Chapter 11. Sustainable Infrastructure
11.1. Sustainable Transportation: Accessibility, Mobility, and Derived Demand*
Learning Objectives
After reading this module, students should be able to explain
how the automobile-based system of transportation is unsustainable in terms of inputs, outputs, and social impacts
how transportation is a derived demand and how making transportation sustainable depends on land use as well as vehicles and infrastructure
the difference between accessibility and mobility, how they are currently treated by our transportation system, and how a more sustainable system might treat them
What is Sustainable Transportation?
Transportation is a tricky thing to analyze in the context of sustainability. It consists in part of the built environment: the physical infrastructure of roads, runways, airports, bridges, and rail lines that makes it possible for us to get around. It also consists in part of individual choices: what mode we use to get around (car, bus, bike, plane, etc.), what time of day we travel, how many people we travel with, etc. Finally, it also is made up of institutions: federal and state agencies, oil companies, automobile manufacturers, and transit authorities, all of whom have their own goals and their own ways of shaping the choices we make.
Most importantly, transportation is complicated because it's what is called a derived demand. With the exception of joyriding or taking a walk or bicycle ride for exercise, very rarely are we traveling just for the sake of moving. We're almost always going from Point A to Point B. What those points are—home, work, school, shopping—and where they're located—downtown, in a shopping mall, near a freeway exit—influence how fast we need to travel, how much we can spend, what mode we're likely to take, etc. The demand for transportation is derived from other, non-transportation activities. So in order to understand transportation sustainability, we have to understand the spatial relationship between where we are, where we want to go, and the infrastructure and vehicles that can help get us there.
Is our current transportation system in the U.S. sustainable? In other words, can we keep doing what we're doing indefinitely? The answer is clearly no, according to professional planners and academics alike. There are three main limitations: energy input, emissions, and social impacts (Black, 2010).
Energy Inputs
The first reason that our current transportation system is unsustainable is that the natural resources that power it are finite. The theory of peak oil developed by geologist M. King Hubbert suggests that because the amount of oil in the ground is limited, at some point in time there will be a maximum amount of oil being produced (Deffeyes, 2002). After we reach that peak, there will still be oil to drill, but the cost will gradually rise as it becomes a more and more valuable commodity. The most reliable estimates of the date of peak oil range from 2005 to 2015, meaning that we've probably already passed the point of no return. New technologies do make it possible to increase the amount of oil we can extract, and new reserves, such as the oil shale of Pennsylvania and the Rocky Mountains, can supply us for some years to come (leaving aside the potential for environmental and social damage from fully developing these sites). However, this does not mean we can indefinitely continue to drive gasoline-powered vehicles as much as we currently do.
Scientists are working on the development of alternative fuels such as biofuels or hydrogen, but these have their own limitations. For example, a significant amount of land area is required to produce crops for biofuels; if we converted every single acre of corn grown in the U.S. to ethanol, it would provide 10% of our transportation energy needs. Furthermore, growing crops for fuel rather than food has already sparked price increases and protests in less-developed countries around the world (IMF, 2010). Is it fair to ask someone living on less then two dollars a day to pay half again as much for their food so we can drive wherever and whenever we want?
Emissions or Outputs
The engine of the typical automobile or truck emits all sorts of noxious outputs. Some of them, including sulfur dioxides, carbon monoxide, and particulate matter, are directly harmful to humans; they irritate our lungs and make it hard for us to breathe. (Plants are damaged in much the same way). These emissions come from either impure fuel or incomplete burning of fuel within an engine. Other noxious outputs cause harm indirectly. Nitrous oxides (the stuff that makes smog look brown) from exhaust, for example, interact with oxygen in the presence of sunlight (which is why smog is worse in Los Angeles and Houston), and ozone also damages our lungs.
Carbon dioxide, another emission that causes harm indirectly, is the most prevalent greenhouse gas (GHG), and transportation accounts for 23% of the CO2 generated in the U.S. This is more than residential, commercial, or industrial users, behind only electrical power generation (DOE, 2009). Of course, as was explained above, transportation is a derived demand, so to say that transportation itself is generating carbon emissions is somewhat misleading. The distance between activities, the modes we choose to get between them, and the amount of stuff we consume and where it is manufactured, all contribute to that derived demand and must be addressed in order to reduce GHG emissions from transportation.
Social Impacts
If the definition of sustainability includes meeting the needs of the present population as well as the future, our current transportation system is a failure. Within most of the U.S., lack of access to a personal automobile means greatly reduced travel or none at all. For people who are too young, too old, or physically unable to drive, this means asking others for rides, relying heavily on under-funded public transit systems, or simply not traveling. Consider, for example, how children in the U.S. travel to and from school. In 1970, about 50% of school-aged children walked or biked to school, but by 2001, that number had dropped to 15% (Appleyard, 2005). At the same time that childhood obesity and diabetes are rising, children are getting less and less exercise, even something as simple as walking to school. Furthermore, parents dropping off their children at school can increase traffic levels by 20 to 25%, not just at the school itself, but also throughout the town in question (Appleyard, 2005). At the other end of the age spectrum, elderly people may be functionally trapped in their homes if they are unable to drive and lack another means of getting to shopping, health care, social activities, etc. Finally, Hurricane Katrina made it clear that access to a car can actually be a matter of life or death: the evacuation of New Orleans worked very well for people with cars, but hundreds died because they didn't have the ability to drive away.
Another serious social impact of our transportation system is traffic accidents. Road accidents and fatalities are accepted as a part of life, even though 42,000 people die every year on the road in the U.S. This means that cars are responsible for more deaths than either guns, drugs, or alcohol (Xu et al., 2010). On the bright side, there has been a steady reduction in road fatalities over the last few decades, thanks to a combination of more safety features in vehicles and stricter enforcement and penalties for drunk or distracted drivers. Nevertheless, in many other countries around the world, traffic accidents are in the top ten or even top five causes of death, leading the World Health Organization to consider traffic accidents a public health problem.
An additional problem with our current unsustainable transportation system is that much of the rest of the world is trying to emulate it. The U.S. market for cars is saturated, meaning that basically everyone who can afford or is likely to own a car already has one. This is why automobile manufacturers vie so fiercely with their advertising, because they know they are competing with each other for pieces of a pie that's not getting any bigger. In other countries such as China and India, though, there are literally billions of people who do not own cars. Now that smaller, cheaper vehicles like the Tata are entering these markets, rates of car ownership are rising dramatically. While the same problems with resources, emissions, and social impacts are starting to occur in the developing world, there are also unique problems. These include a lack of infrastructure, which leads to monumental traffic jams; a need for sharing the road with pedestrians and animals; and insufficient regulation to keep lead and other harmful additives out of gasoline and thus the air.
