A Novel Multicriteria Decision Making Model for Sustainable Stormwater Management

Abstract

Precipitation not absorbed by the soil or local vegetation and remain on the surface leading to stormwater can cause soil erosion, flooding, property damage, and overflow to wastewater treatment facilities. This paper introduces a novel multicriteria decision-making model to choose among various sustainable solutions that can help in managing stormwater. This model is intended to help decision-makers in handling stormwater through proper utilization of precipitation while ensuring public safety and adhering to runoff regulations. The model also aims to present sustainable technologies that can help in reducing harmful stormwater overflows. As a way of constructing and validating the model, precipitation and other relevant data from the North-Eastern region of the United States were used. The model can be altered though to suit other regions in the world. The model was further validated by seeking the opinion of a group of experts on its constructs. Statistical analysis identified high item-to-total correlations for model constructs and a model Cronbach’s alpha value of 0.84 leading to conclude that the model is valid. Yet, green solutions presented in this study and the developed model should be considered as a first step in determining sustainable stormwater solutions and further research in this area is needed.

1. Introduction

Runoff is the result of precipitation that is not absorbed by the soil or local vegetation and remains on the surface. It is a process that usually leads to stormwater and, therefore, has commonly been known as stormwater. Runoff volume is affected by precipitation intensity and local surface composition and permeability. Changing lands for residential and commercial uses inevitably results in a decrease in pervious surfaces and an increase in impervious surfaces, which would result in a change in the hydrologic and hydraulic characteristics of the watershed. This in turn results in the rainwater adding to sewage collection, increasing the amount of sewage production by man-made structures. Common city-development practices encourage the excess water to seek outlets through the main sewage collection system [1].

High precipitation level can be a great threat to people’s safety and infrastructure systems. Impervious surfaces such as concrete and asphalt prevent rainwater from being absorbed into local soil and act as heat sinks, causing surges of stormwater to overflow into aging sewers and rising ambient temperatures. The permeability of a surface region varies based on the amount of urbanization the land has undergone. More land development and less natural ground cover result in a lack of permeable surfaces and produce a significant increase in stormwater volumes [1,2].

A surplus of runoff can lead to various safety issues within a community including, but not limited to, erosion, flooding, property damage, and effluent overflow at wastewater treatment facilities. Rainwater is not initially contaminated, but as stormwater flows across streets, parking lots, buildings, and grassy fields, it may collect and transport contaminants such as oils, fertilizers, pesticides, salts or other industrial and agricultural chemicals [3]. The result is redistribution of this pollution into previously ecologically stable and clean waterways, where contaminated water can become harmful or unsafe to people, animals, and plants [4,5].

To cope with increased amounts of displaced runoff from urban environments, a network of trenches, catch basins, and storm drain networks have been integrated into existing and planned infrastructural developments. Although these systems have proven efficient in redirecting water away from the urban structures, their design predominantly discharges this collected wastewater via one of two methods, each with its benefits and limitations [6]. The first method involves separating stormwater from the sewer system by using two different piping networks. The storm drain network directly sends collected rainwater into streams or other receiving water systems and the sewage collection network accumulates waste and transfers it to the local municipal wastewater treatment plant [6]. This system has proven effective in mitigating the negative consequences of runoff, but due to the lack of early regulations regarding such strategies, many cities currently lack this infrastructure. Installing such a safe and sustainable piping network in those cities is very expensive and may not be feasible.

The second method is a combined sewer system. This is the more common wastewater collection strategy. It is designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same piping network. This combined flow is sent to municipal wastewater treatment plants before being reused by the population or discharged into receiving streams [6]. The downside to this method is when a sanitary facility is overwhelmed by collected sewage and stormwater, it results in clogging, overflowing, and discharging of untreated hazardous sewage and wastewater directly into rivers, lakes, or oceans. These effluent overages of wastewater facilities are often referred to as combined sewer overflows (CSO) [7].

Sanitary analysis is defined as the evaluation of water for public use safety protocols. The US Environmental Protection Agency (US EPA) uses the terms of white, grey, and black water to categorize water qualities [8]. All three categories of water can be collected as stormwater. White water is clean enough to drink and for immediate use in hygienic purposes. Slightly contaminated water with some particulate or contaminated materials such as cooking waste, oils, fats, dirt, and sediments is referred as greywater and can be used for a variety of applications but is not safe to drink. Blackwater is defined as water having contact with industrial waste, fecal waste and sewage, or other hazardous toxicants. Blackwater is considered unsafe and requires treatment and purification before reuse. While water may be initially characterized by one of these classifications, each has the potential to be purified or to degrade. For example, greywater after remaining stagnant can have bacterial and pathogen growth causing it to degrade into the black water category [9].

