The Study on Sabo and Flood Control for Western River Basins of Mount Pinatubo in the Republic of the Philippines Final Report Main Report PART I General and Present Condition CHAPTER 1 INTRODUCTION 1.1 Background and Objectives 1.1.1 Study Background The study area is located in the western river basins of Mount Pinatubo in the mid-western part of Luzon Island, as shown on the attached location map. Luzon Island is situated in the active volcanic and earthquake zone between the East Luzon Trench to the east, and the Manila Trench to the west. In June 1991, Mount Pinatubo erupted and approximately 6.7 billion m3 of pyroclastic material was deposited around the mountain slopes. Of the deposited materials, it is estimated that about 4.7 billion m3 was deposited in the study area. A report issued by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) shows that the amount of pyroclastic material deposited in the respective basins of the Bucao, Sto. Tomas and Maloma Rivers was 2.8 billion m3, 0.6 billion m3 and less than 0.1 billion m3. Deposited materials were then transported down the mountain in the form of mudflow during the rainy season. Since the eruption of Mount Pinatubo, twelve years have passed and the volcanic activity has subsided. However, the lahar materials deposited on the mountain slopes continue to be remobilized with the occurrence of concentrated rainwater resulting in secondary disasters including mudflows, riverbed rising and flood/mudflow inundation. Livelihood in both of lowland and mountainous areas, which has been damaged due to deposition of volcanic ash and pyroclastic flow material and resulted influence to infrastructures, has not been recovered yet in the western river basin. The Government of the Philippines (GOP) made a large effort to implement a rescue mission immediately after the eruption of Mount Pinatubo, and organized the “Mount Pinatubo Commission (MPC)” to rescue approximately 600,000 affected inhabitants. Since then, this Commission has promoted employment searches for the affected people and has constructed approximately 34,000 houses. Meanwhile, the Department of Public Works and Highways (DPWH) has implemented mudflow disaster prevention facilities including levee construction. The Government of Japan (GOJ) has provided assistance in the establishment of a mudflow forecasting and warning system, supply of construction equipment and construction of sabo dams. As it was anticipated that a huge volume of the deposited material has been transported down the mountain and has induced secondary disasters, the GOP requested assistance from the donor countries for the preparation of a flood and mudflow control plan. In response to this request from the GOP, the United States of America (USA) prepared a long-term flood and mudflow control plan for eight major rivers around Mount Pinatubo and the final report was issued in March 1994. This control plan has not been implemented in the western river basins of Mount Pinatubo except for levee construction due to the occurrence of a large-scale mudflow, and also because rehabilitation of the western side has a relatively low priority compared with the eastern side. In view of recovery of the livelihood and equitable regional development as well as the anticipated disasters due to flood and mudflow, the GOP requested a technical assistance to the GOJ to conduct a master plan formulation and feasibility study (the study) for sabo and flood control in the western river basins on the basis of the present basin conditions. In response to the request, the GOJ decided to conduct the study and the implementing arrangement (IA) was signed on December 6, 2001 between JICA and DPWH. In accordance with the IA, the study team was organized by JICA and dispatched to the Philippines to commence the study in April 2002 after some preparatory works in Japan in March 1-1 2002. 1.1.2 Study Objectives The objectives of the study are as follows: (i) To formulate a master plan for sabo and flood control in the major three rivers (Bucao, Maloma and Sto.Tomas Rivers), and to carry out a feasibility study for main priority projects, however, excluding a study on internal drainage issues, (ii) To transfer technology that is used for the formulation of plans for sabo and flood control to the counterpart personnel during the study, and (iii) To assist with disaster rehabilitation and economic development in the study area, as well as other river basins in the Philippines, by transferring planning technology which would reduce regional economic disparity and poverty levels and therefore improve and enhance the welfare of the regional inhabitants. 1.1.3 Study Area The study area is confined to the western river basins of Mount Pinatubo as shown in the location map. The subject rivers of the study are the Bucao, Maloma and Sto.Tomas Rivers. The municipalities concerned are Iba, Botolan, Cabangan, San Felipe, San Narciso, San Marcelino, San Antonio and Castillejos, all of which are located in Zambales Province. 