Reimagining Floodwater Infrastructure
Smaller Wetlands, Larger Area of Effect
Introduction
Wetlands provide, social, ecological, and economic health and wellbeing to every environment, including urban. They have a functional value to ecosystems, as well as aesthetic and unmeasurable values. In this poster, we focus on the functional benefits that wetlands provide, specifically flood mitigation, and how the establishment of series-based constructed wetlands will significantly reduce flooding likelihood and the frequency of severe flooding events in urban environments, especially in the face of climate change. Climate change is predicted to increase the number of storms with higher rainfalls at a higher frequency than urban environments have historically seen (Knutson et al., 2010). Our model focuses on the construction of managed wetlands along roadsides and in high-flood areas to manage increased water inputs, and buffer existing stormwater management systems.
Climate Change and Case-studies
In the coming decade precipitation is predicted to occur at a greater frequency and with a greater intensity, and predictions suggest that this intensity will continue to increase well into 2100 (Hausfather, 2018; IPPC, 2014). Due to warming temperatures, the water-holding capacity of the atmosphere has increased, and according to the Clausius-Clapeyron Equation, the atmosphere can hold 4-7% more moisture for every 1°C increase in global temperature (Climate Central, 2018). This is a result of the increased evaporation in the oceans, lakes, rivers and even plants, meaning more water is available to condense in precipitation (Climate Central, 2018).
Figure 1: Average projected percentage changes in precipitation between the current climate and the end of the century (2081-2100) (Hausfather, 2018; IPPC, 2014). The average precipitation model shows large increases in precipitation near the equator (ie. the Pacific Ocean), as well as the Arctic and Antarctic (Hausfather, 2018).
Figure 2: Percentage changes in heavy or intense precipitation events per degree warming.
Red areas show decreases, while blue areas depict increases in heavy precipitation (Hausfather, 2018; IPPC, 2014).
Following this, a world that is around 4°C warmer than the pre-industrial era would contain ~28% more water vapor in the atmosphere (Hausfather, 2018; Climate Central, 2018). Currently, climate predictor models suggest that by 2100, most of the world will see a 16-24% increase in intense precipitation events (Figure 1 & 2) (Hausfather, 2018). Heavier rainfall equates to more flooding--including flash flooding, which can lead to increased property damage and repair costs. 2017 was a historic year of weather and climate disasters, the United States alone faced 16 separate billion-dollar disaster events, including three tropical cyclones, eight severe storms, and two inland floods (Smith, 2018), which costs upwards of $300 billion in damage clean up and rescue expenses (Climate Central, 2018; Smith, 2018).
Case-study 1: Wilmington, NC.
On Friday, September 14, 2018, Hurricane Florence hit the coast of Wilmington, North Carolina (Corcoran & Brueck, 2018, HFCS, 2018). Over the course of five days, 32 inches of intense rainfall (plus storm surges from nearby water bodies), resulted in tremendous flooding upwards of 15ft in parts of the city (Shah & Gabbatt, 2018; Specht & Eanes, 2018). 32 lives were lost to Hurricane Florence, even with 5000 search-and-rescue operations, 36 helicopters, over 200 boats and 13, 500 military personnel working to aid in relief efforts (Shah & Gabbatt, 2018). More than 20,000 people were living in shelters at the height of the flooding, 80,000 people lost power, and more than 500 roads including downtown core and main interstates leading to the city, became impassible (Specht & Eanes, 2018). The Wall Street Journal reported that Florence would cost North and South Carolina between $38-50 billion in clean up and relief effort costs, which would make Florence the 7th costliest storm in US history ( Rocco, 2018; Specht & Eanes, 2018).
We derived our estimates and calculations for water loading and capacity from this study of Wilmington. By using an intense and extreme storm system, we hope to prove that wetlands can manage extreme-amounts of water, and thus are reliable into an unquantified future.
Case-study 2: Harvey, Irma, and Maria.
