1 Sector H River Conditions, Flooding and Inundation 1
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The Study on Comprehensive Water Management of Sector H Musi River Basin in the Republic of Indonesia Final Report SECTOR H RIVER CONDITIONS, FLOODING AND INUNDATION 1. RIVER CONDITIONS 1.1 Musi River Basin The Musi River Basin is in the southern part of Sumatra Island. The Musi River and the major tributaries originate in the Barisan Range. The Musi River originates at Gunung Dempo (3159m) and flows to the northward, joining the Kelingi, Semangus, Lakitan and Rawas rivers. At the confluence of the Rawas River, the Musi River changes its flow direction toward east and joins the Harileko and Lematang rivers before it reaches at Palembang City. Two big tributaries, the Ogan and Komering rivers, join from the right bank at the Palembang City. The Komering River is the largest tributary of the Musi River followed by the Ogan River. At the Komering junction, the flow of the Musi River changes to the north again and finally empties into Bangka Strait. The Musi River Basin is shown in Figure H1.1.1. Figure H1.1.1 Musi River Basin JICA CTI Engineering International Co., Ltd. H - 1 NIKKEN Consultants, Inc. Sector H The Study on Comprehensive Water Management of Final Report Musi River Basin in the Republic of Indonesia The Musi River has a total catchment area of 59,942 km2 at the river mouth with channel length of about 640 km. At the city of Palembang, it amounts to 34,836 km2 before joining the Komering and Ogan rivers. Catchment areas of the main Musi River and its major tributaries are summarized in the Table H1.1.1. Table H1.1.1 Drainage Areas at Major Points of the Musi River Main river and Basin area Cumulative Location tributaries (km2) area(km2) Musi R. 6,251 Kelingi R. 1,928 8,179 Confluence of Kelingi R. Semangus R. 2,146 Lakitan R. 2,763 Rawas R. 6,026 Residual basins 552 19,666 Confluence of Rawas R. Harileko R. 3,765 Lematang R. 7,340 Residual basins 4,065 34,836 Confluence of Lematang R. Ogan R. 8,233 Residual basins 1,696 44,765 Palembang City Komering R. 9,980 54,673 Confluence of Komering R. Residual basins 5,269 59,942 River mouth (South China Sea) The Musi River Basin is characterized by its wide low-lying lands. Simplified contour map of the basin is shown in Figure H1.1.2. As seen in the Figure, the land lower than 25 m above mean sea level (M.S.L.) shares about one third (32%) of the total basin area and the land lower than 100 m,M.S.L. shares about two thirds (67%). H - 2 JICA CTI Engineering International Co., Ltd. NIKKEN Consultants, Inc. The Study on Comprehensive Water Management of Sector H Musi River Basin in the Republic of Indonesia Final Report Figure H1.1.2 Contour Map of the Musi River Basin 1.2 River Channels (1) Riverbed Slope Longitudinal profiles of major rivers in the Musi River Basin are shown in Figure H1.2.1. Focusing on the lower reaches of these rivers, longitudinal profiles below 200 m,M.S.L. are also shown in the Figure H1.2.2. JICA CTI Engineering International Co., Ltd. H - 3 NIKKEN Consultants, Inc. Sector H The Study on Comprehensive Water Management of Final Report Musi River Basin in the Republic of Indonesia 2500 KELINGI 2000 LAKITAN MUSI LEMATANG OGAN 1500 RAWAS 1000 Elevation (m,M.S.L.) KOMERIN G 500 HARILEKO SEMANGUS 0 0 100 200 300 400 500 600 700 Distance (km) Figure H1.2.1 Longitudinal Profile of Major Rivers in the Musi River Basin 200 Komering River Ogan River 175 Lematang River Harileko River 150 Rawas River Musi River 125 100 75 Elevation ( m,M,S,L) ( Elevation 50 25 0 0 50 100 150 200 250 300 350 400 450 500 550 Distance ( Km ) Figure H1.2.2 Longitudinal Profiles below 200 m, M.S.L. The Musi River has a long stretch of gentle bed slope in the lower reaches. The riverbed slope changes to steep in the upper reaches within relatively short transitional reaches. Flash floods with active erosion take place in the upper reaches of the main Musi River and tributaries, and they are absorbed in the lower gentle slope reaches. Principal features of these rivers are described below in brief. H - 4 JICA CTI Engineering International Co., Ltd. NIKKEN Consultants, Inc. The Study on Comprehensive Water Management of Sector H Musi River Basin in the Republic of Indonesia Final Report Musi River: The average bed slope of the Musi River is 1/40,000 from river mouth to Petaling (200 km from the mouth), 1/12,000 from Petaling to the Semangus River junction (391 km), 1/1,900 from the Semangus River junction to Tebing Tinggi (529 km), and 1/69 in the upstream reaches from Tebing Tinggi. Palembang