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2014

Estimating the benefits of trees in storm water management Ravali Siddam University of Toledo

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A Thesis entitled Estimating the Benefits of Trees in Storm Water Management by Ravali Siddam Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Civil Engineering

________________________________________ Dr. Cyndee Gruden, Committee Chair ________________________________________ Dr. Defne Apul, Committee Member ________________________________________ Dr. Ashok Kumar, Committee Member ________________________________________ Dr. Patricia R. Komuniecki, Dean College of Graduate Studies

The University of Toledo December, 2014

Copyright 2014, Ravali Siddam This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

An Abstract of Estimating the Benefits of Trees in Storm Water Management by Ravali Siddam Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Civil Engineering The University of Toledo December 2014 Urbanization has drastically changed the hydrologic cycle and has negatively impacted water quality. Best Management Practices (BMP) are designed to reduce the effects of pollutants in water and reduce the risk of flooding by controlling the storm water entering the river system. Current green storm water practices focus on herbaceous vegetation. Trees are superior at storm water management since they have an increased ability to capture and store rainfall in their canopy, facilitate evapotranspiration, and improve storm water quality by taking up and transforming pollutants through extensive root systems. The overarching objective of this work was to quantify the performance benefits of incorporating trees in bioretention systems. Two storm water tools were used in this project: (i) EPA National Storm Water Calculator and (ii) Green Values National Storm Water Calculator. The EPA calculator represented annual rainfall, annual runoff, days per year with runoff, rainfall retained. The Green Values Calculator represented volume capture, coefficient and runoff, land use, cost benefits. Both the tools demonstrated that trees in bioretention systems can capture more runoff volume than having only bioretention systems without trees. iii

The literature suggests that evapotranspiration and interception together could address between 0.1% and 11% of annual rainfall in Northwest Ohio. However, neither tool had the capacity to quantify interception and evapotranspiration.

More research is needed for

effective usage of trees in storm water management. Storm water management decision making tools should be expanded by providing tree details like tree canopy size, leaf size, trunk diameter to incorporate the benefits associated with trees.

iv

Dedicated to my Parents. Sri Siddam Ramesh Kumar and Smt. Siddam Sreedevi.

Acknowledgements

This journey would not have been possible without the support of my family, professors, and friends. Firstly, I would like to thank Dr. Cyndee Gruden for giving me a Masters Research opportunity to work under her and for her support and guidance. My sincere thanks to Dr. Ashok Kumar and Dr. Defne Apul for being a part of my thesis committee. I would also like to thank my parents, friends and, my well-wishers for their love and support to achieve my goal.

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Table of Contents

Abstract…………………………………………………………………………………...iii Acknowledgements………………………………………………………………………..v Table of Contents…………………………………………………………………………vi List of Tables……………………………………………………………………………..ix List of Figures…………………………………………………………………………….xi List of Abbreviations……………………………………………………………………xiv 1

Introduction………………………………………………………………………..1 1.1Storm Water Runoff……………………………………………………………1 1.2Storm Water Management in Urban Areas…………………………………….2 1.3 Best Management Practices (BMP)…………………………………………...3 1.4 Pollutant Removal of Structural BMP………………………………………...4 1.4.1 Nutrients……………………………………………………………..4 1.4.2 Total Suspended Solids (TSS)………………………………………7 1.5 Project Objectives……………………………………………………………..8

2

Bioretention Systems…………………………………………………………….10 2.1 Examples of Bioretention Systems…………………………………………..12 vi

2.2 Trees in Bioretention Systems……………………………………………….15 2.2.1 Silva Cells………………………………………………………….16 2.2.2 Storm Water Tree Pit………………………………………………17 2.2.3 Tree Filter Box……………………………………………………..18 2.3 Performance of Trees in BMP……………………………………………….19 3

Trees in Urban Storm Water Management………………………………………21 3.1 Interception…………………………………………………………………..22 3.1.1 Stem flow…………………………………………………………..23 3.1.2 Through Fall………………………………………………………..24 3.1.3 Interception Loss…………………………………………………...25 3.1.4 Interception and Hydrologic Benefits……………………………...26 3.2 Evapotranspiration (ET)……………………………………………………...27 3.3 Quantifying Benefits of Trees…………………………….………………….30 3.3.1 National Tree Benefit Calculator…………………………………..30 3.4 Other and Aesthetic Benefits………………………………………………...34 3.5 Nutrient Uptake by Trees…………………………………………………….35