2017 saw three more storms make US history; Hurricane Harvey, in which the damage costs ran between $125-133.5 billion; Hurricane Irma ($50-84.2 billion); and Hurricane Maria ($90-120 billion) (Smith, 2018; Rocco, 2018). In August 2017 Hurricane Harvey devastated Houston, Texas with an incredible 60 inches of rain in 7 days. This storm displaced over 300,000 people and damaged over 200,000 homes and business. Hurricane Irma hit the Florida Keys area in early September, and the extensive flooding destroyed 25% of homes and damaged up 65% of homes (Smith, 2018). Lastly, Hurricane Maria, hit the Southern Caribbean region (Puerto Rico) in mid-September, causing up to 37 inches of rainfall, tremendous flooding, and mudslides across the island (Smith, 2018).
It is clear that a lot of America's time, money, and efforts are being spent on cleaning up the intensive damages from the more frequent and extreme rainfall events. Furthermore, it is evident that road-way mobility during these times of crisis remains a critical limitation during flood-water storms. Thus, it is suggested that efforts and money would be better spent in pro-actively investing in flood-water mitigation or management plans, such as constructed wetlands, rather than continuously throwing money out to repair flooding damage. Mitigation and/ or management systems could decrease the social, economic, and environmental devastation faced in the USA when extreme rainfall events occur. Flood-water mitigation systems can act as a buffer between Mother Nature and society.
Constructed wetlands
The EPA defines a constructed wetland as a treatment system which uses natural processes to improve water quality (EPA, 2017). Usually, this includes the systematic and controlled arrangement of soils, microbes, and vegetation to help treat water inputs before releasing them back into the city's water system, or into nature. Constructed wetlands have also been documented to provide significant wildlife habitat and specific design features towards sustainable cities.
However, we are reimagining what a constructed wetland can do, and what services it can provide to a community. We want to reimagine wetlands from simply water filtering and treatment facilities to a more rugged, adaptable system capable of integrating with existing urban infrastructure to help facilitate the slowed movement of water during intense storms. What follows are the design features, characteristics, and thought processes that have gone into our research, and will describe why we so strongly believe that constructed wetlands will redefine stormwater management worldwide.
Plants and Substrate
Wetland plants can be defined as any species which are found in or on the water, or where soils are flooded/saturated with water for a time period long enough for anaerobic conditions to develop in the roots (Cronk, 2001). Utilizing these types of plants is crucial so that during a period of high rainfall they survive being partly submerged in the wetland units.
Native phragmites (Spartina cynosuroides): Our primary and first plant of consideration for the establishment in wetland units. These perennial plants which are a member of the grass family, stand anywhere from 1-3 meters when fully grown and have seed heads between 30-45 cm long (Legault, 2018). They have a high tolerance for being submerged in water while having a high rate of water absorption. This is due to their extensive root system which can extend up to several meters. This allows them to be strong and resilient, while also aiding in the management of stormwater. It is important that they not be confused with the invasive phragmites, Phragmites australis, as they have a high tendency to outcompete native plants due to their reproductive capabilities (Legault, 2018).
Sedge (Carex spp): a perennial member of the sedge family. They are found throughout North America in various wetland systems. They stand up to one meter in height and can tolerate wet soils to standing water (North Carolina Department of Environment, Health, and Natural Resources, 1997). This makes them versatile enough to withstand both periods of flooding and dryer times.
Although the aforementioned species are versatile and therefore suited to conditions of the wetland units, the public may want more visually appealing plants in highly populated areas.
Rose mallow (Hibiscus moscheutos): both beautiful and adaptable this perennial member of the mallow family reaches up to 2 meters in height, with pink or white roses 6 – 10 cm in size (North Carolina Department of Environment, Health, and Natural Resources, 1997). This species does well with periodic flooding and is typically found in drainage ditches, among other wetland types. This makes it suitable for those wetland systems established in retail, residential, or high-urban traffic zones.
Pickerelweed (Pontederia Cordata): is another flowering wetland species and a member of the water- hyacinth family. It has long spikes of blue flowers up to 15 cm in length and stands around 1 meter high (North Carolina Department of Environment, Health, and Natural Resources, 1997). It is found in standing water, although can do well in wet soils.
