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Gwinnett County Department of Water Resources Low Impact Development Retrofit

Submitted By: Derek Buffardi Jonathan Callura James McCullough Donald Sheldon Renee Viger

Date of Submission: April 24, 2012 Contact Information: Ferst Environmental Solutions 661 Techwood Drive Atlanta, GA 30313 Phone: (678) 936-2780 E-Mail: FerstEnvironmental@gmail.com


T able of Contents 1.0 Introduction ............................................................................................................................... 1 1.1 Background: Low Impact Development ............................................................................... 1 1.2 Problem Statement: Gwinnett County Department of Water Resources .............................. 1 1.3 Site Map ................................................................................................................................ 2 1.4 Recommended Approach ...................................................................................................... 2 1.4.1 Bioretention .................................................................................................................... 2 1.4.2 Infiltration Trench........................................................................................................... 2 1.4.3 Buried Cistern ................................................................................................................. 3 1.4.4 Rain Barrels .................................................................................................................... 3 1.4.5 Pond ................................................................................................................................ 3 1.4.6 Aesthetic Features........................................................................................................... 3 2.0 Area 1 ........................................................................................................................................ 4 2.1 Functional Features ............................................................................................................... 4 2.1.1 Trench Drain ................................................................................................................... 4 2.1.2 Bioretention .................................................................................................................... 4 2.1.3 Buried Cistern ................................................................................................................. 4 2.2 Aesthetic Features: ................................................................................................................ 4 2.2.1 Pond ................................................................................................................................ 4 2.2.2 Walkway ......................................................................................................................... 5 2.2.3 Picnic Area ..................................................................................................................... 5 2.2.4 Landscaping .................................................................................................................... 5 2.2.5 Descriptive Sign ............................................................................................................. 5 3.0 Area 2 ........................................................................................................................................ 6 3.1 Functional Features ............................................................................................................... 6 3.1.1 Trench Drain ................................................................................................................... 6 3.1.2 Bioretention .................................................................................................................... 6 3.1.3 Rain Barrel ...................................................................................................................... 6 3.2 Aesthetics Features ................................................................................................................ 6 3.2.1 Walkway ......................................................................................................................... 6 3.2.2 Picnic Area ..................................................................................................................... 7


3.2.3 Landscaping .................................................................................................................... 7 3.2.4. Descriptive Sign ............................................................................................................ 7 4.0 Area 3 ........................................................................................................................................ 8 4.1 Functional Features ............................................................................................................... 8 4.1.1 Planter Box ..................................................................................................................... 8 4.1.2 Infiltration Trench........................................................................................................... 8 4.2 Aesthetic Features ................................................................................................................. 8 4.2.1 Materials ......................................................................................................................... 8 4.2.2 Greenery ......................................................................................................................... 8 5.0 Opinion of Probable Cost.......................................................................................................... 9 5.1 Area 1 .................................................................................................................................... 9 5.2 Area 2 .................................................................................................................................... 9 5.3 Area 3 .................................................................................................................................. 10 5.4 Total Cost ............................................................................................................................ 10 6.0 Conclusion .............................................................................................................................. 11 7.0 References ............................................................................................................................... 12 Appendix A: Detailed Opinion of Probable Cost ............................................................................. Appendix B: Drawings ..................................................................................................................... Appendix C: Calculations .................................................................................................................


1.0 Introduction 1.1 Background: Low Impact Development The main purpose of low-impact development (LID) is to imitate the natural hydrology of a site as much as possible. When an area is developed, the amount of impervious surface and stormwater runoff is typically increased. Stormwater management techniques at typical sites often rely heavily on infrastructure to quickly convey this runoff away from a site. Such practices can lead to the degradation of aquatic environments via high velocity flows and heavy pollutant loads. In contrast, LID practices treat the water on-site allowing for increased filtration, infiltration and evapotranspiration. Some common LID methods include rain gardens, bioretention areas, permeable pavements, and rain barrels. Implementing such practices can mitigate or even eliminate the issues associated with development. Additionally, their implementation can often reduce the costs associated with stormwater management and increase the aesthetic value of a site. The Gwinnett County Department of Water Resources is located in Lawrenceville, GA. Due to a large amount of impervious surfaces, the facility has significant runoff volumes. This is a growing problem in rapidly developing areas like Gwinnett County and metro Atlanta as a whole. For this reason, there is a push to require development to implement LID practices to treat all rainfall less than or equal to the 95th percentile 24-hr storm event.

