Wednesday, September 3, 2014

Solid info on rainwater carchment design, construction, estimating

Rainwater Catchment

Local rainwater catchment system designers, installation professionals, and equipment vendors (PDF)

View or download the brochure, "Rainwater Catchment in Monterey County: A Homeowner's Guide to Capture and Re-use of Rainwater" (PDF)

Rainwater Catchment Basics

Rainwater catchment is the collection and storage of rainwater for purposes such as landscape irrigation, non-potable household uses, and storm water abatement. Most often rainwater is diverted off of rooftops and diverted to storage tanks. Catchment water is not considered drinkable.

Rainwater is a valuable, free source of water that we can collect and use for much of our everyday needs, like landscape watering. By collecting rainwater, we reduce our utility bills while reducing the harmful water runoff that can carry toxic chemicals into our streams, rivers, and the ocean. Rainwater catchment is valued as a water conservation tool to reduce demands on more traditional water supply sources.

Additional benefits include:

The end use is located close to the source thereby eliminating the need for costly distribution systems
Rainwater provides a source of water when a more traditional source such as groundwater is unavailable or the quality unacceptable
Rainwater is free of Sodium salts, so there is not salt build up in the soil or on hard surfaces
Rainwater harvesting reduces flow to storm sewers and the threat of flooding
Rainwater harvesting helps utilities reduce peak demands during summer months
By harvesting rainwater, homeowners can reduce their utility bills.

Rainwater has an inherently superior quality. Rainwater has long been valued for its purity and softness. It is slightly acidic, and is free from disinfectant by-products, salts, minerals, and other natural and man-made contaminants. Landscape plants tend to grow better with rainwater.

Design Considerations

When you start thinking about a catchment system for your home or business, you’ve first got to answer the question, “How big?” A catchment system can be as small as a single 50 gallon rain barrel or as large as an underground cistern of 50,000 gallons. From one rain barrel to a linked system of 5 or 10 rain barrels, the thinking is the same: collect the water from a downspout on your roof, and store it until the dry months and use it to water your garden.

With water rates climbing rapidly, and with drought conditions around us, it will make sense to catch and store as much water as possible. We can probably collect more than we can store.

Some Disadvantages to Consider

Because rainfall events are highly unpredictable, rainwater harvesting cannot be relied upon as a long-term, drought-proof source of water supply
The capital cost for a rainwater harvesting system is typically higher than the cost of obtaining water from a centralized distribution system. However, it is comparable to the cost of drilling and installing a new groundwater well
Rainwater harvesting systems require care and maintenance after installation which may not be suitable for all homeowners
Rainwater storage tanks may take up valuable space around the house
Rainwater harvesting systems under 5000 gallons are not subject to state building code and the absence of clear construction guidelines may discourage homeowners and developers from installing these systems.

Types of rainwater catchment systems

The simplest system is a rain barrel with a watering can to scoop out the water. You may choose to add a spigot to the barrel and add a hose to that, then include diverters in the garden, and then a pump to move the water around the yard and to an irrigation system. If you believe the ‘simpler the better’, then a gravity feed rain barrel that requires no pump or piping is just the thing. If you want to catch and store thousands of gallons, and move it around your property, then you’ll need a more elaborate design, permits, etc, and enough roof area to fill the cistern.

Cost Considerations

The cost of a simple gravity feed rain barrel and the downspout to fill it is under $100. A complete rainwater harvesting system for a typical single-family home will generally cost between $4,000 and $10,000.
The single largest cost in a rainwater harvesting system is the storage tank. As expected, the cost of a tank depends on its size and construction material. On a per gallon basis, this cost can range from about $0.7 for a fiberglass tank to more than $4 for a welded steel tank. The cost of a large underground cistern is very roughly $1.00 per gallon. Therefore, a 5,000 gallon underground cistern costs roughly $5,000. Other components such as gutters, downspouts, roof washers, pumps, and pressure tanks will add to the costs of the system. Professionally installed systems can further increase costs. If the intended use of the system is to collect water for drinking, costs for disinfection must be added to the total cost. Any cistern of 5,000 gallons or more requires a permit. Any new wiring for a pump to move your water around will also require a permit.

