Salinity Intrusion Modeling for Sungai Selangor

Norbaya HASHIM, Research Officer, Hydraulic Engineering Division, National Hydraulic Research Institute of Malaysia (NAHRIM)
ABD. Jalil HASSAN, Senior Research Officer, Hydraulic Engineering Division, National Hydraulic Research Institute of Malaysia (NAHRIM)

ABSTRACT

Salinity intrusion into estuary of the Sungai Selangor has been carried out on a hydrodynamic numerical modeling to access the parameter that governed the amount of salt in the river. Issues such as water pollution and extraction of water from Sungai Selangor system has been said to be the cause of ‘fading fireflies’. The berembang trees on the river bank that become the fireflies’ habitat need some amount of salt for proper growth. Living at the lower reaches of Sungai Selangor, the fireflies are affected not only by the activities in their vicinity, but by activities in the entire river basin. Rapid economic development in the basin and the strong demand for the water resources puts pressure on the ecosystem. This research has been carried out to investigate the effect of water extraction along Sungai Selangor towards altering the amount of salt content in the river. The hydrodynamic modeling with regards to the salt content is expected to support long term assessment that may affect the berembang trees as a result of changes in the flow from upstream because of the water abstraction activity for domestic water supply.
Keywords: Salinity intrusion; berembang trees; fireflies; hydrodynamic modeling; water extraction; estuary.

1 Introduction

Kuala Selangor has been synonymous with firefly (Pteroptyx tener) watching (see Figure 1). Just outside Kuala Selangor town is the quiet hamlet of Kampung Kuantan, site of one of the largest firefly colonies in the world. Kampung Kuantan is located 25 km from the river mouth. Tourists from near and far flock to Kampung Kuantan to take a boat trip along the Selangor River to have a closer look at what is considered to be the 'eighth' natural wonder of the world. What is special about these small insects is that they display their flashes of light synchronously while congregating in large numbers on certain trees. They particularly favour 'berembang' trees (Sonneratia caseolaris), the branches of which overhang the riverbank (Figure 2). At a glance, it would seem as if we are looking at a row of Christmas trees lighting up the night, and we cannot help but wonder how on earth such a small creature can produce such amazing light. The firefly has a close relationship with the river ecosystem at Kampung Kuantan. The ecosystem comprises the Selangor River and the different types of vegetation especially berembang trees that glow on its bank. Living at the lower reaches of Sungai Selangor, the fireflies are affected not only by activities in their vicinity, but by activities in the entire river basin. The berembang trees only thrive in weakly saline water and a continuous freshwater outflow is necessary to prevent the water at the firefly habitat from becoming too saline. However rapid economic development in the basin and the strong demand for the water resources puts pressure on the ecosystem. Changes in the river water quality as a result of pollution or the building of the dam and barrage further upstream may eventually have an impact on the survival of the snail and the riverside vegetation on which the fireflies depend. To ensure future sustainability of Sungai Selangor as a reliable source of water, protection of water source alone is not sufficient. An effective planning and control of the whole river basin is essential.In the long term, the survival of this 'eight' natural wonder of the world will be dependent on our ability to understand the ecology and habitat requirements of the firefly, and our determination to manage the river system. Integrated river basin management is an important new strategy and vital ingredient in achieving successful water resource management planning in the country.

  
Figure 2: Berembang Tree and its fruit

The objectives of this study are:

a. To develop a 1-Dimensional flow and salinity model for Sungai Selangor basin using unsteady flow.
b. To carry out hydrodynamic numerical modeling to access the parameter that governed the amount of salt in the river.
c. To investigate the effect of water extraction along Sungai Selangor towards altering the amount of salt content in the river.

2 Study Area

Selangor River Basin commands a catchment area of about 1960 sq. km, nearly a quarter of the total area of the State of Selangor. The Basin is located to the north of Kuala Lumpur City, bounded by Klang river basin in the south and Bernam River Basin in the north. The Selangor river rises in Titiwangsa range bordering the State of Pahang and flows in an approximately southwest direction, before discharging into the Straits of Malacca. The mainstream length is about 110 km. The Basin is reach with natural and ecological systems. The upper Basin provides a green and pristine upland with unique flora and fauna, while the downstream areas have a unique natural ecosystem wonder, i.e. internationally known firefly colony at Kg. Kuantan. The Basin in its natural state is still largely a rural catchment. Figure 3 showed the location of the study area.