What Would Make Transportation Sustainable?
The circular answer to the question is to meet our current transportation needs without preventing future generations from meeting theirs. We can start by using fewer resources or using the ones we have more efficiently. One way to do this is by increasing the efficiency of new vehicles as they are manufactured. Since 1981, automotive engineers have figured out how to increase horsepower in the average American light-duty vehicle (cars and SUVs) by 60%, but they haven't managed to improve miles per gallon at all (see Figure World Oil Production - History and Projections). As gas prices continue to rise on the downside of the oil peak, consumers are already demanding more fuel-efficient cars, and federal legislation is moving in this direction to raise the Corporate Average Fuel Economy (CAFE) standards.
However, simply producing more fuel-efficient vehicles is not sufficient when we consider the embodied energy of the car itself. It takes a lot of energy to make a car, especially in the modern "global assembly line," where parts come from multiple countries for final assembly, and that energy becomes "embodied" in the metal, plastic, and electronics of the car. A study in Europe found that unless a car is over 20 years old, it does not make sense to trade it in for a more efficient one because of this embodied energy (Usón et al., 2011). Most Americans trade in their cars after about a third of that time. A related concept is true for electric cars. In their daily usage, they generate zero carbon emissions, but we should also consider the source of power used to recharge the vehicle. In most parts of the U.S., this is coal, and therefore the emissions savings are only about 30% over a traditional vehicle (Marsh, 2011).
If transportation is a derived demand, another way to meet our current transportation needs is by changing the demand. There are two related aspects to this. First, there is a clear causal link between having more transportation infrastructure and more miles traveled on that infrastructure, and greater economic growth. This is true between regions of the world, between individual countries, and between people and regions within countries. This causal connection has been used as a reason to finance transportation projects in hundreds of different contexts, perhaps most recently in the American Reinvestment and Recovery Act that distributed federal funds to states and localities to build infrastructure in the hopes that it would create jobs. Policymakers, businesspeople, and citizens therefore all assume that we need more transportation to increase economic growth.
However, it is also true that more transportation does not automatically mean more economic growth: witness the state of West Virginia, with decades' worth of high-quality road infrastructure bestowed upon it by its former Senator Robert Byrd, but still at the bottom of economic rankings of states. Furthermore, at some point a country or region gains no significant improvements from additional infrastructure; they have to focus on making better use of what they already have instead. We therefore need to decouple economic growth from transportation growth (Banister and Berechman, 2001). We can substitute telecommunication for travel, work at home, or shop online instead of traveling to a store (although the goods still have to travel to our homes, this is more efficient than each of us getting in our own cars). We can produce the goods we use locally instead of shipping them halfway around the world, creating jobs at home as well as reducing resource use and emissions. All of these options for decoupling are ways to reduce the demand for transportation without also reducing the benefits from the activities that create that demand.
The other way to think about changing the derived demand of transportation is via the concepts of accessibility and mobility. Mobility is simply the ability to move or to get around. We can think of certain places as having high accessibility: at a major intersection or freeway exit, a train station, etc. Company headquarters, shopping malls, smaller businesses alike decide where to locate based on this principle, from the gas stations next to a freeway exit to the coffee shop next to a commuter rail station. At points of high accessibility, land tends to cost more because it's easier for people to get there and therefore more businesses or offices want to be there. This also means land uses are usually denser: buildings have more stories, people park in multi-level garages instead of surface lots, etc.
We can also define accessibility as our own ability to get to the places we want: where we shop, work, worship, visit friends or family, see a movie, or take classes. In either case, accessibility is partially based on what the landscape looks like—width of the roads, availability of parking, height of buildings, etc.—and partially on the mode of transportation that people have access to. If a person lives on a busy four-lane road without sidewalks and owns a car, most places are accessible to him. Another person who lives on that same road and doesn't have a car or can't drive might be literally trapped at home. If her office is downtown and she lives near a commuter rail line, she can access her workplace by train. If her office is at a major freeway intersection with no or little transit service, she has to drive or be driven.
Unfortunately, in the U.S. we have conflated accessibility with mobility. To get from work to the doctor's office to shopping to home, we might have to make trips of several miles between each location. If those trips are by bus, we might be waiting for several minutes at each stop or making many transfers to get where we want to go, assuming all locations are accessible by transit. If those trips are by car, we are using the vehicle for multiple short trips, which contributes more to air pollution than a single trip of the same length. Because of our land use regulations, which often segregate residential, retail, office, and healthcare uses to completely different parts of a city, we have no choice but to be highly mobile if we want to access these destinations. John Urry has termed this automobility, the social and economic system that has made living without a car almost impossible in countries like the US and the UK (2004).
So how could we increase accessibility without increasing mobility? We could make it possible for mixed uses to exist on the same street or in the same building, rather than clustering all similar land uses in one place. For example, before a new grocery store opened in the student neighborhood adjacent to the University of Illinois campus in Champaign, people living there had to either take the bus, drive, or get a friend to drive them to a more distant grocery store. Residents of Campustown had their accessibility to fresh produce and other products increase when the new grocery store opened, although their mobility may have actually gone down. In a larger-scale example, the Los Angeles Metropolitan Transit Authority (MTA) was sued in the 1990s for discriminating against minorities by pouring far more resources into commuter rail than into buses. Commuter rail was used mainly by white suburbanites who already had high levels of accessibility, while the bus system was the only means of mobility for many African-American and Hispanic city residents, who had correspondingly less accessibility to jobs, shopping, and personal trips. The courts ruled that the transit authority was guilty of racial discrimination because they were providing more accessibility for people who already had it at the expense of those who lacked it. The MTA was ordered to provide more, cleaner buses, increase service to major job centers, and improve safety and security. More sustainable transportation means ensuring equitable accessibility — not mobility — for everyone now and in the future.
Making Transportation Sustainable
How do we go about making transportation more sustainable? There are three main approaches: inventing new technologies, charging people the full costs of travel, and planning better so we increase accessibility but not mobility.
