Stormwater MAnagement Systems
"Water is life, and clean water means health."
- Audrey Hepburn
- Audrey Hepburn
introduction
Stormwater is surface water that occurs in abnormally high quantities in response to rain or snow. This water is sometimes called 'nuisance water' because it typically doesn't have anywhere useful to go. For the most part, this water flows across our urban landscapes, picking up toxins, chemicals, salts, and other materials along the way, and drains out to main waterways via street drains, following the natural curves of the land. The presence of human-caused pollution in our local waterways has already caused irreparable damage to the ecosystems that surround us, and continued irresponsibility could lead to the full collapse of our natural landscapes.
From Precipitation to Storage: Rain Barrel Systems
Stormwater enters gardens through precipitation, either from rain or snow (including snow melt). Rainwater is a great resource for gardens because it is free of cost, mostly pure, and can help plants thrive. Landscape plants love rainwater for a few reasons. First, rainwater is 100% soft water. This means that it is free of the salts, minerals, treatment chemicals, and pharmaceuticals that are often found in municipal water, groundwater, and surface water. These chemicals and other minerals build up in soil over time, causing imbalances in salts and, consequently, nutrients. Rainwater is also naturally slightly acidic. Most plants prefer a soil pH of between 5.5 and 6.5, which is on the acidic side of neutral. By contrast, city water is likely treated to be more alkaline in order to protect the pipes that it runs through to get to your garden, and can have a pH as high as 8.5. Using greywater to irrigate gardens can also be problematic, as it can have a pH as high as 10.5, depending on what sorts of soaps and detergents have been dissolved. Stored rainwater (rainwater that has been funneled into a container for later use), contains some organic matter, as well, which can assist in adding nutrients to the garden when this water is used to irrigate. Finally, rain contains nitrates, which are an important group of macro-nutrients which provide bio-available nitrogen for use by plants.
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Depending on soil structure, previous saturation, and soil quality, water can either infiltrate or run off of soil. Ideal circumstances for optimal soil water retention are a mix of sand, silt, and clay. This often refers to 'loamy' soils, which have smaller particle size, plenty of organic matter, and a healthy soil biome. As great as rainwater is for landscapes, there is a point at which the soil becomes fully saturated and physically cannot soak up any more water regardless of how great soil is at holding water. For more information on soil and soil water retention, visit the Soil module.
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Harvesting rainwater can help to ease the pressure on the landscape under high rainfall times of the year. This water can be used for many purposes, from irrigation to clothes washing to drinking. There are many benefits of harvesting rainwater for storage, including:
Rainwater is relatively clean, and it is absolutely free.
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Homeowners have total control over water supply (ideal for water restrictions).
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It is socially commendable and environmentally responsible.
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It promotes self-sufficiency and conserves water.
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Rainwater is better for landscapes due to absence of chlorine.
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It reduces stormwater runoff from homes and businesses.
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It can solve drainage problems on the property.
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It can be used as a main or backup source for well and municipal water systems.
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It uses simple technologies that are cheap and easy to setup and maintain.
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It can be easily retrofitted to buildings or built during initial construction.
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Systems are flexible and modular, allowing changes to be made down the road.
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It is a great source of emergency water.
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Irrigating gardens with rainwater is inexpensive, climate-friendly, and is a lifesaver during times of drought. The practice of rainwater harvesting involves collecting water runoff from structures or other impervious surfaces in order for it to be stored for later use This traditionally involves harvesting runoff from a roof, and is becoming an increasingly popular alternative for supplying our households (and even businesses) with water. In some countries, rainwater harvesting is the norm, and as we develop new technologies and materials is is becoming a more common practice in some parts of the United States. Rainwater harvesting and storage systems can be configured to supply your whole house or simply supply irrigation for a landscape.
The amount of rainfall that can be collected in a landscape varies, depending on many factors. The general formula is:
1" of rain x 1 square foot = 0.0623 gallons
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Using this formula requires a few pieces of information. The first is the average rainfall in your area. There are many sources that can provide this data, or you may already know the number. Many services, like the USGS and NOAA keep this information on an ongoing basis; the map shown here is just one of those resources. The next piece of information you'll need to calculate is the square footage of the area that will be collecting water. With smaller-scale systems, this may equate to the area of open-mouthed buckets and containers; in larger operations, this can be the square footage of the roofs of buildings.
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In order to calculate this square footage, add up the total area that will be able to catch water to harvest. This can be found for larger catchment areas using websites like Google Earth, which allow users to measure areas on a property seen from a birds' eye view. For smaller catchment areas, simply measure with a measuring tape or ruler and calculate the area in square feet. For example, a rain harvesting system using runoff from the roof of a home in the Bay Area, California would be calculated as such:
27 feet x 61 feet = 1,647 square feet of roof
17 (inches of water per year) x 1647 (square footage of roof) = 27,999
27,999 x 0.0623 = 1,744.4 gallons of rainwater per year
17 (inches of water per year) x 1647 (square footage of roof) = 27,999
27,999 x 0.0623 = 1,744.4 gallons of rainwater per year
Calculating the potential water collection from a smaller system, such as a single barrel with a diameter of three feet, it will look more like this:
3.14159 x (1.5)^2 = 7.065 square feet
17 (inches of water per year) x 7.065 = 120.1 gallons of rainwater per year
17 (inches of water per year) x 7.065 = 120.1 gallons of rainwater per year
Obviously, the amount of rainwater that is able to be harvested each year is heavily dependent on on the size of the rainwater catchment area. Rainwater harvesting is a process that can be used on its own or in conjunction with other stormwater management systems, like bioswales and rain gardens.
