Stormwater Harvesting: Accounting of Benefits and Feasibility
MWMO Watershed Bulletin: 2013-3
Prepared for the MWMO by:
Emmons & Olivier Resources Inc.
Stormwater Harvesting : Accounting of Benefits and Feasibility
Prepared for the MWMO by:
Brett Emmons, Principal, CEO, and Water Resources Engineer
Greg Graske, Water Resources Engineer
Meghan Jacobson, Limnologist
MWMO Contact:
Udai Singh, Water Resources Manager
Acknowledgements:
Yvette Christianson, Minnehaha Creek Watershed District, Water Quality Specialist
Jim Calkins, Minnehaha Creek Watershed District, Board President
Pat Byrne – City of Minneapolis – Public Works, Senior Professional Engineer
Ross Bintner – City of Edina, Environmental Engineer
Suggested citation:
Emmons & Olivier Resources Inc. 2013. Stormwater Harvesting: Accounting of Benefits and Feasibility. MWMO Watershed Bulletin 2013 - 3. 78 pp.
Front Cover: Top left: Residential Rainbarrel Bottom left: Drip Irrigation
Right image: MWMO Office in NE Minneapolis, MN (photo by EOR)
2522 Marshall Street NE
Minneapolis, Minnesota 55418
(612) 465-8780
(612) 465 8785 fax www.mwmo.org
Stormwater Harvesting: Accounting of Benefits and Feasibility
MWMO Watershed Bulletin: 2013-3
Prepared for the MWMO by:
Emmons & Olivier Resources Inc.
Abstract Goals and Objectives
Reuse and harvesting of stormwater is not new and This project is intended to address obstacles to has been used in many places and throughout making stormwater reuse a more reliable and time. However, in modern urban settings and cities, common tool for urban stormwater managers. The the proliferation of potable water systems has reuse of stormwater, with emphasis on non-potable reduced the reliance on stormwater reuse. Our team applications, has unique benefits that make it a good has used an applied research grant to better tool for meeting newly emerging stormwater volume understand and define how stormwater can be control rules. The goal of this project is to quantify reused to reduce stormwater impacts and reduce our the benefits or “credits” of stormwater reuse. Reuse use of energy-intensive public potable is especially promising in urban settings as a best water. Challenges of stormwater reuse include management practice (BMP) where there are quantifying stormwater benefits of reuse given the limitations to other practices, such as infiltration, due variability of rainfall and adapting stormwater to to site constraints. The objective of this project is to different climatic regions. operationalize stormwater reuse via a new spreadsheet tool and through a selection process
identifying retrofit opportunities.
2522 Marshall Street NE MWMO Stormwater Harvesting and Reuse Full Report_10-28-2013 Minneapolis, Minnesota 55418
(651) 287 0948 (651) 287 1308 fax
www.mwmo.org
Executive Summary
Stormwater harvesting is consistently identified as a way to conserve potable water resources and reduce the volume of runoff exiting the system. Other potential benefits include irrigation cost reductions (or even revenue), enhanced water quality treatment, and improvements in vegetation. However, there is generally a lack of technical guidance on how to quantify the benefits of stormwater harvesting with respect to runoff volume reduction and water quality treatment. For this project, a literature review of existing stormwater harvesting and reuse programs was conducted, followed by the development and testing of a spreadsheet- based model to estimate the runoff volume reduction and water quality benefits of stormwater harvesting with irrigation reuse.
At the time of this literature review, the most advanced stormwater reuse guidance had been developed in Australia and Texas, with a focus on water conservation in arid climates, and in Florida with a focus on reducing stormwater volumes. There is also considerable stormwater research being conducted that is focused on addressing the key challenges of stormwater reuse: the effects of stormwater infiltration on soil quality (e.g., heavy metals and salts) and the effectiveness of stormwater infiltration in cold climates.
A spreadsheet-based model was developed to estimate the runoff volume reduction and water quality benefits of stormwater reuse using a daily time step mass balance of stormwater runoff volume and phosphorus load assuming non-conservative phosphorous mixing (Walker 1987 phosphorus sedimentation equations) in the storage reservoir. The model simultaneously calculates annual volume reduction and phosphorus removal as a percent of the annual watershed load, in addition to annual evaporation losses and phosphorus sedimentation over a dry, average, and wet year.
The suitability of stormwater harvesting and reuse with irrigation was tested for sites with various percent imperviousness, watershed area, irrigation area, irrigation depth, irrigation season length, basin volumes, and stormwater storage systems. Stormwater harvesting with irrigation reuse was found to be an effective best management practice to reduce runoff volume and phosphorus load to downstream waters. Up to an additional 35% of the annual watershed phosphorus load and up to an additional 80% of the annual watershed runoff volume were removed through irrigation reuse of stormwater in model test runs. Including the phosphorus load lost via sedimentation and runoff volume removed via evaporation, a total of 55-96% of the annual watershed phosphorus load and a total of 20-95% of the annual watershed volume were removed through stormwater harvesting and irrigation reuse.
There were four key findings from this project. First, stormwater harvesting and reuse offers an opportunity to retrofit existing ponds and developments for low impact development stormwater management. Second, stormwater harvesting and reuse has the ability to attain more volume reduction in less space than typical infiltration trenches and basins, and offers a suitable alternative for challenging sites where infiltration is not feasible, such as sites with clay soils, contaminants, or shallow bedrock. Third, the graphical outputs of the spreadsheet-based model is an effective tool for managing overflow volume and timing, sizing storage reservoirs and basins, and identifying the need to route water on or off site for irrigation. And finally, model results from numerous theoretical sites indicate that green space is often limiting in highly developed sites while stormwater runoff volume is limiting in highly undeveloped sites, illustrating the need for cooperative stormwater management at a regional scale.
Table of Contents
Stormwater Harvesting and Reuse Literature Review Literature Review Background ...... 9 Stormwater Reuse Literature Review ...... 11 Feasibility Issues in Minneapolis ...... 26 References ...... 39 Technical Guidance on the Estimation of Runoff Volume Reduction & Water Quality Benefits from Stormwater Reuse Introduction...... 46 I. Modeling Assumptions ...... 47 II. Model Inputs and Outputs ...... 51 III. Model Testing ...... 61 IV. Interpretation of Results...... 63 Appendix A. Model Equations ...... 65 Appendix B. Model Test Graphs ...... 69 References Cited ...... 72 Stormwater Harvesting and Reuse: Report on Models for 8 Theoretical Sites Methodology ...... 73 Summary of key findings ...... 74 Recommendations ...... 74 Summary Table ...... 75 Model tables ...... 84 Preliminary Cost Estimates ...... 92
6 Stormwater Harvesting: Accounting of Benefits and Feasibility
List of Figures Figure 1. Example rate-efficiency-volume (REV) Curve for Meteorological Region 1 in Florida ...... 22 Figure 2. Ten year (2001-2010) daily rainfall summary for Minneapolis, Minnesota...... 27 Figure 3. Ten year (2001-2010) cumulative rainfall event summary for Minneapolis, Minnesota...... 27
Figure 4. Daily sums of transpiration per unit canopy area (EC) in evergreen needleleaf (solid circles) and deciduous broadleaf (open circles) plant functional types in a suburban neighborhood of Minneapolis-St. Paul, Minnesota...... 29 Figure 5. Stormwater harvesting and reuse simplified watershed used in model testing ...... 47 Figure 6. Stormwater storage pond and cistern basic dimensions ...... 48 Figure 7. Water mass balance schematic ...... 49 Figure 8. Example change in basin volume and phosphorus concentration with time ...... 50 Figure 9. Phosphorus mass balance schematic ...... 50 Figure 10. Input parameter model interface ...... 52 Figure 11. Output parameter model interface ...... 52 Figure 12. Annual Rainfall 10-year, 20-year, and 30-year trends ...... 53 Figure 13. Input-Output spreadsheet tab of the stormwater reuse model ...... 59 Figure 14. Stormwater harvesting and reuse model summary figures (1 acre watershed and 5 months of irrigation) ...... 62 Figure 15. Volume mass balance screen capture of the stormwater reuse model spreadsheet...... 65 Figure 16. Phosphorus mass balance screen capture of the stormwater reuse model spreadsheet ...... 67 Figure 17. Benilde St. Margaret ...... 76 Figure 18. Pamela Park ...... 77 Figure 19. Edina Country Club 1 ...... 78 Figure 20. Edina Country Club 2 ...... 79 Figure 21. MCWD New Office ...... 80 Figure 22. Christ Presbyterian Church ...... 81 Figure 23. Van Cleve Park...... 82 Figure 24. Summit Preserve...... 83
List of Tables Table 1. Existing stormwater and rainwater harvesting and reuse programs researched for this literature review...... 11 Table 2. Additional rainwater harvesting programs in the United States identified during this literature review ...... 12 Table 3. Major stormwater reuse and rainwater harvesting program goals, water sources, and reuse applications by program location...... 13 Table 4. Example stormwater rate efficiency volume (REV) curve calculation spreadsheet ...... 21 Table 5. Example stormwater rate efficiency volume (REV) curve empty data table ...... 21 Table 6. Annual mean, maximum, and standard deviation potential evapotranspiration (PET; inches/day) for Minneapolis, Minnesota from 2001 to 2010...... 28
Stormwater Harvesting and Reuse 7
Table 7. Growing season (June – September) mean stormwater pollutant concentrations...... 30 Table 8. Flow-weighted mean snowmelt event pollutant concentrations (in mg/L) in the St. Paul area, Minnesota, by site type ...... 30 Table 9. Stormwater Best Management Practices (BMP) average effluent mean concentrations ± 95% confidence interval and number of samples (N) for detention basins, retention ponds, and wetland basins based on available data drawn from the International Stormwater Best Management Practices Database (Geosyntec Consultants and Wright Water Engineers, Inc. 2008)...... 31 Table 10. Mean (± standard deviation) waterlogging tolerance of urban tree species modified from Appendix A in Niinemets and Valarde 2006. Waterlogging tolerance values were determined according to the waterlogging tolerance scale of Whitlow and Harris (1979): 5, very tolerant (survives deep, prolonged waterlogging for more than one year); 4, tolerant (survives deep waterlogging for one growing season); 3, moderately tolerant (survives waterlogging or saturated soils for 30 consecutive days during the growing season); 2, intolerant (tolerates one to two weeks of waterlogging during the growing season); 1, very intolerant (does not tolerate water- saturated soils for more than a few days during the growing season)...... 32 Table 11. Summary of the waterlogging tolerance of common Minnesota turfgrass cultivars. References for each cultivar is listed in the table...... 33 Table 12. Common Minnesota plants with known salt tolerance. Reprinted from the Minnesota Stormwater Manual (MSSC 2008)...... 34 Table 13. Guidelines for appropriate stormwater BMPs based according to stormwater source area. Reprinted from Table 2.8.1 in Low Impact Development Stormwater Management Planning and Design Guide (STEP 2010)...... 38 Table 14. 10-year (2002-2011) rainfall data summary ...... 54 Table 15. Model test maximum storage pond depths corresponding to minimum runoff volume ...... 57 Table 16. Summary table of input parameter values ...... 60 Table 17. Tested model sites (CN = 74) ...... 61 Table 18. Model test graphs for 3-month irrigation season ...... 69 Table 19. Model test graphs for 5-month irrigation season ...... 70 Table 20. Model test graphs for 7-month irrigation system...... 71 Table 21. Benilde St. Margaret School ...... 84 Table 22. Pamela Park ...... 85 Table 23. Edina Country Club ...... 86 Table 24. Edina Country Club 2 ...... 87 Table 25. MCWD New Office ...... 88 Table 26. Christ Presbyterian Church ...... 89 Table 27. Van Cleve Park ...... 90 Table 28. Summit Preserve ...... 91 Table 29. Preliminary Cost Estimates ...... 92
8 Stormwater Harvesting: Accounting of Benefits and Feasibility
Stormwater Harvesting and Reuse Literature Review
LITERATURE REVIEW BACKGROUND
Stormwater harvesting and reuse is the practice of collecting and reusing stormwater for a potable or non- potable application. For the purposes of this review, stormwater is defined as runoff collected from roof and ground surfaces, rainwater is defined as runoff collected from roof surfaces only, harvesting is defined as collection and storage of runoff, and reuse is defined as the potable or non-potable application of runoff. The purpose of this literature review was to determine the scope of studies and research that has been completed to date on stormwater harvesting and reuse, and to include information related to stormwater harvesting and reuse in cold climates. The organization of this review is described in detail below.
Stormwater Reuse Literature Review
The first section summarizes information related to existing stormwater harvesting and reuse programs, and is divided into the following three sub-sections:
Existing Stormwater Harvesting and Reuse Programs
- a summary of the program locations reviewed and a list of the primary stormwater reuse references. Also included is a summary of the common stormwater reuse goals, water sources, reuse applications, operation and maintenance, and regulatory credits of existing programs.
Water Quality, Human Health, and Environmental Concerns
- a summary of common water quality, human health, environmental concerns related to using harvested stormwater for potable and non-potable applications.
Stormwater Reuse System Design
- a summary of available modeling tools for sizing rainwater cisterns and stormwater ponds and determining the efficiency of stormwater reuse.
There are two existing reviews on stormwater harvesting and reuse (Fletcher et al. 2008; O’Connor et al. 2008) that we recommend reading in addition to our review (see accompanying reference CD).
Feasibility Issues in Minneapolis
The second section summarizes information related to assessing the feasibility of stormwater reuse programs in Minneapolis, and is divided into the following four sub-sections:
Stormwater Availability and Use Rates
− a summary of precipitation, evapotranspiration, and irrigation rate estimates for Minneapolis.
Stormwater Quality
− a summary of available water quality data for Minneapolis stormwater runoff and the efficiency of pollutant removal in stormwater ponds.
Water and Pollutant Tolerance of Cold Climate Plants
Stormwater Harvesting and Reuse 9
− a summary of known waterlogging and pollutant tolerance of common Minnesota tree species and turgrass cultivars.
Minnesota Stormwater Management
− a summary of existing stormwater regulations for Minneapolis and other relevant stormwater management resources.
At the end of the review is a list of important references and websites found during this literature review. Comprehensive reviews of stormwater harvesting and reuse are starred and bolded. These references are organized by sub-section and grouped in corresponding folders on the accompanying reference CD.
10 Stormwater Harvesting: Accounting of Benefits and Feasibility
STORMWATER REUSE LITERATURE REVIEW
Existing Stormwater Harvesting and Reuse Programs
This section of the literature review summarizes the characteristics of existing stormwater harvesting and reuse programs located in Australia, Florida, and Texas and advanced rainwater harvesting and reuse programs located in Virginia, North Carolina, and Washington State (Table 1). For the purposes of this review, stormwater is defined as runoff collected from roof and ground surfaces, rainwater is defined as runoff collected from roof surfaces only, harvesting is defined as collection and storage of runoff, and reuse is defined as potable or non-potable applications of runoff. These programs were highlighted for their advancement in the field or for a particular expertise in modeling, regulatory credits, or operation and maintenance plan. Additional rainwater harvesting and reuse programs known to exist at the time of this review but not included in this report are listed in Table 2.
Table 1. Existing stormwater and rainwater harvesting and reuse programs researched for this literature review. Harvesting and Reuse Program Location Stormwater Rainwater Australia – New South Wales1 Florida2 Texas3 Virginia4 North Carolina5 Washington6 Footnotes indicate corresponding documentation for each program.
1 Natural Resource Management Ministerial Council, Environment Protection and Heritage Council, and National Health and Medical Research Council. July 2009. Australian Guidelines for Water Recycling: Managing Health and Environmental Risks (Phase 2) –
Stormwater Harvesting and Reuse; Department of Environment and Conservation New South Wales. April 2006. Managing Urban Stormwater: Harvesting and Reuse
2 Department of Environmental Protection and Water Management Districts. March 2010. Draft Stormwater Quality Applicant’s Handbook:
Design Requirements for Stormwater Treatment Systems in Florida.
3 Alan Plummer Associates, Inc. March 2010. Final Report: Stormwater Harvesting Guidance Document for Texas Water Development Board.
4 Virginia Dept. of Conservation and Recreation. 1 March 2011. Stormwater Design Specification No. 6: Rainwater Harvesting, Version 1.9.5; Cabell Brand Center. 2009. Virginia Rainwater Harvesting Manual, 2nd Edition
5 North Carolina Division of Water Quality. 22 September 2008. Revised Technical Guidance: Stormwater Treatment Credit for Rainwater
Harvesting Systems
6 Washington State Dept. of Ecology, Water Quality Program. February 2005. Stormwater Management in Western Washington: Volume III – Hydrologic Design and Flow Control Design/BMPs
Stormwater Harvesting and Reuse 11
Table 2. Additional rainwater harvesting programs in the United States identified during this literature review7
Location
Tucson, AZ Oregon
San Francisco, CA Tennessee
Santa Monica, CA River Falls, WI
Kansas Eau Claire County, WI
Maryland
Reuse Program Goals
Stormwater runoff from highly impervious urbanized or developed landscapes disrupts hydrology following storm events by causing large peaks in stream flow, large loads of pollutants, and reduced infiltration in the watershed. As a result, the primary goals of stormwater reuse are to reduce downstream stormwater flows and pollutant loads (DEC-NSW 2006). As the human population and urbanization grows, there is also a need to reduce potable water demand (Hatt et al. 2006). However, this goal is most commonly associated with rainwater and stormwater harvesting reuse programs located in arid environments where the availability of freshwater is limited, as illustrated by the examples of Australia and Texas. Other goals include reducing stress on (or cost of) water supply infrastructure, and reducing the size of other stormwater best management practices (CBC 2009; NC-DWQ 2008). The reuse goals for stormwater in Australia, Florida, and Texas and for rainwater in Virginia, North Carolina, and Washington are summarized in Table 3. The three most common reuse goals are highlighted in yellow.
Water Sources
Stormwater must be temporarily stored after rainfall events prior to reuse. Rainwater is usually stored in above or below ground cisterns while stormwater is most commonly stored in pre-existing stormwater BMPs (e.g., wet detention ponds) or stormwater channels (e.g., urban sewers and waterways). Water sources of stormwater in Australia, Florida, and Texas and for rainwater in Virginia, North Carolina, and Washington are summarized in Table 3. The three most common water sources are highlighted in yellow.
Reuse Applications
The most common reuse application of stormwater and rainwater is urban irrigation (Fletcher et al. 2008). Other common reuse applications include residential toilet flushing and vehicle washing, municipal fire-fighting and water features (e.g., ponds), and industrial cooling towers. The reuse applications for stormwater in Australia, Florida, and Texas and for rainwater in Virginia, North Carolina, and Washington are summarized in
7 Gold, A., R. Goo, L. Hair, and N. Arazan. 2010. Rainwater Harvesting: Policies, Programs, and Practices for Water Supply Sustainability. 2010 International Low Impact Development Conference, San Francisco, CA
12 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 3. The most common reuse applications in each major category (irrigation, residential, municipal, and industrial) are highlighted in yellow.
There was no great difference in the number of reuse applications between rainwater and stormwater reuse programs. While rainwater has less pollutants and particulates than stormwater, stormwater can be pre- treated with settling basins and built-in filtration systems to improve water quality for a larger range of reuse applications. Existing stormwater and rainwater reuse programs did not have clear guidelines for choosing appropriate reuse applications beyond listing applications that were locally desirable or feasible.
Stormwater reuse applications can be divided based on whether the application area has “restricted” or “unrestricted” public access. Restricted reuse applications include irrigation of areas where public access can be controlled, such as golf courses, cemeteries, and highway medians. Unrestricted reuse applications include irrigation of areas where public access is not controlled (e.g., parks, playgrounds, school yards, and residences), use for toilet and urinal flushing in commercial and industrial buildings, air conditioning, fire protection, construction, ornamental fountains, and aesthetic impoundments. Unrestricted reuse applications have more stringent water quality regulations than restricted applications to limit public health risk and exposure to pollutants in stormwater (Alan Plummer Assoc. 2010; NRMMC et al. 2009; USEPA 2004).
Table 3. Major stormwater reuse and rainwater harvesting program goals, water sources, and reuse applications by program location. The most common goals, water sources, and applications are highlighted in yellow. Australia – New South Wales (AUST), Florida (FLOR), Texas (TEX), Virginia (VIRG), North Carolina (NCAR), and Washington State (WASH).