Common Boneset (Eupatorium perfoliatum): is a member of the aster family that stands about 1.5 meters in height, with dense clusters of small white flowers 5 – 20 cm in size (North Carolina Department of Environment, Health, and Natural Resources, 1997). This species is not aquatic, however, is found in roadside ditches, floodplain forests, and various wetlands due to its ability to withstand flooding.
The chosen wetland species are native to North Carolina, where the case study is based upon. The species are all capable of withstanding short dry periods, while also being able to withstand periods of water submergence during periods of high precipitation. The root systems act as a sediment catchment, while also providing water absorption. Although the latter would not be sufficient during storms, they can aid existing stormwater infrastructure during regular precipitation periods.
Substrate
Substrates for the wetland units will vary for the main liner vs the smaller liners, which may be added to house the more attractive plant species. The smaller liners would generally require a loamy soil, with high organic matter to promote plant growth and microbial activity (DuPoldt, 1994). A sandy loam may be used to promote water movement, while a clay loam will hold more water and sediments. However, the latter would be more beneficial during the establishment of the wetlands (DuPoldt, 1994).
For the main liners, a high porosity gravel is the best option for areas with high levels of precipitation. This allows for the highest capacity of stormwater to run through, along with a high amount of nutrient and sediment collection without clogging the drainage pipe (DuPoldt,1994). This collection of nutrients and sediment allows for the plant species to absorb what it needs from the runoff during periods of standing water and general runoff. The top portion consists of finer gravel, roughly 12-15mm, which is where the plants would be established. Whereas the lower portion contains coarse gravel, roughly 30-40 mm in size (Sundaravadivel, & Vigneswaran, 2009). This combination allows for the plants to thrive, while not impeding hydraulic conductivity.
Design
Our wetland design comprises of a series of interconnected and integrated ponds that will appear along highways, high traffic roads, and areas of high-frequency flooding. Each series of wetlands will be integrated with existing management systems filtering and regulating the release of high-volume water inputs and ensure a city is not overwhelmed. Cities already have urban water management and are equipped to handle flood-water; however, high inputs like those estimated in climate change forecasts maybe or are currently overwhelming old systems.
Calculations for Wetland Sizing
As these wetlands are designed to be used in any place that needs them, the calculations for this sizing need to be scalable. Though it is possible to have them sized to whatever size needed, a minimum size should be established.
For our research, using an area the dimensions of an entire city is implausible. Alternatively, we have established a trial plot size of a road-network 500m x 500m for a total area of 250,00m2. Secondly, to estimate the size and scope of wetlands, we needed a measurable rainfall and a specific timeframe with which to base our calculations and wetland construction model. We wanted to establish an extreme climate change scenario, and thus used the case-study of Wilmington, NC to start.
Wilmington saw rainfall of 32 inches over a period of 5 days. So, in total, this area of 250,000m2 must have enough wetlands to handle 32 inches (or 81.28 cm/0.8128 m) of rain. Therefore, the volume in this area (length x width x height/depth of rain) equals 203,200m3 of rain. Though this seems like an excessive amount of rain, our smaller scale constructed wetlands are designed to handle this. On its own, a wetland of 3m in depth, 5m in width, and 15m in length can hold an absolute maximum of 225m3 of rain, which requires far too many wetlands. So instead, and as part of the regulation efforts of our wetlands, each series will be connected by a series of gate valves and pipes which will act to slowly release water into existing urban water management systems. Gated channels can be opened to a dull diameter of 4" diameter before extreme weather events by city staff or on an automated shuttering system so that the full capacity of the system is engaged, and later can be closed to 3", 2" or even fully closed for slower regulation of water release during times of drought. This will ensure that flora within the wetland have access to some water during times of drought and thus increases the longevity of the system, and so that it can be easily and seamlessly fully engaged prior to when a storm-event is predicted to make landfall.