Stormwater management (SWM) is defined as a mechanism for controlling stormwater for the purposes of reducing downstream erosion and flooding, and mitigating the negative effects resulting from urbanization [1]. Many countries have responded by adopting laws and regulations that address potential disruptions to the water cycle and quality of water supplies [1]. For example, the Clean Water Act of 1972 initiated the regulation of pollutants discharged into natural waterways in the United States. Since then, various states in the USA have adopted various laws concerning the quality of water and its handling [10,11,12]. Based on the local government’s legislature regarding stormwater, property owners are usually held responsible for what runoff their properties produce. Typically, this responsibility is assumed in the form of payments to local environmental or water collection agencies. Therefore, owners must become informed of what potential fines they may receive from any surplus of greywater leaving their land.

It is difficult to accurately anticipate the necessary capacity to capture the total runoff while constructing an economically efficient, safe, and sustainable system that operates year-round. Also, the cost to upgrade the existing water collection and treatment infrastructure might be too high for most local governments to implement. The growing instability in local water cycles limits the effectiveness of local wastewater treatment infrastructure, especially when faced with periods of drought immediately followed by intense periods of precipitation. Because most wastewater treatment plants are optimized for certain historical average water cycle patterns, they are often ill-equipped to handle the aftermath of unpredictable periods of sudden heavy precipitations. When the amount of stormwater exceeds the capabilities of local water treatment infrastructure, it causes untreated run-off to flow into waterways leading to damage to ecosystems and surrounding urban or rural infrastructure. This effect is intensified by the fact that many regions often experience frequent episodes of droughts. With the added influence of global warming, these concerns are further exaggerated by the fact that more frequent occurrences of severe rainstorms and droughts are expected [13,14].

A World Meteorological Organization (WMO) survey found that 105 out of 139 countries reported that flash flooding was among the highest two most severe hazards killing more people than any other natural disaster and causing severe damages annually [15]. Moreover, the soil becomes vulnerable to erosion from flash flooding, which can cause catastrophic mutilation to buildings and the nearby infrastructure. Therefore, there is a great need to assess and implement more sustainable solutions that will reduce the amount of excess stormwater.

Coping with stormwater by expanding and conducting necessary maintenance on currently aging stormwater systems can be difficult and costly. Besides this, information on available green and sustainable technologies to deal with stormwater are scattered and can be confusing. Achieving sustainable stormwater management goals will require combining environmentally inspired green technologies, modern infrastructure, and new adaptable treatment systems.

Stormwater can be more collectively dealt with if the city plans for an integrated and separate stormwater collection system. However, for older or poorer regions, some with a blend of urban and rural properties, such improvements in city sewage treatment facilities are overwhelmingly costly and impractical. Thus, it becomes the responsibility of individual property owners to deal with any possible negative impact on the environment through stormwater. Some legislatures have taken steps to impose these standards on private property owners where they are required to control their own runoff or be fined for neglecting the issue.

Based on the literature, it could be concluded that selecting an appropriate sustainable and green technology for dealing with stormwater is a sophisticated, time-consuming, ambiguous, and risky process that involves multiple, often conflicting, factors. The literature review also confirmed that there is no previous research that specifically aimed to suggest a clear framework for decision-makers (mainly property owners and municipalities) on utilizing stormwater and identifying sustainable green technology alternatives for them.

It is also found from the literature that sustainably utilizing stormwater and choosing among available stormwater management alternatives is a multicriteria decision-making (MCDM) problem with multiple (often conflicting) constraints (refer to Çolak and Kaya [16] who stated the same for energy alternatives for example). To fill this gap, this study investigates available green technologies reported in the literature to utilize stormwater and proposes an MCDM model for decision-makers to choose among available sustainable green technologies while ensuring public safety and taking into consideration local weather conditions and legal constraints. This MCDM model—called Green Technologies for Stormwater (GTS) model—is anticipated also to help in minimizing excessive runoff safety risks.

In order to prevent any contribution to the deterioration in environmental quality, stormwater impact should be quantified first and then a sustainable stormwater management system can be implemented. This paper aims to present a framework to do this. It aims to identify available green technologies for dealing with stormwater based on the literature review and to propose an MCDM model to help decision-makers in determining the best sustainable green technology alternative for their case.

2. Methods and Model Building

The GTS model proposed in the current study is meant to help property owners and municipalities in deciding the best sustainable green technology to deal with runoff stormwater based on their circumstances. Since different regions and countries in the world have different environmental conditions and legal requirements, the current study considers environmental conditions and legal requirements of the North-East region of the United States but can be adjusted to suit other regions in the world. At the same time, regional data used in proposing the GTS model also serve as a way of validating the model.

