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JOURNAL OF ENVIRONMENTAL SCIENCES 59 (2017) 39– 47 Available online at www.sciencedirect.com ScienceDirect www.elsevier.com/locate/jes Street floods in Metro Manila and possible solutions Alfredo Mahar Lagmay1,2,⁎, Jerico Mendoza2, Fatima Cipriano2, Patricia Anne Delmendo2, Micah Nieves Lacsamana2, Marc Anthony Moises2, Nicanor Pellejera III 2, Kenneth Niño Punay2, Glenn Sabio2, Laurize Santos2, Jonathan Serrano2, Herbert James Taniza2, Neil Eneri Tingin2 1. National Institute of Geological Sciences, University of the Philippines, Quezon City 1101, Philippines 2. Nationwide Operational Assessment of Hazards Phil-LiDAR 1 Flood Modelling Component, UP NIGS, Quezon City 1101, Philippines ARTICLE INFO ABSTRACT Article history: Urban floods from thunderstorms cause severe problems in Metro Manila due to road Received 14 December 2016 traffic. Using Light Detection and Ranging (LiDAR)-derived topography, flood simulations Revised 1 February 2017 and anecdotal reports, the root of surface flood problems in Metro Manila is identified. Accepted 6 March 2017 Majority of flood-prone areas are along the intersection of creeks and streets located in Available online 10 March 2017 topographic lows. When creeks overflow or when rapidly accumulated street flood does not drain fast enough to the nearest stream channel, the intersecting road also gets flooded. Keywords: Possible solutions include the elevation of roads or construction of well-designed drainage Flood modeling structures leading to the creeks. Proposed solutions to the flood problem of Metro Manila may LiDAR avoid paralyzing traffic problems due to short-lived rain events, which according to Japan Urban flooding International Cooperation Agency (JICA) cost the Philippine economy 2.4 billion pesos/day. © 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V. Introduction Apart from devastating floods like those spawned by Tropical Storm Ondoy in 2009 (Lagmay et al., 2010) and the typhoon- Metro Manila is located on an isthmus between the Manila enhanced southwest monsoon rains in 2012, 2013 (Lagmay et al., Bay and Laguna de Bay. The entire region is composed of one 2014) and 2014, more frequent floods caused by short-lived major catchment called the Marikina River Basin, which thunderstorms are also a problem. Once parts of the road covers 535 km2, and eight smaller, river sub-basins, which network are blocked by floods, traffic develops and paralyzes the cover 683 km2 that drain directly into Manila Bay and Laguna entire city. According to JICA, traffic jams due to thunderstorm- de Bay. The Marikina, Pasig, San Juan and Tullahan rivers related flashfloods costs PhP 2.4 billion a day from wasted serve as the main outlets for a network of tributaries of the gasoline and lost economic productivity (Rodis, 2014). Marikina River Basin and smaller catchments of Metro Manila Flashfloods are traditionally blamed on the loss of infiltra- (Fig. 1). Highly urbanized and populated by almost 12 million tion due to urban concrete, a century-old drainage system, residents (Cox, 2011), the metropolis lies on one of the widest and clogged streams. This study analyses nuisance floods floodplains in the Philippines. caused by brief, heavy downpours. It identifies other factors to ⁎ Corresponding author. E-mail address: [email protected] (A.M. Lagmay). http://dx.doi.org/10.1016/j.jes.2017.03.004 1001-0742/© 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V. 40 JOURNAL OF ENVIRONMENTAL SCIENCES 59 (2017) 39– 47 Fig. 1 – Metro Manila natural drainage. (A) Location of Metro Manila, (B) administrative boundaries of component cities and (C) watersheds and tributaries. find relatively inexpensive solutions to flood-generated traffic where V is the average velocity in m/sec, h is the flow depth in problems. meters, and i is the excess rainfall intensity in mm/24 hr. Other variables are slope (S), acceleration due to gravity (g), pressure 1. Methods Table 1 – Metro Manila Development Authority list of flood-prone places in Metro Manila. The Metro Manila Development Authority (MMDA) released Street name City a list of flood-prone areas in the National Capital Region 1. Espana–Antipolo–Maceda Manila (Table 1), verified by accounts collected from photographs 2. P. Burgos (City Hall) Manila posted in social media. 3. R. Papa, Rizal Avenue Manila Crowd-sourced data (Fig. 2a) were overlaid on a 100-year 4. Buendia Extension–Macapagal Avenue Manila rain return flood-hazard map (Fig. 2b, NOAH, 2013). 