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The devastating Texas coastal flooding caused by Hurricane Harvey's staggering rainfall and exacerbated by levee breaches and emergency reservoir releases, combined with the July 22 and Aug. 5 rainfall floods in New Orleans, have prompted a new focus on how to respond to flash flooding. Among the pressing issues are reducing flood risk before the deluges occur, and neighborhood-level evacuation plans when a flood is imminent.
"While these flooding events are fresh in everyone's mind, we need to have an urban flash flood summit for New Orleans," said Suzanne van Cooten, director of the National Weather Service's Lower Mississippi River Forecast Center, which monitors water heights in rivers and streams throughout the south central United States. "A frank discussion is needed."
Van Cooten said investing in a very high resolution computer model, one that can capture all aspects of the complex flow of water in urban areas such as New Orleans and Houston, will be needed to provide accurate and timely information to emergency managers -- and the public -- about flood risk. It also will help with evacuation decisions.
Houston already has a model to monitor rainfall and river heights. But it's not yet at the resolution where emergency officials can use it days in advance of rain to specify what areas will flood.
"That will take a lot of work and will take a major investment," van Cooten said. "Are we there yet? No. Should we be there? That's a question for New Orleans and other city's leaders to ask. It's a risk/reward issue."
New Orleans officials say they've already begun that process, reviewing evacuation alternatives and looking at how to model rainfall events. The effort took on a new urgency after the Aug. 5 flood, made worse by broken drainage pumps, swept through several neighborhoods and led to the replacement of the management of the Sewerage & Water Board.
Two weeks ago, city emergency management officials and their counterparts with the Louisiana Governor's Office of Homeland Security and Emergency Preparedness took the unusual step of holding a second, tabletop exercise in the middle of hurricane season to consider the unthinkable: how to respond to a Category 3 hurricane that would stress the city's new surge defenses, or to to a storm such as Harvey that produced rainfall flooding, said Tyronne Walker, communications director for Mayor Mitch Landrieu.
The Water Institute of the Gulf, a Baton Rouge-based think tank, has already begun to develop a sophisticated flood early warning model for the S&WB. It is to combine real-time National Weather Service forecasts, detailed information about the flow of water through above-ground drainage canals and underground drainage pipes and as-it-happens information about the flow of water through pump stations, said Ehab Meselhe, vice president of science and engineering The Water Institute. He is now advising the water board on drainage issues.
Meselhe said The Water Institute began talking with city officials about developing a short-term and long-term water management plan for New Orleans just before the Aug. 5 flood. He said the proposal includes developing an early warning system, a flood forecasting system and evaluating the city's existing drainage plans, including the design and operation of the above-ground drainage canals and subsurface peipes.
"The idea would be to have up-to-date and credible information about bottlenecks in the system and areas prone to flooding," he said. "It would be a suite of models that could be used to evaluate solutions and alternatives with problems in the drainage system. And those engines will be used to link to the National Weather Service and rain forecasting system and provide the city with an early warning system."
Meselhe warned that the proposed model system would not solve New Orleans' drainage problems on its own. But it would provide officials with basic information they need to operate the system and to determine what improvements are possible.
He said the new modeling system should not be too expensive, as it can be built to take advantage of the city's existing computer model for subsurface water movement and a separate model for above-ground parts of the system. "We just need to integrate the subsurface with the above-ground information and the pump operations in real time," he said.
That effort was paused, however, as Harvey threatened the city, to allow officials to speed repairs to drainage pumps and generators. Talks will resume on the plan next week, he said.
The first steps in developing the new system will be to evaluate New Orleans' existing monitoring system, determine its rainfall information sources and identify how the subsurface system is monitored for the passage of water, as well as pump operations.
Once the model is complete, Meselhe said, Water Institute researchers will work with city officials to identify weak spots in the drainage system and potential solutions. They also will plan for rainfall events that are greater than the current system can handle.
"How can you actually manage floodwater better? Can you retain water somewhere in the city?" Meselhe said. "Rain gardens, retention areas, use soccer fields, find ways to absorb the water even if you have to replant grass later. And look at how these practices are already being done elsewhere, how other cities are absorbing rainwater within urban areas."
Meanwhile, the National Weather Service is trying a similar upgrade of its modeling for flooding throughout the U.S., with a new National Water Model to simulate actual and forecast stream flow across the country. The program has already increased the locations where it is collecting data, from the existing 3,600 river forecast points to more than 2.7 million spots, and is using that data to predict water heights.
