“The term 500-year flood can be a little misleading,” said Robert Holmes, U.S. Geological Survey (USGS) national flood specialist, in a press release. “We hydrologists realize the term has instant public recognition and we use it to point to the extraordinary nature of such floods. However, the occurrence of a 500-year flood doesn’t depend on what happened last year or 15 years ago or 100 years ago,” he said. “It’s based on the annual likelihood of the degree of flooding.”
In an interview with WE&T, Holmes said, “I’d like to get away from the use of the terms 100- and 500-year floods. I think that sets up something in people’s minds, especially the lay public, that really is not there.”
He added that hydrologists should label these floods as the 1% chance flood or the 1% annual exceedence flood for the 100-year flood because in any particular year, there is one chance in 100 that such a flood will occur. Likewise, a 500-year flood would be called a 0.2% chance flood because there is one chance in 500 that it will occur.
“There’s a lot of talk in technical circles that we should be emphasizing [a percentage-based] kind of terminology as opposed to these T-year terminology names,” Holmes said. “It’s one of the things that it’s hard to educate the public on because they think we’re just playing technical games.”
One challenge with this labeling structure is that not all 100-year floods are created equal. In fact, a pair of 100-year floods in the same place could be different.
A 100-year storm in Virginia can be completely different from a 100-year storm in Illinois. Likewise, a 100-year storm in the Piedmont plane — a plateau region located in the eastern United States between the Atlantic Coastal Plain and the main Appalachian Mountains, stretching from New Jersey in the north to central Alabama in the south — can be completely unlike a 100-year storm along the Atlantic coastal plane, Holmes said.
Holmes explained that all kinds of differences such as geographic region, slope of the basin, soil type, and more, all contribute to the way a particular basin responds to rainfall.
In addition, as the models compile more data over time and develop a better image of the flood likelihood in a certain area, the definition of a 100-year, or 1% chance, flood in a particular area can be identified more or less severe.
“Since we constantly refine our models and revise our statistics based on the observed record, this year’s data in the affected region is likely to raise the value of the flow rate required to designate a 500-year flood,” Holmes said in the USGS news release. “That means it’s possible that the current flood could lose its standing as a 500-year flood as the data is further analyzed.”
Moreover, Holmes told WE&T that urbanization can increase the magnitude of flood peaks for a particular flood designation. “What you might have had 40 years ago for a 100-year flood in a particular basin is going to be a lesser value than you would have today,”’ he said.
From a preparation point of view, this means that a system built 25 years ago to handle a 100-year storm, possibly could not handle today’s 100-year storm, especially in areas where there has been much urbanization, Holmes said.
So far the data do not support increased flood severity on a national scale due to climate change, Holmes said. However, he added, there are some locations throughout the country where more severe flood trends are developing.
More Data Mean Less Errors
Regardless of the variation among 100- or 500-year floods, predicting their occurrence relies on stream-gauge data being fed into probabilistic models. The more data available, the less uncertainty about the predictions, Holmes said.
There can be large amounts of error surrounding floods. Holmes mentioned an instance he was researching in which a stream of 5180 km2 (2000 mi2) in southern Illinois had error bands of plus or minus 283 m3/s (10,000 ft3/s) for the 100-year flood.
Holmes explained that eliminating such uncertainty is why USGS strives to keep its stream gauges active for long periods of time. USGS operates a nationwide network of stream gauges that measure the water level and flow in many rivers and streams.
“The more data that is collected, the better idea we have, probabilistically, of what we can expect,” Holmes said. “The longer the record, the smaller the error bands around the estimates become.”
The U.S. National Weather Service uses this stream-gauge data to develop flood forecasts, and the U.S. Army Corps of Engineers uses the data to manage flood control. State and local agencies rely on USGS data to help plan flood response activities, the USGS news release says.
However, another challenge with using T-year designations to label floods is that the method used for the calculations assumes “stationarity” of the data, meaning that there aren’t trends beginning or ending over the course of the data set, Holmes said.
“If you do have some kind of trend or some kind of change, be it by urbanization or by climate change, then you can’t really use those methods on the entire length of the record,” Holmes said.
The Real Tragedy
“Right now, we think about it in terms of these T-years, this flood frequency approach, but I’m not sure that’s totally a good way to do it because a 10-year flood in certain areas can have some pretty devastating effects on people,” Holmes said.
If a 10-year storm hits in an area where floodplain regulations have not been enforced, homes and businesses may be located in the floodplain.
