Original Research ______________________________________________________________________________ Design and Advantages of a Bioretention Area as a Best Management Practice for Low Impact Development on The University of Texas at San Antonio Felipe Alejandro Flores Supervising Committee: Heather Shipley, Ph.D., Chair; Manuel Diaz, Ph.D.; Marcio Giacomoni, Ph.D May 02, 2016 __________________________________________________________________________ ABSTRACT Rainfall on urban areas causes polluted runoff water to contaminate the ground. A bioretention basin can minimize this problem. In this project a bioretention basin was designed for future precipitation changes regarding climate change. The bioretention basin was designed for new development on The University of Texas at San Antonio Main campus and includes an economic analysis comparing three different scenarios regarding media and materials. The basin includes sand and crushed glass as media and Cedar Elm and Muhly grass plants as flora, which are native to San Antonio, to achieve the pollution removal needed. After calculating the drainage area and future average precipitation, the TSS removal required by the BMP was obtained. The equivalent depth, water quality volume treated, and the footprint area were then calculated. Recycled water from a current building at UTSA was tested and was suitable for irrigation. The results were as expected regarding the future average precipitation and the size of the basin. KEY WORDS: Environmental Engineering INTRODUCTION The design of a bioretention basin on UTSA main campus is addressed in this project. This The construction of the built environment (e.g. basin was designed to be incorporated into new roads, buildings) affects natural ecosystem development on campus to help with problems processes and results in negative impacts to the related to the lack of natural systems by environment. Some examples of negative reproducing predevelopment hydrologic impacts are seen during rainfall events when conditions. This project is important because it runoff creates floods that are generated by deals with conditions that could become major impervious areas causing polluted water to stay issues such as an increase in rainfall intensities on top of the developed area instead of due to climate change and pollution of the percolating into the ground. These problems can environment by contaminated water runoff from be mitigated when making use of stormwater future buildings to be built on campus (Dorman, management strategies such as Low Impact et. al., 2013). To fulfill the goal of this project, Development (LID) (Dorman, et. al., 2013). LID the design includes research on climate change strategies are structural stormwater Best in recent years to address intensity concerns. Management Practices (BMPs) and planning There are four objectives addressed in this techniques intended to reproduce project which are: 1) the calculation of the predevelopment hydrologic conditions. An treatment volume and equivalent depth of the example of a LID is a bioretention basin, which basin, 2) the footprint area of the proposed channels water during a storm event to reduce development, 3) the selection of the basin’s flooding and filters water to clean it from media and flora, and 4) conducting an economic pollutants carried from developed areas such as analysis of the different basin designs. parking lots (Dorman, et. al., 2013). Bioretention is a common LID practice LITERATURE REVIEW because it mimics hydrologic conditions and enhances biodiversity and water quality. In areas Stormwater Management Strategies such as Bexar County, it is important to Rainfall over developed areas carries implement BMPs located on the Edward’s pollutants to the ground by surface runoff, which Aquifer such as The University of Texas at San can become a problem. Therefore, stormwater Antonio (UTSA) to avoid polluted water to management strategies are needed to address percolate into the ground and pollute the aquifer. this problem and to reduce the peak runoff rates The university can take advantage of the use of a and runoff volumes of storms (Center for bioretention basin as a LID strategy to clean Research, 2011). There are several BMPs such polluted water, reduce flooding, and comply as bioretention areas, sand filters, bioswales, with the Edward’s Aquifer regulations (Center green roofs, etc. used to convey, infiltrate, and for Research, 2011). Currently, the university treat stormwater runoff, which differ in has several sand filters built, but this project effectiveness regarding the climate and area in focuses on the design of a bioretention basin, which they are built. These low impact which as an advantage uses vegetation to clean developments (LID) have different maintenance pollutants, in an area where new buildings are cost and overall cost, which need to be proposed to diminish the impact of increased compared to design the most efficient one. In impervious areas. LIDs can easily be integrated addition, the drainage area that the LID will into existing development and built into new receive is an important characteristic that needs development (Dorman, et. al., 2013). In order to to be considered when choosing an LID since effectively select a BMP for a specific area, its they have different drainage area limits. For characteristics and