Flooding results from three primary forces: rainfall, coastal storm surge, and rising sea level, made even worse with by runoff and extreme tides. Recently, hurricanes Harvey, Irma, and Maria showcased the new environmental conditions the world faces as well as the devastating damage that can occur when any combination of these flood types converges on a built community constructed without adequately addressing the increasing threats.
Category 4 Hurricane Harvey rapidly developed within approximately 36 hours in the Gulf. As Harvey made landfall on 25 August 2017, huge waves struck the coastline, storm surge rose even higher in the bays, and a 40-inch deluge of rain caused massive flooding. Flooding worsened over the next few days on the Texas coast as the storm moved slowly eastward toward Houston and Galveston. The shallow warm waters of the northern Gulf and the extensive bays behind the barrier islands nourished and sustained the downpour. Two weeks later, Category 5 Hurricane Irma, with sustained record winds of 185 miles per hour, killed dozens, destroyed nearly all structures on some Caribbean islands, and then threatened the Florida coasts. Less than two weeks after Irma, Category 4 Hurricane Maria, with sustained winds of 155 mph, struck the island of Puerto Rico, causing widespread damage across all of its communities.
Reasons for Unusual & Unpredictable Storms
One reason for these harder to predict, unusual storms is that 93% of the extra heat being trapped in the atmosphere by increased greenhouse gases is being stored in the ocean. The Gulf of Mexico is a relatively shallow isolated body of water that is particularly susceptible to warming. In simple terms, that is why Harvey hit with minimal notice and produced flooding that exceeded models. Higher heat levels can even alter ocean currents like the Gulf Stream and atmospheric currents like the Jet Stream, causing unpredictable storms and weather systems, outside the historical record. Subtle temperature changes in the deep ocean further complicate the picture obtained from the easily measured sea surface temperatures.
Unusual weather patterns, including storms and record-breaking rain, are related to the warmer ocean. Although the usual weather forecasts are becoming extraordinarily accurate, many big pattern phenomena like hurricanes and el Niños are getting harder to accurately forecast. Warmer temperatures are melting vast areas of glaciers and polar ice sheets that will raise global sea level far into the future. Since ice melts at an exact temperature, sea level is a good proxy for global average temperature over centuries and millennia. The natural climate cycles are perhaps best exemplified by the ice ages. Sea level reaching levels unknown for more than 100,000 years add a new dimension to the challenge posed by flooding.
Five Forms of Flooding
Solutions to the extreme flooding from recent hurricanes are complex, with three primary flooding forces to consider: storms pushing water in from the sea; record rainfall; and rising sea level and two secondary level forces, runoff and extreme tides. These forces operate somewhat independently, but can combine for a devastating effect. Communities are caught in the triangulation: deluge from above; base sea level rising from below; and waves and storm surge approaching laterally from the coast. In order to design viable communities for the future, it is imperative to understand the dynamics of each of the five flooding forms and why the design solutions can be quite different.
- Rainfall – Harvey demonstrated record levels of rain, more than a foot in a few hours. The warmer ocean temperature means more water evaporates, putting water molecules and heat energy into the atmosphere in much higher volumes than “normal,” which must come down as more precipitation.
- Runoff – Extreme rainfall can trigger the related problem of runoff, causing far 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 suddenly worsen. (Adding to the problem, Houston does not have zoning restrictions to allocate development density and plans for adequate drainage at the scale of a master plan.)
- Storm surge – 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 wall of water as it moves. That water surge creates a special problem when it is pushed into a confined, or even semi-confined space, such as a bay, harbor, or intracoastal waterway and “piles up” to much greater heights. In Texas during Harvey, this occurred as the large water volume moved behind the coastal barrier islands, and into the bays, canals, estuaries, and harbors from Corpus Christi to Houston/Galveston.
