Due to the rapid urbanization around the world, the natural land covers are changing into impervious surfaces and degrading the natural environment. These adverse effects include the increase runoff and peak flows that cause flash flooding and water quality degradation and other water related problems . In addition, climate change is another big challenge that is making our cities vulnerable. Applications of the Low impact development (LID) practices are proved as the sustainable solutions to mitigate the adverse effects of urbanization and to retrieve the natural hydrology of urban areas [2, 3]. Moreover, retrofitting of Low impact development (LID) or Green infrastructure (GI) practices have been introduced which are becoming the most propitious practices; these practices have the ability to reduce the adverse impacts of urbanization on the natural landscape [1–3]. LID practices benefits include the reducing the pollutants from runoff, decreasing the heat island phenomena and enhancing the infiltration [4, 5]. These practices reuse the urban water through the collection and infiltration; this is the key process of the urban hydrology, as it is close to the natural water cycle . Another advantage of LID practices is the ability to collect a large amount of runoff and to reduce it through infiltration and evaporation . As a result of this, the flash flooding and other water-related problems are reduced .
Traditional method/grey infrastructure is a method that throws the excess amount of water to the far places with the help of pipes and gutters . And if we want to treat this sewage water then we can use the big water treatment plants which significantly increase the cost. The traditional method (pipes and gutters) of controlling runoff is causing many problems. For example, during the big storm events, the volume of the stormwater can exceed the capacity of the system, that results in the overflow of sewage water into nearby lake or river known as combined sewer overflow (CSO) . This is especially true in the areas where the stormwater is handled through the combined system. These traditional methods of handling stormwater adversely affect the water quality of large scale and affect the social, environmental and economic benefits. The traditional method only considers the quantity control of stormwater rather than other social and environmental aspects. While on the other hand, LID practices have a potential for controlling the runoff, enhancing water quality and trying to change the grey infrastructure into green infrastructure [10, 11]. In urban areas, the biggest challenge is the limited space available for the application of LID practice. Retrofitting LID practices is an alternative option to handle the water related problems and make our cities safe and sustainable. These practices reduced the rainfall runoff by collecting, infiltrating, evaporating near the source that makes it possible to avoid the overflow events. Retrofitting LID practices consider in multiple way i.e. social, environmental and economical way to make city safe and self-sufficient. Urban cities already developed; most of the natural land is converted into the impervious surfaces. Under these conditions, retrofitting LID practices is an option to change our grey infrastructure into green infrastructure. This will not only result to solve water related problems but also make our cities safe, sustainable and resilient to climate change. Figure 1, shows that grey infrastructure majorly concern to control the runoff quantity rather than quality and amenity values. The runoff is collected through the pipes and throws to the far off places. On the contrary, the green city gives equal importance to quantity, quality and amenity values.
Little information is available that concern to find the optimum place for retrofit LID practices in urban areas [2, 14, 46]. This paper describes the low impact development practices basic functions and benefits of the retrofitting LID practices in urban areas. Optimal places to get multiple benefits by retrofitting LID practices are also included. At the end, barriers for implanting LID practices and potential solutions are also indicated. This paper also provides the information for individual, public and stakeholder to shows the LID/GI practices functions and their importance for the retrofitting in urban areas to achieve multiple benefits.
2.1 Low impact development (LID) / Green Infrastructure (GI) Practices
LID practices are the innovative stormwater management practices that had shown many benefits in different regions all around the world [1, 3, 14]. These practices handle the stormwater through small decentralize practices. LID practices include green roofs, blue roofs, Bioswales, Bioretention, rain barrel, pervious pavement, and tree box planter [4, 44, 62]. Retrofitting LID practices are providing financial, social and environmental benefits in a developed urban area and trying to make it safe and sustainable. Urban areas are complicated areas, as they consist of many complicated structures so the retrofitting, LID practices require special consideration for the selection of appropriate LID practices as well as the place. Therefore, first of all, user should know about the basic function of each LID practices to apply it at the appropriate place for multiple benefits. To understand each LID practices basic functions they can be divided into following two categories. Tablet shows the categories of LID practices on the basic of their function.