5. Buendia–South Superhighway (northbound) Manila – LiDAR-derived topography was used to create profiles of the 6. Buendia South Superhighway (southbound) Manila 7. Osmeña–Skyway (northbound) Makati main roads in these areas, as well as profiles of the road sides. A 8. Makati Makati Roces Street and CP Garcia Avenue in the University of the 9. Don Bosco Makati Philippines (UP) were also examined. Field work was also 10. EDSA Pasong Tamo, Magallanes Makati conducted to check the drainage crossing the streets in those 11. West Service Road, Merville Paranaque areas. 12. East Service Road–Sales street Muntinlupa Floods were simulated in FLO-2D GDS PRO using the St. Venant 13. McKinley Road Taguig equations for continuity and momentum (Eqs. (1) and (2)) and 14. C-5 Bayani Road Taguig 15. C-5–BCDA Taguig the finite-difference scheme to compute flood velocities: 16. C-5 Bagong Ilog Pasig – ∂ðÞVh ∂ðÞh 17. EDSA SM Megamall Mandaluyong þ ¼ i ð1Þ 18. EDSA–Camp Aguinaldo Gate 3 Quezon ∂ðÞx ∂ðÞt 19. Quezon Ave.–Victory Ave./Biak na Bato Quezon 20. NLEX–Balintawak Cloverleaf Quezon V 1 ∂ðÞV ∂ðÞV 21. North Avenue fronting Trinoma Mall Quezon ∂ðÞh g g S ¼ S− − − ¼ 0 ð2Þ 22. EDSA–North Avenue Quezon ∂ðÞx ∂ðÞx ∂ðÞt 23. Philcoa area Quezon JOURNAL OF ENVIRONMENTAL SCIENCES 59 (2017) 39– 47 41 Fig. 2 – Flood-prone areas (a) plotted by netizens, and (b) overlain on a flood map. ∂ðhÞ ð∂ðVÞÞ ð ∂ðVÞÞ gradient ∂ðxÞ , and the local ∂ðtÞ and convective V ∂ðtÞ Five sites have bridges (Appendix A): Philcoa, R. Papa, C-5 accelerations. These are solved using the finite-difference Bagong Ilog, Osmeña–Skyway, and Don Bosco. The street at scheme to get the velocity across the boundaries in eight Philcoa stands 3.8 m above the creek bottom with a rectangular, potential flow directions of every grid element. 2.37 × 4.4-m culvert perpendicular to the road and two circular, The simulations used 1 × 1 m LiDAR-derived elevation data. 1 m-diameter culverts parallel to the road. R. Papa is 1.38 m The floodplains were delineated into catchment areas based on above the creek bottom. In C-5 Bagong Ilog, the street is the flow direction and accumulation. Manning's coefficient of 3.92 m above the stream bottom. Along Osmeña–Skyway is a 0.03 was assigned to streams, which is the normal value for 22.3-m bridge 3.34 m below street level. A stream with its bed main channels (Chow, 1959), and 0.15 to the floodplains which 4.5 m below South Luzon Expressway in the Don Bosco area is are predominantly concrete. Inflow and outflow nodes were drained by a parallel 4 × 2.5-m drainage structure. Eight places assigned based on where the water flows in from the upper do not seem to have drainage networks, which could be masked watershed and out through the main stream channel. by overlying concrete. Rainfall is simulated as a non-point source carrying water throughout the model. Once flood-prone areas were identified 2.2. Roads in UP Diliman from the 100-year flood-hazard map, higher-resolution simu- lations in sub-basins of concern were conducted for short- Both UP creeks are headwaters of a drainage network. A Roces lived thunderstorms. An hour of rainfall with intensities of 30– is 4 m and CP Garcia is 1.25 m above the banks (Fig. 3). In CP 70 mm/hr was used to simulate thunderstorms. Observations, Garcia, there appears to be a bigger channel, where the road is road profiles and flood simulations revealed the causes of street lower in elevation than the bank of the creek. While it is built flooding and indicated appropriate solutions. up above the lowest portion of the channel, it is still below the main channel banks and remains susceptible to flooding (Fig. 3). The streets each have two 1 m-diameter culverts. 2. Results A Roces is sufficiently elevated to avoid flooding even when the creek swells, as during Typhoon Ketsana in 2009. 2.1. Intersection of creeks and streets In contrast, CP Garcia is flooded and impassable even during short-lived torrential thunderstorms. Here, flood depths reached The flood-prone areas list and flood-hazard maps show floods 1 m during Ketsana, and are around 0.9 m during brief at intersections of streets and creeks, and at ponded areas thunderstorms. such as Padre Burgos in Manila (Fig. 2). Road profiles reveal that they follow topographic lows and are not significantly 2.3. Thunderstorm flash-flood scenarios elevated from roadsides and creek banks (Appendix A). R. Papa is lower than the roadside and lies below the tops of Floods during short-duration thunderstorms block traffic, as three stream segments. EDSA-North Avenue floods at its exemplified by four sites in the MMDA list: Bayani Road, lowest portion, which follows the topography of a 250-m wide, Taguig City, Victory Avenue, Quezon Boulevard, R. Papa 1-m high channel. Street, Manila, and EDSA–North Avenue (Fig. 4). 42 JOURNAL OF ENVIRONMENTAL SCIENCES 59 (2017) 39– 47 Fig. 3 – Profiles of (a) A Roces Street and (b) CP Garcia Avenue. Transect a–a′ is brown in the profile; transect b–b′ is green. (c) A Roces and CP Garcia creek during (d) summer and (e) a short-lived thunderstorm.