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The Investigation of Sediment Accumulation and

Its Distribution over Sentani Lake, Jayapura

Yusuf Bungkang1 and Soemarno

2

1Department of Physic, Faculty of Basic Sciences, University of Cenderawasih, Jayapura

(FPMIPA UNCEN JAYAPURA)

E-mail: yusufbungkang@yahoo.com 2 Dept. of Soil Science, Faculty of Agriculture, Univ. of Brawijaya

This paper is presenting the results of field measurement of sedimentation over Sentani Lake. Sentani

Lake systems are complex and characterized by being in a continuous state of evolution and

change. They can easily being disturbed, leading to major ecological or environmental

disasters. Sediment deposited in Sentani Lake has become increasingly accumulated over the past

2 to 3 decades. Some of them related to natural and to anthropogenic causes. In order to alter the

existing conditions, the mapping of sediment distribution in Sentani Lake is essentially required.

A lake survey together with suspended sediment profiling has provided stratigraphic information

as well as the spatial distribution of sediment in the lake. It shows an important influence on the

catchment degradation processes.

Key words: Sentani Lake, sedimentation, erosion, floods.

1. INTRODUCTION

Sentani Lake, as an irregularly shaped body with

approximate maximum dimensions of 28 km (E-W)

by 19 km (N-S) and a surface area of 10,400 ha is

one of the largest lakes in Papua. The Lake is

located on the western part of the island of New

Guinea, 15 km west of the city of Jayapura, Fig. 1.

It situated in a fault-controlled depression mainly in

Mesozoic mafic and ultramafic rocks of the Cyclops

Ophiolite Belt. The Cycloops Mountains Nature

Reserve has approximately 9,360 hectares of width

area. It is a volcanic lake and is lying 70-90 meters

above sea level.

Sentani Lake has the potential of a large scale of

freshwater. Source of freshwater comes from about

14 large and small rivers with a single river mouths.

Another function of the lake is as flood control for

Jayapura city. The flood control functions by

minimising the flood peak during the flood season.

The upstream catchment of Sentani Lake, northward

and eastward of the Lake is topographically steep

and primary forest. Part of this forest has been

experiencing from deforestation. Excessive forest

clearance in the reserve will increase erosion and

sedimentation. In such cases, deforestation leads to

landslides and soil erosion during the rainy season.

Erosion in upper catchment of Sentani Lake is

being considerable now. Meanwhile, the landuse of

the downstream cacthment area of Sentani Lake is

dominated by lowland and agriculture field, as

example in Tami valley.

The objective of this study is to asses the

potential erosion of the lake watershed. In order to

determine the silting up processes of Sentani Lake.

Here the investigation of sedimentation and its

distribution over the Lake being measured and

mapped.

Figure 1. New Guinea Island and the location of

Sentani Lake.

(Source:http://www.oysteinlundandersen.com

/West-Papua/Lake-Sentani/Lake_Sentani.html)

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Figure 2. The condition of surface water coming

from one of river mouth entering to the

lake. (Source: survey 2011)

The samples of suspended and dissolved solid as

well as bed material have been mapped for further

analysis. The output of this research is supposed to

be useful consideration for future watershed

management.

2. METHODOLOGY

The research was carried out in the eastern part of

the lake body and at the upper catchment area. The

data used in this study consisted of survey data and

secondary data. Survey data was including Total

Suspended Sediment, Total Dissolved Sediment,

and flow velocity at bottom and at lake surface. The

bottom material was obtained by grabbing the bed

floor of the lake. Secondary data used to estimate

the potential catchment erosion were rainfall, land

cover, soil type, slope and slope length.

2.1 Data Collecting

Primary data collections were carried out directly in

the body of the lake. Some point’s measurement

over the lake area has been determined according to

the purpose of this study. The river mouth entering

into the lake was considered included. The primary

collected data were as follows:

a) Water sampling for measuring Total Suspended

Solid (TSS) and Total Dissolved Solid (TDS)

was performed at three depth levels according to

the point water depth, h; 0.2h, 0.5h, and 0.8h. For

bed load samples (typically were mud, sand and

gravel) was grabbed from the bottom of the lake

at the same point TSS and TDS.

b) Flow velocity was measured at water surface

(0.2h) and bottom of the lake (0.8h). The

situation of collecting data was shown in Fig. 3.

2.2 Secondary Data Secondary data such as rainfall, land cover, soil type,

slope and slope length was obtained from the

relevant authorities and project reports.

2.3 Processing of water samples, TSS and TDS Water samples were evaporated using Oven Dryer

(GALENKAMP), so all that remains was TSS and

TDS. Furthermore, TSS and TDS were measured

using the Analytical Balance (OHAUSS) at the

Chemistry Laboratory, Department of Chemistry,

Faculty of Mathematics and Science, Cenderawasih

University, Jayapura.

Figure 3. The lake survey and bottom sediment

sampling. (Source: survey 2011)

Figure 4. Typical bed material of Sentani Lake .

(Source: survey 2011)

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Figure 5. The flowchart of this study

2.4 The Analysis of Bed Load Bed material taken from the bottom of the lake was

analyzed at the Faculty of Marine and Fisheries

Univ. of Brawijaya using the electron microscope to

determine the Shape Factor (SF) of grain material.

The typical bed material was shown in Fig. 4.

2.5 Calculation of Potential Erosion The potential erosion was analyzed using USLE

equation utilizing the secondary data including

topography, rainfall, land use, land cover, and soil

type. The results were the maps of potential erosion

might enter to the lake and hazard erosion map.

