Calapan2 O M Floods

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Calapan2 O M Floods

  1. 1. <ul><ul><li>Flooding & Sedimentation in the </li></ul></ul><ul><ul><li>Magasawang Tubig & Bucayao River Basins </li></ul></ul><ul><ul><li>in Oriental Mindoro: </li></ul></ul><ul><ul><li>A Hydrologic Report on the the Field Inspection </li></ul></ul><ul><ul><li>conducted on January 12, 2006 </li></ul></ul><ul><li>Leonardo Q. Liongson </li></ul><ul><li>Guillermo Q. Tabios III </li></ul><ul><li>Cornelio Q. Dizon </li></ul><ul><li>National Hydraulic Research Center </li></ul><ul><li>College of Engineering </li></ul><ul><li>University of the Philippines </li></ul><ul><li>Diliman, Quezon City </li></ul><ul><li>Presented at Calapan City, Oriental Mindoro </li></ul><ul><li>February 13, 2006 </li></ul>
  2. 2. <ul><ul><li>Aim of the presentation : </li></ul></ul><ul><ul><li>To present the hydrologic report on the initial findings </li></ul></ul><ul><ul><li>of the field inspection conducted by the NHRC team </li></ul></ul><ul><ul><li>on January 12, 2006 within the Magasawang Tubig and </li></ul></ul><ul><ul><li>Bucayao River Basins in Oriental Mindoro, Philippines. </li></ul></ul><ul><ul><li>The report is based on information, maps, plans and other </li></ul></ul><ul><ul><li>technical data gathered in the field and from public officials </li></ul></ul><ul><ul><li>and private citizens. </li></ul></ul><ul><ul><li>The data & information are analyzed in terms of topography, </li></ul></ul><ul><ul><li>stream types & river geometry, flood hydrology and river hydraulics. </li></ul></ul><ul><ul><li>Outline: </li></ul></ul><ul><li>1. Introduction & Area Description </li></ul><ul><ul><li>2. Recent Flooding and Sedimentation History </li></ul></ul><ul><ul><li>Stream Types and River Geometry </li></ul></ul><ul><ul><li>Flow Regimes: Bankfull & Flood Flows </li></ul></ul><ul><ul><li>Sedimentation & Flooding Problems </li></ul></ul><ul><ul><li>6. Identified Mitigation Measures – Comments & Suggestions </li></ul></ul><ul><ul><li>7. Conclusion and Recommendations </li></ul></ul>
  3. 3. Watersheds of Mindoro: (12) Bucayao River (17) Magasawang Tubig River (map from ADB report, 2003). , Introduction & Area Description
  4. 4. Calapan, Oriental Mindoro RAINFALL statistics (PAGASA synoptic station) based on 12 years of record Rainfall Depth (mm.) versus Return Period (years) Duration (hours) 1 3 6 24 hours 2 years 50.3 67.5 80.2 105.0 mm 5 70.6 98.2 122.5 165.0 10 84.1 118.5 150.5 206.0 20 97.0 138.0 177.4 244.6 25 101.1 144.2 186.0 256.8 50 113.7 163.2 212.2 294.5 100 126.2 182.2 283.3 332.0 (data from PAGASA, 1981).
  5. 5. Bucayao River STREAMFLOW statistics DA = 339 sq. km. @ Bucayao, Calapan, O.M. & Bucayao steel highway bridge at km. post 13 along Calapan-Pinamalayan highway. 1952-1968 records: max flood peak = 2288 cu.m./s mean annual flood = 517 cu.m./s mean flow = 59 cu.m./s mean annual min. flow = 13.6 cu.m./s Pangalaan River (a tributary of Bucayao River) DA = 28 sq.km. @ 6 km. u/s of Bucayao R. mouth & 0.500 km d/s of damsite . 1952-1968 records: max flood peak = 872 cu.m./s mean annual flood = 430 cu.m./s mean flow = 46 cu.m./s mean annual min. flow = 9.0 cu.m./s Magasawang Tubig River DA = 435 sq.km. @ Bacoron, Naujan, O.M. & approx. 6 km. u/s from highway bridge along Calapan-Pinamalayan highway. 1952-1968 records: max flood peak = 3248 cu.m./s mean annual flood = 825 cu.m./s mean flow = 29 cu.m./s mean annual min. flow = 1.30 cu.m./s (data from NWRC, 1980).
