Effect of a 100-year Flood on River Stability: Case Study of Kulim River

Transcription

1 Effect of a 100-year Flood on River Stability: Case Study of Kulim River CHANG CHUN KIAT, Research Officer, River Engineering and Urban Drainage Research Centre (REDAC), Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Nibong Tebal, Penang, Malaysia AMINUDDIN AB. GHANI, Deputy Director, REDAC, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Nibong Tebal, Penang, Malaysia ROZI ABDULLAH, Research Associate, REDAC, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, Nibong Tebal, Penang, Malaysia ABSTRACT This paper describes analyses on river stability for Kulim River catchment which is located in the southern part of the state of Kedah in the northwestern corner of Peninsular Malaysia. Frequently floods that occur in Kulim River Catchment has caused extensive damage and inconvenience to the community especially floods event in October 2003 which exceeds 100 year ARI. Previous study for Kulim River (DID 1996, Yahaya 1999, Lee 2001, Koey 2003) are limited to a flood event, therefore it is not able to predict river stability efficiency. Thus, a long-term simulation by using two computer models (FLUVIAL-12 and InfoWorks RS) is proposed for this river stability study. A design of stable channel will be carried out based on the results of the long-term simulation. Keywords: River stability, long-term simulation, FLUVIAL-12, InfoWorks RS. 1 Introduction The study area is located at the southern part of the state of Kedah in the northwestern corner of Peninsular Malaysia (Figure 1). It lies within the district of Kulim and upstream of Seberang Perai in Penang. Kulim River catchment consists of 15 subcatchments, with the total catchment area of 130km 2 (Table 1). Kulim river tributaries, namely Tebuan River, Kilang Sago Monsoon Drain, Wang Pinang River, Keladi River and Klang Lama River drain the urban conurbation of Kulim extending from town to the north. Downstream of Kulim town, the catchment comprises mainly rubber and oil palm estate located mainly at the confluences of rivers. The Kulim Structure Plan, has outlined development strategies for the region. Rapid urbanization at Kulim River catchment especially construction for housing estate and on-going 1450 ha Kulim Hi-Tech industrial Park will cause impacts to the catchment become more impervious. However, these impacts are much greater when the impervious areas of a catchment are connected to the streams by efficient stormwater drainage systems. Flooding at Kulim River has been attributed to overbank spill from rivers and tributaries arising from a number of causes, such as undersized river channel and drains to cater flood discharge, high channel roughness, bank irregularity and in-river vegetation, siltation, blockages by debris and refuse. Siltation at study area has been identified as one of the common causes of such flooding brought about by soil erosion at constructions sites. Department of Irrigation and Drainage (DID) Kedah has reported that the rivers have to be desilted typically every 2 to 3 years with removal of one metre thick of silt. 473

2 Peninsular Malaysia Study Area Figure 1 Kulim River Catchment and Subcatchment (DID, 1996). Table 1 Kulim River Subcatchment. Subcatchment Area (km 2 ) Water Cause Kulim River Kulim River Kulim River Ular River Kulim River Kilang Lama River Kulim River Wang Pinang River Tebuan River Keladi River Keladi River Ayer Merah Tengah River Kulim River & Badak River Lembu River Kulim River 2 Study Sites The study reach covers 15km of Kulim River (Figure 2), from the upstream (ch ) to the state boundary between Kedah and Penang (Ch. 00) and futher 2.5km downstream at the Ara Kuda gauging station. Figure 3 shows the photos taken along Kulim River during site visit on Kulim River arises on the western slopes of Gunung Bongsu Range and flowing in a north-westerly direction, and joined Keladi River in the vicinity of Kulim town. At the headwaters, the Kulim catchment is hilly and densely forested. The river slopes are steep and the channel elevation drops from 500m to 20m average mean sea level (amsl) over a distance of 9km. The central area of the catchment is undulating with elevations ranging from 100m down to 18m average mean sea level. Currently the catchment area is undergoing rapid urban development with oil palm and rubber plantation being replaced by housing estate. The lower area of the catchment is within the coastal plain with elevation down to 5m average mean sea level. 474