“That’s really the tragedy: We’ve got the human element where their homes are devastated and possibly they were behind a levy system and it failed or was overtopped,” Holmes said.
— Steve Spicer, WE&T
Shades of Gray
As the need for water conservation measures grows, so does the use of graywater reuse systems
As water scarcity continues to be a growing concern in many locations across the United States and around the world, the use of graywater recycling has increased dramatically. Graywater reuse systems, which divert shower, bathtub, sink, and laundry water for a variety of applications, including landscape irrigation, toilet flushing, and even fire suppression, have proven to be a resourceful method for maximizing water use. Indeed, with increasing drought and water shortages, using water that otherwise would be lost to the collection system is a valuable method for conserving potable water.
Art Ludwig, owner of Oasis Design (Santa Barbara, Calif.), a consulting and publishing business that specializes in ecological systems design, including graywater reuse and regulations, said the level of interest in graywater recycling has doubled in the last 18 months. “In addition to a general desire among the population to live more of a ‘green lifestyle,’ there is recognition that, in terms of our water management, we can’t continue in the direction we are heading,” Ludwig said.
Despite public awareness about the need for more water conservation and the proven effectiveness of graywater reuse systems in reducing potable water use, graywater systems still carry a certain amount of perceived risk.
In Georgia, where a continued drought has caused severe water shortages in the northern third of the state, homebuilder Brock Built (Atlanta) is building an environmentally responsible demonstration project called the “Green House,” featuring a graywater reuse system that will divert water from showers and laundry and nonkitchen sinks into a parallel plumbing system that will then filter the water and convey it into an underground storage tank. Local regulations, however, do not permit graywater to be used above the surface; it must be pumped to an underground drip irrigation system that will water landscaping at the root level. Plumbing-code regulations in Atlanta also do not allow graywater to be brought back in the home for use in flushing toilets, although this may be changing soon, as Georgia is updating regulations to permit more use of graywater technologies.
“There is still a perception by some of the public that a child or a pet might come in contact with graywater and become sick,” said Chase Broward, director of development for Brock Built and the project manager for the company’s pilot LEED for Neighborhood Development initiative. Possibly adding to this perception, University of Georgia (Athens) hydrologist Todd Rasmussen said last year that graywater carries a certain amount of risk to public health because of various substances that can be found in graywater, including blood, fecal material, and meat products that have some level of pathogenic contribution.
Despite Rasmussen’s concern, according to Ludwig, there is not a single documented case of graywater-transmitted illness in North America, nor in Australia, where graywater was legally distributed through sprinklers with a 1.8-m (6-ft) throw. “Health officials tend to take an overly narrow view of the hazard from graywater,” Ludwig said. “Sure, there are pathogens in there. But, you have to keep perspective. A City of Los Angeles Graywater Pilot Project showed that dog feces were a far greater contributor to pathogens in yards than graywater irrigation.”
Ludwig added that irrigating vegetation is a good use of graywater. “Plants and soil are extremely effective for treating wastewater,” he said. “You can irrigate with water that has bits of compost, and plants like it.”
In addition to its water-saving benefits, widespread graywater reuse holds strong potential for reducing the load on collection systems. “Less volume entering the system translates to lower electricity requirements for pumping at wastewater treatment plants,” Broward said. “A higher amount of graywater reuse also would work to concentrate the [wastewater], leading to more efficient treatment.”
More graywater systems in operation also hold strong potential to increase the capacity of existing infrastructure. Collection systems would less likely become capacity-constrained as a result of new development. “A lower amount of volume is easier to manage and would extend the useful life of sewer pipes,” Broward said.
State Regulatory Issues
As momentum shifts in the direction of greater acceptance of graywater systems, more cities and municipalities are considering or adopting guidelines to regulate graywater reuse. For instance, the City of Tucson, Ariz., is considering making graywater systems mandatory in new homes, a measure that, if passed, would take effect in 2010. According to Tucson Water, graywater systems can save a typical household 49,205 L/yr (13,000 gal/yr) of potable water and result in an annual savings of $52 on water in wastewater bills. Also, in Oregon, the State Plumbing Board recently approved new standards that allow graywater systems to be installed in single-family homes, townhouses, and row houses that reuse wastewater for flushing toilets.
Jonathan Gray, principal at Interface Engineering (Portland, Ore.) and vice chair of the Oregon State Plumbing Board, said the move is an important step for graywater reuse in the state, but more regulatory approval is still needed. “Graywater reuse in commercial and multifamily buildings, where the true water savings can be realized, is still not allowed,” he said.