4

EPA Green Storm Water Calculator……………………………………………..37 4.1 Methods………………………………………………………………………37 4.2 Results and Discussion………………………………………………………52

5

Green Values Storm Water Management Calculator……………………………56 5.1 Methods………………………………………………………………………56 5.2 Results and Discussions……………………………………………………...68 vii

5.3 Comparison of Storm Water Tools ………………………………….……...75 6

Conclusions and Future Work…………………………………………………...81

References………………………………………………………………………………..84 Appendix A………………………………………………………………………………91

viii

List of Tables

1-1

Non-Structural BMP………………………………………………………………4

1-2

Phosphorus Concentrations and their Ranges……………………………………..5

1-3

Typical Phosphorus and Nitrogen Removal Rates for BMP………………………7

1-4

Types of BMP and their TSS Removal Rates……………………………………..8

2-1

Pollutant Removal of Listed BMP……………………………………………….20

3-1

Annual Interception Volumes of Trees…………………………………………..27

3-2

Measured Interception from Deciduous and Coniferous Tree……………………27

3-3

Plant Name and their Transpiration Rates………………………………………..29

3-4

Evapotranspiration by Different Land Cover Types…………………………..…30

3-5

Annual Uptake of Nutrients………………………………………………………36

3-6

Nitrogen Uptake by Trees at Different Ages of Tree……………………………..36

4-1

10 Year Analysis…………………………………………………………………53

4-2

20 Year Analysis……………………………………………………….………...54

4-3

30 Year Analysis…………………………………………………………………54

5-1

Volume Control…………………………………………………………………..70

5-2

Coefficients and Runoff………………………………………………………….70

5-3

Life Cycle Costs………………………………………………………………….71 ix

5-4

Benefits of Vegetative Filter Strip without Tree………………………………….72

5-5

Benefits Vegetative Filter Strip with Tree………………………………………..73

5-6

Benefits of Increasing Number of Tree Plantations………………………………74

5-7

Input Parameters………………………………………………………………….76

5-8

Equations Used…………………………………………………………………...77

5-9

Output Parameters………………………………………………………………..79

A-1

Infiltration vs. Rainfall Retained…………………………………………………92

A-2

Hydraulic Conductivity vs. Annual Runoff………………………………………93

A-3

Porosity vs. Volume Capture……………………………………………………..94

A-4

Runoff Caused by Soil Types…………………………………………………….96

x

List of Figures

2-1

Bioretention System……………………………………………………………...11

2-2

Different Types of Rain Garden Designs…………………………………………13

2-3

Open Channel Bioswale………………………………………………………….14

2-4

Silva Cell…………………………………………………………………………16

2-5

Tree Pit…………………………………………………………………………...18

2-6

Tree Filter Box…………………………………………………………………...19

3-1

Evapotranspiration and Interception……………………………………………..22

3-2

Rainfall Above and Below a Canopy…………………………………………….23

3-3

Evapotranspiration……………………………………………………………….28

3-4

Maple Tree Benefits in Toledo, OH………………………………………………32

3-5

Carbon Dioxide Avoided by Single Tree…………………………………………33

4-1

Location………………………………………………………………………….39

4-2

Soil Type…………………………………………………………………………40

4-3

Soil Survey……………………………………………………………………….41

4-4

Soil Drainage……………………………………………………………………..42

4-5

Topography………………………………………………………………………43 xi

4-6

Precipitation……………………………………………………………………...44

4-7

Evaporation………………………………………………………………………45

4-8

Climate Change…………………………………………………………………..46

4-9

Land Cover……………………………………………………………………….47

4-10

Annual Rainfall Distribution……………………………………………………..48

4-11

LID Controls……………………………………………………………………..49

4-12

Rain Garden……………………………………………………………………...50

4-13

Results with Current and Baseline Scenario……………………………………..51

5-1

Getting Started…………………………………………………………………...57

5-2

Lot Information…………………………………………………………………..58

5-3

Predevelopment…………………………………………………………………..59

5-4

Runoff Reduction Goal…………………………………………………………..59

5-5

Conventional Development………………………………………………………60

5-6

Green Improvements……………………………………………………………..61

5-7

Advanced Options………………………………………………………………..62

5-8

Curve Number Method…………………………………………………………..63

5-9

Runoff Curve Numbers for Urban Area………………………………………….64

5-10

Volume Control…………………………………………………………………..65

5-11

Coefficient and Runoff…………………………………………………………...66

5-12

Costs……………………………………………………………………………...66

5-13

Benefits…………………………………………………………………………..67

A-1

Infiltration vs. Rainfall Retained…………………………………………………92

A-2

Hydraulic Conductivity vs. Annual Runoff………………………………………93 xii

A-3

Porosity vs. Volume Capture……………………………………………………..95

A-4

Runoff for Different Soil Types………………………………………………….96

xiii

List of Abbreviations

BMP……………………………...…..Best Management Practices. EPA…………………………………..Environmental Protection Agency. LID…………………………………...Low Impact Development. NPDES………………………………National Pollution Discharge Elimination Systems.

