Climate Change and Storm Surge

The Forgotten Factor 1

Ricardo A. Alvarez 2

CLIMATE CHANGE TROPICAL CYCLONES

Global Warming

Sea Level Rise Storm Surge

EXACERBATED IMPACTS

Higher sea level = deeper faster flowing storm surge + higher breaking waves + faster floating debris, resulting in higher hydrodynamic pressure, stronger wave impact and higher wave run-up, stronger debris impact.

Continually growing potential for damage to the built environment in coastal regions, from impact of storm surge driven by recurring hurricanes.

TROPICAL CYCLONES

Damage Components

WIND WATER Positive Hydrodynamic pressure Wind-velocity pressure Wave impact Negative Hydrostatic pressure Wind-velocity pressure Drag Vortices Scouring Eddies Erosion Wind stagnation Wave runup Flying debris impact Floating debris impact

Water 800 x density of air @ sea level. In the case of storm surge this ratio could reach 1000:1 because of salt water and sediment content.

Water impact Wind impact x 1.9x10 by volume a

a (Assumes: Surge flow = 8.3 m/s, Wind velocity = 53.3 m/s)

1988 Ricardo A. Alvarez 2005 Ricardo A. Alvarez 2005 Ricardo A. Alvarez

QUANTIFYING THE IMPACT OF STORM SURGE

To establish design criteria for a building in coastal locations engineers must quantify the loads acting on it under the impact of wind and water, during a hurricane.

The design methodology is based on calculating the impact energy of the medium (water or air) to quantify the pressure per unit of area acting on the building. This is based on the well known equation for energy:

or energy = mass x square of velocity

In the case of storm surge a two-step process is involved in calculating the actual load applied to the building by the rushing water:

1. Calculate the hydrodynamic pressure using the following equation:

CdV2

2. Calculate the total hydrodynamic load : F = PA

In the case of wave loads the process is similar, but with the additional step of calculating the height of the wave breaking against the building

Where: P = hydrodynamic pressure Cd= drag coefficient = mass-density of water V = velocity of flow F = total force against building A = submerged area of bldg. face

PROBLEM

Velocity of flow is a critical factor in the calculation of loads acting on a building as a result of storm surge and waves.

Relative to this, standards, such as ASCE7, and codes used by engineers, such as the Florida Building Code or the International Building Code are rather general about this specific topic or do not address it at all. SLOSH, the model used by NOAA to simulate storm surge, does not include velocity of flow or waves.

NOAA’s Hurricane Research Division suggests using the speed at which a hurricane is moving as a proxy for velocity of flow for storm surge, but this could result in large uncertainties.

We lack a reliable source of velocity of flow data to use in calculating forces generated by the most damaging component of a hurricane, one which is being exacerbated by climate change!

RECOMMENDATION:

The scientific community in collaboration with relevant federal agencies would make an invaluable contribution by pursuing research, which would allow engineers to have reliable data on the velocity of storm surge flow on a basin-specific basis. This could be incorporated on a storm-surge flow-velocity map similar to the basic wind speed map in ASCE-7, to be adopted by building codes.

1 2011 RICARDO A. ALVAREZ 2 Florida Center for Environmental Studies (CES), Florida Atlantic University (FAU) ricardoalfonso@mitigat.com; www.mitigat.com; (305)931-0871; FAX (305) 931-4704