features should be example if the LID’s drainage area limit is understood; in addition, the design process must smaller than the drainage area it will receive, be planned and followed in order to accurately then a combination of LIDs is needed to achieve meet all the BMP objectives. effective results. All BMPs have drainage area 2 limits but more than one system can be used in they generally have a high rate removal for parallel to cover all the drainage area necessary phosphorus, BOD, zinc, copper, lead, nitrogen, (Center for Research, 2011). and fecal coliform. Sand filters consist of a bed of sand that removes sediments and pollutants. Complying with the Regulations of the In these filters, bacteria slime is formed and Edward’s Aquifer helps remove nutrients, organics, and coliform BMPs are required when building over the bacteria from the water. Additionally, sand Edward’s Aquifer region (Barrett, 2005). The filters can adapt to thin soils, limited-space Edward’s Aquifer recharge zone is an area of areas, and dry areas. Sand filters do not include about 1500 square miles that includes part of flora in its design (Barrett, 2005). Bexar County (City Council of San Antonio, 1994). In this area, vertical faults occur exposing Bioretention basins use adsorption, plant fractured Edward’s limestone at the land surface. uptake, microbial activity, filtration, and The aquifer receives water from the flow sedimentation to remove pollutants, and provide crossing the drainage area and from water that high removal of sediment, metals, and organic percolates from major streams in the region. The material (Dorman et. al., 2013). Bioretention University of Texas at San Antonio is located basins consist of a pretreatment system, a over the recharge zone of the Edward’s Aquifer. surface ponding area, a mulch layer, and a The Edward’s Aquifer is unique in its ability to planted soil media; Figure 1 shows a schematic recharge large quantities of water quickly, and diagram of a bioretention system. The vegetation most of the water comes from infiltration of that needs to be included in the surface of the rainwater that falls over the recharge zone basin is generally a combination of small to (Barrett, 2005). Consequently, the city of San medium-sized trees, shrubs, and groundcover. Antonio has passed a resolution for protection of The flora can adapt to size constraints and take the aquifer that is enforced as high priority for advantage of the semi-arid climate for construction on the recharge zone. evapotranspiration in the San Antonio area (Barrett, 2005). In addition, several physical, The Edwards Aquifer Rules regulate the biological, and chemical processes are applied in activities that could potentially pollute the a bioretention area to effectively remove waters associated with the aquifer and they pollutants (United States Environmental apply to recharge zones in Bexar County Protection Agency, 2015). (Barrett, 2005). The use of permanent BMPs is required on areas over the aquifer to prevent pollution caused by contaminated stormwater runoff from the site. One of the requirements established by the aquifer’s regulations is that at least an 80% reduction of TSS (total suspended solids) on runoff water needs to be achieved by the BMPs in constructions over the aquifer (Texas Commission on Environmental Quality, 2008). Studies indicate that bioretention basins and sand filters as BMPs remove TSS by 89% in average, which comply with the aquifer’s rules (Dorman, et. al., 2013). Sand Filter and Bioretention Basin Design Sand filters and bioretention basins are two stormwater management strategies that are used Figure 1. Schematic Diagram of a Bioretention to minimize problems related to water runoff. System. Sand filters are basins that capture and filter stormwater runoff by using a layer of sand and 3 The design of a LID, including bioretention Likewise, pollutant removal is more achievable basins and sand filters, includes the following when BMPs include media and robust steps. The size of the basin is first determined vegetation rather than just sand or a single plant followed by the selection of media required to species (United States Environmental Protection achieve the performance necessary. There are Agency, 2015). With the construction of a different approaches to determine the water bioretention basin, the UTSA main campus quality volume and the method discussed here could become more sustainable. Additionally, a follows the rational method (Dorman, et. al., bioretention basin can be incorporated into a 2013). The volume-based method depends on development easily and its vegetation provides the runoff coefficients regarding the hydrologic shade, wind breaks, and absorbs noise (Center soil group. It was also developed to achieve total for Research, 2011 and Dorman, et. al., 2013). suspended solids reduction targets regarding Besides its aesthetic benefits and its flexible annual rainfall volume. For this method, rainfall design, its implementation at UTSA can also depth is needed to get the volume necessary to benefit it economically. meet the treatment goals. Additionally, hydrologic evaluations graphs are used to define Additional benefits include the gaining of rainfall depth that must be treated to meet the credits for the Leadership in Energy and desired pollutant reduction goals (Dorman, et. Environmental Design (LEED) certification. al., 2013). On the other hand, flow based design LEED credits are divided into sections and a methods are usually used for configuring inlets, bioretention basin could earn credits in the sizing, conveyance, or settling hydraulic control Sustainable Sites and Water Efficiency sections. (Dorman, et. al., 2013). First of all, a bioretention basin could help the new project earn up to three credits of Rainwater Bioretention Basin Advantages Management subsection. Also, for the Heat Some of the advantages of a bioretention basin Island Reduction subsection it could earn up to include the removal of suspended solids, metals, two credits by providing shade to nearby pollutants, nitrogen, phosphorus, and pathogens developments with the flora planted. For the from the water. They also reduce the peak runoff Water Efficiency section, the BMP can earn up rates for storms, reduce runoff volumes, and can to two credits in the Outdoor Water Use potentially recharge ground-water after filtering Reduction section (USGBC, 2016). Since the it from pollutants (Dorman, et. al., 2013). In flora used in the basin will be native of the area, addition, they are flexible to adapt to urban not much water is going to be needed; the water retrofits and can be used in recharge zones, that will be needed is going to be obtained from karst, clays, and hotspots. Another important the recycled AC condensate water from the new characteristic of these basins is that they are well building developments on UTSA, as further suited for small areas and if multiple distributed explained in section 3.3. units are constructed, they can provide treatment in large areas (Dorman, et. al., 2013). Economic Benefits of a Bioretention System Bioretention basins also enhance aesthetics and Municipalities usually encourage developers to provide habitat for different species. In addition, incorporate LID by offering incentives for in a bioretention basin the standing water is only planned and existing developments (United present for 12-24 hours, minimizing vector States Environmental Protection Agency, 2015). control concerns (Dorman, et. al., 2013). The four most common categories of local incentives are fee reductions, development Research has shown that removal of TSS, inducements, best management practices phosphorus, nitrogen, and fecal coliform is more installation subsidies, and awards and successfully achieved when vegetation is recognition programs. Also, municipalities often included in a BMP such as in a bioretention charge stormwater fees depending on the basin. Bioretention systems typically achieve a impervious surface area on a property, but when TSS reduction of 89% efficiency (Center for a LID system is used to reduce the amount of Research, 2011 and Dorman, et. al., 2013). runoff and clean for pollutants, the federal 4 government can help with the payments of this increase stormwater infiltration rates and reduce fee (United States Environmental Protection the urban heat island effect. Through this Agency, 2015). Likewise, incentives could be program, the city offers energy tax rebates when available for developments using only LID planting trees on a property (Economides, 2014). practices. This may incorporate propositions to Although the program takes stormwater waive or decrease permit fees, accelerate the management as a secondary benefit, it directly permit procedure, or allow for higher density recognizes the value of tree canopy. The design developments. Furthermore, communities could of a bioretention basin at UTSA includes offer programs that subsidize the cost of the vegetation and media that could be registered in materials that are used to construct the the Tree Challenge Program for CPS energy tax bioretention system. Recognition programs are rebates. held by the community to encourage LID innovation. For example, the university could be LID and green infrastructures result in featured in articles, websites, and utility bill multiple financial, environmental, and social mailings about their implementation of LIDs benefits. Financial case studies for LID increasing its prestige (United States implementations were made for Milwaukee, Environmental Protection Agency, 2015). Portland, Philadelphia, and the Sun Valley watershed of Los Angeles County, which The city of San Antonio has implemented a monetized benefits using non-market economic comprehensive plan for improving the valuation techniques (U.S. Environmental sustainability of the city by 2020. The SA2020 Protection Agency, 2013). In these benefit-cost is a nonprofit organization to allow San Antonio analyses, it was discovered that public benefits citizens to work with the city government to of an LID, such as a bioretention basin, include achieve mutual goals. Moreover, the SA2020 management cost, habitat creation, improved air program’s financial support for green quality, and reduced carbon emissions. On the infrastructure such as LIDs comes from private other hand, private benefits include stormwater rather than public funding (Economides, 2014). volume reduction, reduced energy demand for The San Antonio River Authority is also heating and cooling, and less stormwater facility promoting the construction of LID systems as costs (U.S. Environmental Protection Agency, green infrastructure with several initiatives. To 