"The NWM is a new tool in the tool box, and we've recently established a rapid-onset flooding team to further explore the utility of the NWM for flash flood watch and warning applications," said Thomas Graziano, director of the Weather Service's Office of Water Prediction. "Each hydrologic event -- in this case the record setting rainfall and runoff form Harvey -- provides an invaluable data set to look at the performance of our operational models as well as our newer tools such as the NWM."
A team of researchers, led by scientists at the University of Texas at Austin, is hoping to turn the combined information from the water model, local forecasts and detailed local terrain and water flow studies into color-coded maps. They could be used to warn the public of flash flooding dangers, but that's still several years away.
Indeed, the National Hurricane Center's Jamie Rhome, who oversees the issuance of similar color-coded maps that show the potential height of storm surges above ground in advance of hurricanes, as well as new storm surge watch and warning maps, said it took several decades before those products were ready for public use.
Long long before those maps were introduced to the public in 2015, the Hurricane Center, with the assistance of the Army Corps of Engineers, had been producing a series of maps for emergency managers to outline what-if situations called MEOWs and MOMs:
"Maximum envelope of water," a map showing how high surge will be from an individual hurricane along an individual path
"Maximum of maximums," which combines the highest water elevation from several similar hurricane paths along a segment of the coast on a single map.
Emergency managers use the maps to get an idea of what areas are most at risk, and to design their evacuation plans. And the maps can help communities address risk issues in advance of hurricanes.
"As an example, a hospital has its generator on the first floor and the worst case analysis shows that surge saltwater could affect them," he said. "They can decide to elevate the generators."
But turning such maps into tools the public can use to inform evacuation decisions for rainfall events will require a rethinking by the public of of flood risk, said Brian Wolshon, a Louisiana State University transportation engineer who specializes in hurricane evacuation plans. Understanding evacuation planning requires knowing the hazards one faces, and understanding one's vulnerability to those hazards, he said.
"You can have a lot of rain, but if you're really high you're not going to be vulnerable," he said. "But if you are, what are your choices from there, particularly in the time and space parameter.
"An emergency manager must think, 'Where are my people? Where do they have to go? What routes are safe? What means of transportation are available? When they leave, how long will they travel? And how much advance notice will they need.'"
Houston's a good example of the state of evacuation planning today, where emergency managers have largely focused on the storm surge threat: the threat of high water moving into the city from a particular location on the coast, even if it reaches parts of the community through rivers and bayous. The MEOWs and MOMs govern evacuation decisionmaking, and the community has a tiered plan for such evacuations, if public officials deem the threat greater than the threat posed by the evacuation itself.
"But you're not going to find those formalized, detailed traffic management plans for a rainfall event," Wolshon said. "We don't know where the rain is going to fall. It's localized, and you can't predict the hazard and vulnerability in advance."
"However, anybody living in low-lying Houston or in New Orleans knows where the problem areas are," he said. "And you don't necessarily have to look at an evacuation as an all-or-nothing event; you can't evacuate 6.5 million people from Houston. But there is the idea of specific or flexible evacuation -- not one size fits al, but a framework of alternatives based on the threat and vulnerability you expect neighborhoods to encounter."
Wolshon points to New Orleans as a unique location because it's surrounded by levees and prone to both surge and rainfall flooding. But he said there might be a model that managers could follow: wildfire evacuations.
"That's a situation where you don't know the source, direction, speed or movement of wildfire," he said. "It doesn't move like the wall of storm surge water. It moves with the wind and hopscotches around. Embers blown thousands of yards away can start a new fire."
In that way, it's very much like the effects of heavy rainfall events, which can inundate an individual neighborhood or even just a few street intersections.
"They look at the weather forecasts and the movement of the fire, but also what's in its path, where are the people located and what are their route to safety options," Wolshon said. "It's actually the fire departments that take the lead role. And there's no plan per se."
That's a bit like the tabletop exercise that city and state officials conducted after the Aug. 5 pump debacle. Walker said the exercise included a review of evacuation options, including for special population groups such as those with health and transportation issues.
The review also included a discussion of when it might be necessary to tell residents to shelter in place, as well as the need to be flexible in determining the best evacuation routes out of town because of the possibility that some of the routes may also be affected by flooding. In all cases, Wolshon said, the planners stressed the need for residents already to have their own preparedness plans in place.
In the weeks after the Aug. 5 flood, City Hall also has provided new alternatives for informing residents and visitors of flooding problems. They include:
Extensive updates to its NOLA Ready website
Greater emphasis on using the city's mobile application to receive flood and other weather alerts by phone
Distribution of orange-and-white traffic barricades near often-flooded intersections
Posting of new signs at underpasses that can be used to determine floodwater heights.