VIRGINIA DCR STORMWATER
DESIGN SPECIFICATION No. 6

RAINWATER HARVESTING

VERSION 1.6
September 30, 2009

SECTION 1: DESCRIPTION

Rainwater harvesting systems intercept, divert, store and release rainfall for future use. The term rainwater harvesting is used in this specification, but it is also known as a cistern or rainwater harvesting system.  Rainwater that falls on a rooftop is collected and conveyed into an above or below ground storage tank where it can be used for non-potable water uses and on-site stormwater disposal/infiltration. Non-potable uses may include flushing of toilets and urinals inside buildings, landscape irrigation, exterior washing (e.g. car washes, building facades, sidewalks, street sweepers, fire trucks, etc.), supply for chilled water cooling towers, replenishing and operation of water features and water fountains, and laundry, if approved by the local authority.  Replenishing of pools may be acceptable if special measures are taken, as approved by the appropriate regulatory authority. 

In many instances, rainwater harvesting can be combined with a secondary (down-gradient) runoff reduction practice to enhance runoff reduction rates and/or provide treatment of overflow from the rainwater harvesting system.  Some candidate secondary practices include:. 

Section 4.3 provides more details on tank configurations, including the use of secondary practices.

In addition, the actual runoff reduction rates for rainwater harvesting systems are “user defined” based on tank size, configuration, demand, and use of secondary practices.  A Cistern Design Spreadsheet (CDS) is provided as a companion to this specification, and is discussed in more detail in Section 5.

The overall stormwater functions of the rainwater harvesting systems are described in Table 1.

SECTION 2: LEVEL 1 AND 2 DESIGN TABLES   

Rainwater harvesting system design does not have a level 1 or 2 design table. Runoff reduction credits are based on the total amount of annual internal water reuse, outdoor water reuse, and tank dewatering discharge achieved by the tank system in the Cistern Design Spreadsheet.

SECTION 3:  TYPICAL DETAILS

Figures 1 through 6 of Section 4.3 provide typical schematics of cistern and piping system configurations based on the design objectives (year-round internal use, external seasonal irrigation, etc.). 

Figures 7 through 9 of Section 4.4 provide typical schematics Cistern tank configurations based on the desired Treatment Volume (Treatment Volume only, channel protection, etc.).

SECTION 4:  PHYSICAL FEASIBILITY & DESIGN APPLICATIONS

A number of site-specific features influence how rainwater harvesting systems are designed and/or utilized. These should not be considered comprehensive and conclusive considerations, but rather some recommendations that should be considered during the planning phases of incorporating rainwater harvesting systems into site design.  Some of the key considerations include:

4.1 Site Conditions

These elevation drops will occur along the sloping lengths of the underground roof drains from roof drain leader downspouts at the building all the way to the cistern.  A vertical drop occurs within the filter before the cistern.   The cistern itself must be located sufficiently below grade and below the frost line forcing an additional elevation drop.  When the cistern is used for additional volume detention for channel and/or flood protection, an orifice may be included with a low invert specified by the design user.  An overflow will always be present within the system with an associated invert.  Both the orifice (if specified) and the overflow will drain the tank during large storms routing this water through an outlet pipe with varying lengths and slopes. 

All of these components of the rainwater harvesting system have an elevation drop associated with them.  The final invert of the outlet pipe must match the invert of the receiving mechanism (natural channel, storm drain system, etc.) that receives this overflow.  These elevation drops and associated inverts should be considered early in order to ensure that rainwater harvesting systems are feasible for a particular site.

Site topography and tank location will also affect the amount of pumping needed. Locating storage tanks in low areas will make it easier to route roof drains from buildings to cisterns, however it will increase the amount of pumping needed to distribute the harvested rainwater back into the building or to irrigated areas situated on higher ground. Conversely, placing storage tanks at higher elevations may require larger diameter roof drains with smaller slopes, however it will also reduce the amount of pumping needed for distribution.  In general, it is often best to locate the cistern close to the building, ensuring minimum roof drain slopes and cover over top of roof drain pipes is sufficient.

4.2  Stormwater Uses

The capture and re-use of rainwater can significantly reduce stormwater runoff volumes and pollutant loads. By providing a reliable and renewable source of water to end users, rainwater harvesting systems can also have environmental and economic benefits beyond stormwater management (increased water conservation, water supply during drought and mandatory municipal water supply restrictions, decreased demand on municipal or groundwater supply, decreased water costs for the end-user, potential for increased groundwater recharge, etc).  Rainwater harvesting systems can be combined with other rooftop disconnection practices, such as micro-infiltration practices (Spec No. 8), rain gardens and foundation planters (Spec No. 9), to enhance their runoff reduction and nutrient removal capability.  In this specification, these allied practices are referred to as “secondary runoff reduction practices.”

While non-potable uses of captured rainwater, such as those noted above, are most common, in some limited cases, rainwater can be treated to potable standards. This is predicated on the treatment methods and end use quality all meeting drinking water standards and regulations, and system approval by the Health Department and local governing authority.  Treating to potable standards may drive up installation and maintenance costs significantly.

4.3  Design Objectives and System Configurations

Many system variations can be conceived of to meet user demand and stormwater objectives.  This specification focuses on providing a design framework for addressing the water quality treatment volume (Tv) credit objectives and assessing compliance.  From a rainwater harvesting standpoint, there are numerous potential configurations that could be implemented. However, in terms of the goal of addressing the design treatment volume, this specification adheres to the following concepts in order to properly meet the stormwater volume reduction goals:

Therefore, the rainwater harvesting system design configurations presented in this specification are targeted for a continuous (year-round) utilization of rainwater through (i) internal use, and (ii) irrigation and/or infiltration. A brief description of six different configurations is provided below.

Figure 1. Configuration #1 - Year-round indoor use only

• Year-round indoor use and only seasonal outdoor irrigation use: The second configuration utilizes year round indoor use; however the outdoor irrigation use is only seasonal. Because there is no on-site secondary runoff reduction practice incorporated into the design for non-irrigation months, the system must be designed and treatment credit awarded for the interior use only. (It should be noted however, that the seasonal irrigation will provide an economic benefit in terms of water usage.)



Figure 2. Configuration #2 - Year-round indoor use and only seasonal outdoor use

• Year-round indoor use, seasonal outdoor irrigation, and periodic treatment in a secondary runoff reduction practice: The third configuration provides for a year-round internal non-potable water demand, and a seasonal outdoor, automated irrigation system demand. In addition, this configuration utilizes a secondary practice during non-irrigation months in order to yield a greater stormwater credit. However, the drawdown due to seasonal irrigation is compared to the secondary practice runoff reduction rate specified during non-irrigation months, and the lesser of the two is utilized for system modeling and for stormwater credit purposes.



Figure 3. Configuration #3 - Year-round indoor use, seasonal irrigation, and on-site treatment in secondary practice

• Year-round indoor use and approved year-round treatment in a secondary practice: The fourth configuration provides for a year-round internal non-potable water demand, and a year-round secondary practice. Because both practices are year-round, they are both included in the system model and treatment credit.  