1.1.4 Scope of the Study The study is divided into the following two phases: (a) Phase 1: Basic study and formulation of master plan for sabo and flood control. The high priority projects are selected as the subjects for a feasibility study to be performed in the subsequent phase. (b) Phase 2: Feasibility study for the high priority projects. A supplementary investigation is carried out for the priority project and a feasibility study on sabo and flood control works is performed. 1.2 Study Schedule and Activities 1.2.1 Study Schedule The study is to be carried out over 18 months between the middle of March 2002 and the beginning of September 2003. Figure 1.2.1 shows the overall work schedule for the study. The study is comprised of two phases, namely, Phase 1: Formulation of the master plan between March 2002 and October 2002, and Phase 2: Feasibility study for the high priority project between November 2002 and September 2003. 1.2.2 Activities Figure 1.2.2 shows the progress of the phase 1 and 2 studies. The details are as follows: (1) Explanation and Discussion about Inception Report Upon submission of the inception report to the DPWH on April 1, 2002, the contents of the report were explained by the study team to the counterpart staff of the DPWH and the Technical Working 1-2 Group on April 2 and 3, 2002, respectively. The Steering Committee meeting was held on April 4, 2002, and the study approaches and methods proposed in the inception report were accepted in principle by the committee. (2) Collection and Analysis of Data/Information and Field Reconnaissance Data/information relevant to the study were collected in the National Government Agencies in Manila, in their Regional Office No. 3 in San Fernando, and in Local Government Units (LGUs) in the study area. Field reconnaissance was also conducted in the study area to acquire the necessary information about the natural and social conditions in the basin. (3) Field Investigations Field investigations conducted for the master plan formulation included: a) installation of hydrological gauges and observation, b) topographic survey and mapping, c) geological investigation, d) a traffic volume survey, e) a water quality survey, f) a riverbed material survey, g) a river structures inventory survey, and h) a flood damage survey. All the field investigations were completed during the phase 1 study period, although some delays were experienced in the topographic survey due to severe floods that occurred in July 2002. Four rainfall gauges were installed in the study area and in San Fernando. In addition, three sets of staff gauges were installed on the Bucao, Maloma, and Maculcol Bridges to record the river water level. The recording commenced in the middle of June 2002 at all the stations and still continues. Aerial photogrammetric maps were prepared covering a part of the study area for 793 km2 including 300 km2 of the Mount Pinatubo western slope, 180 km2 of the Bucao River basin, 30 km2 of the Maloma River basin and 283 km2 of the Sto. Tomas River basin. The geological investigation planned at the Maraunot Notch using core drilling was not undertaken by the DPWH in the phase 1 stage because of a contractual problem with the local contractor. The geology of the notch was assessed by the study team at the master plan level using the existing data as well as the results of a visual inspection at the notch. (4) Formulation of a Master Plan for Sabo and Flood Control A master plan for sabo and flood control was formulated for the Bucao, Maloma and Sto. Tomas River basins based on the collected data/information and results of the field investigations as well as discussions with the DPWH counterparts and the other agencies’ officials. The three main components of: structural measures, non-structural measures and community based disaster prevention plans were combined in the master plan. In planning the structural measures, future sediment yield was estimated, and sediment balance and mudflow simulation analysis were conducted. The planning scale was set at a 20-year probable flood as being the most economical within reason. The proposed structural measures in the master plan include: 1) dike heightening, a consolidation dam and sand pockets in the Bucao River, 2) channel improvement in the Maloma River, 3) dike heightening/strengthening, channel training works and sand pockets with a consolidation dam in the Sto. Tomas River, 4) the Maraunot Notch improvement by gabion mattress, and 5) reconstruction of the Bucao, Maloma and Maculcol Bridges. The proposed non-structural measures in the master plan are: 1) a flood/mudflow forecasting and warning system, 2) improvement of evacuation centers, 3) extension of community-based forest management, and 4) basin management of the Mapanuepe Lake.