A3 RIVER BASIN AND RIVER CHANNEL

Transcription

1 Part-A: Existing Conditions A3 RIVER BASIN AND RIVER CHANNEL A3.1 Basin The Limboto-Bolango-Bone (LBB) basin has a total catchment area of about 27 km 2 and is shaped rectangle extending about 1 km from east to west, and about 3 km from north to south. The LBB basin consists of the Lake Limboto basin (89 km 2 ), the Bolango basin (49 km 2 ), and the Bone basin (1,32 km 2 ). General basin map with river system and basin areas at major sections are shown in Figure A3.1.1 and overall longitudinal profiles of the Bone system and the Lake Limboto system in Figures A3.1.2 and A Administratively the LBB basin is located mostly in the south-eastern part of Gorontalo Province (2,4 km 2 ) and small portion in the south-western part of North Sulawesi Province (3 km 2 ). Kabupaten Gorontalo and Kota Gorontalo of Gorontalo Province and Kabupaten Bolaang Mongondow of North Sulawesi Province are related to the basin. Their administrative boundaries in the LBB basin are shown in Figure A (1) Lake Limboto Basin The Lake Limboto basin shares western half of the LBB basin with a total catchment area of about 89 km 2. The major tributaries of the Lake Limboto are the Biyonga (66 km 2 ), the Meluopo (27 km 2 ) and the Marisa (64 km 2 ) from the north, and the Alo-Pohu, which consists of the Alo (342 km 2 ) from the north-west and Pohu (124 km 2 ) from the south-west. There are numerous small tributaries from the southern hill. (2) Bolango Basin The Bolango originates in the Mt. Lowulowu. It passes through the mountainous areas towards south, joining the Bongo (81 km 2 ) and the Mongiilo (262 km 2 ) on the left, until it reaches at the Lomaya weir. Basin area of the Bolango is 388 km 2 at the Lomaya weir. The Bolango joins the Palanggua (58 km 2 ) at the downstream of the Lomaya weir and flows down to the plain land on the northern part of Gorontalo city. Passing on the west of Gorontalo city, it joins the Tapodu, outlet channel of Lake Limboto, and empties into the Bone on the south of Gorontalo city. Basin area of the Bolango is about 49 km 2 in total A3-1

2 Part-A: Existing Conditions excluding that of Lake Limboto. (3) Bone Basin The Moloti which originates in the Mt. Unggiango and the Bulawa which originates in the Mt. Sula meet together and become the Bone in the east of the LBB basin. Passing through the mountainous areas towards the west, the Bone joins the Bulahu (73 km 2 ), the Olama (73 km 2 ) and the Bunano (169 km 2 ) on the right. After flowing towards the west, it reaches at Alale weir. The total basin area of the Bone is around 1,6 km 2 at the weir. The Bone further runs toward west on the south of Gorontalo city and joins the Tamalate (7 km 2 ) and the Bolango on the right. Just at the downstream of the confluence with the Bolango, the Bone changes its direction towards the south, and finally discharges into the Tomini Bay. The total basin area of the Bone basin is about 1,32 km 2 excluding those of the Bolango and Lake Limboto. A3.2 Principal Channel Features system in the plain area of the LBB basin is shown in Figure A Based on the results of latest river survey conducted by the Study Team, channel characteristics of the major river channels in the Study Area were studied and the results are shown in Table A The channel characteristics include the riverbed slope, river width, mean depth, cross-sectional area, channel slope and existing carrying capacity. The channel characteristics are also illustrated for the selected principal rivers as shown in Figure A They are summarized as follows: 1) Bone : bed slope : 1/18~1/25 width : 12m to 22 m Carrying capacity : 21 to 87 m 3 /s 2) Bolango : Lower Reaches: mouth to confluence of Tapodu bed slope : 1/14 width : 34 m Carrying capacity : 175 m 3 /s A3-2

3 Part-A: Existing Conditions Middle Reaches: upstream of confluence of Tapodu bed slope : 1/96~1/15 width : 6m to 9 m Carrying capacity : 2 to 3 m 3 /s 3) Tamalate ( in Gorontalo City): bed slope : 1/12 width : 2m Carrying capacity : 5 to 6 m 3 /s A3.3 Flood and Sediment Disasters (1) Type of Disaster According to the information obtained through field reconnaissance and public consultation, flood, bank erosion and sedimentation are the major types of the disaster in the Study Area. Flooding: Flooding over the riverine lands is the typical disasters observed in the areas along the river course. These flooding occur because of shortage of channel capacity. Poor drainage due to low-lying topography, natural and artificial channel constriction, and backwater from the main river cause the stagnant of flooded water and local storm water, bringing about inundation disaster. Bank erosion: Bank erosions are observed in places of the rivers in the Study Area. Bank protection works were implemented to protect residential areas, trunk roads, etc., although these are implemented as remedial measure with less consideration from basin-wide viewpoint. Sediment: Devastation of watershed due to illegal logging or shifting cultivation increases sediment yield in the watershed and causes sediment deposits in the river channels, farm lands and lake in the lower reaches. A large part of sediment flows in the Lake Limboto and decrease its storage volume. Consequently a haul of fish in the lake decreases and flood regulation function of the lake lowers. (2) Inundated Area and Causes Major inundated areas in the LBB basin are southern part of Gorontalo City, middle A3-3