Stormwater reuse Rainwater harvesting
AUST FLOR TEX VIRG NCAR WASH
Reuse Program Goals
Reduce pollution load to surface waters
Reduce stormwater flows
Reduce potable water demand
Reduce impacts of urbanization on watershed hydrology
Reduce stress on water supply infrastructure
Reduce size of other stormwater BMPs
Water Sources
Roofwater – Residential
Roofwater – Nonresidential
Stormwater – Wet Detention Pond
Stormwater Harvesting and Reuse 13
Stormwater reuse Rainwater harvesting
AUST FLOR TEX VIRG NCAR WASH
Stormwater – Urban sewers
Stormwater – Waterways
Stormwater – Wetlands
Sewage
Greywater
Reuse Applications
Irrigation – Playing fields, golf courses, public parks and gardens, residential, commercial
Irrigation – Agricultural
Residential – Toilet flushing, vehicle washing
Residential – Washing machine use
Residential – Dual reticulation
Municipal – Fire-fighting or fire-suppression
Municipal – Water features and ponds
Municipal – Street cleaning
Industrial – Cooling tower make-up
Industrial – Miscellaneous
Industrial – Dust control
Industrial – Feed lot cleaning
Hydrological – Downstream flow augmentation
Hydrological – Aquifer storage and recovery
Regulatory Credits
Regulatory credits for the benefits (e.g., volume and pollutant reductions) of stormwater and rainwater harvesting and reuse vary greatly by region and are largely undefined. For example, despite the advanced nature of stormwater harvesting and reuse in Australia, we were unable to find specific regulatory credits associated with their programs. When they do exist, credits are most often granted for volume and pollutant load reduction benefits. Some programs offer financial incentives to encourage incorporation of reuse into their stormwater management plans. The following is a summary of regulatory credits established for
14 Stormwater Harvesting: Accounting of Benefits and Feasibility
stormwater harvesting and reuse in Florida and Texas, and for rainwater harvesting and reuse in Virginia, North Carolina, and Washington State.
Florida – The improved nutrient removal efficiency of stormwater wet detention ponds resulting from reuse activities can be used in Florida to meet requirements for nutrient load reductions in stormwater runoff management. These requirements depend on the level of water quality that needs to be achieved in the receiving body of water and range from achieving an 85% reduction of post-development average annual loading of nutrients from the project or a reduction such that post-development average annual loading does not exceed nutrient loading from natural vegetative community types. Nutrient reduction calculation examples can be found on pages 172-178 of the Stormwater Quality Applicant’s Handbook (DEP-WMD 2010).
Texas – We could not find any regulatory credits for stormwater harvesting and reuse in Texas. However, implementation of stormwater harvesting and reuse systems is encouraged through financial incentives, such as property tax exemptions, tax exemptions for rainwater harvesting systems, and rebates and discounts on rainwater harvesting systems (TX-WDB 2005, Chapter 5).
Virginia - In Virginia, rainwater harvesting and reuse can be used to meet stormwater volume reduction and nutrient removal requirements according to the type of water body receiving discharge from a stormwater BMP treatment train that includes a stormwater reuse system (CBC 2009; VA-DCR 2011). Runoff reduction credits are based on the total amount of annual internal and outdoor water reuse for the harvesting and reuse system using the Cistern Design Spreadsheet.
North Carolina - Similar to Virginia, rainwater harvesting and reuse systems can be used to meet stormwater volume reduction requirements (NC-DWQ 2008). The roof area captured may be deducted from the impervious area used to size other stormwater BMPs on the site or the captured rainwater can contribute to the peak runoff attenuation and runoff volume reduction of the stormwater BMPs.
Washington State – We could not find any regulatory credits directly related to rainwater harvesting and reuse in Washington State. But rainwater harvesting and reuse systems can be incorporated into BMPs that address the volume and timing of stormwater flows to help prevent or reduce adverse impacts to downstream waters (WA-DEWQ 2005).
Operation and Maintenance Issues
There are not well-defined operation and maintenance procedures for rainwater and stormwater harvesting and reuse programs. Guidelines for operation and maintenance exist in Australia, Florida, and North Carolina and are summarized below. A review of stormwater harvesting and reuse (O’Connor et al. 2008) recommended that delivery and distribution systems of stormwater reuse require additional devices and regular maintenance to ensure reliable service because of greater corrosion and clogging of pipes resulting from higher sediment and microbial loads in stormwater than treated water. They recommended periodic flushing of pipes to remove sediment build up and chlorination of pump heads to clear microbial scum.
Australia - One of the major concerns associated with stormwater harvesting and reuse programs in Australia is the lack of ongoing monitoring after construction and the potential risk of exceeding water quality guidelines (Fletcher et al. 2008). To address this concern, a recent guideline recommended biannual monitoring of
Stormwater Harvesting and Reuse 15
nutrients and quarterly monitoring of sediments to assess stormwater quality for irrigation (NRMMC et al. 2009).
Florida - Stormwater harvesting and reuse systems in Florida are required to file reports every two years documenting results of annual inspections (DEP-WMD 2010). These reports must include the following information:
1. Inspect operation of the stormwater harvesting system to assure that the pump, flow meter, and filter system are operating properly and achieving desired flow volumes 2. With respect to the irrigation system, inspect the pump, timer, distribution lines, and sprinkler heads to assure they are working properly 3. Maintenance log Stormwater volume harvested using a flow meter specifying the day, time, and volume Stormwater volume irrigated or otherwise used using a flow meter specifying the day, time, and volume used Observations of the stormwater harvesting system operation, maintenance, and a list of parts that were replaced Observations of the irrigation system operation, maintenance, and a list of parts that were replaced Dates on which the stormwater harvesting and irrigation (or other use systems) were inspected and maintenance activities conducted
North Carolina - Rainwater harvesting and reuse systems in North Carolina must be inspected by the owner/operator within 24 hours after each rain event and on a monthly basis (NC-DWQ 2008). Records of operation and maintenance must be kept in a known set location and be available upon request. A copy of their Operation and Maintenance agreement is included in the accompanying reference CD.
Water Quality, Human Health, and Environmental Concerns
Stormwater runoff over developed or managed surfaces collects pollutants and toxins that may cause health risks to plant, animals and humans. In addition, harvesting and storing stormwater in a storage pond or basin may attract unwanted organisms and degrade the surrounding environment. The water quality, human health, and environmental concerns of stormwater harvesting and reuse have been extensively studied. A summary of common concerns is included below.
Water Quality
Common pollutants in stormwater runoff include nutrients, sediments, heavy metals, salinity, pathogens, and hydrocarbons (NRMMC et al. 2009; Alan Plummer Assoc. 2010). Vehicular traffic is a source of heavy metals, hydrocarbons, salt, and sediment; lawn and gardens are sources of nutrients and pesticides; and human and animal wastes are sources of bacteria and viruses. Most stormwater reuse systems rely on the pollutant removal abilities of stormwater best management practices to treat stormwater (Fletcher et al. 2008). For example, constructed wetland effluents can achieve total suspended solid levels as low as 5-10 mg/L and a 60-90% reduction in phosphorus (Rousseau et al. 2008).
16 Stormwater Harvesting: Accounting of Benefits and Feasibility
Secondary treatment is also used to achieve higher water quality for reuse applications, such as biofiltration, water treatment technologies, and disinfection. Biofiltration results in loss of heavy metals and total suspended solids of up to 90%. Nutrient removal is more variable and depends on the filter media, plant type, and presence of an anaerobic zone for denitrification (nitrogen removal). Treatment technologies include mechanical sand or disc filtration, in-pipe treatment filtration, filter screens, sediment tanks, and aeration. Disinfection includes UV dosing and chlorination, with Log10 removal of pathogens related to the dose amount. (Fletcher et al. 2008; NRMMC et al. 2009; USEPA 2004)
A 3-year assessment of water quality in a stormwater detention pond was conducted to assess the feasibility of reusing stormwater to irrigate public lands in Alberta, Canada (He et al. 2008). The stormwater originated from a residential development and was to be used to irrigate parkland predominantly used for strolling, sitting, and walking dogs. Agricultural water quality guidelines were chosen because of the risk of accidental ingestion of pollutants. This study found that detention pond stormwater generally met water quality guidelines for heavy metals and pathogens. This suggests that the pollutant removal ability of stormwater BMPs is effective and that secondary treatment to improve stormwater water quality is not always necessary.
Human Health
The main risk of stormwater reuse to human health is exposure to pathogenic bacteria. The Australian Guidelines for Water Recycling: Managing Health and Environmental Risks Phase 2 (NRMMC et al. 2009) is a comprehensive guide developed to manage the human health risks associated with stormwater reuse. They recommend signage and fencing around an area irrigated with harvested stormwater to limit public exposure. If access cannot be controlled, they recommend secondary treatment (including disinfection) of harvested stormwater. The effectiveness of secondary treatment on pathogen removal was studied at two stormwater reuse sites in Australia by Davies and others (2008). Log10 reduction of pathogens in stormwater by UV disinfection ranged from 2.26 - 4.59, by biofiltration from 1.40 - 1.77, and by sedimentation tanks from 1.39 - 2.16. In the U.S., water quality guidelines for stormwater reuse in unrestricted areas are set by state governments (USEPA 2004). Florida has one of the most stringent guidelines, requiring secondary treatment, filtration, and high-level disinfection such that total suspended solids are less than 5 mg/L and that 75% of Fecal coliform samples are below detection limit with maximum fecal coliform levels less than 25 counts per 100 mL. As of 2004, there were no water reuse guidelines established in Minnesota.
Environmental Concerns
One major environmental concern related to stormwater reuse is the risk of toxic spills. An important guideline developed by Australia (NRMMC et al. 2009) was to incorporate at least a 72-hour residence time of harvested stormwater in a stormwater pond prior to reuse to provide a time buffer to stop the use of harvested stormwater in the event that a toxic spill contaminates the water supply. Other environmental concerns include mosquito breeding and contaminated pond sediments (Alan Plummer Assoc. 2010; VA-DCR 2011).
Stormwater Harvesting and Reuse 17
Stormwater Reuse System Design
Models of stormwater harvesting and reuse systems are important to correctly size the storage pond to meet runoff supply and reuse demand, and to measure the potential volume and pollutant load reduction benefits of reuse. The only publicly available model for stormwater reuse is the Rate-Efficiency-Volume (REV) curve model developed by the University of Central Florida (Wanelista et al. 1991). There are two detailed rainwater harvesting supply and demand models: the Cistern Design Spreadsheet developed in Virginia (VA- DCR 2011) and the Rainwater Harvester, a Windows-based computer model, developed by North Carolina State University (NCSU 2011). Other rainwater harvesting and reuse models exist, but because they are very similar to the Virginia and North Carolina models, they are not included in this review. Finally, the Stochastic and Reliability Estimation Tool (SARET), an academic model developed by Matt Basinger and others (2010), is included in this review due to its potential use to determine the efficiency and reliability of stormwater reuse systems under variable rainfall regimes. One common characteristic of all of these models is that they are based on a mass balance of the stormwater harvesting pond.
Australia and Washington State have requirements for modeling stormwater or rainwater supply and demand, however, details on these models are not publicly available and the applicant must work closely with a local agency to determine if and what modeling is needed on a project by project basis. Australia has many watershed and urban catchment modeling programs available for purchase on the website: http://ewater.net. Washington State recommends using the Western Washington Hydrology Model which uses the EPA HSPF software program to calculate rainfall-runoff and routing computations as part of Low Impact Development design plans.
Stormwater Rate-Efficiency-Volume (REV) Curves (Florida)
Stormwater REV curves (Waneliesta et al. 1991) are a design tool for stormwater reuse ponds that relate runoff volume to reuse rates for a selected reuse efficiency (the amount of stormwater reused as a percentage of total runoff). These curves are normalized to the equivalent impervious area of the watershed. The REV curves were calculated from a 15-year mass balance of storage level on each rainfall event day of a stormwater reuse pond based on long-term rainfall records. The pond inputs were runoff (rainfall) and inflow (supplementary volume necessary to maintain a base level in the pond), and the pond outputs were reuse and discharge. Each rainfall day, the net change in storage was calculated and added to or subtracted from the previous pond storage. If the temporary storage volume exceeded the available storage volume, discharge occurred. If the temporary storage volume was less than zero, supplemental water was needed to replenish the pond and maintain the permanent pool. The stormwater pond mass balance used to calculate REV curves was critically dependent on a supplemental water source.
One strength of this model is that it is normalized to equivalent impervious area and therefore can be applied to watersheds of any size or runoff coefficient. One weakness of this model is that it uses one runoff coefficient as an annual average, but runoff coefficients are event specific. As a result, this model will not accurately estimate variability in stormwater efficiency on an event by event basis. Another weakness of this model is that it depends on using a supplementary source of water to maintain a baseline level in the storage pond.
This model was developed in Florida which has a year-round growing season and rainfall. Several modifications are needed to apply this model to a cold climate like Minnesota. For example, if-then clauses must be added to
18 Stormwater Harvesting: Accounting of Benefits and Feasibility
exclude artificial dry days during the winter season gap in Minnesota precipitation records and to exclude irrigation reuse during the non-growing season (October – April). Additionally, spring snowmelt events should be added to the precipitation record as an equivalent depth of water.
The following section describes how to calculate REV curves and examples of practical applications.
1. Calculating Rate-Efficiency-Volume (REV) curves a. Historic 10-year rainfall record (Columns A, B, and D) i. Before first day of record, assume one dry day with no net change in stormwater storage (Row 5) to begin the calculation ii. Columns A, B, and D are the number month, number date, and amount of rainfall in inches for every rainfall event day during the 10-year record – do not include dates without rainfall b. Dry days (Column C) i. The number of days without rainfall preceding the current rainfall event ii. C6 = if(C6-C5-1>120,0,C6-C5-1) c. Input: Runoff (Column E) i. Equal to the rainfall because all values are normalized to inches on equivalent impervious area ii. E6 = E5 d. Input: Inflow (Column F) i. The amount of supplemental water needed to maintain a baseline storage volume ii. Calculated as the difference between the current days net storage less the previous days net storage volume less the current days runoff plus the current days outputs (reuse + discharge) iii. F6 = I6 – (I5 + E6 – G6 – H6) e. Output: Discharge (Column G) i. If the previous day’s net storage is less than the reuse volume, there will be no discharge from the stormwater pond ii. If the previous day’s net storage is greater than the reuse volume and the current day’s runoff is greater than the difference between the previous day’s net storage and the reuse volume, then the discharge is equal to the previous day’s net storage less the reuse volume. iii. If the previous day’s net storage is greater than the reuse volume but the current day’s runoff is less than the difference between the previous day’s net storage and the reuse volume, then the discharge is equal to the runoff. iv. G6 = if(I5>$B$2, if(E6>I5-$B$2,I5-$B$2,E6),0) f. Output: Reuse (Column H) i. Equal to the reuse rate times the number of dry day preceding the current day plus the current day. ii. =if(A6<5,0,if(A6>9,0,$B$1*(C6+1))) g. Net Storage (Column I)
Stormwater Harvesting and Reuse 19
i. If the previous day’s net storage plus the current day’s runoff less the current day’s reuse and discharge is less than zero, then the change in net storage is zero. ii. If the previous day’s net storage plus the current day’s runoff less the current day’s reuse and discharge is greater than zero, then the change in net storage is equal to that difference. iii. =if(I5+E6-G6-H6<0,0,I5+E6-G6-H6) h. % Reused i. Equal to the sum of all the reuse volumes as a percentage as a sum of all the runoff volumes ii. =sum(H5:Hn)/sum(E5:En)*100, where n = number of rows in the historic rainfall record i. Assumptions i. Net ground water movement into or out of the pond is zero ii. Use rate is constant for each month in a year – presented on the REV curve as an average rate per day and over the equivalent impervious area (EIA) j. Curve Output Table i. Use Excel Solver function to find the reuse rates (in inches on EIA/day) necessary to achieve a certain reuse efficiency based on a certain reuse volume ii. The Florida REV curves were created for reuse efficiencies of 50%, 60%, 70%, 80%, 90% and 95%, and reuse volumes of 0.25, 0.5, 0.75, 1, and every subsequent 0.5 inch increment through 7 inches on EIA ( iii. Table 5). 2. Using REV curves for stormwater reuse design: a. Calculating the Equivalent Impervious Area (EIA) based on the effective runoff coefficient for the watershed (C) i. EIA = C A where
1. CN = runoff coefficient for surface N
2. AN = area of surface N 3. A = area of watershed (acres) b. Converting the available Runoff Volume (V) units to inches over the EIA
i. V = VP / EIA x 12 inches/ ft x 1 acre/ 43,560 ft2
1. VP = maximum storage volume available in pond c. Use REV curves to find the Use Rate (R) in inches on EIA/ day
i. R = I x AI / EIA 1. I = average rate of irrigation (inches/ week)
2. AI = area available for irrigation (acres) d. Use REV curves to calculate the Efficiency (E, percent of runoff reused) i. E = R / V
20 Stormwater Harvesting: Accounting of Benefits and Feasibility
e. Use REV curves to determine the percent nutrient mass removed from direct discharge with harvesting i. Average annual nutrient mass removal
= weighted harvesting efficiency + pond efficiency
= E * 100% - (1- E)* pond efficiency
Table 4. Example stormwater rate efficiency volume (REV) curve calculation spreadsheet
Based on the methods described in Waneliesta et al. 1991.
A B C D E F G H I
1 Reuse rate 0.25778 in. on EIA/d
2 Reuse vol 0.75 in. on EIA Storage: 1.13 Final Net: 1.13 % Reused:
3 Sum: 284.86 206.77 77.79 412.71
INPUT: INPUT: OUTPUT: OUTPUT: Net Storage, 4 Month Date Dry days Rain, in. Runoff, in. Inflow, in. Discharge, in. Reuse, in. in.
5 4 36981 0 0.00 0.00 0.00 0.00 0.00 0.00
6 4 36982 0 0.10 0.10 0.00 0.00 0.00 0.10
7 4 36983 0 0.01 0.01 0.00 0.00 0.00 0.11
Table 5. Example stormwater rate efficiency volume (REV) curve empty data table
Based on the methods described in Waneliesta et al. 1991.
A B C D E F G
1 REV 50% 60% 70% 80% 90% 95%
2 0.25
3 0.5
......
17 7
Stormwater Harvesting and Reuse 21
Figure 1. Example rate-efficiency-volume (REV) Curve for Meteorological Region 1 in Florida
Reprinted from Waneliesta et al. 1991
Rainwater Harvesting Cistern Design Spreadsheet (Virginia)
The Stormwater Design Specification for Rainwater Harvesting (VA-DCR 2011) in Virginia allows runoff reduction credits for rainwater harvesting based on the total amount of annual internal water reuse, outdoor water reuse, and tank dewatering discharge. These values can be calculated using the Cistern Design Spreadsheet. The spreadsheet includes one worksheet of user inputs and two worksheets of runoff reduction volume summary using precipitation data less than or equal to 1 inch/day and all precipitation data.
This spreadsheet is user friendly but lacks adjustment for variable rainfall and is specific to Virginia rainwater harvesting systems. Other spreadsheets are available that are similar to the Cistern Design Spreadsheet.
The following is a list of the input and output variables found in the Cistern Design Spreadsheet.
1. Inputs a. Roof footprint (square feet) b. Irrigation area (square feet) c. Irrigation rate (inches/ week) d. Flushing toilets/urinal indoor demand e. Laundry indoor demand f. Additional daily use (bus washing, street sweepers, etc) g. Chilled water cooling tower use h. Secondary runoff reduction drawdown i. First flush filter diversion and efficiency 2. Outputs for pre-set cistern volumes a. Overflow frequency for storms of 1” or less per year (%)
22 Stormwater Harvesting: Accounting of Benefits and Feasibility
b. Overflow (days per year) c. Dry frequency (%) d. Runoff reduction volume credit e. Demand met by rainwater (%)
Rainwater Harvester Computer Model (North Carolina)
North Carolina State University developed a computer model to assist design of residential or municipal rainwater harvesting cisterns and is available for download from the Rainwater Harvesting website at North Carolina State University (NCSU 2011). This model is not directly associated with stormwater runoff credits. This model is similar to the Virginia Cistern Design spreadsheet but is a Windows based application.
One strength of this model is that it can be applied to other regions by uploading a rainfall file specific to that region into the model. Rainfall files for selected North Carolina cities are available online but can be easily substituted with rainfall files for other cities as long as the rainfall data table is formatted the same and saved with the extension “.rai”. In addition, many of the irrigation and water use input information specific to North Carolina soils and growing season can be manually modified for other regions. Another strength of this model is that it includes cost-benefit estimates in the output.
The following is a list of model inputs and outputs.