New Technology
This is the hardest category to rely on for a solution, because we simply can't predict what might be invented in the next five to fifty years that could transform how we travel. The jet engine totally changed air travel, making larger planes possible and increasing the distance those planes could reach without refueling, leading to the replacement of train and ship travel over long distances. However, the jet engine has not really changed since the 1960s. Is there some new technology that could provide more propulsion with fewer inputs and emissions? It's possible. But at the same time, it would be unreasonable to count on future inventions magically removing our sustainability problems rather than working with what we already have.
Technology is more than just machines and computers, of course; it also depends on how people use it. When the automobile was first invented, it was seen as a vehicle for leisure trips into the country, not a way to get around every day. As people reshaped the landscape to accommodate cars with wider, paved roads and large parking lots, more people made use of the car to go to work or shopping, and it became integrated into daily life. The unintended consequences of technology are therefore another reason to be wary about relying on new technology to sustain our current system.
Charge Full Costs
The economist Anthony Downs has written that traffic jams during rush hour are a good thing, because they indicate that infrastructure is useful and a lot of people are using it (Downs, 1992). He also notes that building more lanes on a highway is not a solution to congestion, because people who were staying away from the road during rush hour (by traveling at different times, along different routes, or by a different mode) will now start to use the wider road, and it will become just as congested as it was before it was widened. His point is that the road itself is a resource, and when people are using it for free, they will overuse it. If instead, variable tolls were charged depending on how crowded the road was—in other words, how much empty pavement is available—people would choose to either pay the toll (which could then be invested in alternative routes or modes) or stay off the road during congested times. The point is that every car on the road is taking up space that they aren't paying for and therefore slowing down the other people around them; charging a small amount for that space is one way of recovering costs.
Traffic congestion is an example of what economists call externalities, the costs of an activity that aren't paid by the person doing the activity. Suburbanites who drive into the city every day don't breathe the polluted air produced by their cars; urban residents suffer that externality. People around the country who use gasoline derived from oil wells in the Gulf of Mexico didn't experience oil washing up on their beaches after the BP disaster in 2010. By charging the full cost of travel via taxes on gas or insurance, we could, for example, pay for children's hospitalization for asthma caused by the cars speeding past their neighborhoods. Or we could purchase and preserve wetland areas that can absorb the floodwaters that run off of paved streets and parking lots, keeping people's basements and yards drier. Not only would this help to deal with some of the externalities that currently exist, but the higher cost of gas would probably lead us to focus on accessibility rather than mobility, reducing overall demand.
Planning Better for Accessibility
The other way we can produce more sustainable transportation is to plan for accessibility, not mobility. Many transportation planners say that we've been using the predict and provide model for too long. This means we assume nothing will change in terms of the way we travel, so we simply predict how much more traffic there is going to be in the future and provide roads accordingly. Instead, we should take a deliberate and decide approach, bringing in more people into the planning process and offering different options besides more of the same. Some of the decisions we can make to try and change travel patterns include installing bike lanes instead of more parking, locating retail development next to housing so people can walk for a cup of coffee or a few groceries, or investing in transit instead of highways.
For example, the school district in Champaign, Illinois, is considering closing the existing high school next to downtown, to which many students walk or take public transit, and replacing it with a much larger facility on the edge of town, to which everyone would have to drive or be driven. The new site would require more mobility on the part of nearly everyone, while many students and teachers would see their accessibility decrease. As gas prices continue to rise, it will cost the school district and parents more and more to transport students to and from school, and students will be more likely to drive themselves if they have access to a car and a driver's license. Putting the new school in a more accessible location or expanding the existing one would keep the school transportation system from becoming less sustainable.
You may have noticed that these proposed changes to increase transportation sustainability aren't really things that one person can do. We can certainly make individual choices to drive less and walk or bike more, to buy a more fuel-efficient car, or to use telecommunications instead of transportation. In order to make significant changes that can reduce overall energy usage and emissions production, however, the system itself has to change. This means getting involved in how transportation policy is made, maybe by attending public meetings or writing to city or state officials about a specific project. It means contacting your Congressional representatives to demand that transportation budgets include more money for sustainable transportation modes and infrastructure. It means advocating for those who are disadvantaged under the current system. In means remembering that transportation is connected to other activities, and that focusing on how the demand for transportation is derived is the key to making and keeping it sustainable.
Review Questions
Explain the concept of a derived demand and how it accounts for the connections between transportation and land use planning.
What is the concept of embodied energy? Why does it suggest that switching to electric cars is not a surefire way to make transportation more sustainable?
Give an example in your daily life that could be used to explain the difference between accessibility and mobility.
References
Appleyard, B. S. 2005. Livable Streets for School Children: How Safe Routes to School programs can improve street and community livability for children. National Centre for Bicycling and Walking Forum, available online: http://www.bikewalk.org/pdfs/forumarch0305.pdf
Banister, D. and Berechman, Y. 2001. Transport investment and the promotion of economic growth. Journal of Transport Geography 9:3, 209-218.
Black, W. 2010. Sustainable Transportation: Problems and Solutions. New York: Guilford Press.
Deffeyes, K. 2002. Hubbert's Peak: The Impending World Oil Shortage. Princeton, NJ: Princeton University Press.
DOE (Department of Energy). 2009. Emissions of greenhouse gases report. DOE/EIA-0573, available online: http://www.eia.doe.gov/oiaf/1605/ggrpt/carbon.html
Downs, A. 1992. Stuck in Traffic: Coping With Peak-Hour Traffic Congestion. Washington, DC: Brookings Institution Press.
IMF (International Monetary Fund). 2010. Impact of high food and fuel prices on developing countries. Available online: http://www.imf.org/external/np/exr/faq/ffpfaqs.htm
Maring, G. 2007. Surface transportation funding issues and options. Presentation to the National Surface Transportation Infrastructure Financing Commission. Available online: http://financecommission.dot.gov/Documents/Surface%20Transportation%20Funding%20Issues%20and%20Options_Gary%20Maring.ppt
Marsh, B. 2011. Kilowatts vs. Gallons. New York Times, May 28. Available online: http://www.nytimes.com/interactive/2011/05/29/weekinreview/volt-graphic.html?ref=weekinreview
UPI (United Press International). 2011. Global biofuel land area estimated. Available online: http://www.upi.com/Science_News/2011/01/10/Global-biofuel-land-area-estimated/UPI-97301294707088/
Urry, J,. 2004. The 'System' of Automobility. Theory Culture and Society 21:4-5, 25-39.