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Quick Question: What's the difference between a Cistern and a Rain Barrel?
Cisterns are similar in function to rain barrels, but are typically much larger. On average, rain barrels will hold up to 50 gallons of water, and cisterns hold between 100 and a few thousand gallons. Cisterns can be either above or below ground, and are the historical form of rainwater collection. They also can hook up to multiple sources of water inputs, whereas rain barrel systems typically link up with a single gutter downspout. |
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The typical basic rain barrel setup has a few parts. It begins with a water catchment area, usually the roof of a building with a gutter system to collect runoff. The rain barrel is placed on a level concrete or gravel foundation under a downspout of the gutter system, which is shortened so that feeds into the barrel. It can be raised up for better water pressure, or left at ground level. The storage container should be covered with some sort of mesh screening that allows water and air to penetrate, but not bugs or debris. A few inches down from the top (typically 3-4 inches), an angled runoff spout is installed with or without an attached hose. Finally, a spigot is typically installed towards the base of the barrel to allow for extracting collected water for use.
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Building a Basic Rainwater harvesting System
Before starting, it is always a good idea to check local laws, regulations, and codes to make sure that everything is above-board. Some states do regulate the harvesting of rainwater, and others even have tax rebates available for homeowners that install them. To find more information about your area, visit the Federal Energy Management Program website and click on your state. The components of rainwater harvesting systems are relatively uniform, varying depending on the size of the system and the preferences of the person guiding the project. The basic components of a rainwater harvesting system include the following:
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Rainfall Catchment Surface(s) This is typically a roof of a home or other structure on the property, but can really be any area that is wide and somewhat elevated. Some rainwater catchment systems, called 'rain roofs' (pictured), are installed without a structure underneath solely for the purpose of maximizing the catchment area on a property. |
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Conveyance System (Gutters and Downspouts) These components run around the sloped catchment areas and transport the water to the storage tank. Horizontal pipes are called gutters, and vertical pipes are called downspouts. These conveyances can be made from a wide variety of materials, with galvanized steel, zinc, and aluminum being the most commonly used. Many homeowners choose to use the systems that already exist around their homes, and simply re-route downspouts to empty into storage tanks instead of simply diverting rainfall away from the structure. |
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Filters and Screens To avoid any backup issues due to debris in the system, these components are installed at multiple points. Regardless of whether a rainwater harvesting system is installed, gutters on a home should have filters installed to reduce the need for cleaning them. It is also important to have screens over the top of open storage tanks. Floating filters are also used in some systems inside of storage tanks to clean water a final time before it enters a pipe or pump. |
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Storage Tank(s) These components can be barrels, buckets, tanks, or cisterns. Their function is to hold water once it has been harvested, and are usually the most expensive part of a rainwater harvesting system. These storage containers can be made of any material and can be placed above or under the soil surface. Some of these containers will have built-in components for use in a rainwater harvesting system, and others are bare-bones and will need to be upgraded. |
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Foundation
Water is heavy; each gallon of water a tank holds can add 8.3 pounds (meaning a 55-gallon barrel will weigh approximately 450 pounds when filled). This weight requires a strong foundation so that the tank stays level and does not sink into the ground. A foundation for a water storage tank should be sturdy and immovable. Consider compacted gravel.
Storage Tank Vent
Storage tanks will always be 100% filled, either with water, with air, or a mixture of both. They should therefore have a opened area or a vent that allows air to pass in and out as the tank empties and fills with water. If a lid on a storage tank is on too tightly, and the tank's water empties without enough air entering and neutralizing the interior pressure, the tank can implode. The opposite can also happen if air is not allowed to exit as the tank fills.
Overflow
In addition to allowing air to pass out of the tank, water also needs to be allowed to leave the tank if it gets too full. All storage tanks in rainwater harvesting systems should have a pipe that routes any excess water away from building foundations as well as the foundation of the storage tank. It is helpful to direct this water to a rain garden or bioswale area, or simply to an area of the garden that could use or handle some extra water.
Outlet
This component is where the harvested water will be taken out for use. Many people choose to attach a spigot and/or hose that is then used to water various areas of the garden. Other systems, especially those that will be used as a potable water source, are connected to a pump or another finer filtration system.
First-Flush System
This optional component serves to flush the water that is harvested from the first rain of the season, which typically carries with it many more toxins and pollutants than subsequent precipitation events. For most outdoor uses, such as for irrigating gardens, this step is not necessary. These diverters are, however, very helpful when collecting water for drinking. These first-flush systems need to be cleaned and inspected on a regular basis, as they can clog up or collect bacteria over time.
Pumps and Indoor Use Filtration and Treatment
These components are not always necessary, especially when the water is only being used for garden irrigation purposes. They are necessary when water pressure is critical for use and the tank will not be elevated enough to create it naturally. To increase water pressure at the point of use without the use of a pump, the tank can be elevated. For every foot of elevation of a water tank, it increases water pressure by 0.43 PSI. If water pressure is still not strong enough after raising the barrel (or if it is impractical to raise the barrel), a pump may be needed. If harvested water is to be used indoors for faucets and appliances, a pump is a necessity.
Treating Stored Rainwater for Use
french drains
bioswales
rain gardens
berms
bioswales
rain gardens
berms