1. Inputs a. System Design i. Rainfall Input file (.rai): 1. 30 yr daily: DOY,YYYY,M,D,Rainfall(in) – 1973,11,1-2003,10,31 2. 10 yr hourly: DOY,YYYY,M,D,Rainfall(in) – 1995,1,1,0-2005,12,30,22 3. Can make file for any other city as long as it is in the same format and date range! ii. Roof area (square feet) iii. Capture factor – default 0.9 iv. Water cost ($/gallon) v. Sewer cost ($/gallon) vi. Atmospheric nitrogen load (mg/L) vii. Water Quality volume depth (inches) viii. Cistern volume (gallon) ix. Cistern cost ($) x. Back-up water supply start trigger and stop backup (% of cistern volume) b. Basic and Custom Water Usage i. People flushing (people/day) + gal/flush ii. Consistent daily usage (gallon/day) – 7 input cells iii. Monthly usage (gallon/day) – 12 input cells iv. Day of week usage by month (gallon/day) – 84 input cells c. Irrigation Estimation
Stormwater Harvesting and Reuse 23
i. Typical daily PET depths (inch) – monthly averages ii. Irrigated area (square feet) iii. Irrigation system (drip irrigation, impact sprinkler, other) – used to generate irrigation efficiency values but can also enter custom value iv. Soil texture (course sand, sand, loamy sand, sandy loam, loam, silt loam, silty caly loam, clay loam, sandy clay loam, silty clay, clay) – used to generate plant available water (inch/ inch) v. Irrigated crop (turfgrass, flowers, corn) – generates effective root depth (inch) but can also enter custom value vi. Months of irrigation 2. Outputs a. Total volume captured (%) b. Usage replaced (%) c. Annual water usage (gallon) d. Annual water savings ($) e. Annual backup water usage (gallon) f. Annual backup water cost ($) g. Overflow frequency (%) h. Dry cistern frequency (%) i. Water quality volume captured (%) j. Annual nitrogen removed (pounds) k. Payback period (years)
Stochastic and Reliability Estimation Tool (SARET; Basinger et al. 2010)
The Stochastic and Reliability Estimate Tool (SARET) is an academic model developed by Matt Basinger and others (Basinger et al. 2010) to estimate the reliability of rainwater harvesting and reuse for a multifamily residential building case study in New York City. The model is based on a nonparametric rainfall generation procedure utilizing a bootstrapped Markov chain. Future precipitation is simulated based on probabilities of rainfall for each day over a 15 day window from historic rainfall records. These precipitation estimates are used to estimate the reliability of a rainwater harvesting system for a user-specified watershed area and tank volume. The SARET model could be used to test the reliability of a stormwater reuse pond with Minnesota historic precipitation data.
The following is a list of model inputs and outputs.
1. Historical Data a. Arrange for each day a sampling array is created that includes all precipitation values from all 25 years of historical data. Example sub array for June 1: ... May 31 June 1 June 2 ...... 1999 0.2 0.5 1 2000 1 0 0 2001 0 .5 .7
24 Stormwater Harvesting: Accounting of Benefits and Feasibility
b. Categorize each day i. W = wet day (rain) ii. D = dry day (no rain) iii. E.g., June 1, 1999 = W; June 1, 2000 = D c. Calculate probability of i. P(W) = wet day ii. P(WW) = wet day following a wet day iii. P(WD) = dry day following a wet day iv. P(DD) = dry day following a dry day v. P(DW) = wet day following a dry day 2. User Inputs a. Specify Input i. Demand: supplied by storage only and/or supplied by direct rain fed or storage, value or range and simulation increment, frequency (days), period (months) ii. Catchment: value or range and simulation increment, irrigated field size iii. Storage: value or range and simulation increment iv. First flush: depth, frequency b. Specify Output i. Storage: maximum storage needed to supply 100% reliability for specified demand(s), and catchment (single value or range) ii. Reliability: 1 (failures/total days) 3. Markov Model a. Storage calculation: 100 of the simulated 25 year data sets are generated. The largest
Storagemax is found:
i. Supplyn = Catchment x Precipitationn
ii. Deficit = Demand – Supplyn
iii. Storagemax = Max(0, Storagei-1 + Deficiti) b. Reliability calculation: 100 of the simulated 25 year data sets are generated. The lowest reliability is found: i. The storage begins full, a mass balance is performed on the storage each day considering the demand(s), catchment, etc. ii. If the storage hits zero, it is counted as a failure iii. Reliability = 1 (failures/total days)
Stormwater Harvesting and Reuse 25
FEASIBILITY ISSUES IN MINNEAPOLIS
Stormwater Availability and Use Rates
Important water availability and use rates for determining the feasibility of stormwater reuse in Minnesota are precipitation, potential evapotranspiration, and irrigation rates. Precipitation patterns determine the volume and timing of stormwater runoff, and the effectiveness of reusing stormwater depends on the ability of plants to uptake water (irrigation rate) which is strongly related to potential evapotranspiration. The effectiveness of using stormwater for supplemental irrigation across the U.S. based on these parameters was examined by Heaney et al. (1999). Compared to other U.S. cities, Minneapolis had a high percent utilization of stormwater, low annual water deficit, and required a small stormwater storage tank. Additional resources relevant to stormwater availability and use rates are a report on annual stream runoff in Minnesota (Vandegrift and Stefan 2010) and Minnesota turfgrass and landscape irrigation (IA-MNLA 2005). The following sections summarize available data for water availability and use rates in Minnesota.
Precipitation Rates
Rainfall in Minneapolis was measured as inches per day between April 15 and September 30 from 2001 through 2010. Summary figures of daily (Figure 2) and cumulative (Figure 3) rainfall during consecutive rainfall days for each year between 2001 and 2010 are shown below. Cumulative rainfall is shown to account for the full rainfall event eliminating the arbitrary split that occurs when looking at 24-hour periods. According to Dr. Mark Seeley at the University of Minnesota, sufficient data exist to support recently observed trends of climate change in Minnesota. Relevant to stormwater, over the last 30 years there has been greater annual precipitation with more snowfall, more frequent heavy rainstorm events, and more days with rain. The increasing precipitation and snowfall trends suggests that recent daily precipitation records must be used to size stormwater harvesting and reuse systems in Minnesota instead of long-term averages.
26 Stormwater Harvesting: Accounting of Benefits and Feasibility
45 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
40
35
30
25
20 Count (days) Count
15
10
5
0 0 - 0.25 0.25 - 0.5 0.5 - 1 1 - 2 2 - 3 3 - 4 4 - 5 > 5 Rainfall Category (inches per day)
Figure 2. Ten year (2001-2010) daily rainfall summary for Minneapolis, Minnesota.
Categorized by rainfall depth (inches) and year.
45 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
40
35
30
25
20
Count (days) Count 15
10
5
0 0 - 0.25 0.25 - 0.5 0.5 - 1 1 - 2 2 - 3 3 - 4 4 - 5 > 5
Rainfall Category (inches over consecutive rainfall days)
Figure 3. Ten year (2001-2010) cumulative rainfall event summary for Minneapolis, Minnesota.
Categorized by rainfall event depth (inches) and year.
Stormwater Harvesting and Reuse 27
Potential Evapotranspiration Rates
Potential Evapotranspiration (PET) is the amount of water that could be evaporated from land, water, and plant surfaces if soil water were in unlimited supply. For the application of stormwater reuse, the actual evapotranspiration of an irrigated area of land will be nearly equal to the potential evapotranspiration because the soil should remain near saturation in order to maximize the stormwater reuse efficiency. PET rates can be calculated based on daily available sunlight and air temperature. Most agricultural irrigation estimators are based on daily PET. In addition, PET is one of the input variables in the Rainwater Harvester Model mentioned above.
PET estimates are calculated for Minnesota and Wisconsin by the University of Wisconsin Agriculture Extension Service from satellite-derived measurements of solar radiation and air temperatures at regional airports. The PET values are a reasonable estimate of daily crop water use for most crops that have reached at least 80% coverage of the ground. Prior to 80% or greater coverage, ET will be a fraction of the map value in proportion to the amount of coverage. The PET estimates are derived from the UW Soil Science model using insolation estimates derived by UW Space Science & Engineering from GOES satellite imagery (Bland and Diak 2011; http://www.soils.wisc.edu/uwex_agwx/sun_water). Average annual PET for Minneapolis is 0.16 ± 0.06 with a maximum of 0.30 (Table 6).
Table 6. Annual mean, maximum, and standard deviation potential evapotranspiration (PET; inches/day) for Minneapolis, Minnesota from 2001 to 2010.
Based on data derived from the UW Soil Science model using insolation estimates derived by UW Space Science & Engineering from GOES satellite imagery (Bland and Diak 2011; http://www.soils.wisc.edu/uwex_agwx/sun_water).
PET Mean PET Max PET SD Year (inches/day) (inches/day) (inches/day)
2001 0.17 0.31 0.07
2002 0.18 0.33 0.06
2003 0.16 0.29 0.07
2004 0.14 0.28 0.06
2005 0.16 0.29 0.07
2006 0.16 0.30 0.07
2007 0.17 0.29 0.06
2008 0.17 0.28 0.06
2009 0.16 0.32 0.05
2010 0.16 0.29 0.07
All 0.16 0.30 0.06
28 Stormwater Harvesting: Accounting of Benefits and Feasibility
Irrigation Rates
The rule of thumb for turfgrass irrigation rate is 1-2 inch/week (or, 0.14 – 0.29 inch/day). Turfgrass irrigation rates can be quantified more specifically from the amount of water lost through evapotranspiration (ET) multiplied by a plant specific coefficient (Brian Horgan, U of M Extension, pers. comm.). In spring and summer, plants usually require only 40-60% of ET, whereas in mid-summer, turfgrass can require up to 100% of ET. Crop coefficients vary by species and can be found in the book, Turfgrass Water Conservation (Leinauer and Greene 2011). Transpiration (water use) by urban trees in Roseville was measured over a growing season by Peters et al. (2010). They found that water use by evergreens was greater than deciduous trees because evergreens transpire more water and have a longer growing season than deciduous trees (Figure 4).
Figure 4. Daily sums of transpiration per unit canopy area (EC) in evergreen needleleaf (solid circles) and deciduous broadleaf (open circles) plant functional types in a suburban neighborhood of Minneapolis-St. Paul, Minnesota.
Symbols represent monthly averages from April to November 2008. Error bars present ±1 standard error. Reprinted from Figure 6 in Peters et al. 2010).
Minnesota Stormwater Quality
Pollutant levels in stormwater runoff determine the level of treatment necessary prior to reuse applications. Stormwater water quality was measured by the Mississippi Watershed Management Organization (MWMO) in 2010. The mean growing season concentrations of total phosphorus (TP), total kjeldahl nitrogen (TKN), total suspended solids (TSS), chloride, zinc, and oil were calculated and summarized in Table 7. MWMO growing season mean stormwater water quality was lower than typical snowmelt water quality in St. Paul (Table 8; Oberts 2000). High pollutant loads, especially TP, TSS, and chloride, during spring snowmelt will be a concern for stormwater reuse in Minnesota.
MWMO growing season stormwater water quality were also compared to mean stormwater BMP effluent concentrations from a 2008 analysis of the Stormwater BMP water quality database (Table 9); GC and WWE 2008). Pollutants found in Minnesota stormwater are higher than pollutants from stormwater BMP effluents from across the U.S, which suggests that pollutant removal by stormwater BMPs will be important for stormwater reuse in Minnesota.
Stormwater Harvesting and Reuse 29
Additional resources relevant to stormwater quality include a predictive model of stormwater runoff volumes, loads, and polluntant concentrations in the Twin Cities (Brezonik and Stadelmann 2002), the nationwide stormwater monitoring data from National Stormwater Quality Database (Maestre and Pitt 2005), and the role of first flush in stormwater harvesting (Sansalone and Cristina 2004). Moreover, several reports have estimated pollutant removal efficiencies of stormwater BMPs (NC-DENR 2007, NRMRL 2002).
Table 7. Growing season (June – September) mean stormwater pollutant concentrations.
Based on stormwater outfall monitoring data published in the 2010 Mississippi Watershed Management Organization Annual Monitoring Report.
TP TKN TSS Chloride Zinc Oil Site (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L)
10SA 0.17 1.09 23.8 37 0.02 4.8
1NE 0.32 1.86 40.4 58 0.06 4.5
2NNBC 0.45 1.56 21.0 81 0.05
4PP 0.15 1.06 29.5 116 0.08 5.4
6UMN 0.15 0.91 29.1 93 0.07 5.4
7LSTU 0.57 1.48 69.2 51 0.12 11.2
All 0.30 1.33 35.5 73 0.07 6.3
Table 8. Flow-weighted mean snowmelt event pollutant concentrations (in mg/L) in the St. Paul area, Minnesota, by site type
(Reprinted from Table 2 in Oberts 2000).
30 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 9. Stormwater Best Management Practices (BMP) average effluent mean concentrations ± 95% confidence interval and number of samples (N) for detention basins, retention ponds, and wetland basins based on available data drawn from the International Stormwater Best Management Practices Database (Geosyntec Consultants and Wright Water Engineers, Inc. 2008).
TP TKN TSS Zinc BMP Type (mg/L) (mg/L) (mg/L) (ug/L)
Detention Basin 0.19 ± 0.07 1.89 ± 0.3 31.04 ± 14.97 25.84 ± 15.09
N 19 10 22 15
Retention Pond 0.12 ± 0.03 1.09 ± 0.22 13.37 ± 6.08 32.86 ± 15.16
N 40 30 43 9
Wetland Basin 0.14 ± 0.1 1.05 ± 0.23 17.77 ± 8.51
N 12 7 14
Water and Pollutant Tolerance of Cold Climate Plants
One of the goals of stormwater reuse is to reduce the volume of runoff to downstream water bodies by reusing stormwater for irrigation. This water is subsequently lost to the atmosphere by plant transpiration. Stormwater reuse application rates will be designed to maximize the saturation of soil without limiting plant growth. Runoff from developed areas also has higher pollutant loads (e.g., nutrient, sediment, heavy metal, chloride, and hydrocarbon) than natural runoff. These pollutants may decrease the productivity of or even kill certain plant species. Therefore, to successfully maximize the rate of stormwater reuse, stormwater irrigated plant species should have high waterlogging and pollutant tolerance. The following is a review of known waterlogging and pollutant tolerance of common Minnesota plant species.
Waterlogging tolerance
Under high rates of irrigation, soils may become saturated or waterlogged for extended periods of time. These conditions result in low soil oxygen levels which negatively affect plant growth. Nearly all research on water uptake by plants is focused on how little soil water is required for survival. However, there are a just a handful of studies that investigate how much soil water plants can tolerate and still survive.
Niinemets and Valarde (2006) quantitatively compared the waterlogging tolerance of 806 North American tree and shrub species and assigned them a unitless rating from 1 to 5, with 5 being the most tolerant to waterlogging and 1 being the least tolerant to waterlogging. Based on this study, common Minneapolis urban trees that are waterlogging tolerant are summarized in Table 10. A few studies have investigated the waterlogging tolerance of turfgrass cultivars and the results from these studies are summarized in Table 11. Ryegrass, Kentucky bluegrass, and creeping bentgrass cultivars appear to have high waterlogging tolerance.
Pollutant tolerance
The Minnesota Stormwater Manual (MSSC 2008) lists plant species with known tolerance to salt, summarized in Table 12. These species may also have some tolerance to sediments and petroleum which are commonly
Stormwater Harvesting and Reuse 31
associated with salt in road runoff. A study by Qu and others (2003) found that some turfgrass cultivars were capable of efficiently absorbing heavy metals and could tolerate high lead concentrations. In addition, there have been a few other studies on using turfgrass as bio-remediators of polluted soils (e.g., Sloan et al. 2006). This suggests that turfgrass may have higher salt and pollutant tolerance than other plant species. Additional studies may be found using Turfgrass Information File, a cooperative project between the United States Golfer’s Association and the Michigan State University Libraries (http://turf.lib.msu.edu/index.html).
One concern with using stormwater for irrigation is that the higher pollutant loads will increase susceptibility to exotic and invasive species, such as common buckthorn, box elder, and reed-canary grass.
Table 10. Mean (± standard deviation) waterlogging tolerance of urban tree species modified from Appendix A in Niinemets and Valarde 2006. Waterlogging tolerance values were determined according to the waterlogging tolerance scale of Whitlow and Harris (1979): 5, very tolerant (survives deep, prolonged waterlogging for more than one year); 4, tolerant (survives deep waterlogging for one growing season); 3, moderately tolerant (survives waterlogging or saturated soils for 30 consecutive days during the growing season); 2, intolerant (tolerates one to two weeks of waterlogging during the growing season); 1, very intolerant (does not tolerate water-saturated soils for more than a few days during the growing season).
Waterlogging tolerance rating Species name Common name (unitless)
Quercus bicolor Swamp white oak 2.58 ± 0.28
Acer negundo Box elder 2.75 ± 0.25
Betula nigra River birch 2.85 ± 0.35
Fraxinus pennsylvania Green ash 2.98 ± 0.25
Larix laricina Tamarack 3.00
Populus deltoides Eastern cottonwood 3.03 ± 0.27
Acer rubrum Red maple 3.08 ± 0.28
Acer saccharinum Silver maple 3.37 ± 0.22
Fraxinus nigra Black ash 3.50
Salix nigra Black willow 4.68 ± 0.17
32 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 11. Summary of the waterlogging tolerance of common Minnesota turfgrass cultivars. References for each cultivar is listed in the table.
Turfgrass cultivar Common name Waterlogging tolerance Reference
Creeping bentgrass High water tolerance Rough and annual bluegrass
Kentucky bluegrass
Canada bluegrass
By Cultivar Redtop grass Moderate water tolerance Lawnirrigation.com
Colonial bent grass
Ryegrass
Buffalo grass Low water tolerance Fescues
All Star Relatively tolerant to Razmjoo et al. Perennial ryegrass Premier flooding 1993
Moonlight Most Tolerant
Serene
Limousine
Champagne Somewhat Tolerant Midnight II Wang and Jiang Kentucky bluegrass Awesome 2007
Julia
Kenube Least Tolerant Eagleton
G-6 Better quality under L-93 waterlogging conditions Jiang and Wang Creeping bentgrass A-4 2006 Lower quality under Penncross waterlogging conditions Pennlinks
Stormwater Harvesting and Reuse 33
Table 12. Common Minnesota plants with known salt tolerance. Reprinted from the Minnesota Stormwater Manual (MSSC 2008).
Salt Tolerance Species name Common name Soil Moisture Type in Soil
Always Wet/ Frequently Ulmus Americana American elm Medium/Low Tree Saturated
Fraxinus pennsylvanica Green ash Always Wet Medium Tree
Frequently Saturated/ Fraxinus americana White ash High Tree Mostly Drained
Frequently Saturated/ Populus spp. Poplars Medium Tree Mostly Drained
Frequently Saturated/ Celtis occidentalis Hackberry Medium Tree Mostly Drained
Elymus canadensis Canada wild rye Frequently saturated Medium Grass
Alopecurus pratensis Meadow foxtail Frequently saturated Low Grass
Karl Foerster reed Frequently Saturated/ Calamogrostos acutifolia High Grass grass Mostly Drained
Pinus banksiana Jack pine Mostly Drained High Tree
Rhus glabra Smooth sumac Mostly Drained Medium Shrub
Rhus typhina Staghorn sumac Mostly Drained High Shrub
Rhus trilobata Cutleaf sumac Mostly Drained High Shrub
Rosa rugosa Rugosa rose Mostly Drained Low Shrub
Lolium perenne Perennial ryegrass Mostly Drained Medium Grass
Bouteloua hirsute Blue grama grass Mostly Drained High Grass
Schizachyrium scoparium Little bluestem Mostly Drained High Grass
Puccinella distans Alkali grass Mostly Drained High Grass
Agrophyron elongatum Tall wheatgrass Mostly Drained High Grass
Western wheat Elytrigia smithii Mostly Drained High Grass grass
34 Stormwater Harvesting: Accounting of Benefits and Feasibility
Salt Tolerance Species name Common name Soil Moisture Type in Soil
MNDOT urban Seed Mix Mostly Drained High Grass prairie
MNDOT western Seed Mix Mostly Drained Medium Grass tall grass prairie
Stormwater Management
There are many published guides on stormwater management in cold climates, including the Minnesota Urban Small Sites BMP Manual (Barr Engineering 2001), Stormwater Practices for Cold Climates (Center for Watershed Protection), Plants for Stormwater Design: Species Selection for the Upper Midwest (Shaw and Schmidt 2003), The Minnesota Stormwater Manual (MSSC 2008), and Pollution Prevention and the MS4 Program: A Guide on Utilizing Pollution Prevention Activities to Meet MS4 General Permit Requirements (Emmons and Olivier Resources). In addition, there are two required regulations for stormwater management in the Minneapolis/ Hennepin County area: the Municipal Separate Storm Sewer System Stormwater (MS4) Permitting Program and the Comprehensive Surface Water Management Plan. The following section summarizes the purpose and requirements of these programs.