1.2 Problem Statement: G winnett County Department of W ater Resources The Gwinnet County DWR aims to act as an example for local communities in implementing LID practices to treat the 95th percentile storm event. The size of the area to be treated is about 5 acres, including two large parking lots, part of the EXLOGLQJÂśV roof, and some greenspace surrounding the building. Some important constraints include minimal public inconvenience, little to no loss of parking spaces, consideration of a $1 million budget, and continued use of current LID features. In creating a solution, the site was divided into sections and a variety of LID techniques were designed accordingly.

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1.3 Site M ap Figure 1 shows outlines for each of the three treatment areas.

F igure 1: Satellite image of site, separated by area

1.4 Recommended A pproach The recommended solution consists of a variety of components outlined below. 1.4.1 Bioretention Bioretention is a method of temporary on-site detention of stormwater. The proposed design defines two separate bioretention zones to be used for stormwater runoff from the two large parking lots and grassed areas. It is also commonly used for filtration and removal of contaminants from stormwater. A typical bioretention area consists of a top layer with a variety of plants, a mulch layer, a ponding area, engineered soil, and a layer of gravel. For the Gwinnett County site, the cross-section of the bioretention area consists of a top layer of plants and mulch, 5 feet of engineered soil and a 2 foot thick bottom layer of #57 stone. The engineered soil serves the purpose of detaining a significant amount of water, as well as housing plants that minimize erosion, aid in pollutant filtration and provide evapotranspiration. The #57 stone layer at the bottom serves to detain additional water, made possible by its higher porosity. 1.4.2 Infiltration T rench An infiltration trench keeps water out of the stormwater system. A small infiltration trench will run along a thin grassed area next to the parking lot. A planter box that runs 120 feet alongside the building catches water from roof and routes it through the box and to the infiltration trench. The planter box itself also holds a portion of the runoff water. This trench will collect water from a section of the rooftop and detain it until it can evaporate or percolate into the surrounding soil. The trench will have a 3-foot layer of #57 stone.

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1.4.3 Buried C istern One buried cistern will be located on the northeast half of the site beneath a large area of greenspace. The cistern will serve three main purposes: storing rainwater from the roof, distributing rainwater for irrigation, and storing water as backup for the pond. The proposed underground cistern can hold up to 25,000 gallons of water. The cistern will be made of fiberglass, and will be 10 feet in diameter along its length of 48 feet. The cistern itself helps the site qualify for LEED certification through both its type of material as well as its purpose of retaining water for recycled use. 1.4.4 Rain Bar rels One rain barrel will be located near the southwest end of the site next to the building. It is about 12 feet in diameter and will hold up to 12,500 gallons. The rain barrel will collect water from the second story roof near the southwest end of the building. The main purpose of the rain barrel is to store water for irrigation of one of the bioretention areas during drier periods and to keep the roof water from flowing directly to the stormwater system. 1.4.5 Pond The pond will allow for some water storage while providing an aesthetic attraction for public enjoyment. It will have a surface area of about 1,300 square feet, with the deepest section of the pond reaching approximately 1.75 feet. It is designed to blend into the surrounding environment by using a natural looking bottom liner and surrounding it with a variety of plants. As a demonstration of sustainable energy, a solar panel will provide power to the aeration and recirculation pumps. 1.4.6 A esthetic F eatures There are several features added to the site serving mostly aesthetic purposes, but they are also designed to draw attention to the low impact development strategies in order to promote awareness and demonstrate the flexibility of such practices. The depth of engineered soil in each of the bioretention areas is actually slightly higher than the minimum required amount. The reason for the added soil is to provide a more plant-friendly environment to allow for a larger variety of trees and shrubs throughout the site. The proposed designs also include a recycled rubber sidewalk winding from one end of the site to the other. The sidewalk allows people to use the site for leisure activities, and to enjoy the green space. The recycled materials are intended to be a way of demonstrating more sustainable options for sidewalks; this particular material allows for better water flow and root growth in the area, easier maintenance and renovation, as well as the sustainability benefits behind using recycled materials. There are two separate proposed picnic areas, one on each large section of greenspace. Like the sidewalk, these picnic areas are intended to promote use of the area for recreational purposes as well as draw attention to the sustainable design. The pond, while storing some stormwater and potentially cistern overflow, primarily draws attention to the area. The pond will be located along the sidewalk and near the seating area for people to enjoy the sight and sound of moving water.

