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Ondoy Tragedy Lessons to be learned…

Ondoy Tragedy Lessons to be learned…. Fernando Siringan ando.msi@gmail.com Marine Science Institute, University of the Philippines Diliman Gemma Narisma, Rosa Perez, Celine Vicente, Antonia Loyzaga Manila Observatory Mahar Lagmay

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Ondoy Tragedy Lessons to be learned…

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  1. Ondoy Tragedy Lessons to be learned… Fernando Siringan ando.msi@gmail.com Marine Science Institute, University of the Philippines Diliman Gemma Narisma, Rosa Perez, Celine Vicente, Antonia Loyzaga Manila Observatory Mahar Lagmay National Institute of Geological Sciences, University of the Philippines Diliman

  2. http://earthobservatory.nasa.gov Observations from TRMM from Sept 21 to 27, 2009

  3. Wind Direction and Wind Speed • The highest rainfall was measured in Manila Observatory between 9 AM and 1 PM. • Highest rainfall of 61.4 mm/hr was measured at 10 AM

  4. Weekly Accumulated Rain = 459.8 mm Daily Rain 26 Sep 2009 368.6 mm • “Ondoy” brought a total of 368.8 mm of rain over Manila Observatory on September 26, 2009. • This daily rain measured in Manila Observatory is higher than the monthly normal (330.3mm) in the Port Area.

  5. 330.3 mm Rain Accumulated in Southern and Central Luzon (21-28 Sep 2009) http://www.nscb.gov.ph/headlines/StatsSpeak/2009/030909_rav_climatechange.asp Accumulated rainfall over a week measured by TRMM was over 500 mm in Metro Manila. This value is higher than the monthly normal. http://trmm.gsfc.nasa.gov/trmm_rain/Events/manila_rainfall_perspective_21-28sep09.jpg Ondoy is an extreme event. But is Ondoy a manifestation of climate change?

  6. Daily rainfall at Science Garden (1961-2004) 334.5 267 276.5 246.4 223 Daily rainfall at Port Area (1961-2004) 403.1 371.4 265.4 257.4 252.8

  7. Essential parts of a warning system … Since rainfall is highly variable even over a small area, it is best to have a good network of weather stations – data can be viewed by anybody through the internet. A similar network of river gages – also viewable through the internet. Water level of dams can be included in the network.

  8. What is causing our worsening floods? River channel capacities are decreased by… • Encroaching houses and fishponds • Increased siltation due to deforestation • Garbage

  9. Fishpond encroachment (from DPWH) Sta. 7+000 250m Malusac River Guagua River 130m River Pasac

  10. Where is the river?

  11. In Metro Manila worsening floods are typically attributed to insufficient drainage … drainage structures are outdatedrelative to degree of urbanization … some are century old channel capacities decreased by… encroachment of structures … filling up of river channels … by sediments and garbage disappearance of about 21 km of small rivers (Zoleta-Nantes, 2000) – converted for housing or other uses by government and private entities Climate change?

  12. Longos area River Where is the river? Letre Rd Missing river?

  13. Other causes of worsening floods • Decrease in floodplain area • Urban sprawl • Flood control dikes • Fishponds • Global sea level rise • Land subsidence. The least understood, but very important.

  14. Land subsidence Lowering of land, even large areas, due to: • Natural compaction of underlying soil and sediment • Compaction sped up by rapid withdrawal of groundwater

  15. Long-lasting floods that now last for months occur in… coastal CAMANAVA and the Pateros-Taguig-Muntinlupa area where floods are due to elevated lake levels during the rainy season But in CAMANAVA, floods can be caused by high tide alone… but this was not so in the past.

  16. 1770 MLD in 2004 (CEST 2004) 778 MLD 250 MLD <20 million liters per day (MLD) At Manila’s South Harbor mean sea level rose at about 2 millimeters per year from 1902 to the early 1960’s. . . . . . then started rising ten times as fast. WHY? 989 MLD in 1990 (JICA 1992) Groundwater withdrawal! Metro Manila`s groundwater demand is still increasing. Consequent subsidence will continue and may even accelerate !