2013). In another analysis, it was demonstrated name one, the Mission Verde Sustainability Plan that for an equal investment amount and similar is investing in energy saving initiatives that overflow volume reductions, LIDs would innovate and encourage green engineering such provide 20 times more benefit than traditional as the LID systems (Office of Mayor Phil stormwater infrastructure such as stormwater Hardberger, 2009). Furthermore, San Antonio pipes (U.S. Environmental Protection Agency, has existing government and non-profit 2013). A bioretention basin will additionally programs each year to implement a green increase community aesthetics, increase wildlife infrastructure plan such as a bioretention basin. habitat, reduce heat island effect, and create a The city of San Antonio Office of Sustainability possible reuse of the water for different activities will also integrate green infrastructure in the that will also benefit UTSA directly. future, saving costs for developers and the city in forthcoming construction of LIDs (Economides, 2014). METHODS LID approaches can be easily integrated into To achieve the goal of designing a bioretention capital improvement programs. One of the city’s basin for future building developments in UTSA initiatives that indirectly support stormwater Main campus, the project has been broken-up management is the Tree Challenge Program into four objectives. This chapter discusses the through the Parks and Recreation Department methods that were used to carry-out these (Economides, 2014). This program aims to objectives. The objectives are: 1) determine the expand the tree canopy in San Antonio to treatment volume and the equivalent depth of 5 water stored following design formulas, 2) obtain the footprint area of the basin, 3) select the media and flora, and 4) conduct economic analysis on the proposed basins. For the fourth objective, three bioretention basins were designed with different media and materials to select the most productive and the most economical design. Climate change has been an important factor for variations in rainfall depth for the last few years and this factor is taken into account in the design of the bioretention basin. Determining the Contributing Drainage Area The contributing drainage area is one of the fundamental values needed to design a bioretention basin. It defines the portion of the site in acres that is contributing runoff to the BMP. This area is utilized to determine the water quality volume. The contributing runoff was obtained based on the drainage contours of the Texas Natural Resources Information System (TNRIS) using Google Earth. First, from the TNRIS webpage, the 2010 TNRIS 5ft Figure 2. Contour Lines and Bioretention Basin Contours Elevation GIS data was downloaded Location. along with the StratMap Elevation Contours. The first represents the green contour lines and Determining the Treatment Volume the second one the red contour lines in Figure 2. The proposed development for which the These contours were imported with a 2013 bioretention basin is designed at The UTSA Bexar Metro 911 Image in ArcMaps version 10. Main campus will cover an area of 7.15 acres, The 2 contours give different approaches to the which will be the contributing drainage area for elevation of the terrain and they both specify the basin. Figure 3 shows UTSA Main campus where the lowest point in the section is going to with some of the current sand filters marked by a be and thus, where the LID is going to be star and the proposed development area for located to receive all the runoff. In Figure 2, the which the basin was designed is circled. The future development location is enclosed by the proposed development will be near roads, black box and the basin’s location and lowest buildings, and parking lots; but the bioretention point in the terrain is where the blue circle is. basin will only receive runoff from the proposed After determining where the water is going to buildings due to the drainage already installed. flow, the contributing drainage area was Consequently, for this project, the basin was obtained using the Google measuring function. designed to handle the runoff received only from the new development. The drainage area limit for a bioretention basin is 10 acres and this project’s drainage area falls inside the parameters (Barrett, 2005). The volume-based method was used to design the basin since it uses annual rainfall precipitation to obtain the water quality volume providing an easier way to determine the volume based on precipitation increases due to climate change (Barrett, 2005). 