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2.0 A rea 1 2.1 F unctional Features 2.1.1 T rench D rain The trench drain in Area 1 is located at the end of the parking lot farthest away from the building. The 1.4% grade of the parking lot allows for sheet flow from the parking lot to enter the 153 foot drain during a rain event. The drain has a grade of 0.5% until it reaches the entrance to the greenspace. During the design storm, the parking lot should deliver about 65,000 gallons of water. The trench drain collects this water before it reaches the stormwater drain and redirects it all into a grassed swale, which feeds into a bioretention zone. 2.1.2 Bioretention The bioretention zone of Area 1 wraps around the northern end of the grassed region. It is an Lshaped space with an area of about 5,500 square feet that can treat up to 85,000 gallons of water. The trench drain reroutes the water from the large parking lot to a grassed swale, which removes some sediment and reduces WKHZDWHUÂśVYHORFLW\7KHZDWHUIURPWKHVZDOHLVWKHQIHGLQWRWKH bioretention zone. Water from the street is also input via 2-foot curb cuts. The bioretention area is graded at 1% with the lowest point in the center of the longer side of the L-shape and 33% designed slopes to existing grade levels. From the top, it has a layer of plants and mulch with 6 inches of allowable ponding area, 5 feet of engineered soil, and 2 feet of #57 stone. In case of a rain event larger than the area can hold, an overflow drain will lead any more than 6 inches of ponding out to the stormwater system. 2.1.3 Buried C istern The buried cistern is located beneath the grassy area. It has a total capacity of 25,000 gallons of water, and is made mostly of long-lasting, rustproof fiberglass. The outside of it is ribbed for added strength, and it stays 10 feet in diameter along its 48-foot length. The cistern can also provide points for LEED certification. All of the roughly 18,000 gallons of rainwater from the front part of the roof of the building is routed via gravity flow through a manifold system into the cistern. The manifold system includes in-line filtration units to prevent clogging of the system and to minimize maintenance. The cistern has hosing leading its stored water to the bioretention area for irrigation purposes. The water can also be switched from the irrigation hoses to an attached spigot for above-ground watering or for maintaining water levels in the pond if necessary. Pressure for the hoses is provided by a pump which is powered by a 1,440-Watt solar generator.

2.2 Aesthetic Features: 2.2.1 Pond The recirculating pond is located southeast of the bioretention area and has an operating capacity of about 9,300 gallons. The oval-shaped pond will be about 50 feet long by 40 feet wide, and 1.75 feet at its deepest point. There will be solar-powered pumps and aerators to maintain clean water in the pond, and to ensure minimal maintenance. The pond-liner along the bottom will be 4


covered with decorative material, and blocks will be used for structural support. The pond is designed to blend in with the surroundings, making it look as natural as possible. It will be surrounded by rocks and plants, and will be close enough to the picnic area for public enjoyment. 2.2.2 W alkway The sidewalk running throughout the site is made of a recycled rubber material. It maintains a five foot width, and is about 940 feet long in Area 1. The sidewalk will skirt the outer perimeter of the green space, showing all of the features of the site. There will be a descriptive sign along the sidewalk explaining the sustainable LID features. It will also lead to a picnic area for recreational purposes and to the rest of the property for those using it for leisure or exercise. 2.2.3 Picnic A rea The picnic space in Area 1 is a round mulched space with a diameter of about 20 feet. The area will have two picnic tables, each about six feet long, potentially seating more than a dozen people. It will be located about 10 feet away from the pond so people can sit and enjoy the water, and the space itself will be connected to the winding sidewalk for easy access. A vine-covered pergola will cover the area to provide both pleasing aesthetics and shade in the summer months. 2.2.4 L andscaping The method of landscaping was an attempt to imitate a natural environment as much as possible. A large variety of trees and plants were chosen to keep the area looking alive and healthy throughout the year. The spacing of the trees and other plants is a more randomized pattern in an attempt to mimic the natural irregularities of nature. 2.2.5 Descriptive Sign The descriptive signs act as both an aesthetic feature as well as an educational tool and will be located near the sidewalk for easy access. The sign will have an animation of the system, with details about and a description of each part. The sign for Area 1 will include drawings of the bioretention area, the underground cistern, and the pond. It will focus on the movement of rainwater in each region: from the trench drains to the bioretention area, from the cistern to irrigation pipes, and to and from the pond area. The sign will explain the main purposes of each functional system to make them more informative and easily understood for potential future designers.