2.6 Distribution of TSS and TDS Spatial data modeling for mapping distribution

patterns of TSS and TDS was carried out by

interpolation method of observed point data using

Geographical Information System (GIS) tools. The

brief summary of data processing was figured out

by a flowchart shown in Fig. 5.

3. RESULT AND DISCUSSION

According to the rainfall data, Fig. 6, the Sentani

Lake is experiencing with rain along the year. The

maximum monthly rainfall occurs on December and

minimum is July – September.

Figure 6. The pattern of monthly rainfall over the

catchment of Sentani Lake. (Source:

survey 2011)

The bed load material deposited on the bottom of

the lake was mainly by mud as product of the past

surface erosion and decomposed organic material

flowing into the stream. The form of the finer

material was sharp and mostly dominated by

breccias released from geological processes at the

lake catchment. The typical forms of finer material

were shown in Fig.7.

Figure 7. The typical size of finer bed material

under the magnificence of the electron

microscope (Source: the laboratory

analysis).

3.1 The potential surface erosion The actual land use classification derived from the

recent satellite image, Fig. 8. was used to calculate

the potential erosion. The map of potential erosion,

Fig. 9 indicated that the locations near the lake was

very high, ranging from 60 to 180 ton per hectare

per year. This suggests that the sediments deposited

in the lake were coming from these areas. The

erosion was highly occurred at the northern side of

the lake where the topography was steep and it was

opened area.

Conversion tools Polygon to Raster

Map of R, K, CP in Raster format

Raster Calculator

Reclassify

Erosion (USLE) and

hazard classification

1 Erosion map

2 Map of surface velocity profile

3 Map of bottom velocity profile

4 Shape factor (SF) of Bed material

5 Map of Total Dissolved Solid (TDS) and

6 Map of Total Suspended Solid (TSS)

R factor K factor CP factor

Rainfall data Soil Map Landuse

LS factor

Topography Map Field survey data of

- TSS, TDS

- bed material

- flow and current

Reclassify

Spatial analyst

Interpolasi

Distribution of TSS, TDS,

SF, surface and bottom

velocity profiles.

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Figure 8. The land use classification for the

catchment area of Sentani Lake.

(Source: the analysis)

Figure 9. The potential erosion of the catchment

area of Sentani Lake and Shape Factor

(SF) of bed material. (Source: the

analysis)

The shape factor of the bed material was well

correlating with rate of erosion. At the north side of

lake, the material dominated by rounded particles

(SF ~ 1). It seemed that the deposited material come

from the particles from upland erosion. Similarly,

when the surrounding area had low rate of erosion,

the bottom material was irregular in shape as

indicating the finer material of ancient erosion

processes or organic materials.

3.2 Distribution of TSS and TDS It was obtained that the concentrations of TDS

among the near-surface layer (upper layer), the

middle layer and the layer near the base (bottom

layer) had no significant difference. However, for

TSS there was a significant difference between TSS

concentrations in the top layer and in the bottom

layer (Fig. 10 and Fig. 11), whereas the top layer

was about 19 milligrams/liter to 373 milligrams/liter

and the bottom layer of 32 milligrams/liter to 407

milligrams/liter. Similarly, at the area of the lake

inlet and outlet, the differences were highly

significant. Namely in the area of the inlet for the

average was ranging from 80 milligrams/liter to 100

milligrams/liter, while at the outlet varied from 20

mg/liter to 30 milligrams/liter. This evidence had a

good agreement with the condition of stream water

entering to the lake. Considering the distribution of

flow velocity at the surface water was higher than

velocity at the bottom layer. The TSS circulation

was driven by current moving vertically as well as

in horizontal direction. When the current was weak

then material tend to move to deeper layer as well as

indicated by the measured data. The distribution of

current was showed by Fig. 12 and Fig. 13 for the

surface and bottom layers respectively.

Figure 10. The concentration of TSS at the surface

layer (source: survey 2011)

Figure 11. The concentration of TSS at the bottom

lake (source: survey 2011)

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Figure 12. The flow velocity at the surface.

(Source: survey 2011)

Figure 13. The flow velocity at the bottom.

(Source: survey 2011)

Figure 14. The sand/gravel mining activity on one

of stream as sediment source entering

into the lake. (Source: Survey 2011)

Figure 15. The condition of run-off from one of

stream entering to the lake. (Source:

survey 2011)

This field investigation summarized that the

dominant material deposited in the Sentani Lake

was muddy, coming from the ancient erosion

processes as well as decomposition of organic

materials (leaf, logs etc). However, the lake was

threatened by material of surface erosion and sand

mining activities at the upland, Fig. 14 and Fig. 15.

It is necessary for the authority to monitor the land

use changing and acting the forest conservation

otherwise the lake silting up processes will be

increase.

4. CONCLUSION

The pilot research of sedimentation and erosion on

Sentani Lake was carried out as part of the Study on

Modeling and Simulation of Sediment Distribution

over Sentani Lake. The results were useful to set up

the baseline condition of the sedimentation and

erosion in the lake catchment. Further, this initial

finding shows an important influence of the

catchment degradation processes.

ACKNOWLEDGMENT: The author would like to

acknowledge to everyone who engaged in this

research, especially to the Doctoral Program of

Water Resources and Marine Environment, Faculty

of Agriculture -UB for supporting this research.

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Budiyanto, Eko.(2005). Sistem Informasi Geografis dengan

Menggunakan ARC-VIEW GIS. ANDI. Yogyakarta.

John Bridge and Robert Demicco. (2008) Earth Surface

Processes, Landforms, and Sediment Deposits, Cambridge

University Press, 2008

S.E. Jorgensen et al. (2005). Lake And Reservoir Management,

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