3 Ch Ch Ara Kuda Ch Ch Ch Figure 2 Kulim River Study Reach. Ch Ch where property damage is move compared with that in agricultural areas. Figure 4(a) shows the extent of flooding as derived from field investigation and interviews at Kulim River catchment. Subcatchment 8, 9 and 10 in Kulim River Catchment shows the visible flood incidences reported. Flood depth ranges are reported from 0.8m to 1.2m. However, the flood map of Kulim River Catchment (Figure 4(b)) shows the extent of flooding under future land use condition due to 100 year flood. This map was correlated with the flood information shown in Figure 4(a). Areas flooded are relatively well defined. The areas at subcatchment 5, 8, 9 and 10 are where extensive flooding occurs. Moreover, flooding at subcatchment 7, 13 and 15 is widespread. Frequently floods occurs in Kulim River Catchment caused extensive damage and inconvenience to the community. This is illustrated by the examples floods event in October 2003 (Figure 5). According to the information given by the DID, floods have occurred in 1-5 October 1989, 31 May - 2 Jun 1991, 24 April 1994, October 1997, Jan 2001, April 2001 and October Ch Ch Ch Ara Kuda Gauging Station Figure 3 Kulim River. 3 Historical Flood Conditions The effects of flooding are felt most in built-up areas such as residential and commercial areas within the urban confine Figure 5 October 2003 Flood (After DID Kulim/Bandar Bahru). 475

4 (a) (b) Figure 4 Flooding Extent Map (DID, 1996). 4 Study Approach Development which takes place in river catchment areas will result in the increase of discharge, bed erosion and deposition and it will caused river channel instability. When this happens, there will be failures to the river bank or in the river protection structure and reduce the channel capacity to flow the flood to downstream. Therefore, it is necessary to predict the river channel stability that will happen due to the existing development or will take place in the future development in a river catchment area. However, previous studies (Yahaya 1999, Lee 2001, Koey 2003) in Kulim River were limited to a single storm event. As a result, it is not enough to predict river stability for the Kulim River. Hence, the main purpose of the study is to evaluate river stability for a long period by considering the effect of changes in cross section and flow capacity. Two models will be used to study Kulim River stability are FLUVIAL-12 (Chang 1993) and InfoWorks RS (River Simulation) developed by Wallingford Software, UK (2003). The main factor to make the study successful is the availability of information and data. The models input data includes river geometry data for year 1991 survey (Figure 6) and historical hydrological data such as rainfall data, flow data (Figure 7) and water level data (Figure 8) for year 1991 and year 1997 to Long-term simulations (from year 1997 until 2003) by using two computer models stated above will be analyzed but three year events were selected for calibration including year 1991 (initial condition), year 1997 and 2003 (Figure 9). The end results from the simulation will be compared to the on-site survey which will be carried on year Figure 10 shows one of the river geometry surveys at Ara Kuda during May Conclusion Rapid development takes place at river catchment area will result high discharge, erosion and deposition which will cause river instability. The 100-year flood occurs during October 2003 caused extensive damage and inconvenience to the community. Therefore, it is necessary to 476

5 predict the river channel stability that will happen due to the existing and future development in Kulim River catchments area. As a result, a design for stable channel for Kulim River based on the long-term simulation by using FLUVIAL- 12 and InfoWorks RS model will be made. Figure 6 Kulim River Cross Section Ch (After DID Kulim/Bandar Bahru). /s) Discharge, Q(m Kulim River Hydrograph Hour Figure 7 Hydrograph for Kulim River (After DID Hydrology & Water Resources Division). Water Level, D (m) Danger Level Alert Level Water Level for Kulim River Year Hour Figure 8 Water Level Chart for Kulim River (After DID Hydrology & Water Resources Division). 477

6 and all information in the completion of this paper. They are also grateful to DID Hydrology & Water Resources Division for providing hydrology data in this stidies. References Figure 9 Flow Chart of Computer Modeling for Kulim River Stability Study. Figure 10 Cross Section Ara Kuda (May 2004). Acknowledgments The authors gratefully acknowledge Department of Irrigation and Drainage (DID) Kulim/Bandar Bahru for providing river survey geometry data, flooded photo 1. Chang, H. H., FLUVIAL-12: Mathematical Model for Erodible Channel. San Diego, California. 2. Department of Irrigation and Drainage Malaysia. (1996). Kajian Pelan induk Saliran Untuk Bandar Kulim Dan Sekirta, Kedah Darul Aman. 3. Koay, B. C. (2004). River Equilibium: Case Study of Kulim River Final Year Project Thesis. Penang : Universiti Sains Malaysia. 4. Lee, C. B., (2001). Application of River Modeling (Fluvial-12): Case Studies of Kulim River and Melaka River. Final Year Project Thesis. Penang : Universiti Sains Malaysia. 5. Yahaya, N. K. (1999). Development of Sediment Rating Curves for Rivers In Malaysia: Case Studies of Pari, Kerayong and Kulim Rivers. MSc. Thesis. Penang : Universiti Sains Malaysia. 478