Figure 3: Location of study area

3 Methodology

This study involved field measurements, data collection and development of salinity model, which include hydraulic and salinity simulation. The salinity model shall be able to describe the present situation as well as predict future trend of salt water intrusion.
A hydrodynamic model is set up and developed in this study on the behaviour of the saline intrusion and movement in the river system. The model is calibrated by using a 1-Dimensional hydrodynamic InfoWorks RS. It deployed full St. Venant equation for shallow water which is also suitable for water quality and sediment transport modeling.
The modeling involves two phases which are developing a flow model and secondly a salinity model. The model covers from river mouth up to Rantau Panjang hydrological station. After the calibration process, various analyses can be carried out to look at the salinity behaviour with the change of flow from upstream of the river.

3.1 Site Visit and Data Collection

Site visit was carried out a few times in order to gather information about the salinity and berembang trees. NAHRIM researchers with the help from DID Kuala Selangor staff conducted sampling of salinity at various location along the river and all location were recorded using GPS. The survey was carried out on the 23 August 2005 and 16 February 2006. It is quite fortunate that the sampling cover both low and high flow. Photo during the site visit are shown in Figure 4.

3.2 Observation

The visual inspection indicates that the growth of the berembang trees started at about 6 km and ended at about 34 km from the river mouth. Therefore salinity sampling was carried out covering this stretch of the river.

3.3 Modeling

The hydrodynamic model is carried out to cover Sg. Selangor river system from its river mouth up to Rantau Panjang which is free from tidal effect. Total distance of the model is 57 km. The main input to the model is the flow measured at the Rantau Panjang station, tide and salinity concentration at the river mouth.

3.4 Calibration

Flow and water level calibration was carried using data taken in November 2005 and comparison was carried out with measured water level at Kg. Asahan. The flow from upstream cover both high and low flows. The most suitable Manning’s coefficient ‘n’ value use in the calibration is 0.020. Figure 5 below shows the input data for the calibration process. Figure 6 shows the tide level at Kuala Selangor while Figure 7 shows the comparison between observed and simulated water level.

4.4 Salinity Calibration

Before the model can be use to analyse the behaviour of the salinity movement, a calibration need to be carried out. Two salinity sampling were done in August 2005 and February 2006. There are three water intakes operating at Batang Berjuntai, the SSP1, SSP2 and SSP3 which extract 950MLD, 950MLD and 800 MLD respectively. The total amount of water extracted from the river is equivalent to 30 cumecs.

The salinity calibration was carried out for the month of August 2005. This can be considering a dry month with average flow from upstream at Rantau Panjang set to 20 cumecs. Final flow after passing through the water intake is assumed at 10 cumecs. The simulated salinity results are shown in Figure 8 and Figure 9 and the comparison of results between observed and simulated salinity at Kg. Bukit Belimbing and Kg. Kuantan is shown in Table 1. Table 2 shows the salinity comparison between observed and simulated at various locations in February 2006

5 ANALYSIS

Preliminary simulation was carried out for year 2000 flow. The result is shown in Figure 10 below at Kg. Bukit Belimbing and Kg Kuantan.

The analysis shows that, salinity at Kg. Kuantan increase during low flow of the year. The salinity patterns also follow as tide reach spring tide.
One simple analysis was carried out to see the effect of water intake. The flow from upstream was reducing to 5 cumecs and comparison at various places was done. For the purpose of this paper, two scenarios were compared. From the result, it can be seen clearly that the salinity along the river increases once the flow is reduce (Fig. 11 to Fig. 14). The model was able to predict the values and extent of the salinity intrusion. It is also interesting to note that saline water does travel further upstream which was estimate to about 5km.

6 CONCLUSION

This paper does not comment on the effect of salinity to the berembang tree. However the result indicates that expert in berembang tree shall be able to analyses the effect of salinity change to the tree. Therefore the hydrodynamic modeling will be useful tools in predicting the salinity change in the river which shall effect the growth of the berembang trees.

The study is still at the early stage. More data is required to enhance the modeling process. However it can be indicate that the output from the modeling will be of a great help and support to in monitoring the effect on salinity changes due to the water extraction in Sg. Selangor. The model shall also be used to predict for the long term effect on salinity to the river system.

Acknowledgments

The authors would like to thank to DID, Kuala Selangor and all River Research Centre & Water Quality & Environment Research Centre staff for their involvement in this project.

References

• Prof. Dr. Ir. F. De Smedt (1989) Introduction to River Water Quality Modeling, VUB Press, Brussels
• Abd. Jalil Hassan (2005) Permodelan Hidrodinamik Sungai- Pendekatan Awal Menggunakan Infoworks RS, Institut Penyelidikan Hidraulik Kebangsaan Malaysia
• DID (2002). Master Plan Study On Flood Mitigation and River Management for Sg. Selangor River Basin, Final Report Volume I.
• Wallingford Software Ltd (2005). InfoWorks RS, Wallingford Software, UK.
• Tyrone Parkinson (2006). Saline Intrusion Modelling, Wallingford Software. Paper presented at NAHRIM Salinity Intrusion Sg. Selangor, Workshop.