4 Part-A: Existing Conditions reaches of the Bolango, Limboto City, western part of the Lake Limboto, and Isimu area as shown in Figure A Brief descriptions are given on these major flood prone areas as follows. Among these the disasters in the southern part of Gorontalo City are most serious. Southern Part of Gorontalo City: In the southern part of Gorontalo city, the Bone, the Bolango and the Tamalate rivers join. Water level is raised due to backwater at the confluence of these rivers and inundation occurs frequently. High tide and sedimentation at the river mouth aggravate the situation. Middle Reaches of Bolango : Inundated area extends along the Bolango from the confluence with the Mongiilo to the northern part of Gorontalo city. terrace is formed in this stretch, and flood flow runs along the Bolango between the river terraces. Limboto City: Limboto city is inundated by flood water from the Biyonga and Meluopo rivers. The flood flow is interrupted by Gorontalo-Isimu road. Western Part of Lake Limboto: Western part of the Lake Limboto is inundated due to rising water level of the lake. Inundation in this area lasts for a long time, since the capacity of outlet channel from the lake is small. There would be enough time for evacuation because water level rises slowly. Isimu area: Isimu area where the tributaries gather is suffering from inundation due to shortage of carrying capacity of river channel and low-lying topography in the area. A3-4

5 Table A3.2.1 CHARACTERISTICS OF EXISTING RIVER CHANNEL System From (m) To (m) Average bed Slope (1/I) Average Width (m) Average Depth (m) Average Area Average Carrying Capacity (m 2 ) (m 3 /s) Alopohu 3, Lake Limboto 3,618 9, System Alo 9,227 13, Reksonegoro 2, Pohu 3, Biyonga 3, ,1 6, ,585 8, Meluopo 2, ,181 4, Marisa 1, ,451 5, Rintenga 7, Bolango 5, Bolango 5,981 16, System 16,884 18, ,725 19, Tapodu 2, Siendeng 1, Limba 2, ,36 8, Bone Bone 969 5, System 5,572 14, ,713 15, Pangimba 6, Tamalate 5, ,653 8, ,318 1, ,52 1, Ulanta 3, Irrigation Canal 1, A3-5

6 N Mt.Molelahu Alo Marisa Pohu Scale 5 1 2km Mt.Lowulowu Biyonga Bolango Bongo Mongiilo Meluopo Mt.Olaola Lake Limboto Lomaya weir Palanggua Tamalate Alale weir Bone Tomini Bay Bunano System Area (sq.km) Bone System Bone (mouth) 1322 Bone (Alale weir) 16 Moloti 1 Bulawa 282 Bulahu 73 Olama 73 Bunano 169 Tamalate 7 Lake Limboto System Tapodu rivermouth 892 Biyonga 66 Meluopo 27 Marisa 64 Alo 342 Pohu 124 Alopohu 477 Bolango System Bolango (mouth) 487 Bolango (Lomaya weir) 388 Bongo 81 Mongiilo 262 Palanggua 58 Total 271 Olama Bulahu Mt.Unggiango Mt.Sula Moloti Bulawa Mt.Taneya Figure A3.1.1 RIVER BASIN BOUNDARIES A3-6

7 Tamalate/Ulanta R. Bolango R. Mongiilo R. Bongo R. Pilolahea Daa Butaiyo Kiki Bone R. Bunano R. Bulawa R /1 1/5 1/1 1/2 1/ Distance from rivermouth (km) Elevation (m,msl) (mouth,tomini Bay) Figure A3.1.2 OVERALL LONGITUDINAL PROFILE : BONE - BOLANGO RIVER SYSTEM A3-7

8 Alo R. Qlilumayango R. Meluopo R. Marisa R. Biyonga R. Molamahu R. Pohu R /1 1/5 1/1 1/2 1/ Distance from Lake Limboto (km) Elevation (m,msl) (Lake Limboto) Figure A3.1.3 OVERALL LONGITUDINAL PROFILE : LAKE LIMBOTO SYSTEM A3-8

9 Kec. Kaidipang Boroko GORONTALO PROVINCE Kec. Kwandang KABUPATEN GORONTALO Kec. Atinggola Kec. Tapa Kec. Tibawa Kec. Limboto Kec. Telaga Bolango Kec. Kabila Kec. Batudaa Kec.Batudaa Pantai Kota Barat Kota Utara Kota Selatan Bone Kota Gorontalo Legend Provincial Boundary Kecamatan Boundary Basin Scale 5 1 2km N Kec. Sangtombolang Kec. Bolangitang Kec. Bintauna NORTH SULAWESI PROVINCE KABUPATEN BOLAANG MONGONDOW Kec. Suwawa Kec. Dumoga Kec. Bolang Uki Kec. Bone Pantai Figure A3.1.4 ADMINISTRATIVE BOUNDARIES OF LBB BASIN A3-9