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3.0 A rea 2 3.1 F unctional Features 3.1.1 T rench D rain Similar to Area 1, the length of the trench drain in Area 2 is 154 feet long. The drain will catch most of the rainwater at the far end of the parking lot and divert it away from the stormwater system and into one end of the bioretention zone. The trench drains will be built to handle approximately 75,000 gallons of water, which is the amount of water coming from the parking lot during the design storm. 3.1.2 Bioretention The bioretention zone in Area 2 is about 170 feet by 30 feet, yielding a total area of about 5,100 square feet. It runs along the length of the green space and has a designed grading of 1% with the boundaries sloping at a constant angle of 33%. The deepest part of the bioretention zone is in the center, where ponding can occur until about six inches is reached. In cases where ponding would exceed 6 inches an overflow drain will catch any excess water and divert it to the stormwater system. The cross-section of the bioretention area includes, from the highest elevation: a layer of mulch and plants (with a ponding zone), 5 feet of engineered soil, and 2 feet of #57 stone. It is equipped to treat over 80,000 gallons of water. The water being treated by this bioretention zone comes from the parking lot (diverted to it by the trench drains), runs through a grassed swale, and to the bioretention zone itself. 3.1.3 Rain Bar rel The proposed rain barrel is about 12 feet in diameter and holds about 12,500 gallons when full. The main purpose of the rain barrel is to capture a portion of the roof water from the southwest end of the building. During the 1.8 inch design storm, the tank will hold all of the 11,500 gallons of water from the roof. The reason for capturing this water is both to keep it out of the stormwater system as well as to use it for on-site irrigation purposes. Having backup irrigation allows for the bioretention zone to house a larger number and variety of plants. The rain barrel will sit on top of a square 13 foot concrete slab for support. Water from the tank can flow to the buried irrigation system or feed into an above-ground hose.

3.2 Aesthetics Features 3.2.1 W alkway As in Area 1, the sidewalk will run around the green space and be made of a recycled rubber material. This section of the sidewalk is about 390 feet total in length and 5 feet wide uniformly. It continues along the same latitude as the sidewalk from Area 1 to promote a sense of continuity throughout the site. The sidewalk runs alongside of the bioretention zone to promote viewing of the area. It also leads to a picnic area to show that a site can serve to be both recreational and functional.

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3.2.2 Picnic A rea The picnic space in Area 2 is a round zone with a diameter of about 20 feet, with an identical design to the picnic area in Area 1. The ground will have a mulch topcoat to separate it from the grass, and it will be connected to the sidewalk for easy access. It will have two picnic tables that are about 6 feet long each, potentially seating more than a dozen people. The area will be partially covered by a pergola, which will provide some shade in the hotter months. Vines will be planted on and around the pergola to keep the area green and visually pleasing. 3.2.3 L andscaping In order to encourage people to use the space as a place of recreation or leisure, the design is intended to mimic a natural environment and create an aesthetically pleasing area, similarly to Area 1. A large variety of trees and shrubs have been chosen in attempt to have some amount of the plants flowering at any given time. Different types of trees were chosen to have a variety of colors, heights, and shade present in the area; they were also placed in a less uniform manner to imitate patterns more typically found in nature. The bioretention area, while subterraneously uniform, exhibits a winding pattern of mulch and river rock on the surface to emulate the flow of water, similar to that seen in rivers. This section of the land will be watered almost exclusively by rainwater. In the event of a drought, backup irrigation is stored in the rain barrel connected to a buried system for irrigation. 3.2.4. Descriptive Sign As in Area 1, the descriptive sign in Area 2 acts as an educational tool. The sign will be near the sidewalk or picnic space to make it easily visible to visitors. For Area 2, the sign will explain the rain barrel being used for irrigation, the trench drain used to capture runoff from the parking lot, and the bioretention zone used for water storage. The sign is an effort to increase awareness of LID techniques as well as show the purposes of the methods present.