Types of Tanks and Tank Stability

While a vast majority of the rainwater collection storage tanks are placed above ground, there are tanks available that can be installed below ground surface. In-ground storage tanks tend to be a lot more expensive than above-ground tanks because of excavation costs and the need to have a more heavily reinforced tank.
Water weighs just over 8 pounds per gallon, so even a relatively small 1,500-gallon tank will weigh 12,400 pounds. A leaning tank may collapse; therefore, tanks should be placed on a stable, level pad. If the pad consists of a stable substrate, such as decomposed granite, a load of sand or pea gravel covering the bed may be sufficient preparation. In some areas, sand or pea gravel over well-compacted soil may be sufficient for a small tank. For any tank over 5000 gallons, a concrete pad should be constructed. When the condition of the soil is unknown, enlisting the services of a structural engineer may be in order to ensure the stability of the soil supporting the full cistern weight.
Consider protecting the pad from being undermined by either normal erosion or from the tank overflow. The tank should be positioned such that runoff from other parts of the property or from the tank overflow will not undermine the pad. The pad or bed should be checked after intense rainfall events.

Calculating Storage Tank Size

In theory, a rainwater harvesting system can collect approximately 0.62 gallons of water per square foot of roof area, per inch of rainfall. In practice, however, there is always some loss due to first flush, evaporation, splash-out, overshoot from gutters, and possible leaks. Most installers use an efficiency of about 75 to 85 percent for the system.
For a Monterey home with a roof surface of 2,000 square feet, using a collection rate of 0.62, a system efficiency of 0.85, and an average annual rainfall of 17 inches, you can expect to collect about 18,000 gallons of rainwater per year (0.62 x 0.85 x 2,000 x 17 = 18,000 gallons per year). So, even if you only collect from half the roof in a drought year (0.62 x 0.85 x 1,000 x 17 = 5270 gallons per year), you still can collect a lot of rain water. You just need a tank large enough to store it.

There are various methods determine the amount of rainwater you should collect to meet your needs. The easiest method to calculate landscape water use is to look at your past water bills and compare winter months water use to summer water use each month. The portion of higher water use in the summer is most likely that amount used for exterior landscaping. Some very rough, but simple daily consumption guidelines are:

Gardens/Lawns 600 gallons per 1,000 square feet
Young Trees 15 gallons
Small Animals .25 gallons per 25 pounds
Dairy Cattle 20 gallons
Range Cattle 15 gallons

Since rainwater is so valuable, based on the effort to collect and store it, it is not advised to use it to water lawns. Watering grass would be wasting this valuable resource. A typical lawn requires about 3,000 gallons a month. This means you would need some large tanks to hold the water, especially in drier climates. Additionally, you would need a very large surface area to capture the rain.
If you choose to water grass and are planning on installing large tanks, consider reducing your outdoor water consumption as much as possible elsewhere. Choose low water use, hearty, native plants and consider irrigating all your landscape, including the lawn, with drip or sub-surface irrigation. But remember, rainwater is still free. The constraint is the cost of the tank.

Provide a Tank Overflow Pipe

Run-off that is not collected in the tank or that overflows should be diverted away from tank foundations, buildings or other structures. This water should be directed onto gardens or into the storm water drain; it should not be allowed to pool or to cause nuisance to neighboring properties or to areas of public access.

Tank Installation

Tanks are often located in remote locations. Consider what type of equipment will be required to access the tank pad. How close can a 40' tractor and trailer combination get to the tank pad? How much reach is needed to lift the tank or components with crane? Is the site accessible by two-wheel drive? How large of a vehicle can the access road accommodate? Does the site have existing structures, tanks or foundations? Is alternative water service available? Is there electricity available at tank site? Are there any overhead obstructions such as power lines or trees? In addition to the physical aspects of the tank site, consideration should be given to piping from water source, gravity flow, site aesthetics, soil stability, drainage, and site security (to name a few)..
From my own experience, having road access to the top of the construction site, or higher, has huge advantages, ie, no more pushing wheelbarrows full of sand, cement, firewood to push uphill!

Consider Installing a Pump

If your tank or rain barrel is at ground level and you need to move the water up any slope, then you will likely need a submersible electric water pump. However, sometimes you can get enough water pressure in an elevated, closed-looped water collection system to supply the pressure required, even enough to drive a sprinkler system.