Figure 4.  Configuration #4 - Year-round indoor use and approved year-round secondary practice

• Intermittent indoor and/or outdoor use and approved year-round secondary practice: The fifth configuration does not include meeting any indoor (non-potable use) or outdoor (irrigation) water demand since it is intermittent or seasonal. Rather, this configuration relies solely on the use of an approved secondary practice. The secondary practice is actually the primary runoff reduction mechanism, and represents the only runoff reduction credit (as it is the only year-round practice).



Figure 5. Configuration #5 - Intermittent (or no) indoor or outdoor use and approved year-round secondary practice

1. Approved year-round secondary practice only: The sixth configuration utilizes only an on-site secondary practice and does not re-use the rainwater to meet any on-site demand. The primary objective with this layout is to meet the stormwater treatment requirement.

                                                             
When a secondary practice is the primary drawdown practice, the storage volume (in gallons) associated with the treatment credit  must be a minimum of 50% of the roof area (in square feet).



Approved year-round secondary practice only:

 

1. The sixth configuration utilizes only an on-site secondary practice and does not re-use the rainwater to meet any on-site demand. The primary objective with this layout is to meet the stormwater treatment requirement.                                                       
   When a secondary practice is the primary drawdown practice, the storage volume (in gallons) associated with the treatment credit  must be a minimum of 50% of the roof area (in square feet).

4.4  Design Objectives and Tank Design Set-Ups

Pre-fabricated rainwater harvesting cisterns typically,range in size from 250 to over 30,000 gallons. There are three basic tank design set-ups to meet the various system configurations that are described in the Section 4.3.

1. The first tank (Figure 7) maximizes the available storage volume associated with the Treatment Volume to meet the desired level of Treatment Credit.  This layout also maximizes the storage that can be used to meet a demand.  An emergency overflow exists near the top of the tank as the only gravity release outlet device (not including the pump, manway or inlets). It should be noted that it is possible to address Channel Protection and Flood volumes with this tank configuration, but the primary purpose is to address the water quality Treatment Volume (Tv).



Figure 7. Tank Design 1 - Storage Associated with Treatment Volume only

• The second tank set-up (Figure 8) utilizes tank storage to meet Treatment Volume (Tv) objectives as well as utilizing an additional detention volume above the treatment volume space to also meet some or all of the Channel Protection and Flood Volume requirements.  An orifice outlet is provided at the top of the design storage for Tv and an emergency overflow at the top of the detention volume.  This specification only addresses the storage for Tv, however the Runoff Reduction spreadsheet in combination with other approved hydrologic routing programs may be used to model and size the Channel Protection and Flood (detention) volumes.



Figure 8. Tank Design 2, - Storage Associated with Treatment,Channel Protection and Flood Volume

1. The third tank design (Figure 9) creates a constant drawdown within the system. The small orifice at the bottom of the tank needs to be routed to an appropriately designed on-site secondary practice (e.g., rain garden, micro-scale infiltration, urban bioretention) that will allow the rainwater to be treated and allow for groundwater recharge over time.  The release should not be discharged to a receiving channel or storm drain without treatment, and maximum specified drawdown rates from this constant drawdown should be adhered to, as the primary function of the system is not for it to act as a detention facility. 

For the purposes of this tank design, the secondary practice shall be considered a component of the rainwater harvesting system with regard to the runoff reduction percentage received in the Runoff Reduction Spreadsheet.  In other words, the runoff reduction associated with the secondary practice shall not be added (or double-counted) to the rainwater harvesting percentage.  The reason for this is that the secondary practice is an integral part of a rainwater harvesting system with a constant drawdown. The exception to this would be if the secondary practice were also sized to capture and treat impervious and/or turf area beyond the area treated by rainwater harvesting (for instance, the adjacent yard or a driveway). In this case, only these additional areas should be added into the Runoff Reduction Spreadsheet to receive credit for the secondary practice.

While a small orifice is shown at the bottom of the tank in Figure 3, the orifice could be replaced with a pump that would serve the same purpose of conveying a limited amount of water to a secondary practice on a routine basis.



Figure 9. Tank Design 3, Constant drawdown, Storage Associated with Treatment, Channel Protection and Flood Volume

4.5. On-Site Treatment in a Secondary Practice

Recent rainwater harvesting system design materials do not include guidance for on-site stormwater infiltration or ‘disposal’.  The basic approach is to provide a dedicated secondary runoff reduction practice on-site that will ensure water within the tank will gradually drawdown at a specified design rate between storm events.  Secondary runoff reduction practices may include:

1. Rooftop Disconnection: Design Specification No. 1; excluding rain tanks and cisterns. This may include release to a compost-amended filter path
2. Front yard bioretention area: Design Specification No.1
3. Grass filter strip: Design Specification No. 2
4. Micro-bioretention (rain garden): Design Specification No. 9
5. Infiltration and Micro-infiltration: Design Specification No. 8
6. Grass channel:  Design Specification No. 3
7. Dry swale: Design Specification No. 10
8. Storage and release in foundation planter (Spec No. 9, Appendix A)
9. Underground infiltration soakaway pit. 

The secondary practice approach is useful when demand is not enough to sufficiently draw water levels in the tank down between storm events to achieve the desired treatment credit.  Of course, if demand for the harvested rainwater is relatively high, then a secondary practice may not be needed or desired. 

While design specifications are available for most of the secondary practices proposed, an “underground infiltration soak-away pit” (or infiltration facility – Design Specification 8) may prove useful in some situations and may be used in conditions where soil has moderate to high infiltration rates.  The soakaway pit must be properly designed to adequately infiltrate the controlled design release rate. Design approval is subject to the reviewing authority.

Use of a secondary practice may be particularly useful to employ in sites that utilize captured rainwater for irrigation during part of the year, but have no other use for the water during non-irrigation season months.  During non-irrigation months, credit cannot be realized unless on-site infiltration/treatment or another drawdown mechanism creates a year-round drawdown, as no stormwater benefit would be realized during non-seasonal periods.

Soil types, ground surface areas, release rates, methods of conveyance (gravity fed or pumped), time periods of operation and inverts should all be accounted and designed for to determine the disposal rate and sizing of the receiving mechanism (both storage volume and surface area).

4.6  System Components

There are six primary components of a stormwater rainwater harvesting system:

1. Rooftop Surface
2. Collection and Conveyance System (e.g. gutter and downspouts)
3. Pre-screening and First Flush Diverter
4. Storage Tank
5. Distribution System
6. Overflow, Filter Path or Secondary Runoff Reduction Practice

Each of these is discussed below:

1.   Rooftop Surface: The rooftop should be made of smooth, non-porous material with efficient drainage either from a sloped roof or an efficient roof drain system.  Slow drainage of the roof leads to poor rinsing and a prolonged first flush and can decrease water quality.  If the harvested rainwater will be used for potable uses, or uses with significant human exposure (e.g. pool filling, watering vegetable gardens), care should be taken in the choice of roof materials.  Some materials may leach toxic chemicals making the water unsafe for humans.  Rainwater can also be harvested from other impervious surfaces, such as parking lots and driveways, however this practice is much less common and will require more extensive pretreatment or treatment, as the quality of water is typically much lower.