Alternately this system could be designed with pressure valves which can be used to release water into storm systems only when the pressure exceeds a predefined limit within ponds, or when water levels reach a predefined limit. This will ensure a series of burst water inputs into existing systems, with rest periods between inputs to stagger the speed at which water enters stormwater management systems. Additionally, these pressure releases could be programmed to do something similar with a timed release of water instead of a pressure release. While programmed release systems may be convenient for large areas, their capacity to function under the pressure of a storm where power-outages are likely is undetermined at this time, and thus an analog pressure system self-contained within each individual wetland block may be preferred.
With this system of outlet pipes, and given that rain in our case-study fell over a period of 5-days, we believe that the level of water estimated in this section is still well under control by our system. Over the 5 days, we estimate that 40,640m3 of rain will fall in one day, equaling 1,693.33m3/hour, and 28.22m3/minute in the given plot (note that 1m3 is equivalent to 1000L). The pipes fitted into these wetlands can vary in size depending on their specific needs, however, in this case, a 4-inch wide pipe is more than sufficient, having an outflow of 984.207L/min, at a velocity of 2.01m/sec.
Given these calculations, at maximum capacity, and with absolutely no fail-safe, our sample area of 250,000m2 would require 3 wetlands to handle the rain in our scenario. But given that rainfalls are unpredictable in intensity, and we want our system to not only function but also to thrive and adapt to changing climatic events, we would instead recommend the following:
250,000m2 of Land =6 wetland units of 225m3 (Or 3m D x 5m W x 15m L)
Design Features
Now that we have established the calculations, we can discuss how these wetlands will be constructed and how they will look. Wetlands can be designed with a variety of plants to help filter water and sediments, retain water during drought, and ensure that water stagnation is eliminated. As discussed above, plants like phragmites whose roots can withstand water of 6ft in depth could be used to filter and absorb water at the bottom of each wetland to limit the risk of waterborne disease and health risks associated with standing water.
Fig 3: Possible designs of wetland liners. Image on left depicts main design, plants are robust wetland species with deep roots, and the right shows a secondary concrete liner placed into gravel for less-hardy shrub and floral arrangements. Alternatively, aesthetically pleasing wetland plants could be planted also, however these may not survive a storm.
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Construction
Construction of wetlands would consist of concrete bed-liners in the dimensions described above. Liners would be placed strategically along roadsides so that water could enter the system both from the open top and the inlet pipes embedded into the roadside storm drains. These liners would feed via 4" pipes into whatever stormwater management system exists in the city in question or into the other wetlands in the series, and it is these pipes that would contain the gated-values to be opened or closed dependent on weather conditions. Within these liners, wetlands would contain 2-2 ½m of 35% porosity gravel, it is within this gravel that our plants would be rooted. To ensure plant matter and gravel does not clog the outlet pipes, fitters or grates would cover inlets large enough to not impede the flow of water, but small enough not to get blocked by gravel (ie. 1" grate).
Vegetation
Given the integrated and flexible nature of our system, we have designed two possible forms for the wetlands. Depending on the location these wetlands can take on forms to focus on aesthetics or maximum absorbency. In the case that a location is close to residential, retail, or high pedestrian traffic area, wetlands could have an additional, smaller liner inserted into the gravel (see fig 3). Within this smaller liner flowers and decorative shrubs could be planted; alternatively, a variety of more decorative, flood and drought tolerant plants could be arranged and maintained directly within the gravel. This design was intended to aid in the integration of constructed wetlands to areas where residents or patrons may be reluctant to introduce a constructed flood-system.
Alternatively, wetlands could be constructed with bulk arrangements of deep-rooting, tolerant plant species such as phragmites whose roots would keep the bottom of the liner dry and free from stagnant water, but are also highly tolerant of flood waters for when the system needs to be fully engaged and water will be moving quickly through the system.