- Sea level – Sea level has just started to rise. The primary driver is ice melt on land (i.e., the glaciers and ice sheets largely on Alaska, Greenland, and Antarctica). Rising sea level has been modest to date, but will soon start to rise at an increasing rate, in fact, almost certainly an exponential one (for further explanation, read “Beware the Doubling Time for Rising Seas”). The key difference with sea level rise from the other two primary flood factors is that it is slow and stealthy. It is often overlooked as communities focus on the big events of storms and heavy rainfall. Yet, as sea level rises over the coming decades, it will essentially cause permanent change to coastal areas around the world. The effects go through marshlands and up tidal waterways rather far inland.
- “King Tides” – Following the pull of the moon and other planets, the oceans change height on a regular tidal cycle, varying somewhat by location. The extreme high tides are often referred to as “king tides” and can be predicted to the minute for various locations. Over the last few decades as sea level rise accelerates, the height and extent of the king tides is getting noticeably greater. From San Francisco Bay to Annapolis and from Vancouver to Miami, this routine flooding is becoming more than a nuisance, even though it recedes in hours. It is driven by and is the harbinger of creeping higher permanent sea level. (“Permanent” here in the sense that sea level is unlikely to go down for millennia.)
At the convergence of these five flooding forms, even engineers, architects, and planners find themselves challenged in terms of how to plan for new flooding extremes, where the past no longer easily predicts the future. In Texas and Louisiana, Hurricane Harvey illustrated the challenge of designing viable solutions for the rapidly changing environment. Drainage systems were overwhelmed. Pavement, concrete, and buildings all stopped water from absorbing into the ground.
The lessons of these three recent storms are profound and powerful. Specifically, Houston needs to implement good zoning, building codes, and storm water management. Bayous are not something to be paved over with urban sprawl, without consequence. Puerto Rico not only needs to rebuild with new building codes, this should be the opportunity to redesign its power system from the ground up. This is also a good opportunity to design a large-scale system of solar, wind, microgrids, and batteries using the latest technology. Despite a potential catastrophic scenario, Florida mostly escaped disaster this time, but may not fare as well with the next storm, and certainly not with sea level rise.
Solutions for rainfall and runoff require more robust drainage systems, including retention ponds, larger storm culverts, pumps, and areas to absorb the water. Storm surge from the sea, though, 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. However, structures designed to stop waves and storm surge would 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 do nothing to solve the problem of record rainfall and runoff. 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 solution to prevent flooding has to be drainage systems, elevating buildings, or even relocating low-lying neighborhoods. Realistic solutions should also include revisions to building codes, zoning, and restoration of wetlands. The key is to design for and adapt to this new reality, recognizing the very different flooding forces.
Though a somewhat separate issue, there needs to be a slowing of carbon dioxide and other greenhouse gas releases that are trapping heat and warming the planet. That means instituting policies to reduce reliance on fossil fuels and reduce the level of greenhouse gases that correlate with the warming temperatures. Slowing the warming process and adapting to changing climates need to be done simultaneously and aggressively.
Adapting communities for the five different forms of flooding will not be easy. The effects are very different, with storms, rainfall, runoff, and king tides causing short-term flooding and sea level rise potentially putting property underwater permanently. Rising sea level is a special problem, as it slowly increases the water levels from other short-lived flooding factors. However, understanding these different aspects clearly can facilitate designs for greater resilience for all of them.
A New Era of Water Challenges
As long as the planet continues warming with increases in carbon dioxide and other greenhouse gases, sea levels will rise, storm surges will likely worsen, and moisture will come down as unprecedented rainfall – or snow, if the local air mass is below freezing. Paradoxically, the above normal temperatures will also cause droughts and wildfires outside of historical patterns.
Preparing for this new era of extreme rainfall will be a huge challenge. Despite differences in preparing for the flooding from storms, extreme tides, rainfall, runoff, and rising sea level, the similarities may help when confronting the “water challenge” in a bold and thorough manner. People, cities, infrastructure, economies, and nations must adapt rapidly to environmental changes. Full recovery for the places affected by these three recent storms will take years. However, perhaps the “1-2-3 punch” of Harvey, Irma, and Maria will be a turning point for discussions and action to deal with the increased flooding in this new era. Those involved with emergency preparedness are on the front lines of flooding response and prevention.