Oregon’s case is a good example of the kind of regulatory environment that graywater advocates must navigate in order to get graywater reuse systems approved. Following that approval, legislation is being prepared in Oregon for a 2009 session that would extend graywater reuse to commercial applications. However, challenges remain. “Oregon’s Department of Environmental Quality and the Department of Health Services have raised concerns about reuse in a commercial setting,” Gray said. “Also, the [Department of Environmental Quality] still does not distinguish between gray[water] and black water. So, to pass this legislation, it is critical to first redefine the definition of graywater in order to establish a separation.”
Ultimately, Gray said he wants to see an all-encompassing and prescriptive graywater law established to allow for best practices to be implemented that also consider public health and safety issues. “Our goal is to be the model for the rest of the U.S. for how graywater reuse can be regulated and utilized,” he said.
In terms of graywater regulation nationwide, Ludwig said there is a “sweet spot” for guidelines that optimize safety and environmental benefit. “If graywater laws are overly stringent, homeowners won’t comply,” he said. “They’ll just install systems secretly, without the benefit of guidance from codes or assistance from professional plumbers, who are more reluctant to break the law.” He said California’s law is a good example of one that is too stringent. “California’s graywater compliance rate is vanishingly low because the laws are unrealistic,” he said. “They mandate design specifics that have little to do with practical application.”
However, Ludwig said Arizona and New Mexico’s approach to regulating graywater systems makes the most sense and is the preferred model to emulate. “Arizona’s graywater laws use a straightforward, commonsense approach,” he said. “Instead of being concerned with how a system is built, Arizona and New Mexico regulators focus on system functionality and performance goals. This creates a favorable climate for innovation and helps keep technical progress from outdating the law. There is one permit which automatically covers everyone in the state who follows the guidelines. Thus, the compliance rate is very high. Most importantly, professional plumbers and builders are free to implement graywater systems and share their experiences with evolving best practices.”
Global Interest Rising
One example of the growth in overseas interest for graywater reuse systems is the expansion of Brac Systems (Montreal), a company that designs and implements residential and commercial graywater recycling systems, mainly for use in flushing toilets. Although the company has been in operation for only 3 years, it already has installed graywater systems in 20 countries and experienced growth of about 300% per year.
“The market for graywater recycling has really only begun,” said Dennis Yasar, company president. “With water demands rising at the same time that our water resources are diminishing, reusing graywater is becoming more important as a tool to increase water efficiency.”
According to Yasar, the water savings that can be realized through graywater reuse are considerable. “There are roughly 1.9 million homes being built every year in the United States,” he said. “Hypothetically, if we could install graywater reuse systems into all of those homes over the next 2 years, the water savings alone would be enough to supply the entire population of United States on a daily basis.”
Recently, Brac received approval for installation in Costa Rica and is also in the process of securing a $200 million contract for implementing graywater systems in the Middle East. “Dubai is experiencing incredible growth, but at the same time, they are setting up for a potential serious water shortage,” Yasar said.
In Dubai, where construction is booming, Brac has identified two unique graywater reuse opportunities. One method includes installing the company’s two largest commercial systems for use in labor camps, which often house as many as 700 workers. “Ten times a day, water is trucked into these camps,” Yasar said. “The workers take about two showers a day, which amounts to an average of 25 gallons [95 L] of water use per person on a daily basis. This water, in addition to laundry water, would be recycled back for use in flushing. Due to the high cost of delivered water, the graywater systems are anticipated to pay for themselves in only 2 months.” For labor camps that use compost toilets, recycled graywater would be used for dust control and concrete mixture.
Another area for graywater implementation in Dubai is with air-conditioning systems. “Due to the extreme heat, Dubai residents have their air conditioners running 24 hours a day,” Yasar said. “There is a considerable amount of water overflow from these systems that can be utilized, which would otherwise be lost to evaporation.”
Back in the United States, the company is working with a detention facility in Georgia to install a laundry-water reuse system that is anticipated to cut water use by as much as 40%.
The system, which would filter water from rinse cycles and reuse it in soaking and washing cycles, is estimated to save the facility nearly 12,490 L (3300 gal) of water per day. “After this water goes through its useful life in the laundry system, it can actually be used yet again for flushing toilets,” Yasar said. “By doing that, the facility could cut its total annual water consumption by as much as 60%.”
— Jeff Gunderson, WE&T