Concave seawalls will worsen storm surge

This is in reaction to the news item “New seawall design pushed.” (Inquirer, 10/03/11)

The item reveals that Francis Tolentino, chair of the  Metropolitan Manila Development Authority, is proposing to change the shape of the Roxas Boulevard seawall from a “straight line to a concave (curving inward) so that the force created by the strong waves slapping against the seawall would be evenly distributed.”

On the contrary, with a curve-shaped seawall, a feature similar to a cove, the waves may funnel towards Roxas Boulevard, which would increase the probability of building up the heights of the waves and the water pressure associated with storm surge or tsunami waves between the narrowing sections of the seawall. Historical storm surge and tsunami events in the Philippines and elsewhere in the world show that adverse effects of these two types of phenomena were worst in coves and river mouths pointing toward the direction of the waves.

The news item also reports that sandbags had been put up in the breached portion of the seawall. If a storm surge similar to the recent one would strike again, the sandbags may not withstand the waves.  Gabions filled with cobbles would serve better than sandbags.

—ROLANDO G. VALENZUELA

4 Pagasa Compound,

Tandang Sora Avenue,

Old Balara, Quezon City