Figure 10. Sample Rainwater harvesting system System Detail


2.  Collection and Conveyance System: The collection and conveyance system consists of the gutters, downspouts and pipes that channel stormwater runoff into storage tanks. Gutters and downspouts should be designed as they would for a building without a rainwater harvesting system. Aluminum, round bottom gutters and round downspouts are generally recommended for rainwater harvesting. Minimum slopes of gutters should be specified.  Gutters should, at a minimum, be sized with slopes specified to contain the 1” storm for treatment credit volume.  They should be designed to convey the 10 year storm, specifying size and minimum slope, if volume credit will be sought for channel protection.  In all cases, gutters should be hung at a minimum of 0.5% for 2/3rd of the length and at 1% for the remaining 1/3rd of the length.

Pipes (connecting downspouts to cistern tank) should be a minimum of 1.5% and sized/designed to convey the intended design storm.  In some cases, a steeper slope and larger sizes may be recommended and/or necessary to convey the required runoff.  Gutters and downspouts should be kept clean and free of debris and rust. 

3.   Pre-Treatment: Screening, First Flush Diverters and Filter Efficiencies:  Pre-filtration is required to keep sediment, leaves, contaminants and other debris from the system.  Leaf screens and gutter guards meet the minimal requirement for pre-filtration of small systems, although direct water filtration is preferred.  All pre-filtration devices should be low maintenance or maintenance-free. The purpose of pre-filtration is to significantly cut down on maintenance by preventing organic buildup in the tank, thereby decreasing microbial food sources. 

For larger tank systems, before rainwater enters the storage tank, the initial first flush diversion, must be diverted from the system. Designers should note that the term first flush in rainwater harvesting design is not the same term that had been historically used in the design of stormwater treatment practices. In this specification, the term “first flush diversion” is used to distinguish it from the traditional stormwater management “first flush” term, and can range anywhere from the first 0.02 to 0.06 inches of rooftop runoff.

A variety of first flush diverters are described below.  In addition to the initial first flush diversion, filters have an associated efficiency curve that estimates the percentage of rooftop runoff that will be conveyed through the filter to the storage tank.  If filters are not sized properly, a large portion of the rooftop runoff may be diverted and not conveyed to the tank at all.  For the 1” storm treatment volume, a minimum of 95% filter efficiency is required. This efficiency includes the first flush diversion.  The Cistern Design Spreadsheet, discussed more in Section 5, assumes a filter efficiency rate of 95% for the 1” storm. 

The diverted flows (first flush diversion and overflow from filter) must be directed to an acceptable pervious flow path, that will not cause erosion during a 2 year storm, or to an appropriate BMP on the property, for infiltration.  Preferably the diversion will be conveyed to the same secondary BMP in series practice that may be used for tank overflows.

1. First Flush Diverters: First flush diverters direct the initial pulse of stormwater runoff away from the storage tank. While leaf screens effectively remove larger debris such as leaves, twigs and blooms from harvested rainwater, first flush diverters can be used to remove smaller contaminants such as dust, pollen and bird and rodent feces (Figure 11). Simple first flush diverters require active management, by draining the first flush water volume to a pervious area following each rainstorm.  However, if water is to be used for indoor uses, this may be a preferred pre-treatment technique. Vortex filters may serve as an effective pre-tank filtration and first flush diverter.  
1. Leaf Screens: Leaf screens are mesh screens installed either in the gutter or downspout and are used to remove leaves and other large debris from rooftop runoff. Leaf screens must be regularly cleaned to be effective; if not maintained, they can become clogged and prevent rainwater from flowing into the storage tanks. Built-up debris can also harbor bacterial growth within gutters or downspouts (TWDB, 2005).
• Roof Washers: Roof washers are placed just ahead of storage tanks and are used to filter small debris from harvested rainwater (Figure 12). Roof washers consist of a tank, usually between 25 and 50 gallons in size, with leaf strainers and a filter with openings as small as 30-microns (TWDB, 2005). The filter functions to remove very small particulate matter from harvested rainwater. All roof washers must be cleaned on a regular basis.

1. Vortex Filters: For large scale applications, vortex filters can provide filtering of rooftop rainwater from larger rooftop areas.  The images below displays two examples of the filter.  The first image in Figure 13 provides a plan view photograph showing the interior of the filter with the top off.  The second image in Figure 14 displays the filter just installed in the field prior to the backfill.





4.   Storage Tanks: The storage tank is the most important and typically the most expensive component of a rainwater harvesting system. Cistern capacities range from 250 to over 30,000 gallons.  Multiple tanks can be placed adjacent to each other to increase overall storage on-site as needed and connected with pipes to balance water levels.  Typical rainwater harvesting systems for residential use are 1500 to 5000 gallons. Storage tank volumes are calculated to meet the water demand and stormwater treatment volume credit objectives, as described in Section 5 of this specification.
The tanks can be made of many materials and configured in various shapes, depending on the type used.  While many graphical representations of cisterns in this specification utilize that of a cylindrical shape, for the purposes of communicating visual concepts, cisterns can in fact be configured in many different shapes.  Many configurations are rectangular, ‘L’ shaped, or ‘step’ vertically to match the topography of a site.  Other factors that should be considered when designing a rainwater harvesting system and storage tank are:

1. Aboveground storage tanks should be UV and impact resistant. 
2. Belowground storage tanks must be designed to support the overlying sediment and any other anticipated loads (e.g., vehicles, pedestrian traffic).
3. Belowground rainwater harvesting systems should have a standard size manhole opening or equivalent to allow access for maintenance purposes. This access point should be secured/locked to prevent unwanted access. 
4. All rainwater harvesting systems should be sealed using a water safe, non-toxic substance.
5. Rainwater harvesting systems may be ordered from a manufacturer or can be constructed on site from a variety of materials. Table 2 compares the advantages and disadvantages of different storage tank materials.
6. Storage tank should be opaque or otherwise protected from direct sunlight to inhibit algae growth and screened to discourage mosquito breeding and reproduction.
7. Tanks should be accessible for cleaning, inspection, and maintenance.
8. Dead storage below the outlet to the distribution system and an air gap at the top of the tank should be added to the total volume. For gravity-fed systems a minimum of 6 inches of dead storage should be provided. For systems using a pump, the dead storage depth will be based on the pump specifications.
9. Any hookup to a municipal backup water supply should have a backflow prevention device to keep municipal water separate from stored rainwater, and/or as specified by code; this may include incorporating an air gap to separate the two supplies.

Table 2 discusses the advantages and disadvantages of various cistern materials.

