87

0.52

4.7

4.0

3.51

106

0.009

0.73

0.58

116 PERTANIKA VOL. 8 NO. 1, 1985

SUSPENDED AND DISSOLVED SEDIMENT CONCENTRATIONS OF TWO FORESTED WATERSHEDS

According to AH Riza (1977), the Air HitamForest Reserve, previous to logging, was a low-land dipterocarp forest after which it was gra-dually replaced by secondary disturbed forestspecies. Three canopy levels could be identified;the emergent canopy level which is dominated bydipterocarpaceae family (Anisoptera, Diptero-carpus, Dryobalanops, Hopea, Shorea, Para-shorea); the middle canopy which represents themain canopy level is dominated by the formercanopy species together with families of Burser-aceae, Gutiferae, Myristicaceae, Myrtaceae andSaportaceae; and the lower-most understorey,which is dominated by species of the two upperstoreys.

Forestry Activities

From past records, logging has been themain activity in WA and WB. Log extractionbegan as early as 1930 and thereafter continuousextraction has been undertaken on a commercialand subsistence basis up to 1983. On record, WAis relatively more disturbed compared to WB;logging operations (including salvage logging)only ceased in July 1983 in WA, a year afteroperations stopped in WB. About 35.3% and13.5% of basin area of WA and WB were affect-ed respectively.

Log extraction was carried out by a crawlertractor-Santai Wong system in which a A-16Komatsu was used. Since the area had been pre-viously logged, the construction of logging roadswere minimal and previous logging roads wereused in most instances.

Collection od Data

The sampling of stream water for suspend-ed and dissolved sediment concentrations in thestudy consisted of the use of a USDH-48 depthintegrating sampler and a multi-stage samplercomprising 4 one-litre plastic bottles fixed firmlyon to a vertical mount at 0.1 m intervals. Eachsample container was fixed with two coppertubes, one as the intake and the other as an airexhaust, inserted through a rubber stopperfollowing the specifications of the single-stagesampler U.S. U-59C (U.S. Inter Agency Com-

mittee on Water Resources Report No. 13 —cited in Ullah, 1972). The USDH-48 sampler wasused to obtain samples during low flow condi-tions while the latter was installed for samplingstorm and high flows in order to improve sedi-ment records.

Steamflow was periodically determinedwhich involved the basic procedures of currentmetering and measuring known cross-sectionalareas of the channel. Each measurement wasthen made reference to a stage gauge reading inorder to facilitate the computation of ratingcurves for the respective rivers.

Steam water samples were analysed in thelaboratory for suspended and dissolved solid con-centrations following the procedure outlined inWang and Ong (1978). However, in order toobtain mineral sediment concentrations, of sus-pended solids and dissolved solids, the sampleswere ashed at 600 °C for 2 hours to removeorganic materials which are non-denudationalcomponents (Peh, 1981).

RESULTS AND DISCUSSION

Suspended and Dissolved Sediment

Tables 2a and 2b summarise some of themeasures of dispersion of suspended and dissolv-ed sediment data obtained during baseflows andstormflows. From the tables, it is obvious thatthe suspended sediment concentration producedin WA during stormflows is more than 4 timesgreater than that of WB with respect to meanvalues. However, the maximum concentration ofsuspended sediment produced during stormflowsis about 150 times greater than the mean base-flow concentration for WA and 52 times for WBrespectively. The importance of storms withrespect to sediment discharge is clearly shown.

For statistical tests, the difference in sus-pended sediment concentration' between water-sheds is significant (P < 0.05). This is expectedsince WA is more recently logged-over comparedto WB. The effect of logging on erosion andsubsequent sedimentation in streams is wellknown (Ow Yang, 1965; Douglas, 1970; Daniel

PERTANIKA VOL. 8 NO. 1, 1985 117

F.S. LAI ANDM. SAMSUDDIN

TABLE 2aSuspended and dissolved sediment concentrations in mg/1 — WA

Minimum Maximum Mean No. of samples

Base flow

Stormflow

Baseflow

Stormflow

2

5

4

12

Suspended

20

1305

Dissolved

136

70

Sediment

8.5

187.3

Sediment

36.7

29.8

32

32

32

26

TABLE 2bSuspended and dissolved sediment concentrations in mg/1 — WB

Baseflow

Stormflow

Baseflow

Stormflow

Minimum

2

1

3

6

Maximum

Suspended

16

292

Dissolved

90

58

Mean

Sediment

5.6

40.7

Sediment

28.4

22.9

No. of samples

32

26

32

26

and Kulasingam 1974; Kunkle, 1974). Therelationship between suspended sediment anddischarge is not firm for both watersheds (Figs.2a and 2b). An examination on the scattergramsindicates considerable scatter about the line ofbest fit as explained by the low r2 values obtainedespecially for WB. In this regard, inherentpotential errors during sampling such as hyste-retic effects related to rising and falling stage,exhaustion effects and varying patterns of tri-butary inflow have some effect on the results(Heidel, 1956; Walling, 1977). For example,sampling done after storms on a few occasionsrecorded low concentrations although at highdischarge levels. The main reason for this occur-rence is largely because of 'flushing' of sedimentsduring early parts of stormflows.