These two categories are explained below.
2.1.1 Storage LID practices
This group is referred as storage LID practices that only have the storage/retention function. These LID practices can apply to the areas where there is need to store the water rather than infiltration into the soil (ground). The optimal places to apply storage LID practices are roof areas, ground areas above the underground rooms or parking lots. Examples of storage LID practices are blue roofs, green roofs, roof garden, rain barrels, permeable pavements above the parking lot and grass swale above the basement rooms. This group LID practices are explained as given below.
Green roofs are the vegetated roof that consisted of different types of vegetations, can be applied to new construction or retrofitting options at residential, commercial or industrial. While applying the green roof, the structural requirements such as the weight of the additional roof on the structure and slope should be considered . These types of roofs have the ability to retain and detain the large amount of the rainwater that helps enhance the air quality, water quality, to reduce flooding and heat island phenomena in urban areas [14–17]. Blue roof are the temporary storage type roof, this kind of roof has the ability to detain the stormwater during storm events . Blue roofs are now implementing into the big project all around the world because it is cost effective and easy for the retrofitting options.
Rain barrel and cistern are simple structures that are capable of collecting and storing the water for later use . A rain barrel can be constructed in two days, the simple one can be a just a barrel having a screen at the top to keep out debris placed under the downspout near the building, but other more complex system according to the requirements can be constructed. They can be constructed above or underground, near inside or outside the building with different capacities to collect water. Rain barrels are the best management practices because they are easy to apply, keep water out from the sewer system and reduce the water demand and stormwater management costs . The evidence of this is that the City of Chicago is estimated to have diverted almost 8.3 million gallons from its sewer system by applying the rain barrel and downspout disconnection .
Blue roof is the non-vegetated source control that use to retain the storm water for the longer duration. This type of roof also called as reservoir type roof because it has temporary storage up to a certain height to stormwater. This roof types can collect the stormwater on the roof for longer duration that results in reducing the peak flow in the urban area. Shafique et al.  installed the blue roof on the Seoul City Hall Annex Seosomun, Korea. They analyzed the results during different storm events to check the performance the blue roof to control the stormwater. During the storm events of September 2014, the results showed that the rainwater runoff from blue roof was 0.45 l/s as compared to the common roof that was 1.55 l/s. This shows that the blue roof as very helpful LID practices to collect the storm water for longer duration in urban areas.
Other LID practices such as permeable pavements above the parking lot and grass swale above the basement rooms commonly used where we only need to collect the stormwater. In these two cases, the water cannot infiltrate into the ground because there is a structure available. These two practices we can apply at the suitable places to control the runoff and to reduce the flash flooding problems in highly developed urban areas.
2.1.2 Storage + infiltration LID practices
This group of LID practices has storage as well as infiltration function. LID practices in this group can retain and infiltrate the runoff water into the ground refer to Table 1.
Different LID practices that already applied at different areas around the globe are explained below which can provide the information about benefits of these facilities. These above-mentioned LID practices can be applied new and retrofitted in existing areas.
Bioretention system also called as rain gardens are the depressed landscape features designed for the rainfall runoff treatment and storage at the site . Bioretention is a little complex system but has proven to be an effective system for the managing a large amount of stormwater runoff volumes and enhancing the water quality . Rain garden reducing the runoff and peak flow as well as it can aesthetically improve the nearby area by providing the trees and plants .
Pervious pavements referred to the pavements types that can infiltrate the water through the pores in the pavements and reduce the runoff. Permeable pavements can be an effective method to reduce or eliminate a large amount of runoff in an area . In impervious surface such as roads, roofs, buildings the amount surface runoff is closely following by rainfall rates during the storm events . On Contrary, the permeable pavements allow the water to infiltrate into the ground, which can reduce the chances of flash flooding . Permeable pavements require special maintenance after the construction to achieve its longterm goals, although in the cold climates permeable pavements may require fewer maintenance costs as compared to the impervious pavements winter costs .