Table 2: Advantages & Disadvantages of Various Cistern Materials (Cabell Brand, 2007, 2009)
Tank Material	Advantages	Disadvantages
Fiberglass	Commercially available, alterable and moveable, durable with little maintenance, light weight, integral fittings (no leaks), broad application	Must be installed on smooth, solid, level footing. pressure proof for below ground installation..expensive in smaller sizes
Polyethylene	Commercially available, alterable, moveable, affordable, available in wide range of sizes, can install  above or below ground little maintenance, broad application	Can be UV- degradable. Must be painted or tinted for above ground installations. Pressure proof for below ground installation
Modular Storage	Can modify to topography, can alter footprint and create various shapes to fit site, relatively inexpensive	Longevity may be less than other materials, higher risk of puncturing of water tight membrane during construction
Plastic Barrels	Commercially available Inexpensive	Low storage capacity (20 to 50 gallons) Limited application
Galvanized Steel	Commercially available, alterable and moveable, available in a range of sizes, film develops inside to prevent corrosion	Possible external corrosion and rust, must be lined for potable use Can only install above ground  soil pH may limit underground applications
Steel Drums	Commercially available, alterable and moveable	Small storage capacity, Prone to corrosion and rust can lead to leaching of metals, verify prior to reuse for toxics, water pH and soil pH may also limit applications
FerroConcrete	Durable and immoveable, suitable for above or below ground installations. Neutralizes acid rain	Potential to crack and leak Expensive
Cast in Place Concrete	Durable, immoveable, versatile, suitable for above or below ground installations. Neutralizes acid rain	Potential to crack and leak. Permanent Will need to provide adequate platform and design for placement in clay soils.
Stone or concrete Block	Durable and immoveable, keeps water cool in summer months	Difficult to maintain Expensive to build




The images below in Figures 15 to 17 display three examples of various materials and shapes of cisterns, discussed in the categories above.



Figure 15. Example of Multiple Fiberglass Cisterns in Series



Figure 16. Example of two Polyethylene Cisterns



Figure 17. Example of Modular Units

5.   Distribution Systems: Most distribution systems require a pump to convey harvested rainwater from the storage tank to its final destination, whether inside the building, the automated irrigation system or gradually discharge to an onsite runoff reduction practice following a storm event. The rainwater harvesting system should be equipped with an appropriately sized pump that produces sufficient pressure for all end-uses.  Separate plumbing, labeled as non-potable, is sometimes required by the municipality.

The typical pump and pressure tank arrangement consists of a multistage centrifugal pump, which draws water out of the storage tank and sends it into the pressure tank, where it is stored for distribution. When water is drawn out of the pressure tank, the pump kicks on and supplies additional water to the distribution system. The backflow preventer is required to separate harvested rainwater from mains.

Distribution lines from the rainwater harvesting system should be buried beneath the frost line.  Lines from the rainwater harvesting system to the building should have shut-off values that are accessible when snow cover is present. A drain plug or cleanout sump, also draining to a pervious area, should be installed to allow the system to be completely emptied, if required for any reason.  Above ground outdoor pipes should be insulated or heat wrapped to ensure operation during winter.

6.   Overflow, Filter Path and Secondary Runoff Reduction Practice : An overflow system should be included in the rainwater harvesting system design in order to handle an individual storm event that exceeds the capacity of the tank or in the case that multiple storms occur in succession to fill the rainwater harvesting system storage capacity. Overflow pipes should have a capacity equal to or greater than the inflow pipe(s) and have a diameter and slope sufficient to drain the cistern maintaining an adequate freeboard height. The overflow pipe should be screened to prevent access to the tank by rodents and birds. 
The filter path is a pervious or grass corridor that extends from the overflow to the next runoff reduction practice, the street, adequate existing or proposed channel or the storm drain system.  The filter path shall be graded to have a slope such that sheet flow conditions are maintained. If compacted or impermeable soils are present along the filter path, compost amendments may be needed (see Stormwater Design Specification No. 4). It is also recommended that the filter path be used for first flush diversions.

In many cases, rainwater harvesting system overflows are directed to a secondary runoff reduction practice to boost overall runoff reduction rates. These options are addressed in Section 1.3.

SECTION 5: DESIGN CRITERIA

5.1. Sizing of Rainwater Harvesting Systems

The rainwater harvesting cistern sizing criteria presented in this section was developed using best estimates of indoor and outdoor water demand, long term rainfall data and rooftop capture area using a spreadsheet model (Lawson and Forasté, 2009) The Cistern Design Spreadsheet is primarily intended to provide guidance in sizing cisterns and to quantify the runoff reduction volume and credit for input into the Runoff Reduction Spreadsheet for stormwater management purposes.  A secondary objective of the spreadsheet is increasing beneficial uses of the stored water, utilizing it as a valuable natural resource, which few other stormwater BMPs can realize.  

5.2. Incremental Design Volumes within Cistern

Rainwater tank sizing is determined by accounting for varying precipitation levels, captured rooftop runoff, first flush diversion (through filters) and filter efficiency, low water cut-off volume, dynamic water levels at the beginning of various storms, storage needed for treatment volume (permanent storage), storage needed for channel protection and flood volume (detention storage), seasonal and year-round demand use and objectives, overflow volume, and freeboard volumes above high water levels during very large storms.  See Figure 18 for a graphical representation of these various incremental design volumes.

This specification does not provide design guidance for sizing the Channel and Flood Protection volume, but rather provides guidance on sizing for the 1” target storm Treatment Volume (Tv) Credit.  See Chapter 11 of the Handbook: Uniform Stormwater BMP Sizing Criteria for more information on design volumes and sizing criteria associated with various target storm events.

Note that the Treatment Volume is different from the “Storage Associated with the Treatment Volume”.  The Treatment Volume, as defined by DCR in Table 11.2, Chapter 11, is calculated by multiplying the “water quality” target rainfall depth (one inch) with a composite of three site cover runoff coefficients (forest, disturbed soils and impervious cover).  In the case of rainwater harvesting, because only rooftop surfaces are captured, only one runoff coefficient is applicable, impervious cover.  Therefore, the only variable for Treatment Volume is surface area captured. 



Figure 18. Incremental Design Volumes associated with tank sizing

The “Storage Associated with the Treatment Volume” is the storage within the tank that is modeled and available for re-use.  While the Treatment Volume will remain the same for a specific rooftop capture area, the Storage Associated with the Treatment Volume may vary depending on demand and runoff reduction credit objectives.  It includes the variable water level at the beginning of a storm and the low water curt off volume that is necessary to maintain for pumping requirements.

5.3 Cistern Design Spreadsheet (CDS)

This specification is intimately linked with the Cistern Design Spreadsheet (CDS).  The spreadsheet utilizes daily rainfall data from September 1st, 1977 to September 30th, 2007 to model performance parameters of the cistern under varying rooftop capture areas, demands on the system and tank size. The precipitation data is the same that was utilized by the Center for Watershed Protection (CWP) to determine the 90th percentile 1” target storm event, as presented and explained in Figure 11.1 of the Handbook, Chapter 11.  Precipitation data for four different regions throughout Virginia can be selected for use within the model.

1. Richmond International Airport
2. Reagan Airport
3. Lynchburg Regional Airport
4. Millgap 2NNW near Harrisonburg

 
A runoff coefficient of 0.95 for rooftop surfaces and a filter efficiency rate of 95% for the 1” storm are assumed.  It is assumed that filters are to be installed on all systems and that the first flush diversion is incorporated into the filter efficiency.  The remaining precipitation is then added to the water level that existed in the cistern the previous day, with all of the total demands subtracted on a daily basis. If any overflow is realized, the volume is quantified and recorded.  If the tank runs ‘dry’ (reaches the cut off volume level), then the volume in the tank is fixed at the low level and a dry frequency day recorded.  The full or partial demand met in both cases is quantified and recorded.  A summary of the water balance for the system is provided below.