However, the sensitivity of WA (Fig. 2) tosuspended sediment disharge is immediatelyclear by comparison; due mainly to greater

erosion within degraded areas of the watershed.Exposure of forest soils, particularly from logg-ing roads and skid trails, are the main sources ofsediment supply. High intensity storms easily dis-lodge recently exposed soil which are subse-quently washed into streams by overland flow.

This action increases the concentration of sus-pended matter and in its form, suspendend sedi-ment. The role of overland flow in this respecthas been noted to be of significance even underforested conditions (Peh, 1980). The area WB,where logging operations ceased a year earlier,records significantly lower suspended sedimentconcentrations primarily because of recoverywith time. Regeneration, especially rapid withthe lower plants has been effective in coveringpreviously exposed tracks. Consequent to which,the vegetation serve as energy dissipators to raindrops, improve infiltration as well as hold thesoil together.

118 PERTANIKA VOL. 8 NO. 1, 1985

SUSPENDED AND DISSOLVED SEDIMENT CONCENTRATIONS OF TWO FORESTED WATERSHEDS

logy = -2.5515 + 1.1294 log*

r* = 0.6188

Fig. 2a. Relationship between suspended sedimentconcentration and discharge for WA,

logy = -0.5636 + 0.4434 logx

r 2 = 0.1246

Dttchoro* ; • V ;

Fig. 2b. Relationship between suspended sedimentconcentration and discharge for WB.

It will also be important to note that someform of watershed protection was carried out inWA and WB to reduce damage from loggingoperations. They include avoiding the use ofroads and logging near major streams, avoidingmajor stream crossings and the maximum use ofprevious logged roads. Additionally, loggingintensity was relatively low, averaging 4 to 5tons/acre. It is certain that these considerationshave resulted in lower than expected sedimentdischarge from the watersheds.

Results from studies conducted elsewhere inPeninsular Malaysia are compared (Table 3).From the table, the range of suspended sedimentconcentrations seem more varied than dissolvedsediment despite different watershed conditions.Compare, for example, results obtained fromDouglas (1968). The behaviour stresses theimportant suspended sediment component intotal sediment load during stormflows. The com-parison between concentration range is however,limited in view of inherent potential errorsinvolved in sampling mentioned earlier, timeperiod of study and watershed conditions.Taking these factors into consideration, sus-pended sediment concentrations from water-sheds in this study are nevertheless relatively lowdue mainly to watershed protection measures,comparatively low timber production and timebetween sampling and logging operations.

Dissolved sediment concentrations however,do not differ significantly between watersheds.The range was 4 -136 mg/1 for WA and 3 - 9 0mg/1 for WB. The relationship between dis-solved sediment and discharge is not significantfor both watersheds (Figs. 3a and 3b). 'indi-cations that dissolved sediment concentrationscan be relatively higher during baseflows thanstormflows are, however, largely attributed todilution effect. Essentially, dissolved sedimentsare derived from chemical action of water incontact with rocks and soils. Water that isdischarged into streams from surface runoffgenerally contains lower concentrations ofdissolved sediments because water that flows overthe surface (during storms) reaches the streamquickly and in relatively large volumes. Thisprocess can result in lower concentrations ofdissolved sediments although the catchmentdischarge is high (Hembree et al., 1964). It willbe important to note that dissolved sedimentloads are also dependent on the geology, soil andgeomorphology of the basin area. Douglas(1968) for example, in a study in Gombak foundthat areas with limestone outcrops yield higherdissolved sediment concentrations with dischargethan granitic areas because of the solubility oflimestone.