xiv

Chapter 1

1. Introduction

1.1 Storm Water Runoff: Storm water runoff is the connection between precipitation, such as rain or snow, and the generation of flows through natural or man-made conveyance systems during a storm event. Where the surface is pervious, water oozes into the ground and eventually replenishes ground water aquifers and surface waters such as lakes. In this way, storm water runoff is part of the hydrologic cycle, involving the distribution and movement of water between earth’s atmosphere, land, and water bodies (Department of Energy & Environmental Protection, 2004). However, urbanization has drastically changed the hydrologic cycle and has negatively impacted water quality. Storm water can be considered as both point source and non-point source pollution. According to EPA’s National Pollutant Discharge Elimination Systems (NPDES) point source discharge is considered storm water runoff which flows into the conveyance system and is discharged through pipe. A non-point source is storm water runoff which flows over land surface and does not concentrate in a defined channel. 1

In most cases, storm water runoff begins as a non-point source and becomes a point source discharge through a storm water collection system. Both point and non-point sources of urban storm water runoff have been shown to be significant causes of water quality impairment (Department of Energy & Environmental Protection, 2004). Habitats have been destroyed due to urban runoff including fisheries, shellfish harvesting, tourism, and other recreational losses. Flooding due to poor management of storm water can cause significant damage to infrastructure. In addition, revenue losses have occurred due to increased maintenance and management due to the loss of the natural stream network (Booth et al., 2006). Damages to ecosystems and infrastructure can be mitigated using storm water management.

1.2 Storm Water Management in Urban Areas: Rapid development of urban areas is affecting water quality and quantity. Due to the development of land, pervious surfaces are converted to mostly impervious surfaces as from the construction of buildings, parking lots, and other structures. This increase in impervious surface proportionally increases storm water runoff. In this way, the natural hydrologic cycle is negatively impacted since storm water is unable to naturally percolate into the ground instead creating significant storm water flows. Due to the increase in storm water runoff, there is an imbalance between physical, chemical, and biological processes resulting in pollution, soil erosion, and flooding. Storm water pollutants are contributed through construction activities, agricultural activities, street pavement, motor vehicles, atmospheric deposition, vegetation, land surface, chemicals, and waste water. The common pollutants in storm water runoff are sediments, nutrients, heavy metals, oxygen demanding 2

substances, pathogens, petroleum hydro carbons, and toxics. Removal of pollutants in storm water can be done through treatment processes including sedimentation, flotation, filtration, adsorption, and biological degradation. These processing mostly affect the particulate and dissolved forms of pollutants (Peluso et al., 2002). Many communities are using best management practices (BMP) to reduce the likelihood of storm water runoff, pollution, and flooding.

1.3 Best Management Practices (BMP): BMP are primarily designed to reduce the effects of pollutants in water and reduce the risk of flooding by controlling the storm water entering the river system. Control of water quantity is a critical part of storm water management using BMP since pollutant concentrations are dependent upon flow quantity. Flow control objectives in storm water management include flow attenuation and volume reduction. These are generally achieved through BMP that encourage infiltration, detention, storage, and evaporation. The three principles applied for water quality treatment by BMP are 1) Prevention: avoiding pollutants 2) Reduction: reducing the pollutants 3) Treatment: capturing and treating pollutants The two types of BMP are structural and non-structural. Structural BMP are constructed to intercept or treat storm water flow prior to release into receiving waters. Examples of structural BMP include: wet ponds, retention ponds, wetlands, bioswales, filter strip, and sand filters. These systems are popular in urban settings, since many are