Water Contribution:

• Precipitation to rooftop: Volume of water contributing to rainwater harvesting system is a function of rainfall and rooftop area captured, as defined by designer. 
• Municipal Backup (optional):  In some cases, the designer may choose to install a municipal backup water supply to supplement tank levels. Note that municipal backups may also be connected post-tank (i.e. connection is made to the non-potable water line that is used for pumping water from the tank for re-use), thereby not contributing any additional volume to the tank.

Water Losses:

• Rooftop Runoff Coefficient – The rooftop is estimated to convey 95% of the rainfall that lands on it’s surface, Rv = 0.95
• First Flush Diversion – The first 0.02-0.06 inches of rainfall that is directed to filters is diverted from the system in order to not clog it with debris.  This value is assumed to be contained within the filter efficiency rate.
• Filter Efficiency – Each filter has an efficiency curve associated with the rate of runoff and size of storm that it will receive from a rooftop.  It is assumed that, after the first flush diversion and loss of water due to filter inefficiencies, the remainder of the 1” storm will be successfully captured.  This means that, a minimum of 95% of the runoff from a 1” storm should be conveyed.  The filter efficiency value is adjustable and can be modified as an input value in the CDS, but should remain a minimum of 95%.  Some localities may require that a minimum filter efficiency for a larger storm event is met (e.g. minimum 90% filter efficiency for 10 year storm), depending on design objectives and local reviewing agency policy.
• Drawdown (Runoff Reduction Volume) – This is the stored water within the cistern that is re-used or directed to a secondary runoff reduction practice.  It is the volume of runoff that is reduced from the rooftop drainage area.  This is the water loss that grants rainwater harvesting the Runoff Reduction Volume Credit.
• Overflow – For the purposes of addressing treatment volume (not addressing channel or flood protections volumes), both orifice outlets for detention and emergency overflows are treated the same.  This is the volume of water that may be lost during large storm events or successive precipitation events.

See Appendix A.1 for a detailed description of Spreadsheet Inputs

5.4. Results for all Precipitation Events

The performance results of the rainwater harvesting system for all days during the entire period modeled, including the full spectrum of precipitation events, is included in the “Results” tab.  This tab is not provided in order to address Runoff Reduction Volume Credit, but rather may be a useful tool in assisting the user to realize the performance of the various rainwater harvesting system sizes with the design parameters and demands specified.

• Demand Met: The demand met for various size cisterns and rooftop area/demand scenarios is reported.  A graph displaying the percentage of demand met for various cistern sizes is provided in this tab.  Normally this graph assists the user in understanding the relationship between cistern sizes and optimal/diminishing returns.  An example is provided below in Figure 19.


19. Percent Demand Met Vs. Storage for Re-use (Example)

At some point, larger cisterns no longer provide significant increases in percentages of demand met.  Conversely, the curve informs the user when a small increase in cistern size can yield a significant increase in the percentage of time demand is met.   

• Dry Frequency:  Another useful measure is the dry frequency.  If the cistern is dry a substantial portion of the time, this can inform that user that they may want to decrease the size of the cistern, decrease the demand on the system or they may want to explore capturing more rooftop area to provide a larger supply, if feasible.  It can also provide useful insight for the designer to determine whether they should incorporate a municipal backup supply if they need to ensure water supply through the system at all times.
• Overflow Frequency: A metric of both overflow frequency and average volume per year for the full spectrum of rainfall events is provided.  This will inform the user on the design parameters and magnitude of demand and associated performance of the system.  If the system overflows at a high frequency, then the designer may want to increase the size of the cistern, decrease the rooftop area captured, or consider other mechanisms that could increase drawdown (e.g. increase area to be irrigated, incorporate or increase on-site infiltration, etc.). 
• Inter-relationships and Curves of Diminishing Returns: Plotting various performance metrics against one another can be very informative and reveal relationships that were previously not evident.  One such inter-relationship is the percentage of demand met versus tank size compared to percentage of overflow frequency versus tank size on the same graph.  There is a range of cistern sizes that tends to emerge informing the Designer where a small increase or decrease in tank size can have a significant impact on dry and overflow frequency.  Conversely, outside this range, changes in cistern sizes would yield small changes to dry and overflow frequencies, yet yield a large trade-off compared to cost of rainwater harvesting systems. 

5.5. Results for Precipitation Events of 1” or less

The amount of rooftop runoff volume that the tank can capture and use and/or drawdown for all precipitation events of 1” or less is also quantified and recorded.  These results are presented on the “Results Treatment Volume” tab.  It is with this information that the Treatment Credit is calculated.  The primary result from this tab is the Treatment Volume Credit that can be transferred and used as input to the Runoff Reduction spreadsheet. 

1. Treatment Volume Credit: A series of Treatment Volume Credit values are calculated for multiple sizes of cisterns.  A trade-off curve plots these results which allows for a comparison of the credit earned versus cistern size.  While smaller tanks may yield less credit than larger tanks, they are more cost effective.  Conversely, while larger tanks yield more credit, they are more costly.  The curve assists the user to choose the appropriate size based on their objectives and site needs, as well as realizing the rate of diminishing returns.

The Runoff Reduction and Treatment Volumes are also quantified, however these results will automatically be calculated in a similar manner on the Runoff Reduction spreadsheet with the use of the credit earned.  Therefore, only the credit needs to be transferred, not the volumetric results.

• Overflow Volume from 1” storm: The frequency that the cisternoverflows and the average volume that it overflows per year as a result of precipitation events of 1” or less are also reported in this tab.  A chart of the Treatment Volume Credit and Overflow Frequency for the 1” storm versus storage size is provided, such as in the example shown below in Figure 20.



Figure 20. Percent Runoff Reduction Credit Vs. Storage for Re-use(Example)

These plotted results enable an establishment of a trade-off relationship between these two performance metrics.  For instance, in this case, a 20,000 gallon cistern optimizes the runoff reduction credit and the overflow frequency (near the inflection point of both curves).

5.6. Results from Cistern Design Spreadsheet to be transferred to Runoff Reduction Spreadsheet

There are two results from this Cistern Design spreadsheet that are to be transferred to the Runoff Reduction Spreadsheet. They are: 

• First Value to Transfer: Once the cistern storage volume associated with the Runoff Reduction Credit has been selected, simply transfer that credit amount into the Runoff Reduction Spreadsheet column called “Credit” in the “2.f. To Rain Barrel, Rainwater harvesting system, Cistern” row in the blue cell (cell F28).
• Second Value to Transfer: Then enter the rooftop area that was used in the Cistern Design Spreadsheet in the same row into the “Credit Area (acres)” column in the blue cell.

See Appendix A.2 for STEP BY STEP INSTRUCTIONS for using the Cistern Design Spreadsheet.

