Initial Assessment Report on Sendong Flood in Iligan...

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Transcript of Initial Assessment Report on Sendong Flood in Iligan...

  • Initial Assessment Reporton

    Sendong Flood in Iligan-Cagayan de Oro

    Philippines

  • ILIGAN Area

    MANDULOG River Basin

    Bayug New Town

    New Bridge

    Old Bridge

  • Drift logs clogged old Mandulog Bridge then Flood was diverted into city proper. After that the bridge was

    destroyed by excessive water pressure.

    Flood Diverted to City Area

  • Flash Flood burst out from the old bridge run over the New Bridge

    Destroyed Old Bridge

    New Bridge

  • Flah Flood swept away Bayug Elmt.school then devastated Bayug New Town

    LowHigh

    Direction o

    f Flash Flo

    odOld Bridge

    New Bridge

    Elmt. SchoolBayug New Town

  • Bayug New Town was totally devastated by flash flood

    Hard Hit

    of

    Flash Flood

  • Upstream of Mandulog River still potential source

    of drift logs

  • Shallow Estuary of Mandulog is threatening upstream area with future flooding

  • Conclusion of Inspection

    One of reasons for flood is clogging the old bridge by drift logs to dam-up the river water level causing flood along the river course and diverting flood toward Iligan city area.

    Surcharged water finally broke the bridge and blew out on the straight line toward to the downstream, causing flash flood at Bayug new town area.

    If there was no clogging of the bridge, flood damage and area may be less but how much can be confirmed by study based on detailed topographic map.

    Still in this area has possible danger of flood caused by clogging the new bridge by drift logs and by shallowness of riverbed in the estuary.

  • Recommendation

    Mark the flood traces and record their locations and elevations for future flood analysis.

    Dredge the estuary of Mandulog River. Construct facilities to trap drift logs Study on flood analysis and

    countermeasure, and on Sabo Works including drift-log trap.

  • Possible sites for drift-log-trap

  • Cagayan De Oro Area

    Flood area illustrated in this paper is copied from CDODEV report.

  • Estuary of Cagayan De Oro River

    Access of Bridge

    Sea wall

  • River flow is obstructed by the bridge approach ramp in the flood plain

  • Flood water intruded from no-seawall section to the land

    Sea wall

  • Isla De Oro : Low Land

    Flood Area

  • Isla de Ore is one of sand bars and very low land area

    Flood mark shows the level

    Flood water level

    Sand bars have reduced flow capacity of the river

    4 or 5 m

  • Bottle Neck Upstream of Ysalina BridgeControlling Flood with Retarding Effect

    Bottle Neck

    Flood Plain with retarding effect

    Flood Area

  • Retarding Effect Upstream of Bottle Neck is lessened by obstructions by making flood plain narrower

    Upstream view of the bottle neck. The photo was taken at the backyard of St. Augustine Metropolitan Cathedral

  • Balulang-Macasandig Town in Flood Plain

    Flood Area

    Macasandig New Town

    Balulang New Town

  • Housing was placed on the floodplain

    Upstream view of Macasandig

    Downstream view of Macasandig

    Macasandig New Town

  • Upstream of Balulang-Macasandig

    Pelaez Bridge

    Flood Flow

  • Flood water level

  • Upstream of Pelaez BridgeBoulders cumulate a large amount

    Boulders LayersBroken Pipe Bridge

  • Conclusion of Inspection

    One of the problem is that there are so many people residing in the flood plain.

    Another problem is that the river flow capacity is not enough for discharge of this time flood.

    There are some obstruction to reduce the flow capacity and retarding effect along the river, like bridge approach, sand bars, bottle neck, some of flood walls and shallow river bed.

    In general, there three kinds of solution in structural measure to mitigate flood; increase flow capacity along the river course, develop storage capacity like dam-reservoir and retarding pond, and divert flood to the other way.

    No possible site for dam-reservoir because of steepness and narrowness of river course upstream of Cagayan de Oro River

    Institutional arrangement is badly required for zoning based on flood risk map.

    Over certain level of flood return period should be covered by flood warning system with evacuation

    Land development should consist with flood analysis and river engineering. Study is required to take into account those solutions and arrangement.

  • Recommendation

    Mark the flood traces and record their locations and elevations for future flood analysis.

    Dredge the estuary of Cagayan de Oro River and excavate sand bars to widen the river course.

    Comprehensive study is required for flood mitigation and management for the river basin.

    Include in the study Sabo works to control sediments, which have caused the river bed rise and retarding effect less.

    Establish authority for Floodplain Management Disseminate river engineering for river maintenance

    to the local government and its personnel concerned.

    Initial Assessment Report on Sendong Flood in Iligan-Cagayan de OroILIGAN AreaDrift logs clogged old Mandulog Bridge then Flood was diverted into city proper. After that the bridge was destroyed by excessive water pressure.Flash Flood burst out from the old bridge run over the New BridgeFlah Flood swept away Bayug Elmt.school then devastated Bayug New TownBayug New Town was totally devastated by flash floodUpstream of Mandulog River still potential source of drift logsShallow Estuary of Mandulog is threatening upstream area with future floodingConclusion of InspectionRecommendationPossible sites for drift-log-trapCagayan De Oro AreaEstuary of Cagayan De Oro RiverRiver flow is obstructed by the bridge approach ramp in the flood plainSlide 15Isla De Oro : Low LandIsla de Ore is one of sand bars and very low land areaBottle Neck Upstream of Ysalina Bridge Controlling Flood with Retarding EffectRetarding Effect Upstream of Bottle Neck is lessened by obstructions by making flood plain narrowerBalulang-Macasandig Town in Flood PlainHousing was placed on the floodplainUpstream of Balulang-MacasandigSlide 23Upstream of Pelaez Bridge Boulders cumulate a large amountSlide 25Slide 26