City flooding: Crux of the matter (First part of a series)

When drainage expert Engineer Maarten Vierhout came to the city for a two-week study of our drainage system, people who knew it or those who cared to know may be excited about the answer to the lone question: Is there a solution to the perennial flooding within the city? By the way, Engr. Vierhout is a national from Netherlands who works with PUM, a non-government organization of his country composed mainly of retired experts in various fields. The expert is well-travelled and has some important drainage projects in Southeast Asia under his control and supervision. In the Philippines, he was involved in the flood control project of the Laguna Lake Development Authority. His credentials may well speak of his authority on the matter, thus, he was commissioned to review our drainage master plan.

As a factual backdraft too, the Philippine Institute of Civil Engineers (PICE) was the one who invited Vierhout. PICE is member to the taskforce drainage formed by Mayor Sara Duterte which she signed as Executive No. 2 in 2010. Among the duties outlined in said memo include the conduct of continuing inspection, studies and examination of the flood problem and the current state of the drainage system of our city. As we are posed to answer that previous question, we are also about to determine whether or not there are beans to spill by the Dutch as regards our drainage master plan of 1998 since he made it as his main reference.

Mr. Vierhout has drafted a Summary Report of his findings on February 6, 2012.  With Davao’s topographic location, a portion of the urban center sprawled in the coastal plain. He stated in his report that the coastal strip of 1 to 2 kilometers from the sea shore, the ground surface elevations range between 0.5 to 3 meters above mean sea level (MSL). He noted that the average high tides range between 1.4 to 1.6 meters above MSL. There are two high tide occurrences in a day with durations of approximately 5 to 6 hours per high tide. He said, the probability that a heavy rainstorm over the city jointly occurs with high sea water level, is quite high. During the period of high tide the discharge capacity of the drainage system is very much reduced and rainfall runoff cannot be drained out by gravity. The excess runoff that cannot flow out of the system will be temporarily stored in the drainage system and in low spots of the urban area, until the high tide starts to recede. This storage in rainfall runoff is experienced as temporary flooding in the streets, paved surfaces and low lying areas, the report said.

The flooding is said to be mostly of a few hours duration and only at shallow flood depth (reportedly 10 cm to 40 cm) or up to knee level. He blamed the flooding to several factors like the insufficiency and the partly dysfunctional storm water drainage infrastructure. Based on his report, water flow in the main, lateral and tertiary drains are impeded by reason of damaged drainage structures; insufficient street inlets and manholes; urban structures built in or near the waterways or drains such houses, buildings, roads and driveways; the very huge problem on garbage and debris in the drains; and increased sedimentation due to reduced flow velocity caused by the garbage and debris. 

A significant fact to consider before answering that previous question is the state of completion of the proposed drainage network under the 1998 storm drainage masterplan. It would be relevant to note that only 6% of the proposed main drainage system in the priority areas has been completed so far. These priority areas include: Lizada-Toril, Dumoy-Bago, Matina Aplaya, Ma-a, Buhangin and Panacan. In other words, out of 23, 830 meters of the proposed main drain, only 1, 480 meters was completed so far, or stated otherwise, with the 3, 410 hectares of area covered in the original plan, only 330 hectares was served in the implementation of said masterplan. According to the City Engineer’s Office (CEO), in said report, the main reasons for the delay in its implementation are right of way issues, budget constraints, capacity of the CEO itself, change in the urbanization which entailed a change in the priorities of the city.

However, granting that the drainage network is completed under the masterplan, it is still no guarantee that the city will no longer experience flood. As stated in the expert’s report, it said: “Even when a complete storm water drainage system is constructed, the gravity flow in the lower parts of the system (outfall) will be temporarily reduced or interrupted by the diurnal high tides in the sea and by the high river water levels during heavy rainfall in their upper catchment areas. (Note: In next week’s issue this column will strive to present what the city has actually done to address this problem on perennial flooding). E-mail for comments at rggmjournal@yahoo.com.