5.7. Completing the Sizing Design of the Cistern

• Low Water Cutoff Volume (Included): A dead storage area must be included so that the pump will not run the tank dry.  This volume is included within the Cistern Design Spreadsheet volume modeled.
• Cistern Storage Associated with Treatment Volume (Included): This is the volume that was designed for with the Cistern Design Spreadsheet.
• Adding Channel Protection and Flood Volumes (Optional): Additional detention volume may be added above and beyond the Cistern Storage Associated with the Treatment Volume for Channel Protection and Flood Volumes.  Typical routing software programs may be used to design for this additional volume. The local reviewing authority has the option of accepting an adjusted curve number, accounting for the volume that has already been reduced as a result of the storage provided within the storage for the Treatment Volume (methodology as presented in the runoff reduction spreadsheet), or requiring that the system be modeled assuming that the Storage associated with the Treatment Volume is full

4.   Adding Overflow and Freeboard Volumes (Required): An additional volume above the emergency overflow must be provided in order for the tank to allow very large storms to pass.  Above this overflow water level will be an associated freeboard volume.  This volume must account for a minimum of 5% of the overall tank size, however sufficient freeboard should be verified for large storms.  These volumes need to be added to the overall size of the cistern tank.

Adding all of the incremental volumes above yields the total size of the cistern tank:

Total Cistern Size = 1 + 2 + 3 + 4

See Appendix A.3 for more notes relating to use and development of the spreadsheet and documentation on the methodology used.

5.8. Design for Potable Water Calculations

In situations with insufficient potable water supply, rainwater can be treated and used for potable water supply subject to state and local health requirements (The Virginia Department of Health maintains regulations pertaining to reuse of water for potable uses). This rainwater harvesting system use is not covered in this specification, although there is growing interest in using harvested rainwater for potable drinking water. If this use is permitted by the appropriate public health authority, and the rainwater harvesting system is equipped with proper filtering equipment, the increased water re-use rate would reduce the demand on municipal water systems sharply.   It would also enable more standard plumbing system, since potable and non-potable water would no longer need to be separated.

 

5.9. Rainwater Harvesting Material Specifications  

The basic material specifications for rainwater harvesting systems are presented in Table 3. Designers should consult with experienced rainwater harvesting system and irrigation installers on the choice of recommended manufacturers of prefabricated tanks and other system components.

Table 3: Design Specifications for Rainwater harvesting systems
Item	Specification
Gutters and Downspout	Materials commonly used for gutters and downspouts include polyvinylchloride (PVC) pipe, vinyl, aluminum and galvanized steel. Lead should not be used as gutter and downspout solder as rainwater can dissolve the lead and contaminate the water supply. The length of gutters and downspouts is determined by the size and layout of the catchment and the location of the storage tanks. Include needed bends and tees.
Pre- Treatment	At least one of the following (all rainwater to pass through pre-treatment) first flush diverter vortex filter roof washer leaf and mosquito screen (1 mm mesh size)
Storage Tanks	Materials used to construct storage tanks should be structurally sound. Tanks should be constructed in areas of the site where native soils can support the load associated with stored water. Storage tanks should be water tight and sealed using a water safe, non-toxic substance. Tanks should be opaque to prevent the growth of algae. Re-used tanks should be fit for potable water or food-grade products. Underground rainwater harvesting systems should have a minimum of 18-24” of soil cover and be located below the frost line. The size of the rainwater harvesting system(s) is determined during the design calculations.
Note:  This table does not address indoor systems or pumps.




SECTION 6: REGIONAL & SPECIAL CASE DESIGN ADAPTATIONS

6.1. Karst Terrain  

Above-ground rainwater harvesting systems are a preferred practice in karst, as long as the rooftop surface is not designated as a stormwater hotspot.

6.2. Coastal Plain

Above-ground rainwater harvesting systems are a preferred practice in the coastal plain, since they avoid the flat terrain, low head and high water table conditions that constrain other stormwater practices.

6.3. Steep Terrain 

Rainwater harvesting systems are ideal in areas of steep terrain.

6.4. Winter Performance:

Rainwater harvesting systems have a number of components that can be impacted by freezing winter temperatures. Designers should give careful consideration to these conditions to prevent system damage and costly repairs.

For above-ground systems, winter-time operation may be more challenging, depending on tank size and whether heat tape is used on piping. If not protected from freezing, these rainwater harvesting system systems must be taken offline for the winter and stormwater treatment credit may not be granted for the practice. At the start of the winter season, vulnerable above-ground systems, that have not been designed to incorporate special precautions should be disconnected and drained. It may be possible to reconnect the former roof leader systems for the winter.

For below-ground and indoor systems, downspouts and overflow components should be checked for ice blockages during snowmelt events.

6.5. Linear Highway Sites

Rainwater harvesting systems are generally not applicable for linear highway sites

SECTION 7:  RAINWATER HARVESTING SYSTEM CONSTRUCTION
SEQUENCE & INSPECTION

7.1. Construction Sequence

It is advisable to have a single contractor to install the rainwater harvesting system, outdoor irrigation system and secondary runoff reduction practices. The contractor should be familiar with rainwater harvesting system sizing, installation, and placement. A licensed plumber is required to install the rainwater harvesting system components to the plumbing system.

A standard construction sequence for proper rainwater harvesting system installation is provided below, which can be modified to reflect different rainwater harvesting system applications or expected site conditions.  

1. Choose tank location on the site
2. Route all downspouts or roof drains to pre-screening devices and first flush diverters
3. Properly Install tank
4. Install pump (if needed) and piping to end-uses (indoor, outdoor irrigation, or tank dewatering release)
5. Route all pipes to the tank
6. Stormwater should not be diverted to the rainwater harvesting system until the overflow filter path has been vegetatively stabilized

7.2. Construction Inspection
The following items should be inspected prior to final sign-off and acceptance of a rainwater harvesting system:
 

• Rooftop area matches plans
• Diversion system is properly sized and installed
• Pretreatment system is installed
• Mosquito screens are installed on all openings
• Overflow device is directed as shown on plans
• Rainwater harvesting system foundation is constructed as shown on plans
• Catchment area and overflow area are stabilized
• Secondary runoff reduction practice(s) installed as per plans

SECTION 8:  RAINWATER HARVESTING SYSTEM MAINTENANCE

8.1. Maintenance Agreements

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program.  The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.    

All rainwater harvesting systems must be covered by a drainage easement to allow inspection and maintenance. The easement should include the tank, the filter path and any secondary runoff reduction practice. If the tank is located in a residential private lot, its existence and purpose shall be noted on the deed of record. Homeowners will need to be provided a simple document that explains the purpose of the rainwater harvesting system and routine maintenance needs. Legally binding maintenance agreements should specify the property owner’s primary maintenance responsibility, require homeowners to pay to have their system inspected by a qualified third party inspector, and authorize local agencies to access the property for inspection or corrective action in the event this is not done.

8.2. Maintenance Inspections 

All rainwater harvesting systems components should be inspected by the property owner in the Spring and the Fall each year. A comprehensive inspection by a qualified third party inspector should occur every third year.

8.3. Rainwater harvesting system Maintenance Schedule

Maintenance requirements for rainwater harvesting systems vary according to use. Systems that are used to provide supplemental irrigation water have relatively low maintenance requirements, while systems designed for indoor uses have much higher maintenance requirements. Table 4 describes routine maintenance tasks to keep rainwater harvesting systems in working condition.