10 Meluopo Biyonga Lake Limboto Tapodu Alo Marisa Alopohu Bolango Tamalate Tomini Bay N Scale km Bone Reksonegoro Rintenga Pohu Figure A3.2.1 RIVER SYSTEM IN PLAIN AREA A3-1

11 Elevation (m,msl) Width (m) Lake Limboto Left bank Ib=1/172 Ib=1/796 Pohu R. Alopohu Weir Reksonegoro R. Road Ib=1/689 Longitudinal profile 2, 4, 6, 8, 1, 12, 14, Right Average riverbed Lowest riverbed width 2, 4, 6, 8, 1, 12, 14, Depth (m) Area (sq.m) Carrying capacity (cu.m/s) Depth 2, 4, 6, 8, 1, 12, 14, 2, 4, 6, 8, 1, 12, 14, 2, 4, 6, 8, 1, 12, 14, Distance (m) Area Carrying capacity Figure A3.2.2 CHANNEL CHARACTERISTICS (1/5 : ALOPOHU RIVER) A3-11

12 3 Elevation (m,msl) Lake Limboto Road Right bank Left bank Average riverbed Huludapitanggo Weir Width (m) Lowest riverbed Ib=1/646 Ib=1/357 Ib=1/245 Longitudinal profile 1, 2, 3, 4, 5, 6, 7, 8, 9, width 1, 2, 3, 4, 5, 6, 7, 8, 9, Carrying capacity (cu.m/s) Area (sq.m) Depth (m) Depth 1, 2, 3, 4, 5, 6, 7, 8, 9, , 2, 3, 4, 5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9, Distance (m) Area Carrying capacity Figure A3.2.2 CHANNEL CHARACTERISTICS (2/5 : BIYONGA RIVER) A3-12

13 Elevation (m,msl) Width (m) Depth (m) Bone R. Siendeng R. Ib=1/1435 Topodu R. Left bank Right bank Road Ib=1/958 Average riverbed Lowest riverbed Pilohayanga Wier Ib=1/153 Ib=1/365 Longitudinal profile 2, 4, 6, 8, 1, 12, 14, 16, 18, 2, width 2, 4, 6, 8, 1, 12, 14, 16, 18, 2, Depth 2, 4, 6, 8, 1, 12, 14, 16, 18, 2, Area (sq.m) , 4, 6, 8, 1, 12, 14, 16, 18, 2, Area Carrying capacity (cu.m/s) Carrying capacity 2, 4, 6, 8, 1, 12, 14, 16, 18, 2, Distance (m) Figure A3.2.2 CHANNEL CHARACTERISTICS (3/5 : BOLANGO RIVER) A3-13

14 25 2 Bone R. Road Ulanta R. Siphon Elevation (m,msl) Right bank Left bank Average riverbed Lowest riverbed Ib=1/1163 Ib=1/692 Ib=1/34 Ib=1/112 Width (m) Longitudinal profile 2, 4, 6, 8, 1, 12, width 2, 4, 6, 8, 1, 12, Depth (m) Area (sq.m) Depth 2, 4, 6, 8, 1, 12, 2, 4, 6, 8, 1, 12, Area Carrying capacity (cu.m/s) Carrying capacity 2, 4, 6, 8, 1, 12, Distance (m) Figure A3.2.2 CHANNEL CHARACTERISTICS (4/5 : TAMALATE RIVER) A3-14

15 4 3 mouth Bolango R. Tamalate R. Bridge Left bank Alale Weir 2 Right bank Elevation (m,msl) 1 Lowest riverbed Average riverbed -1 Ib=1/1783 Ib=1/815 Ib=1/529 Ib=1/247 Carrying capacity (cu.m/s) Area (sq.m) Depth (m) Width (m) Longitudinal profile -2-2, 2, 4, 6, 8, 1, 12, 14, 16, 8 6 width 4 2-2, 2, 4, 6, 8, 1, 12, 14, 16, Depth -2, 2, 4, 6, 8, 1, 12, 14, 16, , 2, 4, 6, 8, 1, 12, 14, 16, Area Carrying capacity -2, 2, 4, 6, 8, 1, 12, 14, 16, Distance (m) Figure A3.2.2 CHANNEL CHARACTERISTICS (5/5 : BONE RIVER) A3-15

16 Scale km Alo Marisa Isimu-Pohu areas Reksonegoro Alopohu Pohu Rintenga Western area of Lake Limboto Inundated area Meluopo Biyonga Lake Limboto Tapodu Southern part of Gorontalo City Limboto City Bolango Tamalate Tomini Bay Middle reaches of Bolango Bone N Figure A3.3.1 FREQUENTLY INUNDATED AREAS A3-16