Municipal Stormwater Permitting Program (MS4)
The Municipal Separate Storm Sewer System Stormwater (MS4) Permitting Program is designed to reduce the amount of sediment and pollution that enters surface and ground water from storm sewer systems to the maximum extent practicable. Stormwater discharges associated with MS4s are regulated and the owners or operators of these systems are required to develop a SWPPP that incorporates best management practices applicable to their MS4. The MS4 general permit is scheduled for adoption in early 2006.
Enabling Legislation: Section 402, Clean Water Act; Minnesota Statutes, Chapter 115; Minnesota Rules, Chapter 7001; Minnesota Rules, Chapter 7050; Minnesota Rules, Chapter 7090
Required Permit(s): NPDES/SDS General Stormwater Permit for Construction; NPDES/SDS General Stormwater Permit for Industrial; NPDES/SDS General Stormwater Permit for Municipal Separate Storm Sewer Systems; NPDES/SDS Individual Stormwater Permit
Regulatory Authority: Minnesota Pollution Control Agency; U.S. Environmental Protection Agency
Applicability: Stormwater
Stormwater Relationship: Stormwater discharge
Comprehensive Surface Water Management
The Board of Water and Soil Resources oversees the adoption and implementation of comprehensive surface water management plans, which are created by watershed districts, watershed management organizations, or county/city/township joint powers organizations within the seven county metropolitan area. After local,
Stormwater Harvesting and Reuse 35
regional, and agency review, plans are approved by the Board of Water and Soil Resources. The WMO/WD/JPO then formally adopts the plan and requires each city or township within the WMO/WD/JPO to create and implement their local water management plan consistent with the WMO/WD plan. Updates are required every 5-10 years.
Enabling Legislation: Minnesota Statutes 103B; Minnesota Rules Chapter 8410
Required Permit(s): NA
Regulatory Authority: Board of Water and Soil Resources; Local Government
Applicability: Watershed Districts, Water Management Organizations, or Joint Powers Organizations in seven-county metro area
Stormwater Relationship: Erosion and sedimentation reduction, storm water design standards, wetland protection
36 Stormwater Harvesting: Accounting of Benefits and Feasibility
Impacts on Soil and Groundwater Quality
With the emphasis on volume control BMPs in recent years, the issue of soil and groundwater contamination is gaining much more attention as reflected in the increasing number of research projects. Stormwater runoff from urban areas has much higher concentrations of pollutants than from natural sources for many constituents. Many stormwater BMPs are designed to filter out pollutants from stormwater runoff, but accumulating pollutants in soils and groundwater adjacent to stormwater BMPs is a growing concern among water resource managers, especially when large amounts of stormwater runoff are infiltrated in areas with sandy soils or shallow water tables. The following is a brief summary of recent research studies that have investigated the impacts of stormwater infiltration on groundwater quality.
Paved areas are sources of metals, hydrocarbons, and chloride, and some studies of stormwater BMPs have found some decrease in groundwater quality due to stormwater infiltration. A recent study on the effects of urban runoff on groundwater quality found a clear link between increased groundwater recharge rates and decreased groundwater quality downstream of an urban area (Carlson et al. 2011). Infiltration basins draining light industrial and residential areas have underlying groundwater that are well within drinking water guidelines for toxic metals, nutrients, and pesticides (Appleyard 1993), but infiltration basins and vegetative filter strips draining major roads often have soils contaminated with heavy metals in depths of 30 cm up to 1.5 m (Barraud et al. 1999, Legret et al. 1999, Winiarski et al. 2006, Mikkelsen et al. 1997). In addition, other studies have shown higher chloride levels (Wilde 1994, USEPA 2009) and greater hydrocarbon and pesticide detection frequency (Fischer et al. 2003) in groundwater, and accumulation of lead (Nightingale 1987), metals, hydrocarbons, and nutrients in soils underlying stormwater BMPs (Barraud et al. 2005; Dechesne et al. 2005). However, the leaching of heavy metals into groundwater may be limited (Mikkelsen et al. 1997) due to the immobility of lead in soil (Norrström 2005). Soil cores below bioretention devices during the first 5 years of operation in the Greater Toronto Area showed comparable metal and hydrocarbon levels to un-impacted sites (TRCA 2008). BMPs receiving runoff from metal roofs have been found to be at high risk for zinc contamination of soils (Zimmermann et al. 2005).
However, recent research has improved the outlook on the risks of soil and groundwater contamination due to stormwater infiltration. While pavements are a source of hydrocarbons to runoff, naturally occurring microbial communities growing on pervious pavements are capable of degrading hydrocarbons. Moreover, research has shown that microbial degradation of hydrocarbons is assisted by the geotextile layer below the base course layer of pervious pavements (Newman et al. 2006a, b). Another study found that the levels of road salt derived chlorides in groundwater dropped quickly during the spring and eventually leveled out in the summer due to dilution with runoff low in chlorides (Kwiatkowski et al. 2007). Finally, long-term (20 years or more) studies of groundwater below infiltration basins have shown no adverse effects from infiltrating stormwater (Salo et al. 1986; Mikkelsen et al. 1994) and other monitoring data indicate that small distributed stormwater infiltration practices do not contaminate underlying soils, even after 10 years of operation (TRCA 2008). Due to the lack of observations of significant contamination of underlying groundwater after 20 years of service (TRCA 2009), it has been noted that “the risk of groundwater contamination from infiltration practices can be properly managed through appropriate screen of suitability, siting, and design.”
To reduce potential for groundwater contamination due to stormwater infiltration, Pitt et al. (1999) recommended diverting the first runoff following periods of pollutant build-up, such as periods of dry weather
Stormwater Harvesting and Reuse 37
and spring snowmelt. In addition, pretreatment of stormwater runoff from critical pollutant source areas was recommended.
The Low Impact Development Stormwater Management Planning and Design Guide (STEP 2010) has developed specific guidelines for the treatment and use of stormwater for infiltration systems based on the quality of stormwater runoff generated from various urban sources. A summary of the risks for groundwater contamination from infiltration BMPs discussed in this guide is provided below:
Stormwater infiltration practices should:
- Not receive runoff from high traffic areas where large amounts of de-icing salts are applied (e.g., busy highways), nor from pollution hot spots (e.g., source areas where land uses or activities have the potential to generate highly contaminated runoff such as vehicle fuelling, servicing or demolition areas, outdoor storage or handling areas for hazardous materials and some heavy industry sites). - Prioritize infiltration of runoff from source areas that are comparatively less contaminated such as roofs, low traffic roads and parking areas. - Apply sedimentation pretreatment practices (e.g., oil and grit separators) before infiltration of road or parking area runoff.
Also included in this guide is a summary table of guidelines for appropriate stormwater BMPs according to stormwater source area and runoff characteristics (Table 13).
Table 13. Guidelines for appropriate stormwater BMPs based according to stormwater source area. Reprinted from Table 2.8.1 in Low Impact Development Stormwater Management Planning and Design Guide (STEP 2010).
38 Stormwater Harvesting: Accounting of Benefits and Feasibility
REFERENCES
1. Existing Stormwater Harvesting and Reuse Programs
Australia
Department of Environment and Conservation New South Wales. 2006a. Managing Urban Stormwater: Harvesting and Reuse. pp. 1-72.
Department of Environment and Conservation New South Wales. 2006b. Managing Urban Stormwater: Harvesting and Reuse. pp. 73-156.
Department of Planning and Local Government. 2010. Chapter 8: Urban Water Harvesting and Reuse. Water Sensitive Urban Design Technical Manual for the Greater Adelaide Region. Government of South Australia, Adelaide.
Fletcher, T. D., Deletic, A., Mitchell, V. G., and Hatt, B. E. 2008. Reuse of Urban Runoff in Australia: A Review of Recent Advances and Remaining Challenges. Journal of Environmental Quality 37: S-116-S-127.
***Summary of global stormwater harvesting and reuse programs
Hatt, B., A. Deletic, and T. Fletcher. 2004. Integrated Stormwater Treatment and Re-use Systems-Inventory of Australian Practice. CRC Catchment Hydrology Industry Report.
Hatt, B. E., A. Deletic, and T. D. Fletcher. 2006. Integrated Treatment and Recycling of Stormwater: A Review of Australian Practice. Journal of Environmental Management 79, no. 1: 102–113.
*** In-depth review of stormwater harvesting and reuse programs in Australia
Natural Resource Management Ministerial Council, Environment Protection and Heritage Council, and National Health and Medical Research Council. 2006. Austrailian Guidelines for Water Recycling: Managing Health and Environmental Risks (Phase 1).
Natural Resource Management Ministerial Council, Environment Protection and Heritage Council, and National Health and Medical Research Council. 2009. Australian Guidelines for Water Recycling: Managing Health and Environmental Risks (Phase 2) - Stormwater Harvesting and Reuse.
Florida
Department of Environmental Protection and Water Management Districts. March 2010. Draft Stormwater Quality Applicant’s Handbook.
University of Florida Program for Resource Efficient Communities. 2008. Florida Field Guide to Low Impact Development: Stormwater Reuse. University of Florida.
Texas
Alan Plummer Associates, Inc. 2010. Stormwater Harvesting Guidance Document for Texas Water Development Board.
Alan Plummer Associates, Inc. 2011. History of Water Reuse in Texas
Texas Water Development Board. 2005. The Texas Manual on Rainwater Harvesting, 3rd Edition. Austin, Texas.
Stormwater Harvesting and Reuse 39
U.S.A. (Stormwater Harvesting)
O’Connor, G.A., H.A. Elliott, and R.K. Bastian. 2008. Degraded Water Reuse: An Overview. Journal of Environmental Quality 37: S157-S168.
***Summary of stormwater harvesting and reuse programs in the USA
United States Environmental Protection Agency. 2004. Guidelines for Water Reuse. EPA 625/R-04/108. USEPA, Cincinnati, OH.
Watereuse.org. 29 June 2011. Total Water Reuse Research Project List.
U.S.A. (Rainwater Harvesting)
North Carolina
North Carolina Division of Environment and Natural Resources. 2007. Chapter 19: Rooftop Runoff Management. In Stormwater Best Management Practices Manual.
North Carolina Division of Water Quality. 2008. Technical Guidance: Stormwater Treatment Credit for Rainwater Harvesting Systems. Raleigh, NC.
North Carolina State University. 2011. Rainwater Harvesting Website. http://www.bae.ncsu.edu/topic/waterharvesting/
North Carolina State University. 2006. Urban Waterways Series. http://www.bae.ncsu.edu/stormwater/pubs.htm
Virginia
The Cabell Brand Center. 2009. Virginia Rainwater Harvesting Manual, 2nd Edition. Salem, VA.
Forasté, J.A. and D. Hirschman. A Methodology for Using Rainwater Harvesting as a Stormwater Management BMP.
Virginia Division of Water Quality. 2008. Stormwater Management Permit Application Form: 401 Certification Application Form – Rainwater Harvesting Supplement.
Virginia Department of Conservation and Recreation. 2011. Stormwater Design Specification No. 6: Rainwater Harvesting, v1.9.5.
Washington
Washington State Department of Ecology and Water Quality Program. 2005. Volume III: Hydrologic Analysis and Flow Control Design/BMPs In: Stormwater Management in Western Washington, Publication No. 05-10-31.
Stormwater Pollution Prevention Plan Template. 2008. Northwest Regional Office, Bellevue, WA.
40 Stormwater Harvesting: Accounting of Benefits and Feasibility
2. Stormwater Reuse Water Quality, Human Health, and Environmental Concerns Davies, C.M., V.G. Mitchell, S.M. Petterson, G.D. Taylor, J. Lewis, C. Kaucner, and N.J. Ashbolt. 2008. Microbial Challenge-testing of Treatment Processes for Quantifying Stormwater Recycling Risks and Management. Water Science & Technology 57.6: 843-847.
He, J., C. Valeo, A. Chu, and N. F. Neumann. 2008. Water Quality Assessment in the Application of Stormwater Reuse for Irrigating Public Lands. Water Quality Research Journal of Canada 43: 145-159.
Rousseau, D.P.L., E. Lesage, A. Story, P.A. Vanrolleghem, and N. De Pauw. 2008. Constructed Wetlands for Water Reclamation. Desalination 218: 181-189.
3. Stormwater Reuse System Design Basinger, M., F. Montalto, and U. Lall. 2010. A Rainwater Harvesting System Reliability Model Based on Nonparametric Stochastic Rainfall Generator. Journal of Hydrology 392: 105-118.
Brodie, I.M. 2008. Hydrological Analysis of Single and Dual Storage Systems for Stormwater Harvesting. Water Science & Technology 58.5: 1039-1046.
Biological and Agricultural Engineering Department. 2011. Rainwater Harvester 2.0. North Carolina State University. http://www.bae.ncsu.edu/topic/waterharvesting/model.html
Virginia Department of Conservation and Recreation. 2010. Rainwater Harvesting Spreadsheet, v1.6.
Wanielista, M. P., Y. A. Yousef, G. M. Harper, and L. Dansereau. 1991. Design Curves for the Reuse of Stormwater. Department of Civil and Environmental Engineering, University of Central Florida.
Wanielista, M. 2006. Stormwater Reuse: A Summary. From: University of Central Florida Stormwater Management Academy Research Publications Website: http://www.stormwater.ucf.edu/research/wanielista_publications.htm
4. Feasibility Issues in Minneapolis – Stormwater Availability and Use Rates Bland, B., R. Wayne, and G. Diak. 2011. Sun/Water: Evapotranspiration Model. University of Wisconsin Extension, Agriculture and Weather. http://www.soils.wisc.edu/uwex_agwx/sun_water
Heaney, J. P., L. Wright, and D. Sample. 1999. Chapter 8: Stormwater Storage-Treatment-Reuse Systems. In: Innovative Urban Wet-Weather Flow Management Systems (EPA/600/R-99/029). Cincinnati, OH.
The Irrigation Association and Minnesota Nursery and Landscape Association. 2005. Minnesota Turf and Landscape Irrigation BMP Overview.
Minnesota Climatology Working Group. 2011. Preliminary Local Climatological Data-Minneapolis/St. Paul, MN. http://climate.umn.edu/doc/prelim_lcd_msp.htm.
Mississippi Watershed Management Organization. 2011. Annual Monitoring Report 2010. MWMO Watershed Bulletin 2011-1. 63pp.
Stormwater Harvesting and Reuse 41
Peters, E.B., J.P. McFadden, and R.A. Montgomery. 2010. Biological and Environmental Controls on Tree Transpiration in a Suburban Landscape. Journal of Geophysical Research 115: G04006.
Peters, E.B., R.V. Hiller, and J. P. McFadden. 2011. Seasonal Contributions of Vegetation Types to Suburban Evapotranspiration. Journal of Geophysical Research 116: G01003.
Vandegrift, T. and H. Stefan. 2010. Project Report No. 543: Annual Stream Runoff and Climate in Minnesota’s River Basins. Anthony Falls Laboratory, University of Minnesota.
5. Feasibility Issues in Minneapolis – Stormwater Quality Brezonik, P. L. and T. H. Stadelmann. 2002. Analysis and Predictive Models of Stormwater Runoff Volumes, Loads, and Pollutant Concentrations from Watersheds in the Twin Cities Metropolitan Area, Minnesota, USA. Water Research 36: 1743-1757.
Geosyntec Consultants and Wright Water Engineers, Inc. 2008. Analysis of Treatment System Performance: International Stormwater Best Management Practices (BMP) Database [1999-2008].
Maestre, A. and R. Pitt, Center for Watershed Protection, and U.S. EPA Office of Water. 2005. The National Stormwater Quality Database, Version 1.1: A Compilation and Analysis of NPDES Stormwater Monitoring Information.
North Carolina Division of Environment and Natural Resources. 2007. Chapter 9: Stormwater Wetlands. In Stormwater Best Management Practices Manual.
North Carolina Division of Environment and Natural Resources 2007. Chapter 10. Wet Detention Basins. In Stormwater Best Management Practices Manual.
National Risk Management Research Laboratory. 2002. Considerations in the Design of Treatment Best Management Practices (BMPs) to Improve Water Quality. U.S. Environmental Protection Agency, Cincinnati, OH.
Oberts, Gary L. 2000. Influence of Snowmelt Dynamics on Stormwater Runoff Quality. Watershed Protection Techniques 1(2): 55-61.
Sansalone, J.J. and C.M. Cristina. 2004. First Flush Concepts for Suspended and Dissolved Solids in Small Impervious Watersheds. Journal of Environmental Engineering November: 1301-1314.
6. Feasibility Issues in Minneapolis – Water and Pollutant Tolerance of Cold Climate Plants
Jiang, Y. and K. Wang. 2006. Growth, Physiological and Anatomical Responses of Creeping Bentgrass Cultivars to Different Depths of Waterlogging. Crop Science 46: 2420.
Jiang, Y. 2008. Responses of Turfgrass to Low-Oxygen. In Handbook of Turfgrass Management and Physiology, Pessarakli, M. ed. Taylor & Francis Group, LLC, Florida.
Niinemets, U. and F. Valladares. 2006. Tolerance to Shade, Drought, and Waterlogging of Temperate Northern Hemisphere Trees and Shrubs. Ecological Monographs 76(4): 521-547.
42 Stormwater Harvesting: Accounting of Benefits and Feasibility
Qu, R. L.; Li, D.; Du, R.; Qu, R. 2003. Lead Uptake by Roots of Four Turfgrass Species in Hydroponic Cultures. HortScience. 38(4): 623-626
Razmjoo, K., S. Kaneko, T. Imada. 1993. Chapter 90: Varietal Differences of Some Cool-season Turfgrass Species in Relation to Heat and Flood Stress. International Turfgrass Society Research Journal, 7: 636-642.
Sloan, J. J.; Genovesi, A. D.; Engelke, M. C.; White, Richard. 2006. Phytoremediation of Heavy Metal Contaminated Soils with Warm Season Grasses. Abstracts: 2006 International Annual Meetings [ASA/CSSA/SSSA]. p. [15]
United States Golf Association and the Michigan State University Libraries’ Turfgrass Information Center. 2011. Turfgrass Information File. http://turf.lib.msu.edu/index.html
Wang, K. and Y. Jiang. 2007. Waterlogging Tolerance of Kentucky Bluegrass Cultivars. HortScience 42(2): 386- 390.
7. Feasibility Issues in Minneapolis – Stormwater Management Barr Engineering. 2001. Minnesota Urban Small Sites BMP Manual: Stormwater Best Management Practices for Cold Climates. Prepared for the Metropolitan Council with support from City of Minneapolis, City of St. Paul, Minnehaha Creek Watershed District – Excelsior, Rice Creek Watershed District – Arden Hills, Six Cities Watershed Management Organization – Fridley. St. Paul, MN.
Center for Watershed Protection. Stormwater Practices for Cold Climates. http://www.stormwatercenter.net/Cold%20Climates/cold-climates.htm
Emmons & Olivier Resources, Inc. undated. Pollution Prevention and the MS4 Program: A Guide on Utilizing Pollution Prevention Activities to Meet MS4 General Permit Requirements.
Gulliver, J.S., A.J. Erickson, and P.T. Weiss (editors). 2010. Stormwater Treatment: Assessment and Maintenance. University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. Online Manual. http://stormwaterbook.safl.umn.edu/
Minnesota Pollution Control Agency. 2006. General Permit No: MNR040000 – Authorization to Discharge Storm Water Associated with MS4 under the NPDES.
Minnesota Rules. 2007. Chapter 7090: Minnesota Pollution Control Agency Storm Water Regulatory Program.
Minnesota Stormwater Steering Committee. 2008. The Minnesota Stormwater Manual, Version 2. Minnesota Pollution Control Agency, St. Paul, MN.
Shaw, D. and R. Schmidt. 2003. Plants for Stormwater Design: Species Selection for the Upper Midwest. Minnesota Pollution Control Agency, St. Paul, MN.