3

compact in design. Non-structural BMP are used to control the amount of contaminants entering storm water. These often involve changing human behavior. Examples of non-structural BMP include source control/maintenance, used oil recycling, storm drain flushing, and public education (Table 1.1). Table 1.1: Non- Structural BMP Non Structural BMP Source control/ Maintenance

Type Street sweeping Used oil recycling Vehicle split control Vegetation control Storm drain flushing Detention device maintenance

Public Education and Participation

Newspaper, brochures

(Transportation Research Board, 2006)

1.4 Pollutant Removal of Structural BMP: There are a variety of storm water contaminants that can cause damage to the ecosystem. One purpose of structural BMP installation is to reduce the concentration of pollutants in the runoff before it is discharged. Typical storm water contaminants that are measured and are of significant concern include nutrients (phosphates and nitrates) and suspended solids. 1.4.1 Nutrients: Phosphorus is a non-metallic element which is essential for plants in land-based ecosystems. It is also essential material for terrestrial plant growth and a common pollutant 4

in fresh water systems. Excess phosphorus causes eutrophication, resulting in excess growth of biomass including algal blooms in surface waters. After decay of the biomass, the level of dissolved oxygen in the river system decreases, which causes death of aquatic life (Faucette et al, 2010). Phosphorus mostly enters the river system through non-point sources attached to sediment particles or in dissolved formn life (Faucette et al., 2010). By reducing runoff volume by bioretention or through filtration, phosphorus content can be reduced (USEPA, 2007). Table 1.2: Phosphorus Concentrations and Their Ranges Phosphorus Concentration Range (mg/L) Waste water treatment plant 0.05 - 5.00 Storm water runoff (urban) Up to 0.4 Natural Sediment P + soluble P 0.03 - 0.10 Natural Soluble Phosphorus 0.03 – 0.09 (USEPA, 2007), (Sharpley, 1980), (Faucette et al., 2010) Table 1.2 highlights the range of phosphorus concentration in different waste water streams as compared to natural concentrations. This table does not include agricultural drain inputs as those values are according to site specific conditions. The concentration of soluble phosphorus in pristine environments is comparatively very low. As expected, the highest range of phosphorus concentration is being carried by a waste water treatment plant. Natural waters have 0.02 mg/L of phosphorus and consider it a limiting factor for plant growth. Per capita contributions estimated at approximately 3.5 pounds of phosphate yearly to the environment (Cameron et al., 2007). Total phosphorus and dissolved phosphorus are both typically measured.

In

general, phosphorus is present in particulate-bond form. Hence, a BMP with permanent settling pools provides effective removal of phosphorus. Phosphorus can be increased in bioretention BMP because of leaching and suspension of particulate phosphorus. So, 5

vegetated BMP should be provided with adequate inlet protection, dense vegetation, and drop structures or check dams to minimize suspension of particulates. The usage of chemical fertilizers should be avoided (Jones et al., 2012). Nitrates are highly oxidized forms of nitrogen. These enter into surface water either by runoff or by groundwater inflow. Microbial denitrification and plant growth are effective nitrate removal procedures (Kelly, 2006). Denitrification is an energy requiring process to convert nitrate into nitrogen gas. It takes place in the presence of abundant carbon. Denitrification is a microbial mediated process; it slows at cold temperatures and increases as water temperature increases. The chemical equation 1.1 is used for calculating denitrification, NO3 + Organic C → N2 , N2 O (or) → NO + CO2 + H2 O ….. (eq,1.1) (Kelly, 2006) (Jones et al., 2012) Total nitrogen as well as nitrates and nitrites are often included in the analysis of nitrogen. It is assumed that retention ponds release nitrogen during the growing season and then release nitrogenous solids during decay of vegetation. The particulate nitrogen bound depends upon the climatic conditions and algal growth. For wetlands in particular, nitrification and de-nitrification processes can result in conversion of nitrates to nitrogen gas, which is then lost to the atmosphere. Aerobic and anaerobic zones for nitrification/ denitrification may be provided by bioretention which includes storage in soil pores. Harvesting vegetation and algal removal may help to remove nitrogen levels in storm water treatment systems (Kelly, 2006).