Try to place your tank at the highest elevation possible. Every foot you raise your storage tank increases the pressure about 0.433 psi (1 psi ~ 3.21 feet of fresh water head). It generally takes only a few feet of elevation to allow the use of a hose or drip system, but it takes an elevation difference of over 50 feet to run a sprinkler or sprinkler system.

Screens and Water Filtration

In California, our first concern is debris. Leaves and algae’s wash off the roof and into the gutters. So, the first defense against this issue is the installation of gutter screens. There are many types available, of varying price and effectiveness. The second line of defense is the diverter. This device screens debris out of the water as it diverts it away from the downspout and toward the storage tank. Then another even finer screen is used in the leaf catcher to catch even smaller bits of debris. Then a first flush device takes the very first water to come off the roof and disposes it in the landscape. It is believed that this first flush of water would contain bird droppings and other contaminates that build up over the dry season. However one study shows that the first rain simply loosens the debris, and then a later hard rain washes it off.
It is recommended that you install an inexpensive in-line filter at the outlet of your tank to collect small debris. Most irrigation stores sell inline sprinkler filters. This is a simple device that screws into the line, prior to your pump or irrigation system, and cleans out the small leaves and other stuff (i.e. sometimes referred to as particulates) so it does not clog your drip irrigation emitters or sprinkler nozzles.
Screens and filters are categorized by the size of filtration. Below is a conversion chart comparing mesh to microns. These terms are typically are used to tell you how small the opening is in the filter.

Tank Sludge

All tanks should be examined for the accumulation of sludge every 2-3 years, or if sediment is evident in the water flow. Sludge can provide an environment for survival and/or growth of micro-organisms and in some cases relatively high concentrations of lead have been detected in sludge even though the body of stored rainwater complied with drinking water guidelines.

Often, a drain valve is provided at the bottom of tanks, and as sludge accumulates, this valve can be opened to allow removal of tank rinse water. Sludge may also be removed by siphoning without emptying the tank. To do this, use a swimming pool vacuum or siphon, and move it carefully across the bottom of the tank.

Tank Cleaning

Where cleaning necessitates entering the tank, care should be taken to ensure adequate ventilation is provided and an additional person is in attendance. Advice on working in confined spaces should be available from Occupational, Health, Safety and Welfare authorities in each State. It is important to check the structural condition of the tank before choosing a method of cleaning. Cleaning agents that might release hazardous fumes or adversely affect water quality after cleaning should not be used. After cleaning it is recommended that the internal walls and floor of the tank be rinsed with clean water. Rinse water and sediment should be run to waste.


Rainwater collected and stored in domestic tanks is likely to contain micro-organisms from one or a number of sources. While most will be harmless, the microbiological safety of rainwater will depend on the exclusion of organisms that can cause infections of the gastrointestinal tract (enteric pathogens). The enteric pathogens include types of bacteria, viruses and protozoa. These organisms are typically introduced into drinking water supplies by contamination with fecal material from humans, animals and birds, with human enteric pathogens more frequently carried in human waste.

The majority of domestic rainwater storage tanks are installed above ground and collect run-off from roofs via gutters. Likely sources of micro-organisms include:

soil and leaf litter accumulated in gutters particularly if kept damp for long periods due to poor drainage.
fecal material deposited by birds, lizards, mice, rats, possums etc.,
dead animals and insects either in gutters or in the tank itself.


Rainwater tanks can provide a very good habitat for mosquito breeding. The most effective control measure is to prevent access of adult mosquitoes. If access has occurred, remedial action can be taken to prevent the release of mosquitoes. Ensure that, unless in use, all access points excluding the inlet and any overflows are kept shut with close fitting lids that will prevent access of mosquitoes.

Inlets and overflows should be covered with closely fitting removable insect-proof screens. The screens should be made of non-rust material formed, typically, with 0.315 mm diameter material and 6x7 mesh openings per cm2. The screens should be readily accessible for regular cleaning.

Mosquito larvae (wrigglers) found in rainwater tanks indicate the presence of an opening through which the female mosquito can enter and lay eggs on the water. The opening should be closed. This will prevent further entry and will also prevent the escape of any hatched mosquitoes.

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