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4.0 A rea 3 4.1 F unctional Features 4.1.1 Planter Box The water used in the planter box will come from a portion of the second story roof; the 5 gutters will feed directly into the planter box rather than entering the stormwater system. The box itself is about 120 feet long, 2.5 feet tall and about 2 feet wide. It contains 6 inches of topsoil, 1 foot and 9 inches of #57 stone, and a perforated pipe running along the bottom. The perforated pipe takes the water around the #57 stone and leads it out to the infiltration trench. Around each of the downspouts is a 3 foot radius of flowering plants, and the rest of the space is mulched. In case of an extreme rain event, there is a 6-inch overflow drain at each downspout leading to an existing manifold, which goes to the storm sewer. The interior perforated pipe is graded at 0.6% with the highest point toward the middle of the building and the low point at the end. At the low point sits a pipe which leads to an infiltration trench near the parking lot. Note that the exact location of the existing manifold is unknown, so the design attempts to be cautious of the unknown parameters. A pond liner will be placed between the planter box and the existing building as well as along the base of the wall to prevent damage to the foundation. 4.1.2 Infiltration T rench The infiltration trench is located along the side of the parking lot opposite Area 1. It can treat up to 9,200 gallons of water, and is fed by the overflow drains of the planter box. The trench is 100 feet long and about 8 feet wide. It is filled with 2.75 feet of #57 stone for storing water. It serves to divert water from the stormwater system, giving it time to evaporate or infiltrate the surrounding soil.

4.2 Aesthetic Features 4.2.1 M aterials The outside of the planter box will be made of recycled railroad ties as a demonstration of reusing and recycling materials. It will also create a different texture along the bottom of the wall, which is otherwise mostly empty. The infiltration trench will be lined with river rocks to mimic the appearance of water even when water is not present. 4.2.2 G reenery Flowering groundcover will be placed within each 3 foot radius of the planter box. These plants were chosen to provide growth and color throughout the year. The other reason for the plants is to provide dimension and contrast against an otherwise plain wall.

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5.0 O pinion of Probable Cost 5.1 A rea 1 Table 1 shows expected costs for Area 1. T able 1: Expected cost summary for Area 1

AREA 1 Site Preparation Soils & Rocks Storage W ater T reatment Plants Piping A esthetics Construction Total A/E Costs A dditional Costs T O T A L C OST T O T A L C OST W I T H C O N T I N G E N C Y

Cost $38,300 $82,000 $51,000 $23,800 $50,000 $109,200 $354,300 $36,400 $14,200 $404,900 $445,400

5.2 A rea 2 Table 2 shows expected costs for Area 2. T able 2: Expected cost summary for Area 1

AREA 2 Site Preparation Soils & Rocks Storage W ater T reatment Plants Piping A esthetics

Cost $18,600 $59,200 $11,500 $16,900 $42,400 $71,100

Construction Total A/E Costs A dditional Costs T O T A L C OST T O T A L C OST W I T H C O N T I N G E N C Y

$219,700 $22,600 $8,800 $251,100 $276,200

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5.3 A rea 3 Table 3 shows expected costs for Area 3. T able 3: Expected cost summary for Area 3

AREA 3 Site Preparation Soils & Rocks W ater T reatment Piping A esthetics Construction Costs A/E Costs A dditional Costs T O T A L C OST T O T A L C OST W I T H C O N T I N G E N C Y

Cost $9,900 $7,400 $12,100 $200 $29,600 $3,000 $1,200 $33,800 $37,200

5.4 Total Cost The total expected cost for the entire site is shown in Table 4. T able 4: Total expected costs

OVERALL Site Preparation Soils & Rocks Storage W ater T reatment Plants Piping A esthetics Construction Total A/E Costs A dditional Costs T O T A L C OST T O T A L C OST W I T H C O N T I N G E N C Y ANNUA L O & M