6

Table 1.3: Typical Phosphorus and Nitrogen Removal Rates for BMP BMP

Phosphorus Removal Rate (%) 20-60 51-55 20-30 50-60 80-90

Nitrogen Removal Rate (%) 20-30 52-57 10-20 Up to 50 40-50

Bioretention Basin Bioswale Extended Detention Basin Infiltration Basin Manufactured-Treatment Devices (International Stormwater BMP Database, 2012)(University Of New Hampshire Storm Water Center, 2012) (Department of Environmental Protection, 2004)(Perry et al., 2009) Table 1.3 shows phosphorus and nitrogen removal concentrations for particular BMP. Highest phosphorus removal BMP is manufactured devices and also bioswales. Least phosphorus removal rate is being carried by extension detention basin. The highest nitrogen removal rate is being carried by porous pavement, and bioswale. The least nitrogen removal rates are being carried by extension detention basin. 1.4.2 Total Suspended Solids (TSS): In most of the surface water, TSS is the most significant pollutant. It causes damage to aquatic life, creates turbidity in water, and acts as vehicle to transport other pollutants. BMP are frequently used to treat TSS. TSS can be controlled by minimizing erosion (Faucette et al., 2010). To control TSS, BMP should include slowing the flow through detention ponds or vegetation. Infiltration can also be an effective approach.

7

Table 1.4: Types of BMP and their TSS Removal Rates Best Management Practice (BMP)

TSS Removal Rate (%)

Bioretention System Up to 80 Constructed Storm water Wetland Up to70 Dry Well Volume reduction up to 40 Extended Detention Basin 40-60 Infiltration Structure 50- 80 Pervious Paving System Volume reduction up to 80 Sand Filter Up to 80 Vegetative Filter 60-80 (Department of Environmental Protection, 2004) (Brown et al., 1997) Table 1.4 explains the total suspended solids removal rate by particular BMP. Highest removal rate occurs through implementing bioretention, such as rain gardens. Least removal rate occurs in a dry well. Constructed wetland, vegetative filter, wet pond, and infiltration filter are also found to be good removal rates for total suspended solids.

1.5 Project Objectives: With urbanization on the rise and impermeable surfaces dominating the environments, and existing storm water infrastructure is often inadequate. Although current green storm water practices focus on herbaceous vegetation, trees are far superior at storm water management since they have an increased ability to capture and store rainfall in their canopy, facilitate evapotranspiration, and improve storm water quality by taking up and transforming pollutants through extensive root systems. Also, root penetration through typically impermeable urban soil layers into more permeable zones can increase storm water infiltration rates. Urban forests have been shown to be effective at intercepting rainfall from small, short duration storms often responsible for the “first flush” of runoff, during which most annual pollutant runoff occurs (Xiao et al., 1998). There are very few experiments 8

conducted on trees in BMP. Most data on trees exists in the urban forestry literature. This work will endeavor to quantify the performance benefits of incorporating trees in bioretention systems. The hypothesis of this project is that, trees will improve the performance of bioretention systems in terms of both water quality and water quantity released to surface waters. Objective 1: To prepare a literature review on current implementation of trees in BMP and their performance. Objective 2: To supplement this literature with fundamental information on trees and their relation to storm water through the urban forestry literature. Objective 3: To attempt to quantify the benefits of trees in bioretention systems using existing BMP planning too.

9

Chapter 2

2.0 Bioretention Systems Bioretention systems are storm water best management practices (BMP) that use infiltration to treat storm water runoff. These systems emulate natural systems since they use vegetation as such trees, shrubs, and grasses to remove storm water pollutants. Runoff enters the bioretention system either directly, through a designed drainage system or swale (Figure 2.1). It consists of a surface layer, mulch layer, and porous media. When the rainfall exceeds the infiltration capacity, instead of discharging into surface area, water will be temporarily held in the ponding area on the surface or sent through an overflow device to nearby surface waters.