8. Feasibility Issues in Minneapolis – Impacts on Soil and Groundwater Quality Appleyard, S.J. 1993. Impact of Stormwater Infiltration Basins on Groundwater Quality, Perth Metropolitan Region Western Australia. Environmental Geology 21(4): 227-236.
Stormwater Harvesting and Reuse 43
Barraud, S., A. Gautier, J. P. Bardin, and V. Riou. 1999. The Impact of Intentional Stormwater Infiltration on Soil and Groundwater. Wat. Sci. Tech. 39(2): 185-192.
Barraud, S., Dechesne, M., Bardin, J-P., and Varnier, J-C. 2005. Statistical analysis of pollution in stormwater infiltration basins. Water Science and Technology. Vol. 51. No. 2. pp. 1-9.
Carlson M., K. Lohse, J. McIntosh, and J. McLain. 2011. Impacts of Urbanization on Groundwater Quality and Recharge in a Semi-arid Alluvial Basin. Journal of Hydrology 409: 196-211.
Dechesne, M, Barraud, S. and Bardin, J-P. 2005. Experimental Assessment of Stormwater Infiltration Basin Evolution. Journal of Environmental Engineering. July 2005. pp. 1090-1098.
Fischer, D., E. G. Charles, and A. L. Baehr. 2003. Effects of Stormwater Infiltration on Quality of Groundwater Beneath Retention and Detention Basins. Journal of Environmental Engineering 129(5):464.
Kwiatkowski, M., Welker, A.L., Traver, R.G., Vanacore, M., Ladd. T. 2007. Evaluation of an infiltration best management practice utilizing pervious concrete. Journal of the American Water Resources Association. Vol. 43. No. 5. pp. 1208-1222.
Legret, M., M. Nicollet, P. Miloda, V. Colandini, and G. Raimbault. 1999. Simulation of Heavy Metal Pollution from Stormwater Infiltration through a Porous Pavement with Reservoir Structure. Wat. Sci. Tech. 39(2):119- 125.
Mikkelsen, P.S., Weyer, G., Berry, C., Walden, Y., Colandini, V., Poulsen, S., Grotehusmann, D. and Rohlfing, R. 1994. Pollution from urban stormwater infiltration. Water Science and Technology. Vol. 29. No. 1-2. pp. 293- 302.
Mikkelsen, P., M. Häfliger, M. Ochs, P. Jacobsen, J. Tjell, and M. Boller. 1997. Pollution of Soil and Groundwater from Infiltration of Highly Contaminated Stormwater - A Case Study. Wat. Sci. Tech. 36(8-9): 325-330.
Newman, A.P., Coupe and Robinson, K. 2006a. Pollution Retention and Biodegradation within Permeable Pavements. In: Proceedings of the 8th International Conference on Concrete Block Paving. November 6-8, 2006. San Francisco. California.
Newman, A.P., Coupe, S.J., Smith, H.G., Puehmeier, T. Bond, P. 2006b. The Microbiology of Permeable Pavements. In: Proceedings of the 8th International Conference on Concrete Block Paving. November 6-8, 2006. San Francisco. California.
Nightingale, H.I. 1987. Accumulation of As, Ni, Cu, and Pb in Retention and Recharge Basins Soils from Urban Runoff. Water Resources Bulletin. Vol. 23, No. 4, pp. 663-672.
Norrström, A.C. 2005. Metal mobility by de-icing salt from an infiltration trench for highway runoff. Applied Geochemistry. Vol. 20, pp. 1907-1919.
Pitt, R., S. Clark and K. Parmer. 1999. Potential Groundwater Contamination from Intentional and Nonintentional Stormwater Infiltration. U.S. Environmental Protection Agency, Risk Reduction Engineering Laboratory, Office of Research and Development, Springfield, V.A.
Salo, J.E., Harrison, D., Archibald, E.M. 1986. Removing Contaminants by Groundwater Recharge Basins. Journal of the American Water Works Association. Vol. 78. No. 9. pp. 76-81.
44 Stormwater Harvesting: Accounting of Benefits and Feasibility
Sustainable Technologies Evaluation Program (STEP). 2010. LID Stormwater Management Planning and Design Guide.
Toronto and Region Conservation (TRCA). 2008. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.
Toronto and Region Conservation Authority (TRCA). 2009. Review of the Science and Practice of Stormwater Infiltration in Cold Climates. Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.
United States Environmental Protection Agency (USEPA). 2009. Drinking Water Contaminants. List of Secondary Drinking Water Regulations. http://www.epa.gov/safewater/contaminants/index.html#listsec. Accessed July 27, 2009.
Wilde, F.D. 1994. Geochemistry and Factors Affecting Ground Water Quality at Three Storm-Water Management Sites in Maryland: Report of Investigations No. 59. Department of Natural Resources. Maryland Geological Survey. Baltimore, Maryland.
Winiarski, T., J.-P. Bedell, C. Delolme, and Y. Perrodin. 2006. The Impact of Stormwater on a Soil Profile in an Infiltration Basin. Hydrogeology Journal 14: 1244-1251.
Zimmermann, J., Dierkes, C., Gobel, P., Klinger, C., Stubbe, H. and Coldeway, W.G. 2005. Metal concentrations in soil and seepage water due to infiltration of roof runoff by long term numerical modelling. Water Science and Technology. Vo. 51. No. 2. pp. 11-19.
Stormwater Harvesting and Reuse 45
Technical Guidance on the Estimation of Runoff Volume Reduction & Water Quality Benefits from Stormwater Reuse
INTRODUCTION
The purpose of this report was to provide technical guidance on estimating runoff volume reduction and water quality benefits from stormwater reuse. Stormwater harvesting is consistently identified as a way to conserve potable water resources and reduce the volume of runoff exiting the system. Other potential benefits include irrigation cost reductions (or even revenue), enhanced water quality treatment, and improvements in vegetation. However, there is generally a lack of technical guidance on how to quantify the benefits of stormwater harvesting with respect to runoff volume reduction and water quality treatment.
A spreadsheet-based model was developed to estimate the runoff volume reduction and water quality benefits of stormwater reuse using a daily time step mass balance of stormwater runoff volume and phosphorus load assuming non-conservative phosphorous mixing (Walker 1987 phosphorus sedimentation equations). The model simultaneously calculates annual volume reduction and phosphorus removal as a percent of the annual watershed load, in addition to annual evaporation losses and phosphorus sedimentation over a dry, average, and wet year.
The suitability of stormwater harvesting and reuse with irrigation was tested for sites with various percent imperviousness, watershed area, irrigation area, irrigation depth, irrigation season length, basin volumes, and stormwater storage systems. Stormwater harvesting with irrigation reuse was found to be an effective best management practice to reduce runoff volume and phosphorus load to downstream waters. Up to an additional 35% of the annual watershed phosphorus load and up to an additional 80% of the annual watershed runoff volume were removed through irrigation reuse of stormwater in model test runs. Including the phosphorus load lost via sedimentation and runoff volume removed via evaporation, a total of 55-96% of the annual watershed phosphorus load and a total of 20-95% of the annual watershed volume were removed through stormwater harvesting and irrigation reuse.
This document describes model development (including assumptions, inputs, and equations) and model testing (including figures from 576 test runs), and is organized into the following sections:
I. Modeling Assumptions
II. Model Inputs and Outputs
III. Model Testing
IV. Interpretation of Results
Appendix A. Model Equations
Appendix B. Model Test Graphs
References Cited
46 Stormwater Harvesting: Accounting of Benefits and Feasibility
I. MODELING ASSUMPTIONS
The following section discusses the specific assumptions used to build and test the stormwater harvesting and reuse model. Assumptions about the watershed and irrigation areas, stormwater storage system dimensions and behavior, and the model partitioning of water and phosphorus will be discussed. The objective of this section is to introduce the basic structure of the model for a non-technical audience. Model equations and screen captures of the model can be found in Appendix A.
Watershed and Irrigation Areas
The modeled watershed area can be divided into 4 types: storage pond surface, pervious surfaces, connected impervious surfaces, and disconnected impervious surfaces (Figure 5). Impervious surface runoff contributes to watershed runoff in one of two ways: 1) without any infiltration by pervious surfaces encountered during runoff transit to the storage pond (connected), and 2) with some infiltration by pervious surfaces encountered during runoff transit to the storage pond (disconnected). To maximize the volume reduction and water quality benefits of stormwater reuse in the testing of the model, all of the pervious surfaces in the watershed were assumed to be irrigated. The total irrigation area was equal to the pervious surface area minus the surface area of the storage pond. However, the model is capable of simulating the benefits of stormwater harvesting and reuse with independent watershed and irrigation areas. Cisterns were assumed to be underground and were not deducted from the total irrigation area. However, irrigation area is a user-defined model input and can be modified based on specific site constraints. For example, some fraction of pervious surfaces may not be irrigated due to forestation, inaccessibility by irrigation systems, or different ownerships.
Figure 5. Stormwater harvesting and reuse simplified watershed used in model testing
Stormwater Harvesting and Reuse 47
Stormwater Storage Systems
The model was based on a pond and a cistern storage system for collected stormwater (Figure 6). Stormwater pond retrofits are feasible when there are preexisting ponds in a development or the construction of a pond is desirable to attract wildlife or improve site aesthetics. If site constraints limit the use of stormwater ponds, cisterns can alternatively be built under or beside buildings to collect roof and ground runoff. The pond was modeled to NURP standards with an elliptical surface area, length (L) equal to two times the width (W), a side slope of 3:1, and a maximum depth (D) of 6 feet. The cistern was modeled as a cylinder with a height (H) of 5 feet and a variable diameter (L=W). The total volume of the basin was equal to the volume of watershed runoff based on a specific rain event. This volume was then used to determine the pond or cistern dimensions listed above. Ponds and cisterns can be many shapes and sizes, therefore the user define storage model inputs are basin volume (VB), sediment storage volume (VS), and basin depth. All other basin sizing calculations are external from the model, allowing future users to use any type of pond or cistern designed for any size storm event.
Sediment Storage Volume
The minimum depth of water in each storage system, the sediment storage volume, is illustrated by shading in Figure 6 below. Sediment storage volume was necessary to maintain a permanent volume of water in which sediment can settle out of the water column and be stored for removal. The sediment storage volume must be sufficient to prevent resuspension of the sediment layer into overlying waters from wind action or mixing, and to prevent clogging of pumps, sprinkler heads, or other water delivery systems. The sediment storage volume depth was modeled as 2 feet for ponds and 1 foot for cisterns. The sediment storage volume was deeper in the pond than in the cistern because the pond was exposed to wind disturbance while the enclosed cistern is not.
Figure 6. Stormwater storage pond and cistern basic dimensions
Evaporation
The main difference between a pond and a cistern in the model is evaporation. Evaporation was modeled from the surface of the pond but was not modeled from the surface of the cistern. This was based on the assumption that evaporation from the surface of an enclosed cistern would be negligible compared to evaporation from the surface of a pond exposed to wind and direct sunlight. Evaporation from the pond surface was based on 10-year (2002-2011) monthly average pan evaporation values reported on the University of
48 Stormwater Harvesting: Accounting of Benefits and Feasibility
Minnesota Climate Group webpage. The period of evaporation based on available pan evaporation values was mid-April to mid-October. Pan evaporation values were converted to a water depth using pan coefficient values interpolated from Figure 8-1 "Average Annual Class A Pan Coefficient in Percent" from the Hydrology Guide for Minnesota 1992.
Water Mass Balance
The water component was modeled using a daily time step mass balance. Daily changes in basin volume (VB) were equal to watershed runoff (QW) inputs less evaporation (QE), overflow (QO), and irrigation (QI) outputs, all divided by the previous days basin volume. Watershed runoff volume was calculated using separate CN values for impervious and pervious surface areas. Watershed flow into the basin was assumed to occur whenever precipitation was in the form of rain. All water flowing into and out of the basin was assumed to occur at the end of the day. Constraints were included in the model to keep the basin volume at least as great as the sediment storage volume but no greater than the total volume of the basin. If watershed flow occurred during the winter, the flow and accompanying phosphorus load were assumed to reach the pond or cistern. The initial basin volume at the beginning of each year (January 1) was set to the sediment storage volume because it is likely that most of the variable storage volume from the prior year would be used for irrigation prior to January 1. For example, the volume of a cistern during an average rainfall year is illustrated in Figure 8. At the beginning of the year, the volume of the cistern was equal to the sediment storage volume. During the growing season, the cistern volume increased following rainfall events and decreased following irrigation reuse events. By the fall, the cistern volume had declined to approximately the sediment storage volume again. However, the initial basin volume can be changed by the user.
Figure 7. Water mass balance schematic
Stormwater Harvesting and Reuse 49 SUMMARY: Annual Basin Dynamics
DRY YEAR (2009) 80,000 50,000
70,000 45,000 40,000 60,000 35,000 50,000 30,000 40,000 25,000 30,000 20,000 15,000 20,000 10,000
10,000 5,000
Overflow Volume (cubic feet) Volume Basin in (cubic feet) 0 0 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin
AVERAGE YEAR (2011) 80,000 50,000
70,000 45,000 40,000 60,000 35,000 50,000 30,000 40,000 25,000
30,000 20,000 15,000 20,000 10,000
10,000 5,000
Overflow Volume (cubic feet) Volume Basin in (cubic feet) 0 0 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin
Figure 8. Example change in basin volume and phosphorusWET YEAR (2007) concentration with time 80,000 80,000
(Modeled test case: 70,0001 acre watershed, 65% impervious, pond, 5 months of irrigation)70,000 60,000 60,000 50,000 50,000
Phosphorus Mass40,000 Balance 40,000 The phosphorus component30,000 was modeled in three steps each day (Figure 9). First, the30,000 watershed phosphorus load and watershed 20,000volume entering the basin on that day completely mixed with the20,000 basin phosphorus load
and basin volume from10,000 the previous day to determine an intermediate phosphorus concentration10,000 in the basin
Overflow Volume (cubic feet) Volume Basin in (cubic feet) with no change in volume0 (i.e., no overflow). For this step only, the calculated basin volume0 may exceed the 1/1 4/11 7/20 10/28 actual basin volume. Next, sedimentation, overflow and irrigation were assumed to occur along with a Overflow Volume at Overflow Minimum Volume Basin corresponding volume and phosphorus load based on the intermediate pond phosphorus concentration. Finally, a new basin concentration was calculated based on the remaining phosphorus load in the basin and the new basin volume calculated for that day from the water mass balance described above.
Figure 9. Phosphorus mass balance schematic
50 Stormwater Harvesting: Accounting of Benefits and Feasibility
Sedimentation
Sedimentation was calculated based on equations developed by Walker (1988). When watershed runoff enters a basin, phosphorus attached to larger particles (usually sediment) falls out (sedimentation) of the water and settles on the basin floor. The longer watershed runoff remains in a basin (i.e., longer residence times) the more phosphorus sedimentation occurs, although most phosphorus sedimentation occurs within the first several days. Therefore, we assumed that if water flowed into the basin on a day without overflow, the maximum amount of phosphorus retention occurred (~60-70%, depending on the maximum depth of the basin). If water flowed into the basin on a day with overflow, the maximum phosphorus sedimentation was decreased in relation to the decreasing residence time (calculated from the overflow rate and maximum basin depth). The model also reduced phosphorus sedimentation for that day if another runoff event occurred on the next 1-3 days, corresponding to a subsequent reduction in basin residence time due to future runoff events. Note that there are three main fractions of phosphorus (P) represented by the model: water column dissolved P, water column particulate P, and sediment bound particulate P. Sedimentation involves only the transfer of water column particulate P to sediment bound particulate P.
Did you know? Phosphorus concentrations in the basin will: Increase when evaporation > inflow; watershed P concentration > basin P concentration Decrease when inflow > evaporation; basin P concentration > watershed P concentration Remain stable when no inflows or evaporation
II. MODEL INPUTS AND OUTPUTS
The first part of this section discusses the input parameters used in the model. All input parameters, except for precipitation data, can be changed by the user using the input parameter interface of the model (Figure 10). A set of values for each input parameter was chosen for model derivation to assess the effect of each parameter on the volume and phosphorous removal performance of stormwater reuse with irrigation. Underlying assumptions used to choose input parameter values are detailed below. Table 16 summarizes all input parameter values used for model derivation and testing.
The second part of this section discusses the output parameters derived from the model. The output parameters reported by the model (Figure 11) were annual runoff volumes (shaded in blue) and phosphorus loads (shaded in orange) according to model component (watershed, overflow, irrigation, basin volume evaporation, and basin phosphorus sedimentation) and precipitation year (dry, average, wet, and the average of all 3 years) in units of total component volume/load and as a percent of the total watershed volume/load. The output parameters used to assess the runoff volume and phosphorus reduction benefits of stormwater reuse were overflow, irrigation, and basin phosphorus and volume loads averaged over all three precipitation years (dry, average, and wet) as a percent of the total watershed volume or load (white cells in Figure 11).
Stormwater Harvesting and Reuse 51
INPUTS A 12,135 2 irrigation ft IRRIGATION Dirrigation 0.5 in/day Begin/End 5 to 9 month
Awatershed 1 acres
%Imp, connected 65% % %Imp, disconnected 0% % WATERSHED CNImp, disconnected
CNpervious 74
CW-TP 410 ppb
CW-orthoP 100 ppb 3 Vbasin 7,693 ft 3 Vstorage 717 ft Lbasin 89 ft
Wbasin 44.5 ft STORAGE Dbasin 6 ft Side slope 3 ratio Evaporation 1 1=on; 0=off 3 Vbasin, initial 717 ft Figure 10. Input parameter model interface
(Modeled test case: 1 acre watershed, 65% impervious, pond, 5 months of irrigation)
OUTPUTS From the To To To To Change in Rainfall Year Units Watershed Overflow Irrigation Evaporation Sedimentation Storage 100% 9% 49% 19% 23% % Total Vol 30,229 2,584 14,865 5,804 6,976 Vol (ft3/yr) DRY 100% 3% 22% 65% 9% % Total P 0.35 0.01 0.08 0.23 0.03 TP (kg/yr)
100% 17% 64% 17% 2% % Total Vol 35,571 6,042 22,624 6,068 837 Vol (ft3/yr) AVERAGE 100% 8% 31% 59% 2% % Total P 0.41 0.03 0.13 0.24 0.01 TP (kg/yr) 100% 38% 37% 12% 14% % Total Vol 51,528 19,329 19,155 6,068 6,976 Vol (ft3/yr) WET 100% 19% 18% 57% 7% % Total P 0.60 0.11 0.11 0.34 0.04 TP (kg/yr)
100% 21% 50% 16% 13% % Total Vol 39,109 9,319 18,881 5,980 4,930 Vol (ft3/yr) ALL 100% 10% 24% 60% 6% % Total P 0.45 0.05 0.10 0.27 0.03 TP (kg/yr) Figure 11. Output parameter model interface
52 Stormwater Harvesting: Accounting of Benefits and Feasibility
(Modeled test case: 1 acre watershed, 65% impervious, pond, 5 months of irrigation)
Watershed Inputs
Precipitation
The volume reduction and water quality benefits of stormwater reuse were based on average reductions during a typical dry, average, and wet rainfall year for the Minneapolis/St. Paul region. Typical dry, wet, and average rainfall years were identified according to the 25th, 50th, and 75th percentiles, respectively, of the most recent 30-, 20-, and 10-years of precipitation data (1982-2011) collected at the Minneapolis/St. Paul airport weather station. Typical dry, average, and wet years have become increasingly drier (Figure 12) with more extreme precipitation events (Saunders et al. 2012). Because the timing of rainfall is equally important as (if not more than) the amount of rainfall, only the most recent 10 years of rainfall data were used to identify typical dry, average, and wet rainfall years. In addition, the amount of rainfall that occurs during the growing season (May through September) will have a strong impact on the performance of a stormwater reuse system. For example, a 4-inch rain event in April may increase the total annual rainfall but because that rainfall occurred prior to the start of a normal growing season, that rainfall may not be able to be used for irrigation. Table 14 summarizes the 25th, 50th, and 75th percentile 10-year annual and growing season rainfall (in inches) and the corresponding years within that 10-year period that have annual and growing season rainfall most similar to the 25th, 50th, and 75th percentile annual and growing season rainfall. The years that were most similar to the growing season rainfall amounts also had similar annual rainfall amounts as the 10-year annual rainfall amounts. As a result, these years were chosen to represent dry, average and wet annual rainfall (2009, 2011, and 2007, respectively).
Figure 12. Annual Rainfall 10-year, 20-year, and 30-year trends
Stormwater Harvesting and Reuse 53
Table 14. 10-year (2002-2011) rainfall data summary
Annual Growing Season (May-Sept) Corresponding GS Annual 10- Similar Rainfall Rainfall 10-year Similar year year year (in.) (in.) Rainfall (in.)