  6. 6. <ul><ul><li>Introduction & Area Description (cont’d) </li></ul></ul><ul><ul><li>Upper sub-basins (upstream of Mt. Muyod): </li></ul></ul><ul><ul><li>Aglubang River & Ibolo River – steep mountain braided </li></ul></ul><ul><ul><li>streams joining to form upper reach of Magasawang </li></ul></ul><ul><ul><li>Tubig River (MTR). </li></ul></ul><ul><ul><li>Bagto River – upper tributary of Bucayao River (BR) </li></ul></ul><ul><ul><li>Middle sub-basins (up to downstream of Calapan-Victoria Highway): </li></ul></ul><ul><ul><li>Alcate River – minor stream on the east side </li></ul></ul><ul><ul><li>Magasawang Tubig River (MTR) – separates at Mt. Muyod </li></ul></ul><ul><ul><li>Pangalaan River – branches out parallel to MTR </li></ul></ul><ul><ul><li>An overflow channel separating from MTR & joining to BR. </li></ul></ul><ul><ul><li>Bucayao River (BR) – meandering river in middle basin. </li></ul></ul><ul><ul><li>Lower sub-basins (up to the coastline): </li></ul></ul><ul><ul><li>Magasawang Tubig River (MTR) – exits at Naujan, OM. </li></ul></ul><ul><ul><li>Pangalaan River – rejoins BR before sea exit </li></ul></ul><ul><ul><li>Bucayao River (BR) – straight channel exit in Calapan City. </li></ul></ul>
  7. 7. <ul><ul><li>Flows from south to north: </li></ul></ul><ul><ul><li>Upper sub-basins </li></ul></ul><ul><ul><li>(upstream of Mt. Muyod): </li></ul></ul><ul><ul><li>Aglubang River </li></ul></ul><ul><ul><li>& Ibolo River – </li></ul></ul><ul><ul><li>two steep mountain braided </li></ul></ul><ul><ul><li>streams joining to form </li></ul></ul><ul><ul><li>the upper reach of Magasawang Tubig River (MTR). </li></ul></ul><ul><ul><li>Confluence near Villa Cerveza. </li></ul></ul><ul><ul><li>Bifurcation downstream of </li></ul></ul><ul><ul><li>Mt. Muyod into three streams: </li></ul></ul><ul><ul><li>An overflow channel to BR (west). </li></ul></ul><ul><ul><li>Upper reach of MTR (center) </li></ul></ul><ul><ul><li>Alcate River (east). </li></ul></ul><ul><ul><li>Satellite photos from </li></ul></ul><ul><ul><li>maps.google.com </li></ul></ul>Overflow channel MTR
  8. 8. <ul><ul><li>Aglubang River & Ibolo River – braided streams joining at confluence near Villa Cerveza to form the upper reach of Magasawang Tubig River (MTR). </li></ul></ul>
  9. 9. <ul><ul><li>Bifurcation downstream of Mt. Muyod into three streams: </li></ul></ul><ul><ul><li>An overflow channel to Bucayao River (BR) (west or to the left). </li></ul></ul><ul><ul><li>Upper reach of Magasawang Tubig River (MTR) (center) </li></ul></ul><ul><ul><li>Alcate River (east or to the right). </li></ul></ul>Mt. Muyod
  10. 10. Alcate River (aligned east of Magsawang Tubig River) - looking upstream.
  11. 11. Middle sub-basins (up to downstream of Calapan-Victoria Highway): Magasawang Tubig River (MTR) – separates at Mt. Muyod; flood-prone area. Pangalaan River – branches out parallel to MTR; flood-prone area. An overflow channel separating from MTR & joining to BR. Bucayao River (BR) – meandering river in middle sub-basin; flood- prone area; more flood flows than MTR. Overflow channel
  12. 12. aa Bucayao River (BR) – meandering river in middle basin; flood-prone area; receives more flood flows than MTR due to overflow from MTR.