10

Figure 2.1: Bioretention System (Jiayuliu, 2013) Bioretention is a general description of BMP that mimic the natural environment and provides a variety of pollutant removal mechanisms including filtration, adsorption, and nutrient uptake. Due to multiple removal mechanisms, they can provide a high degree of treatment. These systems can be easily incorporated into parking lots, and median strips. If designed adequately, they can provide cost effective storm water peak flow control as well as water quality control. Some challenges to bioretention systems include seasonal efficiency in the cold climate regions and excessive sediment loads that might cause clogging (Ten towns, 2012). Bioretention has become the most frequently installed process BMP in many watersheds because it can be built to suit a site. Nitrogen removal by using bioretention is high almost exceeding 40% (Hunt, 1999). A bioretention cell appears as most effective, when they are designed for nutrient load removal. 11

2.1 Examples of Bioretention Systems: Rain Garden. A rain garden is an excavated shallow surface depression planted with native vegetation to treat storm water runoff. These gardens are generally shallow (2’ to 3’) deep and are planted with trees, shrubs and covered with mulch. A rain garden works as both a hydraulic and a unit treatment process BMP. These work by allowing infiltration through selected treatment material or media. In locations where the surrounding natural soils have good infiltration rates, they can recharge ground water by reducing large flow quantities of storm water runoff. By using adsorption process and decomposition process, rain gardens are effective in removing storm water pollutants (Dietza et al., 2007). Design manuals are being developed for implementation of rain gardens. There is a yet still wide spread controversy about media selection to achieve effective removal of storm water pollutants. Typically rain gardens are sized to either capture or store a set quantity of flow (e.g., the first half inch of runoff) or to capture the water quality volume calculated by knowing site specific information (Dietza et al., 2007). Rain garden vegetation serves to filter and transpire water quantity and enhance infiltration. The plants actually uptake pollutants, while soil medium is primarily used to filter out pollutants and provide habitat for the plants themselves. The media layers will create some internal storm water storage, whereas the bed itself provides additional volume control. Properly designed rain gardens provide mimic ecosystem through species diversity resulting in a system that is resistant to pollution. Rain garden function on volume reduction is medium, it provides medium to high recharge, low to medium peak flow control, and also medium to high water quality. It removes 60% of total phosphorus and 30% of total nitrogen (Department of Environmental Protection, 2006). 12

Figure 2.2: Different Types of Rain Garden Designs (Courtney et al., 2013) Rain gardens can be designed with plants, shrubs and also with dry creek using stones and also by creating a depression (Figure 2.2). Rain gardens function effectively when native plants and soils are used. Incase if infiltration is not being carried out quickly then under drains are used. Because these under drains are connected to the existing storm water system. Overall runoff is not significantly reduced, even then it can reduced when underdrains are installed, and however it can effectively reduce pollutant loads and attenuate peak flows. Media with excellent infiltration rates are being designed to

13

maximize the amount of runoff that can be treated without creating standing water (Dietza et al, 2005). Bioswale. A bioswale can be considered as a vegetated ditch which handles storm water runoff. These are generally constructed to control the peak flow of storm water. These control the runoff of storm water, there by reducing the velocity of water over a period of time. These helps us to reduce pollutants mainly nitrogen and phosphorus. There are three types of bioswales are dry swale, wet swale, Grassed channel (Jurries, 2003).

Figure 2.3: Open Channel Bioswale (Sustainable Cities, 2013) Open channel bioswales are generally constructed on the road side and also in parking lots. It should be provided with curb outlet and inlet, and a stone buffer (Figure 2.3). Generally, bioswales are effective in treating storm water runoff with low amount of maintenance. These effectively remove pollutants in storm water. Pollutant reduction is based on retention time entering bioswale. Vegetation selected also influences the removal of pollutants (Courtney et al., 2013). Bioswales are good at removing pollutants in storm water. They are able to remove total suspended solids, metals, and nutrients like nitrogen 14

and phosphorus by following flocculation processes. Nutrients in the storm water can also be removed though natural settings and vegetation, and used as effective tools in reducing peak flow. In the urban areas with large impervious areas, they provide good detention time and can be used in most situations with little restrictions.For example bioswales are often used in treating highway and road side runoff as they are linear practises. They can perform well in both arid and semiarid climate regions (Jurries, 2003).

