John’s Blog/Newsletter: SEA LEVEL RISE NOW

The terrible flooding in Texas associated with Hurricane Harvey is already triggering discussion about solutions. It is complex. There are three primary flooding forces to consider. Storms pushing water in from the sea. Record rainfall. Rising sea level. They operate quite independently but can combine for devastating effect. Communities are caught in the triangulation. Deluge from above. Base sea level rising from below. Waves and storm surge coming in laterally from the coast.

If we are to design viable communities for the future, it is imperative to understand the dynamics of each flooding force and why the design solutions can be quite different.

Rainfall: Harvey demonstrated truly record levels of rain, more than a foot in a few hours. As covered in a post two weeks ago (Rain Rain Go Away…) the warmer ocean temperature means more evaporation, which must come down as more precipitation, in this case rain. Extreme rainfall can trigger the related problem of Runoff. That can cause even greater flooding as water flows to lower elevations in a city, down a valley, or into a stream / river possibly overflowing river banks hundreds of miles away. As the ground saturates and can no longer absorb the rainfall, flooding can get suddenly worse.  Adding to the problem, Houston does not have the kind of zoning restrictions that most cities use to allocate development density and plan for adequate drainage at the scale of a master plan.

Storm surge: At its core, Harvey was a powerful hurricane that took shape rather suddenly in the Gulf of Mexico. Aside from the wind and rain, the special threats from a major storm are the huge waves at the coast and the storm surge. The cyclone force of a hurricane essentially “sucks” a huge quantity of seawater with its low pressure and pushes a virtual giant wave or wall of water as it moves. That surge of water creates a special problem when it is pushed into a confined, or even semi-confined space. This usually happens when the water piles into a bay, a harbor, or inter-coastal waterway. In Texas this happened as the huge water volume got behind the coastal barrier islands, and into the bays, canals, estuaries and harbors from Corpus Christi to Houston / Galveston.

Sea level has just started to rise. The primary driver is the melting of ice on land, the glaciers and ice sheets largely on Alaska, Greenland, and Antarctica. (Thermal expansion of seawater also has been an important component of rising sea level over the last century, but the melting of ice has now overtaken that as the significant factor and will continue to do so.)  Rising sea level has been modest to date, but will soon start to rise at a galloping rate. (Beware the Doubling Time for Rising Sea Level) The key difference with sea level rise from the other two primary flood factors is that it is slow and stealthy. We tend to ignore it as we focus on the big events of storms and heavy rainfall. Yet as it rises inch by inch and foot by foot over the coming decades it will cause essentially permanent damage all over the world. Even now rising sea level is already making its presence known. In coastal areas all over the world, following the full moon and alignment of the planets, the extreme high tides are getting higher and higher — due to rising sea level. Sometimes called “king tides” this routine flooding is referred to as “sunny day flooding”, noting that it is unrelated to bad weather. Rising sea level is a particular problem because we have now passed the tipping point and it will continue to rise for centuries. Rising sea level is a special problem as it creates a higher base level making flooding from rainfall and storms even worse.

We are caught in between, at the convergence of all these flooding forces. Even engineers, architects, and planners find themselves challenged in terms of how to plan for these new flooding extremes where the past no longer predicts the future. The disaster demonstrated by Hurricane Harvey in the Texas and Louisiana area was a strong illustration of the challenge to design viable solutions in the face of the rapidly changing real environment. Solutions have to consider the extremes of each of these flood forces in this new environment, and consider their possible combination.

Harvey demonstrated how quickly drainage systems can be overwhelmed. Pavement, concrete, and buildings all act to stop water getting into the ground. Solutions for rainfall and runoff will require more robust drainage systems including retention ponds, larger storm culverts, pumps, and areas to absorb the water.

Storm surge from the sea is entirely different. One proposed solution to deflect the storm surge at Galveston, at the mouth of the Houston Ship Channel is an $11 billion storm barrier. This would be similar to the giant gates at Rotterdam (Maeslantkering), the Thames Barrier in London, the gate at St. Petersburg-Russia or the more modest one at Providence, Rhode Island. Those are reasonable engineering inventions to be closed temporarily in advance of the storm surge and waves.

But those structures designed to stop waves and storm surge will not stop slowly rising sea level. A dam or water-tight barrier to protect against rising sea level would need to accommodate shipping traffic efficiently, posing a different design challenge. Also gates or barriers also do nothing to solve the problem of record rainfall and runoff as occurred with Hurricane Harvey. In fact, many storm surge barriers or sea walls could actually act to retain the flooding from rainfall, making the problem even worse.

With extreme rainfall, the solutions to prevent flooding has to be drainage systems, elevating buildings, or even relocating low-lying neighborhoods. Given the estimated six million people in the greater Houston area, that is a Texas sized challenge. Realistic solutions should also include revisions to building codes, zoning, and restoration of wetlands. The point is that we need to design for and adapt to this new reality recognizing the very different flooding forces.

Though a somewhat separate issue, we need to take our foot off “the gas pedal.” The carbon dioxide and other greenhouse gases that are trapping heat and warming our planet need to be reduced ASAP. That means instituting policies to get off of fossil fuels and reduce the level of greenhouse gases that correlate with the warming temperatures. We must work to slow the warming and begin adaptation, simultaneously and aggressively.