Usón, A.A., Capilla, A.V., Bribián, I.Z., Scarpellini, S. and Sastresa, E.L. 2011. Energy efficiency in transport and mobility for an eco-efficiency viewpoint. Energy 36:4, 1916-23.
Xu, J., Kochanek, K., Murphy, S., and Tejada-Vera, B. 2010. Deaths: Final Data for 2007. National Vital Statistics Reports, 58:19, available online: http://www.cdc.gov/NCHS/data/nvsr/nvsr58/nvsr58_19.pdf
11.2. Sustainable Stormwater Management*
Learning Objectives
After reading this module, students should be able to understand
how stormwater runoff affects water quality in urban watersheds
how stormwater is currently managed in the United States
some of the conventional and innovative techniques that have been developed to address the water pollution and flood risks associated with urban stormwater runoff
Introduction
This module reviews some of the complex issues of urban stormwater management. It first examines the hydrological issues affecting the discharge of stormwater runoff to our urban rivers and streams, and then provides an overview of how urban stormwater is managed under the Clean Water Act. After describing the conventional approaches to urban stormwater management, the final section provides an overview of various "sustainable" strategies, especially the use of "green infrastructure," that can be considered to reduce the water pollution and flooding risks generated by urban stormwater runoff.
The Hydrological Context of Urban Stormwater
Stormwater runoff (or overland flow) is the portion of precipitation reaching the ground that does not infiltrate into soils, is not taken up and transpirated by plants, nor is it evaporated into the atmosphere. It is an especially important component of the hydrological cycle in urban areas, since it can cause both pollution and flooding risks to nearby waterways and their adjacent communities. It should also be noted that many of the current models of global climate change predict changes in the hydrological cycle in the future. They predict many more severe storms likely in parts of the Midwest as a result of the moisture and energy in the atmosphere increasing over the next century because of increasingly higher concentrations of greenhouse gases. Higher frequencies of more severe storms are likely to further increase the pollution and flooding risks posed by stormwater runoff, especially in urban areas (USGCRP, 2009).
Current strategies to manage these risks employ the concept of a watershed – the variations in natural topography that cause both surface water and surficial ground water to flow downhill towards lower-lying areas or points of discharge, usually to a stream or river. Watershed boundaries are defined topographically by mapping variations in land elevations around waterways that create hydrologic divides between adjacent watersheds and between sub-watersheds. The amount of stormwater that ends up as runoff within a watershed not only depends on the intensity and amount of precipitation reaching the ground in the form of rain or snow, but also on the characteristics of the watershed itself. State and federal environmental protection agencies have developed a number of sophisticated hydrological simulation models that enable the amount and characteristics of stormwater runoff (in terms of its volume and the pollutant load that would be carried by the stormwater to rivers and streams within the watershed) to be forecasted. They forecast this based on historical estimates of the amount of precipitation entering the watershed, the characteristics of a watershed's terrain and soils, the amount and location of impermeable surfaces associated with the development of the watershed, and the extent and types of ground cover within the watershed's drainage area (NRC 2008, Appendix D). A change in any of these factors will affect the amount and extent of flooding and water pollution attributable to the discharge of stormwater runoff into a river or stream.
Since the pattern of precipitation varies seasonally the water pollution and flooding risks posed by stormwater runoff also tend to vary seasonally. Generally, larger flood and pollution risks will occur in the spring, when rapid snowmelt can generate a lot of runoff volume (especially if the ground is still frozen), which can carry pollutants that have accumulated within the snow cover over the winter months to nearby streams and rivers. There can also be storm-related flood and pollution "spikes" when heavy rain strikes the ground at a faster rate than it can be infiltrated into the soils, or when it is prevented from infiltrating into the soils by roofs, paving, or other impermeable surfaces. This initially high volume of stormwater runoff can carry greater amounts of contaminants – a process often described as the "first flush" phenomenon. Usually, the first half-inch of stormwater will be carrying the highest pollution load, so its capture and management becomes a priority for water quality protection.
How some of these features, especially the amount of impervious surface associated with different densities of development, affect the generation of urban runoff are illustrated in Figure Degrees of Imperviousness and its Effects on Stormwater Runoff. Research by the Center for Watershed Protection has found that stream quality becomes impaired when 10% of the stream's watershed is impervious and that an urban stream's ecology is severely impacted when more than 25% of its watershed is impervious.
When flowing downhill within a watershed, stormwater runoff can pick up pollutants from various anthropogenic sources and activities. It can also collect pollutants from the atmospheric deposition of particulates and air pollutants carried to the earth's surface by precipitation, by windblown dust, or by simply settling out of the atmosphere. Urban runoff can also dissolve or transport chemicals that may be found naturally in soil or nutrients which may have been deliberately added to lawns. Common urban pollutants can include such things as pesticides and fertilizers applied to residential lawns, parks and golf courses, enteric microbes from animal waste, industrial chemicals that may have been accidentally spilled on the ground or improperly stored, or oils and greases leaking from cars parked in lots or on driveways.
As stormwater runoff flows towards lower-lying areas of the watershed, it carries these contaminants with it and therefore contributes to the pollution of the stream, river or lake into which it is discharging. Once it reaches a river or stream, the concentrations of pollutants in the receiving waters are naturally reduced as the contaminants are carried downstream from their sources, largely through dilution but also by settlement, by uptake by posure to sunlight and oxygen, and by interactions with various chemical and physical proplants and animals (including bacteria and other microorganisms), through degradation by excesses occurring within the waterway and its streambed.
Regulating Urban Runoff
Water pollution risks within watersheds are managed under the federal Clean Water Act, which requires state environmental protection agencies to regulate the discharge of pollutants into navigable waterways and waterbodies pursuant to federal guidelines (NRC, 2008). The Clean Water Act employs maximum concentration standards for common pollutants that can impair the recreational or ecological functions of a river or stream. One class of polluters regulated under the Clean Water Act consists of those that are directly discharging pollutants into a waterway from an industry or sewage treatment plant through a pipe, ditch, outfall or culvert – these are called point sources.
Point sourcesare managed under the Clean Water Act by the requirement that each direct source have a renewable discharge permit, called a National Pollution Discharge Elimination System (NPDES) permit. NPDES permits set limits for the various pollutants being discharged by that source based on the ambient water quality of the waterway and its proposed use (e.g. its use as a public water supply source, or for fishing, or recreational use). The other regulated class of polluters managed under the Clean Water Act consists of those sources that introduce contaminants into a waterway through overland or subsurface flow – these are called non-point sources, and include most of the water pollution loads carried by urban stormwater runoff.