Grass swales are the landscape features that are using for the different purposes such as to decrease runoff by infiltrating into the ground, moving to the other place and improving the stormwater quality. Different plants/vegetation used for the bio swales for the stormwater management. Grass swale has different slope and erosion control material that make it more functional . This stormwater management practice reduces the gutters and curbs costs as compared to the conventional stormwater management . There are different infiltration swale systems that include grass swale, filter strips or biofilter, bioswale. Grass swales are the cost effective GI practices for the stormwater management in the urban areas. These practices can easily set along the roadways, highways and park lots.
Tree box filters are the in-ground containers usually contained the street trees in urban areas . Runoff of nearby area is diverted to the tree box, where it is collected and infiltrate into the ground. This system adapts the bioretention principle to control runoff and to enhance the filtered water quality. This system is easy to construct and to maintain.
There are also more two possible options such infiltration trench and porous infiltration pipe; can also use for the stormwater management in urban areas. These two practices have ability to collect stormwater and infiltrate into the ground [2, 11]. These practices can be applied near the garden areas where they can control the runoff and infiltrate into the ground. These practices can easily apply where we there is a less space available to apply the LID practices.
3 Places for Retrofitting LID/GI in urban area
As the most of the cities of the world already developed, so that to apply the LID practices as the retrofitting options demand more intentions for selecting available space and suitable LID practice. For the new development, LID practices can easily apply to achieve multiple benefits because there is enough space available to apply LID practices. Therefore, the selection of suitable LID practices for multiple benefits needs special concern. This study provides the possible options to retrofit LID practices at the suitable place for multiple benefits. For this purpose, the catchment area can be divided into three parts as follows: (1) Roof catchment area, (2) Ground catchment area, (3) Artificial ground catchment area. Figure 2, shows the catchment areas where LID facilities can be used for multiple benefits. On the roof catchment area, different LID practices can apply to store water and the excess water can be diverted to the nearby LID facilities on the ground. In this way, the roof can control runoff at the roof as well as the ground surface. Here the ground catchment area is the natural ground surfaces areas that allow storage as well as infiltration through the soil into the ground. In this area, different individual and combination of LID facilities can apply to achieve multiple benefits. Thirdly the catchment area is referred as artificial ground catchment area that is created by our own on the ground surface to apply LID facilities for the storage purposes only. In this area, the water can only store and not infiltrate into the ground. For example, permeable pavements on the underground parking lot where the water in the only store into the voids of soil and surplus water connected to the sewage system.
However, for the multiple benefits of retrofitting LID practices into a developed urban area, there is a need to consider the suitable LID practices in the suitable catchment area that can give multiple benefits with less cost. In retrofitting LID practices, the most important thing is the selection of cost-effective LID practices combination at the suitable place to make city safe, sustainable and resilient to climate change. Figure 3, shows the suitable places for applying the best combination of LID/GI facilities that can give multiple benefits in urban areas.
Retrofitting LID/GI practices at the different catchment areas are explained below in details.
3.1 Roof catchment area
In developed countries, the level of urbanization is continuously growing and forecasted to reach 83% in 2030 . This ongoing urbanization unstable the natural hydrology of an area and creating many water-related problems in cities. In the urban areas, roofs areas are accounts for approximately more than 50% of total area . Therefore, numerous roof areas are very important for the application of the GI/ LID practices to solve the water-related problems. Retrofitting LID/GI practices (green roof) at roof area is a possible option to transfer impervious areas into pervious areas. A green roof can be applied on the rooftop for the multiple benefits as explained below.
