Project Overview 

The central location of the San Anselmo Public Library makes this a highly visible demonstration project that thousands of people will walk past annually. Rainwater harvesting was incorporated into the Town of San Anselmo's existing plans to renovate the landscape at the library. The project consists of a small cistern on one side of the building, and two rain gardens flanking the library's front steps.

Rainwater Catchment System Unique Features

A 305-cistern at the side of the building receives rainwater run-off from one downspout that collects water from approximately 675-square feet of the library's roof. A first flush and debris diverter unit was installed at the downspout to eliminate debris, such as leaves, and minimize pollutants, such as dust or bird droppings deposited on the roof, from entering the cistern. This maintains a high quality of water inside the cistern, and avoids problems with clogged irrigation tubing down the road.

The cistern sits on a concrete pad designed and built by the Town of San Anselmo. Overflow from the cistern along with the first flush water return to the existing storm drain system- due to site constraints it was not practical to install a rain garden directly adjacent to the cistern to handle its overflow.

A low-pressure gravity drip irrigation system was installed to convey stored water from the bottom of the tank to plantings in the rain garden located in the front of the building without the need for a pump. The system uses a Holman battery controller to regulate the flow of water, which then passes through a 400 micron IrriGRAY filter to remove any sediments from the water. 3/4-inch poly tubing carries the water to the planting area, and then connects to 3/8-inch in-line dripper tubing with emitters at 12-inch spacing. This dripper tube is run around the perimeter of the planting area, and has screw-on end caps to allow the system to be easily flushed for maintenance.

Two downspouts at the front of the building drain directly to the rain gardens.  Each downspout collects rainwater run-off from approximately 675 square feet of the roof. 

To build the rain gardens, soil was excavated in a bowl shape to a depth of approximately two feet, and a low concrete wall was built around the edges in order to maximize the amount of space available for collection. The space was filled back in with a "Bio-filtration" soil mix that promotes fast drainage. In the event of a large storm event, overflow from both rain gardens travels via pipes under the sidewalk to the street, where all the run-off originally went prior to the installation of this project.

Square foot area draining to each downspout:
Approximate gallons per downspout per inch of rain:
Total gallons generated per year (44 inch average) per downspout:
Rain catchment capacity in tank:
Rain garden capacity per basin:
Potential stormwater run-off mitigation per rain garden per year:

675 square feet
378 gallons
16,652 gallons
305 gallons
180 square feet
16, 652 gallons each