  17. How groundwater withdrawal causes land to subside Pumps extract water from“aquifers” –layers of sand and gravel soaked with water. Pipe appears to rise out of the ground If too much water is pumped out of the aquifer, the pressure is reduced in the pore spaces between grains of sand and gravel. Water in the clay layers is sucked into the aquifer. This causes the clay layers to shrink . . . We need to lessen our dependence on groundwater. . . . and the ground surface to sink.

  18. Tuff 1.46 0.76 1.40 0.79 0.16 0.88 0.96 0.57 0.77 0.95 0.95 0.64 0.96 0.59 0.73 0.67 0.51 0.61 Coastal/Alluvial 0.57 Lake/Alluvial 0.46 0.46 0.47 0.43 0.46 Benchmarks used for re-leveling (1978 vs 2000) survey (Jacob 2004) Maximum magnitude of subsidence – 1.46 m (6.1 cm/y) Change in height of maximum high tide, from 1991 to 2002 based on social survey ranges from 0.5 - 1.0 m (4.5 – 9.1 cm/y) There appears to be an acceleration in rate of subsidence in recent years. Lateral variation in magnitude of subsidence is not a function of underlying lithology.

  19. Piezometric Surface 2004(CEST, 2004) Piezometric Level 1951 (NHRC, 1991) -120

  20. Trace of West Marikina Valley Fault M 6-7 earthquakes recur every 200 to 400 years (Nelson et al., 2000) and have an associated ~0.5 m vertical displacement (R. Rimando pers. comm.) Subsidence along the Marikina Valley can be due to combined natural, earthquake and human induced compaction. (Sources: 1:50,000 topographic maps of Namria, 1990 and 1992)

  21. Other SE Asian sites experiencing subsidence due to over extraction of groundwater Tokyo, 1918-87 4.5 6.5 Japan Osaka, 1934-68 2.8 8.2 Japan Tokyo Osaka Shanghai, 1921-65 2.63 6 China Shanghai Yun-Lin Yun-Lin, 1989-97 0.66 8.25 Taiwan (Fishpond area!) Hanoi Manila Hanoi, 1988-93 0.1-0.3 2-6 Vietnam Bangkok CAMANAVA1991-2002 0.3 - 12.7 – 9.1 Bangkok, 1980-90 0.5-1 5-10 Thailand Jakarta, 1991-99 0.3-0.8 4-10 Indonesia Jakarta SUBSIDENCE LOCATION PERIOD Meters cm/year

  22. Bathymetric data based on 1997 to 1998 hydrographic surveys Lake surface area 869 km2 Total lake volume 1.95 x 109 m3 Average water depth 2.24 m Bathymetric data based on 1920 to 1939 hydrographic surveys Lake surface area 922 km2 Total lake volume 2.32 x 109 m3 Average water depth 2.52 m

  23. Change in Bathymetry (1939 to 1968)

  24. Change in Bathymetry (1968 to 1998) The Manggahan Floodway is a major source of sediment.

  25. Immediate and long-term solutions to Laguna de Bay inundation … Increase the number of channels draining the lake - widen Taguig River and revive other rivers in Taguig Delta Plain. For the long-term, modification of Napindan structure might be required. Increase the capacity of Pasig River through channel widening – remove major constrictions. … a shallow but wide channel is more efficient than a deep but narrow channel because the gradient is almost flat. … allow planned, episodic high discharges to flush accumulated sediment and other debris within the channel. Control erosion in the watersheds and sedimentation in Laguna de Bay. We need a good forest cover.

  26. The Ondoy tragedy is due to an extreme event … But ….. Magnitude and extent could have been much less Given wider and deeper channel ways Greater capacities of natural retention ponds For the future, among others we should increase, widen and deepen our channel ways, implement easement rules, reforest, allocate retention ponds

  27. FLOOD HEIGHT: 4-5m Houses on stilt in Candaba Photo by J. Ong

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