3.1.1 Green roof
Several researchers had done research on the application of the green roof and their benefits for the stormwater management all around the globe [14, 15, 29–43]. Green roofs applied in various countries and the results proved that green roof is the best option to reduce runoff and peak flow in urban areas [14, 15, 29–43]. Moreover, it also helps to reduce the flash flooding in urban areas [14, 29]. On the hand, the blue roof is another possible and cost effective for the runoff control in the highly developed urban area . However, the green roof is more effective practices for the stormwater management in urban areas. Table 2, represented the different research done by different authors on the green roof performance at various locations. Carter and Rasmussen  constructed the green roof at the University of Georgia building to check the performance for the runoff reduction. They constructed the green roof and black roof for analyzing the runoff reduction. Storm water runoff was monitored for 31 storm events. During different storm events, the results indicated that the green roof can reduce runoff up to 78% as compared to the common roof.
Following the same mechanism, Berghage et al.  retrofitted green roof at Chicago, IL building to measure the performance of green roof to control the runoff control during storm events. In this experiment 75,000 s.f. the extensive green roof was installed with “a low-transmissivity uniaxial drainage mat augmented with the perforated conduit, 3 inches of media and a Sedum groundcover”. 106 precipitation events monitored to check the hydrological performance of the green roof. From the experiments, the results indicated the runoff retained was 74% on the green roof. Shafique et al.  constructed the green blue roof in the highly developed area of Seoul, Korea. Green blue roof was constructed at the school building Seoul, Korea. During the different storm events, the runoffs were measured test beds of green roof and common roof. From the analysis of the results, it is indicated that the green roof have ability to store more water as compared to the common roof. The results also revealed that the green blue roof retained approximately 65% of runoff during different storm events. Table 2, shows the importance of the green roofs as the retrofitting option into existing areas. Wright et al.  used the long-term hydrologic impact assessment low-impact development model (L-THIA LID) to analyze the green roof hydrological performance in urban areas. From the model results, it was revealed that the green roofs have the ability to reduce 23% runoff of that area. This software is very effective to forecast the long-term stormwater and cost benefits of each LID/GI practices . This software also gives very useful information to the stakeholders to estimate the project long term benefits. From the discussion and research results it can be concluded that the green roofs and blue roofs are the potential solutions for the stormwater management in highly developed urban areas.
Presently, there is a need to find more safe and sustainable vegetation for the green roof that can withstand under the extreme weather conditions. Moreover, there is also needs to find the plants/vegetation for the green roof that could produce the food. The other social benefit of the green roofs is the rooftop agriculture for the residents. An example of this Eagle street rooftop farm in Brooklyn New York that operates a small agriculture supported program (CSA). This farm can produce the different food for the community restaurants . This is a best example of the getting food from the rooftop.
Rain barrels or cisterns are the storage tank that used for the collection of rooftop runoff. The rooftop runoff is collected in these storage tanks and uses for the different purposes i.e. toilet flushing, irrigation etc . Tree rain cisterns applied at the King Street Center in Seattle uses the rainwater for toilet flushing . The rainwater is collected in three rain cisterns of size 5,400 gallon. The collected water is used for the toilet flushing. This system is able to provide 60% of the annual water needed for toilet flushing. Rain cistern applied at the “The Solaire, Battery Park City, New York” which covers 357,000 square foot area. Rainwater from the building rooftop is collected in a 10,000-gallon cistern located in the building’s basement. The collected water is treated with a sand filter and reuse for irrigating two green roofs on the building. The treated water also used for the toilet flushing. This system of reuse water reduces the potable water use in the building approximately 50%.
Figure 4, below shows the possible LID practices which can be used to control the rooftop runoff. First, we should try to control the rooftop runoff by applying the green roof on the top of the building. The runoff water quality from the green roof is better as compared to the black roof [41, 45]. Therefore, this water should not waste just flowing into the sewer system. Rooftop runoff can be diverted into two following options.
Disconnecting the downspout of a roof from the sewer reduces the chance of overwhelming the system by keeping the stormwater on site. The runoff first can be retained and collected in the green roofs, blue roofs, roof garden and then the excess runoff can be diverted into the other nearby LID facilities i.e., bioretention, rain barrel and permeable infiltration trench. This will decrease the rain runoff and improve the rainwater utilization in urban areas. Through this approach we can utilize the rooftop runoff is a more efficient and effective way.