2.2 Trees in Bioretention Systems: Bioretention systems preserve forests and wetlands by storing flood water and protecting the downstream conditions. They can also capture runoff. Trees installed in bioretention systems have many environmental and community benefits. Trees can reduce the nitrogen and phosphorus uptake. Roots of the trees play a major role in treating storm water management systems since they incorporate more extensive interception and evapotranspiration than other vegetation (Fazio, 2010). Roots grow and decay leaving open channels which increase soil porosity and infiltration rates. Tree roots create confining layers which also help to maintain infiltration rates over the lifetime of the bioretention system. The rainfall interception and evapotranspiration benefits provided by trees within bioretention are likely to be negligible in terms of annual runoff reduction. However, the benefits provided in individual storms may be significant. Although the research is lacking on tree performance in bioretention systems, there is a vast quantity of literature on the environmental, social, and economic benefits of trees. There is a movement in BMP selection and installation toward the increased installation of trees with a focus on their aesthetic benefits (Herwitz, 1985). The following sections highlight examples of storm water BMP that incorporate trees. 15

2.2.1 Silva Cells: The Silva Cell is a flexible and modular soil containment system that holds unlimited amounts of lightly compacted soil while supporting traffic loads beneath pavement. The soil housed within the Silva Cell serves two important functions, growing large trees and treating storm water on site. As the tree becomes mature, it becomes a powerful form of green infrastructure by providing economic benefits (Deeproot, 2014). Trees growing in forests have access to large amounts of lightly compacted soil, whereas in cities, heavy compaction of soil takes place. Silva Cells eliminate this issue for trees by providing modular soil for tree growth. Trees can be made a functional part in the urban infrastructure, where they can serve as the lungs to a city (Deeproot, 2014).

Figure: 2.4: Silva Cell (Dobbin group, 2013) Generally, Silva Cells are designed to control the storm water up to 2 inches (5 cm) Silva Cells can be constructed on streetscapes (Deeproot, 2014). They can also be installed in parking lots, and on green walls. In the United States Silva Cells are found in many places like New York, Minnesota, Florida, and Illinois. Silva Cells generally receive storm water inflow from direct street runoff or storm water collection systems (Figure 2.4). The deck

16

allows for monitoring of soil moisture and provision of optimum soil volumes resulting in appropriate sized, successful long-term trees (Urban et al, 2011). 2.2.2 Storm Water Tree Pit: Storm water tree pits are similar to rain gardens since they take storm water runoff off line for treatment and typically receive flow from curb cuts. Storm water tree pits can consist of a pre-cast concrete vault containing engineered soil, native vegetation plantings and trees suited for urban conditions. An under drain is installed at the bottom of the basin and connected to the existing storm sewer system if infiltration cannot be allowed due to slow soil infiltration rates. Tree pits may also include storage chambers, which hold additional runoff, available for plant uptake or groundwater recharge (Figure 2.5). The optimal area for root zone is 1,500 cubic feet (multiple trees can share the same root zone area). A minimum of 300 cubic feet of root zone area should be provided. Depth should be below 3 ft., if it is above 3 ft., then it would result in small rooting zone areas and, therefore, significantly limit tree growth (USEPA, 2013).

17

Figure 2.5: Tree Pit (NYC, 2012)

2.2.3 Tree Filter Box: Tree box filters are small bioretention areas installed that can be very effective at controlling runoff. Runoff is directed to the tree box, where it is cleaned by vegetation and soil before entering a catch basin. They are designed to collect only the first flush of storm water. The runoff collected in the tree-boxes helps to irrigate the trees. They can fit into any landscape scheme increasing the quality of life in urban areas by adding beauty, habitat value, and reducing urban heat island effects (UNH Annual Report, 2007).

18

Figure 2.6: Tree Filter Box ( Steffens, 2009) A tree box consists of three primary components: a chamber, soil media, and the plant. The underground storage chamber typically is a precast concrete structure which contains a specially formulated soil media to filter the storm water and the native, non-invasive tree or shrub (Figure 2.6). Tree box filters are filled with specially designed soil media that is designed for rapid infiltration (e.g., a soil media mixture of 80% sand and 20% compost). Some tree boxes are filled with a proprietary media specially designed to remove a particular pollutant such as bacteria (Steffens, 2009). Infiltration capacity is a function of the soil media, gravel size, depth, and the underlying subsoil. An open chamber system with a gravel and stone chamber allows vertical infiltration. The rate of infiltration and depth of filter media play a role in design of the open system.