Tailoring the design of sustainable and green infrastructure projects to local conditions allows a property owner the opportunity to treat potential runoff issues at the source. Runoff laws in the United States commonly designate a property owner as the one responsible for mitigating stormwater leaving their property. However, a typical municipality in the United States may provide tax exemptions or waivers to encourage the usage of sustainable green technologies as innovative and creative solutions to stormwater or may create permit requirements, which address pollution control in landscaped areas. The proposed GTS model, which takes into consideration such regulations in the context of the North-Eastern region of the United States, is presented in Figure 1 while steps followed in developing the model are given in the following section.

3. Results and Model Development

In this section, steps followed in developing the model are given in details based on data from the North-Eastern region of the United States.

3.1. Precipitation Estimation

Figure 2 demonstrates a generic US suburban detached house featuring common government-provided stormwater infrastructure. It also displays natural and artificial structures and the resulting runoff across the various surfaces and grades.

House location’s climate dictates the amount of precipitation and temperatures experienced. Warmer climates and associated temperatures will determine the daily amount of sunlight exposure and evaporation rate of excess rainwater. The North-East region of the US is currently facing increased periods of intense precipitation resulting in considerable pollution of natural rivers and dangerous flooding. Therefore, this region would greatly benefit from stormwater alleviation technologies.

To elaborate, precipitation data were solicited for the Pittsburgh International Airport (PIA) area from the National Oceanic and Atmospheric Administration and evaluated for the resulting intensity storms. Precipitation intensities are commonly measured in terms of their frequency over one or more years. For example, a 25-year storm will occur, on average, once every 25 years. This information, based on the Pittsburgh climate, is presented in Figure 3. It shows that the fewer the precipitation episodes, the greater the intensity and resulting stormwater experienced for that respective storm forecast.

3.2. Estimating Runoff Amounts

Local legislature establishes regulations defining not only the quality of water that can be offset but also the amount of runoff that must be either diverted properly or treated and collected. Therefore, a property needs to be evaluated regarding these laws to determine what runoff mitigation action is required. If the site meets the established laws, then no current action is required. If the site does not meet the laws, then the amount of runoff that must be offset to meet these requirements needs to be determined. To properly redirect runoff from any property and determine what green technology applies to the location, several factors must be considered, which are:

(1)

Are there any existing local storm drains which the site’s runoff may tie into?

(2)

Can the (greywater) precipitation be re-directed towards a nearby natural waterway if a storm drain system is not available?

(3)

Does the topography of the lot allow for cost-effective diversion of stormwater?

(4)

Is there space available for the installation of green stormwater diversion tools and strategies?

When considering the surface runoff from a property, primary factors are related to the surface topography and climate. Surface runoff coefficients represent surface features of the property and are determined based on the degree of impermeability of the ground cover. Then, based on where in the world the property is located will determine the climate that the site will experience as well as the precipitation intensity episodes in its region.

The ability of a soil to absorb moisture is dependent on its infiltration properties. These properties vary based on the various soil types found on the location. The US database for precipitation accumulations in a specific area may be found in the Hydrometeorological Design Studies Center (HDSC) within the Office of Hydrologic Development of National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS).

Using the runoff coefficients data, which designate values representing the impermeability of the surface, allows determining the amount of runoff leaving the property. Highly impervious surfaces yield runoff coefficients approaching “1” while natural pervious surfaces have low runoff coefficients approaching “0”. For example, the city of Austin in Texas considers runoff coefficient between 0.75 and 0.97 for concrete surfaces or roofs, 0.73 and 0.95 for asphaltic surfaces, 0.21 and 0.62 for grass areas, 0.35 and 0.6 for undeveloped cultivated land, and 0.22 and 0.58 for forests [17,18].

The total soil runoff value will designate a net gain or loss in water accumulation. A net gain indicates the presence of runoff while a net loss indicates the complete absorption of the precipitation into the soil. The precipitation flowrate generated on a particular site is a function of the surface runoff coefficient and the precipitation volume per hour. The higher the concentration of precipitation over time, the more quickly the surface becomes saturated and begins to reject rainwater, turning it into a runoff. Based on the location’s topography and surface area, the rational method for evaluating runoff to calculate the peak discharge by using Equation (1) [18]:

$$Q_P=CiA,$$

(1)

where

$Q_P$ 

peak discharge (m³/s)

A 

drainage area (ha) (1 ha = 10,000 m²)

C 

a runoff coefficient based on the ground surface type (0 < C < 1)

i 

average precipitation intensity (mm/h)

If diverting stormwater is not a feasible solution, then more substantial investment options should be considered for collecting and storing the stormwater. Such a decision should be made between artificial and natural water collection systems. The natural system offers resolutions such as retention ponds, which require minimal maintenance beyond installation. If the user wishes to recycle collected water, then an artificial collection system is required where a storage tank is selected based on the desired amount of rainwater collection relative to the amount of precipitation experienced [19]. Greywater must be filtered to acceptable purity level standards before storage. This water can then be reused in watering gardens, washing cars, or even in evaporative technologies.