City is located at about 85 km upstream from the river mouth. Komering River: The riverbed slope of the Komering River is around 1/4,700 in the reaches from the Musi River junction to Rasuan (137 km from the junction), 1/900 from Rasuan to Muara Dua (265 km), and 1/120 in the upper reaches. Ogan River: The riverbed slope of the Ogan River is around 1/8,300 in the reaches from the Musi River junction to Lubuk Rukem (170 km from the junction), 1/450 from Lubuk Rukem to Padang Bindu (255 km), and 1/45 in the upper reaches. Lematang River: The average riverbed slope of the Lematang River is around 1/6,200 in the reaches from the Musi River junction to Beruge (155 km from the junction), 1/480 from Beruge to Pulau Pinang (255 km), and 1/34 in the upper reaches. Harileko River: Watershed of the Harileko River is relatively low in elevation and the riverbed slope is also simple showing around 1/2,400. Rawas River: The average riverbed slope of the Rawas River is around 1/2,000 in the reaches from the Musi River junction to Surolangun Rawas (103 km from the junction), and 1/110 in the upper reaches. (2) Low-lying Lands Bed slope of the lower Musi River is very gentle, and the riverbed lower than 25 m, M.S.L. extends far about 400 km upstream from the river mouth. Major tributaries such as the Komering, Ogan, Lematang, Harileko and Rawas rivers flow into the low-lying Musi River. Stretches of the riverbed lower than 25 m, M.S.L. are summarized for these rivers in the Table H1.2.1. Table H1.2.1 Stretch of Riverbed Lower Than 25 m, M.S.L. River Stretch length Stretch length from river mouth in km from the Musi confluence in km Musi River 400 - Komering River 182 104 Ogan River 258 170 Lematang River 294 129 Harileko River 261 32 Rawas River 354 10 JICA CTI Engineering International Co., Ltd. H - 5 NIKKEN Consultants, Inc. Sector H The Study on Comprehensive Water Management of Final Report Musi River Basin in the Republic of Indonesia Lands are flat and low-lying in these stretches and swampy lands are found everywhere in the low-lying lands. River courses in these stretches are meandering. 1.3 Existing Problems These low-lying lands extending along the main Musi River and its tributaries serve for retardation of flood and sediment runoffs. The existing low-lying lands have been serving significantly for alleviation of the flood and sediment damages in the down stream areas owing to its wide area. However, there still exist flood and sediment problems as described below in the Musi River Basin. (1) Musi River: Riverbed in the lower reaches has been silted up due to sediment deposition mainly caused by erosion in the upper catchments and geological anticline conditions. The rise of riverbed results not only in the reduction of discharge capacity and flooding, but also in the extension of back swamp areas. River banks have been eroded around the bends, and the bank erosion damages roads and houses during very short period. The critical riverbanks are protected by small-scale revetment works made of concrete walls and gabion mattresses. Some urban areas and food-crop lands are located in the low-lying land suffering from inundation due to over-bank flood flows. Large back swamp areas are formed in the downstream reaches of the river caused by long-lasting flood flow and sedimentation. The main problems of the upper Musi River Basin including the Lakitan, Semangus and the Kelingi rivers are inundation due to over-bank flooding and riverbank erosion. (2) Komering River In the middle reaches of the Komering River, elevation of the floodplain is about 10 m higher than that of the Ogan River probably due to difference of sediment yields. Therefore, greater part of the river flow of the Komering River diverges to the Ogan River through several irrigation channels such as Randu, Arisan, Jambu, Sigonang and Anyar which were constructed during Dutch colonial era. In the downstream reaches of the bifurcation of the Randu channel, the river is left almost dry in the dry season, and the riverbed slope becomes very mild and braided with numerous sand bars rising due to the sediment deposition. (3) Ogan River River channel of the Ogan River is meandering with high sinuosity and has a narrow and deep channel. Sediment discharge is mostly suspended load. H - 6 JICA CTI Engineering International Co., Ltd. NIKKEN Consultants, Inc. The Study on Comprehensive Water Management of Sector H Musi River Basin in the Republic of Indonesia Final Report The Ogan River receives diverged flow from the Komering in the middle reaches mainly through the Randu channel.