Tuesday, March 12, 2013

Exposing largest Philippine urban hub to flooding

An expert warned Metro Manila will further sink from continuous groundwater extraction that's being done to meet demand of the ballooning population here, increasingly exposing this largest Philippine urban hub to flooding and other hazards.

"Metro Manila's groundwater demand continues rising as more people live here so subsidence will worsen," noted Filipino geologist and University of Illinois at Chicago Professor Emeritus Dr. Kevin Rodolfo, referring to sinking of land over time due to excessive groundwater withdrawal that leaves soil particles compact.

He noted latest available data already show onslaught of subsidence in various parts of Metro Manila as well as adjacent Bulacan, Cavite and Laguna provinces already.

Dr. Rodolfo warned such occurrence is raising the areas'vulnerability to sea level rise (SLR), which experts identified as one impact of climate change, as well as to salt water intrusion and flooding.

Subsidence, storm surges and liquefaction are the three reasons Dr. Rodolfo cited in opposing Manila Gold Coast Development Corporation's proposal to reclaim part of Manila Bay for mixed-use purposes.

Citing a study he and his colleagues conducted, Dr. Rodolfo reported data show subsidence-plagued Metro Manila areas include Pateros municipality as well as Valenzuela, Caloocan, Malabon, Manila, Taguig, Las Pinas and Muntinlupa cities.

"Navotas City is below seal level already," he warned.

He said subsidence is also gripping Bulacan province's Malolos City as well as Guiguinto, Marilao and Obando municipalities; Kawit and Dasmarinas municipalities in Cavite province and Laguna province's San Pedro-Binan area.

"Land's sinking as more water is being extracted and used - more population creates need for more water," Dr. Rodolfo noted.

Bulacan, Cavite and Laguna provinces are adjacent to Metro Manila and host this urban hub's spill-over population.

Dr. Rodolfo reported magnitude of subsidence in some parts of Metro Manila even hit nearly 1.5 meters with the areas sinking at a rate of around 6.1mm/year.

He isn't surprised, however, noting groundwater extraction in Metro Manila and surroundings rose and exceeded the 20 million liters per day (MLD) volume withdrawn in the 1930s.

"Groundwater extraction back then wasn't very much but this increased to some 250 MLD in the 1980s, 778 MLD in the 1990s and 1,770 MLD in 2004," he said.

Available data placed Metro Manila's average groundwater recharge as of 1990 at around 564 MLD representing rate at which the liquid's replenished daily, Dr. Rodolfo continued.

"As early as the 1970s, however, groundwater was already being extracted at a rate faster than what nature can provide," he noted.

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