Since the 1970s, the principal approach used by state and federal environmental protection agencies to control water pollution is to try to simply reduce the quantity of pollutants being released into our rivers and streams (NRC, 2008). NPDES permits control the direct discharge of contaminants into our waterways, while non-point sources are managed through Best Management Practices (BMPs) that are designed to limit the amount of pollutants released into a watershed, where they could later be carried by stormwater runoff or by groundwater flow to a receiving stream or river. Depending on the pollutant of concern, BMPs could be as simple as requiring pet owners to clean up after their pets or as complex as requiring that industries using toxic materials design, construct and manage loading and storage areas in order to keep spilled materials from being transported off-site by stormwater or groundwater flow. BMPs can even include encouraging some industries to change their production processes in order to reduce the total amount of toxic materials they use, a pollutant reduction strategy known as pollution prevention (since the fewer toxics used, the lower the risk that they will inadvertently be released into the environment).
The strategy of simply reducing the amount of pollutants entering the environment is complicated by the fact that many of the non-point pollutants are not amenable to management through local BMPs. For example, agricultural activities are expressly exempted from the Clean Water Act, even though stormwater runoff from farms and animal feedlots can carry agricultural chemicals, fertilizers and manure into adjacent waterways, along with topsoil from freshly-plowed fields. Pollutants could also be introduced into an urban watershed by the deposition of air pollutants. Airborn particulate matter, for example, can be transported very long distances by the wind, making most locally administered BMPs (except possibly instituting regular street-sweeping programs) ineffective in reducing the distribution and quantities of these types of urban stormwater pollutants.
In response to these challenges, the Clean Water Act was amended to require state environmental protection agencies to calculate pollution budgets for the impaired segments of their streams and rivers. The "impaired segments" were those reaches of a stream or river that did not meet the water quality standards for their intended uses. Models were used to calculate the "total maximum daily load" (TMDL) of pollutants entering the waterway through both point and non-point sources that would enable the stream segments to achieve their highest proposed use. The Clean Water Act's new TMDL program provides a more sophisticated framework for evaluating the impacts of non-point pollution on water quality. However, given the limitations of trying to put more and better BMPs into place, environmental protection agencies have begun to refocus some of their attention from reducing the total amount of pollutants being released within a watershed to also reducing the amount of stormwater runoff.
Environmental protection agencies have developed strategies for urban stormwater management that involve modifying a development site so that more precipitation would be retained on-site rather than flowing off of it into nearby waterways or waterbodies. These stormwater retention strategies initially stressed traditional engineering solutions, such as installing a stormwater collection system that temporarily stores the stormwater on-site in order to reduce the rate and amount of stormwater being released to a waterway. The strategies were later expanded to include various site modifications, such as constructing vegetated buffer strips or swales (ditches),in order to encourage more stormwater to infiltrate into the ground.
Reducing the volume of urban stormwater leaving a site as runoff also offers an additional hydrologic benefit in urban watersheds – reducing flood risks (NRC 2008). Besides having the potential to carry pollutants, stormwater runoff discharge increases the amount of water entering into a lake, stream or river, increasing both the water volume and flow velocity of the waterway. A relatively large amount of stormwater runoff entering a waterway over a relatively short time can quickly raise a stream's water levels beyond its banks, causing flooding that could threaten adjacent development. Stormwater contribution to a river or stream can also increase the velocity of the stream's flow, causing increased channel and bank erosion, undercutting or damaging dikes, levees and other water control structures, and scouring the stream or river bed. Stream edge or streambed erosion can impair water quality by increasing the cloudiness (or turbidity) of the waterway, which can also damage aquatic and riparian habitats.
Stormwater-induced flood risks are managed by the National Flood Insurance Act, where hydrologic models (adjusted by historical flood events) are used to forecast the potential flooding caused by a 100-year storm (a storm that has a one percent chance of occurring in any given year). The Act forces financial institutions to require homeowners within the designated 100-year floodplains to purchase flood insurance in order to get a mortgage, with the federal government subsidizing the insurance premiums if the community adopts a flood management program restricting development from extremely hazardous areas and instituting building code changes to lessen flood damage.
In assessing flood risks, it is important to realize that managing the volume and rate of urban stormwater being discharged from developed areas does not affect the total amount of stormwater that is being discharged to a river or stream within a watershed – they only affect the timing of when a storm's precipitation will be discharged to the waterway (NRC, 2008). Both the conventional and the newer, more sustainable, ways of managing stormwater discussed below seek to delay the time it takes for stormwater runoff to reach a waterway in order to reduce the water levels and flow velocities of the receiving streams after a storm. Slowing the rate by which stormwater is being contributed to a stream spreads out the peak of the resultant flood levels over a longer time period, allowing many flood risks to be substantially reduced.
Conventional Stormwater Management
Urban stormwater is traditionally managed by the construction of engineered stormwater facilities, such as storm sewers and detention basins, as part of the land development process. These engineering processes are specifically designed to modify the natural hydrology of a site. For example, when land is being developed, the parcel is usually graded for development and stormwater infrastructure is installed to channel the stormwater from individual lots into a separate stormwater sewer system connected to a detention basin where it is retained until it can be discharged off-site. Site preparation also includes elevating building sites so that they are constructed on slightly elevated "pads" to encourage stormwater to flow away from building foundations and toward the streets. After reaching the street, stormwater is then directed to the stormwater sewers by curbs and gutters.
Conventional stormwater detention facilities were historically built to reduce off-site flood risks, and were not expressly designed to reduce off-site water pollution risks. Any stormwater detention that was provided was only temporary, often providing an insufficient retention time to allow the natural attenuation of any pollutants that were carried by the runoff into the detention basin – unlike the natural attenuation processes occurring in a river or riparian wetland (where ambient pollution levels are gradually reduced through dilution, oxidation, chemically binding to rocks and soils, being gobbled up by microorganisms, etc.). Stormwater is usually detained on-site after a storm only for a period of hours or, at most, days and then released to a waterway. Some of the particulate contaminants in the stored runoff might settle out if they are large or heavy enough to do so during that short time, some might infiltrate into the soils in the bottom of the detention basin, and some pollutants might be taken up by grass lining the basin, but many pollutants still end up being carried into the waterway along with the released stormwater.