The bulk of our design is focused on the gravel substrate. Our design aims to slow the speed of incoming water, and to hold those waters long enough for existing systems to keep up with demand. Due to this nature, sediments, pollutants, and garbage are likely to fill-up within the gaps of the substrate, the part of our design aimed to stop flood-systems from becoming blocked. As such, and without testing our system in-situ, we can only make estimates that the gravel would need to be changed every 3-10 years depending on how often they are pushed to full-capacity, and what types of sediments and pollutants are entering the system.
However, this gravel is not waste. Once removed from the concrete liners, gravel could be sold for construction fill, or other needs once it was washed and processed to ensure no more harmful contaminants remain - such as heavy metals. Given this, we believe that the system is sustainable in an urban centre and that by-products of the wetland could be reused and recycled in perpetuity. Longevity of the system could be increased through community activism, where community or city workers could routinely check wetlands for sediment and waste build up. With regular maintenance and cleaning, our wetlands are projected to withstand many flood-events before repair work is necessary.
Socio-economic benefits
"Constructed wetlands facilitate a harmonious balance between sustainable development, biodiversity, and waterway health" (Spyrakis, 2007). In other words, constructed wetlands can provide a unique and visually aesthetic environment for the community, one that instills people with perceptions of a cleaner and safer surrounding city (Idris et al., 2008). However, the phrase "visually aesthetic" wetlands, is often in direct contrast with most people's experiences of urban green spaces, which usually consists of a very straightforward and unattractive combination of trimmed grasses and scattered trees (Idris et al., 2008). Here, the indirect social and economic benefits of wetlands in urban areas will be examined, in order to show that wetlands can be more than a "dirty swamp," and in fact actually have a harmonious balance within sustainable social, economic, and environmental development.
First, wetlands can provide the ideal place for nature observation, as they usually contain a wealth of biodiversity (Idris et al., 2008). While the proposed wetland models are meant to within proximity to roadways, these constructed habitats can provide a green space "haven," or a natural corridor, for various species within the patchwork of urban sprawl. The little spot of a constructed wetland, even in proximity to roadways provide an important location for bird watching, especially during migration season (Ghermandi, 2005). Furthermore, the wetlands can provide natural views, for people on their commutes home to relax, breathe, be reminded to slow down, stop to take a picture and to escape from the stresses of everyday life (Idris et al., 2008).
Secondly, constructed wetlands can help people develop an informal connection and learning experience with nature, that increases people appreciation and awareness of the local environment and its issues (Idris et al., 2008). Informative signs, especially near busy roadways, could provide information about the purpose of the wetland, including their scientific benefits and the diverse flora/fauna that can be found there, effectively raising awareness and knowledge around the importance of sustainable infrastructure and preserving the environment (Idris et al., 2008). Furthermore, depending on where the proposed wetland model is to be constructed, the wetlands can provide great locations for scientific research, as local schools can include wetlands as part of the student curriculum in order to learn about the local environment and the ongoing environmental issues (Idris et al., 2008).
Third, considering that the constructed wetland model presented here is just that, a model. It is possible that the wetlands could be constructed in areas within proximity to roadways or more offset in the background of a parking lot. No matter the location, it has been proven that constructed wetlands can increase the real estate and development trends within the local community (Idris et al., 2008). For example, residential accommodations and restaurant developers typical prefer building within scenic locations, often near water-bodies areas that offer panoramic views that appeal to consumers (Idris et al., 2008). A lush wetland filled with cattails, reeds, birds, and various other forms of wildlife is far more visually appealing than a barren landscape with grass, shrubs, and garbage. Properties with views and development buffers (ie. wetlands) tend to sell for more, up to 34% more, than lands lacking these landscape features ( Cabe Space, 2005; Idris et al., 2008). Summatively, the more attractive green space located within the city, can serve to attract new businesses, services, and economic development into the newly rejuvenated area (Idris et al., 2008).