3.2 Ground Catchment Area
In the developed areas, ground areas consist of many complicated infrastructures therefore to retrofitting LID practices needs more intention and care. LID is innovative stromwater management approach that controls the rainfall runoff near the source. In the developed area, the ground surface is already transferred into hard surfaces such as roads, buildings etc . This is causing the flash flooding and reduction in infiltration into the ground . To diminish these adverse effects of urbanization, rain garden, permeable pavements, rain barrel, permeable infiltration trench, grass swales and trees box filter are the best stormwater management practices.
3.2.1 Bioretention / Rain Garden
A rain garden is a landscape depression that consists of several layers of filter media, an overflow weir, different kind of vegetation, and an optional underdrain [48, 50, 51, 64]. Rain gardens are good stormwater management practices to collect large quantity of runoff water in urban areas [49, 50]. The main purposes of this system are to receive the runoff from upgradient store it into different layers, evaporate through vegetations [48, 50–52, 64]. Rain gardens are usually small size LID facilities and mainly use for the stormwater management in area. This system usually treats small catchment area less than 2 hectares . Rain garden mimic the natural hydrology of an area by retaining a large amount of runoff to decrease the flow rate, peak flow and total volume . This system has multiple benefits as examples, improves the aesthetic of the area and habitat for wildlife, and reduces the erosion of an area [18, 19]. Table 3, represented the hydrological performance of the bioretention systems at different locations. It also includes the description of the bioretention cells. The Bioretention system can increase the runoff time of concentration . Davis, A.P.  had done experiment to check the performance of green roof to increase the time of concentration as well as for runoff reduction. From the result, time of concentration can increase up to 5–10 min for a parking lot of 0.2–0.4 ha (size). This will helps to delay the overflow from the rain garden and reduces the chances of flash flooding . The selection of suitable components of rain garden is very important to improve its performance for long period.
Bioretention system requires special design (depth of different layers, special vegetation etc.) for the construction; however, it has many advantages for the stormwater management in urban areas. Brown and Hunt  constructed the bioretention system at Rocky mountain, NC, for the runoff reduction. This bioretention system soil layer has high clay content, which helps to enhance exfiltration and reduce drainage from bioretention. This system was monitored for two year-long periods to measure the runoff reduction. This system increased hydraulic retention time in the media resulted in lower outflow concentrations. From the experiments, the results showed that bioretention reduced the runoff 90 to 100% and also enhanced the water quality by reducing the 92% TSS, 80% TN and 72% of TP. This system is ideal for the retrofitting in urban areas where we have space available usually from 50 to 200 m2. Debusk and Wynn  had constructed the retrofit bioretention cell (BRC) at Virginia, USA for the runoff quality and quality measurement. The dimensions of the BRC were 4.6 m long, 7.6 m wide and 1.8 m deep. BRC was filled with a mixture of 88% washed medium sand, 8% clay and silt fines, and 4% leaf compost. The drainage layer was located 30 cm above the bottom of the BMP. Research results of 28 rain events showed that bioretention cell reduced flow volume up to 97%. Similarly, Jaber, F.H  checked the retrofit bioretention cell performance in clayey soil at Texas. USA. Bioretention system had 4-inch diameter of perforated CPP underdrain. This system was constructed on the clay soil to capture more runoff for longer duration. Results showed that bioretention system reduced runoff volume up to 50% as well as improve the water quality by holding nitrate (78%). From the above results, it can say that bioretention system has the ability to reduce runoff volume and to improve the runoff water quality in urban areas. This system has the ability to retrieve the natural hydrologic cycle of an area. In the individual house the lawn, garden, and available open space can be converted into rain garden to reduce runoff and to enhance the water recharge and quality of life in that area. Bioretention could apply at the space available near apartment, parking lot, near garden and nearby open space. However, there should need to select more appropriate plants for the bioretention that can give produce different food i.e., any vegetables or fruits plant. This will help the community for the food production from rain garden.