Dry Year 25.3 2009 24.8 12.1 2009 12.5 24.8 (25th percentile)
Average Year 27.5 2006 27.6 17.5 2011 17.9 26.9 (50th percentile)
Wet Year 33.3 2005 33.4 23.4 2007 22.7 34.3 (75th percentile)
Land Cover CN
Pervious surface areas were assumed to behave as good condition open spaces with grass cover covering greater than 75% of the total pervious surface area. The USDA Moisture Condition II hydrologic curve numbers (CN) for A, B, C, and D soils of this type were 39, 61, 74, and 80. To simplify the testing phase of the model, it was assumed that all pervious areas had type C soils with a CN of 74. Impervious surface areas were assumed to be connected and behave as paved parking lots, roofs, and driveways with a corresponding CN of 98. However, the disconnected impervious CN and pervious CN can be defined by the user to more accurately model site-specific stormwater reuse feasibility.
Did you know?
You can model the effects of disconnected impervious areas using a composite CN input.
Percent Imperviousness and Drainage Area
Watershed drainage areas and percent imperviousness were chosen to represent typical developed sites: commercial lots, commercial campuses, residential lots, and residential developments. The percent imperviousness was based on the average percent imperviousness listed for the following urban districts in the USDA hydrologic curve number computation sheet: commercial and business (85%) and average residential lot size of 1/8 acre or less (65%). The percent imperviousness for an average residential lot size of 1/3 acre (30%) was used to be the same as a commercial campus to account for its mix of highly developed areas surrounded by large undeveloped areas. Modeled sites were:
1. Small commercial: 1 acre site and 85% imperviousness 2. Large commercial: 5 acre site and 85% imperviousness 3. Small residential lot: 0.125 acre site and 65% imperviousness 4. Residential development: 5 acre site and 65% imperviousness
54 Stormwater Harvesting: Accounting of Benefits and Feasibility
5. Commercial campus: 5 acre site and 30% imperviousness
In addition to those listed above, a 1-acre residential lot and commercial campus were also modeled to assess the effects of watershed size and percent impervious cover on stormwater volume reduction and water quality benefits.
Average P Concentration
The average total phosphorus (TP) and ortho-phosphate (SRP) concentrations of watershed runoff in the model were based on median TP (410 ppb) and SRP (100 ppb) of monitored event-based concentrations from 562 events at 65 sites over 1980-1998 (Brezonik and Stadelmann 2002). These stormwater data were selected from 15 studies representing 68 watersheds in the Twin Cities Metropolitan Area. Catchment sizes ranged from 6.9 to 215 hectares with diverse land uses including residential, public and open space, commercial/industrial, grassland, woods, and wetlands.
Irrigation Inputs
Irrigation Area
To maximize the volume reduction and water quality benefits of stormwater reuse in the model, all of the pervious surfaces in the watershed were assumed to be irrigated. The total irrigation area was equal to the pervious surface area minus the surface area of the storage pond. Cisterns were assumed to be underground and were not deducted from the total irrigation area. However, irrigation area is a user-defined model input and can be modified based on specific site constraints. For example, some fraction of pervious surfaces may not be irrigated due to forestation, inaccessibility by irrigation systems, or different ownerships.
Did you know?
Watershed area and irrigation area are independent inputs in the model so runoff generated from one site
can be used to irrigate another site.
Irrigation Depth
A weekly irrigation depth of 1.5 inches was modeled based on recommendations by the University of Minnesota Horticulture Extension (Taylor 2009). The weekly irrigation depth was equally distributed over a 3- day per week irrigation schedule, or 0.5 inches per day. The daily irrigation depth is the user-defined input parameter in the model. For example, to change the weekly irrigation depth to 2 inches, the user would input a daily irrigation depth of 2/3 inches into the model. Irrigation can occur no more frequently than every other day, and irrigation does not occur if the combined total of irrigation and precipitation over the three previous days exceeds the daily irrigation depth. Irrigation was assumed to occur in the morning before any rainfall events on that day. Therefore, irrigation and precipitation can occur on the same day in this model. This is a reasonable assumption since many local rain events occur in the late afternoon and evening.
Stormwater Harvesting and Reuse 55
Evapotranspiration
Plant evapotranspiration was not included as a separate parameter to keep the model simpler and easier-to- use. However, evapotranspiration rates of the irrigated plant community should be included in the user’s calculation of irrigation depth. For example, if a plant community has high rates of evapotranspiration, the weekly irrigation rate should be increased to accommodate for additional losses of water due to evapotranspiration.
Irrigation Season Length
Annual growing season lengths vary in Minnesota depending on timing of spring snowmelt and the first fall frost. As a result, three different irrigation season lengths were modeled to represent long, average, and short growing season lengths: short (June through August), medium (May through September), and long (April through October).
Storage Inputs
Basin Volume
The basin volume is the maximum volume of the basin before overflow occurs. Basin volume was calculated differently for stormwater ponds and cisterns based on different dimension constraints. The basin volume was calculated based on the watershed runoff from the entire drainage area assuming a CN for impervious areas of 98 and a CN for pervious areas of 74, for rainfall depths of 0.25 to 3.0 inches in 0.25-inch increments. Early iterations of the model showed a much stronger effect of percent imperviousness than soil CN on the runoff volume and phosphorus reduction benefits of stormwater reuse because the maximum irrigation area became limited at high percent imperviousness. To reduce the number of basin volume calculations used to test the model, only a pervious CN of 74 was used. However, this parameter can be defined by the user to more accurately model site-specific stormwater reuse feasibility.
The pond surface area was modeled as an ellipse, with length equal to 2 times the width, and as a series of 1 foot depth slices. The width of the surface area of each slice was adjusted to achieve a given storage design volume while maintaining a side slope of 3:1 and a maximum depth of 6 feet for volumes > 4,128 ft3. The maximum depths for runoff volumes < 4,128 ft3 were set in 1-foot increments according to the minimum volume of runoff needed to fill that basin depth (Table 15). The sediment storage volume was calculated from the volume of the bottom 2 slices (the bottom 2 feet depth of pond).
56 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 15. Model test maximum storage pond depths corresponding to minimum runoff volume
Depth (ft) Vmin (ft3)
6 4,128
5 2,403
4 1,244
3 537
2 170
1 28
Cisterns were more simply modeled as 5 feet deep cylinders, adjusting the diameter of the cylinder to achieve the storage design volume. The sediment storage volume was calculated as the volume of the bottom 1 foot depth of the cistern.
Basin Length and Width
The basin surface area was recalculated every time step from the basin volume and the maximum length, width, and volume of the basin. The basin surface area was used in the model to determine the volume of evaporation from the pond water surface.
Basin Depth
The maximum basin depth was derived from the basin volume calculations described above. Basin depth was a separate model input because it was used to determine the sedimentation retention coefficient of the basin.
Did you know?
The user must define appropriate basin geometry or the model results will be invalid.
Basin Side Slope
The model was tested based on an average basin side slope of 3:1. Evaporation is calculated based on a variable pond surface area with changing water depth assuming a constant side slope of the basin. As a result, evaporation from ponds designed with a safety bench may be underestimated in the model if only the side slope of the storage pool is used as an input to the model. For more accurate model results, an average side slope of the storage pool and the safety bench should be used.
Did you know?
You can set the average side slope of the storage system to account for the design of safety benches.
Stormwater Harvesting and Reuse 57
Sediment Storage Volume
The sediment storage volume was derived from the basin volume calculations described above. Sediment storage volume was a separate model input because it represents the minimum allowable volume of the basin.
Did you know?
You can set the minimum volume of the basin to maintain a desired minimum pond depth.
Storage System
The model was based on a pond and a cistern stormwater storage system. The main difference in the model between a pond and a cistern is evaporation, with evaporation modeled from the surface of the pond but not from the surface of the cistern. This was based on the assumption that evaporation from the surface of an enclosed cistern would be negligible compared to evaporation from the surface of a pond exposed to wind and direct sunlight. Evaporation losses can be turned on (=”1”) or off (=”0”) using the input parameter model interface (Figure 10).
Initial Basin Volume
The model was tested based on the assumption that the initial basin volume as equal to the sediment storage volume. However, the initial basin volume can be changed by the user according to the performance of individual stormwater harvesting and reuse systems.
Did you know?
You can choose the starting volume of the basin to account for winter dynamics.
Model Validation Figures
Several figures are included in the Input-Output spreadsheet tab to assist the user in validating the results from model iterations (Figure 13). On the right are line graphs illustrating basin volume and phosphorus concentration with time during dry, average, and wet years. This allows the user to monitor the behavior of the stormwater harvesting and reuse system under specific rainfall conditions if there are specific site concerns, such as the length of time the basin is full, the frequency with which the basin fills and empties, or the range in basin phosphorus concentrations. On the bottom left are bar graphs illustrating the volume and phosphorus reduction output results for overflow, irrigation, and basin components during dry, average, wet, and the average of all 3 years.
58 Stormwater Harvesting: Accounting of Benefits and Feasibility
INPUTS OUTPUTS SUMMARY: Airrigation 12,135 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units IRRIGATION Dirrigation 0.5 in/day Sedimentation Watershed Overflow Irrigation Evaporation Storage DRY YEAR (2009) Begin/End 5 to 9 month 100% 9% 49% 19% 23% % Total Vol 9,000 2,000 Awatershed 1 acres 30,229 2,584 14,865 5,804 6,976 Vol (ft3/yr) DRY 8,000 1,800 % 65% 100% 3% 22% 65% 9% Imp, connected % % Total P 1,600 % 0% 0.35 0.01 0.08 0.23 0.03 7,000 Imp, disconnected % TP (kg/yr) 1,400 CN 100% 17% 64% 17% 2% 6,000 WATERSHED Imp, disconnected % Total Vol 1,200 5,000 CNpervious 74 35,571 6,042 22,624 6,068 837 Vol (ft3/yr) AVERAGE 1,000 C 4,000 W-TP 410 ppb 100% 8% 31% 59% 2% % Total P 800 C 3,000 W-orthoP 100 ppb 0.41 0.03 0.13 0.24 0.01 TP (kg/yr) 600 V 7,693 3 100% 38% 37% 12% 14% basin ft % Total Vol 2,000 400 3 3
Vstorage 717 ft 51,528 19,329 19,155 6,068 6,976 Vol (ft /yr) 1,000 200 Overflow Volume (cubic feet) WET Volume Basin in (cubic feet) Lbasin 89 ft 100% 19% 18% 57% 7% % Total P 0 0 Wbasin 44.5 ft 0.60 0.11 0.11 0.34 0.04 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 6 ft 100% 21% 50% 16% 13% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope 3 ratio 39,109 9,319 18,881 5,980 4,930 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1 1=on; 0=off 100% 10% 24% 60% 6% % Total P 9,000 2,500 Vbasin, initial 717 ft 3 0.45 0.05 0.10 0.27 0.03 TP (kg/yr) 8,000 60,000 0.70 7,000 2,000
50,000 0.60 6,000 1,500 5,000 /yr) Change in 0.50 3 40,000 Change in Storage Storage 4,000 0.40 1,000 Evaporation Sedimentation 30,000 3,000 0.30 Irrigation P (kg/yr) 2,000 Volume(ft Irrigation 20,000 500 0.20 1,000
Overflow Overflow Volume (cubic feet) Overflow Volume Basin in (cubic feet) 10,000 0.10 0 0 1/1 4/11 7/20 10/28 0 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL DRY AVERAGE WET ALL WET YEAR (2007) 120% 120% 9,000 3,000 8,000 100% 100% 2,500 7,000
80% Change in 80% 6,000 2,000 Storage Change in Storage 5,000 Evaporation 60% 60% Sedimentation 1,500 4,000 Irrigation 40% 40% Irrigation 3,000 1,000
Overflow Overflow 2,000 20% P (% WatershedLoad) 20% 500
Volume (% Volume(% Watershed Flow) 1,000 Overflow Volume (cubic feet) Volume Basin in (cubic feet) 0 0 0% 0% 1/1 4/11 7/20 10/28 DRY AVERAGE WET ALL DRY AVERAGE WET ALL Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid 5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
Figure 13. Input-Output spreadsheet tab of the stormwater reuse model
(Modeled test case: 1 acre watershed, 65% impervious, pond, 5 months of irrigation
Table 16. Summary table of input parameter values
Parameter Values tested
Minneapolis/St. Paul Airport precipitation data: 2009 (Dry year) Precipitation (built-in and unchangeable by user) 2011 (Average year) 2007 (Wet year)
Pervious surfaces = 74
Land Cover CN Impervious surfaces = 98 (connected)
85% Percent Imperviousness 65%
30% Watershed Inputs 0.125 acre (65% imperviousness only) Drainage Area 1 acre 5 acre
TP = 410 ppb Average P Concentration Ortho P = 100 ppb
Irrigation Area All pervious surfaces less pond surface area
1.5 inches per week Irrigation Depth 0.5 inches every other day
3 months (June – August) Irrigation Inputs Irrigation Season Length 5 months (May – September) 7 months (April – October)
Runoff volume from 0.25 to 3.00 inch rain Basin Volume events (for every 0.25-inch increment)
Basin Surface Area Variable
Pond = 6 feet or less depending on volume Basin Depth Cistern = 5 feet
StorageInputs Pond = 2 feet Sediment Storage Volume Cistern = 1 foot
On = 1 Evaporation Off = 2
III. MODEL TESTING
The stormwater model was tested for 12 rainfall events, 3 growing season lengths, and 16 sites (Table 17). A total of 576 model iterations were run to test the feasibility of stormwater reuse. The performance of stormwater reuse was assessed by analyzing the percent annual reduction in phosphorus and volume due to irrigation. The graphs in Figure 14 show example results for a 1-acre watershed, 5-month growing season, and pervious CN of 74. Graphs illustrating all 576 model iterations were included in Appendix B. These graphs were used to assess the performance of stormwater reuse.
Table 17. Tested model sites (CN = 74)
Imperviousness Pond Cistern
85% 1 acre 5 acre 1 acre 5 acre
0.125 acre 0.25 acre 0.125 acre 0.25 acre 65% 1 acre 5 acre 1 acre 5 acre
30% 1 acre 5 acre 1 acre 5 acre
Stormwater Harvesting and Reuse 61
Figure 14. Stormwater harvesting and reuse model summary figures (1 acre watershed and 5 months of irrigation)
IV. INTERPRETATION OF RESULTS
Stormwater harvesting with irrigation reuse was found to be an effective best management practice to reduce runoff volume and phosphorus load to downstream waters. Up to an additional 35% of the annual watershed phosphorus load and up to an additional 80% of the annual watershed runoff volume were removed through stormwater harvesting and irrigation reuse using the modeling assumptions defined in Section II. Including the phosphorus load lost via sedimentation and runoff volume removed via evaporation, a total of 55-96% of the annual watershed phosphorus load and a total of 20-95% of the annual watershed volume were removed (Appendix B). The removal efficiency of stormwater reuse systems designed for large sites (> 5 acres) performed similarly as stormwater reuse systems designed for 1- and 5-acre sites, indicating that optimal reuse efficiency is achieved for site sizes greater than 1 acre. The model parameters with the strongest influence on the performance of stormwater reuse and the limitations of stormwater reuse as identified by the model are discussed below.
Key Drivers
Four model inputs strongly influenced the performance of stormwater reuse: percent imperviousness, irrigation season length, stormwater storage system, and design basin volume.
Percent Imperviousness
- Percent annual volume and phosphorus reduction tended to increase as the percent imperviousness decreased because available irrigation area was limited at high percent imperviousness. In addition, stormwater harvesting ponds in sites with 30% imperviousness tended to have greater variability in percent annual volume and phosphorus reduction due to high evaporation rates and large differences in pond surface area as volume increased.
Did you know? - -Low Impact Development increases the amount of green space available for irrigation, increasing the -annual volume and phosphorus reduction efficiency of stormwater harvesting and reuse.
Irrigation Season Length
- Greater percent annual phosphorus and volume reductions were observed for an irrigation season length of 7 months compared to 5 months. However, increasing the irrigation season length to 9 months did not significantly change the percent annual volume and phosphorus reduction due to mismatched timing between rainfall and irrigation demand. In addition, the model was based on an irrigation depth of 1.5 inches per week, but greater irrigation depths will increase the percent annual phosphorus and volume reduction.
Stormwater Storage System
- Percent annual volume reductions due to irrigation were higher for cistern storage systems than pond storage systems because there was no evaporation from the cistern surface, but overall volume reductions were slightly greater for pond storage systems.
Design Basin Volume
- The percent annual volume and phosphorus reduction of stormwater reuse increased as the basin volume increased from 1” to 2” rainfall events. Basin volumes designed for larger rainfall events provided additional annual phosphorus and volume reductions but at incrementally smaller amounts. - Did- you know?
Regular maintenance is critical for removing accumulated sediment and maintaining performance.
Limitations
Sites with small areas or high percent imperviousness are better suited for a cistern storage system than a pond storage system due to limited irrigation area and/or limited runoff volume. For example, certain sites do not produce enough watershed runoff to provide any percent annual volume and phosphorus reduction benefits from irrigation reuse. A minimum basin volume (sediment storage volume) is required to maintain a permanent volume of water in which sediment can settle out of the water column and be stored for removal. Basins designed for sites with volumes less than the sediment storage volume do not allow for water withdrawal for irrigation. Some stormwater ponds had basin volumes less than the sediment storage volume, but cisterns always maintained basin volumes greater than the sediment storage volume because the surface area could be reduced. Sites tested in the model with stormwater pond volumes less than the sediment storage volume (537 ft3) that resulted in no volume and phosphorus reduction benefits were:
1. 65% imperviousness and 0.125-acre site designed for less than a 2” rain event 2. 65% imperviousness and 0.25-acre site designed for less than an 1.25” rain event 3. 65% imperviousness and 1-acre site designed for less than a 0.5” rain event 4. 30% imperviousness and 1-acre site designed for less than a 0.75” rain event
Stormwater Applications
Stormwater harvesting and reuse depends on many parameters that vary widely from site to site (Section II). Modifying any one of these parameters can change the percent annual volume and phosphorus reduction benefits of stormwater reuse. As a result, the model spreadsheet was designed to automatically run several years of simulations and compile volume and phosphorus reduction results based on site specific parameters entered into the model input interface by a user. This allows a designer to optimize the site and/or a stormwater regulator to assess the volume and phosphorus reduction benefits of a particular system. In addition, the model could be modified in future versions to incorporate other non-irrigation water uses, such as toilet flushing, vehicle washing, or municipal street cleaning.