Lastly, research suggests that contact with nature can improve mental health, mood, and cognition (Capalldi, 2014). For example, those who are more connected to nature tend to experience a more positive outlook (mood), reduced fatigue, increased vitality, more energy, and an increase in life satisfaction or happiness, compared to those less connected to nature (Capalldi, 2014; Nisbet, 2015). The idea the humans feel happier when they are connected to nature is based on Wilson (1984)'s hypothesis that humans have an inborn tendency to focus and associate with other living things, this was later termed the biophilia hypothesis by Kellert and Wilson (1993) (Capalldi, 2014). Generally, for the first time in history, more of the world population now lives in urban instead of rural areas (United Nations Population Division, 2002). Individuals from developed nations and highly urbanized areas spend almost all of their time indoors, and it is argued that this physical disconnection from the natural environment, in which humanity has historically evolved in, maybe having a detrimental impact on our emotional well-being, as exposure to nature is associated with increased happiness ( Nisbet and Zelenski, 2011; Mackerron and Mourato, 2013). A reason why one would believe that nature connectedness may be positively correlated to one's mental/emotional wellbeing, is the feeling of social cohesiveness (Capalldi, 2014) Being and feeling connected generally predicts an individual's well-being; a rich and full social life is commonly related to happy people, while loneliness and shyness are negatively correlated with happiness (Capalldi, 2014). It is believed that having a connection with nature may function similarly as social connectedness, or promote an individual's emotional and mental well-being or happiness (Capalldi, 2014). While a roadside wetland may not be a grandiose conservation park teeming with extensive wildlife and picture-perfect views, even the more mundane landscapes can provide a little slice of nature (Williams, 2017). Peace and happiness could be found even nature's still waters, birdsong, and sunlight (Williams, 2017).
Ultimately, constructed wetlands within urban sprawl or within proximity to roadways may have more socio-economic benefits than anticipated. The wetlands proposed here have potential to act as a prime location for nature observation and a corridor connect for wildlife, a source for natural and/or water education, increase nearby real estate value, and most importantly improve a communities levels of happiness of mental-wellbeing. It is evident that the benefits of constructed wetlands go far beyond stormwater flood mitigation.
Future research and Conclusion
Due to the nature of our research, our constructed wetland system is a theoretical application of ideas that we believe will radically alter the way urban areas handle their stormwater. Our study is untested, and thus a full understanding of its capacity and its cost are thus far unknown. Future applications will test and quantify the capacity of our system in-situ, as well as help to decide what substrates and plant matter should be grown within the wetland. These elements require case-by-case decisions, especially when concern around invasive species and aesthetics are concerned. Plants that are suitable for Wilmington, NC will not be suitable in Houston, TX. However, with adaptive design, our system can be designed to fit any urban environment, for the changing and variable needs of each city. While we believe that a gated water release pipe-system would be more than capable of regulating the water of an intense-storm, a pressure-valve or delayed release system is equally likely and should be taken into consideration.
Further, our wetlands are aimed at highway and roadside applications but could be applied to inner-city limits where space is available and the need high. Such areas include parks, parking lots, roundabouts, and other large wayside sites. If such a site were to be found, our wetland system could have further socio-economic benefits, wildlife habitat and more beyond the cost-mitigation of flood-damages. These benefits have been studied previously, but our integrated system has not been specifically studied. We believe that our system could change the way an urban environment manages and thinks about their storm-water and how urban areas may prevail in the face of a changing climate.
To conclude, our research is theoretical and preliminary. More research and an applied situation are needed to fully understand the capacity of this system. While that comes with undetermined costs, social and economic uncertainties, and faces an uncertain future. We on the team believe that if applied uniformly across road-networks in urban areas, you will see definitive improvements in the number of flood events, higher survival of urban infrastructure in the face of storms, and a City built to withstand and even adapt to a changing future.
Acknowledgements:
We would like to take this time to acknowledge the people who helped make this poster a reality. Barb Elliot, and Mike Fraser who were our mentors throughout this project have been invaluable to us, and their dedication to us as their students is something we will forever appreciate. They dedicate much of their time to making our posters here at the NCSE possible. Further we would like to thank Gordon Balch, who helped us with wetland dimensions and design, and whose availability throughout the project has allowed us to branch out from standard wetland design.
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