Figure 4 demonstrates the relationship between the ground cover type and resulting runoff using Equation (1). To focus on the relationship between the percent of impervious surfaces relative to the resulting peak discharge runoff, precipitation intensity and total lot area were held constant. Runoff coefficients are selected to represent urban and natural ground cover based on available published data [17,18,20]. These runoff coefficients are 0.33 for the natural land and 0.9 for the urban land. The x-axis depicts the percentage of impervious (urbanized) ground cover.

Using Figure 4 and Equation (1), one may calculate the resulting runoff of a certain property based on Pittsburgh International Airport location’s typical precipitation intensity. As mentioned earlier, precipitation intensity and total lot area were held constant, along with the assumed runoff coefficients. When analyzing a specific property, how much of the ground cover is classified as natural and how much as urban (i.e., manmade) should be determined first in percentage terms. The percentage of manmade ground cover is then correlated to a runoff rate, in L/min, the location can expect during a precipitation episode producing runoff coefficients consistent with the assumed precipitation rate. To elaborate, if the precipitation intensity for a region experiencing a two-year storm (i.e., an intense storm predicted to occur bi-annually) is estimated to be 2.1 mm/h, the resulting runoff rate would be as follows. For a lot containing 0% urban surfaces, the produced runoff would be 46.56 L/min, whereas this would increase to 127.55 L/min for a 100% urban property. This demonstrates how significantly the type of ground cover influences runoff production. As the urban surface coverage increases, it directly correlates to a drastic increase in stormwater. In the described example, it produced a runoff increase of approximately 174%. This relationship is further demonstrated in Figure 5, which visually depicts the relationship between the amount of natural versus artificial ground cover as well as the resulting runoff, infiltration, and evaporation trends.

Sustainable green infrastructure may be integrated into a community through a variety of approaches ranging from small-scale elements at local levels up to much larger-scale installations that span entire watersheds [4]. EPA has found that green infrastructure, when properly integrated into cities, will provide sustainable flood protection and cleaner water. Therefore, a variety of approaches must be considered to properly address the runoff impact at the location of interest [21]. After considering existing scenarios in their properties, decision-makers have to choose among various solutions. As depicted from the published scientific literature, those solutions fall under three categories: Diverting runoff, runoff harvesting, and artificial solutions. These three categories will be discussed in detail in the following sub-sections.

3.3. Diverting Runoff

The goal of green diverting (i.e., rerouting) runoff technologies, which usually cost more than runoff harvesting solutions, is to merely divert and redistribute stormwater and its byproducts (silt, pollution, etc.) properly into natural greenery and waterways. Implemented design choices may be subtle and diverse. However, they mainly pivot around downspout (rain gutter) disconnections, rain gardens, planter boxes, bioswales, green pavement, and permeable surfaces, all illustrated in Figure 6.

3.3.1. Downspout Disconnection and Rainwater Harvesting

Although it is ideal to install sustainable green features during the initial design and construction of a project, downspout disconnections and rainwater harvesting are two low-cost strategies that are well suited to retrofit implementation with the existing systems to generate various benefits [22]. Downspout disconnections divert rainwater collected via rooftop drainage pipes and, rather than sending it directly into the main city sewage collection systems, can reroute this water into rain barrels, reservoirs, or other permeable areas where it can be allowed to disperse and/or evaporate away properly.

These adjustments to rooftop drainage decrease the peak discharge of a site by reducing the volume of roof runoff. Therefore, although this may be a small-scale correction, it is believed that the cumulative effect of multiple implementations of this strategy will compound into substantial alleviations to the existing wastewater collections systems. Downspout disconnection could be beneficial to cities where combined sewer systems are too heavily relied on as they will allow for the private and public property owners to utilize this excess runoff for their applications [4].

3.3.2. Rain Gardens and Planter Boxes

Water collection systems such as rain gardens and planter boxes offer further solutions to a property owner with the added benefit of visual appeal. These technologies require little to no maintenance and have an exceptionally long lifetime if implemented properly [4].