64 Stormwater Harvesting: Accounting of Benefits and Feasibility
APPENDIX A. MODEL EQUATIONS
Volume Mass Balance
Stormwater Reuse Volume Reduction and Water Quality Benefits Spreadsheet: 2 Created by: Emmons & Olivier Resources, Inc. (2013) 1 acre = 43,560 ft IRRIGATION WATERSHED BASIN EVAPORATION 2 3 Airrigation 12,135 ft Awatershed 1 ac Vbasin 7,693 ft mo in/day in/mo Days Month A 0.28 ac A 43,560 ft 2 V 717 ft 3 4 0.17 1.71 10 Apr 21-30 irrigation watershed storage Dirrigation 1.5 in/wk %Imp, connected 65% % Lbasin 89 ft 5 0.20 6.10 31 May Dirrigation 0.5 in/day CNImp, connected 98 Wbasin 44.5 ft 6 0.23 7.00 30 June Begin 5 month SImp, connected 0.20 in Dbasin 6 ft 7 0.25 7.81 31 July End 9 month %Imp, disconnected 0% % Side slope 3 ratio 8 0.21 6.46 31 August CNImp, disconnected 0 9 0.15 4.65 30 Sept
SImp, disconnected 0.00 in 10 0.13 1.30 10 Oct 1-10
CNpervious 74 0.795 Pan Evaporation Coefficient
Spervious 3.51 in
CW-TP 410 ppb
MAX: 2.08 CW-orthoP 100 ppb %TOTAL: 19% 9% 49% 58%
TOTAL: 19.19 Fo 0.24 TP: orthoP TOTAL: 30,229 TOTAL: 5,804 2,584 14,865 17,449 Impervious Impervious Pervious Watershed Open Flow into Last 1 Surface Basin Evap- Over Irrigation Surface Mo. Date Snow Rain Connected Disconnected Runoff Flow water? Basin Irrig? Irrig. 3 days Evap? Evap Area Volume oration Flow Flow Flow Qw (1=yes) QB (1=yes) (1=yes) AB VB QE QO QI QS in in in in in ft 3 /day Y/N ft 3 /day Y/N in in Y/N in ft 2 ft 3 ft 3 /day ft 3 /day ft 3 /day ft 3 /day 717 8 8/16 72 0.08 0.01 0.00 0.00 15 1 15 0 0 1.17 1 0.21 2,943 6,446 36 0 0 0 8 8/17 72 0 0.00 0.00 0.00 0 1 0 1 0 0.75 1 0.21 2,940 6,410 36 0 0 0 8 8/18 68 0 0.00 0.00 0.00 0 1 0 0 0 0.75 1 0.21 2,935 6,374 36 0 0 0 8 8/19 66 1.46 1.24 0.00 0.13 3,098 1 3,098 1 0.5 0.08 1 0.21 2,930 7,187 36 1,743 506 2,248 8 8/20 64 0.45 0.27 0.00 0.00 644 1 644 0 0 1.96 1 0.21 3,042 7,693 36 102 0 102 8 8/21 66 0.18 0.06 0.00 0.00 133 1 133 1 0 2.41 1 0.21 3,111 7,693 36 97 0 97 8 8/22 64 0 0.00 0.00 0.00 0 1 0 0 0 2.59 1 0.21 3,111 7,657 36 0 0 0 Figure 15. Volume mass balance screen capture of the stormwater reuse model spreadsheet
(Modeled test case: 1 acre watershed, 65% impervious, pond, 5 months of irrigation)
Watershed Flow, QW:
If rainfall depth (in.), R < 0.2 × S, then runoff (in.),
Q = 0
If rainfall depth (in.), R ≥ 0.2 × S, then runoff (in.), ( ) ( ) ( ) Where the percent imperviousness area = I ( ) ( ) [( ) ( ) ( )]
( ) ( ) [( ) ( ) ( )]
( ) ( ) ( )
Basin Surface Area, AB:
( ) {√[( ) ( )]}
Where L = length of basin and W = width of basin, in feet
Basin Volume, VB:
( )
Where VB-1 = the basin volume on the previous day
Evaporation, QE: ( ) ( )
Overflow, QO:
( )
Irrigation, QI:
( ) , whichever is less
Where AI = irrigation area (ft2) and DI = irrigation depth (ft)
Phosphorus Mass Balance
Overflow Irrigation Sediment %P 3% 22% 65% %Vol 9% 49% 30-day Average % Total P 100% 3% 22% 65% 9% Check: 0.082 Walker 1987 Sedimentation Eqns: 0.65 TP (kg/yr) 0.35 0.01 0.08 0.23 0.03 0.00E+00 Residence Residence Surface Second-order Reaction Retention Pond Pond Watershed P Load P Load P Load P Load Time Time Overflow Decay Rate Rate Coefficient Conc. Conc. TP Load to Overflow to Irrigation to Sediment in Pond T Adjusted T Qs K2 Nr Rp CP Int. CP PW-TP PO PI PS PP yr yr m/yr 1/ppb/yr unitless unitless mg/L mg/L mg/day mg/day mg/day mg/day mg/day 0.10 0.10 0.082 0.082 22.3 0 5 0.64 0.17 0.17 6 0 0 4 1113 0.000 0.000 0.17 0.17 0 0 0 0 1113 0.000 0.000 0.17 0.15 0 0 0 0 1113 0.011 0.145 161.8 0 13 0.76 0.15 0.15 1,270 262 76 959 1086 0.206 0.211 8.9 0 8 0.70 0.15 0.15 264 15 0 186 1149 0.217 0.217 8.4 0 8 0.70 0.15 0.15 55 14 0 38 1151 0.000 0.000 0.15 0.15 0 0 0 0 1151 Figure 16. Phosphorus mass balance screen capture of the stormwater reuse model spreadsheet
(Modeled test case: 1 acre watershed, 65% impervious, pond, 5 months of irrigation)
Residence Time, T:
( )
Surface Overflow, QS:
( ) ( )
Where DB = maximum basin depth (ft)
Second-order Decay Rate, K2:
( )
( )
Reaction Rate, NR:
Retention Coefficient, RP:
( ) [ ]
Pond Phosphorus Concentration, CP: ( )
( )
( ) ( )
Watershed TP Load, PW-TP:
( )
P Load to Overflow, PO:
( )
P Load to Irrigation, PI:
( )
P Load to Sediment, PS:
( )
P Load in Pond, PP:
( )
APPENDIX B. MODEL TEST GRAPHS Table 18. Model test graphs for 3-month irrigation season
Stormwater Harvesting and Reuse 69
Table 19. Model test graphs for 5-month irrigation season
70 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 20. Model test graphs for 7-month irrigation system
Stormwater Harvesting and Reuse 71
REFERENCES CITED
Brezonik, P. and T. Stadelmann (2002) Analysis and predictive models of stormwater runoff volumes, loads, and pollutant concentrations from watersheds in the Twin Cities metropolitan area, Minnesota, USA. Water Research 36(7): 1743-1757.
Pitt, R., S. Clark and K. Parmer. 1999. Potential Groundwater Contamination from Intentional and Nonintentional Stormwater Infiltration. U.S. Environmental Protection Agency, Risk Reduction Engineering Laboratory, Office of Research and Development, Springfield, V.A.
Saunders, S., D. Findlay, T. Easley, and T. Spencer. (2012) Doubled Trouble: More Midwestern Extreme Storms. Prepare for The Rocky Mountain Climate Organization and the National Resource Defense Council.
Taylor, D. (2009). Watering Lawns and Other Turf (WW-02364). University of Minnesota – Extension.
Walker, W. (1987) Phosphorus removal by urban runoff detention basins. Lake and Reservoir Management 3: 314 – 326.
72 Stormwater Harvesting: Accounting of Benefits and Feasibility
Stormwater Harvesting and Reuse: Report on Models for 8 Theoretical Sites
METHODOLOGY
The water reuse model was utilized to test eight different theoretical sites. A variety of sites were chosen in order to evaluate the relative costs and benefits of harvesting and re-using stormwater in different situations. The results give a general idea of the potential for water reuse on each site and provide a way to assess the benefits of each of the sites relative to each other. This exercise was intended as an initial screening process, rather than a complete feasibility assessment for each site. More detailed assessment and feasibility should be conducted prior to considering an improvement project on any of these sites. However, the results do show a range of benefits and a rough analysis of costs/benefits. These results could be used as a guide to select a particular site for further evaluation.
Using aerial imagery and GIS software, each site was analyzed by land-cover type: irrigable area, ponds and other water features, impervious area, and buildings (see maps below). Pond storage capacity was determined by assuming dimensions of a typical NURP pond if none existed on the site. For sites with existing ponds, available information such as construction plans, topography, and aerial imagery was used to establish dimensions. If bathometric data was available the storage was modeled as an approximation of that data because the model does not allow entry of exact bathymetry. If no bathometric data was available it was assumed that the ponds were 6 feet deep with 4:1 average side slopes. It was further assumed that the ponds began the season half-full by depth, and that the irrigation system could not draw the water below a depth of 2 feet. The modeling of the evaporation would still have the potential to bring the water level down below 2 feet.
The sizes of the surface pond at Van Cleve Park and the underground storage at three sites (see Summary Table, page 3) were determined in HydroCAD using a 2.5-inch storm event. Building underground storage for such a large runoff volume is costly but we used these volumes in order to be consistent with NURP pond design parameters. If any of these projects is pursued, engineers should look closely at smaller design storms to limit the cost of underground storage.
This information about land cover and storage was entered into the Stormwater Harvesting and Reuse model. A summary of the model results is provided in a Summary Table (see page 3) with the results for each site provided in Tables 1-8. An accompanying figure is provided for each site in Figures 1-8. A planning level cost assessment was also completed for each site along with generation of a preliminary cost-benefit analysis.
Stormwater Harvesting and Reuse 73
SUMMARY OF KEY FINDINGS
Several key findings were identified as part of this assessment:
The combination of stormwater harvesting, storage, and reuse easily exceeds standard pond treatment performance for total phosphorus removal (55-60%). Reuse P removals range from 70% to 95%. In addition, reuse specifically targets the soluble phosphorous that is typically not removed by BMPs that rely on sedimentation for removal, like ponds. Harvesting and reusing rain water captures and reuses nutrients (phosphorous and nitrogen) needed by turf and vegetation. Lack of sufficient area to irrigate can be a major constraint (for example, in densely populated urban areas); this is an important factor in designing a reuse system. o Finding nearby green space to irrigate is one solution. Construction of underground storage for rain water is more expensive than utilizing ponding especially when existing ponds are available. Building storage and irrigation infrastructure in new areas is typically cheaper as compared to retro- fitting an existing site. Where existing storage can be used, rainwater harvesting is very cost effective; routing off-site drainage to existing storage is one way to increase phosphorous and volume reductions. Reuse performance depends on several variable: o Storage volume; o Percentage of green space available for irrigation – to allow storage to be recovered between events; o Application rates of irrigation; o Feasibility of capturing runoff and transporting it to irrigation/reuse site.
RECOMMENDATIONS
Quantify how much additional irrigation of turf and other vegetation types can be sustained (by soil type) over typical irrigation standards (0.5 inches/day). Establish more pilot demonstration sites to highlight rainwater harvesting and reuse and to accelerate adoption locally (with MCWD and MWMO cost-sharing). Establish test sites to determine best water application rates and water quality benefits. Develop guidance on pre-treatment needs and disinfection needs.
74 Stormwater Harvesting: Accounting of Benefits and Feasibility
SUMMARY TABLE Summary of model results
Phosphorus Reduction Volume Drainage Area Drainage Area Irrigable Reduction (Watershed) Impervious (3) Area Type of Storage (4) Sedimentation Irrigation TP acre % acre % % % % Location City Comments Ideas for improvement
St. Louis 1 Benilde St. Margaret 31 38% 7.7 Existing pond 57% 51% 25% 76% Small pond; large irrigable area Enlarge pond Park
Large off-site drainage from Pamela Park neighborhood; other feasibility Existing 2 (includes off-site Edina 266 43% 16.8 38% 61% 11% 72% considerations: DNR permits for ponds drainage) irrigating from lake; aesthetics of low pond volume.
Very large irrigable area; aesthetic Bring in runoff from surrounding Existing concerns about draining ponds too low; 3 Edina Country Club 1 Edina 125 1% 112.5 100% 64% 0% 64% neighborhood and commercial ponds runoff to ponds not sufficient for area; enlarge ponds irrigation
Edina Country Club 2 Proposed (Draining Club House Limited by size/cost of underground 4 Edina 6.2 100% 4.2 underground 57% 51% 26% 77% roof & parking lot; storage storage irrigating 9th hole)
Proposed Irrigate neigboring grass; team MCWD (New Office) Limited irrigable area; Limited by 5 Minnetonka 0.76 68% 0.24 underground 51% 49% 24% 73% up with neighboring garden (1) size/cost of underground storage storage center (Weber's Westdale)
Proposed Limited irrigable area; large impervious Christ Presbyterian 6 Edina 9.7 71% 2.4 underground 46% 48% 21% 69% area; Limited by size/cost of Church storage underground storage
Proposed Adequate space for a pond; sufficient Bring in runoff from surrounding 7 Van Cleve Park Minneapolis 4.5 14% 4.5 87% 62% 31% 93% pond green space neighborhood
Summit Preserve (2) Large pond and irrigation system are 8 (Proposed Prior Lake 39 72% 15 Planned pond 97% 68% 28% 96% planned development) Notes (1) MCWD: wetland area wasn’t used in any calculations (2) Summit Preserve: total site is 54 acres; 39 acres drain to pond (3) Impervious areas do not include ponds (in this chart) (4) The evaporation accounts for between 3% and 13% of the volume reduction for sites with surface ponds. The one exception is the Edina Country Club 1: in our model evaporation accounts for 100% of volume reduction (i.e. no water is used for irrigation).
Figure 17. Benilde St. Margaret
Figure 18. Pamela Park
Stormwater Harvesting and Reuse 77
Figure 19. Edina Country Club 1
78 Stormwater Harvesting: Accounting of Benefits and Feasibility
Figure 20. Edina Country Club 2
Stormwater Harvesting and Reuse 79
Figure 21. MCWD New Office
80 Stormwater Harvesting: Accounting of Benefits and Feasibility
Figure 22. Christ Presbyterian Church
Stormwater Harvesting and Reuse 81
Figure 23. Van Cleve Park.
82 Stormwater Harvesting: Accounting of Benefits and Feasibility
Figure 24. Summit Preserve.
Stormwater Harvesting and Reuse 83
MODEL TABLES
Table 21. Benilde St. Margaret School
INPUTS OUTPUTS SUMMARY: Airrigation 338,461 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units D 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION irrigation in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 32% 57% 4% 8% % Total Vol 80,000 80,000 Awatershed 30.7 acres 532,019 168,301 301,178 18,888 43,652 Vol (ft3/yr) DRY 70,000 70,000 %Imp, connected 38% % 100% 17% 24% 52% 6% % Total P
%Imp, disconnected 0% % 6.18 1.08 1.46 3.24 0.40 TP (kg/yr) 60,000 60,000 WATERSHED CNImp, disconnected 100% 37% 58% 3% 2% % Total Vol 50,000 50,000 CNpervious 61 622,088 230,703 361,052 18,888 11,445 Vol (ft3/yr) AVERAGE 40,000 40,000 CW-TP 410 ppb 100% 21% 31% 46% 2% % Total P 30,000 30,000 CW-orthoP 100 ppb 7.22 1.55 2.23 3.32 0.12 TP (kg/yr) 20,000 20,000 Vbasin 76,168 ft 3 100% 45% 48% 2% 5% % Total Vol Vstorage 20,816 3 902,470 406,843 433,087 18,888 43,652 3 10,000 10,000
ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin 150 ft 100% 22% 20% 56% 2% % Total P 0 0 Wbasin 100 ft 10.48 2.27 2.12 5.87 0.21 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 6 ft 100% 38% 54% 3% 5% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope 4 ratio 685,526 268,616 365,106 18,888 32,916 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1 1=on; 0=off 100% 20% 25% 51% 3% % Total P 80,000 80,000 Vbasin, initial 32,516 ft 3 7.96 1.63 1.94 4.15 0.24 TP (kg/yr) 70,000 70,000 1,000,000 12.00 60,000 60,000 900,000 10.00
800,000 50,000 50,000 /yr) 3 700,000 Change in Change in 40,000 40,000 Storage 8.00 600,000 Storage Evaporation Sedimentation 30,000 30,000 500,000 6.00 20,000 20,000
400,000 Irrigation (kg/yr) P Volume (ft Volume Irrigation 4.00 300,000 10,000 10,000
feet) (cubic VolumeOverflow
Overflow feet) (cubic Basin in Volume 200,000 Overflow 2.00 0 0 100,000 1/1 4/11 7/20 10/28 0 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL DRY AVERAGE WET ALL WET YEAR (2007) 120% 120% 80,000 80,000
70,000 70,000 100% 100% 60,000 60,000 80% Change in 80% Storage Change in 50,000 50,000 Storage Evaporation 60% 60% Sedimentatio 40,000 40,000 n Irrigation 30,000 30,000 40% 40% Irrigation 20,000 20,000
Overflow Overflow P (% Watershed Load) Watershed (% P 20% 20%
Volume (% Watershed Flow) Watershed (% Volume 10,000 10,000
Overflow Volume (cubic feet) (cubic VolumeOverflow Volume in Basin (cubic feet) (cubic Basin in Volume 0% 0% 0 0 DRY AVERAGE WET ALL DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
84 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 22. Pamela Park
INPUTS OUTPUTS SUMMARY: Airrigation 731,301 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units D 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION irrigation in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 28% 47% 4% 21% % Total Vol 3.E+06 3.E+06 Awatershed 266 acres 3,970,411 1,130,653 1,858,723 145,022 836,013 Vol (ft3/yr) DRY %Imp, connected 24% % 100% 10% 14% 65% 11% % Total P 2.E+06 2.E+06 %Imp, disconnected 19% % 46.10 4.39 6.38 30.12 5.21 TP (kg/yr)
CNImp, disconnected 92 100% 55% 34% 3% 8% WATERSHED % Total Vol 2.E+06 2.E+06 CNpervious 61 4,726,663 2,593,732 1,614,956 145,022 372,952 Vol (ft3/yr) AVERAGE C W-TP 410 ppb 100% 21% 13% 57% 9% % Total P 1.E+06 1.E+06 CW-orthoP 100 ppb 54.88 11.59 7.18 31.22 4.89 TP (kg/yr) 3 Vbasin 2,155,327 ft 100% 63% 23% 2% 12% % Total Vol 5.E+05 5.E+05 Vstorage 617,389 3 6,923,681 4,327,690 1,614,956 145,022 836,013 3
ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin 264 ft 100% 25% 7% 61% 7% % Total P 0.E+00 0.E+00 Wbasin 528 ft 80.39 19.81 5.98 49.37 5.24 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 10 ft 100% 49% 35% 3% 14% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope 4 ratio 5,206,918 2,684,025 1,696,212 145,022 681,659 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1 1=on; 0=off 100% 18% 11% 61% 9% % Total P 3.E+06 3.E+06 Vbasin, initial 1,319,314 ft 3 60.46 11.93 6.51 36.90 5.11 TP (kg/yr) 8,000,000 90.00 2.E+06 2.E+06 7,000,000 80.00 2.E+06 2.E+06
6,000,000 70.00 /yr)
3 Change in 60.00 Change in 5,000,000 Storage Storage Evaporation 50.00 1.E+06 1.E+06 4,000,000 Sedimentation 40.00
Irrigation (kg/yr) P Volume (ft Volume 3,000,000 Irrigation 30.00 5.E+05 5.E+05
2,000,000 feet) (cubic VolumeOverflow Overflow 20.00 Overflow feet) (cubic Basin in Volume 1,000,000 0.E+00 0.E+00 10.00 1/1 4/11 7/20 10/28 0 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL DRY AVERAGE WET ALL WET YEAR (2007) 120% 120% 3.E+06 3.E+06
100% 100% 2.E+06 2.E+06 80% Change in 80% Storage Change in 2.E+06 2.E+06 Storage Evaporation 60% 60% Sedimentatio n Irrigation 1.E+06 1.E+06 Irrigation 40% 40% Overflow Overflow P (% Watershed Load) Watershed (% P 5.E+05 5.E+05
20% 20%
Volume (% Watershed Flow) Watershed (% Volume
Overflow Volume (cubic feet) (cubic VolumeOverflow Volume in Basin (cubic feet) (cubic Basin in Volume 0% 0% 0.E+00 0.E+00 DRY AVERAGE WET ALL DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