Since the 1990s, environmental protection agencies have begun to consider the water pollution impacts of releases from stormwater detention facilities, after the Clean Water Act was amended to require states to treat stormwater discharges from detention basins as a type of direct source and to require that NPDES permits be phased in for discharges from Municipal Separate Stormwater Sewer Systems ("MS4") in cities and urban areas above certain population thresholds (NRC, 2008). The NPDES permits issued under the U.S. Environmental Protection Agency's (U.S. EPA) MS4 program now require the water pollution loads from stormwater detention basin discharges to be assessed through the creation and adoption of local stormwater management plans and that the contaminants carried by the stormwater runoff to the basins for later re-release to a waterway be better managed and reduced through the adoption of local BMPs. MS4 permit regulations issued by state environmental protection agencies usually involve the issuance of a "general permit" by the agency, applying to all applicable Municipal Separate
Stormwater Sewer Systems Located within the State's Designated Urban Areas
A different set of stormwater management issues arise in older urban areas that are already developed. Most of the United States' older cities and suburbs, especially those established in the late-19th and early 20th centuries, do not have Municipal Separate Stormwater Sewer Systems. Instead, they have what are known as combined sewer systems – sewers that carry both the stormwater runoff from paved streets and the wastewater (sewage) from homes, stores and factories. These combined sewers transport the mixed wastewater and stormwater to municipal sewage treatment plants where the diluted sewage is treated and then discharged to a waterway under an NPDES permit (NRC, 2008).
Water quality problems arise when rainstorms deposit more precipitation in the city than can be handled by the sewage treatment plant. As the diluted wastewater begins to fill up the combined sewer system at a faster rate than it can be treated, the sewage treatment plant operators are faced with a difficult choice – they can either allow the diluted sewage to continue to back up in the sewers, eventually flooding residents' basements (a politically unpopular as well as unhealthy option), or they can allow the diluted wastewater to bypass the sewage treatment plant and be discharged directly into the waterway, with the untreated wastewater's pollutant levels usually exceeding the limits set forth in the plant's NPDES permit. Most treatment plant operators choose the more politically acceptable option of releasing the wastewater in violation of their NPDES permit, creating water pollution incidents called combined sewer overflows (CSOs).
Strategies to Manage CSOs
CSO problems are very difficult and expensive to resolve in older cities. One approach to managing stormwater off-site is to tear up the city's streets, digging up the old combined sewers and replacing them with separate stormwater and wastewater sewer systems. The high costs of retrofitting new separate sewer systems are often prohibitively expensive, especially in these times of stressed state and local budgets. Moreover, the extensive traffic disruptions involved in replacing most streets would not make this a politically popular choice.
A second approach to managing CSO issues off-site in developed areas is to keep the combined sewer system, but to construct a reservoir system large enough to store the diluted wastewater until it can be treated by the sewage treatment plant. This is the approach used by both the City of Milwaukee, Wisconsin and by the Metropolitan Water Reclamation District of Greater Chicago in its Tunnel and Reservoir Plan, or TARP. Although most of TARP has been built, all of the reservoirs have not yet been completed because of federal budgetary cutbacks. The tunnels themselves and one reservoir are currently able to temporarily store the combined sewage and the runoff from only the first 3/8-inch (.95 cm) of rain falling in the Metropolitan Water Reclamation District's service area. The extremely high expense of installing such a supplementary sewage and stormwater storage system would make it unaffordable to most cities unless very substantial federal and state grants are provided.
A third way to address CSO issues off-site is to use the streets themselves to temporarily store stormwater by installing low speed bump-like structures at intersections and by restricting the streets' sewer intakes to the combined sewer system (US EPA, 2000). This urban retrofit strategy would allow stormwater to flow from lots into the streets, which would flood up to their gutter tops during heavy storms, functioning as stormwater reservoirs. The stored stormwater would then slowly be discharged to the combined sewers through the restricted grates over a period of hours after the storm, reducing the amount of diluted sewage flow to a quantity that could be adequately treated by sewage treatment plants. The flooding of streets, impairing automobile access, and the possibility of stormwater overflowing the curbs and damaging parked cars and adjacent property during very heavy rainstorms may not make this a politically popular option, though.
Managing Urban Stormwater More Sustainably
There is a fourth approach to dealing with CSO problems, which involves intercepting and delaying the discharge of precipitation from a parcel of land before it flows off-site to a separate or combined sewer system, or to an adjacent waterway. Encouraging on-site storage or infiltration reduces the stormwater contribution to a combined sewer's flow in developed areas, thereby reducing the amount of diluted wastewater being generated and enabling combined sewer systems to better handle their wastewater loads during rainstorms. These decentralized on-site approaches to managing stormwater could also be used to reduce the amount of conventional stormwater infrastructure needed in new developments using separate stormwater sewer systems. Because these on-site approaches are less resource-intensive and more cost-effective than conventional stormwater management approaches, they are also more sustainable investments.
On-site stormwater management techniques are also often known as "green infrastructure" (Jaffe et al., 2010). Development projects using "green infrastructure" for urban stormwater management are commonly known as "Low Impact Developments." Low Impact Development projects using green infrastructure usually allow stormwater to be managed at lower costs than by using conventional detention practices (US EPA, 2007).
There are essentially three strategies for on-site stormwater management: (1) techniques that encourage the infiltration of stormwater into soils to reduce its volume before it reaches a sewer system, or which employ more selective grading and the planting of vegetation to reduce its rate of flow from the site; (2) techniques that encourage the temporary storage of stormwater on-site, instead of transporting it off-site for centralized detention within a development project or a municipality; and (3) techniques, such as the construction of artificial wetlands, which also allow some degree of longer-term retention and treatment of the stormwater by natural processes before it is discharged. Infiltration techniques might also provide some water treatment capabilities due to the longer retention times of groundwater before discharge, but the degree of such treatment would largely depend on soil characteristics, the amount of overlying vegetation and the depth of the soil's unsaturated zone.
Increasing Stormwater Infiltration
Techniques to decrease the volume of stormwater runoff and to reduce the rates at which it is discharged include the use of permeable paving and the construction of "rain gardens" and vegetated swales (see Figure Permeable Paving & Vegetated Swales). Permeable paving uses materials which are specially formulated to have air voids in their matrix, allowing water to flow into and through the paving materials after they are installed. It also includes the more common installation of precast porous pavers that are designed with holes through their surfaces, allowing stormwater to flow through their holes into the soils beneath them. Permeable paving needs to be periodically maintained because its pores can be clogged by fine grains of topsoil or with other small particles (such as soot from atmospheric deposition) carried along by the runoff. Maintenance includes periodically sweeping or vacuuming the paving to control the build-up of clogging particles.