Rain gardens are green projects that can be installed in almost any unpaved space. Also known as bioretention or bioinfiltration cells, these are shallow vegetated basins that collect and absorb runoff from rooftops, sidewalks, parking lots, and streets. Design of these gardens consists of depressions in a property’s landscape to collect and store stormwater, holding this until it properly infiltrates back into the local soil. It should be noted that rain gardens are not ponds; they do not merely gather and stagnate the collected water, rather their purpose is to allow the runoff to gradually absorb into surrounding soils and plant root systems. By doing this, a rain garden can prevent flooding and erosion problems. Rain gardens are typically easy to construct, require minimal land consumption, are flexible in their design manipulation for versatile applications, and are easy to maintain and upkeeps [18].

Rain gardens and planter box systems are ideal for space-limited sites that will be more common in dense urban areas [4]. Additionally, planter boxes are commonly used as a visually appealing addition to an urban property as they bring in the esthetic of natural vegetation back into hard and cold urban developments.

Still, rain gardens have several disadvantages associated with their implementation and potential for a return, which must be noted. Firstly, they are ill-suited to limited spaces as their effectiveness depends on the number of available properties. Secondly, proper landscaping and planning are required to successfully integrate them into a property, as they are susceptible to clogging if the surrounding terrain is not designed or managed properly. Lastly, they cannot be successfully constructed in areas with steep sloping topographies making them impractical for more mountainous regions. In such situations, the rain gardens are more pragmatic alternatives to the planter boxes.

3.3.3. Bioswales, Infiltration Trenches, and Infiltration Basins

Infiltration basins, infiltration trenches, and bioswales are adaptations of vegetated stormwater collection devices that are commonly placed alongside properties, buildings, roadways, and parking lots [23]. During intense storms, water follows the local topological gradients until captured in and around the vegetation or flowing into the larger waterways. The root systems of plants will absorb some rainwater as well as provide pathways for water movements. This allows some precipitations to pass through the root zone so that it can move farther down to the zone of saturation and towards the water. After an amount of time passes, this water re-emerges on the surface through geologic features such as creeks, rivers, and springs. These green infrastructure techniques take advantage of this natural behavior and have all been found to be technically feasible depending on site-specific conditions.

Among available green infrastructure techniques, bioswales are vegetated, mulched, or landscaped channels that convey stormwater from one place to another while providing treatment and retention to promote natural pollutant removal [24]. These vegetated swales are designed as linear features which slow, infiltrate, and filter stormwater, making them particularly well suited to being placed along streets and parking lots where a majority of human waste tends to be collected by rainwater runoff [4].

When compared to other runoff management strategies, often referred to as best management practices (BMPs), bioswales are considered preferable as they are more feasibly instituted when space is limited since they can be easily situated in small spaces such as medians and shoulder areas. Moreover, bioswales and other vegetative surface controls have less rigorous maintenance requirements than traditional BMPs, making them a low-cost and low-upkeep investment for a property owner [22]. Also, a report by the National Cooperative Highway Research Program [25] indicated that vegetated swales are among the least expensive green devices, and among the best performers in reducing sediment and heavy metals in the runoff.

3.3.4. Permeable Pavement

Due to the large runoff coefficients of paved surfaces, low-impact development strategies recommend that permeable paving can be utilized as a substituting surface for parking areas and other paved surfaces [3]. Permeable pavements allow rainwater to transition directly into the soil where it falls instead of being redirected, collecting various forms of pollution, and entering a centralized collection system. Design options include pervious concrete, porous asphalt, or permeable interlocking pavers [26]. The use of pervious concrete is among BMP recommendations by EPA [7].

Unfortunately, though, these benefits bring a significant cost increase. Based on information solicited from permeable paving suppliers, in the case of porous asphalt and pervious concrete, construction cost may increase by up to 50% than conventional asphalt and concrete. Furthermore, as with any site improvement, property owners should provide a budget for maintenance of permeable paving at an annual rate of 1–2% of construction costs [5]. Although the material itself and contracting its construction may cost more, this technology allows for more sustainable and efficient land use by eliminating the need for rain gardens, retention ponds, bioswales, or other stormwater management devices, especially where land values are high and flooding or icing is a problem [5]. Again, the collective goal of these green strategies allows imitating the natural hydrological cycle by infiltrating, evaporating, and transpiring the stormwater safely back into the environment [4].

3.4. Runoff Harvesting and Storage Systems

If diversion is not sufficient to mitigate stormwater issues for a site, then artificial rain-harvesting and storage systems must be considered. Knowing the variability in the intensity of precipitation episodes will have a great impact on any diversion or storage implementation strategy and budgeting. When determining artificial storage options, it is practical to select a stormwater containment system with a minimum capacity, which can withstand a one- or two-year precipitation accumulation record. Higher precipitation events occur at times, but the containment of large stormwater events is not practical due to the volume of precipitation generated in such events. Based on this information, a property owner can then properly implement sustainable technologies, which allow users to store stormwater to be repurposed at a later time.