2.3 Performance of Trees in BMP: Although there is not an exceptional amount of performance data available on trees in BMP, some data is collected (Table 2.1). Of course, performance will vary in accordance with the design parameters established for each BMP installation. However, it should be noted that these

19

BMP have very good performance in reduction of nutrients and suspended solids with the exception of the tree filter box which does cause significant reduction in solids, but cannot remove as many nutrients due to limited residence time in the tree filter box. Its primary capability is infiltration (filtering out solids) for improving water quality. Table 2.1: Pollutant Removal of Listed BMP Total Phosphorus (%)

Total Nitrogen (%)

Total Suspended solids (%)

Expected Life Span (years)

Silva Cell

>80

60-70

80-90

5-10

Tree Pit

70-75

60-68

70-85

25

Tree Filter Box

>63

>48

>85

20-25

BMP

References (Building green.com) (www.stormh2 o.com) http://www.crw a.org/projects/b mpfactsheets/cr wa_treepit.pdf http://stormtree.com/ourstorm-waterfiltrationsystems/

(a)Silva Cell Facilitates Urban Tree Growth (Building green.com) (Deep Root Partners, L.P). (b)Project Profile-Reshaping Downtown Minneapolis 52 Storm water march/April 2010(www.stormh2o.com) (c) http://storm-tree.com/) (d) http://www.crwa.org/projects/bmpfactsheets/crwa_treepit.pdf

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Chapter 3

3.0 Trees in Urban Storm Water Management: Planting trees in storm water treatment practices can increase pollutant uptake and reduce storm water runoff through rainfall interception and evapotranspiration (ET), enhance soil infiltration, provide soil stabilization, increase aesthetic appeal, provide wildlife habitat, and provide shade. A typical tree in suspended pavement can hold up to 2.54 cm (1 inch) storm event from impervious surface area, significantly greater than just the area under the tree canopy (Marritz, 2011). It is estimated that the urban forest can reduce annual runoff by 2 – 7 percent (Fazio, 2010). When trees are combined with other natural landscaping, studies have shown that as much as 65 percent of storm runoff can be reduced. In this chapter, it is described about how trees can provide an added reduction of storm water flow and pollutants primarily through evapotranspiration and interception (Figure 3.1).

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3.1 Interception: Interception refers to precipitation that does not drive into the soil, but is instead seized by the leaves and branches of plants and the forest floor. It occurs in the canopy and in the forest floor by direct root uptake. Interception is mathematically defined as the rainfall beneath canopy plus the precipitation running down the trunk less the measured rainfall outside the drip line (Center for Watershed Protection, 2012). Interception varies by tree type.

Figure 3.1: Evapotranspiration and Interception (Marritz, 2011)

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Previous studies on trees showing that interception by conifers is greater than interception by deciduous trees because of the greater foliage surface area of conifers (Center for Watershed Protection, 2012). Also, the presence of foliage on conifers during winter month’s results in increased annual interception. In general, mature trees with a wide crest will intercept the most rainfall and, in urban areas, storm water runoff reduction benefits will be maximized when trees are located so they overhang impervious surfaces. Once rain falls onto a vegetation canopy, it effectively partitions the water into separate modes of movement: through fall, stem flow and interception loss. Below (Figure 3.2) shows through fall, interception loss and stem flow. Each process is described individually below.

Figure 3.2: Rainfall Above and Below a Canopy (Ronson, 2012). 3.1.1 Stem flow: Stem flow is the process that directs precipitation down the plant branches and stems. It is the flow of intercepted water down the trunk or stem of a plant (Center for Watershed Protection, 2012). This process causes the plant's stem to receive additional 23

moisture. Leaf shape, stem, and branch architecture establishes the amount of stem flow. In general, deciduous trees have more stem flow than coniferous trees. The transfer of precipitation and nutrients from the canopy to the soil is possible in both stem flow and through flow. Stem flow can act like a funnel, as it collects water from a large area of canopy, but can transport it to the soil in a much smaller area. This is most obvious for the deciduous oak-like tree. At the base of a tree, it is possible for the water to rapidly enter the soil through flow along roots and other macro pores surrounding the root structure. This can act as a rapid conduit of water sending a significant pulse into the soil water (Xiao et al., 2003). 3.1.2 Through Fall: It is the process of precipitation passage through the plant canopy which varies with the type of vegetation. This process is controlled by factors like plant leaf and stem density, type of the precipitation, intensity of the precipitation, and duration of the precipitation event. This is the water that falls to the ground either directly or indirectly, directly through gaps in the canopy, and indirectly, having dripped off leaves, stems or branches. The amount of direct through fall is controlled by the canopy coverage for an area, a measure of which is the leaf area index (LAI) or ratio between leaf area and ground surface area. If LAI