PERTANIKA VOL. 8 NO. 1, 1985 119

TABLE 3Summary of suspended and dissolved sediment concentrations of selected watersheds in Peninsular Malaysia

o

Watershed

Upper Sg. Gombak1) Field Centre2) 12V4 Milestone3) Jalan Pekeliling4) Sg. Pasir

Sg. Manan Kanan

Sg. Tekam1) Basin A2) Basin B3) Basin C

Sg. Tok Pawang

Sg. Bujang

Sg. Rasau

1) WA2) WB

Location

SelangorSelangorSelangorSelangorSelangor

Negeri Sembilan

PahangPahangPahang

Kedah

Kedah

SelangorSelangor

SuspendedSediment

(mg/1)

8 (mean)2.1-1609.24.2-1080.9

21.1-1071.12.1-5788.9

2-76

21-11231-11028-90

17.5 (mean)

3.1-25.4

2-13051-292

DissolvedSediment

(mg/l)

7.5-55.010.0-82.517.5-95.020.0-110.0

80 (mean)

19-6641-7225-65

-

4-1363-90

Remarks

Forested conditionForested conditionForested conditionSome mining activitiesEffects of acceleratedrunoff from road surfacesignificant

Forested condition

Forested conditionForested conditionForested condition

Forested (controlcatchment)

Disturbed Forest

Disturbed ForestDisturbed Forest

Source

Norris and Chartson (1962)- cited in Peh (1981)Douglas (1968)

Leigh et al. (in press) —cited in Peh (1981)

Peh (1981)

Salleh et al. (1983)

Salleh et al. (1983)

Present studyPresent study

>

2

3

SUSPENDED AND DISSOLVED SEDIMENT CONCENTRATIONS OF TWO FORESTED WATERSHEDS

logy = - 0.0005 log* + 36.1054

r*= 0.0062

l 04

Dwchorgt h / « )

Fig. 3a. Relationship between dissolved sedimentconcentration and discharge for WA.

.oV

f i o a

logy = -0.0011 log* + 29.6003

r*= 0.0206

Fig. 3b. Relationship between dissolved sedimentconcentration and discharge for WB.

Suspended and Dissolved Sediment Loads

The volume of suspended and dissolvedsediment discharge can be determined sepa-rately by the equation:

Q, = 0.3448 CQw

where, = sediment load in m Vkm Vyr= suspended or dissolved matter in

mg/1- stream discharge in ft Vs

A d = drainage area in mJ

G = specific gravity of sediment/rocksin basin area.

Although the rate of denudation, expressedin volume per unit area per year, would providean importang indication on soil loss from thewatersheds, it is not permissible to estimate theannual load in the absence of continuous stream-flow records. However, estimations on the maxi-mum and minimum sediment load dischargewere computed to demonstrate the differences inthe instantaneous denudation rate at the time ofsampling; suspended sediment load varies from alow 2.7 mVkmVyr to a high 18375.6 mVkmVVT for WA and 4.7 mVkmVyr to 1828.6 mVkmVy* for WB respectively; dissolved sedimentload varies from 5.8 mVkmVyr to 397.4 mVkm2/ yr to 314.3 mVkmVyr for WA and WBrespectively.

The results indicate rather high rates ofdenudation from the instantaneous suspendedsediment samples, especially for WA. By com-parison, the maximum limit of sediment produc-tion from streams under natural or undisturbedconditions in the humid tropics is in the region ofabout 100 mVkmVyr (Douglas, 1972 - citedin Peh, 1981) which is clearly below the estimat-ed maximum instantaneous sediment dischargeload in the study. It is unfortunate however thatlong term steamflow records were not availableto enable a meaningful estimation and com-parison on the total annual sediment loads.Nevertheless, the results obtained will be usefulindicators with respect to future changes in thewatersheds.

CONCLUSION

The study suggests time difference with res-pect to logging operations to be of great impor-tance with respect to comparative suspendedsediment concentrations and loads after loggingoperations. The impact on the dissolved sedi-ment component is somewhat lesser in extent.The study also indicates the importance ofstream recovery during post-logging periods in-sofar as suspended and dissolved sediments areconcerned. It is within this scope that this study

PERTANIKA VOL- 8 NO. 1, 1985 121

F.S. LAI AND M. SAMSUDDIN

will be continued after a one-year lapse with theprimary aim of assessing the extent of furtherchange with time.

ACKNOWLEDGEMENTS

The authors wish to thank the Dean of theFaculty of Forestry, Universiti Pertanian Malay-sia for the facilities, Saad Nyan for technicalassistance, Encik Wan Saufi Wan Mustapha,Encik Abdul Kadir bin Jaafar and Encik Bashribin Razali for their kind cooperation andRamlah Md. Zin for typing the manuscript. Theauthors are also grateful for the financial assis-tance granted by Universiti Pertanian Malaysia.

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122 PERTANIKA VOL. 8 NO. 1, 1985