Tamalpais Community Services District Rainwater Harvesting System

Acknowledgements: This project was designed and installed in 2010 as part of the 10,000 Rain Gardens Project. Special thanks to the Tamalpais Community Services District (TCSD), the Urban Farmer Store and Americorp Watershed Steward Program interns who helped make this project possible.

Interpretive sign at project site
1,550 gallon cistern at rear of building

Project Overview

The Tamalpais Community Services District (TCSD) provides a range of services to it approximately 7,000 constituents in Tam Valley. This rainwater harvesting demonstration project is located at the TCSD's Tamalpais Valley Community Center, an active hub which hosts a range of public activities and events, and is adjacent to a well-used bicycle route up the Tennessee Valley to the Marin Headlands.

The system is design to capture rainwater run-off form the Community Center roof. At the southwest side of the building, a downspout directs run-off into a 1,550-gallon cistern, and at the northeast corner of the building, a downspout directs run-off into a 50-gallon rain barrel. In both cases, overflow is directed back to the existing drainage system. Space limitations, mature landscaping and slope impacted this design and eliminated the possibility of installing a rain garden for overflow water from the cistern and barrel. Stored water is saved for the summer to help establish new native plantings and to irrigate established landscaping.

Unique Features and Installation Details
The area for the cistern was prepared by first leveling the ground, and a foundation ring was installed then filled with pea gravel to create a solid base for the cistern. The existing downspout was cut off at a height that allowed the downspout diverter to be inserted with adequate space to run the water to the tank through a "wet conveyance" system. This type of system uses the self-leveling properties of water to move water from the downspout to the cistern in a continuously sealed pipe. Restrictions in the flow of water, called friction loss, which cause the water to back up a certain amount during a heavy rain event must be taken into account. 
Cistern with pump box


'Wet' conveyance system
As the name implies, the pipe stays "wet," or full of water, in between rain events, so it is important to provide a way to drain the line at a low point (visible at bottom of photo- a threaded cap was added for easy draining). This type of conveyance system is useful when aesthetics are a concern, if there is a path, patio or other hardscape feature between the tank and the downspout, or for moving water longer distance where an elevated pipe (called a "dry conveyance" because it drains dry between rain events) is impractical.

New water-wise native plantings


First flush system
A downspout diverter has a screen which prevents debris from entering the cistern and is easily cleaned off. This manufactured unit also has a first flush diverter that allows the initial rain that carries a greater amount of dust, bird droppings and other potential pollutants to bypass the tank. This system is plumbed so that the first flush water goes into the existing drain system where all the water was previously going. This particular first flush has a foam insert that expands as it becomes wet, and plugs the bypass so that the cleaner water is directed to the cistern via the pip coming out the side. Air vents allow the unit to dry out between rain events.


Water enters the cistern through the manufacturer-installed bulkhead fitting on the side. An additional two-inch bulkhead fitting was installed just below the level of the inflow to provide for an overflow. The overflow pipe provides ventilation and is piped to an existing drain system, where all the water was originally going. The overflow vent and discharge end are covered with wire mesh secured by a pipe clamp to prevent mosquitoes or other critters from entering the tank.

In California, a rainwater cistern is considered an auxiliary water source and requires the installation of a reduced pressure backflow prevention device at the water meter to prevent any potential contamination of the domestic water supply by the non-potable rainwater cistern. Annual inspection and testing of the device is required. TCSD had an existing backflow prevention device.


Drip irrigation system
Drip Irrigation System and Native Plantings
A pump is used to distribute water from the tank to a drip irrigation system. A SMART controller with an on-site weather station is used to program the irrigation system and adjusts the amount of water applied based on weather conditions. This system allows the precious stored rainwater to be used as effectively as possible based on the rate that plant-available water in the soil is transpired by plants and evaporated from the soil.

Native yarrow, coffeeberry, and monkey flower grown in SPAWN's native nursery were planted in the area around the tank. The coffeeberry will eventually provide additional visual screening of the tank and is beneficial to local wildlife. The yarrow and monkey flower will fill in the gaps between the existing plantings, providing beautiful flowers and foliage as well as wildlife benefits. Once these plants are established, usually in one to three years, the stored rainwater can be re-purposed to establish additional native plantings, supplementing irrigation demand for other landscaping at the Community Center, or be piped over to the Center's nearby demonstration vegetable garden to showcase the use of rainwater for growing food.