3.2.2 Permeable pavements
Permeable pavements or porous pavements are the pavements that can infiltrate runoff into subsoil. It reduces the runoff and improves the water quality in an area. There are much research had done on the hydrological and water quality performance of the permeable pavements around the globe [21, 22, 55]. Jaber, F.H  constructed the permeable pavements at Texas A&M AgriLife Research to know the hydrological performance and water quality of this system. A parking lot with 4 permeable and 1 impermeable pavements was analyzed to check for the runoff reduction. Each pavements size is 18” × 10”. The runoff from the four pavements such as grass pavers, permeable interlocking concrete pavers, gravel parking, and an impervious concrete (control) were measured by using the A flow meter (ISCO Bubbler Flow Meter). From the experiments, results revealed that permeable pavements improve the water quality of infiltrating as wells as reduced the runoff to 65% respectively. Wright et al.  analyzed the permeable pavements long-term hydrological assessment by using low-impact development (L-THIA LID). L-THIA LID different parameter such rain events value, characteristics of each LID facilities, geographical locations etc. can be added. By using this model, he analyzed the runoff reduction of the permeable pavements in urban areas. The results showed that permeable pavements reduced runoff approximately 40%. There is a little information available that suggest the optimum place for the application of permeable pavements. Optimal places to apply permeable pavements to achieve multiple benefits are parking lots, sidewalks, driveways along roads and small streets of communities. Because these places are easily available in highly developed urban areas and can change to permeable pavements for multiple benefits (rainfall runoff reduction, storm water collection and infiltration into the soil). These are the ideal and suitable places to apply the permeable pavements for multiple benefits.
3.2.3 Grass swale/Bio swale
Grass swale is the green landscape feature that uses for the slow, infiltrate runoff into ground and improve the water quality in urban areas . This LID practice is very useful because it transfers the impervious areas into pervious and requires very less repair and maintenance costs. Grass swale could apply along the roadways, streets, residential pathways, along with the garden/ lawn of a single house and in or around the park areas.
For the retrofitting LID practices in urban areas, there are also needs to find the suitable places to apply the infiltration trench and underground rain cisterns which can allow the infiltration into the ground. From the literature, there is no research results found in these two practices. So there is a need more research on these practices in urban areas.
3.3 Artificial ground catchment area
The artificial ground area is referred to the area that is created by own self to retrofit the LID facility. For example, porous road that only stores the water into their pores with no infiltration of water into the ground. The excess runoff passes to the sewage system. A rain garden can apply on the underground room that can only store water and use it for different purposes. We can utilize the upper surface of the underground parking lot just to store water and to reduce the runoff on the nearby surfaces. Flowers gloves can also put along the road to store the rainwater that helps to reduce overall runoff and to improve the environmental value of that area. By this way, we can create the artificial area where we can retrofit LID facilities for the different purposes to make our cities safe and sustainable. From the literature, there is no research available that describes these practices and their benefits. Therefore, there is more research need for the application of these practices in highly developed urban areas.
4 Optimal places for retrofitting LID/GI to achieve multiple benefits
In urbanized areas, the difficult task is to find the suitable place to apply the GI/LID technologies to achieve multiple benefits. As we know that in urban areas, different kind of infrastructure already laid on or under the ground so it makes it more complicated. Under this situation, the optimal places to retrofitting LID/GI are single house, schools/colleges/universities and parks, where we can apply the different GI/LID facilities for sustainable stormwater management [56, 57].
4.1 Single House Area
A single house is consists of simple infrastructure and has different open spaces that make retrofitting LID facilities easy. Low impact development (LID) practices can apply easily at the individual single house at different locations. For example, roofs can be transferred into green roof/blue roof or roof garden. In single house, open space or lawn can change into rain garden for achieving the multiple benefits (runoff control, water utilization and increase the enmity value) . From rooftop water can collect into the rain barrel and use it for different purposes i.e., irrigation, toilet flushing etc. Figure 5, shows how we can apply the different LID practices at different location in a single house. This also explains the suitable combination of different LID/GI practices can be used to make our home green and sustainable.