Table 4: Suggested Maintenance Tasks for Rainwater harvesting systems
Activity	Frequency
Keep gutters and downspouts free of leaves and other debris	O: Twice a year
Inspect and clean pre-screening devices and first flush diverters	O: Four times a year
Inspect and clean storage tank lids, paying special attention to vents and screens on inflow and outflow spigots. Check mosquito screens and patch holes or gaps immediately	O: Once a year
Inspect condition of overflow pipes, overflow filter path and/or  secondary runoff reduction practices	O: Once a year
Inspect tank for sediment buildup	I: Every third year
Clear overhanging vegetation and trees over roof surface	I: Every third year
Check integrity of backflow preventer	I: Every third year
Inspect structural integrity of tank, pump, pipe and electrical system	I: Every third year
Replace damaged or defective system components	I: Every third year
O = Owner   I = qualified third party inspector

SECTION 9: COMMUNITY AND ENVIRONMENTAL CONCERNS

Although rainwater harvesting is an ancient practice, it is enjoying a revival due to the inherent quality of rainwater and the many beneficial uses that it can provide (TWDB, 2005). Some common concerns associated with rainwater harvesting that must be addressed during design include:

• Winter Operation: Rainwater harvesting systems can be used throughout the year if they are located underground or indoors to prevent problems associated with freezing, ice formation and subsequent system damage. Alternately, an outdoor system can be used seasonally, or year round if special measures and design considerations are incorporated. See Section 6.4 for further guidance on winter operation of rainwater harvesting systems.
• Local Plumbing Codes:  Designer and plan reviewers should consult local building codes to determine if they explicitly allow the use of harvested rainwater for toilet and urinal flushing. Rainwater harvesting systems should be required to have backflow preventers or air gaps to keep harvested water separate from the main water supply, in the cases where a municipal backup supply is used.  Pipes and spigots using rainwater must be clearly labeled as non-potable.
• Mosquitoes: In some situations, poorly designed rainwater harvesting systems can create habitat suitable for mosquito breeding and reproduction. Designers should provide screens on above- and below-ground tanks to prevent mosquitoes and other insects from entering the tanks. If screening is not sufficient in deterring mosquitoes, dunks or pellets containing larvicide can be added to cisterns when water is intended for landscaping use.
• Child Safety: Above grade home rainwater harvesting systems cannot have unsecured openings large enough for children to enter the tank. For underground cisterns, manhole access should be secured to prevent unwanted access.

SECTION 10: PLAN SUBMITTAL REQUIREMENT
AND CHECKLIST RECOMMENDATIONS

It is highly recommended that designers of rainwater harvesting systems coordinate design efforts and communicate intent to both site designers and building Architects as rainwater harvesting systems link building to site.  The effectiveness of such systems, in terms of use for demand and as a stormwater management tool is also highly dependent on the efficiency of capturing and conveying rainwater from building rooftop to storage tank. 

The following lists are recommended items that plan reviewers may want to consider and/or require for submittals of rainwater harvesting systems being used as a stormwater management tool.  To ensure effectiveness of design, the following items should be considered for inclusion with plan submittals:

A. Incorporation of Rainwater Harvesting System into Site Plan Grading and Storm Sewer Plan construction documents:

1.  Roof plan of building that will be used to capture rainwater showing slope 
     direction and roof material. 
 
      2.  Display downspout leaders from rooftops being utilized to capture rainwater.
 

1. Storm drain pipe layout (pipes between building downspouts and tank) in plan

            view specifying material, diameter, slope and lengths to be included on typical  
            grading and utilities or storm sewer plan sheets.

4.  Detail or note specifying minimum gutter size, shape configuration and slope to
     convey rainwater

B. Rainwater Harvesting System Construction Document sheet to include:

1. Cistern or Storage Unit material, dimensions and scaleable detail. (Cut sheet detail from Manufacturer if appropriate)
2. Specific Filter performance specification and filter efficiency curves included.  Runoff estimates from rooftop area captured for 1” storm should be estimated and compared to filter efficiencies for 1” storm.  It is assumed here that first flush diversion is included in filter efficiency curves.  A minimum of 95% filter efficiency for treatment volume credit should be met.  If this value is altered (increased) in cistern design spreadsheet, the value should be reported.  Filter curve cut sheets are normally available from the manufacturer.
3. Specified material and diameter of inflow and outflow pipes.
4. Inverts of orifice outlet, emergency overflows, receiving secondary practice or on-site infiltration facility.
5. Incremental volumes specified for: a.) low water cut-off volume level, b.) storage volume associated with Treatment Volume Credit, c.) storage volume associated with Channel Protection Volume (if applicable), d.) storage volume associated with Flood Protection Volume (if applicable), e.) overflow freeboard volume.
6. Cross section of storage unit displaying inverts associated with incremental volumes (if requested by reviewer

C. Supporting Calculations and Documentation

1.  Provide drainage area map delineating rooftop area to be captured indicating SF,
     1” storm, 1 year and 10 year storm peak discharge values on plan (11x17 is 
      sufficient).

2.  Provide calculations showing that gutter at specified size and slope will convey
     design storm, as specified by regulatory authority.

3.  Provide calculations showing that roof drains at specified size, slope and material
      will convey design storm, as specified by regulatory authority.

        4.  Cistern Design Spreadsheet: Printout of input tab as modeled.
      

1. Cistern Design Spreadsheet: Printout of Results-Treatment Volume Credit tab as  modeled
2. Cistern Design Spreadsheet: Printout of Results tab, as mode

D. Stormwater Management Forms

1.  Owner to treat rainwater harvesting system as they would any other stormwater 
   management facility.  If stormwater management agreement forms are required in  
   jurisdiction, then the same form should be submitted for rainwater harvesting 
    system.

2. Agreement Form or Note on Plans should be included to ensure the minimum 
    demand that was specified in the stormwater management plan submittal 
    documents are being met.  Likewise, if the property (and rainwater harvesting 
    system) is transferred to a different owner, the system should continue to ensure a 
    year-round drawdown.  If year-round drawdown is not being met as specified, an 
    alternative stormwater management plan may be required. 

SECTION 11: DESIGN REFERENCES

The following references and resources were used to develop this master specification:

Cabell Brand Center. 2009. Virginia Rainwater Harvesting Manual, Version 2.0. Salem, VA. (Draft Form) http://www.cabellbrandcenter.org

Cabell Brand Center. 2007. Virginia Rainwater Harvesting Manual. Salem, VA.
http://www.cabellbrandcenter.org

Center for Watershed Protection (CWP). 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Ellicott City, MD.

City of Portland, Environmental Services. 2004. Portland Stormwater Management Manual. Portland, OR. http://www.portlandonline.com/bes/index.cfm?c=dfbcc

City of Tucson, AZ. 2005. Water Harvesting Guidance Manual. City of Tucson, AZ. Tucson, AZ.

