Numerical modeling of storm surge and waves and preliminary engineering against overtopping of Roxas Boulevard Seawall

Author: Eric C. Cruz, Ismael Aragorn D. Inocencio, Jose Carlo Eric L. Santos

Presenter: Eric C. Cruz

Built in the 1960s, RB Seawall is a coastal structure that has protected the embankment road Roxas Boulevard, which is a crucial backbone of the urban road transport system of Metro Manila, and the inland areas of Manila against the storm surges and typhoon waves from Manila Bay (Figure 1). In 2011, this aging infrastructure collapsed at the height of Typhoon Pedring (international name: Nesat). In 2012, it was again overtopped, though not crushed, by Typhoon Gener/Saola. In 2013, a coastal engineering study was commissioned by the national public works and highways agency DPWH to study the causative processes of wave overtopping of the seawall and serve as basis of the subsequent engineering design of the rehabilitated seawall. 

This paper presents the methodology for the study of the storm hazards and the quantification of their intensities on the coastal structure. Seabed modeling was based on a consolidated bathymetry of the offshore region using GEBCO data, and of the nearshore region based on a local bathymetric survey carried out by a PPA team. The study identified the 30 strongest historical typhoons that tracked the seawall, and a cyclonic wind model was used to determine their wind shear stress and atmospheric pressure loadings on the sea surface. The Advanced Circulation numerical model was used to simulate the storm tides resulting from historical storm surges, the envelopes of which along the 1.35-km-long seawall were used to synthesize the highest sea surface elevation. Then a quasi-stationary nonlinear wind wave model was used to determine the typhoon waves occurring simultaneously with the storm tides to determine the storm wave heights along the seawall’s toe line and their resulting wave runups along the seawall.

The synthesis of the intensities of these coastal hazards suggested a re-entrant geometry of the rehabilitation design of the seawall, and NOCE (non-overtopping crest elevation) condition necessitated a crest elevation of MSL+4.7m at the seawall’s southern end (towards the Army-Navy marina). Geotechnical investigation data carried out after preliminary engineering phase indicated that raising the seawall crest by at most 2.2 m will be more cost-effective than a scheme involving jointly increasing the wall to less than NOCE but requiring an offshore breakwater to reduce the wave hazard component. After the engineering design and analyses, the site development of the promenade area was assimilated into the construction drawings that allows for a continued view of the bay’s sunset when the new seawall is in place.

Conference/s:

First National Scientific Conference on Water, 2019 November 7-9

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