4.2 School/College/ University Area
In schools areas, there is a large open space available where LID/GI facilities can be applied for the stormwater management. These places consist of large areas and have open free spaces that offer an opportunity to retrofitting LID practices for multiple benefits. There are many case studies, where the different LID practices are retrofitted at school/colleges to reduce the overall runoff and to improve the environment of surroundings. An example of this is found in USA where a lot of example of this can be seen from USA where they applied many LID facilities at different schools/universities. Carter and Rasmussen  constructed the green roof at the University of Georgia building rooftop for the runoff reduction. From the experiments, results showed 78% runoff reduction during different rainfall events. Shafique et al.  constructed the green blue roof at Cheong-un middle school Seoul, Korea to reduce the runoff and heat island phenomena. Results indicated that green blue reduced the runoff up to 65% as compared to common roof . Green roofs areas can change into the blue roof or roof garden to control the runoff to avoid the flash flooding in that area. Bioretention should apply near the playground area in the schools so that more water can enter into the rain garden and helps for runoff reduction. Large runoff from playground areas can divert toward the rain garden for the collection, storage and infiltration into the ground through the soil. In the Playground, sandy soil should use to infiltrate a large amount of water into the soil. There is need to select the best combination of different LID/GI practices to apply at suitable place to make our area green, safe and sustainable. By doing this, the total runoff in urban area and overall load on the sewage system can decrease.
4.3 Parks Areas
Large and easy available areas for the application of rain garden/grass swales and other LID practices in urban areas are the parks areas. In urban areas, Parks are the places that offer to apply different LID practices easily. Therefore park areas can control a large amount of stormwater, to reduce the overflow runoff from the sewage system and to improve the environment of the surroundings . Many authors applied rain garden at different parks in the different countries. Davis, A.P  constructed the rain garden at the College Park, MD to improve the hydrological performance. He used the media depth from 90 to 120 cm with media composition of 50% sand and 30% of topsoil. The research results indicated the runoff reduction from 52 to 65% during the different rainfall events. Li et al.  monitored the six bioretention cells at the College Park, MD by using the different media depth. These different media depths captured the runoff from 10 to 60% during different storm events. In parks, permeable pavements could apply at the pathways. This will help in infiltrate the rainfall into the soil and improve the water quality. Grass swales can use in around the garden areas, along with the pathways. Rain barrel can be applied to capture the rainfall and store it. These different LID/GI practices could easily construct at different parts of the parks to make our surrounding areas safe, sustainable and resilient to climate change.
Retrofitting LID practices improve quality of life as well as recreational areas. Retrofitting LID practices are the cost effective practice than the traditional stormwater system . LID practices cost is less than the conventional development costs of different projects. For example, garden valley USA; the LID cost is $260,700 that is less than 20% of total conventional development cost that was $324,400 . Another project that is 2nd Avenue SEA street USA; the total LID cost was $ 651,548 that is 25% less than the total costs of the conventional development that was $868,803.The total costs include the construction, repair and maintenance and management costs of the projects . Retrofitting LID practices are the cost effective stormwater management practices to make an area safe and sustainable. LID practices benefits follow the triple bottom line approach. As the Retrofitting LID practices give financial , social and environmental benefits [60, 61].
Application LID practices into existing area follow the triple bottom line approach to shows the project is sustainable with respect to financially, socially and environmentally. This proves that the LID practices are the safe and sustainable practices in an urban area [60, 62].
5 Limits for Retrofitting LID/GI practices in Urban Areas
Low impact development (LID) technology is the use of various stormwater management practices to mimic the natural hydrology of an area. LID/GI practices manage the stormwater runoff near the site. Retrofitting LID practices in the highly developed urban areas are very useful because it has the ability to transfer traditional stormwater approach to green stormwater management approach. It gives multiple benefits for the rainfall runoff control and tries to retrieve the natural hydrology. However, common misperceptions about the retrofitting LID are difficult to apply and high repair and maintenance costs [63, 65]. Another big challenge to retrofit the LID practices is to find the suitable place into the existing complex area.