"Rain gardens" can also be used to encourage stormwater to infiltrate into the soils, where it can be taken up by plants and transpired to the atmosphere, evaporated from the soils, or allowed to infiltrate deeper into the soils to become groundwater. Rain gardens are created in areas of low-lying terrain that are expressly designed for, or engineered with, well-drained soils and are usually planted with deep-rooted native vegetation that often can survive the drier soil conditions between rains. Rain gardens can be quite effective in intercepting and infiltrating stormwater being discharged from roofs, with roof downspouts directing the discharge of stormwater into a rain garden instead of allowing it to flow across the lot and into the street sewer system. Some native vegetation, however, may have special maintenance requirements, such as the periodic burning needed to manage some prairie plants.
Vegetated ditches or swales can also be used to transport stormwater runoff to a conventional stormwater management system, with the vegetation planted in the ditch slowing the rate of stormwater flow while also allowing a portion of the runoff to be infiltrated into the soils or taken up by plants. In many cases, vegetated swales and rain gardens can provide less-expensive alternatives to the installation of separate stormwater sewer system, since it reduces the need for the construction of street gutters, grates, street catchment basins and sewer pipes (US EPA, 2007). Interception of the stormwater by infiltration and plant uptake in a rain garden or vegetated swale may also reduce the amount, capacity and size of the sewers that would have to be built to manage a predicted volume of stormwater, if these green infrastructure techniques are used to supplement a conventional stormwater collection system.
Increasing Interim On-site Storage
Sustainable management techniques that can temporarily store stormwater on-site until it can be released off-site to a sewer system or to conventional stormwater detention facilities include the use of "green roofs" and rain barrels connected to roof downspouts. Rain barrels allow precipitation to be collected and stored, and then used for non-potable purposes (lawn irrigation, for instance) allowing the captured stormwater to substitute for more expensive, treated water (see Figure A Rain Barrel Collection System).
A green roof is a flat roof surface that uses amended soil materials installed above a layer of waterproof roofing materials to allow shallow-rooted plants to be planted. While still being an impermeable feature of a development site (because of its waterproof layer), a green roof can temporarily store rainwater before it is discharged to the ground by the roof gutters and downspouts (see Figure A Green Roof). Just as a rain barrel can store (and re-use) a portion of the stormwater precipitation being discharged from impervious roofs, the soils of a green roof can capture and temporarily store stormwater precipitation as the pores between the soil particles fill up with rainwater. Green roofs can even partially reduce the runoff's pollution load through plant uptake and by other biological and physical processes within the roofs' soil materials while they are saturated. Because of the need to both water-proof the roof while installing a biological system on top of it, green roofs tend to cost more than conventional roofs, even ignoring the additional structural engineering that might be necessary to accommodate the weight of the green roof's soil and plantings.
The stormwater management benefits of rain barrels and green roofs depend on their storage capacity relative to the amount of impervious surface area of the roof with which they are associated. Rain barrels might be able to capture only a fraction of an inch of the stormwater falling on a roof and being discharged from a downspout, while several inches of amended soils on a rooftop might be able to store substantially more precipitation before it evaporates, is taken up by the roof's plants, or is discharged from the green roof via its gutters and downspouts. In both cases, however, the interception and temporary retention of stormwater by these green technologies may allow conventional stormwater management systems to function more efficiently by reducing the amount of stormwater being discharged into the systems. They would also certainly reduce some of the "peakiness" of stream flooding by being able to temporarily store and then release stormwater from impermeable roof surfaces later after a storm event.
Treating Urban Stormwater
Some sustainable stormwater management approaches have the potential to actually treat the water to remove pollutants as well as control its volume and rate of discharge. These strategies include constructing wetlands and planting trees. Wetlands have proven to be very effective in both temporarily storing stormwater runoff and reducing flooding risks, while also reducing the pollutant load carried to the wetland (because of its high biological activity that can capture and degrade the contaminants). As a result, the federal government has adopted a "no net loss" policy with respect to protecting existing wetlands. Section 404 of the federal Clean Water Act requires that the U.S. Army Corps of Engineers (under U.S. EPA oversight) review any proposals to fill or damage any wetlands that are directly hydrologically associated with navigable waterways. Any actions affecting existing wetlands will need a Corps 404 permit in addition to any local or state approvals.
Besides preserving existing wetlands, new wetlands can also be designed, created and maintained as part of a "green" stormwater management strategy (NRC, 2008). The constructed wetland can be designed and used to intercept, temporarily store and treat stormwater runoff before it is released to a stream or river. Water control structures are also usually installed to ensure that the constructed wetlands remain flooded for long enough periods of time to support wetland vegetation. If appropriate plants are selected, they can also provide important habitats. Wetland maintenance involves the control of invasive plant species (e.g. Purple Loosestrife) and the management of any sediment that can be carried by stormwater runoff into the wetland, since the sedimentation of wetlands can fill them in, impairing their ecological and treatment functions.
The planting of trees is an especially valuable strategy to manage urban stormwater, especially when the trees become mature. Tree canopies break rain velocity, reducing runoff flow rates, while tree roots can stabilize soils against being eroded by urban runoff. Tree canopies reduce temperatures, mitigating urban heat island effects, by providing shade and through their transpiration processes. Their leaves and roots can also capture some stormwater contaminants and provide carbon sequestration to reduce climate change impacts. Moreover, trees provide a valuable soil amendment as their fallen leaves decay into mulch, improving the infiltration rate and biological activity of surrounding soils, while larger broken branches falling into urban streams can slow stream velocities and provide improved riparian and aquatic habitat. The shading of streams by riparian trees is particularly important in ensuring that a stream's ecological functions remain resilient in the face of rising temperatures caused by global climate change.
Conclusions
All of the green infrastructure and Low Impact Development techniques that provide interim on-site stormwater storage to reduce flood risks can also provide some pollution removal capabilities, as well. The American Society of Civil Engineers and U.S. EPA maintain an International Stormwater BMP Database of development projects using green infrastructure. This on-line resource reviews the effectiveness of various stormwater management practices and makes these sustainable techniques more accessible to local officials and municipal public works departments charged with managing stormwater runoff in their communities.