  13. 13. Lower sub-basin (up to the coastline): Bucayao River (BR) – straight channel exit in Calapan City. Bucayao River
  14. 14. <ul><ul><li>Recent Flooding & Sedimentation History </li></ul></ul><ul><ul><li>Upper Basins (upstream of Mt. Muyod): </li></ul></ul><ul><ul><li>Aglubang River & Ibolo River – steep mountain braided </li></ul></ul><ul><ul><li>streams joining to form upper reach of Magasawang Tubig River. </li></ul></ul><ul><ul><li>According to local folks, heavier sediment load (sand, gravel, cobble, & boulder sizes) started to come down after the major Mindoro earthquake of 1994 had occurred. The earthquake must have loosened the rocks, now easily eroded by rains from the mountains. </li></ul></ul><ul><ul><li>Bed sediment deposits at the braided channels </li></ul></ul><ul><ul><li>of Aglubang River (below) : </li></ul></ul>
  15. 15. Flashback to the Mindoro Earthquake (7.1 magnitude) of 1994
  16. 16. The Mindoro Earthquake of 1994
  17. 17. <ul><li>According to the DPWH District Engineer Serafin V. Santos </li></ul><ul><li>(shown above) in a meeting with the NHRC team: </li></ul><ul><li>the bifurcation of flood flows from upper MTR to an overflow channel </li></ul><ul><li>joining Bucayao River has prevailed after the Mindoro earthquake of 1994. </li></ul><ul><li>this has resulted in higher flood flows passing through Bucayao River </li></ul><ul><li>and much less flows through Magasawang Tubig River. </li></ul>
  18. 18. The Mindoro Flood of December 2005
  19. 19. The Mindoro Flood of December 2005 … . From NDCC
  20. 20. The Mindoro Flood of December 2005…. 2005 Flood Archive
  21. 21. Stream Types and River Geometry
  22. 23. Mountain stream sub-types & Flow Velocity vs. Sediment size diagram e.g. Aglubang & Ibolo Rivers
  23. 26. Bankfull stage is river water level below overflow range. Flood-prone area gets inundated by river overflow.
  24. 27. Stream profile view (top). Stream plan view (middle). Stream cross- section (bottom).
  25. 28. Maintenance of equilibrum river meander geometry – based on bankfull flow, which is below flood overflow levels.
  26. 29. Design criteria for maintenance of equilibrium river meander geometry – based on bankfull flow level. Top: wavelength, amplitude Left: scour depth, radius of curvature
  27. 30. Meandering river at bankfull levels - e.g. Bucayao River. Cut-off channels are not recommendable since meandering will reestablish again.