A particularly notable benefit of rain-harvesting and storage systems is in dry regions where limited water supplies impose heightened demand for water conservation [4]. Harvested rainwater can be employed for various outlets that include landscaping, in-home uses, wildlife, livestock, fire protection, or other stormwater management strategies. Two notable systems for rainwater harvesting and storage are suggested here, green roofs and retention ponds. These solutions are discussed in the following sub-sections and illustrated in Figure 7.

3.4.1. Green Roofs

As the name suggests, green roofs are roofs covered with vegetation that can absorb large quantities of water (stormwater). Their design requires a layer of insulation, a waterproof membrane, a layer of growing media, an irrigation system, and the desired flora [27]. Not only will this design allow for alleviation of stormwater, but also offer potential energy benefits for the structure that will be constructed onto. The rooftop garden will intercept and dissipate considerable amounts of solar radiation, reducing cooling costs in summer and warmer climate regions, as well as by increasing the lifespan of the structure itself [28]. When compared to other green rainwater storage solutions available, green roofs require the least amount of physical property, making them optimal in industrialized regions where property costs are high and often lacking greenery.

Although green roofs are designed to be self-sustaining and require minimal maintenance costs, initial installation costs can become considerable and these systems require some maintenance infrastructures. It is expected to spend 25–30% more on a green roof installation, compared to a traditional roof. To note, green roof setups can cost anywhere from $320 to $2100 per square meter. Still, a properly installed and maintained green roof may extend the lifespan of a conventional roof by nearly 20 years [29].

Green roofs are considerably heavier than conventional roof designs so the structure will require reinforcement if a green roof is to be installed. This added weight not only comes from the storage of collected rainwater but also the growing media and plant life. Because of this factor, most existing green roofs have been implemented in new construction, making them poor retrofitting solutions to alleviate stormwater [28].

3.4.2. Retention Ponds

Retention ponds accumulate rainwater runoff from a site into a permanent pool of water. The pond holds the water until it may naturally seep back into the surrounding soils as well as evaporate, typically over several days. Unlike detention ponds, retention ponds constantly maintain a pool of water. This process allows for a small reduction in stormwater while simultaneously purifying any collected runoff. This natural filtration process utilizes gravitational settling of suspended particulates, biological absorption of pollutants by plants, algae and bacteria growing in the pond, as well as the decomposition of some contaminants [30].

As the effectiveness of the pond is contingent on the size and inflow volume of the pond, this is not a good solution in locations where the land area is limited and/or expensive. If designed properly, though, maintenance and associated costs are minimal when compared to the green roof solution [31]. Retention ponds are expensive stormwater BMPs. Installation and operation of these ponds include one-time capital costs, one-time installation costs, recurring maintenance costs, and recurring waste disposal costs.

3.5. Artificial Solutions

Installation of an artificial storage system, which is usually more costly than both diverting runoff and runoff harvesting solutions, allows a property owner to recycle collected stormwater for both evaporative processes as well as an additional water resource. Potential artificial storage solutions include rainwater collection, treatment, and recycling as well as evaporative systems. Figure 8 demonstrates how an artificial collection tank may be placed either above or below ground, depending on the available area and the amount of financial investment. Furthermore, smaller and less costly alterations, such as rain barrels, may be easily incorporated into existing gutter systems. Shrubbery and/or other plants may be used to conceal unsightly rainwater collection infrastructure.

The design of an artificial storage system requires (1) the selection of a tank size to be implemented, (2) selection of a filtration system, (3) determination of how the water will be repurposed, and (4) the design of a piping system to integrate the first three components.

To determine the selection of the tank size, a property owner must first know the amount of stormwater, which must be offset or collected from the property and based on that, an artificial storage tank could be selected. The relationship between the costs of these tanks concerning storage capacity is shown in Figure 9.

A linear representation of the cost vs. storage capacity trend can be made based on Figure 9. Tank costs were based on the market value of these tanks based on information solicited from local fiberglass tank suppliers. Following this, Equation (2) was derived from the data plotted in Figure 9. This equation can be used to estimate the cost of fiberglass tank required. Although the relationship was based on average market values, it should be noted that prices will vary based on location and, possibly, other factors.

y = 0.8974x + 2.5757

(2)

where,

x:

the storage capacity (m³)

y:

cost of fiberglass tank (USD)

3.5.1. Filtration

The potential use of runoff or stormwater is dependent on the water quality being collected. It is categorized by the quality and degree of contamination. Possible pollutants in stormwater include solids, volatile organic compounds, and even heavy metals. These toxins are primarily collected when stormwater drains over urbanized or industrial sites. Federal regulation NPDES 40 CFR 122.26 sets the standards regarding pollutants in stormwater and their control [32].