6 Excel file, the yearly average was obtained for each year and then the total average was obtained for each projection. After that, graphs were plotted for each projection showing an increase of the precipitation average throughout the years (section 4.1). In the design formulas for the bioretention basin the yearly average precipitation in inches was required so the values were converted into in/yr. To calculate the water quality volume of the basin, the required TSS removal was obtained using equation 3-1 followed by the load remove d by the bioretention basin as shown in equation 3-2. For the required TSS removal Figure 3. The University of Texas at San calculations, it was assumed that the appropriate Antonio Main campus from Google Earth. runoff coefficient of impervious areas is 0.9 and 0.03 for natural areas (Barrett, 2005). The new The bioretention basin was designed to development on campus is of 7.15 acres, which account for future variations in climate. Climate include pervious and impervious areas that were change has proven to increase average measured specifically for the calculations. The precipitation throughout the years and so TSS concentration increases to 170 mg/L in an projections were done to account for the increase impervious area, and this is what was used for in precipitation (Downscaled CMIP3 and the design. In addition, it was assumed that the CMIP5 Projections). The projections showed an bioretention basin achieves an 89% of TSS increase in future average annual precipitation reduction according to Table 1 (Barrett, 2005). due to climate change and the basin was The rainfall depth was obtained using equation designed to be able to function under future 3-3, which is the fraction of annual rainfall precipitation increases. The projections were treated by the best management practice that downloaded from the Lawrence Livermore also determines if the BMP selected was good National Laboratory archive. The data from this enough for the treated area, and table 2 (Barrett, archive is based on global climate projections 2005). from the World Climate Research Programme’s (WCRP’s) Coupled Model Intercomparison Equation 3-1 LM= 27.2 (AN x P) Project Phase 3 (CMIP3). First, downscaled CMIP3 daily climate and hydrology projections where: developed using bias-correction and constructed analogs (BCCA) were downloaded for the LM= Required TSS removal (pounds) specific area in which the project is located. AN= Net increase in impervious area Daily climate and precipitation data for years (acres) 1961-2000, 2046-2065, and 2081-2099 was P= Average annual precipitation obtained. (inches) There were several different historical Equation 3-2 LR= (BMP efficiency) x P x (Ai projections used to obtain the precipitation x 34.6 + Ap x 0.54) average for the future and included the possible where: alterations due to climate change. The historical projections obtained included five historical LR= Load removed by BMP projections from years 1961 to 2000, three BMP efficiency= TSS removal variable projections including years 2046 to efficiency (from table 1) 2065, and three future projections from years 2081 to 2099. After downloading them into an 7 Ai= impervious tributary area to the BMP (ac) Ap= pervious tributary area to the BMP Table 2. Relationship between Fraction of (ac) Annual Rainfall and Rainfall Depth (in) (Barrett, P= average annual precipitation 2005) Equation 3-3 F= LM/ΣLR where: F= Fraction of the annual rainfall treated by the BMP LR= Load removed for each BMP (from equation 3-2) (pounds) LM= Required load reduction (from equation 3-1) (pounds) Table 1. TSS Reduction of Selected BMPs (Barrett, 2005) The water quality volume was calculated using equation 3-4 by multiplying the rainfall depth from Table 2 by the runoff coefficient from Figure 4 and by the contributing drainage area (7.15 acres). Figure 4. Relationship between Runoff Equation 3-4 WQV= Rainfall depth x Runoff Coefficient and Impervious Cover Coefficient x Area The water quality volume needed to be increased by a factor of 20% to account for reductions in storage due to deposition of soils that can occur in maintenance activities (Barrett, 2005). In addition, a modification in the average annual rainfall was made due to studies relating to the changing climate in San Antonio. In San 8 Antonio specifically, the change in precipitation necessary. First, soil should be free of stones, lacks evidence that relates to climate change but uniform mix, and free of other objects. The it is certain that precipitation will not be steady recommended sand is ASTM C-33 with a grain over time, it will decrease or increase size of 0.02 to 0.04 inches. Clay content should (Schmandt, 2011). Since precipitation has be less than 5% and filtration media must have a increased over five percent over the last 50 years minimum of 3 ft thickness if soil mixture is 50 in the United States it is expected that to 60% sand, 20 to 30% compost, and 20 to 30% precipitation will increase by 10 percent in the topsoil (Barrett, 2005). For a smaller soil media next 100 years in Texas (Karl, et. al., 2009). depth of 2 to 3 feet, then soil mixture should be 85 to 88% sand, 8 to 12% fines, and 2 to 5% Footprint Area and Equivalent Depth plant delivered organic matter. The underdrain The next step in the design requires the depths layer including the underdrain pipe should have of the media as recommended in the LID ASTM No. 8 stone over a 1.5 feet envelope of Technical Guidance Manual to obtain the ASTM No. 57 stone separated from the soil by 3 required footprint. A temporary ponding depth inches of washed sand (Dorman et. al., 2013). of 9 inches was used (Dorman, et. al., 2013). To make the basin more sustainable, crushed Also, soil media depth of 48 inches was used recycled glass can be used instead of sand; if this with a media porosity of 0.35 (Dorman, et. al., design is preferred, then more organic matter, 2013). The depth of gravel used was 8 inches from 20 to 30%, should be used. Additionally, since the underdrain pipe diameter should have a only mature, low-nutrient compost should be minimum of 4 inches diameter; the porosity of used for all the designs (Center for Research, the gravel will be 0.4 (Dorman, et. al., 2013). 