Some barriers for retrofitting the LID practices in urban are described below.
The biggest challenge is to find the suitable places for LID practices in the existing complex infrastructure.
Lack of the design standards of LID practices that can acceptable all around the world. As most of the research has done in cold climate regions i.e. the US, UK, Germany, it cannot apply in Asian regions due to different climate conditions and geographical locations.
Another big barrier is the lack of knowledge about the LID technology to the local resident and stakeholder. Lack of government agencies programs related to the importance of the LID / GI projects for safe and sustainable city. Less participation of the stakeholder in the LID/GI projects.
Many misperceptions about the retrofitting LID practices are that these practices are difficult to maintain or maintenance cost is very high. Insufficient data about the maintenance costs and long term benefits. No trust on the long term trust goals of LID technology.
Lack of technical, human capabilities and less financial and other resource for the implementation of LID practices.
6 Potential Solutions for Retrofitting LID practices
Before retrofitting LID facilities into existing areas, there is a need to understand basic functions of each LID facilities. Then we should select the best combination of LID/GI practices to apply at suitable place to achieve multiple benefits (social and environmental).
Consideration of complete local and federal design standards and methods of applying LID practices that facilitate the design and implementation of these practices on the broader regions [66, 67]. These standard and codes acceptable all around the world by just putting the information (climate change, geographical location etc.). Share the information to the rest of world of successfully applied LID practices in urban areas .
There is a need to prepare some general manual which includes all the information of LID practices (Construction, management and maintenance) [63, 64]. New projects should include the retrofitting LID practices in urban areas.
More educational workshops should arrange for the awareness of the importance and benefits of retrofitting LID/GI practices in urban area. Tell the public that how these LID practices can save our resources i.e. water, energy and food etc. for our future generation. Encourage the stakeholders and people to start the projects of LID [63, 65].
Make the standard operation and maintenance plan, manuals to achieve the long-term benefits of LID practices [65, 66]. Exchange the information with each other to determine the best management practices approach that needs less maintenance cost. Share the information of successfully applied LID project that has fewer maintenance costs but more benefits.
Nowadays, the main issue that arises while applying the LID practices is the lack of cooperation and collaboration between engineers (Civil Engineers, Transportation Engineers, Water Engineers, Land Engineers and LID experts). This issue should be eliminated by cooperating and collaborating for applying LID practices for the safe and sustainable city.
Another issue is the management of the LID facilities that can eliminate by co-operation and collaborating between the local government, stakeholders and local residents. They should decide who will take care the management of LID facilities after constructions.
Retrofitting LID practices are very useful technique to achieve social, environmental and financial benefits and to transfer grey city to the green city. There is need to apply different LID practices into existing areas to make our cities self-sufficient in every aspects such as food, energy etc.
This review paper shows that the retrofitting low impact development (LID) practices are very helpful for stormwater management in urban area. Based on the literature, retrofitting LID/GI practices has a great potential to reduce the runoff, improves the water quality as well as natural environment of urban areas. However, the retrofitting LID practices require special concern about the suitable place and combination of the LID practices to achieve the multiple storm water management benefits in urban areas.
Although there are many successful case studies of urban agriculture in the northern hemisphere the US, it is surprising that South Korea has so few examples of urban food production and to date, no empirical studies as to its viability. Therefore, it is necessary for the governments to take initiatives for the food production from different LID practices (green roof, bioretention).There is also need to select the most suitable combination of LID practices for multiple benefits (social, environmental and economical) and to check their performance for longer period at different locations. In addition, life cycle assessment should be performed at each geographical location to understand the individual and stakeholders about the long-term benefits of LID practices.
This research was supported by a grant (15technology innovationC04) from Advanced Water Management Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
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About the article
Published Online: 2017-06-20
Citation Information: Open Geosciences, Volume 9, Issue 1, Pages 240–254, ISSN (Online) 2391-5447, DOI: https://doi.org/10.1515/geo-2017-0020.
© 2017 Muhammad Shafique and Reeho Kim. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0