Coombes, P. 2004. Water Sensitive Design in the Sydney Region. Practice Note 4: Rainwater Tanks. Published by the Water Sensitive Design in the Sydney Region Project.
http://www.wsud.org/planning.htm

Credit Valley Conservation. 2008. Credit River Stormwater Management Manual. Mississauga, Ontario

Forasté, A. and S. Lawson. 2009. Cistern Design Spreadsheet, McKee-Carson, Inc., Rainwater Management Systems, Inc., and Center for Watershed Protection, Inc.

Gowland, D. and T. Younos. 2008. Feasibility of Rainwater Harvesting BMP for Stormwater Management. Virginia Water Resources Research Center. Special Report SR38-2008. Blacksburg, VA

North Carolina Division of Water Quality. 2008. Technical Guidance: Stormwater Treatment Credit for Rainwater Harvesting Systems. Revised September 22, 2008. Raleigh, NC.  

Northern Virginia Regional Commission. 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia

Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD

Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD www.chesapeakestormwater.net

SECTION 12: APPENDICES

APPENDIX A.1. Cistern Design Spreadsheet Inputs

The spreadsheet model requires the following user inputs: 

• Regional location – Indicate the region that is closest to where the practice is being installed.  Rainfall data associated with that region will automatically provide the relevant precipitation data for the design storm for that area. 
• Roof area – The user must estimate the total rooftop area that will be captured for contribution to the system; this combined with the target storm (1”) yields the volume of rooftop runoff to be managed.
• Irrigation use –  The user must supply the total pervious area (in SF) that will be irrigated; the spreadsheet will automatically calculate the demand based on a 1” per week watering during the appropriate season, unless user specifies different watering rate.  The user can specify a start and end date in the year to specify season (i.e. March 30th to September 1st).  If an on-site infiltration system is to be designed, the lesser drawdown rate (irrigation or on-site infiltration during off-season) shall be used to quantify the treatment volume credit.
• Indoor use – The user then needs to define the parameters relating to indoor use of water; the spreadsheet will automatically calculate the demand as described below:
  • Number of bedrooms – the user enters the number of bedrooms in the home; the spreadsheet uses the same approach that currently determines drainfield size by estimating the use required to accommodate 1.5 people per bedroom
  • Laundry use – the user selects either yes/no as to whether harvested rainwater will be used for laundry; the spreadsheet calculates laundry use as 1 load per person per week with an estimated water usage of 20 gallons per load (the upper end of use for Energy Star washers), with the number of people determined by the number of bedrooms determined above
  • Toilet use – the user selects either yes/no about whether harvested rainwater will be used for toilets; the spreadsheet calculates use based on a low flow toilet (1.2 gallons per flush) with three flushes per person per day, with the number of people determined by the number of bedrooms determined above
  • Optional additional input – the user may enter an additional demand, such as bus or fire truck washing, street sweeper filling, etc.
• Chilled Water Cooling Towers - The user may enter a quantity of water that will be needed for use in chilled water cooling towers.
• Secondary Runoff Reduction Practice Drawdown – A cell is provided to enter an additional drawdown for secondary practices.  This rate will be specified by the designer and based on a practice that has been designed to properly accept and infiltrate, store, and/or treat this drawdown amount.

APPENDIX A.2. STEP BY STEP INSTRUCTIONS
for Using the Cistern Design Spreadsheet (CDS)

TAB 1: INPUT

• Select Region in drop down menu that is located closest to proposed site.
• Enter rooftop area to be captured and routed to cistern (SF).
• Enter Irrigation data as described in Section 6.3: Spreadsheet inputs.
• Enter Indoor Demand – Flushing toilets/urinals as described in Section 6.3.
• Enter Indoor Demand – Laundry as described in Section 6.3.
• Enter Additional Daily Uses (gallons per day).
• Enter amount that will be used for Chilled Water Cooling Towers (gpd).
• Enter On-Site infiltration design drawdown rate (gpd).
• Enter filter efficiency percentage for the 1” storm.  A minimum of 95% must be achieved and is assumed as the base value, however if a higher efficiency rate is realized, this value can be adjusted.

TAB 2: JULIAN DAY CALENDER

This tab is included for assistance in selecting a start and end date for any demand practices.  The day of the year should be selected according to the julian day dates specified in this tab.

TAB 3: RESULTS – TREATMENT VOLUME CREDIT

• Select the Results-Treatment Volume Credit (TVC) tab to view modeling results for the 1” storm.
• Observe the results for the Treatment Volume Credit highlighted in the green column, the dry frequency and the overflow frequency as they relate to the cistern storage associated with TVC.  If the TVC level is much higher or lower than design objectives for many of the cistern storage sizes, the input values should be assessed to determine if the demand can be increased or decreased.

TAB 4: RESULTS

• Select the Results tab to view modeling results for all storm events.
• Observe the results for overflow frequency, dry frequency and percent of demand met by rainwater.
• If the demand met for a particular storage size is adequate, observe the dry frequency, overflow frequency and TVC.  If all of the design parameters meet design objectives and balance trade-offs reasonably well, move to the next step.  If any of the resulting performance metrics are not acceptable design objectives, then re-visit the input spreadsheet to assess whether lower or higher demands can be achieved (e.g. decrease/increase SF of irrigation area, increase/decrease rooftop area captured, if feasible, add to/subtract from On-site infiltration facility).

RESULT TO BE TRANSFERRED TO RUNOFF REDUCTION SPREADSHEET

• First Value to Transfer: Once the cistern storage volume associated with the TVC has been selected, simply transfer that credit amount into the Runoff Reduction Spreadsheet column called “Credit” in the “2.f. To Rain Barrel, Rainwater harvesting system, Cistern” row in the blue cell (cell F28).
• Second Value to Transfer: Then enter the rooftop area that was used in the same row and in the Cistern Design Spreadsheet into the “Credit Area (acres)” column in the blue cell.
  

Appendix A.3. Notes on spreadsheet use and methodology

If a use is only seasonal (e.g. summer irrigation), the spreadsheet must set the input for irrigation to zero for the purposes of Treatment Volume credit, unless an On-site Infiltration facility is designed to infiltrate an equivalent volume of water during non irrigation season periods. 

With each documented daily use, the runoff volume is reduced.  The Treatment credit is a percentage that is the sum of all the stored water that is used/disposed during the entire 30 year period divided by the entire volume that is generated during that same period for all storm events of 1” or less.  That is:

 



Where:



And



 

 = Treatment Volume Credit (%)

 = Runoff Reduction Volume.  This is the total volume of runoff that has been removed from the runoff for storms of 1” or less for the entire 30 year period.  It is calculated adding the contribution all precipitation of 1” or less times the runoff coefficient minus first flush diversion, minus overflow.

= First flush diversion and filter overflow due to filter inefficiency

= Overflow from precipitation events of 1” or less

= Runoff Coefficient of Rooftop = 0.95

 = Precipitation of 1” or less (inches)

 = Surface Area of rooftop that is captured and conveyed to cistern (SF)

i = Start day of modeling (First day modeled in 1977)

n = End day of modeling (Last day modeled in 2007)

The spreadsheet calculations should always be included with the stormwater management submittal package for local plan review.  See Section 9.0 for more information on recommended submittal package checklists and materials.