2011). These values were used to get the equivalent Crushed recycled glass can be used instead of depth of water stored in the bioretention and sand as media for the bioretention basin. The use with that, the required bioretention footprint area of the crushed glass has several advantages over following equations 3-5 and 3-6. the use of sand, which include: 1) it is less expensive, 2) it is recycled so it is more Equation 3-5 Deq= (D surface)+(n media x D environmentally friendly, and 3) it can be media)+(n gravel x D gravel) pulverized to meet the size the design specifies. The cost of crushed glass is 38% of the price of where: regular sand used for filtration and it can save Deq= equivalent depth of water stored in money in maintenance since it gets clogged representative cross section (ft) more slowly due to the shape of its particles D surface= average depth of temporary surface (Rutledge, 2010). Additionally, recycled crushed ponding (maximum 12 in) glass filters have shown similar results in n media= porosity of soil media removal of particles than sand filters, which D media= depth of soil media does not affect the design of the bioretention n gravel= porosity of gravel drainage basin (Rutledge, 2010). If crushed glass is going layer to be used as media, then an extra mulch layer D gravel= depth of gravel drainage layer should be included in the design due to Equation 3-6 A= WQV/Deq specifications (Barrett, 2005). Also, due to the fact that the crushed glass is a recycled material, where: the project could earn up to 2 credits for the Leadership in Energy and Environmental Design A= required bioretention footprint (ft2) (LEED) certification (USGBC, 2016). WQV= water quality treatment volume (ft3) The plants used for the basin must be able to Deq= equivalent depth (ft) adapt to the San Antonio climate. Examples are Muhly grass, and Cedar Elm plant. These Selection of Media and Flora species are suitable for the LID features and can A bioretention basin should have a soil provide the specific characteristics needed to mixture adequate to filter all the pollutants clean pollutants (Center for Research, 2011). 9 The irrigation of plants can be done using The resulting footprint of 13,145.5 ft2 is going to recycled water from the proposed buildings or be used to estimate three different costs of the from the already existing buildings. Therefore, basin depending on the different characteristics the AC condensate water, reclaimed water, and used. Different materials result in different costs. blowdown water from a current building on The economic analysis was made based on the campus were sampled and tested to determine if media used such as soil or crushed glass, the water quality was suitable for the plants in concrete or geomembrane as a liner, and the order to reuse the water for irrigation. The water different depths of mulch required depending on was analyzed for turbidity using a turbidity the crushed glass. meter, pH using a pH probe and meter, conductivity using a conductivity meter, RESULTS AND DISCUSSION alkalinity and hardness following the titration method, copper, zinc, and sodium using the Average Precipitation Projections inductively coupled plasma mass spectrometry The average precipitation from the projections (ICP-MS), and phosphate measured is 33.64 in/yr for years 2081-2099, which is spectrophotometrically. adopted in the basin’s calculations. The average annual precipitation in Bexar County is of Economic Analysis approximately 30 in/yr historically and an An economic analysis was performed to select increase of 5% has been observed since 1950 the most economical, efficient, and sustainable (Barrett, 2005 and Karl, et. al., 2009). The 30 basin alternative. To achieve this, three basins in/yr average compared to the 33.64 in/yr were designed. The designs costs were calculated for the future shows a percentage calculated using sand and recycled crushed increase of 12.1% in average annual glass, and concrete or a geomembrane as liners. precipitation, which was within expectations These scenarios give a better idea of the since there was a 5% increase observed from differences in cost regarding the material used. 1950 to 2000 (5% increase for 50 years) (Karl, In addition, the maintenance cost was analyzed. et. al., 2009). Figures 5 to 15 correspond to the results of the projections and its averages. In accordance to the depth of the design, the approximate cost is broken down as follows: ) Historic Projection 1 r Cost a e Excavation with $5/ft2 y/ n 60 underdrains i( n 50 Soil or crushed glass $5/ft2 or $2/ft2 o it 40 respectively at ip 30 Aggregate $0.28/ft2 ic e 20 Pipe with underdrain $3.6/ft2 rP y = 0.073x -112.93 R² = 0.0096 Gutter $18/ft2 eg 10 a 0 Mulch $0.32/ft2 or $0.42/ft2 re v 1950 1960 1970 1980 1990 2000 2010 (the latter is used A when using crushed Year glass) Concrete barrier or $12/ft2 or $0.45/ft2 Average= 31.6 in/year geomembrane liner respectively Vegetation $2/ft2 Figure 5. Historic Projection 1 The maintenance cost to keep the basin working properly is of $1.91/ft2 every 2 years, $2.5/ft2 every 10 years, and of 10.11/ft2 every 20 years. The quantities represent the cost depending on the depth needed for the design. 10