Table 23. Edina Country Club
INPUTS OUTPUTS SUMMARY: Airrigation 4,898,300 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units D 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION irrigation in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 0% 0% 302% -202% % Total Vol 600,000 600,000 Awatershed 125 acres 99,351 0 0 300,291 -200,940 Vol (ft3/yr) DRY %Imp, connected 1% % 100% 0% 0% 64% 36% % Total P 500,000 500,000 %Imp, disconnected 0% % 1.15 0.00 0.00 0.74 0.42 TP (kg/yr) 400,000 400,000 WATERSHED CNImp, disconnected 100% 0% 0% 289% -189% % Total Vol CNpervious 61 103,955 0 0 300,291 -196,336 Vol (ft3/yr) AVERAGE 300,000 300,000 CW-TP 410 ppb 100% 0% 0% 64% 36% % Total P CW-orthoP 100 ppb 1.21 0.00 0.00 0.77 0.44 TP (kg/yr) 200,000 200,000 Vbasin 570,966 ft 3 100% 0% 0% 179% -79% % Total Vol 100,000 100,000 Vstorage 463,950 3 168,008 0 0 300,291 -132,283 3
ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin 264 ft 100% 0% 0% 64% 36% % Total P 0 0 Wbasin 528 ft 1.95 0.00 0.00 1.24 0.71 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 6 ft 100% 0% 0% 257% -157% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope 4 ratio 123,771 0 0 300,291 -176,520 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1 1=on; 0=off 100% 0% 0% 64% 36% % Total P 600,000 600,000 Vbasin, initial 463,950 ft 3 1.44 0.00 0.00 0.92 0.52 TP (kg/yr) 400,000 2.50 500,000 500,000
300,000 400,000 400,000 2.00
/yr) 200,000 3 Change in Change in 300,000 300,000 Storage 1.50 Storage 100,000 Evaporation Sedimentation 200,000 200,000 0
Irrigation (kg/yr) P 1.00 Volume (ft Volume Irrigation DRY AVERAGE WET ALL 100,000 100,000 -100,000
feet) (cubic VolumeOverflow
Overflow feet) (cubic Basin in Volume 0.50 Overflow -200,000 0 0 1/1 4/11 7/20 10/28 -300,000 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL WET YEAR (2007) 340% 120% 320% 600,000 600,000 300% 280% 260% 240% 100% 500,000 500,000 220% 200% 180% 160% 140% Change in 80% 400,000 400,000 120% Storage Change in 100% 80% Storage 60% Evaporation 40% 60% Sedimentatio 300,000 300,000 20% 0% n -20% Irrigation -40% DRY AVERAGE WET ALL Irrigation 200,000 200,000 -60% 40% -80% -100% Overflow Overflow -120% Load) Watershed (% P -140% 20% 100,000 100,000
Volume (% Watershed Flow) Watershed (% Volume -160% Overflow Volume (cubic feet) (cubic VolumeOverflow
-180% feet) (cubic Basin in Volume -200% -220% -240% 0% 0 0 DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
86 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 24. Edina Country Club 2
INPUTS OUTPUTS SUMMARY: Airrigation 184,822 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units D 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION irrigation in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 32% 60% 0% 7% % Total Vol 40,000 40,000 Awatershed 6.2 acres 279,227 90,225 168,179 0 20,823 Vol (ft3/yr) DRY 35,000 35,000 %Imp, connected 100% % 100% 18% 25% 51% 6% % Total P %Imp, disconnected 0% % 3.24 0.57 0.81 1.66 0.19 TP (kg/yr) 30,000 30,000 WATERSHED CNImp, disconnected 100% 38% 60% 0% 2% % Total Vol 25,000 25,000 CNpervious 61 327,514 125,480 197,121 0 4,913 Vol (ft3/yr) AVERAGE 20,000 20,000 CW-TP 410 ppb 100% 22% 32% 45% 1% % Total P 15,000 15,000 CW-orthoP 100 ppb 3.80 0.85 1.21 1.70 0.04 TP (kg/yr) 10,000 10,000 Vbasin 36,373 ft 3 100% 45% 50% 0% 4% % Total Vol Vstorage 7,275 3 473,699 214,746 238,130 0 20,823 3 5,000 5,000
ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin ft 100% 22% 21% 56% 2% % Total P 0 0 Wbasin ft 5.50 1.19 1.16 3.06 0.10 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 6 ft 100% 39% 57% 0% 4% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope ratio 360,147 143,483 201,144 0 15,520 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1=on; 0=off 100% 21% 26% 51% 3% % Total P 40,000 40,000 Vbasin, initial 15,550 ft 3 4.18 0.87 1.06 2.14 0.11 TP (kg/yr) 35,000 35,000 500,000 6.00 30,000 30,000 450,000 5.00
400,000 25,000 25,000 /yr) 3 350,000 Change in Change in 20,000 20,000 Storage 4.00 300,000 Storage Evaporation 15,000 15,000 250,000 3.00 Sedimentation 10,000 10,000
200,000 Irrigation (kg/yr) P Volume (ft Volume Irrigation 2.00
150,000 5,000 5,000 Overflow Volume (cubic feet) (cubic VolumeOverflow
Overflow feet) (cubic Basin in Volume 100,000 Overflow 1.00 0 0 50,000 1/1 4/11 7/20 10/28 0 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL DRY AVERAGE WET ALL WET YEAR (2007) 120% 120% 40,000 40,000
35,000 35,000 100% 100% 30,000 30,000 80% Change in 80% Storage Change in 25,000 25,000 Storage Evaporation 60% 60% Sedimentatio 20,000 20,000 n Irrigation 15,000 15,000 40% 40% Irrigation 10,000 10,000
Overflow Overflow P (% Watershed Load) Watershed (% P 20% 20%
Volume (% Watershed Flow) Watershed (% Volume 5,000 5,000
Overflow Volume (cubic feet) (cubic VolumeOverflow Volume in Basin (cubic feet) (cubic Basin in Volume 0% 0% 0 0 DRY AVERAGE WET ALL DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
Stormwater Harvesting and Reuse 87
Table 25. MCWD New Office
INPUTS OUTPUTS SUMMARY: Airrigation 10,491 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units D 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION irrigation in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 37% 53% 0% 11% % Total Vol 3,500 3,500 Awatershed 0.763 acres 23,452 8,602 12,340 0 2,509 Vol (ft3/yr) DRY %Imp, connected 68% % 100% 21% 23% 50% 7% % Total P 3,000 3,000 %Imp, disconnected 0% % 0.27 0.06 0.06 0.14 0.02 TP (kg/yr) 2,500 2,500 WATERSHED CNImp, disconnected 100% 39% 57% 0% 4% % Total Vol 2,000 2,000 CNpervious 61 27,482 10,638 15,741 0 1,104 Vol (ft3/yr) AVERAGE CW-TP 410 ppb 100% 23% 30% 45% 2% % Total P 1,500 1,500 CW-orthoP 100 ppb 0.32 0.07 0.10 0.14 0.01 TP (kg/yr) 1,000 1,000 Vbasin 3,136 ft 3 100% 49% 44% 0% 6% % Total Vol Vstorage 627 3 39,784 19,591 17,684 0 2,509 3 500 500
ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin ft 100% 26% 19% 52% 3% % Total P 0 0 Wbasin ft 0.46 0.12 0.09 0.24 0.01 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 4 ft 100% 42% 51% 0% 7% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope ratio 30,239 12,944 15,255 0 2,041 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1=on; 0=off 100% 23% 24% 49% 4% % Total P 3,500 3,500 Vbasin, initial 627 ft 3 0.35 0.08 0.08 0.17 0.01 TP (kg/yr) 3,000 3,000 45,000 0.50 40,000 0.45 2,500 2,500 0.40
35,000 2,000 2,000 /yr)
3 Change in 0.35 Change in 30,000 Storage 0.30 Storage 1,500 1,500 25,000 Evaporation 0.25 Sedimentation 20,000 1,000 1,000
Irrigation (kg/yr) P 0.20 Volume (ft Volume Irrigation
15,000 0.15 500 500 Overflow Volume (cubic feet) (cubic VolumeOverflow
Overflow feet) (cubic Basin in Volume 10,000 0.10 Overflow 0 0 5,000 0.05 1/1 4/11 7/20 10/28 0 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL DRY AVERAGE WET ALL WET YEAR (2007) 120% 120% 3,500 3,500
100% 100% 3,000 3,000 2,500 2,500 80% Change in 80% Storage Change in Storage 2,000 2,000 Evaporation 60% 60% Sedimentatio n 1,500 1,500 Irrigation Irrigation 40% 40% 1,000 1,000
Overflow Overflow P (% Watershed Load) Watershed (% P
20% 20% 500 500
Volume (% Watershed Flow) Watershed (% Volume
Overflow Volume (cubic feet) (cubic VolumeOverflow Volume in Basin (cubic feet) (cubic Basin in Volume 0% 0% 0 0 DRY AVERAGE WET ALL DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
88 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 26. Christ Presbyterian Church
INPUTS OUTPUTS SUMMARY: Airrigation 103,256 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units D 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION irrigation in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 43% 51% 0% 7% % Total Vol 45,000 45,000 Awatershed 9.69 acres 310,825 132,323 157,243 0 21,258 Vol (ft3/yr) DRY 40,000 40,000 %Imp, connected 71% % 100% 23% 21% 50% 6% % Total P 35,000 35,000 %Imp, disconnected 0% % 3.61 0.83 0.75 1.82 0.21 TP (kg/yr) 30,000 30,000 WATERSHED CNImp, disconnected 100% 51% 49% 0% 1% % Total Vol 25,000 25,000 CNpervious 61 364,291 184,107 178,304 0 1,880 Vol (ft3/yr) AVERAGE CW-TP 410 ppb 100% 30% 28% 41% 1% % Total P 20,000 20,000 CW-orthoP 100 ppb 4.23 1.27 1.19 1.72 0.05 TP (kg/yr) 15,000 15,000 Vbasin 42,515 ft 3 100% 58% 38% 0% 4% % Total Vol 10,000 10,000 Vstorage 8,503 3 527,292 303,879 202,155 0 21,258 3 5,000 5,000 ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin ft 100% 31% 16% 52% 2% % Total P 0 0 Wbasin ft 6.12 1.87 0.95 3.16 0.14 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 8 ft 100% 50% 46% 0% 4% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope ratio 400,802 206,770 179,234 0 14,799 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1=on; 0=off 100% 28% 21% 48% 3% % Total P 45,000 45,000 Vbasin, initial 21,257 ft 3 4.65 1.32 0.96 2.23 0.14 TP (kg/yr) 40,000 40,000 600,000 7.00 35,000 35,000
500,000 6.00 30,000 30,000 25,000 25,000 /yr) 5.00
3 Change in 400,000 Change in Storage Storage 20,000 20,000 Evaporation 4.00 300,000 Sedimentation 15,000 15,000 3.00
Irrigation (kg/yr) P Volume (ft Volume Irrigation 10,000 10,000 200,000
2.00 5,000 5,000 Overflow Volume (cubic feet) (cubic VolumeOverflow Overflow Overflow feet) (cubic Basin in Volume 100,000 1.00 0 0 1/1 4/11 7/20 10/28 0 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL DRY AVERAGE WET ALL WET YEAR (2007) 120% 120% 45,000 45,000 40,000 40,000 100% 100% 35,000 35,000 80% Change in 80% 30,000 30,000 Storage Change in Storage 25,000 25,000 Evaporation 60% 60% Sedimentatio n 20,000 20,000 Irrigation 40% 40% Irrigation 15,000 15,000
Overflow Overflow 10,000 10,000 P (% Watershed Load) Watershed (% P 20% 20%
Volume (% Watershed Flow) Watershed (% Volume 5,000 5,000
Overflow Volume (cubic feet) (cubic VolumeOverflow Volume in Basin (cubic feet) (cubic Basin in Volume 0% 0% 0 0 DRY AVERAGE WET ALL DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
Stormwater Harvesting and Reuse 89
Table 27. Van Cleve Park
INPUTS OUTPUTS SUMMARY: Airrigation 196,504 ft 2 From the To To To To Change in Annual Basin Dynamics Rainfall Year Units D 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION irrigation in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 4% 63% 15% 17% % Total Vol 16,000 16,000 Awatershed 13.29 acres 62,369 2,674 39,347 9,655 10,692 Vol (ft3/yr) DRY 14,000 14,000 %Imp, connected 7% % 100% 2% 27% 63% 8% % Total P %Imp, disconnected 6% % 0.72 0.01 0.19 0.46 0.06 TP (kg/yr) 12,000 12,000 WATERSHED CNImp, disconnected 92 100% 5% 84% 13% -2% % Total Vol 10,000 10,000 CNpervious 61 73,321 3,682 61,392 9,649 -1,402 Vol (ft3/yr) AVERAGE 8,000 8,000 CW-TP 410 ppb 100% 2% 35% 62% 1% % Total P 6,000 6,000 CW-orthoP 100 ppb 0.85 0.02 0.30 0.53 0.01 TP (kg/yr) 4,000 4,000 Vbasin 14,527 ft 3 100% 7% 76% 9% 8% % Total Vol Vstorage 1,929 3 108,733 8,134 82,699 9,655 8,245 3 2,000 2,000
ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin 113 ft 100% 3% 31% 63% 3% % Total P 0 0 Wbasin 56 ft 1.26 0.04 0.39 0.79 0.04 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 6 ft 100% 6% 74% 13% 8% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope 4 ratio 81,474 4,830 61,146 9,653 5,845 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1 1=on; 0=off 100% 2% 31% 62% 4% % Total P 16,000 16,000 Vbasin, initial 3,835 ft 3 0.95 0.02 0.29 0.59 0.04 TP (kg/yr) 14,000 14,000 120,000 1.40 12,000 12,000 100,000 1.20 10,000 10,000
/yr) 80,000 1.00 3 Change in Change in 8,000 8,000 Storage Storage 60,000 0.80 Evaporation Sedimentation 6,000 6,000 40,000 0.60 4,000 4,000
Irrigation (kg/yr) P Volume (ft Volume Irrigation
20,000 0.40 2,000 2,000 Overflow Volume (cubic feet) (cubic VolumeOverflow Overflow Overflow feet) (cubic Basin in Volume 0 0.20 0 0 DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 -20,000 0.00 Overflow Volume at Overflow Minimum Volume Basin DRY AVERAGE WET ALL WET YEAR (2007) 120% 120% 16,000 16,000
14,000 14,000 100% 100% 12,000 12,000 80% Change in 80% Storage Change in 10,000 10,000 60% Storage Evaporation 60% Sedimentatio 8,000 8,000 40% n Irrigation 6,000 6,000 40% Irrigation 20% 4,000 4,000
Overflow Overflow P (% Watershed Load) Watershed (% P 20%
Volume (% Watershed Flow) Watershed (% Volume 0% 2,000 2,000 Overflow Volume (cubic feet) (cubic VolumeOverflow DRY AVERAGE WET ALL feet) (cubic Basin in Volume -20% 0% 0 0 DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
90 Stormwater Harvesting: Accounting of Benefits and Feasibility
Table 28. Summit Preserve
INPUTS OUTPUTS SUMMARY: 2 Airrigation 653,400 ft From the To To To To Change in Annual Basin Dynamics Rainfall Year Units Dirrigation 0.5 Watershed Overflow Irrigation Evaporation Sedimentation Storage IRRIGATION in/day DRY YEAR (2009) Begin/End 5 to 9 month 100% 0% 91% 16% -7% % Total Vol 800,000 800,000 Awatershed 39 acres 1,268,425 0 1,156,858 204,684 -93,117 Vol (ft3/yr) DRY 700,000 700,000 %Imp, connected 72% % 100% 0% 31% 68% 1% % Total P %Imp, disconnected 0% % 14.73 0.00 4.59 9.99 0.15 TP (kg/yr) 600,000 600,000 WATERSHED CNImp, disconnected 100% 8% 97% 14% -19% % Total Vol 500,000 500,000 CNpervious 61 1,486,669 124,304 1,442,925 204,684 -285,244 Vol (ft3/yr) AVERAGE 400,000 400,000 CW-TP 410 ppb 100% 3% 32% 68% -3% % Total P 300,000 300,000 CW-orthoP 100 ppb 17.26 0.48 5.60 11.72 -0.54 TP (kg/yr) 200,000 200,000 Vbasin 706,892 ft 3 100% 16% 62% 10% 12% % Total Vol Vstorage 40,249 3 2,151,796 346,051 1,341,618 204,684 259,444 3 100,000 100,000
ft Vol (ft /yr) feet) (cubic Volume Overflow WET feet) (cubic Basin in Volume Lbasin 350 ft 100% 6% 21% 68% 6% % Total P 0 0 Wbasin 300 ft 24.98 1.42 5.18 16.89 1.49 TP (kg/yr) 1/1 4/11 7/20 10/28 STORAGE Dbasin 20 ft 100% 8% 84% 13% -5% % Total Vol Overflow Volume at Overflow Minimum Volume Basin Side slope 4 ratio 1,635,630 156,785 1,313,800 204,684 -39,639 Vol (ft3/yr) ALL AVERAGE YEAR (2011) Evaporation 1 1=on; 0=off 100% 3% 28% 68% 1% % Total P 800,000 800,000 Vbasin, initial 447,448 ft 3 18.99 0.64 5.12 12.87 0.37 TP (kg/yr) 700,000 700,000 2,500,000 30.00 600,000 600,000 25.00 2,000,000 500,000 500,000
/yr) 20.00
3 Change in 1,500,000 Change in 400,000 400,000 Storage Storage Evaporation 15.00 300,000 300,000 1,000,000 Sedimentation 10.00 200,000 200,000
Irrigation (kg/yr) P Volume (ft Volume Irrigation 500,000
5.00 100,000 100,000 Overflow Volume (cubic feet) (cubic VolumeOverflow Overflow Overflow feet) (cubic Basin in Volume 0 0.00 0 0 DRY AVERAGE WET ALL DRY AVERAGE WET ALL 1/1 4/11 7/20 10/28 -500,000 -5.00 Overflow Volume at Overflow Minimum Volume Basin WET YEAR (2007) 140% 120% 800,000 800,000
120% 700,000 700,000 100% 100% 600,000 600,000 80% 80% Change in Storage Change in 500,000 500,000 60% Storage 60% Evaporation Sedimentatio 400,000 400,000 40% 40% n Irrigation 300,000 300,000 20% Irrigation 20% 200,000 200,000 Overflow Overflow 0% Load) Watershed (% P DRY AVERAGE WET ALL
Volume (% Watershed Flow) Watershed (% Volume 0% 100,000 100,000 Overflow Volume (cubic feet) (cubic VolumeOverflow
-20% feet) (cubic Basin in Volume DRY AVERAGE WET ALL -40% -20% 0 0 1/1 4/11 7/20 10/28 Overflow Volume at Overflow Minimum Volume Basin 1. The irrigation and watershed areas can be independent 3. The watershed P concentrations and fractions can be set to site specific data 6. An average side slope can be used to account for the design of safety benches User notes: 2. The user must composite the CN for disconnected impervious 4. The user must define appropriate basin geometry or model results are invalid 7. The initial basin volume can be set by the user to account for winter dynamics 3. Turn off evaporation until all basin geometry is entered and valid5. The storage volume can be set to maintain any desired minimum pond depth 8. Model results assume regular maintenance to remove accumulated sediments!
Stormwater Harvesting and Reuse 91
PRELIMINARY COST ESTIMATES
Table 29. Preliminary Cost Estimates
Planning Level Cost Estimates (1)
Storage Irrigation 30 Year Removal (6) WQ retro-fit Underground Surface Pumping/ General contingency Professional services (3) Total cost Location (2) Size Storage cost (4) Area Irrigation cost Phosphorous Volume tank (5) pond (7) irrigation
ft^3 $/ft^3 $/ft^3 $ Lump sum acre $/acre $ % $ % $ $ $/lbs TP $/ac-ft
Benilde St. $ 1 76,168 $ 15,000 7.77 7,500 $ 58,303 20% $ 11,661 20% $ 11,661 $ 753 $ 384 Margaret 96,625 2 Pamela Park 2,155,327 $ 20,000 16.79 5,000 $ 83,942 20% $ 20,788 20% $ 20,788 $ 145,519 $ 338 $ 125
Edina Country $ $ 3 570,966 $ 50,000 112.45 20% 20% $ 10,000 na na Club 1 4,000 64,000 Edina Country 4 36,373 8 $ 290,984 $ 15,000 4.24 20% $ 58,197 20% $ 58,197 $ 422,378 $ 6,025 $ 3,049 Club 2 MCWD (New $ $ 5 3,136 10 $ 31,360 $ 15,000 0.24 15,000 $ 3,613 20% 20% $ 6,995 $ 3,868 $ 6,088 Office) 6,995 63,962 Christ 6 42,515 8 $ 340,120 $ 15,000 2.37 15,000 $ 35,556 20% $ 75,135 20% $ 75,135 $ 540,947 $ 2,564 $ 4,382 Presbyterian
7 Van Cleve Park 14,527 2 $ 29,054 $ 15,000 4.51 7,500 $ 33,833 20% $ 12,577 20% $ 12,577 $ 103,042 $ 1,770 $ 2,113
8 Summit Preserve 706,892 1.50 $ 1,060,338 $ 15,000 15.00 15,000 $ 225,000 20% $ 257,068 20% $ 257,068 $ 1,814,473 $ 1,525 $ 1,735
Notes
(1) These estimates only take into consideration capital costs; operation and maintenance costs are not included. These costs are cursory estimates. Further feasibility study is needed prior to doing one of these projects.
(2) These estimates assume clean soils that do not require remediation or special handling.
(3) Professional services includes: engineering, legal, geo-technical, and administrative.
(4) Disinfection may be wanted by property owner or required by local codes. This water quality retro-fit is the cost of class A UV filters. The Pamela Park retro-fit also incluces costs for hydraulic structures.
(5) These estimates include the costs of pavement removal and bituminous re-paving.
(6) For those sites with existing ponds, these calculations do not take into account TP removal to sedimentation or volume removal to evaporation because these removals occur under existing conditions.
(7) The costs for surface storage include: excavation and hauling as well as construction related costs like erosion control and restoration. For Summit Preserve the cost also includes a retaining wall and hydraulic structures.