There is increasing public interest in using sustainable stormwater management techniques to replace or supplement conventional stormwater facilities. The U.S. federal government, for example, is now requiring that green infrastructure be used in all federal projects above a certain size to manage urban stormwater runoff. Local officials are also showing a greater interest in these sustainable approaches, since they are often less expensive to install and maintain over their life-spans than conventional stormwater sewer systems and detention facilities. Finally, state governments are beginning to set aside money in their revolving loan funds for public infrastructure that is earmarked for green infrastructure projects. It is likely that this interest in sustainable urban stormwater management will continue to grow.
Review Questions
Which of the sustainable urban stormwater management practices can best be used in existing neighborhoods, and which are best suited for new development?
The performance of many of the green infrastructure practices often depends on how well they are maintained over their life-spans. What are some effective strategies that local officials can consider in order to ensure that the green infrastructure being used to manage urban stormwater in their communities is adequately maintained and continues to perform as designed?
Resources
For more information about the:
Clean Water Act, visit http://www.epa.gov/agriculture/lcwa.html.
International Stormwater BMP Database, visit http://www.bmpdatabase.org.
Metropolitan Water Reclamation District of Greater Chicago, visit http://www.mwrd.org/irj/portal/anonymous/tarp.
Milwaukee Metropolitan Sewerage District, visit http://v3.mmsd.com/DeepTunnel.aspx.
National Flood Insurance Act, visit http://www.fema.gov/library/viewRecord.do?id=2216.
References
Gulliver, G.S. & Anderson, J.L. (eds.). (2008). Assessment of Stormwater Best Management Practices. Stormwater Management Practice Assessment Study. Minneapolis: University of Minnesota.
Jaffe, M., Zellner, M., Minor, E., Gonzalez-Meler, M., Cotner, L., Massey, D., Ahmed, H., Elbert M., Wise, S., Sprague, H., & Miller, B. (2010). Using Green Infrastructure to Manage Urban Stormwater Quality: A Review of Selected Practices and State Programs. Springfield, IL: Illinois Environmental Protection Agency. Retrieved June 23, 2011 from http://www.epa.state.il.us/green-infrastructure/docs/public-act-recommendations.pdf
National Research Council. (2008). Urban Stormwater Management in the United States. Washington, DC: National Academies Press. Retrieved June 23, 2011 from http://www.epa.gov/npdes/pubs/nrc_stormwaterreport.pdf
U.S. Environmental Protection Agency. (2000, October). Street Storage for Combined Sewer Surcharge Control: Skokie and Wilmette, Illinois (Factsheet). (EPA Publication No. EPA-841-B-00-005C). Washington, D.C. Retrieved May 17, 2011 from http://www.lowimpactdevelopment.org/pubs/Street_Storage_Factsheet.pdf
U.S. Environmental Protection Agency. (2007, December). Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices. (EPA Publication No. EPA 841-F-07-006). Washington. D.C. Retrieved June 23, 2011 from http://www.epa.gov/owow/NPS/lid/costs07/documents/reducingstormwatercosts.pdf
U.S. Global Climate Change Research Program (USGCCRP). 2009. Global Climate Change Impacts in the United States. Cambridge: Cambridge University Press. Retrieved May 18, 2011 from http://downloads.globalchange.gov/usimpacts/pdfs/climate-impacts-report.pdf
Glossary
- "First Flush" Phenomenon
The higher pollutant concentrations found at the beginning of a storm or spring snowmelt.
- "Peaky" Waterways
The "peakiness" of a waterway describes the more rapid increase and decline in stream flow and the higher stream levels after a storm in urbanized watersheds compared to the more gradual rise and decline in stream volumes and lower water levels in less-developed drainage basins after the same storm event, largely because of the greater amounts of impervious surfaces and runoff generated within urban areas.
- Accessibility
In transportation, a measure of the ease with which people are able to get places they want or need to go.
- Ambient Water Quality
The concentration of pollutants found within waterbodies and waterways.
- Combined Sewer Overflows (CSOs)
The overflow and discharge of excess wastewater to surface waters during storms, when diluted wastewater flows exceed the capacity of a combined sewer systems or sewage treatment plant.
- Combined Sewer Systems
Sewer systems that are designed to collect stormwater runoff and domestic and industrial wastewater within the same sewer pipes.
- Derived demand
Demand for a good or service that comes not from a desire for the good or service itself, but from other activities that it enables or desires it fulfills.
- Embodied energy
The sum of all energy used to produce a good, including all of the materials, processes, and transportation involved.
- Externality
Cost of an activity not paid by the person doing the activity.
- Green Roof
Vegetation and planting media installed on a rooftop in order to store and delay stormwater runoff from the roof's surface.
- Hydrology
The scientific examination of the occurrence, distribution, movement and properties of water within the natural environment.
- Low Impact Development
An approach to land development (or re-development) that uses natural drainage and environmental processes to manage stormwater as close to its source as possible.
- Mobility
The ability to move or to get around.
- Native Vegetation
"Wild" plants that have naturally evolved and successfully adapted to a region's environmental conditions.
- Non-Point Source
The term "nonpoint source" is defined to mean any source of water pollution that does not meet the legal definition of "point source" in section 502(14) of the Clean Water Act (see "Point Source" definition below)
- Point Source
Defined by Section 502(14) of the Clean Water Act as any single identifiable and discrete source of pollution from which pollutants are discharged, such as from a pipe, ditch, channel, culvert, confined animal feeding operation, or discharged from a floating vessel.
- Pollution Prevention
Reducing or eliminating waste at the source by modifying production processes, promoting the use of non-toxic or less-toxic substances, implementing conservation techniques, and re-using materials rather than putting them into the waste stream.
- Rain Barrel
A cistern, barrel or storage system that collects and stores the rainwater or snowmelt from roofs that would otherwise be diverted to storm drains and streams as stormwater runoff.
- Stormwater Runoff
The overland flow of precipitation generated by that portion of rain and snowmelt that does not infiltrate into the ground, is not taken up by plants, and is not evaporated into the atmosphere.
- Swales
Graded and engineered landscape features designed as vegetated, shallow, open channels or ditches that are usually planted with flood tolerant and erosion resistant plants.
- Watershed
A geographic area that naturally drains to a specific waterway or waterbody.