  28. 34. <ul><li>Flow Regimes: Bankfull & Flood Flows </li></ul><ul><li>Bankfull Flows – </li></ul><ul><li>non-overflow floods with return period between 1 and 1.5 years. </li></ul><ul><li>basis for design of equilibrium river meander geometry. </li></ul><ul><li>basis for scour protection works on river bends. </li></ul><ul><li>Flood Flows (above bankfull) – </li></ul><ul><li>floods which overflow to the banks and floodplain. </li></ul><ul><li>with return period of 2, 5, 10, 25, 50, 100 years etc. </li></ul><ul><li>basis for design of river walls and dikes, including piles & revetments. </li></ul><ul><li>high lateral dikes are aligned along the sides of the meander belt </li></ul><ul><li>so that flooding is accommodated within the meander belt. </li></ul>
  29. 35. <ul><li>Sedimentation & Flooding Problems </li></ul><ul><li>Extensive sediment deposition in the system of braided channels </li></ul><ul><li>are formed by Aglubang and Ibolo rivers; </li></ul><ul><li>Sediment deposition or aggradation during large flood redefines </li></ul><ul><li>the river geometry so that bifurcation or branching out of rivers </li></ul><ul><li>takes place; </li></ul><ul><li>Bifurcation of flood channel from Magasawang Tubig River MTR) </li></ul><ul><li>goes towards the Bucayao River, increasing flood flow in the latter; </li></ul><ul><li>Bucayao River in effect captures the upper basin of MTR during floods; </li></ul><ul><li>Floodplain inundation arises from river overflows and local rainfall; </li></ul><ul><li>The meander pattern of Bucayao River is submerged during flood </li></ul><ul><li>overflow which is contained within the meander belt and river valley; </li></ul><ul><li>High flow velocities during floods may attack and breach the lateral dikes; </li></ul><ul><li>high water levels during floods may overtop inadequate dike heights. </li></ul>
  30. 36. . <ul><li>Identified Mitigation Measures – Comments & Suggestions </li></ul><ul><li>For both Bucayao ang Magasawang Tubig river basins – </li></ul><ul><li>a comprehensive basinwide flood hydrology/hydraulic study, including </li></ul><ul><li>rainfall, streamflows, sediments and river geometry, to form the </li></ul><ul><li>hydrological basis of the riverbasin flood management master plan. </li></ul><ul><li>For the uppermost steep mountain streams of Aglubang & </li></ul><ul><li>Ibolo Rivers – a special study on erosion control and possibile </li></ul><ul><li>application of “sabo dam technology” to contain the upstream </li></ul><ul><li>sediments (for possible JICA assistance). </li></ul><ul><li>For the braided stream system and overflow channel from MTR to BR – </li></ul><ul><li>the DPWH plan to construct porous dikes (made of gabion cages) to </li></ul><ul><li>reduce the flood flow going to Bucayao River is at best a short-term </li></ul><ul><li>measure because deposition of coarse sediments will persist in the area </li></ul><ul><li>and will eventually fill and cover the structure. Its hydrological design </li></ul><ul><li>criteria (flood return period & sediment load capacity) must be defined. </li></ul><ul><li>A long-term solution to prevent or reduce the flood flows to Bucayao </li></ul><ul><li>River will have to await the effects of upstream measures such as sabo </li></ul><ul><li>dams and upstream erosion control measures. </li></ul>
  31. 37. <ul><li>Identified Mitigation Measures – Comments & Suggestions (cont’d) </li></ul><ul><li>For the meandering middle river reach of Bucayao River and its adjoining </li></ul><ul><li>flood-prone area – </li></ul><ul><li>The plan to construct a cut-off channel to straighten two bends of Bucayao </li></ul><ul><li>River is not recommendable , not only because there are many other </li></ul><ul><li>meander bends to be left alone, but also because the river meander will be </li></ul><ul><li>reestablished again causing new unwanted scouring and deposition </li></ul><ul><li>patterns. The cut-off channel will also not prevent flood overflows. </li></ul><ul><li>For bankfull flow levels of BR & MTR: </li></ul><ul><li>It is more advisable to utlilize the results of a basin-wide flood </li></ul><ul><li>hydrology/hydraulic study in order to determine the bankfull discharge, </li></ul><ul><li>which shall be the basis for long-term design of the equilibrium meander </li></ul><ul><li>river geometry , which shall consist of design wavelength, radius of </li></ul><ul><li>curvature, meander belt width and prescribed scour protection works. </li></ul><ul><li>For flood overflow levels of BR & MTR : </li></ul><ul><li>It is more advisable to utlilize the results of a basinwide flood hydrology/ </li></ul><ul><li>hydraulic study in order to determine the flood flows discharges with high </li></ul><ul><li>return periods, which shall be the basis for the long-term design of the </li></ul><ul><li>lateral dikes to be provided with adequate heights and scour protection </li></ul><ul><li>works. </li></ul>
  32. 38. <ul><li>Conclusion and Recommendations </li></ul><ul><li>The presence of at least three stream types (braided, meandering & </li></ul><ul><li>straight) in the Bucayao and Magasawang Tubig Rivers; </li></ul><ul><li>The nature of the sedimentation and flooding problems caused to a </li></ul><ul><li>large extent by upstream deposition of coarse sediments and overflows; </li></ul><ul><li>Recommendation on the mitigation measures to undertake, the basis of </li></ul><ul><li>whose design must be a basin-wide hydrologic/hydraulic flood study , </li></ul><ul><li>to be incorporated in a flood management master plan for the river basins. </li></ul><ul><li>Special surveys and studies (short-term for immediate implementation): </li></ul><ul><li>Floodplain topographic & river hydrographic surveys of river systems preparatory </li></ul><ul><li>to the hydrologic/hydraulic study and flood management master plan; </li></ul><ul><li>Erosion control measures and sabo dam technology in the uppermost mountain </li></ul><ul><li>streams (to enable effective downstream gabion dam & dike construction); </li></ul><ul><li>Immediate short-term mitigation measures: </li></ul><ul><li>Scour protection works in present meander river bends to protect bank properties. </li></ul><ul><li>Minor repairs (no retrofit yet) of damaged existing dikes in lowland reaches of the </li></ul><ul><li> basins. </li></ul>
  33. 39. Example: Kissimmee River Restoration. Florida, USA Backfilling of man-made straight channel and restoration of the natural meandering form.