Using proper filtration techniques can preserve water quality at the status of greywater for timeframes ranging from weeks to months. This degree of water quality is much more viable, useful, and economic in applications such as water-recycling and/or evaporative technologies. The flow rate must be determined for a two-year storm rain event, and the filter must be sized according to this level of flow. If the flow rate is high, more than one filter may be required to adequately remove pollutants. If rainwater is collected directly from the roof of a structure (before making contact with the ground), a catch basin filter system could be used to catch and remove heavy metals, organics, hydrocarbons, and solids [33].

3.5.2. Evaporative Cooling

To determine if evaporative technologies will offer a return on a property owner’s installed runoff alleviation infrastructure, the owner must consider climate conditions (i.e., temperature and humidity profiles) and the cooling surface area. Evaporative spray roof cooling (ESRC) adds the potential for cost savings through repurposing collected runoff for cooling benefits [34].

ESRC can redirect stored water onto roofs during peak heating conditions to act as a potential cooling system to structures and the versatility of applications to buildings of various scales—ranging from massive corporation warehouses to small residential homes.

ESRC has the potential for a positive return on investment to the building owner or developer because the roof is cooled passively via evaporation instead of using an actively driven process like an air-conditioning (AC) system [35]. During summer months, solar radiations and atmospheric temperatures drive dry roof temperatures as high as 50 °C or more, depending on the color and composition of the roof [36].

The ESRC technologies work to remove thermal energy contained in a building’s roof. As stated by a study on evaluating ESRC potentials, each kilogram of water evaporated absorbs approximately 2257 kJ of heat, this translates to a 22.2 °C to 33.3 °C reduction in the roof surface temperature [37]. This corresponds directly to a reduced need to actively cool down the interior spaces and, in turn, can reduce air conditioning costs and other negative effects on the environment. It also should be noted that too much water on the roof diminishes cooling potential because proportionately less of the water will evaporate. Therefore, to achieve optimal heat transfer via evaporation, no more than a thin film of water should be applied at any one time [35].

Although ESRC has been shown to lower roof temperatures several degrees, these systems are not a direct substitute for the AC one. AC cools the air inside the building directly while ESRC cools the structure of the building and relies on heat transfer from the internal air to the structure. It dissipates the substantial heat gained from solar radiations, convection from the surrounding warm air, and conduction through the roof itself. The ESRC works best when combined with other “environmentally clean” systems to integrate multiple green technologies into a system that prevents excessive stormwater, collects a portion of that runoff, and uses it to keep buildings cool while minimizing loads on the existing water treatment infrastructure and minimizing utility costs in the long term [38].

4. Model Validation and Discussion

On top of implementing the GTS model in the context of North-Eastern region of United States as a way of validating it, it was vital to do content and face validity of the model to ensure its comprehensiveness and consistency. Content validity is defined as the extent to which items are representative of the construct of interest [39] while face validity is defined as appearing to be valid in addition to being valid [40].

Netemeyer et al. [41] recommended seeking the opinion of experts in the field of study to evaluate the content and face validity and suggested that all parts of any survey, including concepts, items, response formats, scale points, and directions are to be reviewed for representativeness by those experts. In developing similar models, the number of experts whose opinion is sought might range, according to their scarcity, between one and nine or sometimes more [42,43,44]. In the current study, the opinion of six experts in stormwater and green technologies was sought through face-to-face interviews. Those six were senior engineers and researchers who were involved with stormwater and relevant green technologies for at least five years in the North-Eastern region of USA.

The experts were asked to examine the taxonomy of items that form the basis of the GTS model. These items include coherence and format of the GTS model, logical sequence of the steps followed in the model, adequacy and comprehensiveness of runoff solutions under all three categories (runoff rerouting, runoff harvesting and artificial storage). The experts also were asked to comment on the overall implementation of the GTS model. They were asked to rate each item on a 7-point Likert scale. Likert scale was deemed appropriate in this case as it gives a good opportunity to capture views and thoughts of respondents [45]. Assessing reliability is also very important within the validation process. Reliability of the GTS model was checked using two methods, Cronbach’s alpha value [46] and item-to-total correlation values. DeVellis [45] and Nunnally and Bernstein [47] recommended that Cronbach’s alpha value should be greater than or equal to 0.7 for the model to be considered valid. As there is no single agreed-upon method to judge the strength of item-to-total correlations [45,48], it was decided to consider item-to-total correlations with alpha values greater than or equal to 0.6 to be strong.

In

Table 1. Mean and correlation α values of items.