Cost $66,800 $148,600 $62,500 $40,700 $104,600 $180,400 $603,600 $62,100 $24,100 $689,800 $758,800 $21,000

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6.0 Conclusion The proposed site design is capable of handling the entirety of a 95th percentile rain event. The design includes 2 bioretention areas, an infiltration trench, an above-ground tank, a subsurface tank, and a retention pond. Based on our calculations and analysis of the past 6 years of rain data, our site released an average of 5 million gallons of water into storm sewers annually. Had the recommend design been in place, we estimate that annually, 4.8 million gallons would have been captured on site, resulting in a runoff volume of less than 250,000 gallons annually which represents a reduction of 95 percent. The aesthetic features of the design, while expensive, illustrate to developers, designers, and the public that stormwater can be successfully treated as a resource rather than a waste with proper planning. Large portions of the cost were due to it being a retrofit rather than initial design. In most cases, utilizing the previously mentioned technologies would only moderately increase cost while drastically reducing runoff and improving water quality. The implementation of this design adds value to the site itself while decreasing the impact of the site on the surrounding ecosystems.

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7.0 References Georgia Department of Natural Resources. (2001, August). Georgia Stormwater Management Manual. (1st edition). [Online]. Available: http://www.georgiastormwater.com/GSMMVol2.pdf Grainger. Grainger Pumps. [Online]. Viewed 2012 April. Available: http://www.grainger.com/Grainger/pumps/ecatalog/N-bir Green Bros Earth Works. Price List: Prices effective March 7, 2012. [Online]. Viewed 2012 April. Available: http://www.greenbrothersearthworks.com/price-list.html Ohio State University. Horticulture and Crop Science: Mulching and Landscape Plants. 2001. [Online]. Viewed 2012 April. http://ohioline.osu.edu/hyg-fact/1000/1083.html RSMeans, RSMeans Site Work & Landscape Cost Data , 29th Annual Edition, Kingston, MA, 2010. Rubbersidewalks, Inc. Rubbersidewalks. [Online]. Viewed 2012 April. Available: http://rubbersidewalks.com/products/rubbersidewalk/ Xerxes. Water Conservation Tanks for Green Applications. [Online]. Viewed 2012 April. Available: http://www.xerxes.com/products/water-products/water-conservation-greenbuilding.html

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T reatment Volume C alculation V = Treatment Volume (ft3) P = Design Precipitation (ft) C = Runoff Coefficient A = Treatment Area (ft2) Sample C alculation: A rea 2 ! Bioretention Sizing C alculation V = Treatment Volume (ft3) k = Native Soil Percolation Rate (in/hr) n = Basin Media Porosity d = Basin Fill Depth (ft) T = Rainfall Duration

Sample C alculation: A rea 2

!


Runoff C alculation

Location Area of Front Roof: Area of Left Roof: Area of Right Roof: Area 2 Greenspace Area 1 Greenspace Parking Lot Area 2 Parking Lot Area 1 Sum

Rainfall Runoff Without System Runoff With System

Area (sqft) 16800 9997 8154 23800 40000 64000 65000 227751

Runoff Coefficient 0.95 0.95 0.95 0.25 0.25 0.95 0.95

Total Runoff Average Annually (gallons) 6,700,000 5,000,000 230,000

Adjusted Area (sqft) 15960 9497.15 7746.3 5950 10000 60800 61750 171703.5 Percent Runoff (%) 75% 3.5%

Note: All storms are broken up by individual runoff coefficients. All storms below 1.8 inches completely infiltrate the system and all storms greater than 1.8 inches initially fill the system and the rest is considered runoff. This consideration and calculation is conservative for our design with expected results anticipated higher. Source of data: NCDC Station Number 722196 in Dekalb County. Accurate rainfall data at Briscoe Field for the desired time frame was unavailable.

Low Impact Development Retrofit Technical Report  

This is the technical report of the Low Impact Development Retrofit of the Gwinnett County Dept of Water Resources Central Facility.

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