  34. 40. Example: Thames River in Oxfordshire, UK. above summer high water level - Use of bank revetment using low steel sheet piling and coir rolls –  coconut coir,e.g.
  35. 41. Examples: above low summer water level: Use of mattress revetment (top) & log toe revetment (right).
  36. 42. From the US Natural Resource Conservation Service (NRCS): for rural areas above baseflow level - Use of rootwad and boulder technique (left) & brush mattress technique (right).
  37. 43. Examples of Sabo Dams: Sabo Dams in Japan – sediment-retaining structures which keep sediments but allow water to pass through.
  38. 44. Examples of Sabo Dams : A cascade or series of Sabo Dams, once filled up with sediments, behaves as terraced river steps which attenuates or slows down the velocity of flood flows.
  39. 45. Examples of Sabo Dams : The concrete and steel barriers can stop large boulders but are open to flow of water..
  40. 46. ORMOC CITY Sabo Dam constructed by the DPWH
  41. 47. <ul><li>Conclusion and Recommendations (cont’d) </li></ul><ul><li>Basin-wide hydrologic/hydraulic flood study (long-term) </li></ul><ul><li>(duration = 6 months) </li></ul><ul><li>Components: </li></ul><ul><li>1. Rainfall, streamflow & sediment data collection. </li></ul><ul><li>Topographic and river geometry determination. </li></ul><ul><li>Storm rainfall analysis and synthesis. </li></ul><ul><li>Water balance & rainfall-runoff analysis and synthesis. </li></ul><ul><li>Flood frequency analysis. </li></ul><ul><li>Sediment balance and transport studies. </li></ul><ul><li>Watershed hydrologic modeling for mountainous river sub-basins. </li></ul><ul><li>Flood hydraulic modeling for lowland rivers, and flood-prone & coastal areas. </li></ul><ul><li>Identification & preliminary design of alternative mitigation measures </li></ul><ul><li>(e.g. sediment retention dams, lateral dikes, spur dikes, scour protection, etc). </li></ul><ul><li>10. Simulation of watershed hydrologic and flood hydraulic models for </li></ul><ul><li>impact studies of alternative mitigation measures. </li></ul><ul><li>11. Recommendations of selected & optimized flood mitigation measures. </li></ul><ul><li>Basin-wide flood management master plan (long-term) </li></ul><ul><li>(duration = 12 months, following the hydrologic/hydraulic flood study) </li></ul><ul><li>Components: </li></ul><ul><li>1. Feasibility-level design of recommended flood mitigation measures. </li></ul><ul><li>Prioritization, sequencing and phasing of flood mitigation measures. </li></ul><ul><li>Cost analysis of flood mitigation measures. </li></ul><ul><li>Economic analysis of flood mitigation measures </li></ul><ul><li>Financial analysis of flood mitigation measures. </li></ul>

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