Questions	Item	Mean	α Value of Item-to-Total Correlation
Does the GTS model describe to the decision-makers (homeowners and/or municipalities) how to deal with stormwater in the context of the North-Eastern region of USA?	Demonstrates how to deal with stormwater	5.14	0.86
Does the GTS model present to the decision-makers main available solutions to deal with stormwater?
Does the GTS model consider local regulations?
Does the GTS model describe when runoff rerouting is possible?	Demonstrates when runoff rerouting is possible and its options	4.92	0.79
Does the GTS model describe the main runoff rerouting options?
Does the GTS model describe when runoff storage is required?	Demonstrates when runoff storage is necessary	5.12	0.86
Does the GTS model describe when runoff harvesting is possible?	Describes when runoff harvesting is possible and its options	5.02	0.81
Does the GTS model describe the main runoff harvesting options?
Does the GTS model describe when artificial storage is possible?	Describes when artificial storage is possible and its options	5.02	0.81
Does the GTS model describe the main artificial storage options?
Does the GTS model consider the main procedures and mechanisms to deal with stormwater and reduce the risk of flooding?	Demonstrates model’s contribution to reducing the risk of flooding and improving safety	4.97	0.77
Is the GTS model implementable in real life and does it contribute to improving the safety of household residents in cases of heavy rain?	Demonstrates model applicability in real-life situations	5.14	0.86

, questions asked to seek the opinion of the six experts on the adequacy of model items in capturing the constructs in concern, are presented. In the same table, the mean and α values of item-to-total correlations of items investigated are also given. As can be seen in Table 1, α values of item-to-total correlations of the items explored ranged between 0.77 and 0.86 indicating that the scale was internally consistent and reliable [46,47]. Moreover, the GTS model’s Cronbach’s alpha value was found to be 0.83 indicating that the model is valid and coherent [45,47].

5. Conclusions

In most developed and developing countries, property owners and municipalities have legal obligations related to dealing with stormwater. This study presented a multicriteria decision making model that can help property owners and municipalities in their efforts to deal with potential harmful stormwater overflow and flooding effects. One of the main available options is utilizing green technologies to deal with this runoff. Initial costs for implementing green technologies may be disconcerting, but green technologies offer significant payback over time, which makes them self-sustaining and attractive additions to a property. Green stormwater technologies can be both fiscally and environmentally responsible choices. In particular, collecting, filtering, and reusing rainwater provides a property owner with a cheap water source and reduces the amount of water purchased from utility companies. As the savings associated with this recycled water from green technologies will vary, each region will define its respective costs and savings. Furthermore, integrating runoff management utilities on a municipal basis provides dedicated revenue for stormwater management through equitable measurements and billing, similar to that of a water meter used for drinking and wastewater [22,49].

The validity of the GTS model was then checked further by seeking the opinion of a group of experts who were asked to rate the adequacy of the model items. Statistical analysis of responses gave high item-to-total correlations of all items explored. Validity analysis of expert opinion also gave a Cronbach’s alpha value of 0.83. Both results, deducted from face-to-face interviews with experts, lead to the conclusion that the model is valid and delivers what it was intended for.

GTS model can also be seen as a decision-making tool for the Leadership in Energy and Environmental Design (LEED) program. LEED is a well-known green building certification program developed by the United States-based non-for-profit Green Building Council (USGBC). It evaluates and recognizes best-in-class building strategies and practices. For property owners, LEED certification could improve public and community relations, and lower operating costs. Recent studies have shown that most corporate leaders believe that sustainability leads to market differentiation and improved financial performance [50,51]. This paper introduced a MCDM model to serve as a tool for property owners, builders, and municipalities when deciding on green technologies to deal with stormwater. It is believed that the GTS model presented in the current study will also serve as a decision-making tool for those property owners, builders, and builders on LEED certification options when dealing with stormwater.

Surface elevations and the type of ground cover determine the movement of water across the site, while the climate will alter the state of the rainwater. Buildings are often constructed with highly impermeable materials resulting in increased amounts of immediate runoff. If any ground has been disturbed via construction, this will also affect the movement of groundwater, as well as the settling of soils across the property. Therefore, the most ideal time to implement the above-mentioned sustainable green technologies into a property surface is at the beginning of new construction. This will minimize the overall costs associated with installation as well as allow for more successful integration in the overall design of the property [52].

6. Limitations

It should be noted that the GTS model and its implementation should be taken with caution. As it is the first model of its kind in this area, more research is needed to enhance and further validate the model. Adding other emerging solutions and doing some adjustments can also be considered. Details and steps of the GTS model presented in the current paper concentrate on the North-East region of the United States. Therefore, other regions and countries would need to be studied separately taking into consideration their weather conditions, local legislations, and other factors. This was beyond the scope of the current study but is recommended for future research.