The document summarizes a hydrologic report on flooding and sedimentation in the Magasawang Tubig and Bucayao River Basins in Oriental Mindoro, Philippines. It describes the area, stream types and geometry, flow regimes, sedimentation and flooding problems, and identifies mitigation measures. Key findings include extensive sediment deposition from upstream rivers loosening after a 1994 earthquake, flood flows bifurcating from one river to another, and recommendations for comprehensive basin studies and erosion control.

3. Watersheds of Mindoro: (12) Bucayao River (17) Magasawang Tubig River (map from ADB report, 2003). , Introduction & Area Description

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. 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).

10. Alcate River (aligned east of Magsawang Tubig River) - looking upstream.

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. aa Bucayao River (BR) – meandering river in middle basin; flood-prone area; receives more flood flows than MTR due to overflow from MTR.

13. Lower sub-basin (up to the coastline): Bucayao River (BR) – straight channel exit in Calapan City. Bucayao River

15. Flashback to the Mindoro Earthquake (7.1 magnitude) of 1994

16. The Mindoro Earthquake of 1994

18. The Mindoro Flood of December 2005

19. The Mindoro Flood of December 2005 … . From NDCC

20. The Mindoro Flood of December 2005…. 2005 Flood Archive

21. Stream Types and River Geometry

23. Mountain stream sub-types & Flow Velocity vs. Sediment size diagram e.g. Aglubang & Ibolo Rivers

26. Bankfull stage is river water level below overflow range. Flood-prone area gets inundated by river overflow.

27. Stream profile view (top). Stream plan view (middle). Stream cross- section (bottom).

28. Maintenance of equilibrum river meander geometry – based on bankfull flow, which is below flood overflow levels.

29. Design criteria for maintenance of equilibrium river meander geometry – based on bankfull flow level. Top: wavelength, amplitude Left: scour depth, radius of curvature

30. Meandering river at bankfull levels - e.g. Bucayao River. Cut-off channels are not recommendable since meandering will reestablish again.

39. Example: Kissimmee River Restoration. Florida, USA Backfilling of man-made straight channel and restoration of the natural meandering form.

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.

41. Examples: above low summer water level: Use of mattress revetment (top) & log toe revetment (right).

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).

43. Examples of Sabo Dams: Sabo Dams in Japan – sediment-retaining structures which keep sediments but allow water to pass through.

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.

45. Examples of Sabo Dams : The concrete and steel barriers can stop large boulders but are open to flow of water..

46. ORMOC CITY Sabo Dam constructed by the DPWH